CuSbSe₂/TiO₂: novel type-II heterojunction nano-photocatalyst

Abstract

In this study, we report band gap engineering via hybridization of the group I–V–VI compound semiconductor, i.e., copper antimony di-selenide (CuSbSe₂) nanoparticles (NPs) with TiO₂ NPs for the effective degradation of organic dyes. The formation of a novel type-II heterojunction nanostructured material was accomplished by thermal and microwave (MW) methods using separately synthesized CSSe and TiO₂ NPs. The changes observed in the characteristic properties of the hybrid as compared to those observed in its parent semiconductors proved the successful combination. The optical properties of the hybrid material showed the combined effects of CSSe and TiO₂, resulting in absorption in the visible region with new band gap energy. XRD analysis indicated the presence of diffraction peaks due to anatase TiO₂ and orthorhombic CSSe NPs, which confirmed the effective nano-hybrid formation; also, the crystal structure of the parent materials was retained. Surface attachment of smaller TiO₂ particles around CSSe NPs with spherical morphology is evident from the TEM images. As-formed CSSe/TiO₂ nanohybrids obtained using MW and thermal methods were evaluated as photocatalysts against three major organic dyes, viz., methylene blue (MB), methyl orange (MO) and Rhodamine B (RhB). The degradation efficiency obtained against all three dyes was higher in the case of the CSSe/TiO₂ hybrid prepared using the MW method. The rate of degradation was evaluated using first order kinetics. This is the first report on a novel CSSe/TiO₂ type-II heterojunction as a photocatalyst for the efficient degradation of organic dyes.

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1. Introduction

Rapidly growing urbanization and industrialization are the major concerns for environmental problems. The release of by-products and industrial or urban waste in water bodies without any pre-treatment causes many health problems and disturbances in aquatic biota.¹ In most of the cases, the by-products are considerably hazardous and may lead to various diseases via inhalation, skin contact, chronic exposure, etc. In particular, by-products or wastes that are easily soluble in water may cause many dangerous diseases including cancer. The contaminants present in drinking water may easily react with body fluids, leading to cell damage or cell overgrowth. Many organic compounds are being employed in the pharmaceutical, textile, paper, and paint industries, and most of these organic compounds are water-soluble.² The aqueous media used by industries may lead to the release of unprocessed water containing high percentage of such organic compounds. Particularly, various organic dyes such as methylene blue, methyl orange, indigo carmine, Rhodamine B, and Eriochrome Black T are commonly used in industries for dying. Such organic dyes are highly soluble in water and considered as persistent pollutants. The high solubility of dyes in water limits their easy disintegration from polluted water; thus, for water treatment, we are facing numerous challenges for converting contaminated water to potable or reusable water.³

In the last three decades, various processes have been reported for water treatment, and they include chemical, biological and physical methods. Most of them have also been exploited at the commercial level to alleviate environmental pollution. However, recently, advanced oxidation processes (AOPs) have gained tremendous attention.^4,5 Such processes include the use of O₃/H₂O₂, UV/O₃, UV/H₂O₂, and H₂O₂/Fe²⁺ for the oxidative degradation of organic compounds present in water, air and soil. AOPs involve the in situ generation of free hydroxyl radicals, which are considered as highly reactive, non-selective, short-lived and powerful oxidants.⁶ Amongst various AOPs, semiconductor nanoparticle-driven photo-catalysis has been extensively applied for water treatment in the recent past. Photocatalysis is one of the most promising photo-conversion technologies because of its ability to control environmental issues and generate solar fuels such as hydrogen (H₂), methanol (CH₃OH), and formaldehyde (HCHO). Prominent materials used for such a purpose are ZnO, TiO₂, CdS, ZnCdS, Bi₂WO₆, NaNbO₃, Ag₂Mo₂O₇, etc.^7–10 Despite impressive research on such nano-photocatalysts, the photo-conversion efficiencies of some of these nano-photocatalysts are not very satisfactory, which is possibly due to their wide band gaps, fast recombination of photo-generated charge carriers and poor light harvesting ability of semiconductors.¹¹

Ideally, a semiconductor should possess low crystalline defects with minimum trap states, which can facilitate photoreactions with increased photo-conversion efficiency. Recently, it has been described that decrease in the recombination rate via suitable textural design, doping, metal loading and forming type-II and/or Z-scheme heterojunction systems can enhance the photo-conversion efficiency.¹² The formation of type-II heterojunctions by combining two semiconductors has emerged as an attractive option to reduce the recombination rate. In such materials, the electrons and holes accumulate in the conduction band of one semiconductor and the valence band of the other semiconductor, respectively, leading to spatial separation of the charge carriers, which enhances the photo-catalytic efficiency.¹³ Generally, type-II heterojunctions comprise two semiconductors possessing small and wide band gaps; out of them, one should possess a more negative conduction band value. Therefore, this allows the small-band-gap semiconductor to act as a sensitizer for the wide-band-gap semiconductor. Such a situation can eventually facilitate easy electron transfer to the conduction band of the wide-band-gap semiconductor.¹⁴

