Introduction
Research Objectives
This research investigates the development of catalyst materials utilizing a Z-scheme photocatalytic system that demonstrates high photocatalytic activity and stability under visible and solar light The study emphasizes synthesis methods aimed at practical applications in wastewater treatment The synthesized photocatalysts were tested for their effectiveness in degrading organic pollutants in water, with specific objectives focused on enhancing environmental remediation.
To synthesize various highly efficient Z-scheme photocatalytic systems
To characterize the morphology and properties of the as-prepared materials
To study the photocatalytic activity of the as-prepared samples in the degradation of organic pollutants in water
To understand the charge transfer mechanism of the as-prepared photocatalyst.
Dissertation Outline
This dissertation consists of seven chapters, covering different photocatalytic systems, their synthesis methods, characterizations, and photocatalytic activities in the degradation of organic pollutants in water
Chapter 1 describes the background, objectives, and outline of the research
Chapter 2 presents the synthesis of ZnO nanoparticles (NPs) with three morphologies deposited on rGO nanosheets using hydrothermal method and their use as photocatalysts for the degradation of MB and RhB dyes under UV light and solar light irradiation
Chapter 3 presents the synthesis of Z-scheme photocatalyst of CMoS2/TiO2 octahedra using a two-step hydrothermal method and their application in the degradation of MB, RhB, MO dyes, and TCH under solar light
Chapter 4 presents the synthesis of Z-scheme ternary photocatalyst of silk cocoon-like BiPO4 on BiOBr/Bi2O3 nanosheets using a rapid microwaveassisted one-step method and their application in the degradation of RhB, MO, TCH, and
Chapter 5 presents the synthesis of Z-scheme ternary photocatalyst of layer- structured BiOCl/BiOBr/Bi2O3 nanocomposite using a rapid microwave-assisted one-step method and their use as photocatalyst in the degradation of RhB, MO dyes, and TCH under visible and solar light
Chapter 6 presents the synthesis of Z-scheme quaternary photocatalyst of
Ag/AgBr@BiOBr/Bi2O3 nanocomposite using rapid microwave-assisted one-step method and their use as a photocatalyst in the degradation of RhB, MO dyes, and
TCH under visible and solar light
Chapter 7 provides an overall summary of the research with some recommendations for future works.
Solvent-driven morphology-controlled synthesis of highly
Introduction
Titanium dioxide (TiO2) is widely recognized as a versatile photocatalytic semiconductor, utilized in applications such as self-cleaning surfaces, antibacterial processes, anti-reflective coatings for solar cells, and anti-corrosion agents The production of TiO2 involves various methods, including sol-gel, electrochemical synthesis, chemical vapor deposition (CVD), and physical vapor deposition (PVD), which often require advanced equipment and complex procedures In contrast, zinc oxide (ZnO) has emerged as a cost-effective and environmentally friendly semiconductor, suitable for applications like photodetectors, energy-storage materials, supercapacitors, and photocatalysts Despite its advantages, ZnO faces challenges such as the recombination of photo-generated electron-hole pairs, photocorrosion, and low photostability, which significantly hinder its photocatalytic performance.
To address the challenges in preparing ZnO, various strategies have been employed to enhance its specific surface area, boost catalytic activity, reduce photocorrosion, and improve electron-hole separation This is achieved through the development of hybrid composite materials that incorporate carbon compounds, including C3N4, C60, and chitosan.
[22], and monolayer polyaniline [24], or doping with non-metal [27] or metal elements [28]
Graphene, an allotrope of carbon, significantly enhances the photocatalytic performance of metal oxides due to its large specific surface area, high electronic conductivity, chemical stability, and effective charge transportation This has led to increased interest in combining ZnO with graphene, aiming to improve electron-hole separation efficiency and facilitate the movement of charges generated by light excitation Recent studies have explored various ZnO structures, including particles, rods, flakes, flowers, rings, and bowls, integrated with graphene sheets Notably, Cai et al demonstrated the preparation of ZnO rods on 3D graphene oxide using chemical vapor deposition followed by hydrothermal synthesis.
[1] However, the corresponding synthetic procedure is complicated, and the photodegradation efficiency of that composite was only 92% for methyl orange
After three hours of UV irradiation, Xu et al synthesized ZnO flowers on reduced graphene oxide (RGO) using hydrazine as a reducing agent under hydrothermal conditions Hydrazine is known for its high toxicity and instability The study reported the highest rate constant observed in the degradation of methylene blue.
(MB) under UV light was only 0.0395 min –1 [31] Moreover, Wang et al grew
ZnO nanoparticles, nanosheets, nanospheres and nanorods on RGO by a hydrothermal method for photocatalytic H2 production [19]
While numerous studies have explored ZnO/RGO nanocomposites and their applications, there is a notable lack of research focusing on how different morphologies of ZnO nanoparticles anchored on RGO sheets influence the photocatalytic efficiency of these composites This gap in the literature highlights the need for further investigation into the relationship between nanoparticle morphology and photocatalytic performance in ZnO/RGO systems.
In this study, we successfully synthesized three distinct morphologies of ZnO particles—short nanorods, nanodisks, and nanospheres—on reduced graphene oxide (RGO) sheets by adjusting the solvent ratio of ethanol to deionized water using a straightforward two-step method The orientation and distribution of the nanoparticles on the RGO sheets were significantly influenced by the ZnO seeds, which were deposited under a flow of hot argon gas.
The photocatalytic performance of the as-prepared composites was investigated by measuring the degradation of the two dyestuffs, MB (C.I 52015) and rhodamine B (RhB, C.I 45170).
Experimental
Graphite powder (99.9995%, Alfa Aesar), zinc acetate dihydrate (Zn(CH3COO)2∙2H2O, 99.0%, Sigma Aldrich), hexamethylenetetramine
((CH2)6N4, HMTA, 99+%, solid, Alfa Aesar), MB (high purity, Alfa Aesar),
RhB (90%, Sigma Aldrich), sodium hydrogen carbonate (NaHCO3, >99%,
DaeJung Chemicals), tert-butyl alcohol (TBA, >99.5%, DaeJung Chemicals), and p-benzoquinone (p-BZQ, 98%, Sigma Aldrich) were used as received
Deposition of ZnO seeds on RGO sheets
The synthetic process, illustrated in Scheme 2.1, involves two primary steps Initially, ZnO seeds are deposited on reduced graphene oxide (RGO) using an adapted method from earlier studies This begins with the dispersion of 50 mg of graphene oxide (GO), prepared through a modified Marcano-Tour method, in absolute ethanol at a concentration of 1 mg/ml while stirring vigorously Following this, 54.9 mg of the necessary materials are introduced to complete the process.
Zn(CH3COO)2∙2H2O was added to a graphene oxide (GO) suspension and stirred for 60 minutes The mixture was then centrifuged at 12,000 rpm to obtain a precipitate, which was dried overnight in a vacuum oven at room temperature Subsequently, the precipitate was calcined in a quartz-tube furnace at 300 °C under argon flow for 60 minutes, with a ramp rate of 9 °C min ‒1 and a working pressure of 107 to 120 kPa, resulting in the formation of ZnO crystal seeds on reduced graphene oxide (RGO) sheets.
Growth of various ZnO morphologies on RGO sheets
The growth of various ZnO morphologies from seeds was successfully achieved under mild conditions by varying the ratios of absolute ethanol to deionized water In a 20-ml vial, 15 mg of prepared ZnO seeds/RGO was combined with 15 ml of absolute ethanol and stirred for 2 hours Following this, 98.8 mg of Zn(CH3COO)2∙2H2O and 63.1 mg of (CH2)6N4 were added to the mixture and stirred for an additional 30 minutes The resulting suspension was then transferred to a 30-ml Teflon-lined stainless steel autoclave and heated in an oven at 105 °C for 24 hours under autogenous pressure.
The reaction mixture was cooled rapidly to room temperature, and the precipitate was collected by centrifugation at 7000 rpm for 5 min and washed five times with
The sample was initially treated with deionized (DI) water and subsequently with ethanol It was then dried in a vacuum oven at room temperature for 24 hours, resulting in a powder of spherical ZnO nanoparticles, designated as sZG Additionally, ZnO nanodisks (dZG) and short nanorods (rZG) were also synthesized and anchored.
RGO sheets were synthesized using ethanol to water ratios of R = 1:0.25 and 1:1 The impact of ZnO seeding on the morphology and photocatalytic efficiency of the photocatalysts was assessed through the synthesis of sZG*, dZG*, and rZG* samples, which were produced without the calcination step Additionally, the function of hexamethylenetetramine (HMTA) in the reaction was explored by replicating the synthesis process without incorporating HMTA.
Scheme 2.1 Schematic illustration of the synthesis of the ZnO/RGO nanocomposites with three different ZnO morphologies
The morphology of the samples was observed by scanning electron microscopy (SEM, Hitachi, S-4800) and transmission electron microscopy (TEM,
The crystallinity of the samples was analyzed using powder X-ray diffraction (XRD), while high-resolution transmission electron microscopy (HRTEM) images were obtained at an acceleration voltage of 200 kV with a TITAN G2 ChemiSTEM Cs Probe electron microscope.
The study utilized a PANalytical X’Pert-PRO MPD system with Cu Kα radiation (λ= 0.154 nm), operating at an accelerating voltage of 40 kV and a current of 30 mA, with a scan rate of 0.101° s ‒1 Fourier-transform infrared (FTIR) spectra were obtained using a Bio-Rad Excalibur Series FTS 3000 spectrometer, while Raman spectra were recorded with a HORIBA XploRA plus spectrometer, employing laser excitation at 532 nm Additionally, the elemental composition and chemical state of the materials were analyzed through X-ray photoelectron spectroscopy (XPS) using an AXIS Nova system with aluminum radiation.
Kα monochromatized radiation The Brunauer-Emmett-Teller (BET) surface area and pore-size distribution were obtained using a N2 adsorptiondesorption apparatus (3Flex, Micromeritics Instruments Corp.) at ‒196 °C with degassing at
The samples were heated at 200 °C for 24 hours before undergoing the adsorption-desorption process The BET specific surface areas were determined using the BET equation, while the pore-size distribution was analyzed through Barrett-Joyner-Halenda (BJH) methodology Additionally, UV-vis diffuse reflectance spectra for solid samples and UV-vis spectra for liquid samples were recorded across a specified wavelength range.
The study utilized a Varian Cary 5000 UV-vis-NIR spectrophotometer to analyze wavelengths ranging from 200 nm to 800 nm Thermogravimetric analysis (TGA) was conducted using a TA Instruments SDT Q600, covering temperatures from room temperature to 900 °C at a heating rate of 10 °C min⁻¹ in an air atmosphere Additionally, photoluminescence (PL) spectra were recorded with a HORIBA scientific photoluminescence spectrometer, employing a laser for excitation.
Electrochemical impedance spectroscopy (EIS) measurement
EIS was performed using an Autolab PGSTAT 302 N (Metrohm) instrument equipped with a conventional three-electrode cell system The Ag/AgCl electrode
(reference electrode), Pt electrode (counter electrode), and working electrode
The electrodes were fabricated by depositing various active materials, including sZG, dZG, rZG, and pure ZnO, onto fluorine-doped tin oxide (FTO) glass using the drop-casting method Specifically, 2 mg of the prepared catalyst was combined with 1 ml of ethanol and 10 μl of Nafion as a binder to create a slurry A volume of 15 μl from this mixture was then drop-casted onto a 1 cm × 1 cm area of the FTO glass, followed by drying the electrode at ambient conditions for 24 hours The electrochemical performance was evaluated in a quartz cell containing a 5 mM K3[Fe(CN)6] solution in a 0.5 M Na2SO4 electrolyte, with impedance spectra recorded over a frequency range of 10^5 Hz to 0.01 Hz.
The photocurrent of the samples was measured on an Autolab PGSTAT302
N (Metrohm) instrument at 0.5 V vs the saturated calomel electrode (SCE) [35]
In this experiment, working electrodes were fabricated on FTO glass following the same method as the EIS study A conventional three-electrode cell system was submerged in a 1 M Na2SO4 electrolyte, and the photocurrent response was recorded over multiple on/off irradiation cycles for 300 seconds using a 10 W UV lamp (λ = 254 nm) as the light source Each working electrode featured a uniform illuminated area of 1 cm × 1 cm.
