Enhanced photocatalytic degradation of congo red by solvothermally synthesized CuInSe₂–ZnO nanocomposites

Abstract

Efficient photocatalysts of CuInSe₂–ZnO nanocomposites were prepared via a solvothermal method using a mixed solvent of ethylenediamine and ethanol (volumetric ratio of 1 : 1). The products were characterized by SEM, EDX, XRD, PL, BET surface area, PZC and DRS UV-vis techniques and used for photo-degradation of congo red. The characterization results showed that crystallite size, BET surface area and optical absorption of the samples varied significantly with the addition of CuInSe₂ to ZnO. The nanocomposites show absorption edges in the ultraviolet and visible regions depending on their CuInSe₂ content. The optical band gap values of these nanocomposites were calculated to be about 3.37–2.1 eV, which show a red shift from that of pure ZnO. These red shifts indicate the incorporation of CuInSe₂ in the zinc oxide lattice. In the investigation of the photocatalytic activity of the samples, the effects of the experimental parameters including pH, congo red concentration, CuInSe₂ content and irradiation sources of UV and visible light have also been studied. Addition of CuInSe₂ was effective in improving the photocatalytic activity remarkably. The highest percentage removals of 99.8% and 80.3% are observed for the photocatalyst containing 10 wt% CuInSe₂ after 90 and 120 min under UV and visible irradiation, respectively. Also, a possible removal mechanism of the samples is proposed. It could be considered as a promising photocatalyst for dye degradation.

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Introduction

Currently, there is growing interest in the development of new techniques in the field of environmental protection due to the global pollution problems.¹ Industrial wastes have caused too much pollution for the environment and further cause health problems for human beings.^2,3 Approximately 10–15% of total world production of dye is lost during the production process and is released in the textile effluent.⁴ The usage of synthetic dyes such as azo-dyes especially in the textile industries and the discharge of the industrial wastes containing these compounds into the aquatic systems have continued to increase. It is known that azo dyes are highly toxic and even carcinogenic to animals and human beings and they are not readily degradable and could create dangerous byproducts through oxidation, hydrolysis, or other chemical reactions occurring in the wastewater stream.⁵ Congo red (CR) is a typical and the first anionic synthetic dye which has two azo (–N=N–) chromophores. Like other azo dyes, this compound is very stable because of its complex aromatic structure, hence it is not easily degradable.⁶

Several traditional physical and chemical methods have been used for the removal of dye pollutants. However, these methods only succeed in transferring organic pollutants from water to other phases. Moreover, secondary pollution could entail additional and costly solid-waste treatment.^1,7

In the two last decades, heterogeneous photocatalysis has received much attention as a promising advanced oxidation process for its capability to completely mineralize recalcitrant contaminants in water or air, which cannot be effectively removed by conventional methods.^5–8

Many studies have been devoted to the synthesis of metal oxide semiconductors with special properties that have potential applications in electronics, optics, thermoelectronics, photoelectronics and photocatalysis. It is interesting to note that various properties could be observed depending on their metal ions, morphology, size and structure.^9,10

ZnO-based semiconductors with a wide band gap (3.37 eV) have been identified as active photocatalysts for organic pollutants in gaseous or aqueous phases due to their non-toxicity, photochemical stability, low price, abundance in nature and being environmentally friendly.^11–13 In comparison with TiO₂, ZnO is a better alternative because of the numerous point defects mainly from oxygen vacancies, higher production of hydroxyl ions and higher photoactivity (by a factor of 2–3) in both UV and sunlight irradiation for the decontamination of water.⁴ Also, ZnO is an interesting example of materials having the capability of low temperature growth with many different kinds of morphologies including wires, rods, tubes, particles and flower shape at nano scale.^14–18 Among many kinds of morphologies for ZnO, nanoparticles provide better characteristics due to larger surface area and the ability to be suspended in a solution.⁴

It is desirable that photocatalysts such as ZnO absorb both UV and visible lights, especially since only a small fraction of the solar spectrum (5%) is UV while visible light accounts for 45% of the solar radiation energy.¹² Thus, various modifications are of considerable interest to enhance the photocatalytic efficiency of metal oxides.^19–22