Ternary chalcogenides because of high absorption coefficients have been widely studied for their applications in solar cells, LEDs, bio-imaging, etc.¹⁵ Recently, researchers have employed them alone or in combination with other semiconductors for photo-catalytic dye degradation; for example, 100% photocatalytic degradation of crystal violet (CV) has been reported using Ag₄Sn₄Se₁₂ NPs (a ternary selenide semiconductor with band gap of 1.80 eV) under visible-light irradiation.¹⁶ Chen et al.¹⁷ demonstrated efficient visible-light photo-catalytic dye degradation of methyl orange, Congo Red and Rhodamine B using ternary sulphide (ZnIn₂S₄) microspheres. Similarly, HgSb₂S₅ (band gap, 1.89 eV) containing organic-decorated quaternary compounds was employed as a photocatalyst against Rhodamine B by Wang et al., and it showed 100% degradation in 300 min.¹⁸ The usefulness of anatase TiO₂ NP-coated CuInS₂ (band gap, 1.80 eV) QDs has been reported by Shen et al. for enhanced degradation of methyl orange under ultraviolet and visible-light irradiation, which was found to be much more effective as compared to those of pure TiO₂ NPs and CuInS₂ QDs.¹⁹ Likewise, Bagheri et al. reported a CuInSe₂–ZnO nanocomposite prepared at 185 °C in a stainless steel autoclave for 36 h for enhancement in the photocatalytic degradation efficiency against Congo Red.²⁰ Zang et al.²¹ documented ZnS–CuInS₂-alloyed nanocrystals for effective degradation of Rhodamine B; they reported 55% degradation in initial 30 min of light exposure and 100% degradation in 120 min. Similarly, enhanced photocatalytic dye degradation was also reported by combining ZnO microspheres having 500 nm size domain with CuInSe₂ having particle size of 20 nm and CuInS₂ with smaller size of about 3.5 nm.²² Furthermore, Liao et al.²³ showed increased efficiency of degradation by the decoration of CuInSe₂ NPs over self-organized anodic TiO₂ NPs. Recently, we explored the photocatalytic activity of hybrid AgInSe₂/TiO₂ and CuInSe₂/TiO₂ NPs against three major dyes MB, MO and RhB, where the degradation was in the range of 50–90%.^24,25

All the above reports emphasize the usefulness and potential of ternary chalcogenides as photocatalysts for dye degradation. Amongst the ternary metal chalcogenides, selenides have great potential as photocatalysts and are reasonably un-explored in the field of photocatalytic energy conversion. CuSbSe₂, a compound semiconductor belonging to the I–V–VI group of the periodic table, possesses bulk band gap of 1.1 eV with absorption coefficient greater than 10⁵ cm⁻¹. In comparison with other ternary metal selenides (e.g., AgInSe₂ and CuInSe₂), CSSe contains relatively less toxic and earth-abundant elements.^26a–c Copper antimony selenide has three possible combinations, i.e., CuSbSe₂, Cu₃SbSe₃ and Cu₃SbSe₄, and they exhibit distinct crystal structures from each other as well as different thermal properties.²⁷ The low thermal conductivities of CuSbSe₂ and Cu₃SbSe₃ NPs have opened avenues for their applications in thermoelectrics.²⁸ Even though these compounds are very attractive, the synthesis of phase-pure CSSe NPs is challenging because of the high degree of uncertainty, which results from the probability of the formation of a mixture with all three phases.²⁹ In the present case, pre-synthesized phase-pure CuSbSe₂ NCs have been employed for the formation of a heterojunction with TiO₂ NPs. TiO₂ NPs are already popular in the field of photocatalysis;³⁰ however, their use is restricted to the UV region, especially up to the UV-B region. To make TiO₂ more versatile and to design novel near visible or visible-light photocatalysts, hybridized heterojunction materials can play a major role. The excellent photo-physical properties of both TiO₂ and CSSe NPs can be exploited to produce nano-hybrid photocatalysts, especially for dye degradation in the near visible region (long UV or UV-A region). To the best of our knowledge, CuSbSe₂ in the pure or combined form has never been explored for photocatalytic dye degradation. In view of above observations, the present study focuses on an idea of generating nano-hybrids of CSSe and TiO₂ NPs for photocatalytic dye degradation.

The article highlights the development of type-II heterojunctions using pre-synthesized CSSe and TiO₂ NPs via thermal and microwave (MW) methods to isolate a novel inorganic nano-hybrid. Two different methods have been employed to understand the effect of heat or MW energy and the time required on the particle size, shape, surface area, agglomeration and eventually on the photocatalytic efficiency. As-formed CSSe/TiO₂ was characterized by UV-visible, FTIR, Raman, XRD, SEM/EDS, TEM, BET and XPS analysis. Organic dyes, viz., MB, MO and RhB were used for degradation study under UV-A light (long UV, λ = 365 nm). The process of dye degradation was monitored using a UV-visible spectrophotometer. The first order reaction rate and activation energy were also calculated for each degradation process. The degradation results obtained using CSSe/TiO₂ NPs and parent semiconductors at similar conditions were compared to understand the effectiveness of the nano-hybrid material for the degradation of organic dyes. For the first time, the present study highlights the synthesis and use of a CuSbSe₂/TiO₂ nano-hybrid for the photocatalytic degradation of organic dyes.

