Oxidative removal of rhodamine B over Ti doped layered zinc hydroxide catalysts

Highlights The successful substitution of Zn2+ by Ti4+ in layered hydroxides/ Ti4+ in octahedral sites of hydroxide layers is prerequisite for the destruction of rhodamine B/ Effective d

Accepted Manuscript

Title: Oxidative Removal of Rhodamine B over Ti-doped

Layered Zinc Hydroxide Catalysts

Author: Nguyen Tien Thao Doan Thi Huong Ly Han Thi

Phuong Nga Dinh Minh Hoan

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Oxidative Removal of Rhodamine B over Ti-doped Layered Zinc Hydroxide Catalysts

Nguyen Tien Thao*, Doan Thi Huong Ly, Han Thi Phuong Nga, Dinh Minh Hoan

Faculty of Chemistry, Vietnam National University, Hanoi

19 Le Thanh Tong Street, Hanoi, Vietnam 100000

*Corresponding author: Tel (+84) – (093) 789 8917; Fax: (+84) – (04) 3824 1140

Email: ntthao@vnu.edu.vn (N Tien Thao)

Photodegradation efficiency of rhodamine B over Ti-Zn layered hydroxide catalyst uncalcined and

calcined at 450 oC for 2 hours under visible-light irradiation at room temperature

Highlights

The successful substitution of Zn2+ by Ti4+ in layered hydroxides/

Ti4+ in octahedral sites of hydroxide layers is prerequisite for the destruction of rhodamine B/

Effective degradation efficiency of rhodamine B at a near neutral pH conditions/

Abstract Ti-doped layered zinc hydroxide materials with different molar ratios of Ti/Zn have been

synthesized through the coprecipitation method at pH of 9.0 - 9.5 The materials possess layered structure with carbonate anions in the interlayer regions The catalysts have uniform particle sizes and high surface area An isomorphous substitution of Zn2+ by Ti4+ in the brucite-like sheets makes

a promotional effect on the photocatalytic activity in the degradation of rhodamine B aqueous solution The catalytic results indicated that the intra layer sheet Ti4+ ions are more active than the extra –TiO2 components in the complete removal of rhodamine B The degradation efficiency is dependant on the intra lattice Ti4+ contents and reaction variables

Keywords: Ti-doping, degradation, rhodamine B, layered hydroxide, photocatalysis

1 Introduction

Photocatalysis has been well known as an advanced oxidation process used for the treatment of organic compound pollutants in water This is the most economic and promising industrial effluent treatment processes today [1-3] Among photocatalytic materials, semiconductor materials have been extensively studied for the effective removal of toxic organic dyes [4-8] And, TiO2-based material is one of the most popular semiconductors used as active heterogeneous catalysts for the wastewater treatment, air purification, water disinfection, hazardous waste remediation, and water purification [1,3,4] Titanium may be usually existed as TiO2 anatase dispersed on matrices, but it has a large band gap and thus its photocatalytic activity is usually limited to the UV region [2,3,6] Furthermore, the photocatalytic performance of TiO2 depends on its crystal structure, particle size

as well as effective surface area [6-10] Therefore, a great effort of researchers has been made for the improvement of the TiO2 performances in the sunlight by loading on appropriate support, doping agents, and reducing TiO2 particle sizes [8,9,11,12] In practice, TiO2 nanoparticles have been recently shown excellent activity in the photocatalytic treatments of polluted water and the applications in nanotechnology [12-16] Nevertheless, the usage of nanosized TiO2 also faced numerous drawbacks such as hazardous human during the preparation and application [17] In other scenarios, TiO2 nanoparticles were easily aggregated, resulting in a rapid decrease of the overall activity after a short reaction period [16-19] Thus, researchers have been looking for other applicabilities of Ti-based heterogeneous catalysts Another promising way is to immobilize Ti4+ into the framework of structured materials such as TiO2 doped by noble metals [12,18], zeolite [20,21], and mesoporous materials [22-24] Indeed, titanium-substituted molecular sieves have shown good adsorption ability of dyes and catalytic activity in the decoloration of organic compounds in wastewater [20,22,24]

