Synergistic effect of graphene as a co-catalyst for enhanced daylight-induced photocatalytic activity of Zn_0.5Cd_0.5S synthesized via an improved one-pot co-precipitation-hydrothermal strategy

Abstract

In this study, a series of reduced graphene oxide (RGO)–Zn_0.5Cd_0.5S nanocomposites was synthesized via an improved one-step co-precipitation-hydrothermal strategy using thiourea as an organic S source. The experimental results demonstrated that thiourea facilitated heterogeneous nucleation of Zn_0.5Cd_0.5S and in situ growth of Zn_0.5Cd_0.5S nanocrystals on the RGO sheets via electrostatic attraction. Moreover, the addition of NaOH as a precipitating agent in the reaction environment was found to reduce the aggregation of Zn_0.5Cd_0.5S on the RGO sheets. Such an intimate interfacial contact between Zn_0.5Cd_0.5S and RGO resulted in well-dispersed nanoparticles decorated on RGO sheets. Photocatalytic performances of the RGO–Zn_0.5Cd_0.5S were evaluated by the degradation of Reactive Black 5 (RB5) under a low-power 15 W energy-saving daylight bulb at ambient conditions. Compared with pristine Zn_0.5Cd_0.5S, 20RGO–Zn_0.5Cd_0.5S (20 wt% of RGO) displayed an enhanced RB5 degradation of 97.4% with a rate constant of 0.0553 min⁻¹ after 60 min of visible light irradiation. 20RGO–Zn_0.5Cd_0.5S exemplified a 1.3-fold enhancement after RGO incorporation relative to that for pristine Zn_0.5Cd_0.5S. The remarkable photocatalytic performance was ascribed to the efficient migration efficiency of the photoinduced electrons from Zn_0.5Cd_0.5S to RGO to inhibit the charge carrier recombination. Additionally, to systematically verify the role of each active species in the degradation of RB5, trapping experiments for radicals and holes were individually explored. It is confirmed that photogenerated ˙O₂⁻, ˙OH and h⁺ were responsible for the degradation of RB5 in the 20RGO–Zn_0.5Cd_0.5S system. Lastly, a postulated visible-light photocatalytic mechanism for the RB5 degradation was discussed.

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1. Introduction

Recently, environmental problems caused by organic contaminants have become a severe threat to humans. It has emerged that the textile industry is considered one of the largest fields in the world due to high consumer demand. However, in the process of manufacturing such products, the pollution in water downstream of production locations has attracted incessant worldwide attention as it could harmfully affect human and aquatic life. Furthermore, the consumption of reactive azo dyes has been very popular, totalling up to 50% of the textile production market.¹ The discharge of highly coloured wastewater into the ecosystem is a dramatic source of aesthetic pollution, eutrophication and perturbations in the aquatic life.

Dealing with pollutants demands the development of effective and sustainable decontamination technologies. Advanced oxidation process (AOP), particularly heterogeneous photocatalysis, has been shown to be a promising solution to the remediation of environmental pollution due to the fact that it can be conducted at ambient conditions resulting in desired complete mineralization of dye degradation.² Heterogeneous photocatalysis for environmental decontamination is considered to be a green strategy without creating harmful by-products. In the past few years, a number of remarkable progresses on semiconductor photocatalysis have been achieved in the environmental degradation.^3–7 TiO₂ and ZnO have been widely used to degrade various organic pollutants in water and air due to their high stability, resistant to corrosion and relatively inexpensive.^8–10 Although the sun provides an abundant source of photons, they are effective only under UV light with moderate performance due to their wide band gap energies, which comprises a small fraction (5%) of solar energy that reaches the Earth's surface as compared with visible light (45%).^11,12 In addition to wide band gap, high recombination rate of charge carriers imposes severe limitations on the photocatalytic efficiency. Therefore, considering energy saving and utilization, engineering of highly efficient visible-light-induced photocatalyst is an urgent and challenging matter hitherto.

Metal chalcogenides are emerged as good candidates for visible-light photocatalysts due to their suitable band positions and improved catalytic functions. Among them, CdS has received immersed attention due to the narrow band gap resulting in more responsive to the visible light region.¹³ However, it suffers from high charge recombination rate and photocorrosion under visible light irradiation, inhibiting the application of CdS to a large extent.¹⁴ On the other hand, the band gap of ZnS is ca. 3.66 eV, which exhibits remarkable ability in redox and durability, but fails in utilizing the visible light effectively.¹⁵ To overcome the aforementioned problems, engineering Zn_xCd_1−xS photocatalyst is a viable approach because CdS possesses similar coordination mode with ZnS and metal atoms could be mutually substituted in the same crystal lattice.^16,17 The fabrication of Zn_xCd_1−xS could circumvent the weak oxidation ability and photocorrosion of CdS as well as shift the absorption edge of ZnS into visible region. As a result, the band gap energy of Zn_xCd_1−xS can be flexibly tuned by changing the molar ratio of Zn and Cd precursors.

