Synthesis, characterization of novel ZnOCuO nanoparticles, and the applications in photocatalytic performance for rhodamine B dye degradation

ZnO pha tạp Cu được tổng hợp bằng phương pháp solgel kết hợp xử lý nhiệt với hàm lượng Cu pha tạp khác nhau. Vật liệu được ứng dụng làm vật liệu quang xúc tác phân hủy Rhdamine B, các yếu tố ảnh hưởng tới quá trình quang xúc tác, động học và cơ chế quá trình đã được nghiên cứu đầy đủ.

RESEARCH ARTICLE

Synthesis, characterization of novel ZnO/CuO nanoparticles,

and the applications in photocatalytic performance for rhodamine B

dye degradation

Thi Thao Truong 1  · Truong Tho Pham 2,3  · Thi Thuy Trang Truong 4  · Tien Duc Pham 4

Received: 9 August 2021 / Accepted: 14 October 2021

© The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature 2021

Abstract

Photocatalytic deg radation of environmental pollutants is being up to date for the treatment of contaminated water In the present study, ZnO/CuO nanomaterials were successfully fabricated by a simple sol-gel method and investigate the photo-degradation of rhodamine B (RhB) The synthesized ZnO/CuO nanoparticles were characterized by X-ray diffraction (XRD), energy-dispersive spectroscopy (EDS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV-Vis diffuse reflectance spectroscopy (UV-UV-Vis-DRS), thermal analysis (TGA), surface charge, and Fourier transform infrared spectroscopy (FTIR) The photo-degradation of the dye RhB was followed spectroscopically The overall composition of ZnO/CuO material was found to be wurtzite phase, with particle size of 30 nm, and the Vis light absorption increased with

an increase of Cu content The ZnO/CuO nanomaterials were highly active leading to a photo-degradation of 10 ppm RhB reaching 98% within 180 min at 0.1 g/L catalyst dosage The change in surface charge after degradation evaluated by ζ poten-tial measurements and the differences in functional vibration group monitored by Fourier transform infrared spectroscopy (FTIR) indicates that the RhB adsorption on the Zn45Cu surface was insignificant And scavenging experiments demonstrate that the RhB degradation by ZnO/CuO nanomaterials involves to some degree hydroxyl radicals

Keywords ZnO/CuO · Photocatalyst · Solgel method · Rhodamine B · Degradation mechanism

Introduction

Metal oxides are a widely used material for various applications

in industrial Among numerous metal oxides, ZnO is known as a multifunctional material due to the potential application in many fields, such as electronics, optoelectronics, sensor, converter, energy generator, and photocatalyst in hydrogen production, event for biomedicine and pro-ecological systems (Kolodzie-jczak-Radzimska et al 2014) The ZnO is high chemical stabil-ity, high thermal and mechanical stability at room temperature, hardness, rigidity, and piezoelectric constant while its hybrid property is low toxicity, biocompatibility, and biodegradability (Ghahramanifard et al 2018) ZnO is also a well-known semi-conductor in groups II–VI, whose covalence is on the boundary between ionic and covalent semiconductors with a broad energy band of 3.37 eV (Chou et al 2017) and large exciton binding energy (Shashanka et al 2020) One advantage is that ZnO has quantum yield and easily controlled synthesis processes (Singh and Soni 2020) Furthermore, the structural, morphology, opti-cal, and electrical properties of the nanoscaled ZnO can also be easily modified or improved for many applications (Belkhaoui

Responsible Editor: Sami Rtimi

* Tien Duc Pham

tienduchphn@gmail.com; tienducpham@hus.edu.vn;

ducpt@vnu.edu.vn

1 Department of Chemistry, TNU-University of Sciences,

Tan Thinh Ward, Thai Nguyen City, Thai Nguyen 250000,

Vietnam

2 Laboratory of Magnetism and Magnetic Materials, Science

and Technology Advanced Institute, Van Lang University,

Ho Chi Minh City, Vietnam

3 Faculty of Technology, Van Lang University,

Ho Chi Minh City, Vietnam

4 Faculty of Chemistry, University of Science, Vietnam

National University, Hanoi, 19 Le Thanh Tong, Hanoi,

Hoan Kiem 1000 00, Vietnam

et al 2019) Therefore, an extensive study investigated the ZnO

as a potential photocatalytic degradation of various both organic

and inorganic pollutants such as ionic dyes, antibiotics,

pesti-cides, peptis, and heavy metal ion (Boon et al 2018; Pirhashemi

et al 2018; Raizada et al 2019) A high number of studies using

ZnO for photocatalysis is increasing every year to approximately

2400 at 2019 (Frederichi et al 2021)