For the purpose of the preparation of Ti-containing heterogeneous catalysts, we have incorporated some Ti4+ions into the layered zinc hydroxides It is well known that layered metal hydroxides contain a single type of metal cations with a positive charge usually accompanied by the appearance of hydroxyl vacancies [25-29] The representative hydroxide formulae are MII(OH)2-y(Xn−)x/n·mH2O (M = Zn2+, Cu2+, Ni2+) A typical example is a layered zinc hydroxide salt of Zn5(OH)8(Xn−)2/n·2H2O in which OH– deficiency on the host layers is compensated by coordinated guest anions (Xn-) [26,29,30] In the zinc hydroxide layers, one-quarter of octahedrally coordinated zinc ions is replaced by two tetrahedrally coordinated zinc ions located below and above the plane and water molecules are coordinated at the apexes of the tetrahedra Thus, they are widely used as inorganic adsorbents for the removal of anionic dyes or photocatalysts for the oxidation reaction

[19,24,25] A substitution of Zn2+ by Ti4+ in the hydroxide layers leads to a variation in the solid composition formulated as [(Zn1−yTiy(OH)2)+y∙(X2y/nn−)2y−]∙mH2O and creates more OH- deficiencies

in the hydroxide layers This modification allows to prepare several modified Zn-based catalysts with some desired photocatalytic properties due to the ability of hydroxide layers to accommodate some cations of various sizes and valences and the appearance of more foreign anions in the interlayer domains [14,25,26,32,33] In this work, the Zn–Ti LDHs have been synthesized by the co-precipitation method for the purpose of the preparation of effective catalysts used in the effective decomposition of rhodamine B in water under visible-light irradiation

hours in oven For the sake of brevity, the prepared catalysts are denoted as xTi-yZn in which x/y is

a molar ratio of Ti/Zn as reported in Table 1

2.2 Characterization

Powder X-ray diffraction (XRD) patterns were recorded on a D8 Advance-Bruker instrument using CuKα radiation (λ = 0.1549 nm) Fourier transform infrared (FT-IR) spectra were obtained in 4000 – 400 cm-1 range on a FT/IR spectrometer (DX-Perkin Elmer, USA) UV–vis spectra were collected with UV-Visible spectrophotometer, JASCO V-670 BaSO4 was used as a reference material The spectra were recorded at room temperature in the wavelength range of 200-800 nm The scanning electron microscopy (SEM) images were obtained with a JEOS JSM-

5410 LV TEM images were collected on a Japan Jeol Jem.1010 The specific surface area was calculated by the Brunauer–Emmett–Teller (BET) method, and the pore size distribution and total

pore volume were determined by the Brunauer–Joyner–Hallenda (BJH) method using a an Autochem II 2920 (USA)

2.3 Degradation of rhodamine B aqueous solution

Rhodamine B (RhB) has been chosen as the degrading pollutions to test the photocatalytic activities of the as-prepared samples The catalyst of 0.3 g was dispersed in Rh B aqueous solution (100 mL, 20 ppm) and air was bubbled through the reaction mixture at the flowrate of about 5.0 mL/min A 40 Watt- visible- light -compact lamp was used as a light irradiation source and running water was circulated to ensure constant temperature of the reaction mixture, which was stirred magnetically The degradation efficiency of RhB was monitored using the UV-vis absorption spectra by measuring the peak value of a maximum absorption of rhodamine h B solution During the irradiation, about 2 mL of suspension was continually taken from the reaction solution at given time intervals for the determination of rhodamine B concentration (C) The degradation efficiency (%) can be calculated as 100%

0

C

C C efficiency  t

where C0 is the initial concentration of dye and Ct is the measured concentration of rhodamine B in aqueous solution at a given time (t) The concentration of the target dye is calculated by a calibration curve The maximum absorption of RhB was at a wavelength of 553 nm The UV-vis spectrophotometer (Shimazdu, Cary 100 UV-VIS spectrophotometer) with quartz cuvettes was used for the determination of color intensity in the range of 300-600 nm Calibration based on the Beer-Lambert law was used to quantify the dye concentration