Chalcopyrite ternary copper indium diselenide (CuInSe₂) with low band gap (∼1.00 eV),²³ which is regarded as a promising material specially for photovoltaic applications due to its high absorption coefficient, optimal band gap energy, good radiation stability and low toxicity,²⁴ has attracted much attention over the last two decades.²⁵ To the best of our knowledge there have been few reports about photocatalytic activity of ternary semiconductors such as CuInSe₂.²⁶

In this study, photo-degradation activity of CuInSe₂–ZnO nanocomposites, containing various percentages of CuInSe₂ synthesized via solvothermal method using three solvents of ethylenediamine, diethylamine and a mixed solvent of ethylenediamine and ethanol (volumetric ratio of 1 : 1), in presence of CR is investigated. The photocatalytic activity of the samples significantly changes with their CuInSe₂ content. Furthermore, influence of different factors such as pH and congo red concentration and irradiation source has also been studied.

Experimental

Reagents and chemicals

Analytical grade of copper(II) chloride dehydrate (CuCl₂·2H₂O, Merck), indium(III) chloride tetrahydrate (InCl₃·4(H₂O), Aldrich), selenium powder (Se, Merck), zinc chloride dehydrate (ZnCl₂·2H₂O, Merck), ethylenediamine (C₂H₄(NH₂)₂, Merck), anhydrous diethylamine (C₄H₁₁N, Merck) and absolute ethanol were purchased and used without further purification. CR (purity 99%, Aldrich) has been used as received.

Synthesis of materials

Synthesis of CuInSe₂. CuInSe₂, was synthesized via a solvothermal method using three solvents of ethylenediamine, diethylamine and a mixed solvent of ethylenediamine and ethanol (volumetric ratio of 1 : 1).

CuInSe₂ obtained using ethylenediamine solvent was synthesized according to the procedure reported in the literature.¹ Also, synthesis of CuInSe₂ using diethylamine solvent was done in a similar procedure, the only difference being that the autoclave was maintained at 180 °C oven for 48 h. While for synthesis of CuInSe₂ with a mixed solvent of ethylenediamine and ethanol (volumetric ratio of 1 : 1) another procedure reported in the literature³ was used in which capacity of autoclave was 50 mL instead of 25 mL.

Synthesis of ZnO. In a typical experiment, 350 mg of zinc chloride dihydrate, ethylenediamine and absolute alcohol (1 : 1) were stirred in a teflon lined stainless steel autoclave of 50 mL capacity for 30 min. After sealing, the autoclave was heated at 200 °C for 24 h and then cooled to room temperature. The white precipitate was collected by centrifugation and then washed several times with double distilled water and ethanol, and then dried in an oven at 60 °C under vacuum for 6 h.

Synthesis of CuInSe₂–ZnO samples. In a typical procedure, enough amounts of stoichiometric mixtures of copper(II) chloride dihydrate, Indium(III) chloride tetrahydrate and selenium powder to achieve the following percentages of CuInSe₂ (3, 5, 10, 25 and 50 wt%, respectively) were first dissolved in 8 mL ethylenediamine and absolute alcohol (1 : 1) under vigorous stirring at room temperature (RT) for 10 minutes. An appropriate amount of zinc chloride dihydrate was separately stirred with 25 mL of the same mixed solvent. Both mixtures were taken in a stainless steel autoclave (250 mL), heated slowly to 185 °C and maintained at that temperature for 36 h. The light/dark gray precipitate thus formed was separated from excess solvent by centrifugation and washed thoroughly with double distilled water and ethanol and dried at 60 °C under vacuum for 6 h. The samples were denoted as ZC-x, where “x” stands for percentage of CuInSe₂.