Nevertheless, the photocatalyst of ZnO has several

limita-tions For example, the absorption in the visible (Vis) region

is less than 5% (Yu et al 2019) and the high recombination

rate of photo-induced charge carriers against the movement

of electron and hole to material surface reacts (Kumari et al

2020) To improve this feature, ZnO was combined with

many metal oxides to form effective materials, such as Cu2O

(Mamba et al 2018; Yu et al 2019), WO3, NiO, CoFe2O4,

Au, Pt/Ga, Sr–Au, graphene, Mn, Co, Ce, Nd, Gd (Koe et al

2019; Zhai and Huang 2016), Sn (Venkatesh et al 2020), and

Ag (Ramasamy et al 2021) Among these composite

materi-als, a mixed oxide system of ZnO and Cu has attracted many

researchers (Huo et al 2019; Jiang et al 2019; Maleki et al

2015; Vaiano et al 2018) That is because CuO is a low-cost

metal oxide with a narrow bandgap (1.2–2.1 eV) (Singh and

Soni 2020) Nevertheless, CuO shows a low photocatalytic

performance (Pirhashemi et al 2018) because of the high

recombination rate of charge carriers in the CuO system

Therefore, the integration of ZnO and CuO could decrease

the rate of recombination of photogenerated carriers (Sahu

et al 2020) Moreover, S Rtimi indicated that the signal for

the iso-energetic charge transfer among Cu2O and ZnO and

the electrostatic interaction between p-type Cu2O and ZnO

accelerated the electron migration to the ZnO n-type

semi-conductor (Mamba et al 2018), enhanced the response to Vis

light, and increased the photocatalytic performance under the

sunlight Many methods are studied to fabricate the ZnO/CuO

materials known as metallurgical process, mechanochemical

process, or chemical processes as precipitation, solgel,

sol-vothermal, and hydrothermal methods, using an emulsion

or microemulsion, or growing from a gas phase, pyrolysis

spray, sonochemical method, or synthesis using microwave

Among them, the sol-gel method allows the using of a wide

selection of solvents, surfactants, and heat treatment, making

it easier to control the particle size and shape For instance,

the hollow microsphere with a diameter of approximately 5

μm composed of uniform nanoparticles with a diameter of

approximately 20 nm was observed in the work of Chen et al

(2020); the quasi-sphere shape with uniform morphologies

has been reported (Acedo-Mendoza et al 2020); the hybrid

nanocomposite in which CuO nanoparticle is attached to the

ZnO–T tetrapod surface (Sharma et al 2020), ZnO nanorod

(Patil et al 2019), nanoflower-like structure (Mardikar et al

2020), nanowires (Chou et al 2017), and thin film (Asikuzun

et al 2018) was investigated The composite of ZnO and CuO

was studied with various ratio of Zn/Cu:Cu content with very

low Zn/Cu ratio in the range 1000/3÷1000/9 (Singh and Soni

2020), with the mass ratios as 99.9/0.1, 98.0/2.0, and 95.0/5.0 (Ruan et al 2020), or different percentages of Cu in the cata-lyst of 1, 3, 5, and 10% (Harish et al 2017), event component from 100% Zn to 100% Cu (Lavín et al 2019) The previous work indicated that the ZnO/CuO composite showed a better photocatalytic efficiency than the ZnO Nevertheless, to the best of our knowledge, the optimum composition of CuO and the influence of Cu content have not been reported Kavita Sahu et al (Sahu et al 2020) founded that the formation

of p-n 2D CuO–ZnO hybrid nanoheterojunctions enhanced the photogenerated charge carrier separation, so that they exhibited excellent photocatalytic decomposition for meth-ylene blue (MB), methmeth-ylene orange (MO), and 4 nitrophenol (4-NP) under sunlight radiation Ruan et al (2020) indicated that the ZnO/CuO n-n heterojunction photocatalysts, electron

on the conduction band (CB) of ZnO, move to the valence band (VB) of CuO by the electrostatic attraction, and form electrons in the CB of CuO and holes in the VB of ZnO, respectively Thus, the recombination of the electrons and hole pairs was reduced on the surface of ZnO, and the pho-tocatalytic activity of ZnO/CuO for Acid Orange 7 under the solar light was improved compared to pure ZnO

Rhodamine B (RhB) is a well-known fluorescent cationic dye in organic chemistry and biological studies RhB is usually used as a colorant in many industries such as the plastic, textile,

or paint, or illegally used for coloring different confectionery

by sweet markets or bakers RhB is soluble in water, and sta-ble with light, temperature, chemicals, or microbes However, RhB is an extremely toxic pollutant for water environment that strongly affects humans and organisms (Yen Doan et al 2020) RhB has no deadly effects on the ecosystem as pesticides (Rani

et al 2021) but in the body, RhB can cause oxidative stress,

injury, increase in cell apoptosis, and brainstem (Sulistina and Martini 2020) In this study, for the first time, we synthesized the series of ZnO/CuO nanomaterials with different ratios by sol-gel method The characterization of the materials was sys-tematically examined by different physicochemical methods including X-ray diffraction (XRD), energy-dispersive spectros-copy (EDS), scanning electron microsspectros-copy (SEM), transmis-sion electron microscopy (TEM), and UV-Vis diffuse reflec-tance spectroscopy (UV-Vis-DRS), thermal analysis (TGA), Fourier transform infrared spectroscopy (FTIR), and surface charge evaluation Their photocatalytic performance for RhB degradation under the solar light was thoroughly investigated The mechanisms for RhB removal using ZnO/CuO were also studied based on the presence of different radicals as well as the changes in charging behavior and surface functional group after RhB degradation

