In situ fabrication of highly crystalline CdS decorated Bi₂S₃ nanowires (nano-heterostructure) for visible light photocatalyst application

Abstract

In situ synthesis of the orthorhombic Bi₂S₃ nanowires decorated with hexagonal CdS nanoparticles (nano-heterostructure) has been demonstrated by a facile solvothermal method. The tiny 5–7 nm CdS spherical nanoparticles are decorated on the surfaces of 30–40 nm Bi₂S₃ nanowires, successfully. Structural, morphological and optical studies clearly show the existence of CdS on the nanowires. A possible sequential deposition growth mechanism is proposed on the basis of experimental results to reveal the formation of the nano heterostructure. The heterostructures have been used as a photocatalyst for hydrogen production as well as degradation of methylene blue under solar light. The maximum hydrogen evolution i.e. 4560 and 2340 μmol h⁻¹ 0.5 g was obtained from H₂S splitting and glycerol degradation for Bi₂S₃ NWs decorated with CdS nanoparticles (nano-heterostructure) which is higher than that of the Bi₂S₃ NWs (3000 and 1170 μmol h⁻¹ 0.5 g, respectively). The enhanced photocatalytical hydrogen evolution efficiency of the heterostructures is mainly attributed to its nanostructure. In the nano heterostructure, the CdS nanoparticles control the charge carrier transition, recombination, and separation, while the Bi₂S₃ nanowire serves as a support for the CdS nanoparticles. The photogenerated electron's migration is faster than the holes from the inside of a CdS nanoparticle to its surface or to the phase interface, resulting in a relatively higher hole density inside the CdS nanoparticle leaving electron density at surface of the Bi₂S₃ NWs. This influences the photocatalytic activity under solar light. Such nano-heterostructures may have potential in other photocatalytic reactions.

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1. Introduction

One-dimensional (1D) oxide and sulphides nanostructures, such as nanorods, nanotubes, nanowires and nanobelts, have attracted intensive interest due to their unique optical, electronic and mechanical properties, and potential application in the fabrication of photocatalysts, nanoscale electronics and optoelectronics devices.^1–3 One dimensional (1D) semiconductors are considered as the most encouraging photosensitive materials due to the large surface-to-volume ratio and a Debye length comparable to their small size.^4,5 The number of surface trap states and prolong photo-carrier lifetime with increase of surface-to-volume ratio, ultimately enhance the photocatalytic performance of material.^6,7

The photocatalytic hydrogen generation as a clean energy via abundant water, hydrogen sulphide splitting and biomass degradation using semiconductor photocatalyst has attracted much attention due to the global energy crisis and environmental problems. Till date, most of the semiconductor photocatalysts have low efficiency and instability for hydrogen evolution under the solar light. In view of this, different semiconductor heterostructures^8–11 have been investigated and designed to enhance the photo-induced charge carrier's separation unlike to single phase photocatalysts.^12,13

Considering the importance of the one dimensional as well as heterostructured materials for photocatalysis, many efficient and stable heterostructures for photocatalytic hydrogen generation are investigated based on different electron transfer mechanisms.^14–18 For example, CdS/CeO_x heterostructures show superior hydrogen generation than single CdS or CeO_x due to the efficient light harvesting and fast separation of photogenerated carriers compared to CdS or CeO_x. Jum Suk Jang¹⁹ and group have also reported that the molar concentration of TiO₂ in CdS NW/TiO₂ nanocomposite photocatalyst exhibit highest activity for H₂ evolution. This configuration of photocatalyst results in an efficient charge separation due to fast diffusion of photoelectrons generated from CdS NWs toward surrounding TiO₂ NP, leading to high photocatalytic activity of hydrogen production. The CdS accompanied with host material such as ZnO and CdLnS₄, alter the energy levels of the conduction and valence bands in the coupled semiconductor system which helps to enhanced hydrogen generation via water splitting.^20,21 Recently, Zhen Fang et al. demonstrated that a growth of CdS on Bi₂S₃ nanostructures may exhibit high hydrogen evolution by prolonging lifetime of charge carriers.²² The heterostructured CdS/Bi₂S₃ has been also investigated for photocatalytic organic dye degradation and enhanced photoelectrochemical activity.^23–25 However, there are some issues regarding the synthesis and characterization of such heterostructure and is very essential to skirmish them by thorough investigation. Also, photocatalytic hydrogen production using CdS/Bi₂S₃ has not been reported yet. In view of this, we have demonstrated CdS/Bi₂S₃ nano-heterostructure with formation and growth mechanism as well as with justified enhanced hydrogen evolution.