The photocatalytic activity of the ZnO/RGO samples was evaluated through dye degradation under UV-light irradiation using an ultraviolet (UV) lamp
In this study, the photodecomposition efficiency of photocatalysts was evaluated using MB and RhB dyes as model contaminants in wastewater, utilizing a Philips 40 W lamp with a centered wavelength of 365 nm and a solar simulator (ABET Technologies, 150 W) under one sun illumination The reactions were conducted in a photochemical reactor at room temperature, where 5 mg of photocatalyst was added to 50 ml of a 10 mg L ‒1 aqueous dye solution, achieving a photocatalyst loading of 0.1 g L ‒1 The mixture was stirred in a dark chamber for 30 minutes to reach adsorption-desorption equilibrium before irradiation commenced During the photodegradation process, 3 ml aliquots were taken at regular intervals, centrifuged, and the residual dye concentrations were measured spectrophotometrically at λ = 664 nm for MB and 553 nm for RhB The reusability of the sZG catalyst was tested over 15 cycles for the degradation of a 10 mg L ‒1 MB solution, with the catalyst being washed and dried after each cycle Post 15 cycles, the catalyst's surface morphology and structure were analyzed To elucidate the photocatalytic mechanism and identify active oxidizing species, trapping reagents such as 0.2 M TBA, 0.2 M NaHCO3, and 1 mM p- were introduced to the dye solutions prior to UV illumination.
BZQ [23,35] were used as scavengers for the hydroxyl radicals, holes, and superoxide radicals, respectively.
Results and Discussions
Characterization of ZnO/RGO nanocomposites
The synthesis of ZnO/RGO nanocomposites involves a two-step process Initially, graphene oxide (GO) is created through the oxidation and exfoliation of graphite flakes In the first step, ZnO seeds are deposited onto reduced graphene oxide (RGO) sheets, followed by the growth of ZnO nanostructures under mild conditions, utilizing various ratios of absolute ethanol to deionized water.
The synthesis procedure is simple and efficient for obtaining different morphologies of ZnO deposited on RGO, namely short nanorods (rZG), nanodisks
(dZG), and nanospheres (sZG) The XRD pattern of ZnO seeds on RGO (Fig
2.1e) shows characteristic broad ZnO peaks with low intensities at 2θ = 31.1°,
33.5°, 35.4°, 46.8°, 56.0°, 62.3°, and 67.6°, indicating low crystalline quality The
ZnO seeds have a mean crystallite size d of 0.42 nm, which was estimated from the (101) peak using the DebyeScherrer equation [36]:
(2.1) where λ is the wavelength of Cu K irradiation ( = 0.154 nm), β is the full width at half maximum (FWHM) of the diffraction peak, and θ is the diffraction angle
The XRD analysis revealed a prominent peak at 2θ = 26.0° for graphite, indicating the (002) crystalline plane, along with a weaker peak at 2θ = 54.5° corresponding to the (004) plane, confirming the presence of hexagonal graphite In contrast, these peaks were absent in the XRD pattern of graphene oxide (GO), demonstrating that the oxidation and exfoliation processes effectively transformed graphite into GO Additionally, the XRD pattern of the ZnO seeds/reduced graphene oxide (RGO) sample prior to calcination in argon gas did not exhibit any ZnO peaks.
The XRD pattern comparison reveals that the peak at 2θ = 10.9° associated with the (001) plane of graphene oxide (GO) is absent after interaction with Zn²⁺ ions This disappearance is due to the restacking of GO layers caused by the interaction with oxygen functional groups, which significantly reduces the interlayer distance Consequently, the (001) peak of GO vanishes during the formation of ZnO seeds, indicating that GO has undergone reduction in this process.
Before the ZnO growth step, RGO is indicated by a broad peak at 2θ 23.3° (Fig 2.1d and e) [37] The nanohybrid samples exhibited distinct XRD peaks that correspond to the lattice planes of (100), (002), (101), (102), (110), and (103).
(200), (112), (201), (004), and (202) (Fig 2.1f‒h) of the hexagonal wurtzite structure (JCPDS no 36-1451) of ZnO The peak of RGO was not observed in the
XRD patterns of all the ZnO/RGO samples because the peak intensity of RGO at
2θ = 23.3° is quite weak compared to that of the ZnO peaks in the nanocomposites
The XRD patterns of the nanocomposites indicated no presence of crystalline impurities, while a notable shift in the XRD peaks for the (100), (002), and (101) planes was observed This shift, highlighted in the magnified view of Fig 2.1, is attributed to the solvent effects during the reaction, particularly a reduction in DI water content from 50% to 20%, which caused the peaks to move further from the reference positions These shifts are linked to changes in the size and structure of the ZnO particles, as well as lattice deformation resulting from interactions between ZnO and RGO or lattice strain within the structure.
Figure 2.1 XRD patterns of (a) graphite, (b) GO, (c) ZnO seeds/RGO before calcination under Ar gas, (d) RGO, (e) ZnO seeds/RGO after calcination under
Ar gas, (f) rZG, (g) dZG, and (h) sZG The magnified XRD pattern on the right shows the (100), (002) and (101) crystalline planes of the samples
The transformation of abundant oxygen-containing functional groups in graphene oxide (GO) to reduced graphene oxide (RGO) was confirmed through FTIR spectroscopy analysis The characterization encompassed pristine GO, ZnO seed/RGO, and ZnO/RGO nanocomposites, covering a spectral range from 4000 to 400 cm −1.
(Fig 2.2a) For GO, the broad absorption peak centered at 3209 cm −1 was assigned to the O‒H stretching vibrations The FTIR spectrum of GO revealed peaks at
1730 cm −1 (C=O stretching vibrations of –COOH on the edges of GO sheets) and
1627 cm −1 (configurable vibrations from sp 2 hybridized C=C group [20,33,39], O‒
The vibrational spectrum analysis revealed significant peaks associated with various functional groups, including O‒H deformation vibrations of COOH groups and skeletal vibrations of graphitic domains Notable peaks were observed at 1394 cm −1 for tertiary C‒OH deformation, 1160 cm −1 for C‒O stretching, 1043 cm −1 for alkoxyl C‒O vibrations, and 876 cm −1 for epoxy vibrations The absence of peaks related to oxygen-functional groups in the ZnO seed/RGO and ZnO/RGO nanocomposites indicated the successful reduction of graphene oxide (GO) to reduced graphene oxide (RGO) Additionally, a red shift was noted, with the peak at 1627 cm −1 in GO shifting to 1551 cm −1 in ZnO seed/RGO, 1565 cm −1 in rZG and dZG, and 1553 cm −1 in sZG.
The observed shift is linked to the change in π-electron interactions within polarizable aromatic rings, which transition to cation-π interactions Additionally, this shift involves the coordination of hydroxyl and epoxy groups remaining on the surface of graphene oxide (GO).
The study identified a new absorption peak at 1230 cm −1 in all samples containing ZnO, attributed to the C‒O‒Zn linkage Additionally, Zn-O stretching vibrations were detected below 500 cm −1, specifically at 453 cm −1.
403, 405, and 406 cm −1 for ZnO seed/RGO, rZG, dZG, and sZG, respectively [41]
The observed changes in peak intensities and the emergence of new peaks suggest that the intercalation of ZnO nanoparticles between reduced graphene oxide (RGO) sheets during the growth process effectively reduced graphene oxide (GO) to RGO, leading to the formation of ZnO/RGO composites.
The FTIR spectra presented in Figure 2.2(a) illustrate the characteristics of GO, ZnO seed/RGO, rZG, dZG, and sZG nanocomposites In part (b), the Raman spectra of ZnO/RGO nanocomposites, along with the spectra of GO, highlight the magnified area focused on ZnO Additionally, part (c) details the intensity ratios of the G and 2D bands between ZnO/RGO for the three ZG nanocomposites and GO, providing insights into their structural properties.
Raman spectroscopy is a valuable method for confirming structural changes in graphene oxide (GO) and reduced graphene oxide (RGO), as well as the incorporation of ZnO in nanocomposites The Raman spectrum of GO exhibits two key peaks, known as the D and G bands The D band is linked to the disorder and defects at the edges of graphene sheets, while the G band corresponds to the sp² bond vibrations of carbon atoms Additionally, the intensity ratios of the D band to G band (I D/I G) are calculated to evaluate the disorder in GO and RGO structures Notably, these ratios are higher in ZnO/RGO nanocomposites (I D/I G = 1.01 for sZG and dZG, and 1.07 for rZG) compared to GO.
= 0.86), which indicates an increase in the degree of disorder and defects in the
RGO structure as well as the formation of sp 2 domains Therefore, the larger number confirms the reduction of GO to RGO in the nanocomposite [30,35]
The formation of C‒O‒Zn linkages in nanocomposites is evidenced by the blueshift of the G band from 1591.5 cm −1 for graphene oxide (GO) to 1597.4, 1598.9, and 1598.9 cm −1 for sZG, dZG, and rZG, respectively, indicating strong chemical interactions between zinc and the oxygen atoms in hydroxyl or epoxy groups on the reduced graphene oxide (RGO) surface Raman spectra of ZnO/RGO samples reveal two characteristic vibration modes at 329 cm −1 and 435 cm −1, typical for wurtzite ZnO, confirming the growth of ZnO particles on the graphene surface The observed Raman signal enhancement is attributed to localized surface plasmon resonance (LSPR), influenced by the morphology and size of ZnO in the composites Additionally, the peak intensity ratios (I ZG/I GO) among the three ZnO/RGO nanocomposites indicate that the smaller spherical ZnO structure in sZG provides better interfacial contact with RGO, which enhances photodegradation efficiency.
X-ray Photoelectron Spectroscopy (XPS) is an effective analytical technique used to assess the oxidation state and elemental composition of the ZnO/reduced Graphene Oxide (sZG) sample, confirming the reduction of Graphene Oxide (GO) to reduced Graphene Oxide (RGO) The survey spectrum of the sZG sample reveals the presence of three primary elements: Zinc (Zn), Oxygen (O), and Carbon (C), whereas the GO spectrum only displays peaks for O and C The intensity ratios of I O 1s/I C 1s are notably different, measuring 1.48 for sZG and 1.76 for GO, indicating a significant reduction in oxygen-containing functional groups on the graphene surface These functional groups on GO serve as anchoring sites for ZnO, facilitating the formation of covalent bonds and resulting in robust interactions between the metal oxide and the support, which enhances electron transport.
Zn 2p core level showed two main peaks centered at 1022.4 eV (Zn 2p3/2) and
1045.5 eV (Zn 2p1/2) with a spin-orbit splitting (∆E) of 23.1 eV (Fig 3b), suggesting that only the Zn II oxidation state exists in the nanocomposite [30,38]
The C 1s and O 1s spectra were fitted using the Gaussian functions after background correction to evaluate the reduction of GO and the formation of
The synthesis of the ZnO/RGO nanocomposite reveals significant chemical bonding, as evidenced by the O 1s core-level peak, which displays three major peaks at 530.9 eV (indicating lattice oxygen binding with Zn and Zn‒O bonds), 532.2 eV (related to C‒O‒Zn, C‒O, and C=O bonds or surface-adsorbed oxygen components), and 533.2 eV (representing C‒O bonds) Additionally, the deconvoluted C 1s spectrum shows four peaks at 284.5, 285.4, 288.3, and 290.3 eV, with the first three peaks corresponding to nonoxygenated C‒C or C=C bonds, C‒O‒C (epoxy) bonds, and C=O (carbonyl or carboxylate) bonds, respectively The peak at 290.3 eV signifies the π‒π* satellite peak, indicating the restoration of aromatic structures through self-repair of defects or modification with other functional groups.
Conclusions
Inexpensive ZnO on RGO nanocomposites with three morphologies
Nanospheres, nanodisks, and short nanorods were synthesized using a simple two-step method that employed solvent mixtures with varying ratios of ethanol to water, resulting in distinct photocatalytic activities The formation of seeds on the nanoparticles played a crucial role in this process.
RGO surface and the use of HMTA were necessary to develop the initial structures and to bind the metal ions to them, respectively
These two factors, therefore, were critical in determining the morphology and performance of the ZnO photocatalyst Without the seed and HMTA, only a few
The nanospherical ZnO/RGO composite (sZG) catalyst demonstrated the highest photodegradation efficiency, achieving 99% for methylene blue and 98% for rhodamine B, with a low catalyst loading of 0.1 g L−1 under low-power UV light for 60 minutes In contrast, the short nanorod (rZG) and nanodisk (dZG) catalysts showed significantly lower efficiencies, ranging from 62% to 69% and approximately 79%, respectively.