To further enhance the photocatalytic performance of Zn_xCd_1−xS, carbonaceous nanomaterials are employed as the supports of catalysts for the application in the photocatalytic fields.^13,18 In particular, graphene, which is known as a single atomic layer of graphite arranged in six-membered rings of carbon atoms, offers a two-dimensional (2D) superior electrical conductivity, making it an excellent electron transport matrix.¹⁹ This favors the effective transfer of the photogenerated electrons from the surface of photocatalyst to graphene to prolong the lifetime of charge carriers. Recently, there have been several studies on the development of reduced graphene oxide (RGO)–Zn_xCd_1−xS via different synthetic routes as effective visible-light photocatalysts for H₂ production from water splitting. For instance, Yan et al.²⁰ developed RGO–Zn_xCd_1−xS by a facile one-pot co-precipitation reaction using H₂S as a S source and reducing agent. Besides that, Zhang et al.¹⁸ synthesized RGO–Zn_0.8Cd_0.2S nanocomposites by a one-pot co-precipitation-hydrothermal approach using Na₂S for enhanced H₂ production compared to pure Zn_0.8Cd_0.2S. Nevertheless, the influence of different S sources on the synthesis of RGO–Zn_xCd_1−xS has recently been explored by Li et al.²¹ It was found that Na₂S- or H₂S-based reaction promoted rapid formation of Zn_xCd_1−xS, which resulted in large grain size due to quick aggregation, and thus poor contact between RGO and Zn_xCd_1−xS. Very recently, Min et al.¹⁵ prepared RGO–Zn_xCd_1−xS by a two-step co-precipitation-hydrothermal method using thiourea as a precursor. However, it was found that the Zn_0.5Cd_0.5S nanospheres were decorated on the RGO sheets with a large particle size of ca. 100–200 nm. It is known that a large particle size results in an inefficient harvesting of light and higher electron–hole recombination rate due to increased migration distances en route to the surface reaction sites.²²

In addition, to effectively suppress the recombination of photogenerated charge carriers, the excellent interfacial contact between RGO and Zn_xCd_1−xS is of paramount significance. Hence, inspired by previous works, we envision for the first time that, an improved one-pot co-precipitation-hydrothermal technique with thiourea as the organic S source in the synthesis of RGO–Zn_0.5Cd_0.5S was employed for the photodecolorization of dye, in which limited attention has paid to in the past literature. Besides the low toxicity, the diamine structure of thiourea could reduce graphene oxide (GO) to RGO.²¹ Our results demonstrated that among pure Zn_xCd_1−xS with different stoichiometries, Zn_0.5Cd_0.5S exhibited the greatest degradation of Reactive Black 5 (RB5) under a low-power 15 W energy-saving daylight bulb. Thus, Zn_0.5Cd_0.5S was used in the following study. The relative quantity of RGO to Zn_0.5Cd_0.5S can prominently affect the visible-light-responsive property and photocatalytic activity. Thus, it is indispensable to optimize the relative mass for the heterogeneous growth of Zn_0.5Cd_0.5S on the RGO sheets for effective visible light response and attaining the maximum photocatalytic efficiency. Also, mechanism of the enhanced activity over RGO–Zn_0.5Cd_0.5S is still unclear till now. As a result, the role of active species for the degradation of RB5 was thoroughly investigated due to the lack in evidence on confirming the main oxidative and reductive species (holes, electrons or radicals), which have been relatively scarce and still in its infancy in the RGO–Zn_0.5Cd_0.5S system.

2. Experimental

2.1. Materials

Sulphuric acid (H₂SO₄, 95–97%), hydrochloric acid (HCl, 37%) and ethanol (C₂H₅OH, 96%) were purchased from Chemolab. Graphite powder (average particle size of 45 μm, 99%), potassium persulfate (K₂SO₈, ≥99%), phosphorus pentoxide (P₂O₅, ≥98%), hydrogen peroxide (H₂O₂, 30%), zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O, 98%), cadmium nitrate tetrahydrate (Cd(NO₃)₂·4H₂O, 98%), thiourea (≥99%), sodium hydroxide (NaOH, 98%), terephthalic acid (TA, 98%) and RB5 (55%) were purchased from Sigma Aldrich. Aeroxide P25 was used as the control semiconductor photocatalyst. All reagents were of analytical grade and used without further purification. Deionized (DI) water (>18.2 MΩ cm resistivity) was used in the whole experiment.

2.2. Fabrication of RGO–Zn_0.5Cd_o.5S photocatalysts

Graphite oxide was synthesized from graphite by a modified Hummers' method as reported in our previous work.²³ Since the initial results showed that pristine Zn_xCd_1−xS with x = 0.5 presented the highest degradation percentage of RB5 (see Fig. S2 in ESI†), this stoichiometry was chosen in the present study. The RGO–Zn_0.5Cd_0.5S nanocomposites were synthesized by an improved one-pot co-precipitation-hydrothermal approach using thiourea as an organic S source. In a typical synthesis, a measured amount of graphite oxide (1, 5, 10, 15, 20, 25 and 30 wt%) was dispersed in a mixture of DI water and ethanol (90 mL) with a volume ratio of 2 : 1 and ultrasonicated for 1 h in a table-top ultrasonic cleaner to fully exfoliate graphite oxide into GO sheets. The weight ratios of GO to the obtained Zn_0.5Cd_0.5S were 0, 1, 5, 10, 15, 20, 25 and 30 wt%. Next, Zn(NO₃)₂·6H₂O (5 mmol) and Cd(NO₃)₂·4H₂O (5 mmol) were added into 5 mL of DI water and dissolved before being added dropwise into the mixture and stirred for another 30 min. After that, 10 mL of 6 M NaOH was added dropwise and stirred for additional 30 min. Thiourea (20 mmol) was subsequently added slowly into the mixture and further stirred for 1 h to obtain a homogeneous mixture. Successively, the mixture was transferred into a Teflon-lined stainless steel autoclave and held at 180 °C for 12 h. The mixture was then allowed to cool to room temperature. The precipitates were collected by centrifugation and rinsed with ethanol and DI water several times. The as-obtained product was dried in an oven at 70 °C for 12 h. The resulting products were denoted xRGO–Zn_0.5Cd_0.5S (x = 0, 1, 5, 10, 15, 20, 25 and 30 wt%, respectively). For comparison, pure Zn_0.5Cd_0.5S was prepared under the similar experimental condition in the absence of RGO. Meanwhile, the starting material without adding NaOH as a co-precipitating agent (denoted as RGO–Zn_0.5Cd_0.5S–H) was produced in the same hydrothermal conditions.