Recent studies on metal sulfides reveals that the group of highly efficient catalysts for photochemical reactions due to photogenerated charge carriers can rapidly move to the surface of the catalysts, reducing or oxidizing organic molecules. CdS nanostructures were demonstrated as an effective photocatalyst to degrade methylene blue, methyl orange, and rhodamine B under visible irradiation conditions.^26,27 Bi₂S₃ nanostructures with different sizes show good activity in photodegradation of rhodamine B.²⁸ Whereas the heterostructures like CdS/Bi₂S₃ also tested for degradation of methylene red.²²

In the present work, we report synthesis of Bi₂S₃ nanowires (NW) decorated with CdS nanoparticles (nano-heterostructure) as an efficient photocatalyst for methylene blue degradation and hydrogen evolution from hydrogen sulphide splitting and the glycerol degradation. The heterostructure has characterized thoroughly for their structural optical properties. The Bi₂S₃ nanowires decorated with CdS nanoparticles showed very stable hydrogen under the solar light which is hitherto unattempted.

2. Experimental

2.1 Materials and synthesis

All raw materials in our experiments were purchased from S. D. Fine Chemical Ltd and used without further purification.

2.2 Synthesis of Bi₂S₃ NWs and Bi₂S₃ NWs decorated with CdS nanoparticles

In a typical procedure, 0.04 mol of anhydrous BiCl₃ added to 70 mL of ethylene glycol (EG) and the mixture was vigorously stirred to form a homogeneous solution. Simultaneously, 0.06 moles of thioacetamide and 0.2 g of polyvinylpyrrolidone (PVP) were dissolved in the 10 mL of methanol. Both solutions were transferred into a 100 mL Teflon-lined autoclave. The autoclave was sealed, maintained at 120 °C for 12 h, and cooled to room temperature naturally. The black precipitate was collected and washed three times with water and absolute ethanol, respectively. Then the sample was dried in a vacuum at 50 °C for 2 h. The same procedure has been followed to synthesize Bi₂S₃ nanowires decorated with CdS with addition of 0.01 and 0.02 mol of Cd(NO₃)₂·4H₂O.

2.3 Characterization

The phase purity of the heterostructure was investigated by XRD (Bruker (D8)). Morphological study was performed using FESEM (Hitachi S-4800). Transmission electron microscopy (TEM) images and the corresponding selected area electron diffraction (SAED) patterns were obtained on a JEOL JEM-2010 instrument. Raman spectrum of the samples was performed using HR 800-Raman Spectroscopy, Horiba JobinYvon, France. The room-temperature UV-Vis absorption spectrum was recorded on a Perkin Elmer λ-950 spectrophotometer in the wavelength range of 200–800 nm.

2.4 Photo-response study

A photoresponse device was made by sandwiching an as prepared Bi₂S₃ NWs and Bi₂S₃ decorated with CdS nanoparticles in FTO glass substrate with a device configuration of FTO/Bi₂S₃–CdS/FTO as shown in the Fig. ESI I.† A Xenon Arc lamp (300 W cm⁻²) was used as the illumination source, and a bias voltage of 500 mV was applied on the FTO electrode. The light was interrupted at 15 s intervals, and the currents were measured at crono galvanostatic mode (Autolab, PGST30). With the light regularly chopped, current spikes were recorded.

2.5 Photocatalytic study for H₂S splitting

The cylindrical quartz photochemical thermostatic reactor was filled with 700 mL of 0.5 M aqueous KOH and purged with Ar for 1 h. H₂S was bubbled through the solution at a rate of 2.5 mL min⁻¹ at 298 K. H₂S was continuously fed into the system during the photo-reduction. The 0.5 g sample was introduced into the reactor and irradiated with 300 W visible light source with constant stirring. The UV light has been blocked using 420 nm optical filter. The excess H₂S was trapped in the NaOH solution. The amount of hydrogen evolved was measured using a graduated gas burette.