3 Results and Discussion

3.1 Characteristics of Catalysts

XRD patterns for the synthesized solids with Ti4+/Zn2+ molar ratios of 0/5.0 – 3.0/5.0 and 1.0/6.0 in the preparation solutions were represented in Figure 1 All samples show the observable reflection signals at 2-theta of 13.31, 28.37, and 38.89o corresponding to the basal planes (003), (006), and (009) respectively Furthermore, the additional weaker peaks at 2-theta of 31.01, 33.45, and 35.99o are respectively assigned to the reflections of some non-basal planes of (100), (101), and

(012) All these diffraction peaks exhibit symmetric, strong peaks at low 2θ values and weaker, less symmetric peaks at high 2θ values which are typical characteristics for the layered-double

hydroxide-type structure [14,30,31,33] Thus, XRD analysis indicates that the (Zn/Ti) layered

double hydroxides have a lamellar structure in which Ti4+ ions were inserted into the brucite-like sheets of Zn-based layered materials [33,34] This is indeed confirmed by a small decrease of the

thickness of the unit layer (d003) when the tetravalent metals (Ti4+) replace the divalent metals (Zn2+)

in the framework of layered metal hydroxides (Table 1) As the layered hydroxide material is suggested to be a hexagonal close-packed structure, the cell parameter can be calculated using the equations c = 3d003 (three times the interlayer distance (0 0 3)) and a = 2d110 (average metal–metal

distance in the interlayer structure (1 1 0)) The a cell parameters for a series of the synthesized

samples increases from 3.106 to 3.182 Å as Ti/Zn molar ratio changes from 0/5 to 2/5 and the average cation-cation distance in the brucite-like sheets tends to decrease at a higher Ti/Zn molar ratio (Table 1) [34,35] In theory, a high amount of Ti4+ causes a higher density of tetravalent cations in the layers, resulting in the stronger electrostatic repulsion between positive charges Thus, there is only a certain amount of Ti4+ ions existed in the brucite-like sheets [35-37] Thus, a slight

decrease of the parameter c value can be attributed to the stronger electrostatic interactions between

Ti4+ cations and carbonate anions when Ti4+ ions are introduced in the lattice framework In principle, the charge density on the brucite-like sheets increases with the insertion of the Ti4+ ions, giving rise to a higher amount of carbonate anions required to maintain the electro-neutrality of the final material [17,36] However, the amount of carbonate anions in the Ti-richer samples (Table 1)

is lower than the theoretical amount necessary for the charge neutralization because a fraction of

Ti4+ species presents as the extra-framework hydroxide components in the Ti-richer samples 5Zn, 3Ti-5Zn) [14,33,38] Figure 1 also exhibits a weaker intensity of the peak of 13.27o and the low signal-to-noise ratio for the Ti-rich samples, which is interpreted by the lower crystallinity degree of the corresponding solids Furthermore, the peak at 2-theta of 13.27o is vanished as

(2Ti-calcination of Ti-doped samples at 450 ◦C and new peaks at higher 2θ values were observed (Figure

1S) A series of 2-theta peaks at 31.8, 34.5, 36.3, 47.5, 56.8, 62.8, 57.9o are solely attributed to the reflections of zinc oxide phase (JCPDS Card No 00-036-1451) and the observable 2-theta peak at 25.5o is likely attributed to the TiO2 anatase (JCPDS Card No 00-021-1272) on the calcined Ti-rich samples [29,32,33,39,40] (see Figure 1S in Supplementary Materials)

Figure 2 shows the UV-vis spectra of three Zn-Ti layered hydroxides in the region of 220–

800 nm The most intense band was observed at 305 nm, characterizing for Ti atoms in the hydroxide –like sheets [34-36] The broad adsorption band with maxima at 260 nm corresponds also to the Ti4+ species in an octahedral coordination The intensity absorption at higher wavelength (between 320–360 nm) can be due to the presence of the Zn2+ cations in the layered hydroxide solids as well as the presence of some Ti atoms in an octahedral environment participating in Ti–O–

Ti bonds as part of small TiO2 particles [33,35] This absorption band slightly shifts to a longer

wavelength as increasing amount of titanium A band at 216 nm is firmly assigned to the metal- charge transfer characterizing isolated Ti4+ ions in an octahedral environment