2. Experimental

2.1. Materials and methods

Oleyl amine (OLA), antimony(III) chloride (SbCl₃), titanium tetra-chloride (TiCl₄), absolute ethanol and methanol were all purchased from Sigma Aldrich India Ltd, while N,N′-dimethyl formamide (DMF) was purchased from Merck India, Ltd. Potato starch was procured from Loba Chemie Pvt. Ltd India. For the synthesis of CSSe NPs, cyclohexeno-1,2,3-selenadiazole (SDZ) was used as a Se source and was synthesized as per the reported procedure.³¹ UV-visible absorption spectra at room temperature were recorded using an Analytikjena SPECORD @ 210 plus spectrophotometer (Germany). The samples were dispersed in DMF for UV-visible absorption measurements. For Raman analysis, an EZ Raman spectrometer (USA, λ_Em is 780 nm) operated in the range from 4000 cm⁻¹ to 400 cm⁻¹ was used. The X-ray diffraction (XRD) analysis was carried out using Bruker D8 an Advanced diffractometer (Germany) with Cu_Kα radiation (1.5405 Å) at 45 kV and 40 mA. FEI Technai G2 (300 kV) instrument was used for Transmission Electron Microscopy (TEM). Scanning Electron Microscopy (SEM) was performed on Zeiss Gemini (Germany) operated at 300 kV. A domestic microwave oven of the SAMSUNG company was used for the microwave reactions (Model: MW73AD, capacity 800 W) at an energy of 100 W. Photo-catalytic degradation reactions were carried out in a UV chamber composed of a 365 nm irradiating lamp. Solutions of methylene blue (MB), methyl orange (MO) and Rhodamine B (RhB) (100 ppm concentration) in water were prepared for the photocatalytic degradation study. A LUX meter (LX-101A) of Digital instruments was used for intensity measurement of the UV lamp. X-ray photoelectron spectroscopy (XPS) analysis was carried out using ESCA+ (Oxford Instrument, Germany) equipped with a monochromator aluminum source (Al_Kα radiation hν = 1486.7 eV) operated at 15 kV and 20 mA.

2.2. Synthesis of CSSe NPs²⁹

A mixture of copper chloride dihydrate (1.0 g) and antimony(III) chloride (1.33 g) in oleyl amine (15 mL) was heated at 95 °C for 30–35 min under a nitrogen atmosphere. Separately dissolved cyclohexeno-1,2,3-selenadiazole in oleyl amine (1.63 g/5 mL) at room temperature was then swiftly injected in the above reaction mixture. The reaction temperature was further raised to 220 °C for proper growth and nucleation and stirred for 60 min. Subsequently, the reaction mixture was brought down to room temperature and n-hexane (15–20 mL) followed by ethanol (30–40 mL) was added to precipitate out the product. The black colored precipitate of CSSe was collected by centrifugation and was isolated by repeated washing with ethanol. Finally, the black powder was dried in an oven.

2.3. Synthesis of TiO₂ NPs²⁵

TiCl₄ (10.0 mL) was added in ice cooled iso-propanol (60.0 mL) with continuous stirring to form a homogeneous mixture. Meanwhile, potato starch (0.25 g) was dissolved in distilled water (5.0 mL) and was then added to the above mixture. The reaction mixture was stirred at room temperature for 2 h and was then allowed to stand for another 8–12 h for gelation. The white gel was washed with distilled water and methanol followed by drying in a vacuum oven at 50–60 °C for 8–10 h to get an off-white powder of TiO₂ nanoparticles.

2.4. Preparation of CSSe/TiO₂ nano-hybrids

2.4.1. Thermal method. A two-neck round bottom flask was charged with CSSe (1.0 g), TiO₂ (0.25 g) and DMF (10.0 mL) and the flask was flushed with nitrogen gas. The generated mixture was then stirred initially at room temperature for 15 min and then at 100 °C for 4 h under a nitrogen atmosphere. After cooling the reaction mixture to room temperature, the nano-hybrid material was precipitated by adding methanol (20.0 mL). The black colored powder was collected by centrifugation and to remove excess DMF, it was washed several times with methanol. Finally, the powder was dried in an oven for 3–4 h.

2.4.2. Microwave-assisted method. CSSe (1.0 g), TiO₂ NPs (0.25 g) and DMF (5.0 mL) were ground together in a mortar pestle to generate a homogeneous mixture. The mixture was then transferred to a borosilicate beaker, which was irradiated with 100 W microwaves for 4 min (four cycles each of one minute with a gap of 30 seconds) using a domestic SAMSUNG microwave oven. Subsequently, the mixture was cooled down to room temperature followed by addition of methanol (10 mL). A black precipitate was collected by repeated centrifugation (5000 rpm) with methanol, and the process was repeated several times to remove DMF. The precipitate was dried in an oven for 3–4 h.