Characterization

The size and morphology of the CuInSe₂–ZnO nanocomposites were observed by scanning electron microscopy (SEM) at an acceleration voltage of 15.0 kV by a Holland Philips XL30 microscope instrument. Energy Dispersed X-ray analysis (EDX) was performed by the same microscope to investigate the elemental composition of the samples. The structure and phase purity of the samples were identified by wide-angle powder X-ray diffraction (XRD) on Holland Philips X'pert diffractometer X-pert using Cu Kα radiation (λ = 1.5418 Å). Specific surface area (SSA) of the nanocomposites was determined by nitrogen adsorption using CHEM-BET 3000 instrument using BET equation where the samples were degassed for 30 min at 300 °C at 1 atm. The photoluminescence (PL) spectra were measured using Perkin Elmer LS-5 luminescence spectrometer with a wavelength of 254 nm as the excitation source at room temperature. UV-vis diffuse reflectance spectra were recorded on an Avantes, reflection probes fcr-7uv400 using BaSO₄ as a reference. Surface charge densities – point of zero charge (PZC) – of the sample were measured using Zeta Pals Brookhaven U.S.A instrument.

Evaluation of photocatalytic activity

The photocatalytic activity of CuInSe₂–ZnO nanocomposites was evaluated via a probe reaction on the degradation of CR in aqueous medium under UV and/or visible light irradiation.

The photocatalysis of CR was carried out in a cylindrical quartz UV-reactor with an effective volume of 100 mL. The UV and visible illumination was provided by a 30 W lamp (UV-C, λ = 253.7 nm, photon provides 4.89 eV, manufactured by Philips, Holland) and/or a 500 W lamp (high-pressure mercury-vapor lamp, 400 W and λ = 546.8 nm, Yaming Company, Shanghai), cooled by water flow and pH was adjusted using dilute hydrochloric acid or sodium hydroxide. 50 mL of CR dye solution was implemented to the photoreactor, and 25 mg of CuInSe₂–ZnO photocatalyst was also added to the reactor. An air diffuser (air pump, flow: 4.5 L min⁻¹) was placed at the bottom of the reactor to uniformly disperse air into the solution while the mixture was stirred. The suspension was sonicated for 5 min and then stirred in the dark for 30–45 min (depending on the type of samples and their darkness times found based on the absorption experiments), to ensure an adsorption/desorption equilibrium on the semiconductor surface prior to irradiation. Perpendicular UV and/or visible irradiation with a distance of about 15 cm between the light source and the reaction mixture were applied. Samples for analyses were taken from the reaction suspension at specified reaction times and immediately centrifuged at 6000 rpm for 10 min to remove the particles and were further analyzed by monitoring the absorbance at 498 nm using UV-Vis spectrophotometer (Shimadzu UV 2100). The concentration of dye in each degraded sample was determined at λ_max = 498 nm, using a calibration curve. The percentage removal of CR is calculated as follows:

%Removal = (C_i − C_t)/C_i × 100 (1)

where, C_i is the initial concentration of dye before exposure to light source and C_t is the concentration of dye at any specified time (min).

The chemical oxygen demand (COD) test is widely used as an effective technique to measure the organic strength of wastewater. This test allows the measurement of waste in terms of the total quantity of oxygen required for the oxidation of organic matter to CO₂ and water. The open reflux method was applied for COD determination.²⁷

The photocatalyst stability tests were performed in the same way as the photocatalytic activity tests but they were repeated four times.

Results and discussion

Characterization of CuInSe₂–ZnO nanocomposites

Elemental analyses of the CuInSe₂–ZnO samples determined by EDX are presented in Fig. S1 and Table S1† which are in good agreement with the nominal composition of the samples. The BET specific surface area (SSA) of the samples containing various proportions of CuInSe₂ is presented in Table 1. Generally, the surface area of the samples increases upon adding more CuInSe₂ to zinc oxide. A sharp increase in SSA is observed for the sample containing 10 wt% CuInSe₂. Among the obtained samples the ZC-50 shows the highest SSA of 37 m² g⁻¹.

