Synthesis and characterization of a CuS–WO₃ composite photocatalyst for enhanced visible light photocatalytic activity

Abstract

WO₃ nanorods and flower-like CuS were synthesized by a hydrothermal process. The visible light driven CuS–WO₃ photocatalyst was prepared by adding different weight ratios (10–40%) of CuS on WO₃ by a wet impregnation method. The synthesized photocatalysts were characterized by powder X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, Field emission scanning electron microscopy (FE-SEM), High-resolution transmission electron microscopy (HR-TEM), energy dispersive X-ray spectroscopy (EDAX) and UV-vis diffuse reflectance (DRS) spectroscopy. The photocatalytic performance of synthesized photocatalysts was evaluated for the photodegradation of methylene blue (MB) under visible light irradiation. The 10% CuS–WO₃ photocatalyst showed higher photocatalytic degradation activity than others which could be due to the increased absorption of light in the visible region and also a lower recombination of charge carriers. Further, the photoelectrochemical measurements carried out for 10% CuS–WO₃ revealed the faster migration of photo-induced charge-carriers. A possible reaction mechanism for the enhancement of photocatalytic activity of CuS–WO₃ has been proposed.

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Introduction

The development of highly ordered architectures of semiconductor photocatalysts has received great attention due to their natural growth with unique morphology and their potential applications for energy and environmental issues.^1,2 The pollution of the environment by industrial and agricultural sectors is increasing. The textiles, cosmetics and food industries use organic dyes which are the primary sources for contamination of the environment due to their toxicity and the non-biodegradability of the organic dyes.³ The photocatalytic degradation is an effective way for the removal of organic pollutants from wastewaters. Therefore, it is necessary to develop highly effective, visible light active catalytic materials to solve most of the critical problems related to energy and environment. Over the past few decades, numerous active semiconductor photocatalysts such as metal oxides,^4,5 sulfides,^6,7 oxynitrides⁸ and graphitic carbon nitride⁹ have been synthesized and utilized for the treatment of organic pollutants in aqueous environment. Among these materials, tungsten oxide (WO₃) is a well-known and an important semiconductor photocatalyst because of its wider response in the electromagnetic spectrum of solar radiation, mechanical strength and good metal support interactions.^1,10 Because of these unique properties, WO₃ is recognized as not only a visible light active photocatalyst but also extensively investigated for potential applications in photochromic, electrochromic devices and secondary batteries.¹¹ The photocatalytic activity of WO₃ is still limited due to the fast recombination of charge carriers and the conduction band electrons can not be efficiently trapped by O₂ to yield superoxide radicals.¹⁰ Researchers have modified the band energy positions of WO₃ by the addition of low bandgap semiconductor catalysts to reduce the recombination of charge carriers and thereby improving its photocatalytic activity.^12–16 The difference in conduction band energy between the two semiconductors reduces the charge recombination and extends its absorption range in the visible region. Further, it leads to a higher yield of photo-generated electrons in the conduction band that contribute to achieve a higher photocatalytic degradation.¹⁷

To our knowledge, there have been no reports on the photocatalytic degradation of organic pollutants using CuS–WO₃. In this investigation, CuS is chosen to dope WO₃ because of its interesting optical, electrical properties and also it is available at low cost. CuS exhibits low reflectance in visible and relatively high reflectance in the near-infrared region, which makes it a prime material for solar energy absorption.¹⁸

The morphological features of the photocatalyst such as nanorods, nanowires and flower-like have been preferred to improve the photocatalytic activity. In order to control the morphology of CuS and WO₃ photocatalysts, hydrothermal route was employed as well the organic additives like cetyl trimethylammonium bromide (CTAB) was used as a surfactant.

In this work, WO₃ nanorods and flower-like CuS were successfully synthesized by hydrothermal route and different weight percentage CuS-doped WO₃ photocatalysts were prepared via a simple wet impregnation method. The visible light photocatalytic performance of the synthesized photocatalysts was evaluated by the photodegradation of methylene blue (MB). A possible reaction mechanism for the photodegradation of MB over CuS–WO₃ has been proposed.