2.4.3. Measurement of photocatalytic activity. Typically, the as-prepared hybrid CSSe/TiO₂ (0.1 g) was added in 50.0 mL, 100 ppm dye solution (0.005 g MB or MO or RhB per 50.0 mL water) and the resultant mixture was continuously stirred in the dark for 1 h to ensure an adsorption–desorption equilibrium. A known volume fraction was taken out to record the zero-minute reading by UV-visible absorption spectroscopy. The mixture was then irradiated with long UV light (365 nm, intensity 28 lux) for degradation of the respective dye. At fixed time intervals of 30 min, known volumes of aliquots were drawn and the degradation process was monitored by recording the UV-visible absorption spectra of the reaction mixture.

Recycling of the photocatalyst was carried out to evaluate its chemical stability and photostability. This study involves repeated monitoring of the degradation process using fresh dye solution of the same concentration and a previously employed photocatalyst. The collection of the photocatalyst after the degradation process was executed by centrifugation at 5000 rpm and it was reused five times. The photo-catalyst was then washed thrice with methanol and acetone every time before its reuse, ensuring complete removal of the adsorbed dye from the surface of the photo-catalyst.

3. Results and discussion

Separately synthesized, oleyl amine-capped and spherical CSSe NPs were successfully hybridized with wide-band-gap starch-capped spherical TiO₂ NPs to generate a novel nano-hybrid. Two different methods, viz., thermal and microwave methods were employed for effective processing. In case of the thermal method, prolonged heating (4 h) at 100 °C was applied using DMF as the solvent to facilitate strong surface interaction between CSSe and TiO₂ NPs. However, a rapid energy burst within a short duration of about 4 min was sufficient under MW irradiation for effective processing of the same combination. Oleyl amine being a long chain amine is believed to create steric repulsion between the particles, whereas starch due to the presence of hydroxyl groups (–OH) may impart hydrophilicity to the nano-hybrid material. The hydrophilic surfaces²⁹ provided better dispersion and thus were beneficial for the photo-degradation process in an aqueous medium. A homogeneous dispersion during the photodegradation process is important because it can provide more surface area for the adsorption of dye molecules on NPs. Scheme 1 represents the processing and application of the hybrid material for degradation of three different dyes (MB, MO and RhB).

Scheme 1 Process of CSSe/TiO₂ nanohybrid formation and its utility as a photocatalyst for dye degradation.

The change in the optical properties of the as-prepared nanohybrid photocatalyst can be studied by UV-visible spectroscopy. The as-prepared TiO₂ NPs showed a peak at 323 nm, whereas CSSe NPs exhibited a broad absorption profile at about 700 nm with extended absorption in the NIR region (Fig. 1a). Effective hybridization by thermal and MW methods resulted in a change in the absorption profile. In case of the nano-hybrid material (CSSe/TiO₂) obtained using the MW method (hereafter termed as CST-M) a broad absorption profile from 380 nm to 800 nm was observed with peak maxima of 472 nm (2.62 eV) (Fig. 1b). Similarly, another sample prepared by the thermal method (hereafter termed as CST-T) showed a broad peak at 423 nm (2.93 eV). Both profiles showed new absorption patterns for the hybrid material in the visible region, thus making it suitable for application as a visible-light photocatalyst.

Fig. 1 UV-visible absorption profiles of (a) CSSe and TiO₂ NPs and (b) CST-M and CST-T.

The XRD pattern of CSSe NPs showed major peaks at 2θ of 27.42°, 45.49° and 53.95°, confirming their orthorhombic crystal phase (Fig. 2, CSSe). The comparison of these peaks with the XRD results of nano-hybrid materials revealed the presence of other low-intensity peaks at 2θ of 26.51°, 36.29°, 48.78° and 53.38°. All these peaks were considered to be due to the presence of anatase TiO₂.³² The low intensity obtained for TiO₂ NPs in the XRD profile could be due to their low concentration in the nano-hybrid since the weight by weight ratio was 1 : 0.25 (CSSe to TiO₂). The presence of all the expected peaks for orthorhombic CSSe and anatase TiO₂ confirmed effective coupling between these two without change in the crystal structure (Fig. 2, CST-M and CST-T). However, slight shift in 2θ values with broadening was observed in the case of both hybrid samples due to reduction in the crystalline nature mainly because of the presence of smaller nano-particles around CSSe. Similar observations have also been reported in literature for ZnS–CuInS₂-alloyed nanostructures.²¹ Such an observation suggests only surface attachment between CSSe and TiO₂ NPs without change in the crystal structure.

Fig. 2 XRD patterns of CSSe NPs, CST-M and CST-T nano-hybrid materials.

SEM images of the hybrid materials are shown in Fig. 3(a) and (b), which demonstrate the spherical shape of the nanoparticles. The FESEM image of CST-M [Fig. 3(a)] shows a cluster of spherical nanoparticles, which form a network due to random surface attachment. In the case of CST-T [Fig. 3(b)], the formation of large globules with a rough surface is observed. The formation of large globules in this case can be due to agglomeration caused by excess heating for a longer duration. Even though CST-T exhibited agglomeration, the spherical shape of the nanoparticles could be easily recognized. EDS analysis of the nanohybrids revealed a near stoichiometric ratio between the elements (Fig. S1, ESI†). The higher percentage of oxygen could be due to the presence of starch, which was employed as a capping agent during the synthesis of TiO₂ NPs.