Materials

Oxalic acid (H2C2O4) (purity ≥ 99.5%), cupric nitrate

trihy-drate (Cu(NO3)2·3H2O) (purity = 99.0–101.0%), zinc

ace-tate dihydrate (Zn(CH3COO)2·2H2O) (purity ≥ 99.0%), and

ethanol (C2H5OH) (96%, for HPLC), rhodamine B (≥ 97%,

for HPLC), n-butanol (n-C4H9OH) (purity ≥ 99.5%),

diam-monium oxalate monohydrate ((NH4)2C2O4·H2O) (purity

≥ 99%), and silver nitrate (AgNO3) (purity ≥ 99.8%) were

purchased from Sigma-Aldrich, India, to be used without

further purification Deionized water was used throughout

the whole study

Synthesis of ZnO/CuO nanoparticles

ZnO/CuO nanoparticles were prepared by sol-gel method

(Kolodziejczak-Radzimska et al 2014; Siddiqui et al 2018)

Firstly, zinc acetate dihydrate and cupric nitrate trihydrate

were dissolved in ethanol at 60°C for 30 to 60 min, leading

to the formation of a clear and homogeneous solution The

Zn:Cu molar ratios were 45:1; 30:1; 20:1; 15:1; and 10:1

(labeled as ZnxCu) Secondly, a solution of oxalic acid in

ethanol was added dropwise into the initial solution under

vigorous stirring at 60°C and maintained for 2 h to obtain the

milky white suspension gel form The mole ratio of oxalic

acid:total metal ions was 1:1 The gel formed was dried at

80°C for 36 to 48 h to completely dry (xerogel) The xerogel

was subjected to thermal analysis to determine the sample

annealing temperature After that, the xerogel was finely

ground and annealed at 450°C in air for 2 h, heating rate

200 °C/h

Characterization

After heat treatment, crystal structure and phase

com-position of obtained products were studied by XRD using

a X-ray diffractometer (Max 18XCE, Japan) with Cu Kα

radiation (λ = 0.154056 nm) at a scan rate of 0.02° s−1 in

the 2θ range from 20 to 80° The morphology and particle

size of the materials were evaluated by SEM (Leo 1430VP)

and TEM (Jeol JEM 2100F microscope) at an accelerating

voltage of 200 kV Elemental analysis of samples was

exam-ined using a JSM-7900F SEM attached with EDS Optical

absorption, reflectance, and bandgap properties were

ana-lyzed using UV-Vis diffuse reflectance spectroscopy (DRS)

by a Scinco 4100 instrument The surface charge before and

after interaction with RhB was determined by zeta potential

using Zetasizer Nano ZS (Malvern, England) The zeta (ζ)

potential was calculated from electrophoretic mobility with

Smoluchowski’s equation Fourier transform infrared (FTIR)

spectroscopy was used to evaluate the change of vibration

functional surface groups The FTIR spectra were conducted

on JASCO, Japan (FT/IR-4600 type A), using TGS detector

with a solution of 4 cm−1 The wavenumber was recorded from 400 to 4000 cm−1

Evaluation of photocatalytic activity

The photocatalytic performance of materials was inves-tigated in aqueous RhB solution using natural sunlight (on

a sunny day, between 9:00 am and 15:00 pm), at different concentrations of RhB from 10 to 50 ppm with the range of photocatalyst dosage of 0.1–0.5 g/L In the photodegrada-tion, different amounts of catalyst were dispersed in 250-mL RhB solutions Before sunlight irradiation, the suspensions were agitated using magnetic stirrer in the dark for 60 min

to achieve the contacted RhB molecules with the photocata-lyst About 7 mL RhB solution was withdrawn, centrifuged, and filtered, and the solution was collected to determine the RhB concentrations by using a UV-Visible spectrophotom-eter (UV-1700 Pharma Spec, Shimadzu, Kyoto, Japan) The

relative RhB concentration (C/C0) was determined using the

relative absorbance (A/A0) at a wavelength of 554 nm (Jiang

et al 2019), where A0 and A were the absorbances of RhB solutions at the lighting start time (t0) and at any time t,

respectively The photocatalytic efficiency was calculated using Eq (1) (Venkatesh et al 2020):

where H is the photocatalytic efficiency, C0 is the initial

concentration, and Ct is the concentration of RhB after

illu-mination t min.