Experimental section

Materials

Copper(II) nitrate trihydrate, thiourea, CTAB, tungstic acid, hydrogen peroxide (30% w/v of H₂O₂), Triton X-100 and sodium sulphate were purchased from SDFCL, India. Nitric acid and deionised water were obtained from Spectrum reagents and chemicals Pvt. Ltd. MB was obtained from Sigma-Aldrich. All reagents were used without further purification.

Synthesis of photocatalysts

Synthesis of flower-like CuS. In a typical synthesis, copper nitrate trihydrate (1 g), thiourea (0.945 g) and CTAB (0.452 g) were dissolved in 30 mL of distilled water. Then, the solutions were mixed homogeneously under constant stirring and then transferred into a Teflon-lined stainless-steel autoclave with a volume capacity of 150 mL. The content was finally diluted with water to a volume of 50% of autoclave capacity. The hydrothermal reaction was carried out in hot air oven at 130 °C for 12 h and then allowed to cool to room temperature naturally. The obtained black precipitate was collected, washed with deionized water and ethanol several times to remove impurities. Finally, the prepared sample was dried in hot air oven at 60 °C for 6 h.

Synthesis of WO₃ nanorods. Tungstic acid (2 g) and CTAB (0.895 g) were dissolved in appropriate amount of distilled water. Then, the solution was transferred into a autoclave. The hydrothermal reaction was carried out in hot air oven at 120 °C for 12 h and then allowed to cool to room temperature. The obtained yellow color precipitate was collected, washed with deionized water and ethanol several times to remove impurities, and dried in hot air oven at 60 °C for 6 h.

The different weight percentage of CuS added WO₃ were prepared as follows: 10, 15, 20, 30 and 40% (0.05, 0.1, 0.15, 0.2, 0.3 and 0.4 g) weight ratios of CuS were added separately to 0.5 g of WO₃ and dissolved in 15 mL of ethanol. The resulting solution was continuously stirred and subsequently heated to evaporate the solvent at 60 °C. Then, the obtained CuS doped WO₃ photocatalysts were dried in hot air oven for about 5 h.

Characterization of photocatalysts

X-ray diffraction (XRD) patterns of the synthesized photocatalysts were recorded using a X-ray diffractometer (Mini Flex II, Japan) with Cu Kα radiation (λ = 0.154 nm) at a scan speed of 3°/min. The phase purity was ascertained using X-ray diffraction. The Fourier transform infrared (FT-IR) spectra were recorded to study the interaction among CuS and WO₃ with a wavenumber ranging from 4000–400 cm⁻¹ using an JASCO 460 plus FT-IR instrument. The morphology of the synthesized photocatalysts were examined by field emission scanning electron microscopy (FE-SEM) with an FEI QUANTA-200 system at an accelerating voltage of 30 kV. The distribution of particles was examined using high-resolution transmission electron microscopy (HR-TEM) (JEOL-JEM 1011, Japan). Elemental analysis was examined by using EDAX-Bruker Nano GmbH, X Flash Detector, model 5010. UV-vis diffuse reflectance spectra (DRS) were recorded using a Shimadzu 2100 spectrophotometer in the range of 200–800 nm.

Photocatalytic degradation studies

The photocatalytic degradation of MB was studied under visible light irradiation to evaluate the photocatalytic efficiency of synthesized photocatalysts. Visible light irradiation was provided by a 100 W tungsten-halogen lamp. The distance between the sample and lamp was 20 cm, and the wavelength range of light source was ∼380–800 nm. Typically, a known amount of photocatalyst was added to a 75 mL of dye solution. Prior to light irradiation, the suspension was magnetically stirred in the dark to establish adsorption–desorption equilibrium of MB onto the photocatalyst. During irradiation, 4 mL of aliquots were collected at regular intervals and the photocatalyst was removed by filtration through a 0.45 μm membrane filter (Pall Corporation). The concentration of MB at different time intervals were determined at its characteristic absorption wavelength, viz., 664 nm using a Shimadzu UV-2450 UV-vis spectrophotometer.