80%, respectively, for the two dyes The sZG catalyst has the smallest particle size
The sZG catalyst, with a size range of 15‒35 nm and the largest specific surface area among the tested catalysts, facilitates the fastest transport of excited electrons In a 30-minute photodegradation reaction of methylene blue (MB) using 0.3 g L−1 of the sZG catalyst, the photocatalytic efficiency remained impressive at 98.5%, even under low-power UV illumination.
Under solar light irradiation, nearly 99% of the MB in solution was degraded within 100 minutes using a catalyst loading of 0.2 g L ‒1 Remarkably, the sZG composite preserved its morphology after 15 cycles of photodegradation, demonstrating its durability and effectiveness.
96% of its initial photocatalytic activity, which is a reduction of only
The presence of reduced graphene oxide (RGO) enhanced the performance of nanospherical ZnO by preventing electron-hole recombination, resulting in a catalyst lifetime increase This improvement is attributed to the strong synergistic effect between ZnO and RGO, which boosts the overall efficiency of the catalyst.
RGO composite (sZG) is an effective and economical solution for the degradation of organic pollutants, offering excellent recyclability and a long lifespan as a nanocomposite catalyst.
C as visible light driven Z-scheme photocatalyst for the degradation
Introduction
Water pollution poses serious risks to human health and the environment, necessitating effective solutions Semiconductor photocatalysts, particularly titanium dioxide (TiO2), have emerged as a promising low-cost method for decomposing toxic organic pollutants TiO2 is favored for its non-toxic nature, affordability, excellent redox capabilities, and photostability Extensive research on TiO2's nanostructure, especially its unique crystal facets, has highlighted its potential in energy production and photocatalysis applications.
Recent studies have highlighted the potential of rutile TiO2 and anatase TiO2 with specific exposed facets for enhanced photocatalytic performance Amano et al demonstrated that decahedral single-crystalline anatase featuring (001) and (101) facets exhibits excellent photocatalytic properties Similarly, Gai et al introduced a surfactant-free synthesis method for producing anatase TiO2 octahedra with (101) facets, showing superior photocatalytic activity compared to commercial P25 due to its high crystallinity However, the practical use of TiO2 as a photocatalyst is constrained by its wide bandgap of 3.2–3.3 eV and the rapid recombination of electron-hole pairs To address these limitations, various strategies have been explored to improve TiO2's light-harvesting capabilities under visible light.
Designing a Z-scheme photocatalyst is an effective strategy to prevent the recombination of electrons and holes while enhancing charge carrier transfer at the interface of two semiconductors This approach involves the incorporation of solid-state electron mediators, such as metal components (like Au, Ag, and Pt) or carbon materials (including carbon nanotubes, graphene, and activated carbon), between semiconductors with wide and narrow bandgaps For instance, Zou et al developed a gC3N4/Au/C-TiO2 photocatalyst that utilizes Au nanoparticles as an electron mediator, achieving high efficiency in charge transfer and separation for photocatalytic hydrogen evolution Similarly, Jang et al created TiO2/ZnTe/Au nanocorncob structures through a simple solution-phase method aimed at water splitting applications.
Wei et al [63] fabricated Zscheme TiO2/MoS2 using a complicated approach consisting of two steps of hydrothermal synthesis: calcination and ultrasonication for photocatalytic hydrogen evolution Jo et al [64] prepared a Z-scheme g-
The C3N4/TiO2/MoS2 photocatalyst, incorporating MoS2 nanosheets as an electron transporter and acceptor through a wet impregnation method, demonstrates high degradation efficiency for organic water pollutants Additionally, Jia et al [65] developed a Z-scheme rGO/TiO2-B/W18O49 ternary nanocomposite using a two-step solvothermal method, which showed remarkable photodegradation of rhodamine B (RhB) in water, with rGO playing a crucial role in enhancing the separation of photoinduced carriers.
Although the aforementioned nanomaterials exhibited good photocatalytic activity, the use of high-cost noble metals and complicated fabrication routes limit their practical applications
Molybdenum disulfide (MoS2) is a transition metal dichalcogenide with two distinct crystalline phases: 2H phase (semiconductor) and 1T phase (metallic conductor with 10 7 fold higher electrical conductivity than the 2H phase) [66]
MoS2, characterized by hexagonal (H) and trigonal (T) symmetry, is recognized for its excellent conductivity, high activity, and good chemical stability, making it an effective low-cost non-Pt catalyst for the hydrogen evolution reaction Research by Wang et al indicates that the thermodynamically metastable 1T-MoS2 phase does not naturally occur and tends to transition to the more stable 2H phase, leading to their coexistence This chapter proposes the development of a Z-scheme heterojunction by combining (101) exposed octahedral TiO2 with C-MoS2, utilizing potassium titanate nanowires and carbon disulfide (CS2) through a hydrothermal method.
The photocatalyst material, consisting of both 1T and 2H phases of MoS2, demonstrated significant enhancement in photocatalytic activity due to its Z-scheme configuration This improvement is attributed to the effective light-harvesting abilities of TiO2 and 2H-MoS2, which promote rapid charge transfer and efficient separation of electrons and holes The presence of carbon and 1T-MoS2 serves as electron mediators at the interface between TiO2 and 2H-MoS2, further facilitating these processes The photocatalytic performance of this heterojunction photocatalyst was evaluated by assessing the degradation of various pollutants, including methylene blue (MB), RhB, methyl orange (MO), and tetracycline hydrochloride (TCH).
Experimental
Sodium molybdenum oxide dihydrate (Na2MoO4∙2H2O) ( ≥ 99.5%, Sigma Aldrich), carbon disulfide (CS2) (99%, Kanto Chemicals), TiO2
(ACROS Organics P25), and potassium hydroxide (KOH) (85%, Alfa Aesar), hexadecyltrimethylammonium bromide ([(C16H33)N(CH3)3]Br; CTAB) (98%,
In the synthesis of photocatalysts, high-purity chemicals were utilized, including MB from Alfa Aesar, RhB from Sigma Aldrich, MO from Junsei Chemicals, and tetracycline hydrochloride (TCH) from Alfa Aesar, all employed as target organic pollutants without additional purification Additionally, sodium hydrogen carbonate (NaHCO3) and isopropyl alcohol (IPA) were used in the process, ensuring the integrity and effectiveness of the photocatalytic applications.
(≥ 99.5%) were purchased from Daejung Chemical and p-benzoquinone (p-
BZQ) (98%) was obtained from Sigma Aldrich
Potassium titanate (K2Ti6O13) was synthesized through a modified procedure, beginning with the mixing of 1.5 g of P25 and 60 ml of 10 M KOH for 2 hours This mixture was then subjected to a reaction in a Teflon-lined stainless steel autoclave at 200 °C for 24 hours After the reaction, the resulting white solid was thoroughly rinsed with deionized water and ethanol to eliminate any KOH residue, and subsequently dried overnight at 60 °C.
To synthesize C-MoS2/octahedral TiO2 (CMST), 0.2 g of K2Ti6O13 was combined with 15 ml of a 0.5% aqueous CTAB solution The mixture was sonicated for 30 minutes and subsequently stirred for 2 hours, resulting in a milky suspension.
Solutions of Na2MoO4∙2H2O (0.25 mmol) in 10 ml DI water and 100 l of CS2 in
A solution of Na2MoO4∙2H2O was gradually introduced into a milky suspension of K2Ti6O13 and CTAB, which had been prepared using 5 ml of absolute ethanol After stirring the mixture for 10 minutes, a CS2 solution in ethanol was added dropwise.
The mixture was stirred continuously for 10 minutes before being transferred to a 50 ml Teflon-lined stainless steel autoclave, where it was maintained at 200 °C for 24 hours After this process, a black precipitate formed, which was then isolated from the solution through filtration The precipitate was washed multiple times with deionized water and ethanol, and subsequently dried in an oven at 60 °C.
12 h Scheme 3.1 presents the detailed synthesis For comparison, MoS2/TiO2 (MST) was synthesized using the same steps above using thiourea as the S source C-
MoS2 (CMS) was also synthesized using the procedure above but without the addition of K2Ti6O13 TiO2 was also produced using K2Ti6O13 only
Scheme 3.1 Schematic illustration of the synthesis of the CMoS2/octahedral
Bulk X-ray diffraction (XRD) (PANalytical, X’Pert-PRO MPD) using Cu Kα radiation (𝜆 = 0.154 nm) was carried out to examine the crystallinity of the samples The morphology of the samples was observed by scanning electron microscopy (SEM) (Hitachi, S-4800) and transmission electron microscopy
(TEM) (Philips, CM-200) High-resolution transmission electron microscopy
The crystalline structures of the samples were examined using the HR-TEM (FEI, TITAN G2 ChemiSTEM Cs Probe), while the elemental composition and chemical states of the materials were assessed through X-ray photoelectron spectroscopy (XPS).
The study utilized Kratos AXIS Nova for monochromatized Al Kα radiation and recorded Raman spectra with a HORIBA XploRA plus Raman spectrometer, employing laser excitation at 532 nm Photoluminescence (PL) spectra were obtained using the HORIBA iHR550 with 325 nm laser excitation Additionally, UV-vis diffuse reflectance and UV-vis spectra were collected using a Varian Cary 5000 UV-vis-NIR spectrophotometer Thermogravimetric analysis (TGA) was conducted with a TA Instruments SDT Q600 at a specified heating rate.
To evaluate the photodecomposition efficiency of photocatalyst samples, model pollutants MB, RhB, MO, and TCH were tested using a solar simulator (ABET Technologies, LS 150, with a 150 W Xe arc lamp) under one sun illumination In each experiment, 10 mg of the photocatalyst was combined with 50 ml of a 10 mg L ‒ 1 aqueous solution of the model pollutant in a photochemical reactor The mixture underwent sonication for 5 minutes and was stirred in a dark chamber for 30 minutes to achieve adsorption-desorption equilibrium before being exposed to light.
During the photodegradation reaction, a 3 mL aliquot was taken at specific time intervals, followed by centrifugation at 13,000 rpm for 10 minutes to isolate the catalyst The concentration of the residual pollutant was then assessed spectrophotometrically by measuring the light absorbance of methylene blue (MB).
RhB, MO, and TCH solutions at λ = 664, 553, 465, and 357 nm, respectively, using UV-vis spectrophotometer
The CMST catalyst demonstrated its photostability over five cycles while degrading a 10 mg L ‒1 RhB solution, with the catalyst being collected, washed with deionized water, and dried at 60 °C after each cycle Post the fifth cycle, the catalyst's surface morphology was analyzed To investigate the photocatalytic mechanism and identify the primary oxidizing species involved in the photodegradation, trapping experiments were performed Scavenging reagents, including 0.2 M IPA, 0.2 M NaHCO3, and 1 mM p-BZQ, were introduced to the RhB dye solution prior to irradiation under simulated solar light to target hydroxyl radicals, holes, and superoxide radicals, respectively.
Electrochemical impedance spectroscopy (EIS) and photocurrent response measurement
The electrochemical impedance spectra of the samples were measured under dark and solar irradiation conditions using an electrochemical workstation
(Metrohm, Autolab PGSTAT 302 N) equipped with a conventional three- electrode cell system The Ag/AgCl electrode, Pt electrode, and working electrode
The working electrode was constructed by applying active materials, including P25, CMS, MST, or CMST, onto fluorine-doped tin oxide This setup was placed in a quartz cell containing a 0.5 M Na2SO4 electrolyte, allowing for effective interaction with the as-prepared catalyst.
(FTO) glass with an active area of 1 cm × 1 cm using the drop-casting technique
Briefly, 2 mg of the asprepared catalyst was mixed with 1 ml ethanol and 10 μl of
Nafion to obtain a slurry Subsequently, 15 àl of this mixture was drop cast onto
FTO glass and dried in an oven for 24 h The EIS spectra of the samples were recorded over the frequency range, 10 5 Hz to 0.01 Hz
The photocurrent response of the samples was evaluated using the same equipment and setup at a working potential of 0 V Measurements were taken during multiple on/off irradiation cycles for 50 seconds, utilizing a solar simulator as the light source All working electrodes maintained an identical illuminated area of 1 cm × 1 cm.