FT-IR technique has been used to identify the nature and symmetry of interlayer anions and

the metal-oxygen bonding Figure 3 displays the FT-IR spectra for some Ti-doped layered zinc hydroxide catalysts before and after photocatalytic reactions It is noted all IR spectra are similar shapes, but the peak intensities of the spent catalysts are much lower than those of the fresh samples The broad intense bands between 3600 and 3200 cm-1 are ascribed to the OH stretching mode of layer hydroxyl groups and of interlayer water molecules [28,34,37] A weak shoulder peak recorded around 2920 cm-1 is ascribed to the OH stretching mode of interlayer water molecules [37,40] Simultaneously, the band at 1510 cm-1 is ascribed to the bending mode of water molecules

In a lower wavenumber region, the band at 1365 cm-1 is assigned to the asymmetric stretching mode

of the carbonate species [29,34,37,40] A weak band observed 831 cm-1 could be ascribed to plane bending vibration mode of carbonate anions It is observed some differences in signal intensities between the Ti-richer and -poorer samples probably due to different orientations of carbonate anions in the interlayer gallery as a result of different electrostatic forces in Ti-substituted samples The sharp band observed at 465 cm-1 for Ti–O–Zn in the framework of the solids

out-of-[29,32,33]

The catalyst morphology was investigated by SEM and TEM techniques Figure 4 represents SEM micrographs of all Ti-Zn samples and a TEM image of a selected 1Ti-5Zn layered hydroxide material SEM micrograph of 1Ti-5Zn shows the presence of roughly hexagonal particles [20,27,36] These uniform primary particles aggregate into larger disk-like platelets with the mean crystal domain of 100-200 nm [17,28,36] In other context, TEM image of 1Ti-5Zn presents lamellar structure which is essentially characteristic for layered materials and the hexagonal packing structure (Fig 4B) [39,40] The primary particles with diameter ranging from 20

to 50 nm are clearly observed For the Ti-richer samples, the small particles agglomerate together to form larger plates (Fig 4) Thus, it is expected that there are many slit-shaped spaces between catalyst particles In practice, nitrogen adsorption/desorption measurement for some representative samples shows isothermal curves with a plateau from 0 to 0.5 and a hysteresis loop in the range of 0.55 – 0.95 (Fig 2S) The patterns are likely classified to the IV type and the hysteresis loops are closely to the H3-classification, suggesting that these solids are either mesopores or nonporous materials [28,30,41] In the present work, the H3-like hysteresis loop is described to the nitrogen condensation/evaporation phenomena in slit-shaped pores created by the agglomeration of uniform plate-like particles with average pore width positioned at 5.4 nm (Fig 2S) The specific surface area

of samples is in the range of 98-125 m2/g

Scanning electron microscopy and energy-dispersive X-ray spectrometry (SEM-EDS) analysis provides local information of the concentrations of different elements in the outermost layers of the catalyst platelets EDS patterns of Zn–Ti –layered materials are shown in Fig 5 It is clearly observed that EDS patterns display observable signals of Zn, Ti, C, O No major difference

in the percentage of elements on four spots of each sample indicates a good dispersion of elements

in the Ti-doped layered zinc hydroxides at the micrometer scale [33,35,36,38] It is also revealed that carbonate anions are the interlayer compensating anions Furthermore, a Ti/Zn molar ratio is slightly beyond the theoretical value (Table 1), especially for the Ti-richer samples This is resulted from the different solubility products of ionic solids at a constant pH during the precipitation course, leading to the formation of Ti-Zn layered double hydroxides and amorphous TiO2 phase [26,28,34,36] It is expected the differences between the intra brucite –like sheet Ti- ions and TiO2 particles in the decoloration of rhodamine B

3.2 Oxidative removal of rhodamine B aqueous solution

The catalytic oxidation of rhodamine dye aqueous solution has been carried out using air flowrate at room temperature and atmospheric pressure All the experiments were conducted at the natural pH of 6.0 with a catalyst amount of 0.30 g After a given time interval, the corresponding concentration of the dye (as measured by UV spectrophotometer and the concentration evaluated using Beer-Lambert’s law) was taken as the initial concentration of the dye for all the catalyzed reactions