Photoelectrochemical measurements

The photoelectrochemical properties were investigated with a CHI608E electrochemical workstation in a conventional three electrode system with a Pt-wire as a counter electrode, Ag/AgCl (in saturated KCl) as a reference electrode and synthesized catalyst coated on FTO conducting glass as a working electrode and the electrolyte was 0.1 M Na₂SO₄ solution. In a typical working electrode preparation, 5 mg of the catalyst was ground with 10 μL of Triton X-100 and 20 μL of deionised water to make slurry. The slurry was coated with conducting side of FTO glass surface by doctor blade method. The active area of the electrode was 0.5 × 0.5 cm² and for a uniform coating scotch tape was used as a spacer. Then, it was dried in hot air oven at 100 °C for about 5 h.

Results and discussion

XRD studies

The XRD patterns of the synthesized CuS, WO₃ and CuS–WO₃ photocatalysts are shown in Fig. 1. The diffraction peaks of the synthesized CuS are indexed to the data of the hexagonal phase of CuS (JCPDS no. 06-0464) and are in good agreement with the previous report by Cheng et al.¹⁹ The XRD pattern of pure WO₃ displayed monoclinic crystal structure corresponding to the (002), (020), (200), (120), (112), (022), (202), (222), (004), (400) and (420) diffraction at 2θ = 23.1°, 23.7°, 24.4°, 26.7°, 28.8°, 33.7°, 34.2°, 41.9°, 47.5°, 49.9° and 55.4° (JCPDS no. 43-1035). The XRD pattern of 10–40% weight ratio of CuS–WO₃ are shown in Fig. 1(c)–(f), which demonstrates that the XRD pattern of WO₃ is not changed due to the presence of CuS. Further, when the amount of CuS is increased to 20%, the intensity of the peaks are slightly decreased but it is interesting to note that the CuS peaks do not appear because it is present in below detectable limit of XRD.²⁰ Also, It can be observed that a peak corresponding to CuS appears with an increase in CuS content, which confirms the formation of CuS–WO₃ composite material.

Fig. 1 XRD pattern of CuS, WO₃ and CuS–WO₃ photocatalysts.

FT-IR spectroscopy studies

The FT-IR spectra of pure CuS, WO₃ and CuS–WO₃ are recorded in the region of 500–4000 cm⁻¹ and the results are shown in Fig. 2. In the FT-IR spectrum of pure CuS, the absorption peaks at 608 cm⁻¹ and 1074 cm⁻¹ are assigned to Cu–S stretching modes and bending modes of H–O–H of absorbed water molecules in sulfide products.²¹ In the spectrum of WO₃, the broad band at 793 cm⁻¹ corresponds to the stretching vibration modes of O–W–O bonds. The absorption peaks at 1616 and 1469 cm⁻¹ corresponds to the asymmetric and stretching vibration mode of CH₃–N⁺ bond of CTAB in the sample.²² The characteristic peak at 3418 cm⁻¹ may be due to the stretching frequency of the O–H group.²³ The new distinct peak observed at 1065 cm⁻¹ for the CuS doped WO₃ photocatalysts corresponds to asymmetric valence S=O vibration.²⁴ The FT-IR results clearly indicate the existence of CuS in the WO₃ photocatalyst.

Fig. 2 The FT-IR spectra of pure CuS, WO₃ and CuS–WO₃ photocatalysts.