2.3. Characterization of photocatalysts

The surface morphology of the samples was observed on a Hitachi SU8010 field emission scanning electron microscopy (FESEM) and energy-dispersive X-ray (EDX) spectroscopy. Scanning transmission electron microscopy (STEM) images were taken on a FESEM at an accelerating voltage of 30 kV. Transmission electron microscopy (TEM) and high resolution TEM (HRTEM) images were obtained using a JEOL JEM-2100F microscope operated at 200 kV. A single drop of diluted suspensions in ethanol was dropped onto a lacey-film coated copper grid to prepare the TEM sample. Optical properties of the samples were conducted using a Cary 100 UV-Vis spectrophotometer (Agilent) where BaSO₄ was used as a reflectance standard. The absorbance spectra of the samples were taken from 200 to 800 nm. X-ray diffraction (XRD) patterns were characterized using Bruker D8 Discover X-ray iffractometer with Cu Kα (λ = 1.54056 Å) at 2θ ranging from 5 to 90° and a scan rate of 0.02° s⁻¹. The accelerating voltage and the applied current were 40 kV and 40 mA respectively. Fourier transform infrared (FTIR) spectra of the samples were collected using the transmission mode on a Thermo-Nicolet iS10 FTIR Spectrometer in the spectral range of 4000–400 cm⁻¹ using KBr Die Model 129. Renishaw inVia Raman Microscope was used for the measurement of photoluminescence (PL) emission spectra at ambient temperature using 325 nm He–Cd laser as the excitation light source. The emission spectra were scanned from 400 to 800 nm.

2.4. Evaluation of photocatalytic activity

The photocatalytic activities of the as-prepared samples for degradation of RB5 were conducted under visible light irradiation using a 15 W energy-saving daylight bulb (Philips, TORNADO 15 W WW E27 220–240 V 1CT). The average intensity of the irradiated light was 8.5 mW cm⁻² measured using a pyranometer (Kipp and Zonen type CMP 6). The distance apart between the light source and photoreactor was fixed to be 5 cm. For the degradation of RB5, 30 mg of as-synthesized photocatalysts was dispersed in the RB5 solution (40 ppm, 50 mL) under ambient temperature and pressure and continuous stirring. Prior to light irradiation, the suspension was stirred for 30 min to reach the adsorption–desorption equilibrium. 3 mL of aliquots was sampled every 15 min for 1 h, centrifuged (13 500 rpm, 30 min) to completely separate the catalyst and then tested using the UV-Vis spectrophotometer (Agilent, Cary 100) at the maximal absorption wavelength of RB5, where the characteristic absorption peak of 597 nm was found.

The durability and stability tests of the as-synthesized RGO–Zn_0.5Cd_0.5S nanocomposites were also conducted using similar procedure. The product underwent three consecutive cycles with each lasting for 1 h. In order to make up for the loss of catalyst during the recycling process, two first runs were conducted separately and its remaining catalysts were mixed to obtain 30 mg of photocatalysts before conducting the 2^nd then 3^rd stability tests. After each cycle, the photocatalyst was centrifuged (13 500 rpm, 30 min) and washed thoroughly with DI water, and then it was added to fresh RB5 solution. A minimum of two duplicate runs were conducted under the same conditions and the results were averaged to ensure the reproducibility of photodegradation results. The experimental error was found to be in the range of ±5%.

2.5. Trapping experiments for radicals and holes

The effects of various reactive species such as holes (h⁺), hydroxyl (˙OH) and superoxide radicals (˙O₂⁻) on the degradation of RB5 were investigated to understand the photocatalytic mechanism in the RGO–Zn_0.5Cd_0.5S system. Various scavenging reagents such as isopropanol, tert-butanol, triethanolamine, benzoquinone and N₂ purging were used in this study. The concentrations of isopropanol, tert-butanol, triethanolamine and benzoquinone in the reaction system were all 10 mM. Various scavengers were added to the RB5 solution before adding the photocatalysts. The analysis method was similar to the photodegradation process.

2.6. Analysis of hydroxyl radicals (˙OH)

The PL technique, which uses TA as a probe molecule, was employed to detect the formation of ˙OH radicals at the illuminated catalyst/water interface. TA readily reacts with ˙OH to generate a highly fluorescent product of 2-hydroxyterephthalic acid, which can be detected at a PL signal of 425 nm.^24,25 The amount of ˙OH produced in water is proportional to the PL intensity. The experimental methods were identical to the photodegradation of RB5 except that RB5 solution was replaced by the 5 mM TA aqueous solution with a concentration of 10 mM NaOH. Upon light irradiation, 3 mL of the reaction solution was obtained every 15 min and centrifuged (13 500 rpm, 30 min). The PL spectra of the produced 2-hydroxyterephthalic acid were measured on a Perkin Elmer LS55 fluorescence spectrometer with an excitation wavelength of 315 nm and the emission spectra were scanned from 350 to 550 nm.

3. Results and discussion

3.1. Synthesis approach

During the synthesis, graphite oxide was exfoliated in a mixture of DI water and ethanol to form GO sheets. The presence of carboxyl and hydroxyl groups on the GO sheets makes the surfaces negatively charged. Furthermore, the C=S bonds activated the amino groups in thiourea and thus thiourea molecules could be adsorbed onto GO sheets through surface functional groups at the molecular level.²¹ It is known that thiourea, which is an organic S precursor, releases S²⁻ ions in a more sustained manner at a comparatively slower rate than conventional Na₂S.¹⁵ Owing to that, metal cations (Zn²⁺ and Cd²⁺) firstly grafted on the negatively charged GO sheets via electrostatic attractions in the precursor solution, and then reacted with S²⁻ ions to form strongly coupled nanocomposites through the heterogeneous nucleation pathway.