Table 1 BET surface areas, crystallites sizes estimated from Scherrer's equation, crystallinity and band gap of the samples

Samples	dXRD	Crystallinity (%)	BET area (m^2 g^−1)	dBET	Band gap (eV)
CISe	5.7	90.3	1.3	7.3	—
Z	40	92.9	16.3	45.4	3.37
ZC-3	—	—	23	46.4	3.2
ZC-5	37.3	85.64	25.3	42.2	3.1
ZC-10	30.8	88.97	31.5	33.8	2.6
ZC-25	25.6	81.58	34.5	30.8	2.4
ZC-50	23.2	80.12	37.1	28.5	2.1

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The PL spectra of pure ZnO and ZC-10, containing 10 wt% CuInSe₂, samples are shown in Fig. 1. The PL spectrum of pure ZnO consists of a broad band centered at around 410 nm and a sharp band centered at around 500 nm with a shoulder at 489 nm. The UV emission resulted from the recombination of free excitons and the green emission is related to the recombination of a photo-generated hole with a singly ionized charge state of specific defect specially oxygen vacancies.^28,29 The PL spectrum of ZC-10 is significantly quenched in the UV range 338–437 nm, indicating a lower rate of recombination between the photo-generated hole and electrons on the surface of the pure ZnO.³⁰ It should be noted that the green emission for ZC-10 sample is more intense than that for the pure sample.^31,32 We suppose that an increase in emission intensity is an indication of an increase in concentration of the oxygen vacancies.³³

Fig. 1 PL spectra of samples: (a) Z and (b) ZC-10 samples.

Fig. S2† shows that pure CuInSe₂ can be obtained only using the mixed solvent while by-products remain with CuInSe₂, when using the two other solvents. Some impurities such as In₂Se₃ were detected in the XRD patterns. According to the literature,³⁴ amine can activate selenium as Se²⁻ by a nucleophilic attack in the solvothermal process together with In₂Se₃ precipitate as follows:

2InCl₃ + 3Se²⁻ → In₂Se₃ + 6Cl⁻ (2)

These results show that solvents play a major role in the formation of the chalcopyrite CuInSe₂. Only solvents with suitable reducing or coordination ability such as alkylamines can reduce Cu²⁺ ions and form activated selenium.

As mentioned in the literature³⁵ mixed solvent of ethylenediamine–ethanol appeared to be the optimal reaction medium to build up CuInSe₂ samples. In₂Se₃ intermediate has low solubility in ethylenediamine. In presence of mixed solvent with reduced polarity, the solubility of In₂Se₃ increases. As a result, the site of nucleation becomes bigger and fast individual nuclei growth suppresses the anisotropic growth of the particles, leading to the formation of CuInSe₂. Furthermore, substitution of toxic ethylenediamine to ethanol leads to a decrease in toxicity of the reaction medium.

As presented in Fig. 2, for these samples containing 0–5 wt% CuInSe₂, Bragg diffraction peaks in the range of 2θ = 20–80°, show the typical patterns of a wurtzite structure of ZnO which is in good agreement with the reported data by JCPDS card no.43-1012, with lattice parameters a = b = 3.2 and c = 5.2 Å. The strongest peak for hexagonal was observed at 2θ = 33.2°. No peak in the XRD patterns of the samples, with 0–5 wt% CuInSe₂, can be assigned to any known phase of CuInSe₂, probably because of law content of CuInSe₂ and/or small crystallite sizes not detected in XRD patterns. For other samples containing 10–50 wt% CuInSe₂ both of the ternary compound with tetragonal structure CuInSe₂ and wurtzite ZnO structure are observed. In the case of ZC-10 sample, the peaks of CuInSe₂ are weak as compared to those of ZnO. However, in ZC-25 and ZC-50 samples, the peaks of CuInSe₂ are strong as compared to the ZC-10 peaks.

Fig. 2 XRD patterns of (0–50 wt%) CuInSe₂–ZnO samples in the mixed solvent.

The crystallinity of the samples, determined by ratio of crystalline area to total area of each XRD pattern. The crystallinity tends to decrease for up to 5 wt% CuInSe₂, while an increasing in the sample with 10 wt% CuInSe₂, and decreasing again for samples of above 10 wt% CuInSe_2.