Morphology and elemental analysis

The shape and morphology of the synthesized photocatalysts were examined by FE-SEM. The typical SEM images obtained for CuS, WO₃ and 10% CuS–WO₃ are displayed in Fig. 3. The CuS particles exhibited the hierarchical flower-like morphology, whereas the WO₃ formed nanorod structure. The morphology of optimized 10% CuS–WO₃ clearly showed that the nanorod shape of WO₃ and flower-like structure of CuS were well distributed. Morphology and surface state of the materials can play an important role in affecting the photocatalytic activity. When the electrons and holes are trapped in surface states, the spatial overlaps of charge carriers are reduced and further their recombination is retarded due to the localized nature of surface area.²⁵ In the present study, the formation of the hierarchical flower-like CuS and nanorods of WO₃ photocatalyst are more favorable for the diffusion and separation of photoexcited electron–hole pair. Thus, the synthesized flowers-like CuS and WO₃ nanorods can be promising materials for photocatalytic activity. Further, the distribution of CuS and WO₃ in CuS–WO₃ were explored by TEM and the corresponding images are shown in Fig. 4. The TEM images show agglomeration of CuS nanoparticles on the surface of WO₃. The elemental analysis of CuS, WO₃ and 10% CuS–WO₃ are ascertained by energy dispersive X-ray spectroscopy (EDAX) and the obtained results are shown in Fig. 5. The EDAX spectrum clearly showed that the CuS was composed of copper and sulfur atoms, and WO₃ consisted of W and O atoms only with no other impurities. Further, the EDAX data of CuS doped WO₃ confirmed the existence of peaks corresponding to W, Cu, S and O atoms. The weight and atomic percentages of species detected by EDAX are shown in Fig. 5 inset. This observation supports the existence of CuS in WO₃ phases. The obtained EDAX results showed the excess of weight percentage of Cu in 10% CuS–WO₃, due to copper grid used for sample preparation.

Fig. 3 FE-SEM images of (a) flowers-like CuS, (b) WO₃ nanorods and (c) 10% CuS–WO₃.

Fig. 4 (a–d) TEM images of 10% CuS–WO₃ photocatalyst.

Fig. 5 EDAX spectra of (a) flowers-like CuS, (b) WO₃ nanorods and (c) 10% CuS–WO₃.

Optical absorption studies

The light absorption properties of CuS, WO₃ and CuS–WO₃ photocatalysts were investigated by UV-vis diffuse reflectance spectroscopy and the spectral characteristics are shown in Fig. 6. The absorption sharp edges for the synthesized CuS, WO₃, 10% and 20% of CuS–WO₃ are found to be at 661, 496, 645 and 549 nm, respectively and the corresponding calculated band energy gap values are 1.87, 2.49, 1.92 and 2.25 eV, respectively. It should be noted that the visible light absorption capacity is increased for the WO₃ with the introduction of CuS.

Fig. 6 UV-vis diffuse reflection spectra of CuS, WO₃ and CuS–WO₃ photocatalysts.

Photocatalytic degradation studies

The photocatalytic activities of the synthesized flowers-like CuS and WO₃ nanorods and CuS doped WO₃ catalysts were evaluated for the degradation of MB under visible light irradiation. The stable nature of the dye was evident from the blank test in the absence of a catalyst. There was no significant change in the absorption of MB under 2 h of visible light irradiation without the catalyst. Also, adsorption properties of MB dye on the catalyst were tested in the dark and it was observed that the adsorption equilibrium of the dye in the catalyst was reached after 60 min for all the photocatalysts (results not shown). The photocatalytic degradation of MB with CuS, WO₃, 10–40% of CuS added WO₃ photocatalysts under visible light irradiation are shown in Fig. 7(a). It can be observed that CuS doped WO₃ exhibited a higher activities for MB degradation compared to pure CuS and WO₃. The photodegradation rate of MB over the synthesized photocatalysts showed the following order: 10% CuS–WO₃ > WO₃ > 15% CuS–WO₃ >20% CuS–WO₃ >30% CuS–WO₃ >40% CuS–WO₃ > CuS, and the corresponding rate constant values are 0.013, 0.008, 0.005, 0.003, 0.003, 0.002 and 0.000 min⁻¹, respectively. It can also be observed that 10% CuS–WO₃ showed the highest photocatalytic activity. The reason for the enhancement of the photocatalytic degradation rate of MB with 10% CuS–WO₃ may be due to the extended absorption of light in the visible region, higher separation of charge carriers and a subsequent diffusion of carriers to the surface of the catalyst.²⁶ The formation of nanorod and flowers-like structures may reduce the charge recombination at the photocatalyst surface and thereby participate in the dye degradation process. However, the decrease in the photodegradation rate on increasing the CuS content to 20% may be due to excess CuS covered on WO₃ surface which may shield the incident light, thus preventing the light absorption by WO₃. This ultimately reduces the amount of photogenerated electrons on WO₃ and also at higher CuS content, it may act as a charge recombination center, which results in a decrease in the photocatalytic degradation rate.²⁷