Results and Discussions
The crystallographic phase and structure of the prepared samples were analyzed using X-ray diffraction, as illustrated in Figure 3.1 The results in Figure 3.1a indicate that the P25 utilized in the synthesis process comprised both anatase (JCPDS #211272) and rutile phases.
(JCPDS #21-1276) phases The K2Ti6O13 formed from the hydrothermal reaction of P25 in alkali media showed the characteristic peaks at 11.5°, 24.1°, 29.3°, 33.8°,
The XRD pattern of the CMST sample, synthesized from K2Ti6O13 as a precursor, reveals distinct diffraction peaks at 43.1°, 47.9°, 59.5°, and 66.3° 2θ (JCPDS #400403), indicating the presence of a single anatase phase in the formation of the TiO2 octahedral structure.
2θ (JCPDS #21-1272) Carbon and MoS2 exhibit broad peaks with low intensities compared to TiO2, making them undetectable in the XRD profile of the CMST
Figure 3.1 XRD patterns of commercial P25 (a), K2Ti6O13 (b), spindle-like TiO2
In comparison, the TiO2 produced from the hydrothermal reaction of K2Ti6O13 showed XRD peaks for anatase only (Fig 3.1c), which is in agreement with
JCPDS #21-1272; no rutile phase was found Fig 3.1d shows the XRD pattern of
CMS with broad peaks at 17.6°, 32.4°, and 56.9° 2θ corresponding to the (002)*,
The analysis of MoS2 samples reveals low crystallinity, as evidenced by the (100) and (110) planes, due to the absence of high-temperature annealing Notably, the (002)* plane corresponds to the characteristic (002) reflection peak of MoS2, originally observed at 14.4° 2θ, which has shifted to 17.6°.
The expansion of interlayer spacing is indicated by the 2θ values In the case of CMST, carbon and MoS2 were synthesized in situ on TiO2 surfaces through hydrothermal reactions, resulting in the presence of predominant peaks corresponding to the anatase phase, which align closely with established data.
The XRD analysis of JCPDS #211272 reveals that bulk MoS2 typically shows peaks at 14.4° 2θ; however, the XRD patterns for CMS and CMST do not exhibit this peak, indicating the successful formation of MoS2 nanosheets with a limited number of thin layers.
XPS analysis of the CMST sample was performed to assess its surface chemical composition and the oxidation states of its elements The survey spectrum, illustrated in Fig 3.2a, indicated the presence of molybdenum (Mo), sulfur (S), titanium (Ti), oxygen (O), and carbon (C) within the composite Additionally, the Mo 3d region was deconvoluted into two distinct doublet peaks.
The binding energy peaks at 231.7 eV and 232.7 eV are attributed to Mo 4+ 3d3/2 and Mo 4+ 3d5/2, respectively, indicating spin-orbital splitting The lower energy doublet is associated with the 1T phase, while the higher energy doublet corresponds to the 2H phase Additionally, the S 2p region spectrum reveals two doublets, with peaks at 162.6 eV and 161.4 eV representing S 2p1/2 and S 2p3/2, respectively.
The X-ray photoelectron spectroscopy (XPS) analysis of the CMST sample reveals significant findings regarding the presence of 1T and 2H-MoS2 phases The survey spectrum and high-resolution spectra for Mo 3d, S 2p, C 1s, Ti 2p, and O 1s demonstrate key peaks, with the S 2p doublet peaks at 163.9 eV and 162.7 eV confirming the existence of 2H-MoS2 Additionally, a peak at 168.8 eV indicates sulfate species resulting from sulfur oxidation The C 1s spectrum displays three distinct peaks at 285.2 eV, 284.5 eV, and 286.4 eV, corresponding to C‒S, C‒C/C=C, and C=O bonds, respectively, further supporting the composition of the nanocomposite.
[75,76] The high-resolution XPS spectrum of Ti 2p showed two typical peaks at
464.7 eV (Ti 2p1/2) and 459.1 eV (Ti 2p3/2), indicating the Ti (IV) state in the nanocomposite (Fig 3.2e) [77] The peaks of the deconvoluted O 1s spectra at
529.4 and 530.3 eV correspond to Ti‒O‒Ti and Ti‒O‒C bonds [78], respectively
The peak for the surface adsorbed water or surface hydroxyl group O‒H/C‒O bond was located at 531.6 [78,79]
Raman spectroscopy is a powerful tool for probing the phase components, chemical elements, and the nature of carbon present in a carbonaceous composite
The Raman spectra of CMS (Fig 3.3 a1) and CMST (Fig 3.3 b1) showed two signals typical of carbonaceous materials: the D band at 1366
The presence of carbon in the composite is confirmed by the peaks observed at 1368 cm−1 and 1575 cm−1 for CMS, and 1580 cm−1 for CMST Additionally, the two dominant peaks of 2H-MoS2 were identified at 381 cm−1 and 409 cm−1 for CMS.
CMST, they were observed at 385 and 413 cm −1 [72] These two
The Raman spectra of CMS and CMST samples reveal distinct peaks corresponding to the E2g 1 (in-plane) and A1g (out-of-plane) modes Additionally, the vibration modes J1, J2, and J3 of the S‒Mo‒S structure in the 1T-phase are also identified, as illustrated in the magnified images of the highlighted areas.
MoS2 were observed at 152, 226, and 325 cm −1 [80,81] in the CMS sample (Fig
3.3a2) On the other hand, these three peaks were observed with low intensities at
150, 221, and 321 cm −1 [66,81] in the CMST sample (Fig 3.3b2) This proves the existence of 1T and 2H-MoS2 in the nanocomposites
The Raman peaks of anatase TiO2 at 149, 205, 519, and 640 cm −1 for CMST in
The peaks identified in Fig 3.3b1 correspond to the Eg 1, Eg 2, A1g+B1g 2, and Eg 3 vibrational modes of the anatase phase Notably, the intense peak at 149 cm −1 suggests a potential overlap between the symmetric stretching modes of O‒Ti‒O and the weaker J1 peak.
This study presents the first successful direct synthesis of carbon (C) and molybdenum disulfide (MoS2) on octahedral titanium dioxide (TiO2) using K2Ti6O13 as a precursor in hydrothermal conditions with CS2 and Na2MoO4∙2H2O The process involved the creation of K2Ti6O13 nanowires, measuring 10 nm in diameter and several microns in length, which then transformed into an octahedral TiO2 structure through a dissolution-nucleation mechanism Additionally, the hydrothermal reaction facilitated the formation of carbon and MoS2 The P25 sample exhibited irregularly shaped TiO2 particles ranging from 10 to 50 nm, while the bulk MoS2 showed a stacked layer-like morphology measuring 1 to 2.5 micrometers.
3.4b) The C-MoS2 (CMS) sample synthesized using CS2 showed the lamellae of the layers gathered into clusters, 1–2 m in size (Fig 3.4c) The morphology of the TiO2 in the MST sample was relatively irregular (particle and bar shapes) with particle sizes of
Figure 3.4 SEM images of P25 (a), bulk MoS2 (b), CMS (c), MST (d), CMST
(e), spindle-like TiO2 (f), and EDX elemental mapping of CMST
50–100 nm The distribution of MoS2 on TiO2 in the MST sample was not uniform
(Fig 3.4d), which adversely affected the photocatalytic activity In contrast, Fig
3.4e presents octahedral TiO2 nanoparticles at different magnifications The rough surface of the nanoparticles is visible The width and length of the edges of the nanoparticles ranged from 30 to 80 nm and 200 to 220 nm, respectively Direct observations of carbon and MoS2 on the surface of the octahedral structure through
The SEM images revealed challenges due to the extremely thin MoS2 nanosheets surrounding the TiO2 nanoparticles EDX elemental mapping validated the presence of carbon (C), molybdenum (Mo), sulfur (S), titanium (Ti), and oxygen (O) in the CMST sample, as illustrated in Figs 3.4g−m Notably, the transformation of K2Ti6O13 nanowires to TiO2 under hydrothermal conditions, without CS2 and Na2MoO4∙2H2O, resulted in a markedly different morphology, producing spindle-like anatase TiO2 (Fig 3.4f) rather than the anatase octahedral structure, aligning with prior research findings.
The TiO2 spindles were approximately 300 nm wide and 1 to 1.5 m long
Conclusions
The heterogeneous Z-scheme C-MoS2/octahedral TiO2 catalyst was developed using a hydrothermal method, integrating the wide bandgap semiconductor TiO2 with the narrow bandgap semiconductor 2H-MoS2 The synthesis incorporated carbon and metallic 1T-MoS2 as electron transmission bridges, significantly improving the catalyst's light absorption in the visible spectrum This enhancement facilitated rapid charge transfer and efficient separation of electron-hole pairs As a result, the photocatalytic activity of the C-MoS2/TiO2 catalyst for methylene blue degradation showed remarkable increases of 10%, 140%, and 390% compared to pristine P25 and the anatase phase.
The octahedral structure of TiO2, featuring a (101) facet and high crystallinity, significantly enhances its photocatalytic activity through effective charge separation Additionally, the phase components in the Z-scheme catalyst are crucial for optimizing the design and application of photocatalysts in environmental wastewater treatment.
Facile microwave-assisted synthesis of Z-scheme of silk cocoon-like BiPO 4 on BiOBr/Bi 2 O 3 nanosheets for degradation of organic
Introduction
In the previous chapters, the synthesis of photocatalysts from the widely used semiconductors ZnO and TiO2 was explored through energy-intensive solvothermal and hydrothermal methods, which involved complex two-step processes and lengthy reaction times exceeding 24 hours Unfortunately, the photocatalytic activity achieved with these catalysts fell short of industrial applicability Consequently, this chapter introduces a next-generation bismuth-based photocatalyst, developed through a simple and efficient microwave-assisted one-step method.
Microwave heating is widely used in materials synthesis as an alternative to conventional heating because it is an efficient, rapid, and controllable method
Microwave heating effectiveness is influenced by the dielectric properties of solvents used in reactions Water is an advantageous solvent due to its ability to enhance reactivity and selectivity while maintaining mild reaction conditions and a simplified workflow In contrast, utilizing alternative solvents poses risks of toxicity, environmental pollution, and complications in waste treatment.
Bismuth phosphate (BiPO4) has emerged as a highly effective non-metal oxyacid photocatalyst, showcasing impressive photocatalytic activity and stability This performance is attributed to the efficient separation of electrons and holes, facilitated by the induced effect of highly negatively charged oxyacid anions.
BiPO4 has exceptional optical properties, no deactivation during the photocatalytic process, non-toxicity, and low cost Similar to titanium dioxide,
BiPO4 is only active under UV light due to its wide bandgap, which significantly restricts its application [97]
Bismuth oxyhalides (BiOX (X = I, Cl, Br, F)) have attracted much attention in photocatalysis owing to their high chemical stability, unique layered structures, and highly favorable photocatalytic performance under UV–Vis irradiation [98]
The BiOX family features a tetragonal matlockite-type structure characterized by unique layered [Bi2O2 2+] slabs interspersed with halogen atoms Among its members, BiOBr has garnered significant attention due to its low bandgap, making it a focal point of extensive research.
The material exhibits a bandgap of 2.3–2.9 eV, which contributes to its superior photocatalytic activity and stability Its unique layered crystal structure enhances the effective separation of electron-hole pairs, thanks to the induced dipole.
[101], the photocatalytic performance is still hindered by the low light absorption ability [100]
Bismuth oxide (Bi2O3) is characterized by a low bandgap of 2.1–2.8 eV and possesses exceptional properties, including a high refractive index, affordability, visible-light activity, and enhanced oxidation power of holes Additionally, it is non-toxic and non-carcinogenic, making it a valuable material for various applications.
[104] With these properties, Bi2O3 is considered as a potential photocatalyst
Despite these advantages, its photocatalytic activity differs significantly depending on the crystal structure [105] In general, water purification using a
Bi2O3 photocatalysts are often considered ineffective due to their high rates of photoinduced electron-hole pair recombination and susceptibility to photocorrosion To enhance their photocatalytic performance and fully utilize their properties, numerous strategies have been developed.
In this Chapter, a Z-scheme heterojunction photocatalyst of
BiPO4/BiOBr/Bi2O3 was prepared using a rapid microwave-assisted one-step hydrothermal method at low-temperature The photocatalytic activity of
BiPO4/BiOBr/Bi2O3 composite was investigated by the degradation of rhodamine
B (RhB) and tetracycline hydrochloride (TCH), hydroxylchloroquine (HCQ), and methyl orange (MO) under solar light.