Morphologies of CuInSe₂–ZnO samples are recorded using scanning electron microscopy shown in Fig. 3a–f for Z, ZC-3, ZC-5, ZC-10, ZC-25 and ZC-50. For pure ZnO the morphology is nanoparticle. Morphology difference between samples is insignificant. The addition of CuInSe₂ decreases the particle size so the only difference is particle size of samples. These observations confirm that CuInSe₂ may control the particle growth and thus influence particle size. These results are also in accordance with the results of BET.

Fig. 3 SEM images of (a) Z, (b) ZC-3, (c) ZC-5, (d) ZC-10, (e) ZC-25 and (f) ZC-50 samples.

UV-vis diffuse reflectance spectra of Z and ZC-10 samples are shown in Fig. 4. The optical band gaps of samples are estimated by extrapolating the linear portion of the square of absorption coefficient against photon energy using the equation:

(αhν)² = B(hν − E_g) (3)

where B is a constant. Fig. 4 shows the (αhν)² versus photon energy (hν) plots of the above samples. Pure ZnO presents strong absorption in the short-UV region at around 364 nm which has an optical band gap of at around 3.37 eV, which are in good agreement with the value reported in previous works.^36,37

Fig. 4 (a) Optical absorption spectra of Z and ZC-10 samples and (b) (αhν)² versus photon energy plots of the mentioned samples.

This is due to the absorption of light caused by the excitation of electrons from the valence band to the conduction band of zinc oxide.³⁷ The optical band gap of samples also decreases with an increase their CuInSe₂ content. This substantiates the dramatic effect of CuInSe₂ on decreasing the band gap (Table 1). Thus the DRS UV-vis spectra confirmed the framework incorporation of CuInSe₂ in ZnO lattice. In other words, addition of CuInSe₂ can form new states level into the ZnO. The decrement of the band gap is at about 1.3 eV from 0 to 50 wt% CuInSe₂, which could narrow down the band gap of ZnO by impurity energy, much smaller than that of pure ZnO.

Photocatalytic activity

Effect of pH on removal of dye. Fig. 5a presents the removal efficiency related to different pHs (5, 6, 8 and 10) which is estimated at 30 ppm concentration of CR under UV irradiation for 2 h at room temperature for Z and ZC-10 samples, respectively. For Z sample the removal efficiency percentage of CR was enhanced by increasing pH up to 8 and the reverse performance was observed at about pH of 10. While the removal efficiency percentage of CR was enhanced by decreasing pH in acidic solution of ZC-10 sample, increasing pH of solutions up to 8 caused the reverse performance and no adsorption was observed at pH 10. In presence of CuInSe₂, the maximum removal efficiency of CR dye occurs in pH 5.

Fig. 5 Removal efficiency percentage versus pH for (a) Z and (b) ZC-10 samples, condition: 0.025 g photocatalyst, 20 mL of CR dye with 30 ppm concentration, irradiation time 2 h at room temperature, (b) the surface charge versus pH for ZC-10 sample.

Therefore, all further solutions with various wt% of CuInSe₂ were adjusted at optimum pH of 5 to improve adsorption.

The surface charge data plotted in the form of δ₀–pH curves for ZC-10 sample are presented in Fig. 5b. In the pH limit 5–10, there is a linear decline in surface charge with pH which is then followed by a sharp decrease at about pH 10. Also, the surface charge densities, point of zero charge (PZC), for pure ZnO are at about pH 8.9 (ref. 38) which decrease with an increase in CuInSe₂ content of sample. The PZC values determined from the δ₀–pH curves are 5.5 for ZC-10 sample.

On the other hand, addition of CuInSe₂ resulted in a more acidic ZnO surface.