Moreover, the addition of NaOH as a precipitating agent in the reaction medium was found to further reduce the nucleation rate by inhibiting the tendency of quick growth and aggregation of Zn_0.5Cd_0.5S. This could be evidenced by a drastic decrease in the mean particle size of Zn_0.5Cd_0.5S from 303 nm (without NaOH in the reaction medium) to 31.6 nm (with NaOH in the reaction medium) in the 20RGO–Zn_0.5Cd_0.5S hybrid system as shown in Fig. S3 in ESI.† Therefore, a small grain size was attained due to reduced surface energy of Zn_0.5Cd_0.5S nanoparticles via a one-pot co-precipitation-hydrothermal process. The in situ generation of Zn_0.5Cd_0.5S nanoparticles on the RGO sheets improved the interfacial close contact between the two phases, which agreed well with the TEM images, as discussed in the latter section. Meanwhile, GO was simultaneously reduced to RGO since ethanol is a good reducing agent under hydrothermal process.²⁶ Overall, the RGO sheets provided a fertile platform for the heterogeneous nucleation of Zn_0.5Cd_0.5S to take place. This is in accordance with previous studies on the role of RGO with tunable surface characteristics as a desirable scaffold for the growth of guest materials.^27–29 The whole synthetic protocol of the RGO–Zn_0.5Cd_0.5S hybrid nanocomposites is depicted in Fig. 1.

Fig. 1 Schematic illustration of the synthesis process of RGO–Zn_0.5Cd_0.5S nanocomposites (for clarity, sizes are not represented proportionally.).

3.2. Structural and chemical characterizations

The surface morphology and microstructure of the GO and RGO–Zn_0.5Cd_0.5S were characterized by FESEM, STEM and TEM images. Fig. 2a shows the TEM image of GO. GO has a 2D structure sheets ranging from hundreds of nanometers to several micrometer-long wrinkles at the edge. Fig. 2b–d revealed the fairly well distribution of Zn_0.5Cd_0.5S on RGO sheets in the 20RGO–Zn_0.5Cd_0.5S system. The STEM images also inferred the existence of compact and abundant interfacial contacts between Zn_0.5Cd_0.5S and RGO sheets. It can be viewed that a mean particle size of 31.6 nm was successfully achieved in the 20RGO–Zn_0.5Cd_0.5S hybrid nanocomposites (Fig. 2e). Interestingly, the magnified STEM image (Fig. 2d) displayed that the Zn_0.5Cd_0.5S nanoparticles were distributed separately on the RGO sheets as nanoislands. This could not only improve the dispersion of layered composites, but also offer more photocatalytic reaction sites, and thus heterojunction was formed between Zn_0.5Cd_0.5S and RGO. This could be further evidenced by the HRTEM image (Fig. 2f). The lattice fringes could be vividly seen in the HRTEM image, implying the well-defined crystal structure of Zn_0.5Cd_0.5S. The lattice spacing was calculated to be 0.32 nm, which was corresponded to the (101) lattice plane of hexagonal Zn_0.5Cd_0.5S nanocrystals.^16,18 In addition, a number of 4–5 RGO layers was found in the 20RGO–Zn_0.5Cd_0.5S nanocomposites with a lattice spacing of 0.35 nm.³⁰ This is not surprising since restacking of RGO sheets is a common phenomenon when the oxygen-containing functional groups are mostly removed during the reduction of GO to RGO.²¹ More importantly, the bonding between Zn_0.5Cd_0.5S and RGO was strong and did not peel off easily from the RGO sheets even under sonication during the preparation of the FESEM, STEM and TEM specimens. The EDX spectrum of 20RGO–Zn_0.5Cd_0.5S (Fig. 3) revealed the presence of C, Zn, Cd and S elements in the nanocomposites. This confirmed that C, Zn, Cd and S elements were distributed over the hybrid nanocomposites. On the other hand, a larger particle size (mean value of 41.6 nm) of pristine Zn_0.5Cd_0.5S was observed (Fig. S4b in ESI†), highlighting that RGO had a significant influence on the crystal growth of Zn_0.5Cd_0.5S via heterogeneous nucleation process on the RGO sheets. Therefore, this ascertains the role of RGO as a promising host substrate for the in situ growth of Zn_0.5Cd_0.5S.

Fig. 2 (a) TEM image of GO. (b) FESEM image and (c and d) STEM images of 20RGO–Zn_0.5Cd_0.5S. (e) Particle size distribution of Zn_0.5Cd_0.5S supported on RGO sheets. (f) HRTEM image of 20RGO–Zn_0.5Cd_0.5S.

Fig. 3 EDX mapping of 20RGO–Zn_0.5Cd_0.5S composite: elemental distribution of (a) carbon (purple), (b) Zn (red), (c) Cd (green) and (d) S (yellow). Inset shows the corresponding FESEM image of 20RGO–Zn_0.5Cd_0.5S used for elemental mapping.