2.6 Photocatalytic study for methylene blue (MB) degradation

Photocatalytic activities of the samples were evaluated by the degradation of methylene blue (MB) under visible light irradiation using a same light source as mentioned above. In each experiment, 0.1 g of the photocatalyst was added to 600 mL of MB solution (1 × 10⁻⁵ and 1.6 × 10⁻⁵ M, respectively). Before illumination, the suspensions were vigorously stirred in the dark for 1 h to ensure the establishment of an adsorption–desorption equilibrium between photocatalyst and dye. Then the solution was exposed to visible light irradiation. At certain intervals, a 10 mL solution was sampled and centrifuged to remove the remnant of photocatalyst. Finally, the adsorption UV-vis spectrum of the filtrates was recorded using a λ-950 (Perkin Elmer) spectrophotometer.

2.7 Photocatalytic study for glycerol degradation

The apparatus used for photocatalytic experiments consists of solar light-simulating source, quartz photoreactor and an on-line analysis system. Same light source is used for photocatalytic glycerol degradation, as mentioned above. The photoreactor is of cylindrical shape and its top cover has provisions for measurements of solution pH and temperature, as well as connections for inlet/outlet of the carrier gas (Argon). The gas outlet is equipped with a water-cooled condenser which does not allow vapors to escape from the reactor. The outlet of the reactor is connected to the measuring cylinder via CO₂ trap and further hydrogen gas is collected and analysed on gas chromatography. The 1 g of photocatalyst has been used for glycerol degradation.

3. Result and discussions

The phase purity and crystal structure of as-obtained Bi₂S₃ NWs and Bi₂S₃ NWs decorated with CdS was examined with powder XRD and shown in Fig. 1A. The XRD pattern of Bi₂S₃ NW is shown in Fig. 1A(p) which shows existence of well-indexed orthorhombic Bi₂S₃ (JCPDS card no. 17-0320: Fig. 1B). After, addition of the 0.01 and 0.02 mol of Cd(NO₃)₂ in reaction, a new widened peak, denoted by an asterisk, appears at 2θ of 26.47° (Fig. 1A(q) and (r) and inset of Fig. 1A),which originates from the characteristic reflection of (002) planes of hexagonal phase of CdS (JCPDS card no. 41-1049: Fig. 1C). From XRD pattern, it is also concluded that the intensity of (002) peak of CdS increased with increase in the concentration of Cd(NO₃)₂ (Fig. 1A(r)). The relatively weak diffraction peak of CdS can be ascribed to the lower loading of CdS in the heterostructures.

Fig. 1 XRD patterns (A) of the as-obtained Bi₂S₃ NW (p) and Bi₂S₃ NWS decorated with CdS ((q) and (r)).The JCPDS standard patterns of Bi₂S₃ (B) and CdS (C).

The morphological studies of the nanostructures were investigated using Field Emission Scanning Electron Microscopy (FE-SEM). Fig. 2 shows FE-SEM images of the pristine Bi₂S₃ NW and Bi₂S₃ NWs decorated with CdS. Fig. 2A and B shows typical FESEM image of the Bi₂S₃ NWs, and most of them are 30–40 nm and few micrometers in length. Typical images of Bi₂S₃ NWs decorated with CdS are shown in Fig. 2C–F. The Bi₂S₃ NWs are decorated with 5–7 nm CdS nanoparticles (Fig. 2C and D) using 0.01 mol of the Cd(NO₃)₂.Fig. 2C and clearly shows rough surface of Bi₂S₃ NW due to the growth of the CdS which is also confirmed by XRD. In images of samples obtained at higher concentration of Cd(NO₃)₂(0.02 mol), along with the 5–7 nm CdS, the bigger sized i.e. 60–70 nm CdS nanoparticles on Bi₂S₃ NWs are also observed via secondary growth (Fig. 2E and F). At higher concentration, the crystal growth is quite faster due to higher ionic concentration. Hence, the bigger particle formation via secondary growth is quite obvious.

Fig. 2 FESEM images of Bi₂S₃ NW (A and B), Bi₂S₃ NW decorated with CdS using 0.01 mol of Cd(NO₃)₂·4H₂O (C and D) and Bi₂S₃ NW decorated with CdS using 0.02 mol of Cd(NO₃)₂·4H₂O (E and F).