Fig. 3 (a and b) FESEM images of CST-M and CST-T, (c–e) TEM images of CST-M and (f–h) TEM of CST-T. Insets of image (e) and image (h) are SAED patterns of CST-M and CST-T nanohybrid materials.

TEM analysis of the as-prepared hybrid materials confirmed the presence of both TiO₂ and CSSe NPs in a coupled state. The TEM images of CST-M [Fig. 3(c) and (d)] show large spherical CSSe NPs surrounded by smaller TiO₂ NPs, forming a hybrid nanostructured material. HRTEM of sample CST-M [Fig. 3(e)] revealed lattice fringes with a spacing of 0.32 nm, which matched well with the values reported for the (013) crystal plane of CSSe NPs and the (110) crystal plane of TiO₂. The inset of Fig. 3(e) shows the SAED pattern of CST-M with strong diffraction from the selected area, which confirms the crystallinity of the sample. Similarly, TEM images of CST-T show the presence of both NPs with spherical to oval morphology. Fig. 3(f) shows slight agglomeration of nanoparticles forming clusters of TiO₂ and CSSe NPs. An enlarged image at a scale bar of 20 nm [Fig. 3(g)] shows the presence of small TiO₂ NPs around CSSe NPs, confirming the surface attachment between the two. HRTEM images [Fig. 3(h)] reveal a similar observation as that for the CST-M sample. The inset of image (h) is the SAED pattern of CST-T and confirms the crystalline nature of the material.

The FTIR spectra show the presence of organic capping around the particles. Both samples (CST-M and CST-T) showed the presence of all expected functional groups [Fig. S2(a), ESI†]. Since oleyl amine and starch were employed as capping agents during the synthesis of CSSe and TiO₂ NPs, vibrational peaks due to various functional groups such as NH₂, CH, and C–O were observed in the FTIR spectra. We observed two peaks at 3311 cm⁻¹ and 3239 cm⁻¹ for CST-M and two peaks at 3302 cm⁻¹ and 3205 cm⁻¹ in the case of CST-T due to stretching vibrations of the NH₂ group of oleyl amine.³³ Asymmetric and symmetric C–H stretching peaks were obtained at 2918 cm⁻¹ and 2842 cm⁻¹ for CST-M and at 2922 cm⁻¹ and 2851 cm⁻¹ for CST-T.

Peaks due to NH₂ and C–H bending were also observed for CST-M at 1538 cm⁻¹ and 1429 cm⁻¹, whereas similar stretching modes of vibration were observed at 1547 cm⁻¹ and 1428 cm⁻¹ in the case of CST-T. The C=C stretching frequency was observed at 1636 cm⁻¹ in both samples. All these peaks confirmed the presence of oleyl amine and starch capping around CSSe and TiO₂ NPs. However, the absence of OH stretching could be due to a very low concentration of starch remaining due to thermal as well as MW processing.

Raman analysis of CST-M and CST-T revealed the presence of several broad peaks due to the vibrational modes of CSSe and TiO₂ NPs [Fig. S2(b), ESI†]. Compound semiconductor CSSe NPs were expected to show Raman active modes of B_3g, A_g and B_2g at 140 cm⁻¹, 154 cm⁻¹, 210 cm⁻¹ and 216 cm⁻¹.³⁴ Among these peaks, B_3g and A_g overlapped and appeared at 142 cm⁻¹. Broad peaks observed at 200 cm⁻¹ and 216 cm⁻¹ were considered to be due to the B_2g vibrational mode of CSSe NPs. Furthermore, TiO₂ NPs are known to exhibit peaks at 144 cm⁻¹, 198 cm⁻¹ and 398 cm⁻¹ due to E_g and B_1g vibrational changes.^25,32 It was considered that the E_g vibrational peak of TiO₂ overlapped with the broad intense B_3g vibrational peak of CSSe NPs at 142 cm⁻¹. Other peaks due to E_g and B_1g modes of vibrations were observed at 198 cm⁻¹ and 398 cm⁻¹ for TiO₂ NPs. Therefore, the presence of all the expected Raman-active peaks for CST-M and CST-T confirmed their purity and effective hybridization.

The average particle size distribution of the as-prepared nano-hybrid materials dispersed in DMF was evaluated by a particle size analyzer working on the principle of dynamic light scattering. The sample CST-M showed average particle size distribution of 17 nm, whereas CST-T exhibited average size of 19 nm (Fig. S3, ESI†). Both samples exhibited wide particle size distribution in the range from 2 to 35 nm, which was due to the presence of varied sizes of CSSe and TiO₂ NPs in the nano-hybrid. Marginally wider particle size distribution of the nano-hybrid prepared using the thermal method than that of the nano-hybrid obtained using the MW method could be due to the overgrowth of particles promoted by prolonged heating during synthesis.

Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) analyses of both samples were carried out to obtain their specific surface area as well as porosity. The N₂ adsorption and desorption of CST-M and CST-T showed a type III adsorption isotherm according to the IUPAC classification, corresponding to the microporous nature of the materials [Fig. 4(a) and (c)]. The adsorption on the microporous material according to a type III isotherm suggested weak interaction between the adsorbent and the adsorbate. Both samples showed H1 type adsorption hysteresis, indicating the presence of agglomerates or compactly arranged nanoparticles. The surface areas obtained for CST-M and CST-T are presented in Table 1.