The XRD patterns of the as-prepared graphite oxide, pure Zn_0.5Cd_0.5S and xRGO–Zn_0.5Cd_0.5S are illustrated in Fig. 4. For graphite oxide, the XRD peak at 9.5° corresponded to the (001) interlayer spacing of 0.93 nm, which indicated the presence of oxygen-containing functional groups.^31,32 Interestingly, no apparent peak of graphite oxide at 2θ = 9.5° was observed in all the xRGO–Zn_0.5Cd_0.5S hybrid nanocomposites. This could be attributed to the exfoliation of graphite oxide in the nanocomposites and reduction of GO to RGO with significantly fewer oxygen-containing functionalities after the one-pot synthesis process, which agreed well with the previous studies.^33,34 In addition, no characteristic diffraction peak of graphene (2θ = 24.5°) was observed due to the low amount and relatively low diffraction intensity of graphene.³⁵ The reduction of GO to RGO could be further evidenced by FTIR analysis, as discussed later. Meanwhile, all diffraction peaks of xRGO–Zn_0.5Cd_0.5S could be well-indexed to the hexagonal wurtzite of the pristine Zn_0.5Cd_0.5S, with the distinct peaks corresponding to (100), (002), (101), (110), (103) and (112) planes.^16,36 No peak shift was shown when RGO was hybridized with Zn_0.5Cd_0.5S, manifesting that RGO had a negligible effect on the crystal phase of Zn_0.5Cd_0.5S during the formation of the composites. By comparing 20RGO–Zn_0.5Cd_0.5S with 20RGO–ZnS and 20RGO–CdS, all these composites demonstrated an apparent shift in the XRD patterns (Fig. S6 in ESI†). When Zn was incorporated into the CdS crystal, the diffraction peaks of 20RGO–Zn_0.5Cd_0.5S presented an obvious shift towards the higher angle. This implied that Zn²⁺ incorporated into the lattice of CdS crystal and decreased the fringe lattice distance of the CdS due to smaller radius of Zn²⁺ ion (0.74 Å) than that of Cd²⁺ (0.97 Å).^37,38 This deduced that the Zn atoms in the CdS crystal influenced the position of Cd atoms and subsequently varied the lattice structure of CdS during the fabrication of Zn_xCd_1−xS. The calculated lattice constants a and c of 20RGO–ZnS, 20RGO–Zn_0.5Cd_0.5S and 20RGO–CdS are summarized in Table S1 in ESI.† The changes in lattice constants for these nanocomposites proved that a solid solution of Zn_0.5Cd_0.5S was formed instead of physical coupling between ZnS and CdS crystals.

Fig. 4 XRD patterns of (a) graphite oxide, (b) Zn_0.5Cd_0.5S, (c) 1RGO–Zn_0.5Cd_0.5S, (d) 5RGO–Zn_0.5Cd_0.5S, (e) 10RGO–Zn_0.5Cd_0.5S, (f) 15RGO–Zn_0.5Cd_0.5S, (g) 20RGO–Zn_0.5Cd_0.5S, (h) 25RGO–Zn_0.5Cd_0.5S and (i) 30RGO–Zn_0.5Cd_0.5S.

Since the oxygen-containing functional groups are active in the IR region, FTIR analysis was employed to qualitatively investigate the degree of deoxygenating in the xRGO–Zn_0.5Cd_0.5S hybrid nanocomposites. As indicated in Fig. 5, apart from the C=C skeletal vibration of sp² unoxidized graphitic domains at 1625 cm⁻¹, the FTIR spectrum of graphite oxide divulged the presence of oxygenated functional groups at 853 (O–C=O stretching), 1054 (C–O–C stretching), 1204 (phenolic C–OH stretching), 1728 (C=O stretching vibrations in carboxyl or carbonyl groups) and 3415 cm⁻¹ (structural O–H groups on the RGO).²³ The peak at 2360 cm⁻¹ was clearly recorded for all the samples, which was ascribed to the physically adsorbed CO₂ from the atmosphere. Compared with graphite oxide, for all the xRGO–Zn_0.5Cd_0.5S hybrid nanocomposites, the vibration of O–C=O, C–O–C and C=O peaks disappeared, whereas the intensity of the C–OH and O–H peaks considerably reduced after the one-pot co-precipitation-hydrothermal route, signifying that most of the oxygen-containing functionalities in GO have been effectively reduced.

Fig. 5 FTIR spectra of (a) graphite oxide, (b) 1RGO–Zn_0.5Cd_0.5S, (c) 5RGO–Zn_0.5Cd_0.5S, (d) 10RGO–Zn_0.5Cd_0.5S, (e) 15RGO–Zn_0.5Cd_0.5S, (f) 20RGO–Zn_0.5Cd_0.5S, (g) 25RGO–Zn_0.5Cd_0.5S and (h) 30RGO–Zn_0.5Cd_0.5S.

UV-Vis diffuse reflectance spectra (DRS) of the as-prepared photocatalysts are presented in Fig. 6A. For all the xRGO–Zn_0.5Cd_0.5S samples, a substantial increase in the absorption at wavelengths shorter than 530 nm was attributed to the intrinsic band gap of Zn_0.5Cd_0.5S. According to the Kubelka–Munk (KM) function, the band gap energy of pristine Zn_0.5Cd_0.5S was estimated to be ca. 2.53 eV by extrapolating the linear region of the [F(R)hv]² vs. photon energy (Fig. 6B). Furthermore, when a small amount of RGO was incorporated with Zn_0.5Cd_0.5S, a background absorption in the visible light region (550–800 nm) was enhanced with increasing RGO content for the xRGO–Zn_0.5Cd_0.5S. Evidently, the corresponding colour of the samples became darker, which was from yellow to dark green, with increasing RGO content (inset of Fig. 6A). This decreased the reflection of light and hence improved the light absorption ranging from 500 to 800 nm. The surface electric charge of the Zn_0.5Cd_0.5S was enhanced due to RGO hybridization, resulting in the electronic transition between excited Zn_0.5Cd_0.5S and RGO. Further observation implied that all the xRGO–Zn_0.5Cd_0.5S nanocomposites exhibited almost similar absorption edge as that of the pure Zn_0.5Cd_0.5S, claiming that the layered-structured RGO in the xRGO–Zn_0.5Cd_0.5S system only served as a scaffold for the immobilization of Zn_0.5Cd_0.5S nanocrystals.¹⁷ Hence, the estimated band gap energies of the xRGO–Zn_0.5Cd_0.5S nanocomposites were found to be very similar (ca. 2.50–2.53 eV) (Fig. 6B). However, too high RGO content in the nanocomposite would impose a negative effect on the photocatalytic performance because RGO will absorb most of the light, resulting in a reduced light utilization by Zn_0.5Cd_0.5S. Thus, an appropriate RGO loading in the xRGO–Zn_0.5Cd_0.5S samples is of paramount significance for maximizing light utilization and photocatalytic activity.