The microstructural information of the heterostructure was further analysed by HRTEM. HRTEM image (Fig. 3) shows Bi₂S₃ NW decorated with CdS using 0.01 mol of Cd(NO₃)₂. Fig. 3 clearly reveals that the CdS nanostructures of size 5–7 nm are grown on the 30–40 nm Bi₂S₃ NW which is in agreement with FESEM results. Fig. 3c and d clearly resolve the lattice fringes of the heterostructure. The interplanar spacing i.e. 0.33 nm can be indexed to the (002) plane of hexagonal CdS, while the 0.30 nm correspond with that of the (211) lattice plane of orthorhombic Bi₂S₃. The corresponding SAED pattern of the heterostructure represents the single crystalline nature of material (Fig. 3b).

Fig. 3 HRTEM image of Bi₂S₃ NW decorated with CdS using 0.01 mol of Cd(NO₃)₂·4H₂O (a), magnified HRTEM image of Bi₂S₃ NW (c), HRTEM image of CdS nanoparticle (d) and SAED pattern of heterostructure (b).

To realize the band structure and the charge carrier transition inside the as-prepared Bi₂S₃ NWs decorated with CdS, the diffuse reflectance (UV-DRS) and photo luminance spectra of the Bi₂S₃ NWs and Bi₂S₃ NWs decorated with CdS were recorded and analysed.

Fig. 4 shows diffuse reflection spectra of Bi₂S₃ NWs and Bi₂S₃ NWs decorated with CdS. Fig. 4 shows that the Bi₂S₃ decorated with CdS strong absorption invisible as well as near infrared region. It suggests that these samples have good visible light activity. The band gap values were determined from these spectra by converting the absolute reflection values to the Kubelka–Munk function (F(R)) (see eqn 1).^29,30

where F(R) is equivalent to the absorption coefficient. Figure SII shows plot of F(R) vs. photon energy (hν). As per Kubelka–Munk function, band gap energies of Bi₂S₃ NWs (Fig. 4c) is estimated to be ∼1.29 eV. The E_g of the and Bi₂S₃ NWs decorated with CdS using 0.01 (Fig. 4a) and 0.02 mol (Fig. 4b) of Cd(NO₃)₂ is estimated to be 1.31 and 1.37 eV, respectively, which can be attributed to the loading of CdS nanoparticles. It is quite obvious that at higher concentration, the loading of CdS is more with secondary growth. Magnified spectra (inset of Fig. 4) clearly shows the broad peak from 450–490 nm (2.5 eV) which is due to the presence of CdS nanoparticles on Bi₂S₃ NWs.

Fig. 4 Diffused reflectance spectra of Bi₂S₃ NW decorated with CdS using 0.01 mol of Cd(NO₃)₂·4H₂O (a), Bi₂S₃ NW decorated with CdS using 0.02 mol of Cd(NO₃)₂·4H₂O (b) and Bi₂S₃ NW (c).

The charge carrier transition can be also demonstrated by comparing the PL spectra of commercial CdS nanoparticle, Bi₂S₃ NWs and Bi₂S₃ NWs decorated with CdS. As shown in Fig. 5, prominent peak in PL spectrum of CdS nanoparticle at ∼522 nm is becoming weaker for Bi₂S₃ NWs decorated with CdS, suggesting the improved charge carrier separation in the heterostructure.^31,32

Fig. 5 Photoluminescence spectra of commercial CdS (a), Bi₂S₃ NW decorated with CdS using 0.02 mol of Cd(NO₃)₂·4H₂O (b), Bi₂S₃ NW decorated with CdS using 0.01 mol of Cd(NO₃)₂·4H₂O (c) and Bi₂S₃ NW (d).

3.1 Growth mechanism

As discussed in our previous work,³³ the solvothermal method provides a uniform heating environment under pressure, which results in more simultaneous nucleation than conventional methods. The solvents and precursors have an influence on the morphologies of the products and favor the synthesis of Bi₂S₃ with different pathways. The formation reaction may be described as follows.

In solvothermal process, the reaction has been occurred in two steps. Bismuth(III) from BiCl₃ solution coordinated with the TAA and a dimmer of bismuth(III) sulphide complex has been formed.³⁴ The probable structure of the complex for bismuth(III) sulphide is presented in Scheme 1.

Scheme 1 The formation of dimmer of bismuth(III) sulfide complexes.

During the reaction, at the initial stage, the decomposition occurs through the cleavage of the carbon–sulfur bond in Bi–TAA complex clusters and later on the formed Bi₂S₃ nuclei exhibits one dimensional growth and continue to grow along rod-shaped micelles via Ostwald ripening to form one-dimensional Bi₂S₃ nanorods or nanowire.

The possible reaction mechanism can be proposed as the following.