For cycle tests, the used photocatalyst was washed several times with ethanol and deionized water and dried at 80°C for 12h after each run

For the scavenger tests, di–ammonium oxalate monohy-drate (AO) (Sakib et al 2019), tert–butanol (BuOH) (Raja

et al 2019), and silver nitrate (AgNO3) (Osotsi et al 2018), were used as h+, ·OH, and e− reactive species, respectively

Results and discussion

The TGA/DTA of Zn10Cu and ZnCu0 xerogels have been done and shown in Fig. 1 The TGA-DTA curves of two samples are found to be similar with two weight-loss seg-ments, corresponding to the endothermic process The weight-loss segment between 30 and 200 °C, containing three endothermic processes at 81.21, 125.15, and 162.27°C (ZnCu0 sample) and 83.34, 143.73, and 169.41°C (Zn10Cu sample), is about 52.275% and 20.77% for ZnCu0 and

Zn10Cu, respectively This effect can be attributed to the removal of water and residual solvent (Mel’nik et al 2006), the decomposition of non-carbonized anion NO3−, and other nitrogen-containing molecules (Xu et al 2009) The second weight-loss segment was located between 366–427°C and

(1)

H(%) =

C o − C t

C o 100% = A o − A t

A o 100%

312–394°C for ZnCu0 and Zn10Cu xerogel, respectively The

weight loss in this segment is about 22.49% for ZnCu0 and

39.96% for Zn10Cu, and it may concern to several

mecha-nisms: the volatilization of excess oxalic acid (the boiling

point 365.1±25.0 °C), the decomposition of the gel network,

or a combustions of organic materials A decrease in the

weight is insignificant and thermal effect is observed in the

temperature range after those segment, indicating that the

calcination temperatures at above 450°C is the

crystalliza-tion process Therefore, we have decided to treat all samples

at 450°C

Characterization of ZnO/CuO materials

The crystal structures of ZnO/CuO nanoparticles were

analyzed by XRD method The diffraction patterns of ZnO/

CuO nanoparticles with different Zn/Cu ratio are shown in

Fig. 2 In the ZnO sample, there are a total of nine

diffrac-tion peaks in the 2θ ranging from 25 to 70° These peaks

locate at 31.76°, 34.61°, 36.29°, 47.56°, 56.98°, 62.84°,

66.20°, 68.00°, and 69.08° and match well with the PDF

card (JCPDS No.36-1451) of the Wurtzite structure of ZnO

(Shukla and Shukla 2018) The Miller indices of ZnO of the

Wurtzite structure are denoted in Fig. 2 There are not any

peaks concerning the impurity phase which can be observed

within the XRD detection limit In other samples, the change

in the Zn/Cu ratio influences the shifting and rising a new

peak in the XRD patterns but the Wurtzite structure remains

unchanged By increasing the Cu content, the diffraction

pat-terns of ZnO/CuO nanoparticles tend to shift toward a high

angle, meaning a shrinkable of the volume of unit cell, as

seen in the inset of Fig. 2 This observation is highly con-trary to what was observed in the work of Lu et al (2017), where the lattice parameters of ZnO/CuO nanocomposites had not varied with changing the Zn/Cu ratio Ping et al explained this phenomenon by considering a similar ionic radius of Zn2+ (0.074 nm) and Cu1+ (0.074 nm) ions (sup-ported by XPS measurement) It is worth noting that Cu has doped at the Zn site of wurtzite structure of ZnO to form

0

20

40

60

80

100

-4 -2 0 2

-30 -25 -20 -15 -10 -5 0 5 10

0 20 40 60 80 100

DTA-ZnO

TG-ZnO

Temperature ( o C)

(b)

(a)

DTA-Zn10Cu

Temperature ( o C)

TG-Zn10Cu TG (%

Fig 1 Schematic of thermal analysis of ZnCu0 (a) and Zn10Cu (b) xerogels

Zn15Cu

Zn30Cu

2θ (degree)

ZnO

ZnO: Z-(hkl) CuO: C-(hkl)

ZnO-(101)

Fig 2 XRD pattern of synthesized ZnO/CuO materials The inset show the diffraction profile of ZnO-(101)

the tetrahedral coordination of Zn/Cu surrounded by four

oxygens In general, a partial substitution of Cu at the Zn

site can vary the oxidation state of Cu ions in either Cu2+

(0.071nm) or Cu1+ (0.074 nm) depending on the

synthe-sis conditions (Lu et al 2017; Rooydell et al 2017) The

different oxidation state of Cu ions is a crucial reason for

changing the lattice parameters (the shift of XRD pattern)

of ZnO/CuO nanoparticles Therefore, the shift of the XRD

patterns observed in Fig. 2 is mainly due to the

incorpora-tion of Cu2+ into the ZnO lattice This also suggests that

oxygen vacancies do not play a crucial role on the optical

properties of our samples Our observation is consistent

with the previous reports (Rooydell et al 2017)