With regard to PZC of photocatalyst surface and the nature of dye (cationic, anionic or neutral), solution pH is a significant effect on adsorption efficiency dyes on adsorbent. In pH < pzc, the adsorbent surface is positively charged and due to the anionic nature of CR, electrostatic attractions result in dye adsorption and improvement of removal efficiency of CR, while in pH > pzc, the surface is negatively charged³⁹ and following enhancement in electrostatic repulsion, removal efficiency of CR decreases. Another explanation regarding the behavior of acidic pH is the existence of hydrogen bonds between photocatalyst's hydroxyl groups and sulfonate or amine sites of dye molecules. As pH of the CR solution increased, an extreme decrease in adsorption took place. Generally at higher pHs, the anion OH⁻ can compete with anionic sites of CR dye which is adsorbed onto positive charges of photocatalyst. This leads to blocking of activated sites.⁴⁰

Effect of the initial dye concentration. The influence of initial concentration of the dye solution on the photocatalytic degradation is a significant aspect of the study. The initial concentrations of CR are selected in the range of 5–50 ppm for Z and ZC-10 samples. As shown in Table 2, the experiments related to blank (without catalyst in presence of light) showed no change in the concentration of CR up to 30 ppm. Also, for 40 and 50 ppm of CR we observed negligible change (0.05% change in comparison to initial concentrations). For Z sample, the percentage removal of dye decreases as initial concentration of dye solution increases. While in the case of ZC-10 sample, by increasing dye concentration up to 20 ppm only adsorption takes place. The maximum percentage removal of dye is observed in presence of 30 ppm of dye. The percentage removal starts to decrease in presence of 40 and 50 ppm of dye.

Table 2 Photocatalytic properties of Z and ZC-10 samples in presence of various concentrations of CR

[CR] (ppm)	Removal%	Tdark (min)		Tremoval (min)		Removal%
	(No catalyst)	Z	ZC-10	Z	ZC-10	Z	ZC-10
5	0	20	10	120	—	99.7	99.9 (Ads.)
10	0	20	20	120	—	89.4	99.9 (Ads.)
20	0	25	30	180	—	79.8	99.9 (Ads.)
30	0	30	30	180	90	43.7	99.8
40	0.05	30	30	180	120	40.3	70.2
50	0.05	30	30	180	120	37.3	63.9

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One possible explanation of such circumstances is that as initial concentration increases, more and more organic substances are adsorbed on the surface of sample. Scarcity of active sites in the system causes little adsorption of hydroxyl ions which in turn leads to a decrease in generation of hydroxyl radicals. Further, as the concentration of dye increases, the photons get intercepted before they can reach the catalyst surface, thus the photon adsorption by the catalyst decreases. Consequently, percentage removal is reduced.¹

Therefore, the photocatalytic tests for all samples containing various wt% of CuInSe₂ were carried out in presence of 30 ppm of CR.

Effect of CuInSe₂ content on CuInSe₂–ZnO nanocomposite as photocatalyst. Some absorption spectra of a solution of 30 ppm CR in the presence of photocatalysts, i.e. Z, ZC-10, ZC-25 and ZC-50, under UV light irradiation are presented in Fig. S3a–d.† For Z sample, the maximum percentage removal of CR is about 44% under UV light irradiation for 180 min, so there is a small change in the CR concentration in the solution with the nanoparticle ZnO sample. Introduction of 3 to 10 wt% CuInSe₂ enhances percentage removal of CR in the range of 83.7–99.8%. However, percentage removal decreases steadily afterwards (Fig. S3 and Table S2†). Interestingly, ZC-10 sample has the highest removal efficiency among other samples after irradiation for 90 min. Durations at which the maximum removal occurs vary with the CuInSe₂ content. Also, the experimental results show that the percentage removal of CR using CuInSe₂–ZnO photocatalysts is higher than that for pure zinc oxide “Z”. The similar behavior was reported for ZnO microspheres via hybridization with CuInSe₂ and CuInS₂ in presence of rhodamine B.²⁶ On the other hand, the percentage removal of CR for synthetic ZnO is higher than that for commercial ZnO (Table S2†).

The photocatalytic efficiency of ZC-10, ZC-25 and ZC-50 nanocomposites in presence of 30 ppm CR under visible light irradiation for 2 h is investigated. As shown in Table S2,† ZC-10 sample has the best efficiency among the two other samples at about 80.3% for 2 h. As mentioned above, the highest percentage removals of 99.8% and 80.3% are observed for the photocatalyst containing 10 wt% CuInSe₂ after 90 under UV and 120 min under visible irradiations, respectively.