(i) The formation of dimmer of bismuth(III) sulphide complex or B(III)-TAA complexes via coordination reaction between the sulphur and the Bi(III) source at the initial stage of the solvothermal process.

(ii) Decomposition of bismuth(III) sulfide complex dimmer and the successive nucleation of Bi₂S₃ nanocrystals to form nuclei of Bi₂S₃ nanowires.

(iii) Formation of one-dimensional Bi₂S₃ nanowires originated from continuous growth of nuclei along rod-shaped micelles by Ostwald ripening.

Because of the sulfur-alkane structure, TAA is easily transformed into the resonance structure of allylmercaptan with relatively strong nucleophilicity through the resonance. The TAA coordinates to Bi³⁺ions to form bismuth(III) sulfide complex dimmer with relative stability in the form of allylmercaptan structure (Scheme 1), resulting in low generation rate of S²⁻. On the other hand, the bismuth(III) sulfide complexes dimmer also has a certain spatial orientation. These two factors are beneficial for the formation of uniform long Bi₂S₃ nanowires (Scheme 2). The CdS is also formed in the same fashion as discussed for Bi₂S₃. According to the K_sp of Bi₂S₃ (1.0 × 10⁻⁹⁷) and CdS (8.0 × 10⁻²⁷), Bi₂S₃ will preferentially deposit and form crystal seeds before the formation of CdS. The PVP molecules could attach to some specific crystal facets of the seeds, which limits its growth along [211] direction, and finally confer Bi₂S₃ NWs. However, in the reaction system, the positively charged ions, such as Cd²⁺ and so forth, could weaken the affinity between PVP molecules and nanowires, promote their desorption from the surface of nanowire,³⁵ and make the following epitaxial growth of CdS nanoparticles possible. Along with the prolonged reaction time, when Bi³⁺ was depleted after that there is an initiation of nucleation process of CdS. The as-produced CdS nanoparticles preferentially deposited at the surfaces of preformed Bi₂S₃ nanowires via epitaxial growth process to reduce their surface energy. Instead of a complete and uniform coating, isolated spherical CdS nanoparticles are formed at the surface of the Bi₂S₃ NWs. As the concentration of the Cd(NO₃)₂ increases, the size of decorated CdS nanoparticles also increased. The mobility of ions is quite hastier at higher concentration which leads into fast growth of nanoparticles. Hence, there is a secondary growth of the CdS on surface of Bi₂S₃ NWs as per standard crystal growth mechanism. The Scheme 3 represents the growth mechanism of Bi₂S₃ NWs and Bi₂S₃ NW decorated with CdS.

Scheme 2 Pyrolysis of bismuth(III) sulfide complexes dimmer.

Scheme 3 Schematic representation of growth mechanism of Bi₂S₃ NWs and Bi₂S₃ NW decorated with CdS.

3.2 Photoresponse study

Fig. 6A shows the current–voltage (I–V) plot of prepared Bi₂S₃ NWs and Bi₂S₃ NW decorated with CdS in the dark and under irradiation with white light. Both of these nanostructures showed enhancement of the current flow under white light illumination, Bi₂S₃ NW decorated with CdS showed higher photo conductance. Upon irradiation at 300 W cm⁻², the photocurrent increased approximately from 6.3 × 10⁻⁶ A cm⁻² to 6.5 × 10⁻⁵ A cm⁻² (10 times) for Bi₂S₃ NW decorated with CdS, whereas for Bi₂S₃, it increases from 1.10 × 10⁻⁴ to 1.45 × 10⁻⁴ A cm⁻² (1.33 times) under similar conditions (at 1 V). This dramatic enhancement was further confirmed by the time dependent photoresponse measurements on both materials at 500 mV (bias voltage) by periodically turning the light on and off (Fig. 6B). We obtained considerably high photosensitivity. The current increased by 1 × 10⁻⁶ A and 15 × 10⁻⁶ A for Bi₂S₃ NW and Bi₂S₃ W decorated with CdS, respectively in response to the on-and-off operation for duration of 15 s. Even after a number of cycles, the photocurrent could still be changed by illumination switching; thus, the constructed device was fairly stable and reversible.

Fig. 6 Comparative I–V curves of the Bi₂S₃ NWa and Bi₂S₃ NW decorated with CdS (A) under dark and under light conditions. Time-dependent response of the devices measured in air at a bias of 100 mV.