Experimental
Bismuth (III) nitrate pentahydrate (Bi(NO3)3∙5H2O, ≥ 98%, Sigma Aldrich), hexadecyltrimethylammonium bromide ([(C16H33)N(CH3)3]Br, CTAB, 98%, Alfa
Aesar), nitric acid (70% HNO3, Daejung Chemicals), monosodium phosphate
(NaH2PO4, 96%, Alfa Aesar) were used as received in the synthesis of photocatalysts Rhodamine B (RhB, 90%, Sigma Aldrich), methyl orange (MO,
(C22H24N2O8∙HCl, TCH, 96%, Alfa Aesar), and hydroxychloroquine sulfate
(C18H26ClN3OãH2SO4, HCQ, ≥ 98%, Sigma Aldrich) were used as the target organic pollutants without further purification Sodium hydrogen carbonate
(NaHCO3, >99%, Daejung Chemicals), tert-butyl alcohol (TBA, ≥ 99.5%,
Daejung Chemicals), and p-benzoquinone (p-BZQ, 98%, Sigma Aldrich),
Lhistidine (≥ 99%, Sigma Aldrich) were used as received
Synthesis of BiPO 4 /BiOBr/Bi 2 O 3
The BiPO4/BiOBr/Bi2O3 nanocomposite was synthesized using a microwave-assisted one-step method Initially, 0.251 g of CTAB was sonicated in 50 ml of deionized water to create a transparent solution, followed by the addition of 1 ml of the necessary reagents.
A solution was prepared by adding 70% HNO3, followed by dissolving 1.063 g of Bi(NO3)3∙5H2O in it using a sonication bath for 5 minutes at room temperature, resulting in a milky suspension Concurrently, separate 0.1 M NaH2PO4 solutions were prepared, and a specific volume of this solution was added to the milky mixture, which was stirred for 30 minutes The final mixture was then transferred into a microwave reactor for further reaction at a designated power level.
180 W at 90 °C for 30 min with continuous stirring (2.45 GHz Microwave, CEM,
After cooling to room temperature, the final product underwent multiple filtrations and washes with deionized (DI) water, followed by overnight drying at 60 °C in an oven The optimized experiments utilized varying volumes of 0.1 M NaH2PO4, specifically 0, 1, 3, 6, and 12 ml, referred to as S0, S1, S2, S3, and S4, respectively.
BiPO4, Bi2O3, and BiOBr samples were synthesized for comparative analysis To prepare the BiPO4 sample, 0.485 g of Bi(NO3)3∙5H2O was dissolved in a mixture of 30 ml diethylene glycol and deionized water in a 2:1 volume ratio, resulting in a clear solution Subsequently, 0.156 g of Na2HPO4 was added, leading to the formation of a milky suspension This mixture was then transferred to a reactor and subjected to microwave conditions at 90 °C and 180 W for 60 minutes After cooling to room temperature, a white solid was collected through filtration and thoroughly washed with deionized water The final product was dried overnight in an oven at 60 °C.
For the BiOBr sample, 0.36 g of KBr was dissolved in 20 ml of DI water Then,
Scheme 4.1 Schematic illustration of the synthesis of the BiPO4/BiOBr/Bi2O3 catalyst added to the KBr solution The mixture was stirred for 30 min before being transferred to a 30 ml-stainless steel autoclave The reaction was conducted at 160 °C for 12 h The white solid obtained after the reaction was collected and washed several times with DI water to remove the chemical residues Finally, the sample was dried overnight in an oven at 60 °C For the Bi2O3 sample, 0.075 g of CTAB was completely dissolved in 50 ml of DI water, followed by the addition of 1 ml of 70% HNO3 Then, 1.063 g of Bi(NO3)3∙5H2O was added to the above solution
The reaction occurred at room temperature for three hours, after which the resulting white solid was filtered and washed with deionized water until a pH of 7 was achieved The final step involved drying the sample in an oven at 60 °C for twelve hours.
A powder X-ray diffraction (XRD, PANalytical, X’Pert-PRO MPD) using
Cu Kα radiation (λ= 0.154 nm) was utilized to analyze the crystallinity of the samples The morphology of the samples was examined using scanning electron microscopy (SEM, Hitachi, S-4800) Additionally, high-resolution transmission electron microscopy (HRTEM) images were obtained with a TITAN G2 ChemiSTEM Cs for detailed structural analysis.
Probe electron microscope UV-vis diffuse reflectance spectra and UV-vis spectra of the samples were collected using a Varian Cary 5000 UV-vis-NIR spectrophotometer
The photocatalytic activity of the catalysts was assessed through the degradation of pollutants using a solar simulator (ABET Technologies, Xe lamp 150 W) as the light source The pollutants tested included RhB, TCH, MO, and HCQ, which have absorbance peaks at 553 nm, 357 nm, 465 nm, and respective wavelengths.
In this study, model pollutants with absorption peaks at 342 nm were utilized to evaluate the photodecomposition efficiency of a catalyst in wastewater treatment The experiments were conducted in a photochemical reactor at room temperature, where 50 ml of a 10 mg L ‒1 aqueous pollutant solution was mixed with a photocatalyst at a concentration of 0.5 g L ‒1 To establish adsorption-desorption equilibrium, the mixture was stirred in the dark for 30 minutes prior to irradiation During the irradiation process, 3 ml aliquots were periodically extracted and centrifuged at 10,000 rpm for 5 minutes to isolate the catalyst The remaining concentrations of pollutants, including RhB, TCH, MO, and HCQ, were then analyzed spectrophotometrically by measuring their absorbance.
The photocatalyst's reusability was tested through the photocatalytic degradation of a 10 mg L ‒1 RhB solution After each cycle, the catalyst was collected, washed with deionized water, and dried at 60 °C for subsequent use Following the fifth cycle, the surface morphology and structure of the used catalyst were analyzed To elucidate the photocatalytic mechanism and identify the primary active oxidizing species involved in the photodegradation process, scavenging reagents, specifically 0.2 M TBA, were introduced to the dye solutions before light exposure.
0.2 M NaHCO3, 0.2 M Lhistidine and 1 mM p-BZQ were used as scavengers for the hydroxyl radical, hole, singlet oxygen, and superoxide radical, respectively.
Results and Discussions
X-ray diffraction analysis was used to study the phase components in the asprepared samples Fig 4.1 depicts XRD patterns of BiPO4/BiOBr/Bi2O3,
BiOBr/Bi2O3 samples The XRD pattern of BiOBr/Bi2O3 sample showed the existence of BiOBr phase with peaks at 25.3°, 46.6°, and 57.2° 2θ
Figure 4.1 XRD patterns of samples: BiOBr/Bi2O3 (S0),
BiPO4/BiOBr/Bi2O3 (Samples S1-S4) corresponding to 1, 3, 6, and 12 ml of
0.1 M NaH2PO4 solution corresponding to (011), (020), and (221) planes, respectively (JCPDS # 011004), while the existence of Bi2O3 phase with peaks at 28.0°, 32.4°, 46.4°,
67.7°, and 77.0° 2θ with respect to the (111), (200), (220), (400), and (420) planes
(JCPDS # 52-1007), was recorded (Fig 4.1a) Meanwhile, the XRD pattern of the
BiPO4/BiOBr/Bi2O3 samples (with samples S1 to S4) not only showed the peaks of BiOBr and Bi2O3 phases in the composites but also the peaks of BiPO4 appeared at 14.6°, 20.1°, 25.5°, 29.1°, 29.5°, 31.3°, 37.9°, 39.4°, 41.9°, 48.7°, 52.0°, 53.5°,
62.3°, 63.3°, and 71.4° 2θ with respect to the (100), (101), (110), (111), (200),
(JCPDS # 15-0766) The results confirmed the formation of BiPO4, BiOBr, and
Bi2O3 components in the nanocomposite under microwave conditions without any impurities (Figs
4.1b-e) The XRD patterns of BiPO4, Bi2O3, and BiOBr were shown in Fig
The FTIR spectrum of the BiPO4/BiOBr/Bi2O3 sample is shown in Fig
4.2a The peak at 3487 cm −1 was due to the stretching vibration of O−H of the absorbed water in the sample [106] The characteristic peak related to hydroxyl groups was at 1608 cm −1 , which are typical vibrations of δ (H−O−H) [107]
The BiPO4 component in the composites exhibited characteristic peaks at 588 and 979 cm −1, corresponding to the δ (P−O−P) and υ3 asymmetric stretching vibrations of the (–PO4) group Additionally, a peak at 528 cm −1 was linked to the 4 (–PO4) asymmetric bending vibrations The composites also displayed peaks at 2918, 2844, and 1461 cm −1, which were associated with the symmetric and asymmetric stretching vibrations of −CH2− and the asymmetric stretching vibrations of (CH3–N + ) from residual CTAB.
Figure 4.2 FTIR (a) and Raman (b) spectra of BiPO4/BiOBr/Bi2O3 (sample S3)
The Raman spectra of the BiPO4/BiOBr/Bi2O3 sample, illustrated in Fig 4.2b, reveal significant peaks at 108 cm−1, which correspond to the A1g internal Bi–Br stretching mode Additionally, peaks at 57 cm−1 and 160 cm−1 are identified as the first-order vibration modes of Bi metal and the E1g stretching mode of Bi–Br, respectively.
[109,110] In addition, the Eg and B1g at around 403 cm −1 were produced by the motion of oxygen atoms [109,111] or the peak at 403 cm −1 could be due to the 2
(PO4) bending mode [112] Meanwhile, the peak at 969 cm −1 corresponds to the
This study introduces a novel one-step microwave synthesis method for the simultaneous production of three components: BiPO4, BiOBr, and Bi2O3 The process enables the formation of a nanocomposite by utilizing nanosheets of BiOBr, showcasing an innovative approach in material synthesis.
Bi2O3 components were incorporated into the the silk cocoon-like BiPO4 component (Fig 4.3) The width of BiPO4 was 200–400 nm and the length was
500–700 nm Meanwhile, the BiOBr/Bi2O3 nanosheets with irregular shape have a rough surface, which provides more active sites for degradation reactions
HR-TEM images and elemental mapping analysis demonstrated the presence of silk cocoon-like BiPO4 on the surface of BiOBr/Bi2O3 nanosheets, highlighting a uniform distribution of the composite's elemental components.
Figure 4.3 SEM images of BiPO4/BiOBr/Bi2O3 (sample S3) and its magnified image from rectangular area
Figure 4.4 HR-TEM (a-b) and EDX mapping spectra (c-d) of
The structure of BiPO4 is integrated within the layered framework of BiOBr/Bi2O3, enhancing rapid charge transfer between components and significantly boosting the composite's photocatalytic activity High-resolution transmission electron microscopy (HR-TEM) images further illustrate this interaction.
The BiPO4/BiOBr/Bi2O3 nanocomposite exhibits distinct interplane lattice spacings, with a measurement of 0.44 nm associated with the (101) plane of BiPO4, and a 0.32 nm spacing corresponding to the (111) plane of Bi2O3 Additionally, the presence of BiOBr is validated by a lattice spacing of 0.19 nm.
The elemental mapping of the nanocomposite, as illustrated in Fig 4.4c, reveals a uniform distribution of Bi, Br, P, and O elements within the selected region This uniformity confirms the presence of BiPO4, BiOBr, and Bi2O3 in the material.
The EDX spectrum (Fig 4.4d) confirmed the presence of Bi, Br, P, and O elements, while a peak around 8 eV indicated the presence of Cu from the TEM grid These results demonstrate that the synthesized composite is free from any impurities.