Fig. 6 (A) UV-Vis DRS (inset shows the colour of the photocatalysts) and (B) plot of transformed KM function vs. hv for the Zn_0.5Cd_0.5S and xRGO–Zn_0.5Cd_0.5S (x = 1, 5, 10, 15, 20, 25 and 30).

3.3. Photocatalytic performance of RB5 degradation

The photocatalytic activity of the as-prepared photocatalysts was evaluated by the degradation of RB5 under visible light irradiation as depicted in Fig. 7A and B. The activities of GO and pristine Zn_0.5Cd_0.5S were also performed under similar conditions for comparison. The black experiment showed that RB5 was not degraded under visible light in the absence of photocatalysts, inferring that RB5 is a stable molecule and self-photolysis of RB5 could be ruled out. Furthermore, it could be observed that the adsorption–desorption equilibrium of RB5 was attained within 30 min in the dark as evidenced in Fig. S7 in ESI.† When GO itself was used as a catalyst acting as a control sample, there was no substantial photocatalytic degradation of RB5, implying that GO was inactive under visible light due to the absence of holes, electrons and active ˙OH radicals as discussed in the latter section on PL analysis. From the photocatalytic results (Fig. 7B), the order of degradation efficiency of RB5 was summarized as follows: 20RGO–Zn_0.5Cd_0.5S (97.4%) > 15RGO–Zn_0.5Cd_0.5S (95.4%) > 10RGO–Zn_0.5Cd_0.5S (93.1%) > 25RGO–Zn_0.5Cd_0.5S (92.6%) > 5RGO–Zn_0.5Cd_0.5S (92.6%) > 1RGO–Zn_0.5Cd_0.5S (92.2%) > Zn_0.5Cd_0.5S (92.0%) > 30RGO–Zn_0.5Cd_0.5S (90.8%). Generally, we attributed the enhanced photocatalytic activity of RGO–Zn_0.5Cd_0.5S under visible light to the improved visible light absorption with increasing RGO content and the inhibition of charge carrier recombination in comparison to pristine Zn_0.5Cd_0.5S to generate free electron–hole pairs.

Fig. 7 (A) Photocatalytic degradation of RB5 over GO, pristine Zn_0.5Cd_0.5S and xRGO–Zn_0.5Cd_0.5S (x = 1–30 wt%) under visible light irradiation. The photolysis of RB5 in the absence of photocatalysts was shown for comparison. (B) Percentage of RB5 degradation for the samples after 60 min irradiation with visible light. (C) PL spectra of pristine Zn_0.5Cd_0.5S and 20RGO–Zn_0.5Cd_0.5S nanocomposites. (D) Time-dependent UV-Vis absorption spectra of RB5 solution in the presence of 20RGO–Zn_0.5Cd_0.5S under visible light (inset shows colour change sequence of RB5 solution during the photodegradation process. (E) Pseudo first order kinetic reaction of RB5 degradation under visible light irradiation.

With the hybridization of RGO to the Zn_0.5Cd_0.5S, the photocatalytic activity of RGO–Zn_0.5Cd_0.5S was markedly enhanced, elucidating that the addition of an appropriate amount of RGO could noticeably improve the overall photocatalytic efficiency. Among all the RGO–Zn_0.5Cd_0.5S photocatalysts, 20RGO–Zn_0.5Cd_0.5S presented the highest photodegradation efficiency of 97.4% relative to the pure Zn_0.5Cd_0.5S (92.0%). This was accredited to the unique features of RGO in improving the lifetime of photoinduced electron–hole pairs from Zn_0.5Cd_0.5S and thus boosting the separation of charge carriers.³⁹ This is due to the fact that RGO possesses a remarkably high charge carrier mobility of more than 200 000 cm² V⁻¹ s⁻¹ at room temperature and endows a superior 2D π-conjugation structure to efficiently retard the charge recombination.^40–42 This statement was well-supported by PL analysis in which a strong fluorescence emission peak at ca. 530 nm was detected, which was attributed to the intrinsic band gap energy of Zn_0.5Cd_0.5S (Fig. 7C). In comparison to pure Zn_0.5Cd_0.5S, the remarkably decreased PL emission intensity of the 20RGO–Zn_0.5Cd_0.5S photocatalyst represented a lower recombination probability of photogenerated charge carriers with more efficient interfacial charge transfer due to the interaction between excited Zn_0.5Cd_0.5S and RGO sheets. Thus, the RGO sheet in the RGO–Zn_0.5Cd_0.5S nanocomposite acted as the separation center of the charge carriers due to its highly conductive electron transport “highway”. More importantly, the incorporation of RGO enhanced the adsorptivity of RGO–Zn_0.5Cd_0.5S nanocomposites via π–π conjugation between RB5 and aromatic regions of RGO.⁴³ Analogues features were also observed in the case of RGO–TiO₂ and RGO–ZnO for the degradation of dye.^44,45

In addition, the absorption intensity of the RGO–Zn_0.5Cd_0.5S enhanced with increasing RGO content because RGO is known to be a good light-harvesting material as supported by UV-Vis analysis. However, with the excess addition of RGO content, the photocatalytic activity was deteriorated, revealing that a synergistic interaction between RGO and Zn_0.5Cd_0.5S is necessary to improve the photoactivity. Evidently, when the RGO content in the RGO–Zn_0.5Cd_0.5S was beyond 20 wt%, a decrease in photocatalytic activity was observed for 25RGO–Zn_0.5Cd_0.5S and 30RGO–Zn_0.5Cd_0.5S nanocomposites. This was due to the presence of excess RGO, resulting in an increase in the opacity and light scattering. In other words, the introduction of excess RGO into the Zn_0.5Cd_0.5S matrix led to shielding of the active sites on the catalyst surface and decreased the light intensity through the depth of the RB5 solution. Despite the increase of absorption intensity in the visible region, the total amount of separated electrons from Zn_0.5Cd_0.5S reduced although the addition of RGO was excellent for electron storage. Hence, this concludes that high addition amount of RGO is not favourable for the enhanced photoactivity, which is generally a universal problem for the RGO-based composites.⁴⁶