3.2.1 Catalytic oxidative removal of rhodamine B over Ti-Zn layered hydroxide catalysts

The rhodamine B aqueous solution was stirred under a simulated compact light irradiation for catalytic reaction A blank test (no catalyst) was carried out under the same conditions for the purpose of comparison and its UV-vis absorption spectra was presented in Fig 3S (Supplementary Materials) The degradation efficiency of rhodamine B in the blank test is likely negligible On the contrast, the degradation efficiency of rhodamine B sharply increases as the catalyst is added into the dye aqueous solution Indeed, Figure 6 draws a variation in degradation efficiency profiles of rhodamine B over Ti-doped layered zinc hydroxides, TiO2-reference sample (Wako), and the Ti-free catalyst under visible-light irradiation and in dark to investigation of the catalytic ability of Zn–

Ti samples in the decoloration reaction The degradation efficiency of rhodamine B increases linearly during 2 hours and gradually reaches a plateau as reaction time was kept for 4 hours except for sample of TiO2 The highest degradation of rhodamine B was observed over sample 1Ti-5Zn

(white light) with the decoloration level almost of 96 % after 6 hours The degradation efficiency of catalyst sample decreases in the order of 1Ti-5Zn (white light) > 0Ti-5Zn (white light) > 1Ti-5Zn (dark) > 0Ti-5Zn (dark) To elucidate the photocatalytic behavior of Ti-doped layered hydroxide samples, we have kept track of the temporal evolution of absorption changes during the catalytic degradation of rhodamine B at different reaction times (Fig 4S in Supplementary Materials) Some important issues would be withdrawn from the changes in the UV-vis absorption spectra of the reaction solutions Firstly, it is known that UV-vis spectrum of rhodamine B displays two main absorption bands at 553 and 352 nm The longer wavelength peak is ascribed as the n → π* electron transition while the shorter one is attributed to the benzene ring structure of rhodamine B [1,3,6,11,42] For the two tests performed in dark, the intensities of these two absorbing peaks monotonically decrease with increasing reaction time from 0 to 2 hours while no absorption wavelength shift is observed, indicating a very small amount of rhodamine B was adsorbed by the two catalysts under reported conditions (Fig 4S) On the contrary, the UV-vis spectra of the reaction solution change both intensity and absorption wavelength as the experiments are illuminated by white light (Fig 4S) The absorption band shifts to the blue wavelength suggesting the occurrence of the de-ethylation of rhodamine B [2,18,42-45] In the latter cases, the degradation efficiency of rhodamine B approaches to almost 90 and 98% after 8 hours over the catalysts, reflecting a coupling effect of Ti and Zn in the synthesized catalysts on the photodegradation of rhodamine B under visible light [46] The presence of Ti in layer zinc hydroxide framework has therefore promoted the photocatalytic activity of layered zinc hydroxide catalysts in the oxidation of rhodamine B with air

In order to shed light on the promotional effect of Ti –species in the oxidation reaction in the present study, a number of Ti-doped layered hydroxide catalysts with different Ti4+/Zn2+ molar ratios have been tested for the oxidation of rhodamine B aqueous solutions with air Figure 7 depicts the rhodamine B degradation efficiency varying with the reaction time It is observed that the decoloration velocity of rhodamine B occurs quickly at the beginning time because both adsorption and photodegradation processes simultaneously occur (Fig 7A) [2,10,42] The decoloration efficiency for all catalysts reaches a plateau after 8 hours For sample 1Ti-5Zn, the efficiency reaches almost 99.8% [35] A major change in UV-vis spectral shapes of the reaction solution with reaction time suggests the decomposition of rhodamine B during visible light irradiation (Fig 7B) [33,36] In practical, the dye is presumably decomposed in a stepwise manner with the solution dye changing from an initial red color to a light green-yellow under visible light irradiation Indeed, the UV-vis adsorption wavelength shifts to the shorter wavelengths, reflecting the decomposition of rhodamine B into de-ethylated-intermediates such as N,N,N’-triethylated rhodamine B (~534 nm), N,N’-diethylated rhodamine B (~520 nm), N-ethylated rhodamine B (~