The UV-vis diffuse reflectance spectra (DRS) and Tauc plots of the different samples (BiPO4, BiOBr, Bi2O3, and BiPO4/BiOBr/Bi2O3) were compared in Fig
4.5 The BiPO4 sample showed stronger UV absorption at wavelengths below 350 nm compared to other samples Meanwhile, the BiOBr, Bi2O3, and
BiPO4/BiOBr/Bi2O3 samples showed a strong red-shift to the visible region, and
BiPO4/BiOBr/Bi2O3 sample also exhibited strong photoabsorption properties in the UV range because of the presence of the BiPO4 component (Fig 4.5a)
Furthermore, the bandgap energy of the samples is calculated according to the following equation:
ℎ𝜈𝛼 = 𝐴(ℎ𝜈 − 𝐸𝑔) (4.1) Where, hν is the photon energy (eV), α is the absorption coefficient, A is a proportionality constant, and Eg is the bandgap (eV) The exponent n value for
BiOX and BiPO4 is 4 which characterizes the indirect band transition [113,114]
Therefore, the above equation could be rewritten as:
The good approximation of bandgaps were estimated from the Tauc plot
(hνα) 1/2 vs hν by the intersection of the tangent line with the x-axis (hν- axis)
As shown in Fig 4.5b, the bandgap energies of BiPO4, BiOBr, Bi2O3, and
BiPO4/BiOBr/Bi2O3 were estimated to be 4.10, 2.72, 2.80, and 2.74 eV, respectively
Figure 4.5 UV-vis diffuse reflectance spectra (a) and Tauc plots (b) of BiPO4,
BiOBr, Bi2O3, and BiPO4/BiOBr/Bi2O3 samples
Photodegradation tests were performed using a 10 mg L −1 RhB solution and a catalyst loading of 0.5 g L −1 under a solar simulator at room temperature to evaluate the photocatalytic activities of the synthesized nanocomposites A series of experiments were conducted to identify the optimal volume of 0.1 M NaH2PO4 for synthesizing the most effective photocatalyst, testing samples S0, S1, S2, S3, and S4 under consistent conditions The findings indicated that the ideal volume of 0.1 M NaH2PO4 was 6 ml, corresponding to sample S3, which exhibited the highest photodegradation efficiency.
100% after 25 min Meanwhile, the degradation efficiencies of samples S2, S1,
The photocatalytic activity of the prepared nanocomposite was found to be significantly influenced by the volume of 0.1 M NaH2PO4 solution used, with optimal performance observed at 6 ml Deviations from this volume, either higher or lower, resulted in decreased efficiency Additionally, an excess of the BiPO4 component on the BiOBr/Bi2O3 nanosheets hindered the catalyst's ability to effectively utilize visible light, impacting overall photodegradation performance.
84.6% in the absence of the BiPO4 component In comparison, the photodegradation efficiencies towards RhB solution of pristine BiPO4 and BiOBr were 9.1 and
57.7%, respectively The RhB molecule was very chemically stable with a slight decrease (1%) in concentration for 25 min under the solar light
Figure 4.6 Photodegradation of 10 mg L −1 RhB (catalyst loading of 0.5 g L −1 ) using different samples: BiPO4, BiOBr, So, S1, S2, S3 and S4 represented for the
BiPO4/BiOBr/Bi2O3 were synthesized using varying volumes of 0.1 M NaH2PO4 solution (0, 1, 3, 6, and 12 ml) The study evaluated the rate constants for the photodegradation reaction with these prepared samples Additionally, the impact of catalyst loading and pH levels on the photodegradation of 10 mg L−1 RhB was investigated using sample S3 under solar light conditions.
Fig 4.6b shows the rate constants (k) for the degradation reaction of 10 mg L — 1
RhB solution in the presence of different photocatalysts, which increased in the following order: BiPO4 (k = 0.003 min −1 ) < BiOBr (k = 0.021 min −1 ) <
The investigation into the effect of catalyst loading on the degradation efficiency of a 10 mg L−1 RhB solution revealed a direct correlation between catalyst loading and degradation efficiency Specifically, after 25 minutes of solar light irradiation, degradation efficiencies were recorded at 52.9%, 73.7%, and 81.5% for catalyst loadings of 0.1, 0.2, and 0.3 g L−1, respectively Notably, a catalyst loading of 0.5 g L−1 achieved nearly complete degradation of RhB, reaching approximately 100% These findings indicate that increasing catalyst loading enhances the availability of active sites, thereby facilitating more effective pollutant degradation.
The pH medium significantly influences the photocatalytic activity of BiPO4/BiOBr/Bi2O3 when degrading 10 mg L−1 RhB solution Research indicates that higher degradation efficiencies are achieved in acidic conditions compared to basic ones, with efficiencies recorded at 99.6%, 99.5%, and 99.9% for pH levels of 2, 4, and 6, respectively In contrast, degradation efficiencies drop to 64.1% and 30.7% at pH levels of 8 and 10 This decline in efficiency in basic environments is attributed to a reduced concentration of H+ ions, which impedes the formation of reactive species such as HO•2 and 1O2, as discussed in the mechanism section.
Figure 4.7 Time-dependent UV-vis absorption spectra of RhB, TCH, HCQ, and
MO in the presence of BiPO4/BiOBr/Bi2O3 (sample S3) (Conditions: Catalyst loading of 0.5 g L −1 , pollutant concentration of 10 mg L −1 , solar light)
The effects of the four different pollutant types (RhB, TCH, HCQ, and
The study investigated the degradation of pollutants under solar light, using initial concentrations of 10 mg L−1 and a catalyst loading of 0.5 g L−1 The time required for complete degradation varied based on the type of pollutant, with reaction times of 25 minutes for RhB, 60 minutes for TCH, 120 minutes for another pollutant, and 210 minutes for the last one.
Conclusions
The BiPO4/BiOBr/Bi2O3 photocatalyst was successfully synthesized by a microwave-assisted one-step method The combination among BiOBr, Bi2O3, and
BiPO4 semiconductors facilitate rapid charge transfer and effective separation of electrons and holes through two key processes: band-to-band transfer and the Z-scheme mechanism This catalyst demonstrates outstanding photocatalytic performance, achieving nearly 100% degradation of pollutants at a concentration of 10 mg L -1.
Under solar light, the degradation efficiencies of RhB, TCH, HCQ, and MO were achieved at 25, 60, 120, and 210 minutes, respectively, with a catalyst loading of 0.5 g L−1 at room temperature The catalyst demonstrated excellent photostability, maintaining 98.5% removal efficiency after five cycles, indicating its potential for industrial applications Trapping tests revealed the presence of 1O2, h+VB, and O•2− species in the photocatalytic reactions, with 1O2 being the key player in the degradation process The Z-scheme mechanism contributed to high degradation efficiency and robust catalyst stability, positioning it as a promising solution for wastewater pollution challenges.
Chapter 5 Facile microwave-assisted synthesis of
Zscheme photocatalyst of layer-structured
BiOCl/BiOBr/Bi 2 O 3 nanocomposite for degradation of organic pollutants under visible light
Introduction
In Chapter 4, the silk cocoon-liked BiPO4 deposited on the surface of
BiOBr/Bi2O3 nanosheets showed good photocatalyst activity under solar light
The limited reusability of catalysts can hinder their industrial applications This chapter focuses on replacing BiPO4 with BiOCl to enhance both photocatalytic activity and catalyst reusability The bismuth oxyhalide family (BiOX, where X = I, Cl, Br, F) is chosen for its exceptional chemical stability, unique layered structures, low toxicity, and impressive photocatalytic performance under UV–Vis irradiation The BiOX family features a tetragonal matlockite-type structure, characterized by [Bi2O2 2+] slabs interleaved with halogen atoms.
As a member of the BiOX family, BiOCl compound has an open layered- structure which is favorable for the separation of photoinduced electron-hole pairs between
BiOCl, characterized by its [Bi2O2] 2+ and Cl ‒ layers, demonstrates remarkable electrical, optical, magnetic, and luminescent properties, alongside superior photocatalytic activity compared to TiO2 when exposed to UV light However, like BiPO4, BiOCl has a wide bandgap ranging from 3.17 to 4.10 eV, which limits its photocatalytic effectiveness under visible light.
The advantages of BiOBr and Bi2O3 were mentioned in Chapter 4 The
BiOBr exhibits a low bandgap of 2.3–2.9 eV, exceptional photocatalytic activity, and high stability, thanks to its unique layered crystal structure However, its photocatalytic performance is hindered by the high recombination rates of photogenerated charge carriers Similarly, bismuth oxide (Bi2O3) is a significant semiconductor with a low bandgap of 2.1–2.8 eV, visible-light activity, and a high refractive index, making it cost-effective, non-toxic, and non-carcinogenic Despite these advantages, pure Bi2O3 photocatalysts demonstrate low photocatalytic activity due to inefficient charge separation of photoinduced electron-hole pairs and susceptibility to photocorrosion.
Therefore, in this Chapter, a ternary BiOCl/BiOBr/Bi2O3 photocatalyst was prepared by a rapid microwave-assisted one-step synthesis at low temperature
The photocatalytic activity of the catalyst was carried out towards the decomposition of TCH, MO, and RhB under visible light.
Experimental
Bismuth (III) nitrate pentahydrate (Bi(NO3)3∙5H2O, ≥ 98%, Sigma Aldrich), hexadecyltrimethylammonium bromide ([(C16H33)N(CH3)3]Br, CTAB, 98%,
Alfa Aesar), nitric acid (70% HNO3, Daejung Chemicals), potassium chloride
(KCl, > 99%, Alfa Aesar) ) were used to synthesize the photocatalysts RhB
(90%, Sigma Aldrich), MO (Junsei chemicals), TCH (C22H24N2O8∙HCl, 96%, Alfa
Aesar) were used as the target organic pollutants without further purification
Sodium bicarbonate (NaHCO3, >99%), tert-butyl alcohol (TBA, ≥ 99.5%) were purchased from Daejung Chemical and p-benzoquinone (p-
BZQ, 98%, Sigma Aldrich), L-histidine (≥ 99%, Sigma Aldrich) were used as received
Synthesis of BiOCl/BiOBr/Bi 2 O 3
The BiOCl/BiOBr/Bi2O3 nanocomposite was successfully synthesized using a microwave-assisted one-step method Initially, a transparent 0.15% solution was prepared by sonicating 0.075 g of CTAB in 50 ml of deionized water Subsequently, 1 ml of 70% was incorporated into the synthesis process.
HNO3 was added to the solution Then, 0.2 g of Bi(NO3)3∙5H2O was dissolved
Scheme 5.1 Schematic illustration of the synthesis of the
The preparation of the BiOCl/BiOBr/Bi2O3 nanocomposite involved creating a milky suspension through a 5-minute sonication at room temperature Following this, KCl was incorporated into the mixture and stirred for 30 minutes The resulting mixture was then transferred to a microwave reactor, where the reaction was carried out at 180 W and 90 °C for 30 minutes with continuous stirring using a 2.45 GHz microwave system After the reaction, the product was cooled to room temperature, filtered, and thoroughly washed.
DI water and dried overnight in an oven at 60 °C The optimized experiments were performed with different amounts of KCl (0, 15,
30, and 75 mg, designed So, S1, S2, and S3, respectively) For comparison, a
BiOCl/Bi2O3 sample was synthesized by the same procedure without addition of
CTAB The BiOCl sample was prepared by a hydrothermal method
Typically, 0.22 g of KCl was dissolved in 20 ml of DI water Then, 0.97 g of
Bi(NO3)3∙5H2O was added to the KCl solution The mixture was stirred for
The reaction was conducted in a 30 ml Teflon-lined stainless steel autoclave at 160 °C for 12 hours After the reaction, the resulting white solid was collected and thoroughly washed with deionized water to eliminate any chemical residues.
Finally, the sample was dried overnight in an oven at 60 °C
A powder X-ray diffraction (XRD, PANalytical, X’Pert-PRO MPD) with Cu
Kα radiation (λ= 0.154 nm) was used to investigate the crystallinity of the samples
A scanning electron microscopy (SEM, Hitachi, S-4800) was used to observe the morphology of the samples Raman spectra were recorded using an XploRA plus
(HORIBA) Raman spectrometer with laser excitation at 532 nm UV-vis diffuse reflectance spectra and UV-vis spectra of the samples were collected using a
Varian Cary 5000 UV-vis-NIR spectrophotometer
The photocatalytic activity of the catalysts was evaluated by pollutant degradation using a solar simulator (ABET Technologies, 150 W, Xe lamp), an overhead projector (OHP, 400 W, halogen lamp), and a fluorescent lamp (BLBA,
10 W) as visible light sources TCH, MO, and RhB with absorbance at = 357,
465, and 553 nm, respectively, were used as model pollutants in wastewater to determine the photodecomposition efficiency of the catalyst
The reactions were conducted in a photochemical reactor at room temperature, where a photocatalyst was added to a 50 ml solution of a 10 mg L ‒1 aqueous pollutant, achieving a photocatalyst loading of 0.3 g L ‒1 To establish adsorption-desorption equilibrium, the mixture was stirred in the dark for 30 minutes before irradiation At specified irradiation intervals, 3 ml aliquots were taken and centrifuged at 10,000 rpm for 5 minutes to separate the catalyst from the solution.