Fig. 7 (a) Photocatalytic degradation of MB with CuS, WO₃, and CuS–WO₃ composite photocatalysts and (b) first-order kinetics plot for the photodegradation of MB over CuS, WO₃ and CuS–WO₃ photocatalysts.

The reaction kinetics of the photodegradation of MB was investigated. The experimental data of photodegradation of MB over CuS, WO₃, 10–40% CuS–WO₃ photocatalysts were fitted by a first-order kinetics equation.

ln C₀/C = kt (1)

where C₀ is the initial concentration of MB, C is the concentration of MB at various time intervals, k is the apparent first-order rate constant (min⁻¹) and t is the irradiation time (min). The plots of ln(C₀/C) against irradiation time are shown Fig. 7(b). It can be seen that the plots are linear with good correlation values and photodegradation curves fit very well with the first-order rate equation. Under our experimental conditions, a higher rate constant of 0.013 min⁻¹ is observed for the 10%-CuS–WO₃ photocatalyst, which is comparable with that previously reported by Houas et al. They obtained a rate constant of 0.025 min⁻¹ for the MB degradation using TiO₂/UV based photocatalyst.²⁸ The reaction was carried out using cylindrical shape pyrex reactor and the UV-irradiation was provided by high pressure mercury lamp (Philips HPK 125 W) with transmitting wavelength >340 nm. The dosage of photocatalyst was 50 mg of TiO₂ (2.5 g L⁻¹) in 20 mL of MB dye (Co = 72 μmol L⁻¹).

Effect of catalyst amount

In this study, the best performing catalyst viz., 10% CuS–WO₃ was chosen and a series of experiments were carried out for the photodegradation of MB (2 × 10⁻⁵ M) by varying the amounts of catalyst from 0.25, 0.5, 1.0, 1.5, 2.0 and 2.5 g L⁻¹. The obtained results are shown in Fig. 8. The observed photodegradation rate constants for the different amounts of the catalyst from 0.25, 0.5, 1.0, 1.5, 2.0 and 2.5 g L⁻¹ are as follows; 0.007, 0.007, 0.007, 0.008, 0.009, and 0.011 min⁻¹, respectively. It can be seen that the photodegradation efficiency increases with an increase in the amount of catalyst loading and this may be due to an increase in the quantity of photon adsorbed. Moreover, the availability of active sites on the surface of the catalyst is also increased and it consequently increases the degradation rate.²⁹

Fig. 8 Photocatalytic degradation of MB with catalyst loading by varying the amount (0.5–2.5 g L⁻¹) of 10% CuS–WO₃.

Photoelectrochemical studies

The photoelectrochemical measurements were carried out to investigate the charge separation efficiency of electrons and holes. The transient photocurrent response of the pure flower-like CuS, WO₃ nanorods and 10% CuS–WO₃ was recorded under visible light irradiation and the obtained results are shown in Fig. 9(a). The photostability of all the catalysts were also confirmed from a reproducible photocurrent response for few on/off cycles under light illumination.³⁰ The photocurrent initially generated by CuS is higher than WO₃. However, after 40 s, the photocurrent response of CuS droped below that of WO₃ and 10% CuS–WO₃. This observation indicates the instability of CuS. The photodegradation of MB over CuS confirms that CuS alone does not cause the photocatalytic activity, which may be due to the instability of CuS and fast recombination of electron–hole pairs. The 10% CuS–WO₃ catalyst exhibited higher photocurrent than the pure CuS and WO₃, which indicates that the charge separation efficiency in 10% CuS–WO₃ was higher than other catalysts and hence it can generate more photo-induced charge carriers.³¹

Fig. 9 (a) Transient photocurrent response of the pure CuS, WO₃ and 10% CuS–WO₃ under visible light illumination and (b) the Nyquist plots for CuS, WO₃ and 10% CuS–WO₃ and Inset: The zoomed-in view of selected region of Nyquist plot.