Bi₂S₃ NWs decorated with CdS prepared by the hydrothermal method creates more structural defects and surface states in materials than bare Bi₂S₃ NWs. Once a material with such defect energy levels is illuminated by light, the photo-generated carriers in it can efficiently be separated. Such spatial separation of the photo-generated carriers reduces the recombination rate of the electron–hole pairs, which in turn causes a significant increase of the carrier density, and thus produces a higher photocurrent.^36,37

3.3 Photocatalytic hydrogen evolution via H₂S splitting

Considering the good spectral response to solar light, the photocatalytic activities of Bi₂S₃ NWs and Bi₂S₃ NWs decorated with CdS nanoparticles for hydrogen evolution from H₂S under solar light have been investigated. Different series of experiments were performed to compare the rate of hydrogen evolution by prepared samples and these results are summarized in Table 1 and Fig. 7. The maximum hydrogen evolution (4560 μmol h⁻¹ 0.5 g) has been obtained using Bi₂S₃ NWs decorated with CdS nanoparticles using 0.01 mol of Cd(NO₃)₂ which is higher than that of the Bi₂S₃ NWs (3000 μmol h⁻¹ 0.5 g) and Bi₂S₃ NWs decorated with CdS nanoparticles using 0.02 mol of Cd(NO₃)₂ (4140 μmol h⁻¹ 0.5 g), respectively. However, Bi₂S₃ NWs decorated with CdS nanoparticles using 0.01 mol of Cd(NO₃)₂ resulted in slightly higher (10%) hydrogen evolution compared with sample Bi₂S₃ NWs decorated with CdS nanoparticles using 0.02 mol of Cd(NO₃)₂. Both samples contain CdS nanoparticles interfaced with Bi₂S₃ NWs structures and have good hydrogen evolution. However, slightly lower activity in case of heterostructure prepared using 0.02 mol of Cd(NO₃)₂ is due to the larger CdS particles. The reusability of the Bi₂S₃ NWs decorated with CdS nanoparticles using 0.01 mol of Cd(NO₃)₂ as a catalyst for hydrogen generation via H₂S splitting has been tested at same reaction conditions (See ESI III†). From reusability study, we conclude that our material is more stable after photocatalytic reaction. Surprisingly, it is observed the that the rate of hydrogen generation is higher than the pure CdS nanoparticles and commercial P25.^38–40

Table 1 Summary of hydrogen evolution data

Catalyst	Hydrogen evolution rate (μmol h^−1 0.5 g)
Bi2S3 NW	3000
Bi2S3 NWs decorated with CdS nanoparticles using 0.01 mol of Cd(NO3)2 4H2O	4560
Bi2S3 NWs decorated with CdS nanoparticles using 0.02 mol of Cd(NO3)2 4H2O	4140

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Fig. 7 Time dependent hydrogen evolution using Bi₂S₃ NW decorated with CdS using 0.01 mol of Cd(NO₃)₂ (a), Bi₂S₃ NW decorated with CdS using 0.02 mol of Cd(NO₃)₂ (b) and Bi₂S₃ NW (c).

The mechanism of evaluation of hydrogen gas via H₂S splitting is discussed in our previous article in detail.³³ In 0.5 M KOH solution, the weak diprotic acid, H₂S dissociates and maintains equilibrium with the hydrogen disulfide (HS⁻) ions. The sulphide semiconductors absorb light and generate electron–hole (e⁻–h⁺) pairs. The valence band hole (h_VB⁺) photogenerated after the band gap excitation of the Bi₂S₃ powder oxidizes the HS⁻ ion to the disulfide ion (S₂⁻²), liberating a proton from the HS⁻ ion. The conduction band electron (e_CB⁻) from the Bi₂S₃ photocatalyst reduces the protons to produce molecular hydrogen.⁴¹