The photocatalytic performance could be attributed to the high surface area, the value of band gap and the difference in the rate of recombination, the crystallinity of a photocatalyst, the morphology, low degree of agglomeration, the type of polymorph and lattice structure, defects specially oxygen vacancies and adsorptive affinity.^41,42 With regards to the PL, BET and XRD results, on one hand, the greater concentration of oxygen vacancies cause a greater proper value for SSA and on the other hand, the presence of two different phases (ZnO and some new weak reflections of CuInSe₂ in its structure) with a proper band gap leads to the lower recombination rate. As a result ZC-10 shows the highest activity compared to other samples.

Fig. 6a presents the COD values for the best photocatalyst sample of “ZC-10”. As shown in the figure, in each case, the COD value of the initial color solution initially increases due to cleavage of rings in dye molecule and then significantly decreases after 72 h, indicating the high potential of the CuInSe₂–ZnO photodegradation process for the removal of CR from wastewater. The photo-degradation efficiency is found to be 70.6%. This result confirms that ZC-10 sample is a good candidate for photo-degradation of CR in wastewater. Also, ZC-10 photocatalyst is very stable during four repeated experiments (Fig. 6b). The XRD pattern of ZC-10 photocatalyst before and after repeating the reaction four times is shown in Fig. 6c which clearly indicates that the structure remained the same and no adsorption was observed.

Fig. 6 (a) Change of COD removal efficiency in process of ZC-10 photocatalyst. (b) Cycling run in the removal of CR in presence of ZC-10 under UV light irradiation and (c) XRD pattern of ZC-10 a) before and b) after photocatalytic test.

Possible removal mechanism. The reaction proceeds via bulk processes, such as photoexcitation, charge separation, migration, and recombination, and also via surface redox reactions.^43,44 In a photocatalytic process, adsorbed photons with a proper energy, lead to create the hole–electron pairs in valence band (VB) and conduction band (CB), respectively. The positive hole could either oxidize adsorbed organic contaminants directly or produce very reactive hydroxyl (OH˙). The electron in the CB reduces the adsorbed oxygen on the photocatalyst.^14,45 As shown above, CuInSe₂–ZnO samples are more photoactive than pure zinc oxide for CR removal.

ZnO is an n-type semiconductor while CuInSe₂ is a p-type one. Hence Fermi energy level is distinct of each other. For ZnO, Fermi energy level is located to the conduction band, while Fermi energy levels of CuInSe₂ lie close to valence band. When ZnO and CuInSe₂ are in contact with each other, their Fermi energy levels reach equilibration. Fermi energy levels of ZnO and CuInSe₂ tend to shift downward and upward, respectively. The resulting photo-generated electrons transfer from the conduction band of CuInSe₂ to the conduction band of ZnO, whereas the photo-generated holes transfer in the opposite direction along with valence bands.²⁶ CuInSe₂–ZnO nanocomposite prevents from the recombination of the electron–hole pairs, thus resulting in more holes can participate in the photooxidation process.

Conclusions

In summary, we synthesized UV and visible light sensitive photocatalysts of CuInSe₂–ZnO nanocomposites with various proportions of CuInSe₂ by a simple solvothermal method using a mixed solvent of ethylenediamine and ethanol (volumetric ratio of 1 : 1). The crystallite size, BET surface area and optical absorption of the nanostructures are strongly affected by addition of CuInSe₂ to ZnO. The addition of CuInSe₂ to ZnO semiconductor gives lower band gap values with a red shift for nanocomposites compared to pure ZnO. When treated with 30 ppm of congo red, 10 wt% CuInSe₂–ZnO nanocomposite exhibits the highest photo-degradation due to proper values of SSA and band gap that cause greater concentration of oxygen vacancies and a lower recombination rate, which is promising for applications in water detoxification.

Acknowledgements

Financial support of this work by Tarbiat Modares University is gratefully acknowledged.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra01735f

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