Fig. 4 BET adsorption–desorption isotherms (a and c) and pore size distribution (b and d) for CST-M and CST-T.

Table 1 BET and BJH experimental results

BET results
Sample	Constant ‘C’	Monolayer adsorption volume (Vm) (cm^3 g^−1)	BET surface area (m^2 g^−1)
CST-M	15.523	3.2573	14.17
CST-T	32.806	14.725	64.089

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BJH results
Sample	Surface area of pores (m^2 g^−1)	Pore volume (cm^3 g^−1)	Mean pore diameter (nm)
CST-M	22.589	0.067749	18.27
CST-T	59.261	0.097191	06.31

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CST-M and CST-T showed mean pore size diameters of 18.27 nm and 06.31 nm, respectively [Fig. 4(b) and (d)]. The BJH plots of both samples showed agglomerated particles, resulting in pores with diameter as large as 60–70 nm with random pore size distribution. The total pore volumes were found to be 0.0677 cm³ g⁻¹ and 0.0971 cm³ g⁻¹ for CST-M and CST-T, respectively. Table 1 presents the BET and BJH results.

XPS analysis of the sample synthesized by the microwave method (CST-M) showed the presence of all elements with the expected binding energy values and oxidation states. A full scan of a typical sample, CST-M, revealed the presence of all elements, i.e., Cu, Sb, Se, Ti, O and C [Fig. 5(a)]. HRXPS of copper showed the presence of two peaks at binding energies of 935.5 eV and 955.2 eV with peak splitting of about 19.7 eV, which suggested the presence of monovalent copper in the sample³⁵ [Fig. 5(b)]; these peaks corresponded to the 2p_3/2 and 2p_1/2 states of Cu⁺. The monovalent nature of copper could be due to the reduction of Cu²⁺ to Cu⁺ during the reaction. Antimony showed two peaks at 532.8 eV and 542.3 eV, corresponding to the 3d_5/2 and 3d_3/2 states of trivalent antimony (Sb³⁺) [Fig. 5(c)]. A peak observed at 56.2 eV [Fig. 5(d)] confirmed the presence of the 3d state of selenium with −2 oxidation state (Se²⁻). HRXPS of titanium revealed two different peaks at 459.96 eV and 465.3 eV, which suggested the presence of 2p_3/2 and 2p_1/2 states of tetravalent titanium (Ti⁴⁺) [Fig. 5(e)]. Other elements such as oxygen and carbon showed peaks at 532.84 eV and 286.0 eV for 1s states [Fig. 5(f) and (g)]. The presence of a single peak for oxygen rules out the involvement of oxygen from the lattice (Ti–O), surfactant (C–O) and adsorbed moisture (O–H).²⁵

Fig. 5 XPS analysis of CSSe/TiO₂ prepared using the MW method (CST-M).

3.1. Photocatalytic degradation of organic dyes

The degradation of organic dyes basically depends on the generation of superoxide and hydroxyl radicals upon light irradiation over an aqueous solution in the presence of a photocatalyst. The photocatalyst plays a pivotal role as it provides charge carriers for the effective production of essential radicals as well as surface area for the adsorption of dyes. The photocatalyst is considered as the most important material in the degradation process since the production of radicals is completely dependent on a low recombination rate of the charge carriers³⁶ and therefore, the choice of the photocatalyst is an important aspect.

In the present study, the CSSe/TiO₂ nano-hybrid containing wide- and small-band-gap semiconductors can help reduce the rate of recombination and thus enhance the rate of radical formation for efficient degradation of organic dyes. Irradiation of such a hybrid system by light of the desired wavelength leads to the generation of electron–hole pairs. Electrons from the conduction band of CSSe are probably transferred to the conduction band of TiO₂ because of the lower energy conduction band; they are further utilized for the generation of hydroxyl radicals. On the other side, holes are considered to be responsible for the oxidation of water and they eventually form hydroxyl radicals. Therefore, the degradation of organic dyes can occur due to the attack of such in situ-generated hydroxyl radicals. Overall, the reaction sequences are depicted in Scheme 2.

Scheme 2 The process highlighting the formation of reactive radicals for the degradation of organic dyes.

Solutions of three different dyes MB, MO and RhB (100 ppm) were used for the degradation study under long UV-A irradiation (365 nm). The UV-visible absorption spectra of the as-prepared nano-photocatalysts showed absorption in the visible region and therefore, a UV-A source was chosen for the study. The degradation of the dyes was monitored by UV-visible absorption spectroscopy at fixed time intervals. A known volume fraction was withdrawn and monitored for any changes in the absorption intensity of the dyes. The continuous decrease in the absorption maxima with respect to time corresponded to the degradation of dyes. In the present case, degradation reactions were monitored for five hours with time intervals of 30 min. The decrease in the absorption maxima after continuous irradiation with respect to the initial (zero time) absorption was considered as a change in the concentration of the dye due to its degradation. Therefore, the percentage degradation in this case can be obtained by employing the following equation:^19,33