The time-dependent absorption spectra of RB5 solution in the presence of 20RGO–Zn_0.5Cd_0.5S was shown in Fig. 7D. The intensity of the peak at 597 nm, which was the characteristic UV-Vis absorption of RB5 molecule, reduced significantly with time and diminished after 60 min of light irradiation, confirming the complete decomposition of RB5 molecule. This could be further evidenced by the colour change from blue to colourless after 60 min of light irradiation as shown in the inset of Fig. 7D. From Fig. 7E, there was a linear relationship between ln(C_o/C) and irradiation time, confirming that the photocatalytic degradation was indeed pseudo first order kinetics based on the Langmuir–Hinshelwood model: ln(C_o/C) = kt, where C_o is the equilibrium concentration of RB5, C is the concentration at time t and k is the apparent first order reaction rate constant.^25,47 The calculated k value for 20RGO–Zn_0.5Cd_0.5S was 0.0553 min⁻¹, which exhibited a 1.3-fold enhancement after RGO hybridization relative to that for pristine Zn_0.5Cd_0.5S. The order of k value was found to be 0.0553 min⁻¹ (20RGO–Zn_0.5Cd_0.5S) > 0.0487 min⁻¹ (15RGO–Zn_0.5Cd_0.5S) > 0.0454 min⁻¹ (10RGO–Zn_0.5Cd_0.5S) > 0.0448 min⁻¹ (25RGO–Zn_0.5Cd_0.5S) > 0.0444 min⁻¹ (5RGO–Zn_0.5Cd_0.5S) > 0.0439 min⁻¹ (1RGO–Zn_0.5Cd_0.5S) > 0.0433 min⁻¹ (Zn_0.5Cd_0.5S) > 0.0396 min⁻¹ (30RGO–Zn_0.5Cd_0.5S) > 0.0002 min⁻¹ (GO), which was concordant with the results shown in Fig. 7A and B.

3.4. Possible reaction mechanisms of photocatalytic enhancement

To examine the photocatalytic mechanism for the degradation of RB5 over 20RGO–Zn_0.5Cd_0.5S, the influence of main reactive species such as h⁺, ˙OH and ˙O₂⁻ in the photodegradation process was explored. Various scavengers such as tert-butanol (˙OH scavenger), isopropanol (˙OH scavenger), triethanolamine (h⁺ scavenger), benzoquinone (˙O₂⁻ scavenger) and N₂ purging (˙O₂⁻ scavenger) were employed in this study. As a result of quenching, the photocatalytic activity was suppressed. From Fig. 8A, the photodegradation efficiency of RB5 was 97.4% without scavengers. It can be seen that the photocatalytic degradation of RB5 reduced drastically to 8.8% and 40.0%, respectively after adding benzoquinone and N₂ purging. This highlighted that dissolved O₂ played a key factor in RB5 degradation in the presence of 20RGO–Zn_0.5Cd_0.5S, resulting in the formation of ˙O₂⁻ via direct reduction of O₂ and formation of ˙OH via multi-step reduction of O₂.⁴⁸ On addition of triethanolamine, the photocatalytic activity was substantially decreased to 62.5% compared with no scavenger. Moreover, a reduction in the degradation of RB5 was observed after the addition of tert-butanol and isopropanol, which strongly implied that photogenerated ˙O₂⁻, ˙OH and h⁺ were the important active species which contributed to the overall enhanced photocatalytic performance.

Fig. 8 (A) Effects of different scavengers on the photodegradation of RB5 in the presence of 20RGO–Zn_0.5Cd_0.5S under visible light irradiation for a duration of 60 min. (B) ˙OH trapping PL spectra changing with irradiation time in the aqueous basic solution of TA for the case of 20RGO–Zn_0.5Cd_0.5S. (C) ˙OH trapping PL spectra of (a) 20RGO–Zn_0.5Cd_0.5S, (b) pure Zn_0.5Cd_0.5S and (c) GO in the aqueous basic solution of TA for a duration of 60 min. (D) Schematic diagram of the possible photocatalytic mechanism of 20RGO–Zn_0.5Cd_0.5S for the degradation of RB5 under visible light irradiation.

The formation of ˙OH could be further confirmed by the PL technique using TA as a probe molecule. The experiment of measuring the active ˙OH could avoid the self-photosensitization effect of dye. As depicted in Fig. 8B, a gradual increase in PL intensity at 425 nm was observed with increasing irradiation time, manifesting that the ˙OH was formed in the photocatalytic degradation process, which was in agreement with the results of tert-butanol and isopropanol quenching. In the absence of light or 20RGO–Zn_0.5Cd_0.5S photocatalyst, the PL intensity did not increase. This result signified that the fluorescence was excited by the 2-hydroxyterephthalic acid from the chemical reactions between TA and ˙OH formed on the illuminated 20RGO–Zn_0.5Cd_0.5S surface. Therefore, ˙OH was originated from both pathways, namely (1) from the reaction of photogenerated electrons through multistep reduction of O₂ and (2) from the oxidation of water via holes. Furthermore, Fig. 8C illustrates a comparison of the PL intensity for pure Zn_0.5Cd_0.5S and 20RGO–Zn_0.5Cd_0.5S. It is clear that the amount of ˙OH produced from the 20RGO–Zn_0.5Cd_0.5S was higher than that of pure Zn_0.5Cd_0.5S, inferring that the former exhibited higher photocatalytic activity than the latter owing to the inhibition of charge carrier recombination by the RGO sheets, which verified well with the photocatalytic results and PL emission spectra in Fig. 7A–C. Overall, these results confirmed the evidence of ˙OH formation and participated in the photodegradation process.