Further-more, an increase in the Cu content also enhances intensity

of the diffraction peak at 38.56° This peak belongs to the

main intense peak (111) of the monoclinic structure of CuO

(JCPDS card no 45-0937) The appearance of this peak in

the ZnO/CuO nanoparticles approves that the Cu doping

on the ZnO is not only incorporation inside the ZnO

lat-tice to construct the Zn1−xCuxO compounds but also buildup

of CuO lattice for forming the Zn1−xCuxO/CuO

nanocom-posites Obviously, an increase in Cu doping concentration

prefers to form the Zn1−xCuxO/CuO nanocomposites as

evidence from an enhanced intensity of the (111) peak of

CuO and an unchanged peak position of (101) of ZnO in the

Z15Cu and Z10Cu samples The crystallite sizes calculated

from Sherrer’s equation indicates that the ZnO crystallite

sizes of the Zn0Cu, Zn45Cu, Zn30Cu, Zn20Cu, Zn15Cu, and

Zn10Cu system are about 28.0, 25.2, 23.6, 18.0, 27.7, and

29.0 nm, respectively

The presence of Cu in materials was confirmed by the EDX spectra (Fig. 3) The sharp peaks of Zn, Cu, and O were obtained; no other peaks related to any other element were detected in the spectrum within the detection limit The calculated Zn/Cu ratio from the EDX spectrum of

Zn10Cu is about 47.1/4.2 (11.2/1.0), which is quite close to the design value The other samples also show a matching between theoretical and calculated Zn/Cu ratio values, which are 15.57/1.00, 32.4/1.00, and 47/1 for the Zn15Cu, Zn30Cu, and Zn45Cu samples, respectively

The morphologies of some ZnxCu materials are presented

by SEM and TEM images in Figs. 4 The ZnO and system

of CuO–ZnO are uniformly spherical The SEM images on the larger scale (μm) show that the ZnO particles are aggre-gated but the aggregation of CuO–ZnO did not occur All the materials with different Zn/Cu ratios were in the size of

27 ± 8 nm

The optical nature of synthesized materials was analyzed through the UV-Vis diffused reflectance spectra technique, corresponding to the results in Fig. 5 The optical spectrum

of pure ZnO exhibits with strong absorption spectra in range

of 200–400 nm, and the sharp absorption edge around 400

nm The characteristic edge of ZnO was observable in the

Znx Cu material, and the band-gap energy (Eg) of ZnO and the Znx Cu (x = 45, 30, 20, 15, 10) system was calculated

from the UV-visible absorption spectra of ZnO by a Tauc plot (Senasu et al 2020; Souza et al 2017) (Fig. 6b) The Eg

values decreased with increasing Cu content and found to be 3.07, 3.05, 2.99, 2.92, 2.84, and 2.62 eV

Element Weight

(%) Atomic(%)

Totals 100.00 (Zn15Cu)

Element Weight

(%) Atomic (%)

Totals 100.00

(%) Atomic(%)

Totals 100.00 (Zn45Cu)

t Weight (%) Atomic (%)

Cu L 06.47 04.20

Zn L 74.64 47.10 Totals 100.00

Fig 3 EDX spectra of different ZnxCu materials

Fig 4 A. SEM images of a Zn15Cu, b Zn30Cu, and c Zn45Cu; B TEM images of a ZnO and b Zn45Cu

Fig 5 UV-Visible DRS spectra

of pure and Cu doped ZnO

nanoparticles

2.6 2.7 2.8 2.9 3.0 3.1 3.2 3.3 0.2

0.4 0.6 0.8

0.2 0.4 0.6 0.8

1.0

Zn10Cu

Zn15Cu

Zn20Cu

Zn30Cu

Zn45Cu

ZnO

Eg (eV)

(b)

Wavelength (nm)

ZnO

Zn45Cu

Zn30Cu

Zn20Cu

Zn15Cu

Zn10Cu (a)

Furthermore, all ZnO/CuO materials have a high value

of Abs in the Vis region compared to pristine ZnO: the abs

for ZnO is 0.2 while abs for Znx Cu samples (x = 45, 30, 20,

15, 10) are about 0.35 to 0.7; the abs for ZnxCu samples in

the region below 370 nm is also sharply reduced compared

to pristine ZnO This may be related to their morphologies,

particle size, and surface nanostructures, improving the

crystallinity and reducing the defects (Ungula et al 2017);

another reason may be due to the strong sp-d exchange

inter-action between the band electrons of ZnO and the localized

electrons of Cu2+ ions substituting for the Zn2+ ions (Ramya

et al 2018) or the substitution of Cu ions in the ZnO lattice

(Kama rulzaman et al., 2016) or a separating phase between

ZnO and CuO These results confirm the formation of

CuO-loaded ZnO hierarchical structures Also, the formation of

coupled CuO/ZnO nanocomposite shifted the band gap energy into the visible light region

Photocatalytic activity

To apply the ZnO/CuO materials as photocatalysts in natural environment, we investigated their photocatalytic activities for the RhB degradation in a solution with a pH of approximately 6 under solar light The catalyst dosage of 0.1 g/L was fixed to remove 20 ppm RhB from aqueous solution The results are shown in Fig. 6