Residual pollutant concentrations were spectrophotometrically monitored by measuring the absorbance of TCH, MO, and RhB solutions The reusability of the photocatalyst was assessed through the photocatalytic degradation of a 15 mg L ‒1 RhB solution After each degradation cycle, the catalyst was collected, washed with deionized water, and dried at 60 °C before the subsequent cycle Following the seventh cycle, the surface morphology and structure of the used catalyst were analyzed.
To understand the photocatalytic mechanism and identify the main active oxidizing species in the photodegradation process, scavenging reagents were added to the dye solutions before light illumination; 0.2 M TBA, 0.2 M
NaHCO3, 0.2 M L-histidine and 1 mM p-BZQ were used as scavengers for hydroxyl radicals, holes, singlet oxygen, and superoxide radicals, respectively
Electrochemical impedance spectroscopy (EIS) measurement and photocurrent response measurement
The EIS of the samples was measured using Autolab PGSTAT 302 N
(Metrohm) instrument equipped with a conventional three-electrode cell system
The Ag/AgCl electrode, the Pt electrode, and the working electrode (coated with the as-prepared catalysts) were set up in a quartz cell in the presence of 1 M
The Na2SO4 electrolyte was utilized in the experiment, where the working electrode was fabricated by applying various active materials, including BiOBr, BiOCl, BiOCl/Bi2O3, and BiOCl/BiOBr/Bi2O3, onto fluorine-doped tin oxide (FTO) glass with an active area of 1 cm × 1 cm using the drop-casting technique.
Briefly, 2 mg of the as-prepared catalyst was mixed with 1 ml of ethanol and 5 μl of Nafion to obtain a slurry Then, 15 àl of this mixture was drop-casted onto an
FTO glass substrate and dried in an oven for 12 h The EIS spectra of the samples were recorded over the frequency range from 10 5 Hz to 0.01 Hz.
The photocurrent response of the samples was evaluated using consistent equipment and setup at a working potential of 0.5 V Measurements were conducted over multiple on/off irradiation cycles lasting 20 seconds, utilizing a solar simulator as the light source All working electrodes featured an identical illuminated area of 1 cm × 1 cm.
Results and Discussions
X-ray diffraction analysis was performed on as-prepared samples to investigate phase components Fig 5.1 shows the XRD patterns of
BiOCl/BiOBr/Bi2O3, BiOBr/Bi2O3, BiOCl, and BiOBr samples The XRD pattern of BiOCl/BiOBr/Bi2O3 sample shows the existence of BiOBr phase with peaks at
32.3° and 46.3° 2θ corresponding to (100) and (200) planes, respectively and
Bi2O3 phase with two weak peaks at 22.9° and 59.7° 2θ regarding to (400) and
(860), respectively while the rest of the peaks at 11.9°,
The peaks at (220), (301), and (310) indicate the presence of the BiOCl component, confirming the successful synthesis of BiOCl/BiOBr/Bi2O3 under microwave conditions without impurities Previous studies have highlighted the challenges in achieving a highly crystalline phase of Bi2O3 at low temperatures without additional annealing.
(113), (211), (104), (212), (114), (220), (214), and (302), respectively, which belongs to the BiOCl sample in agreement with JCPDS # 06-0249 (Fig
5.1b) As shown in Fig 5.1c, the XRD pattern of the BiOBr sample with characteristic peaks at 10.9°, 21.9°, 25.2°, 31.8°, 32.2°, 39.4, 44.8°, 46.3°,
(212), (105), (220), (214) and (310), respectively, are consistent with JCPDS # 78-
0348 profile For comparison, the XRD pattern of BiOCl/Bi2O3 was shown in Fig
Figure 5.1 XRD patterns of the BiOCl/BiOBr/Bi2O3 (a), BiOCl(b), and BiOBr
The FITR spectra of the as-prepared samples was depicted in the Fig 5.2a
The BiOCl/BiOBr/Bi2O3 sample demonstrated a significant peak at 3543 cm−1, indicating the stretching vibration of O−H from absorbed water Additionally, a peak at 1617 cm−1 was linked to residual hydroxyl groups (Bi2O3−OH), while the peak at 1463 cm−1 corresponded to Bi−Cl vibrations The most prominent peak at 508 cm−1 was attributed to Bi−O stretching vibrations Furthermore, the sample displayed characteristic bands at 2912, 2849, and 964 cm−1, associated with the symmetric and asymmetric stretching vibrations of −CH2− and the out-of-plane −CH vibration of CH3 from residual CTAB, respectively.
BiOCl samples were recorded with weak peaks at 1463 cm −1 attributed to the
Bi−Cl vibrations The significant peaks at 510 or 518 cm −1 corresponding to Bi−O stretching vibrations were also observed in BiOCl/Bi2O3, BiOCl, and BiOBr
The Raman spectra of the samples are also shown in Fig 5.2b The
The analysis of BiOCl/BiOBr/Bi2O3 revealed four distinct peaks, notably at 59 cm −1 and 141 cm −1, which correspond to the external A1g stretching modes Additionally, a weaker peak at 199 cm −1 was identified as the Eg internal Bi–X stretching modes, while the motion of oxygen atoms produced the broadened Eg and B1g bands at 393 cm −1 Similar peak patterns were observed in the BiOCl/Bi2O3 and BiOCl samples, confirming their structural characteristics.
Figure 5.2 FTIR (a) and Raman (b) spectra of the BiOCl/BiOBr/Bi2O3,
The analysis of BiOCl/Bi2O3, BiOCl, and BiOBr samples revealed that the pure BiOBr sample exhibited five distinct peaks Notably, two peaks at 56 cm−1 and 92 cm−1 correspond to the first-order vibration modes of the BiOBr sample.
Bi metal The most pronounced peak at 113 cm −1 was from A1g internal Bi–Br stretching mode [109] while the band at 160 cm −1 was assigned to E1g internal Bi–
Br stretching mode [111] The Eg and B1g bands from the motion of oxygen atoms were also detected in the pure BiOBr sample with an inconspicuous peak around
Figure 5.3 SEM images (a, d); EDX elemental mapping (b-c, e-f) and EDX analysis (g) of the BiOCl/BiOBr/Bi2O3 nanocomposite
The morphology of the as-prepared catalyst was characterized by SEM images as shown in Figs 5.3a and d The SEM images confirmed that the
The BiOCl/BiOBr/Bi2O3 sample is characterized by numerous nanosheets with a layered structure, measuring between 0.2 to 1 micrometer High-magnification SEM imaging reveals that these nanosheets have a rough surface, which enhances porosity and increases reactive sites, potentially improving the degradation process Additionally, elemental mapping images provide further insights into the composition of the sample.
5.3b-c and e-f) revealed a uniform distribution of Bi, Br, Cl, and O elements in the catalyst Also, the EDS spectrum indicated the presence of Bi, Br, Cl, and O elements
The formation path of the as–prepared catalyst can be proposed as following the equations [113]
BiONO3 +H2O ↔ Bi2O2(OH)NO3 + 2 H + + NO3 — (5.6)
Bi2O2(OH)NO3 + OH — → Bi2O3 +3 NO3 — + H2O (5.7)
Bi(NO3)3∙5H2O, a strongly acidic-weak base salt, undergoes hydrolysis in acidic conditions to form BiONO3 This species subsequently generates BiO + ions, which can react with Br ‒ and Cl ‒ ions to produce BiOBr and BiOCl, respectively In this process, CTAB serves as the source of Br ‒, while KCl provides Cl ‒ ions, facilitating the formation of these bismuth compounds.
Bi2O3 component was present in the catalyst, which was explained by the formation process from BiONO3 species through two reactions (Eqs 5.6 and 5.7)
The incorporation of BiOCl, BiOBr, and Bi2O3 into the heterostructure significantly enhanced the separation of electron-hole pairs in the photocatalytic system, resulting in remarkable photocatalytic activity when exposed to visible light.
The photoabsorption ability of the materials plays an important role in the photocatalytic performance Therefore, different samples (BiOBr, BiOCl,
BiOCl/Bi2O3, and BiOCl/BiOBr/Bi2O3) were evaluated by UV–Vis–NIR spectrometer in the range of 200 to 800 nm (Fig 5.4a) It can be seen from Fig
5.4a, the pure BiOBr sample had strong absorption in the visible region, which showed the low value of bandgap In contrast, the pure BiOCl had strong absorption only in the UV range while the absorption edge of the BiOCl/Bi2O3 sample recorded a red-shift compared to the pure BiOCl sample due to the presence of Bi2O3 phase Due to the conjugation with BiOBr and Bi2O3, the
BiOCl/BiOBr/Bi2O3 sample exhibited a longer absorption edge in contrast to pure
BiOCl or BiOCl/Bi2O3 and showed a broader absorption in the UV-Vis light region Furthermore, the bandgap energies of the samples are calculated according to the following equation [125-128]
Figure 5.4 UV-vis diffuse reflectance spectra (a), Tauc plots (b), Nyquist plots
The photocurrent response of various samples, including BiOCl, BiOBr, BiOCl/Bi2O3, and BiOCl/BiBr/Bi2O3, was analyzed under a solar simulator In this context, hν represents the photon energy in electron volts (eV), α denotes the absorption coefficient, A is a proportionality constant, and Eg indicates the bandgap energy measured in eV The exponent n value plays a crucial role in understanding the materials' photocurrent behavior.
BiOX is 4 which represent an indirect transition [127] Therefore, the above equation could be rewritten as:
The bandgap energies of the samples were analyzed using the Tauc plot method, specifically by plotting (hνα) 1/2 against hν and extrapolating the linear regions The estimated bandgap energies for BiOBr, BiOCl, BiOCl/Bi2O3, and BiOCl/BiBr/Bi2O3 were found to be 2.72 eV, 3.26 eV, 3.20 eV, and 2.75 eV, respectively, as illustrated in Fig 5.4b.
To understand the separation and transport of charge carriers in various samples, Electrochemical Impedance Spectroscopy (EIS) and photocurrent response measurements were conducted EIS effectively assesses the charge transfer resistance of photoelectrodes As illustrated in Fig 5.4c, the ternary nanocomposite BiOCl/BiOBr/Bi2O3 exhibited a smaller arc radius than BiOBr, BiOCl, and BiOCl/Bi2O3, indicating it has the lowest charge transfer resistance and the highest efficiency in separating electrons and holes The relative arc sizes of the samples were ranked as follows: BiOCl > BiOBr.
> BiOCl/Bi2O3 > BiOCl/BiOBr/Bi2O3, demonstrating a significant improvement in charge carrier separation as well as rapid charge transfer in the
BiOCl/BiOBr/Bi2O3 ternary sample compared to the pristine samples and binary composites The anti-recombination of photoinduced electrons and holes in the
BiOCl/BiOBr/Bi2O3 conmposite would provide long life-time of charge carriers, which are favorable for photocatalytic reactions on the surface of the catalyst
Furthermore, the separation and transport ability of these photoinduced electrons and holes were investigated by photocurrent response measurement The experiment was carried out at a fixed bias of 0.5 V vs
Ag/AgCl with 20 s intermittent on-off cycles of solar light irradiation From Fig
5.4d, it is distinctly seen that the photocurrent density over BiOCl/BiOBr/Bi2O3 reached the highest value compared to the other samples, which in turn means that the ternary composite BiOCl/BiOBr/Bi2O3 performed the most effective separation and fastest transfer of charge carriers The highest photocurrent density of BiOCl/BiOBr/Bi2O3 was 0.91 A/cm 2 while the
BiOCl, BiOBr, BiOCl/Bi2O3 samples obtained the photocurrent densities of 0.41,
The study reveals that the BiOCl/BiOBr/Bi2O3 nanocomposite achieves the highest photocatalytic activity, with current densities of 0.49 and 0.70 A/cm² This enhanced performance is attributed to the synergistic interaction among the three components, which significantly improves the separation efficiency and prolongs the lifetime of photoinduced electrons and holes.