Based on the above information, a synergetic effect photocatalytic mechanism of 20RGO–Zn_0.5Cd_0.5S on the enhancement of visible light activity was demonstrated in Fig. 8D. It is evident that the interaction between Zn_0.5Cd_0.5S and RGO was responsible for the efficient charge separation under visible light irradiation. Firstly, upon light irradiation, Zn_0.5Cd_0.5S with a band gap energy of 2.53 eV was excited by visible light and induced the formation of electron–hole pairs. Secondly, the photoexcited electrons were transferred from conduction band (CB) of Zn_0.5Cd_0.5S (−0.42 eV vs. NHE) to RGO (−0.08 eV vs. NHE) via a percolation mechanism,^25,49 which has a lower band-edge position to achieve charge equilibrium and stabilization between these two components. It has been widely reported that the delocalized π structure of RGO facilitates the transfer of electrons and serves as a superior electron acceptor.^34,50 As a result, the transfer of electrons to RGO significantly reduced the electron trapping in the lattice of Zn_0.5Cd_0.5S to suppress the recombination of electron–hole pairs. The accumulated electrons on the RGO sheets reacted with molecular O₂ in solution to form reactive ˙O₂⁻ followed by the formation of oxidative ˙OH species (eqn (2) and (3)). The RB5 molecules were adsorbed with offset face-to-face orientation via π–π conjugation between RB5 and the aromatic regions of RGO,⁴³ leading to mineralization of RB5 to other degraded products. On the other hand, the holes in the valence band (VB) of Zn_0.5Cd_0.5S (2.11 eV vs. NHE) have strong oxidizing power to oxidize RB5 directly as evidenced by the results of triethanolamine quenching. At the same time, holes could oxidize OH⁻ or H₂O to ˙OH since the VB position of Zn_0.5Cd_0.5S was more positive than the standard redox potential of ˙OH/OH⁻ (1.99 eV vs. NHE).^51,52 The adsorbed RB5 was finally oxidized by the ˙OH radicals to produce mineralized products, which was in line with our results of tert-butanol and isopropanol quenching. The plausible photocatalytic reaction process was shown in eqn (1)–(6).

RGO (e⁻) + O₂ → RGO + ˙O₂⁻ (2)

˙O₂⁻ + 2H⁺ + 2e⁻ → ˙OH + OH⁻ (3)

h⁺ + H₂O → H⁺ + ˙OH (4)

h⁺ + OH⁻ → ˙OH (5)

RB5 + h⁺(˙OH, ˙O₂⁻) → mineralized products (6)

3.5. Stability evaluation

The stability and reuse of the photocatalyst are vital for the practical application. In our experiments, 20RGO–Zn_0.5Cd_0.5S was employed as a representative sample to examine the reusability of the nanocomposites towards the RB5 degradation for three times under visible light irradiation by adding fresh RB5 solution with the same initial concentration after each run. As shown in Fig. 9, the RB5 degradation efficiency after each 60 min was 96.6%, 91.4% and 81.4%, respectively. This demonstrated that the as-prepared 20RGO–Zn_0.5Cd_0.5S photocatalysts exhibited a good catalytic stability, maintaining about 85% of reactivity after three consecutive cycles. The slight decrease in the photocatalytic activity was ascribed to the inescapable loss of catalysts during washing and centrifugation process. Therefore, 20RGO–Zn_0.5Cd_0.5S serves as a highly efficient visible-light-active photocatalyst in the energy- and environmental-related applications.

Fig. 9 Recycling efficiency of 20RGO–Zn_0.5Cd_0.5S in the photodegradation of RB5.

4. Conclusions

In summary, highly effective RGO–Zn_0.5Cd_0.5S photocatalysts were successfully synthesized via an improved one-pot co-precipitation-hydrothermal route using thiourea as an organic S precursor. The 20RGO–Zn_0.5Cd_0.5S demonstrated an unprecedented high RB5 photodegradation of 97.4% with a rate constant of 0.0553 min⁻¹ after 60 min of visible light irradiation using a low-power 15 W energy-saving daylight lamp at ambient temperature and pressure. It exhibited a 1.3-fold enhancement after RGO hybridization relative to that for pristine Zn_0.5Cd_0.5S. The improved photocatalytic performance was associated with the synergistic interactions between Zn_0.5Cd_0.5S and RGO, resulting in a high separation efficiency of electron–hole pairs to suppress the charge recombination. Additionally, the trapping experiments confirmed that photogenerated ˙O₂⁻, ˙OH and h⁺ were all responsible for the degradation of RB5 in the 20RGO–Zn_0.5Cd_0.5S system. Our present results highlight the significance of the charge transfer between light harvesting semiconductor and conductive RGO for the enhancement in the photocatalytic applications. More importantly, this study not only offers an efficient photocatalyst for environmental decontamination, but also sheds new insights for engineering heterojunction nanocomposites in designing advanced photocatalysts in the light of combating the ever-increasing environmental concerns and fossil fuel depletion crisis.

Acknowledgements

This work was funded by the Ministry of Science, Technology and Innovation (MOSTI) Malaysia under the e-Science Fund (Ref. no. 03-02-10-SF0244).

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Footnote

† Electronic supplementary information (ESI) available: Synthesis procedures of GO, photocatalytic degradation of RB5 using pristine Zn_xCd_1−xS, FESEM images, particle size analysis and XRD patterns of the studied photocatalysts as control experiments, XRD patterns of pristine Zn_xCd_1−xS, adsorption–desorption equilibrium of RB5 on the photocatalysts in the dark and electronegativity calculation method. See DOI: 10.1039/c4ra10467f

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