The electrochemical impedance spectroscopy (EIS) measurements for the photocatalyst were performed to determine the charge transfer resistance. The Nyquist plots for CuS, WO₃ and 10% CuS–WO₃ are shown in Fig. 9(b). The inset of Fig. 9(b) shows the zoomed-in view of selected region of the Nyquist plot to identify the charge transfer resistance value. It can be seen that the 10% CuS–WO₃ catalyst has a small arc radius on the impedance plot than the pure CuS and WO₃ and this suggests that the addition of CuS into WO₃ is beneficial to the separation of electrons and holes.³² Smaller the arc radius means lower the charge transfer resistance and faster the interfacial charge transfer reaction. Thus, the smaller charge transfer resistance of 10% CuS–WO₃ catalyst is most favourable for higher photocatalytic activity.

Photocatalytic degradation mechanism

Fig. 10 shows the general mechanism of the photocatalytic degradation of MB using CuS–WO₃ catalyst. The valence band (VB) and conduction band (CB) potentials of a photocatalyst were calculated using the following equation.^9,26,33

E_VB = X − E^e + 0.5E_g (2)

E_CB = E_VB − E_g (3)

where E_VB and E_CB are the valence and conduction band potentials, X is the electronegativity of the semiconductor, E^e is the energy of free electrons on the hydrogen scale (∼4.5 eV) and E_g is the band-gap energy of the semiconductor. The absolute electronegativity of CuS and WO₃ is 5.29 and 6.59 eV.^34,35 The calculated VB and CB of CuS is 1.70 and −0.16 eV, whereas VB and CB of WO₃ is 3.33 and 0.84 eV, respectively. The conduction band potential of CuS is more negative than that of WO₃. Thus, the photoinduced electron transfer from CB of CuS to WO₃ and at the same time there is a transfer of the hole from the VB of WO₃ to CuS. Therefore, the electron–hole recombination is reduced and it leads to an increase in the interfacial charge-transfer reactions for the degradation of adsorbed dye molecules.²⁴

Fig. 10 Schematic illustration of the photocatalytic separation and transfer of photogenerated charge carriers on the CuS–WO₃ catalyst under visible light irradiation.

Conclusions

In summary, flower-like CuS and WO₃ nanorods were synthesized individually by a hydrothermal route and a novel visible light-active CuS–WO₃ photocatalyst was developed by adding CuS as a co-catalyst on WO₃ by a wet impregnation method. The morphology and phase purity of the synthesized photocatalysts were revealed by FE-SEM, powder XRD and EDAX analyses. The light absorption properties of the synthesized photocatalysts were observed by UV-vis DRS measurement. The experimental results demonstrated that the photocatalytic degradation of MB over CuS added WO₃ was much higher than that of the pure CuS and WO₃. The maximum photocatalytic activity for the photodegradation of MB was obtained from the 10% CuS–WO₃ catalyst. The observed high photocatalytic activity of CuS added WO₃ is due to the extended absorption of light in the visible region and also due to the reduced recombination of the photo-induced charge carriers. The photo-electrochemical study also confirmed the higher charge carrier separation efficiency. A possible reaction mechanism has been proposed based on the photodegradation results. This study provides a new CuS–WO₃ visible light-driven photocatalyst for the treatment of polluted wastewaters and other environmental remediation work.

Acknowledgements

We gratefully acknowledge the financial support received from Department of Atomic Energy-Board of Research in Nuclear Sciences (DAE-BRNS), Mumbai and Department of Science and Technology (DST), New Delhi, India.

Notes and references

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