H₂S + OH⁻ ↔ 4HS⁻ + H₂O

Semiconductor invisible light: Bi₂S₃ or Bi₂S₃/CdS ↔ h_VB⁺ + e_CB⁻

Oxidation reaction: 2HS⁻ + 2h_VB⁺ → S₂²⁻ + 2H⁺

Reduction reaction: 2H⁺ + 2e_CB⁻ → H₂

3.4 Hydrogen generation via glycerol degradation

Photocatalytic hydrogen evolution study was performed using Bi₂S₃ NWs and Bi₂S₃ NWs decorated with CdS catalysts from aqueous glycerol (50% solution) under visible light irradiation. Fig. 8 shows the effect of morphology on photocatalytic hydrogen evolution from glycerol. Utmost hydrogen generation i.e. 4.38 mmol h⁻¹ g⁻¹ was observed to be for the Bi₂S₃ NWs decorated with CdS nanoparticles using 0.01 mol of Cd(NO₃)₂ 4H₂O and 2.34 mmol h⁻¹ g⁻¹ for Bi₂S₃ NWs. The Bi₂S₃ NWs decorated with CdS showed high hydrogen evaluation rate than Bi₂S₃ NWs which may be due to the high surface area and improved charge carrier separation inside the composite i.e. Bi₂S₃ NWs decorated with CdS which ultimately support the enhanced photocatalytic activity. The reason for enhanced photocatalytic activity is already described in above section.

Fig. 8 Hydrogen production as a function of irradiation time using Bi₂S₃ NWs and Bi₂S₃ NWs decorated with CdS.

Photocatalytic production of hydrogen from glycerol–water mixture consists of two distinct mechanisms, that is, photo-splitting of water and photo-reforming of glycerol.⁴² First, glycerol is oxidized, and then, it produces several intermediate compounds followed by hydrogen as product. Second, glycerol acts as sacrificial agent in photocatalytic water splitting.⁴³ Glycerol, which acts as a sacrificial electron donor, is to rapidly remove the photo-generated holes (hydroxyl radicals) and/or photo-generated oxygen in an irreversible fashion, thereby suppressing electron–hole recombination and/or H₂–O₂ back reaction. By doing so, glycerol is progressively oxidized toward CO₂, with intermediate formation of partially oxidized products. When complete oxidation of glycerol (and reaction intermediates) is achieved, oxygen can no longer be removed from the photocatalyst surface and the rate of hydrogen production drops to steady-state values comparable to those obtained in the absence of glycerol in solution.

The overall process, which may be described as photo-induced reforming of glycerol at room temperature, can be expressed by the following equation, which predicts the observed H₂ : CO₂ = 7 : 3 molar ratio of products formed:

C₃H₈O₃ + 3H₂O → 3CO₂ + 7H₂ (2)

3.5 Photocatalytic methylene blue degradation

Methylene blue (MB) dye is used in textile industries massively and hence is of the major source of water pollution. Characteristic absorption peak of methylene blue appears at around 663. This absorption peak have been used to monitor the photocatalytic degradation process.^44,45 The photodegradation efficiencies of MB mediated by the Bi₂S₃ NWs and Bi₂S₃ NWs decorated with CdS nanoparticles as photocatalyst as well as without photocatalyst under visible-light illumination (λ > 400 nm) are displayed in Fig. 9. Fig. 8 also shows the ln(C/C₀) as a function of time under visible light (λ > 400 nm) irradiation, where C is the concentration of MB at the irradiation time t and C₀ is the concentration in the adsorption equilibrium of the photocatalysts before irradiation. The degradation rate of dyes is expressed as degradation-rate constant k which is summarized in Table 2.

Fig. 9 Photocatalytic degradation of MB (initial concentration 1.0 × 10⁻⁵ M) using Bi₂S₃ NW decorated with CdS using 0.01 mol of Cd(NO₃)₂ (a), Bi₂S₃ NW decorated with CdS using 0.02 mol of Cd(NO₃)₂ (b), Bi₂S₃ NW (c) and without catalyst.

Table 2 Reaction rate constant k for dye degradation

Catalyst	Rate constant (k)
Bi2S3 NWs	−0.042
Bi2S3 NW decorated with CdS using 0.01 mol of Cd(NO3)2	−0.094
Bi2S3 NW decorated with CdS using 0.02 mol of Cd(NO3)2	−0.065
Without catalyst	0.002

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Rate constant obtained for different nanostructures indicates that the photocatalytic performances are strongly dependent on shape, size, and structure. The Bi₂S₃ NW decorated with CdS using 0.01 mol of Cd(NO₃)₂ showed higher photocatalytic activity for the degradation of MB Bi₂S₃ NWs as well as Bi₂S₃ NW decorated with CdS using 0.02 mol of Cd(NO₃)₂ as shown in Fig. 9 and Table 2. The enhanced photocatalytic activity Bi₂S₃ NWs decorated with CdS nanoparticles can be attributed due to creation of more the structural defects and surface states in materials than bare Bi₂S₃ NWs.