Figure 6 a shows the UV-Vis spectra of the RhB aque-ous solute taken out at different reaction times during the photodecomposition process using the Zn45Cu material As can be seen, the maximum Abs of 20 ppm RhB solution

at 554 nm before and after presence of Zn45Cu placed in the dark only slightly decreased (abs from 1.50 down 1.44, corresponding to 4%) When irradiation time increased, the

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

Wavelength (nm)

(a)

Initial RhB 20 ppm

Ads 60 min in the dark

= Lighting 0 min

Lighting 30 min

Lighting 60 min

Lighting 90 min

Lighting 120 min

Lighting 180 min

0 20 40 60 80 100

Time (min)

(b)

Zn10Cu

Zn15Cu

Zn20Cu

Zn45Cu

Fig 6 a Absorption spectra of RhB as a function of irradiation time after the photocatalytic degradation using 0.1 g/L Zn45Cu exposed to the

sun light b Photocatalic efficiency decompose 20 ppm RhB under sunlight by 0.1 g/L synthetic materials

0

20

40

60

80

100

Time (min)

(a): 0.1 g/L Zn 45 Cu

10 ppm RhB

20 ppm RhB

30 ppm RhB

50 ppm RhB

0 20 40 60 80

100

Time (min)

(b) 20 ppm RhB

0.05 g/L Zn45Cu 0.1 g/L Zn45Cu 0.2 g/L Zn45Cu 0.5 g/L Zn45Cu

Fig 7 Photodegradation efficiency of RhB under sunlight at different initial concentrations of RhB and Zn45Cu

maximum absorbance decreased gradually After 240 min

upon sunlight irradiation, RhB degradation reached to 82%

Figure 6b indicates that Cu content was in the sample, and

the light absorbance in the visible region of ZnO/CuO

sys-tems increases while the photocatalytic efficiency of RhB

degradation by the material did not increase accordingly

The ZnO/CuO system with ratio of Zn/Cu = 45 shows the

highest photocatalytic efficiency compared to other Zn/

Cu ratio samples, followed by Zn10Cu; the efficiencies of

Zn15Cu, Zn20Cu and Zn30Cu were insignificant The

influ-ence of doped Cu content to the photocatalytic performance

of ZnO/CuO materials did not follow a rule in the previously

published papers (Acedo-Mendoza et al 2020; Harish et al

2017; Kumari et al 2020) Maybe, in the region of Zn/Cu

atom ratio = 10÷20, the excess amount of Cu cannot be

incorporated in the ZnO host lattice sites, CuO was

segre-gated from the ZnO crystal lattice leading to the new phase,

and the photocatalytic behavior in the visible light of ZnO/

CuO system is mainly due to CuO activity, so the higher

the Cu content, the higher the photocatalytic efficiency is

However, in the region of Zn/Cu atom ratio = 20÷45, that is,

the lower the Cu content, the Cu ion readily penetrates the

ZnO crystal lattice during phase information, causing some

structural deviations, and enhance the RhB decomposition

photocatalytic efficiency of the ZnO/CuO system; the

pho-tocatalytic behavior in the visible light of CuO/ZnO system

may be due to the combined action between CuO and ZnO,

or the interaction between ZnO and CuO Therefore, the

optimum content of Cu in Zn45Cu is the important factor to

affect the photocatalytic activity of the coupled ZnO/CuO

photocatalyst

The Zn45Cu sample will be used in the next studies

Pho-todegradation efficiency of RhB under sunlight at different

initial concentrations of the Zn45Cu catalyst and RhB is shown in Fig. 7

Figure 7 shows that the RhB degradation efficiency using 0.1 g/L Zn45Cu under the solar light decreased significantly when increasing initial RhB dye concentrations from 10 to

50 mg/L The RhB degradation efficiencies after 180 min with 10, 20, 30, and 50 ppm decreased to about 98, 82, 73, and 21% respectively Furthermore, RhB degradation effi-ciencies gradually increased after 180 min with increasing the catalyst from 0.05 to 0.5 g/L It implies that the reaction rate depended on both the initial concentrations of Zn45Cu and RhB To understand this dependency, we used the

pseudo-first-order reaction to describe the ln(C/Co) against the time (Fig. 8) As can be seen in Fig. 8, all correlation

coefficient (R2) values were higher than 0.9, demonstrating that the RhB degradation behavior using Zn45Cu catalyst was in accordance with the pseudo-first-order kinetic The stability of the Zn45Cu nanoparticles was evaluated

by catalytic degradation of RhB recycles The material was recovered and reused three cycles After photocatalytic experiments, the catalyst was taken out from the reaction vessel by centrifugation, rinsed with ethanol and deionized water, before drying in the oven at 80°C for 12h The RhB degradation efficiencies after the regenerations are shown in Fig. 9 As can be seen, the RhB degradation efficiencies at all times decreased insignificantly It means that the photo-catalytic activity of Zn45Cu nanoparticles is relatively stable