here, A₀ is absorbance of the dye before irradiation (at t = 0 min), and A_t is the absorbance of the dye after a certain time ‘t’. Fig. 6 presents the monitoring of dye degradation. The hypochromic shift observed in all cases corresponded to degradation of the dyes. The photocatalysts CST-M and CST-T were found to be effective against all three dyes. Fig. 6(a–c) represent the degradation results of MB, MO and RhB in the presence of the CST-M photocatalyst, whereas Fig. 6(e–g) show the degradation of MB, MO and RhB using the CST-T photocatalyst. Careful observation of Fig. 6(a–c) and (e–g) revealed more decrease in the absorption maxima in the case of RhB, indicating more degradation as compared to that of MB and MO. However, a comparison between the photocatalysts suggested that CST-M is more effective for degradation than CST-T. Indeed, the degradation efficiency calculated using eqn (1) also suggested the same trend and confirmed that both the photocatalysts were more effective against RhB than against MB and MO. The degradation efficiency of both the photo-catalysts is presented in Table 2.

Fig. 6 Monitoring the degradation of MB (a and e), MO (b and f) and RhB (c and g) using photocatalysts CST-M and CST-T, respectively. (d and h) C/C₀versus time plots of degradation of MB, MO and RhB.

Table 2 Comparative description of degradation efficiency, rate constant and activation energy

Dye	MB		MO		RhB
Cat.	CST-M	CST-T	CST-M	CST-T	CST-M	CST-T
% eff.	43.14	42.72	58.26	51.70	75.93	68.18
k (min^−1)	0.0652	0.0718	0.0708	0.0795	0.0552	0.0672
E a (J mol^−1)	6810	6569	6605	6313	7224	6735

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The highest degradation efficiency of 75.93% was obtained against RhB using the CST-M photocatalyst, whereas the lowest value was 42.72% in the case of MB for CST-T. The more effective degradation of RhB than that of MB and MO could be due to its effective surface adsorption driven by the presence of –COOH functional groups and a large surface area and also probably due to low recombination rate of the charge carriers of as-prepared photocatalysts. The comparison of the present findings with degradation carried out using TiO₂ as a photocatalyst under similar conditions³² emphasizes that the combination of CSSe and TiO₂ is effective against MO and RhB. The degradation efficiency of TiO₂ against RhB was 54.85%, whereas CSSe/TiO₂ prepared by the MW technique showed 75.93% degradation of RhB under similar conditions. The increase in the degradation efficiency by 21.08% revealed the effectiveness of the hybrid material via separation of the charge carriers and its utilization for redox reactions.

Fig. 6(d) and (h) show C/C₀versus time plots for the degradation of dyes using CST-M and CST-T, respectively, which reveals continuous decrease in concentration with respect to time. For better understanding and clarity, the degradation process was studied by first order kinetics and rate constant values were estimated using the following equations:^19,32

C = C₀e^−kt (2)

here, k is the rate constant (min⁻¹), t is the time in min, C₀ is the initial concentration of dye (ppm), and C is the concentration at time t (min). The calculated rate constant values are presented in Table 2. The rate of reaction was slightly lower as compared to that observed for other hybrid photocatalysts (CISe/TiO₂) employed in our previous study²⁵ and therefore, the time required for the degradation of the dyes was more. However, the rate of reaction was higher than the photocatalytic dye degradation carried out using anatase TiO₂ as the photocatalyst.³² The activation energy was calculated using the following equation:³⁷here, k is the rate constant of degradation reaction, E_a is the energy of activation, R is the gas constant (8.314 J mol⁻¹ K⁻¹) and T is temperature in Kelvin (K) (300 K). The activation energies estimated for the degradation of the respective dyes are presented in Table 2 and Fig. 7. The highest energy of activation was required for the degradation of RhB using the CST-M photo-catalyst (7224 J mol⁻¹), whereas the lowest energy of activation was required for the degradation of MO (6313 J mol⁻¹) carried out using the CST-T photocatalyst.

Fig. 7 Comparative analysis of the degradation efficiency, rate constant and energy of activation values obtained from degradation study.

3.2. Recycling study of CSSe/TiO₂

The photostability and chemical stability of the as-prepared photocatalyst were evaluated by a recycling study. For the recycling study, the photocatalyst CST-M was employed against RhB. CST-M was used initially for the degradation of RhB; after completion of the reaction, CST-M was recollected by centrifugation at 5500 rpm for 20 min and washed several times to ensure the removal of the adsorbed dye from CST-M. The CST-M photocatalyst was reused for the degradation of RhB and the same procedure was repeated five times. A continuous decrease in the absorption maxima with respect to time corresponded to the degradation of RhB [Fig. 8(a)]. The degradation of RhB was found to be nearly constant since the same photocatalyst under similar conditions was employed. The C/C₀vs. time plot for each cycle represents the effective degradation of RhB during each cycle [Fig. 8(b)]. The degradation efficiency calculated for each cycle demonstrated that there was not much loss even after the fifth cycle of the degradation study. The first cycle showed 72.23% degradation efficiency, which was found to slightly decrease after each cycle. The slight decrease in degradation efficiency was attributed to the decrease in the adsorbing power of the photocatalyst after each cycle because washing after each cycle to remove the adsorbed dye failed to completely remove all the adsorbed dye and resulted in decrease in the degradation efficiency after each cycle. The degradation efficiency, rate constant, energy of activation and percentage loss observed after each cycle are given in Table 3. The rate of degradation of RhB and the energy of activation were found to be almost constant. Therefore, the CSSe/TiO₂ photocatalysts can be employed for efficient and effective degradation of dyes even after five cycles. The present findings, therefore, emphasize the photostability, chemical stability and effectiveness of the CSSe/TiO₂ photocatalyst for degradation of different organic dyes.