Fig. SI–IV (See EIS IV†) reveals the temporal evolution of the absorption spectra of an MB aqueous solution catalysed by the Bi₂S₃ NWs and Bi₂S₃ NWs decorated with CdS under visible light irradiation (λ > 400 nm).

3.6 Possible mechanism of the enhancement of photocatalytic activity of Bi₂S₃ NWs decorated with CdS photocatalyst

It is well reported that the photocatalytic activity is depend of some vital factors such as optical absorption, adsorption ability, phase structure and separation efficiency of photo-generated charge carriers.^46,47 However, it is necessary to find out the band edge positions of the valence band (VB) and conduction band (CB) of both Bi₂S₃ and CdS to understand the photocatalytic reaction mechanism because they have strong relation with the photocatalytic redox process. The valence band potentials and conduction band potential of Bi₂S₃ and CdS were calculated according to the following empirical equations:⁴⁸where E_VB and E_CB are the valence band and conduction band edge potentials respectively, χ is the electronegativity of the semiconductor, which is the geometric mean of the electronegativity of the constituent atoms. As per literature, the values of χ for Bi₂S₃ and CdS have been calculated as 5.27 eV and 5.04 eV, respectively.^49–51 E_fe is the energy of free electrons on the hydrogen scale (about 4.5 eV) and E_g is the band gap energy of the semiconductor. The band gap of Bi₂S₃ and CdS is about 1.32 and 2.40 eV, respectively. On the basis of band gap positions, the band edge potentials of VB and CB for Bi₂S₃ are 1.43 eV and 0.11 eV while for CdS are 1.72 eV and −0.66 eV, respectively.

Based on above values and our photocatalysis results, a possible mechanism for enhanced photocatalytic activity based on photo-carrier transfer is shown in Scheme 4.

Scheme 4 Schematic representation of photocatalytic hydrogen generation using Bi₂S₃ NWs decorated with CdS nanoparticles.

In the presence of solar light irradiation, both Bi₂S₃ and CdS with band gap 1.37 and 2.40 eV can be photo-excited up to higher potential edge i.e. −1.53 and −1.31 eV for Bi₂S₃ and CdS due to the higher photon energy.^52,53 However, bare Bi₂S₃ NWs nanorods show poor photocatalytic activity on expenses of the high recombination of the photoinduced carriers. In the case of Bi₂S₃ NWs decorated with CdS, due to higher conduction band potential (E_CB) of Bi₂S₃ NWs compared to that of CdS, the transfer of the photo-excited electrons from the surface of Bi₂S₃ NWs to CdS nanoparticles take place. Simultaneously, the photo-excited holes created by the visible light irradiation on CdS can also migrate to the Bi₂S₃ NWs surface and react with the HS-ions. This transfer of electron–hole subsequently reduces the recombination of electrons and holes. The reduction of recombination admits more chances for electrons to participate in the reduction reaction to form H₂ molecules which leads to a higher photocatalytic activity and excellent stability. Yet, with an increase in the amount of CdS, the excessive photoinduced electrons and holes can unavoidable recombine thus, decrease the photoactivity. The recombination of electron hole pair is also discussed in photoluminescence study. The optimized Bi₂S₃ NWs decorated with CdS prepared using 0.1 M Cd(NO₃)₂ which showed the best photocatalytic activity.

4. Conclusions

In nutshell, Bi₂S₃ NWs decorated with CdS nanoparticles have been successfully synthesized in solution phase at mild temperature. The growth mechanism of the formation of the nanoscale heterostructure has also been discussed. The prepared nanoscale heterostructure exhibits superior photocatalytic hydrogen generation activity and is strongly dependent on its composition and microstructure. Experimental studies reveal that the size of the CdS nanoparticle determines the band gap energy of the nanoscale heterostructure, controlling the charge carrier transition, recombination and separation in heterostructure. Hence the Bi₂S₃ NWs decorated with CdS showed an efficient hydrogen generation rate than bare Bi₂S₃ NWs. The enhanced photocatalytic results indicate potential application of nanoscale chalcogenide heterostructures in organic waste degradation. The methodology can also be extended to the preparation of other nanoscale heterostructures.

Acknowledgements

The authors would like to thank, Executive Director, C-MET, Pune for technical support and DeitY, New Delhi financial support.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra01488g

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