It should be noted that RhB removal in the presence of 0.1 g/L synthesized materials for 60 min without the light was only 4% (lighting 0 min in Fig. 6a), suggesting the negligible RhB adsorption on Zn45Cu nanoparticles Similar experi-ments were also carried out with all ZnnCu materials We found that RhB concentrations were only reduced below 6%

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

0.05 g/L; y = - 0.05704 + 0.00214x; R 2 = 0.95307

0.1 g/L; y = - 0.16141 + 0.00486x; R 2 = 0.97397

0.2 g/L; y = - 0.41004 + 0.00815x; R 2 = 0.97895

0.5 g/L; y = - 0.02257 + 0.00604x; R 2 = 0.98899

(a)

o /C)

Time (min)

0.0 0.5 1.0 1.5

2.0

(b)

RhB 10 ppm; R 2 = 0.96722

y = - 0.40518 + 0.01127x; RhB 20 ppm; R 2 = 0.97397

y = - 0.16141 + 0.00486x; RhB 30 ppm; R 2 = 0.96091

y = - 0.44227 + 0.00632x; RhB 50 ppm; R 2 = 0.99423

y = - 0.1032 + 0.00138x;

o /C)

Time (min)

Fig 8 Kinetic study of RhB photodegradation process with Zn45Cu catalyst under the sunlight a 0.1 g/L Zn45Cu and different initial RhB con-centrations b 20 ppm RhB and differrent Zn45Cu concentrations

after 60 min without the light It again implies that all the

synthesized ZnnCu material has very low RhB adsorption

effectiveness To confirm the effect of adsorption process,

we evaluate the changes in surface functional group by FTIR

spectra (Fig. 10) and surface charge change by zeta potential

of the material before and after RhB degradation

Figure 10 shows that FTIR spectra of Zn45Cu present

a peak around 450 cm−1 and 750 cm−1, which are

gener-ally assigned to the stretching vibration of Zn–O and Cu–O

bonds (Andrade et al 2017); the broad peak at about 3500

cm−1 was assigned for the –OH group In addition, two small

peaks at around 2900 cm−1 and some peaks at around 1000

cm−1 show the C–H bonds (Kadam et al 2018) and the

peaks at 1400 cm−1 and 1500 cm−1 correspond to the

band-ing of C–H bonds (Manohar et al 2020) All these bands

were presented on the FTIR spectra of Zn45Cu after RhB

adsorption and degradation process, and the intensity of

peaks slightly decreased compared with the FTIR spectra of RhB Furthermore, the FTIR spectra of Zn45Cu after interac-tion with RhB only appeared a new peak at 1638 cm−1 It can

be seen that the 1649 cm−1 peak of RhB shifted to shorter wavenumber, while other characteristic peaks of RhB were not observed The results of FTIR spectra indicate that the RhB adsorption on the Zn45Cu surface was insignificant The results of zeta potential of Zn45Cu sample at pH 6 and 8 were found to be + 14.6 and + 16 mV, respectively Nevertheless, after interaction process with RhB at neutral media, the zeta potential of Zn45Cu sample was found to

be + 19 mV Since RhB is a cation dye, if the RhB adsorp-tion occurred on the surface of material, the zeta potential would increase significantly However, in our case, the zeta potential of Zn45Cu sample before and after degradation changed slightly It implies that the adsorption of RhB onto the surface of material was negligible In other words, the RhB removal from aqueous solution was mainly by photo-catalytic mechanism

The photocatalytic reaction mainly occurred due to the presence of the active species of electrons (e−), holes (h+),

superoxide radical anions ( O·−

2 ), hydroxiperoxyl radical

( HO·

2) , and hydroxyl radicals (·OH) (Kumaresan et al

2020) Among them, hydroxyl radicals (·OH) are most active (Anitha and Muthukumaran 2020; Lavín et al 2019) To study the photodegradation mechanism, AO, AgNO3, and BuOH were conducted during photoreaction respectively The result is shown in Fig. 11

The presence of AO and Ag+ in the reaction system (decomposition of 10 ppm RhB solution by 0.1 g/L Zn45Cu under the sunlight at room temperature) at the first 90 min leads to significantly increased RhB decomposition com-pared to the reaction system without them, and slightly increased from 100 to 180 min (compared to the reaction system without them) These results revealed that the loss

0

20

40

60

80

100

Time (min)

(a)

Run (1)

Run (2)

Run (3)

Fig 9 The reusability of Zn45Cu for RhB degradation

4000 3500 3000 2500 2000 1500 1000 500

Zn45Cu+RhB

Wavenumber (cm -1 )

RhB

Zn45Cu

Fig 10 FTIR spectra of RhB and Zn45Cu sample before and after

adsorption

0 20 40 60 80

100

Time (min)

(b)