510 nm), and rhodamine (~ 498 nm) (Fig 7B) [3,11,40,44,47] Furthermore, the peak intensity in the UV region is markedly reduced after a short irradiation time, substantiating the degradation of both the dye chromosphores and the aromatic rings under these reaction conditions In other words, the catalyzed de-ethylation might occur simultaneously with the self-photosensitization of rhodamine B molecule and its de-ethylated products over Ti-Zn layered catalysts [3,7,11,44,45] The photocatalytic activity increases as the follows of 1Ti-5Zn > 0.5Ti-5Zn > 2Ti-5Zn ≈ 3Ti-5Zn > 1Ti-6Zn A lower activity over the Ti-richer catalysts (3Ti-5Zn) indicates a low activity of the extra layered hydroxide lattice Ti4+ ions in the solids as confirmed by XRD analysis [9,10,16] The activity on 2Ti-5Zn is on a par with that over 3Ti-5Zn, interpreting that the zinc hydroxide layers would only accommodate a certain amount of Ti4+ ions in the hydroxide frameworks

3.3.2 Catalytic activity of calcined Ti-Zn layered hydroxide samples

Two Ti-Zn layered hydroxide samples were calcined at 450 oC for 2 hours in air prior to use

as a catalyst for the photodegradation of rhodamine B XRD pattern of calcined 3Ti-5Zn sample presented in Fig 1S (Supplementary Materials) indicates the thermal decomposition of Ti-Zn layered hydroxides into mixed oxides of TiO2 and ZnO The catalytic activity of both fresh and calcined Ti-Zn catalysts is shown in Figure 8 for the purpose of comparison It is noted that the calcined Ti-Zn layered hydroxide sample gives a rather low degradation efficiency of rhodamine B after 10 hours while their parent samples exhibit almost 80-98% of degradation efficiency at the same conditions Thus, the catalytic activity is strongly related to the intra-layered hydroxide lattice titanium ions The extra-lattice Ti4+ or TiO2 would be much less active for the decoloration of rhodamine B in the present study Moreover, UV-vis spectra of the reaction solution catalyzing by the calcined Ti-Zn samples clearly show no changes in the spectral shapes apart from an observable decrease of peak intensity (Fig 8B) The differences in the UV-vis spectra of the reaction solution over the fresh (Fig 4S) and calcined photocatalyst (Fig 8B) gave useful information about catalytic behavior For the fresh sample, its adsorption ability, decomposition of rhodamine B (both the de-ethylation and de-ethylated products) all take place as quick dynamic processes Meantime, the calcined sample exhibits a weaker activity in the decoloration of rhodamine B through step of the degradation of the aromatic chromospheres and adsorption [2,16,38,43-45]

3.2.3 Effect of pH

Since the photocatalytic activity has been improved by the presence of certain amount of the intra –lattice Ti4+ in Ti-Zn layered hydroxide catalyst, pH of the reaction solution may effect on the removal oxidation of rhodamine B [1,5,43] The effect of pH on the decoloration of rhodamine B

over Ti-doped layered hydroxide catalysts is investigated through the dye aqueous solution adjusted

in the range of pH values between 3 and 11 Figure 9 presents the variations of decoloration efficiency of rhodamine B with reaction time at different pH values over 1Ti-5Zn layered hydroxide catalysts It is not surprisingly observed that the highest degradation efficiency was observed at the

pH value of 3.0 and 6.0 At higher value of pH, the decoloration rate becomes worse possibly due to the reduction the positive surface charge of Zn-Ti layered sheets, and the increase of electrostatic repulsion between rhodamine ions and anions present in the interlayer region and solution (OH-, CO32- ), a change in reaction mechanism, and the hindrance of light illumination pathway [1,29,42-45] The degradation efficiency reaches almost 100% as carried out at pH of 3.0 and 6.0 It is very interesting to observe a good activity at pH of 6.0 because the most appropriate pH value for the degradation of rhodamine B reported in the literature is about 3.0-4.9, which means the oxidative reaction of rhodamine B in the present work occurring at weak to neutral conditions [1,4,5] As Ti4+ions are immobilized in layered hydroxide sheets, they could catalyze the decoloration of rhodamine B at slightly acidic-neutral conditions