To achieve the highest degradation efficiency, photodegradation experiments were conducted on So, S1, S2, and S3 samples under identical conditions These experiments utilized a 10 mg L−1 RhB solution exposed to visible light from a 400 W halogen lamp at room temperature to evaluate the photocatalytic activities of the prepared nanocomposites The results indicate that the optimized amount of KCl significantly influences the degradation process, as illustrated in Figs 5.5a and b.
30 mg (denoted as sample S2) with the
The photodegradation of Rhodamine B (RhB) at a concentration of 10 mg L −1 was investigated under OHP light, utilizing different catalyst samples: So, S1, S2, and S3, which correspond to BiOCl/BiOBr/Bi2O3 synthesized with varying amounts of KCl (0, 15, 30, and 75 mg) The study measured the rate constants of the photodegradation reaction, revealing a maximum degradation efficiency of 99.4% with a rate constant (k) of 0.2443 min −1 Additionally, the effects of varying catalyst loadings on the photodegradation of RhB were assessed under both fluorescent lamps and solar simulators.
Facile microwave-assisted synthesis of sustainable Z-scheme
Introduction
In Chapters 4 and 5, a ternary nanocomposite was developed by combining UV-responsive photocatalysts BiPO4 and BiOCl with the visible light-responsive photocatalysts BiOBr and Bi2O3 This chapter introduces a quaternary photocatalyst synthesized through a rapid one-step microwave method, incorporating Ag/AgBr alongside BiOBr/Bi2O3 to enhance photocatalytic activity and photostability under visible light The advantages of BiOBr and Bi2O3, highlighted in earlier chapters, support the effectiveness of this approach To further improve light absorption, photocatalytic efficiency, and reusability, the Ag/AgBr system was identified as the optimal choice for integration with BiOBr/Bi2O3 in this study.
Herein, noble metallic Ag nanoparticles can act as a strong absorber for harvesting visible light owing to their surface plasmon resonance (SPR)
Silver bromide (AgBr) is a crucial light-sensitive material and plasmonic photocatalyst, but it exhibits optical instability as a single-phase photocatalyst due to partial decomposition into silver nanoparticles (Ag NPs) under visible light To enhance photocatalytic activity and stability, coupling Ag with AgBr is an effective solution.
[140,141] Nevertheless, the photocatalytic performance of the plasmonic
Ag/AgBr composite could be hampered by their micrometer-scale particle sizes resulting in low surface area and the rapid recombination of electrons and holes
Numerous strategies have been extensively researched to improve the performance of visible-light responsive photocatalysts, focusing on enhancing their capability for visible light harvesting, photocatalytic activity, and stability under light exposure For instance, Yang et al have contributed significantly to this field.
A two-step hydrothermal method was employed to fabricate an AgBr/Bi2O3 catalyst aimed at degrading methyl orange (MO) In a separate study, Li et al developed Ag-doped Bi2O3 nanosheets through co-precipitation at 90 °C for 24 hours, followed by calcination at 500 °C for 4 hours, also targeting the treatment of MO.
[100] fabricated AgBr-Ag-BiOBr composites by a hydrothermal route
(160 °C, 16 h) and calcination (450 °C, 4 h) for the removal of MO Yan et al
[142] used a solvothermal approach (160 °C, 10 h) to obtain
Ag/AgBr/BiOBr hollow hierarchical microspheres have been developed for the effective degradation of rhodamine B (RhB) Zhao et al successfully synthesized an Ag/AgCl/BiOCl nanocomposite through a solvothermal reaction at 150 °C for 24 hours, targeting the removal of RhB and tetracycline hydrochloride (TCH) Additionally, Qiu et al reported the creation of Z-scheme SnS2/BiOBr photocatalysts using a hydrothermal method, enhancing photocatalytic efficiency for environmental applications.
(180 °C, 10 h) showed better activity in RhB decomposition than pristine
A Z-scheme Ag/AgBr/Bi4O5Br2 photocatalyst was synthesized via a hydrothermal method at 160 °C for 12 hours to effectively decompose pollutants such as acid orange II, 4-tert-butylphenol, and aniline Despite numerous studies aimed at enhancing photocatalytic activity, challenges remain in practical applications due to high energy consumption, lengthy production times, complex synthesis procedures, and environmental concerns related to toxic solvents Among various photocatalyst designs, the Z-scheme configuration, which incorporates solid-state electron mediators like noble metal components such as Pt, shows promise for improving efficiency.
Au, and Ag) is considered a practical design to improve the visible-light absorption ability, effective electron-hole separation, and accelerated charge transfer at the interface [13]
With undeniable advantages of heating by microwave as mentioned in
Chapter 4, a Z-scheme quaternary heterojunction photocatalyst of
In this chapter, Ag/AgBr@BiOBr/Bi2O3 was synthesized using a low-temperature, rapid microwave-assisted one-step hydrothermal method The photocatalytic activities of the resulting catalyst were evaluated under visible light using various light sources, including a halogen lamp in an overhead projector, light-emitting diodes (LEDs), and a xenon lamp in a solar simulator The effectiveness of the catalyst was tested against three challenging pollutants: tetracycline hydrochloride (TCH), methylene orange (MO), and rhodamine B (RhB).
Experimental
Bismuth (III) nitrate pentahydrate (Bi(NO3)3∙5H2O, ≥ 98%, Sigma Aldrich), hexadecyltrimethylammonium bromide ([(C16H33)N(CH3)3]Br, CTAB, 98%,
Alfa Aesar), nitric acid (70% HNO3, Daejung Chemicals), and silver nitrate
(AgNO3, ≥ 99.8%, Daejung Chemicals) were used to synthesize the photocatalyst
RhB (90%, Sigma Aldrich), MO (Junsei chemicals), and TCH (C22H24N2O8∙HCl,
96%, Alfa Aesar) were used as the target organic pollutants without further purification Sodium hydrogen carbonate (NaHCO3, >99%,Daejung Chemicals), tert-Butyl alcohol (TBA, ≥ 99.5%, Daejung Chemicals), p-benzoquinone (p-BZQ,
98%, Sigma Aldrich), and Lhistidine (≥ 99%, Sigma Aldrich) were used as received.
Synthesis of Ag/AgBr@BiOBr/Bi 2 O 3
The Ag/AgBr@BiOBr/Bi2O3 nanocomposite was synthesized using a microwave- assisted one-step approach Typically, 0.075 g of CTAB was dissolved in 50 ml of DI water to obtain a transparent solution Subsequently,
100 l of 70% HNO3 was added to the solution Then, 0.2 g of
Bi(NO3)3∙5H2O was sonicated in a solution for 5 minutes at room temperature, resulting in a milky suspension Separately, a 0.1 M AgNO3 solution was prepared, and a specific volume was added to the milky mixture, which was stirred for 30 minutes This mixture was then transferred to a microwave reactor, where the reaction took place at 180 W and 90 °C for 30 minutes, with constant stirring using a 2.45 GHz Microwave (CEM, Discover System).
After rapidly cooling to room temperature, the final product underwent filtration and was thoroughly washed multiple times with deionized (DI) water It was then dried overnight in a vacuum oven set at 40 °C The experiments utilized varying volumes of a 0.1 M solution.
AgNO3 solutions (100, 300, 500, 700, and 1000 l denoted by S1, S3, S5, S7, and
A BiOBr/Bi2O3 sample was synthesized using a specific procedure, excluding the addition of AgNO3 solution for comparison Additionally, a pure AgBr sample was created through a reaction between 0.1 M AgNO3 and KBr in a 1:1 molar ratio at room temperature.
Scheme 6.1 Schematic illustration of the synthesis of the
Ag/AgBr@BiOBr/Bi2O3 catalyst
The crystallinity of the samples was analyzed by powder X-ray diffraction
(XRD, PANalytical, X’Pert-PRO MPD) using Cu Kα radiation (λ= 0.154 nm)
The morphology of the samples was examined using scanning electron microscopy (SEM, Hitachi S-4800) and transmission electron microscopy (TEM, Philips CM-200) High-resolution transmission electron microscopy (HRTEM) images were obtained with a TITAN G2 ChemiSTEM Cs Probe electron microscope.
The elemental composition and chemical state of the materials were analyzed by
X-ray photoelectron spectroscopy (XPS, Kratos, AXIS Nova) using monochromatized Al Kα radiation A HORIBA scientific photoluminescence spectrometer (PL, HORIBA, iHR550) with a laser wavelength excitation of 325 nm was used to record the photoluminescence (PL) spectra UV-vis diffuse reflectance spectra and UV-vis spectra of the samples were collected using a
Varian Cary 5000 UV-vis-NIR spectrophotometer
The photocatalytic activity of the catalysts was assessed through pollutant degradation experiments utilizing a solar simulator (ABET Technologies, LS 150, 150 W Xe arc lamp) under one sun illumination, alongside an overhead projector (OHP 400 W, Liesegang Trainer Deluxe).
400, halogen lamp), and light-emitting diode (LED) lamp (6 ea×0.4 W) as the visible light sources TCH, MO, and RhB with an absorbance at = 357, 465, and
In this study, 553 nm contaminants were utilized to evaluate the photodecomposition efficiency of a catalyst in wastewater treatment The experiments were conducted in a photochemical reactor at room temperature During each trial, a specific photocatalyst was combined with 50 ml of a 10 mg L ‒1 aqueous pollutant solution, achieving a photocatalyst loading of 0.5 g L ‒1 The reaction mixture was stirred in a dark chamber prior to exposure to light.
30 min to ensure the adsorption-desorption equilibrium before irradiation A 3 ml aliquot was withdrawn at a given light-irradiation time and then centrifuged at
10,000 rpm for 5 min to separate the catalyst The residual pollutant concentrations were monitored spectrophotometrically by measuring the absorbance of the TCH,
The reusability of the photocatalyst was assessed through the photocatalytic degradation of a 10 mg L ‒1 MO solution, with the catalyst being collected, washed with deionized water, and dried in a vacuum oven at 40 °C after each cycle Following the ninth cycle, the surface morphology and structure of the used catalyst were analyzed To elucidate the photocatalytic mechanism and identify the primary active oxidizing species involved in the photodegradation process, trapping agents such as 0.2 M TBA, 0.2 M NaHCO3, 0.2 M L-histidine, and 1 mM p-BZQ were introduced to the dye solutions prior to visible light irradiation, targeting hydroxyl radicals, holes, singlet oxygen, and superoxide radicals, respectively.
Electrochemical impedance spectroscopy (EIS) measurements and photocurrent response measurement
EIS of samples was performed using Autolab PGSTAT 302 N (Metrohm) instrument equipped with a conventional three-electrode cell system The
In this study, an Ag/AgCl electrode, a Pt electrode, and a working electrode coated with a prepared catalyst were assembled in a quartz cell containing a 0.5 M Na2SO4 electrolyte The working electrode was created by depositing the active material onto fluorine-doped tin oxide (FTO) glass with an active area of 1 cm × 1 cm using the drop-casting technique Specifically, 2 mg of the catalyst was combined with 1 ml of ethanol and 10 μl of Nafion to form a slurry, from which 15 μl was drop-casted onto the FTO glass substrate and subsequently dried in a vacuum oven for 12 hours The electrochemical impedance spectroscopy (EIS) spectra of the samples were then recorded across a specified frequency range.
The photocurrent response of the samples was assessed at a voltage of 0.5 V with a solar simulator as the light source This response was recorded through multiple on/off irradiation cycles lasting 20 seconds each.
The working electrodes had the same illuminated area of 1 cm × 1 cm.
Conclusions
Facile microwave-assisted synthesis of sustainable Z-scheme heterojunction photocatalyst of Ag/AgBr nanoparticles on
BiOBr/Bi 2 O 3 nanosheets for efficient degradation of organic pollutants under visible light
In Chapters 4 and 5, a ternary nanocomposite photocatalyst was developed by combining UV-responsive BiPO4 and BiOCl with visible light-responsive BiOBr/Bi2O3 The study further advanced to synthesize a quaternary photocatalyst using Ag/AgBr alongside BiOBr/Bi2O3 through a rapid one-step microwave method, aimed at enhancing photocatalytic activity and photostability under visible light Previous chapters highlighted the advantages of BiOBr and Bi2O3, while the Ag/AgBr system was identified as the optimal choice for improving light absorption, photocatalytic efficiency, and reusability of the quaternary catalyst.
Herein, noble metallic Ag nanoparticles can act as a strong absorber for harvesting visible light owing to their surface plasmon resonance (SPR)