Zn45Cu 0.1 g/L RhB 10 ppm

Zn 45 Cu 0.1 g/L RhB 10 ppm + AgNO 3

Zn45Cu 0.1 g/L RhB 10 ppm + AO

Zn45Cu 0.1 g/L RhB 10 ppm + BuOH

Fig 11 Effect of different scavenger on photodegradation process

of electron or hole or the present of Ag could accelerate

photodegradation process We all know that Ag absorbs

light at about 420 nm, and Ag has been also adsorpted on

the surface of material (ZnO–CuO); therefore, it accelerates

photodegradation process Only the presence of n-butanol

in the reaction system immediately significantly reduced the

decomposition efficiency of RhB at all times; after 180 min,

the efficiency was only about 35% We suggest that hydroxyl

radicals are mainly activated species involved for RhB

pho-tocatalytic activity of ZnO/CuO system (Zn45Cu) This result

was also confirmed by the influence of the reaction medium

on the photocatalytic efficiency The photocatalytic

degrada-tion of 20 ppm RhB by 0.1 g/L Zn10Cu was carried out at pH

3, 7, and 10 (Fig. 12) The 20 ppm RhB removal efficiency in

different media is very obvious, at 120 min is 30%, 53%, and

92% respectively for pH 3, 7, and 10 At pH 10, the effect

was almost maximum after only 90 min

According to Anitha et al and Lavín et al (Anitha and

Muthukumaran 2020; Lavín et al 2019), the photocatalysis

process takes place according to the following reactions:

(2) 2O2+ 2e−→ 2 O2⋅−

(3) 2H++ 2O2⋅−

→ 2HO⋅2

(4) 2HO⋅

2→ O2+ H2O2

(5)

H2O2+ 2e−→ OH⋅+ OH−

(6) 1∕2 O2+ H2O+ 2e → OH⋅+ OH−

(7)

h++ H2O → H++ OH⋅

(8)

OH⋅+ RhB → degradation products

In the acidic environment, it is favorable for the reactions from (2) to (6) and (8) and in the alkaline medium, it is favorable for the Reactions (7) and (8) And RhB could exist between two forms in acidic and alkaline media as shown in the previously published paper (Birtalan et al 2011).

At low pH, RhB exists in cationic form, the material sur-face is also positively charged, and the electrostatic repulsion makes it difficult for them to come close for a reaction occur And the presence of Ag+ (e− scavenger) did not reduce pho-tocatalytic efficiency; it means that Reactions (2), (5), and (6) which occur insignificantly lead to (8) reaction which is weak In alkaline medium, RhB exists in neutral form, and the electrostatic repulsion makes it easier for them to transfer

to the material surface for reaction to occur; additionally, the RhB molecular structure has bond angles below 90°, which are unstable and easy to decompose So, the reaction

in alkaline medium occurs more easily

The band gap energies (Eg) of ZnO and CuO are reported

to be about 3.23 and 1.4 eV, respectively, whereas the elec-tron affinity (χ) is 4.35 and 4.07 eV, respectively (Harish

et al 2017) During sunlight irradiation, electrons in the valence band (e−VB) of CuO and ZnO were excited (e* VB), and jump into the conduction band (CB), leaving holes in the

VB of CuO However, this energy is not enough for e*VB of

ZnO to pass the Eg = 3.23 eV to jump into the CB to gener-ate electrons and holes at the CB and the VB, but e* VB of CuO can induce it As the above discussion, when the Cu content in the ZnO/CuO system is very low, the Cu atom can penetrate into the ZnO crystal structure, causing structural deviation, giving up the VB overlap (VBO) between ZnO and CuO rather than bandwidth changes (Liu et al 2008)

Thus, it leads to increase in Eg of the ZnO/CuO system fall-ing below 1.4 eV Then, initial e* VB of ZnO migrated on the VBO, and easily moved to the CBO of the system The electrons and holes were generated in both the VB and CB

of CuO and ZnO At the same time, the overlap makes the holes and electron migrate from CuO to ZnO and vice versa that increases photocatalytic capacity of Zn45Cu These e*

VB react with dissolved oxygen molecules, and form super oxide radical anion (O2·−), which further indirectly turn into highly reactive hydroxide radicals (OH·) Moreover, the holes in the valence band of CuO which can react with

OH− ion form highly reactive hydroxyl radicals Hydroxide radicals react strongly with oxidants, and generate either photogenerated electrons or holes which finally oxidize the RhB molecules, or hydroxide radicals oxidize directly with RhB ZnO/CuO system may be a favorable p–n junction which helps the separation of generated electron-hole pairs under visible light irradiation (Harish et al 2017)

Based on the above detailed discussion, we can suggest the mechanism of the photocatalytic process of RhB by

Zn45Cu 450°C under the sunlight follows the reactions:

0 20 40 60 80 100 120 140 160 180

0

20

40

60

80

100

Time (min)

Zn10Cu 0.1 g/L, RhB 20 ppm

pH 3

pH 7

pH 10

Fig 12 Effect of pH on photocatalytic degradation of RhB