3.3.4 Effect of catalyst dosage

For Ti-doped catalysts in the present work, the Ti-sites in octahedrally coordinated titanium ions on the outermost surface may be active sites for the decoloration of rhodamine B, the degradation efficiency would be therefore affected by the catalyst dosage [1,16,21,43] Four reactions were carried out at different catalyst dosages of 0.5Ti-5Zn catalysts The experimental results are presented in Figure 10 The decoloration efficiency increases from 36-76% to 77-98% with an augmenting amount of catalysts used This demonstrates that the decoloration of rhodamine

B is proportional to the amount of active sites on catalysts, which photogenerates electrons that could theoretically react with the absorbed oxygen molecules to generate oxidative intermediate species like super oxide (O2•–) and/or hydroxyl radicals (HO•) [2,29,42, 46] These oxidative agents would further attack the chromospheres and rhodamine B molecules [3,42,43,45]

3.3.5 Effect of reaction temperatures

The effect of reaction temperatures on the oxidative removal of rhodamine B over 1Ti-5Zn catalyst was carried out at 287, 301 and 318 K Figure 11A shows changes in the degradation of rhodamine B aqueous solution at different reaction temperatures on a representative 1Ti-5Zn catalysts while the results of other samples are presented in Fig 5S (Supplementary Materials) The degradation efficiency of rhodamine B markedly increases with the reaction temperatures The decoloration increases from 33.6 to 100% % after a 2-hour-illumination with an increased reaction temperature from 287 to 318 K After a 10-hours treatment, the degradation efficiency of rhodamine

B reaches about 76.8, 98.1, and 100% at 278, 301, and 318 K respectively This strongly indicated

that the elevated reaction temperature would have a positive effect on the oxidation destruction of rhodamine B [3,9,10,42,47] Indeed, the temporal UV-vis spectral changes of rhodamine B aqueous solution during the catalytic degradation reactions at 318K are represented in Fig 11B It is interesting to see that the main absorbance at 553 nm almost disappeared after 2 hours under visible light irradiation in the presence of 1Ti-5Zn catalyst, suggesting that rhodamine B was quickly destructed under experimental conditions In addition, a new absorbing peak at 510 nm is observed, interpreting the presence of N-ethylated rhodamine [3,42,43,48,49] This absorption peak shifts gradually into shorter wavelength and positions at 498 nm after a 6 hour-irradiation; after that time the peak position is almost unchanged, but its intensity continuously decreased The final peak at

498 nm is ascribed to the absorbance of the completely de-ethylated product of rhodamine B [3,6,7,43,50]

3.3.6 Catalyst Reusability

The catalytic stability performance of the two Ti-Zn layered hydroxide catalysts was tested for the decoloration of rhodamine B aqueous solution The catalysts have been recovered from the reaction mixture by filtration, washed with ethanol, dried at room temperature prior to reuse for the reaction under the same reaction conditions Figure 12 shows the photocatalytic activity of the two catalyst samples after five repeated application cycles While sample 1Ti-5Zn shows a constant degradation efficiency of rhodamine B after five recycles, the sample 0.5Ti-5Zn gives a small decrease of degradation values in the fourth cycle The results demonstrate that that the photocatalytic activity of Zn–Ti layered hydroxide is rather stable during the photocatalytic process

of the pollutant molecules [1,18,29] This demonstrated that the intra layered hydroxide sheet Ti+4ions are not leached out during the wet oxidation of organic molecules Rhodamine B is possibly degraded under visible-light irradiation over Ti-Zn layered hydroxides by O2•- oxygen active species formed on the surface adsorbed O2 molecules [3,22,29]

Moreover, this material can be easily separated from the reaction system due to the microscopic particle sizes of Zn–Ti layered materials (Fig 4) The results thus indicate that the Ti-doped- layered zinc hydroxides can be used as an attractive photocatalyst for large-scale environmental treatment with high photocatalytic activity, long-term sustainability and excellent recyclables

4 Conclusion

A number of Ti-Zn layered hydroxide materials with different molar ratios of Ti/Zn have been synthesized at a constant pH The materials possess layered structure with carbonate anions in the interlayer regions of Ti-Zn layered hydroxides as interlamellar anions The catalysts have uniform particle sizes and high external surface areas with the average pore width of 5.4 nm An