Fig. 8 (a) Monitoring of RhB degradation using the recycled photocatalyst CST-M; (b) C/C₀vs. time plot for degradation of RhB using CST-M.

Table 3 Degradation efficiency (deg. eff.), percentage loss in efficiency, rate constant and energy of activation values

Attempt	Deg. eff. (%)	% loss in eff.	Rate constant (min^−1)	E a (J mol^−1)
1	72.23	0.0	0.06189	6940.9
2	71.03	1.20	0.06175	6946.5
3	68.15	4.08	0.05822	7093.4
4	67.78	4.45	0.05935	7045.4
5	67.90	4.33	0.06065	6991.4

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The present findings highlighted the applicability of the novel type-II heterojunction photocatalyst (CSSe/TiO₂) for effective degradation of various organic dyes. The combination of two semiconductors possessing small and wide band gaps was shown to successfully result in a photocatalyst under UV-A light. The combination of two such semiconductors helped to reduce the recombination rate and thus enhanced the photocatalytic degradation. The comparison of the present findings with those reported in literature revealed the absence of any report on the applicability of CuSbSe₂ NPs or their hybrid as a photocatalyst for the degradation of MB, MO and RhB. However, there are some reports on the use of other ternary metal selenides containing hybrids such as CuInSe₂/TiO₂ nanotubes, CuInS₂/TiO₂, CuInSe₂/ZnO and CuInS₂/ZnO for the degradation of various organic dyes.^19–23

Fig. 9 represents the comparison of the present study with our previous study. The degradation carried out using a hybrid composed of ternary metal selenide showed varied performances and amongst all results, the results obtained in the present study stand out. The composition of the individual components in the hybrid is mentioned on the respective bars. Different factors such as the source of light, its intensity and the type of dye are responsible for effective degradation. Comparison of experimental results with those of literature for the degradation of dyes under different experimental conditions is difficult. Therefore, the results obtained in the present study were compared with our previous results, where the previous study was carried out under similar experimental conditions. The comparative analysis suggested better performance of CuSbSe₂/TiO₂ than those of AgInSe₂/TiO₂, CuInSe₂/TiO₂ and AgInSe₂/Ag₂Se nano-hybrids, especially against RhB. Therefore, this emphasizes the applicability of ternary metal selenides in photocatalysis; however, CuSbSe₂/TiO₂ showed better performance against RhB and the present finding is a first of its kind. Therefore, the present outcome is of great importance, highlighting the effectiveness of the CuSbSe₂/TiO₂ hybrid as a photocatalyst.

Fig. 9 Comparison of the degradation efficiency against RhB reported in literature with the present findings. The values presented on the bars are ratios between two components of the hybrid.

4. Conclusion

The combination of CuSbSe₂ and TiO₂ NPs by thermal and microwave methods resulted in nano-hybrids having different properties than their parent semiconductors. The remarkable optical properties of the as-prepared hybrids encouraged us to employ such nano-hybrid materials for photocatalytic dye degradation studies. For the degradation study, three different dyes MB, MO and RhB were selected to test the effectiveness of the photocatalysts. The degradation of the three dyes was carried out under long UV-A irradiation and monitored by UV-visible spectroscopy. According to the degradation results, it was observed that the degradation efficiency was higher against RhB (75%) using the CST-M photocatalyst as compared to that against MB and MO. The comparison of degradation results with those of previously synthesized CuInSe₂/TiO₂ and AgInSe₂/TiO₂ revealed a higher degradation percentage of RhB using the current photocatalyst, which confirmed its effectiveness against RhB. The comparison with the photocatalytic activity of parent semiconductors (CSSe and TiO₂ NPs) also suggested the robust nature of the nano-hybrid and resulted in better performance. The rate of reaction was found to be enhanced relative to that of the parent semiconductors. The recycling study highlighted the effective reuse of the as-prepared photocatalyst without significant loss in the degradation efficiency. Overall, it can be stated that this novel type-II heterojunction photocatalyst is suitable for the degradation of organic dyes and can be further tested as a photocatalyst for H₂ generation by water splitting.

Conflicts of interest

Authors declare no conflict of interest.

Acknowledgements

Authors thank Vice-Chancellor, Defence Institute of Advanced Technology (DIAT), Pune for permission and for a Senior Research Fellowship (SRF) to ASK. Anuraj gratefully acknowledges his college teachers namely Mr R. B. Nikam, Dr D. V. Jagtap and Dr Sande for their constant motivation, encouragement and advice to pursue research oriented higher studies in Chemistry/Chemical Sciences.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8qm00537k

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