Controlled synthesis of Zn_xCd_1−xS nanorods and their composite with RGO for high-performance visible-light photocatalysis

Abstract

Monodisperse and ultrafine Zn_xCd_1−xS (ZCS) nanorods with hexagonal phases were controllably synthesized by a facile one-pot approach. The band gap of these alloyed nanocrystals can be tuned in a broad range from 2.41 to 3.78 eV by simply changing the molar ratio of the two precursors. All the ZCS samples exhibit a band gap-related and aspect ratio-dependent photoresponse to visible light. Zn_0.5Cd_0.5S nanorods with a suitable band gap and aspect ratio display the highest photoresponse, even 25 times higher than that of Zn_0.875Cd_0.125S. Graphene was chosen as a co-catalyst for 1D Zn_0.5Cd_0.5S nanorods due to its 2D structure and excellent conductivity. The Zn_0.5Cd_0.5S/RGO nanocomposites with a RGO content of 2.0 wt% showed the highest photocatalytic activity for the degradation of methylene blue (MB), which is mainly due to the uniform dispersion of ZCS nanorods on RGO and the enhanced separation rate of photoinduced electrons and holes by fast transfer of the photogenerated electrons through the contact line-to-line interface between ZCS nanorods and RGO nanosheets.

---

Introduction

Metal sulfides are an important class of semiconductors, which exhibit unique optical properties and have multiple potential applications in biological labels,^1,2 light emitting devices,³ photovoltaic devices,⁴ and catalyst,⁵ etc. The optical properties of metal sulfides are fundamentally determined by the intrinsic band gap energy of metal sulfide. Therefore, adjusting the band gap is extremely useful for developing metal sulfides with novel optical properties and applications. Generally, the band gap of a metal sulfide can be tuned by varying the particle size due to the quantum size effect.^6–9 However, a prominent problem of the extremely small nanocrystals with excellent optical properties is their instability in some practical applications. Furthermore, the improvement and multifunctional optical properties of metal sulfides usually cannot be achieved only by size control. In contrast, composition control was proved a convenient way for tuning the band gap of metal sulfide solid solution in a large range even under a fixed size.^10–29 Metal sulfide solid solution with different band gap have been widely used in the band gap engineering, such as Zn_xCd_1−xS,^11–19 Zn_xCd_1−xSe,²⁰ CuIn_xGa_1−xS₂,^21,22 AgInS₂–ZnS,^23–25 and CuInS₂–ZnS.^26–29 Their composition may be continuously tuned between the constituent limits and consequently their band gap and light emissions can be tuned gradually.

Zn_xCd_1−xS (ZCS) is one of the mostly studied metal sulfide solid solution, because its band-gap can be adjusted by simply adjusting the molar ratio of Zn and Cd precursors.^11–19 Moreover, its conduction band and valence band can shift to more positive and negative position, which makes it more suitable as visible-light-driven photocatalyst. In recent years, ZCS nanocrystals with different shape and tunable band gap have been synthesized. An early example of synthesis ZCS nanocrystals was reported by Zhong and co-workers through the reaction of a mixture of CdO- and ZnO-oleic acid complexes with sulfur in the noncoordinating solvent system at high temperature. The obtained spherical ZCS nanocrystals exhibited extremely narrow and tunable photoluminescence spectra within 474–391 nm.¹¹ Later, Li and coworkers found a special composition-inducing shape change (from dot to single-armed rod then to multi-armed rod) and phase-structure transition (from wurtzite to zincblende) as well as composition-dependent optical properties.¹² Porous Zn_xCd_1−xS nanosheets were also synthesized by a facile cation-exchange strategy with the inorganic–organic hybrid semiconductor sheets as the starting materials. The as-prepared porous Zn_0.5Cd_0.5S nanosheets exhibited a high activity for photocatalytic H₂ evolution from water splitting.¹³

To enhance the photocatalytic performance of semiconductor photocatalysts, co-catalysts were usually adopted as efficient electron-transport matrices to retard the recombination of photogenerated electron–hole pairs. In most cases, noble metal species such as Pt and RuO₂ were used as co-catalyst.^30,31 But the noble metals are expensive and rare in the earth. To reduce the cost of photocatalytic process, it is necessary to explore alternative co-catalysts. Graphene is a single sheet composed of sp²-hybridized carbon, which has been heralded as the next-generation material for nanoelectronic devices owing to its outstanding electronic, optical, thermal and mechanical properties.^32,33 The excellent electrical conductivity and the flexible atom-thin 2D feature of graphene also make it an excellent electron-transport matrix. For example, Gong and coworker prepared RGO/Zn_0.8Cd_0.2S composite via co-precipitation-hydrothermal reduction route. The as-prepared RGO/Zn_0.8Cd_0.2S composite exhibited a high H₂-production rate of 1824 μmol h⁻¹ g⁻¹, which is 4.5 times higher than Zn_0.8Cd_0.2S and also much better than Pt–Zn_0.8Cd_0.2S.³⁴

Despite these achievements, there are still some urgent issues for rational design ZCS based photocatalysts, including tailoring the morphologies of ZCS photocatalyst, choosing proper co-catalyst and rational design interface between photocatalyst and co-catalyst. Here, we report the controlled synthesis of Zn_xCd_1−xS with ultrafine rod-shape by using a facile one-pot approach. The lattice parameter, aspect ratio and band gap of ZCS were found to vary with composition. Interesting, all the ZCS samples exhibit band gap-related and aspect ratio-dependent photoresponse to visible light. The Zn_0.5Cd_0.5S nanorods with band gap of 2.73 eV and aspect ratio of 7.8 exhibited the highest photoresponse to visible light. Graphene was chosen as co-catalyst for 1D Zn_0.5Cd_0.5S nanorods due to its 2D structure and excellent conductivity. The Zn_0.5Cd_0.5S/2.0 wt% RGO nanocomposites with a contact line-to-line interface exhibited enhanced photocatalytic activity for the degradation of methylene blue.

Experimental section

Chemicals

Sodium diethyldithiocarbamate ((C₂H₅)₂NCS₂Na·3H₂O, Na(DDTC)), zinc acetate (Zn(Ac)₂·2H₂O; 99.0%), cadmium nitrate (Cd(NO₃)₂,99.0%), acetic acid (AC, 99.5%, Aladdin), cyclohexane (AR, 99.5%) and ethanol (AR, 99.7%) were purchased from Sinopharm Chemical Reagent Company. Oleylamine (OM; 80.0–90.0%) and dimethyl sulfoxide (DMSO, 99.0%) purchased from Aladdin Reagent Company. All chemicals were used without further purification.

Preparation of Zn(DDTC)₂ and Cd(DDTC)₂

Zn(DDTC)₂ and Cd(DDTC)₂ were prepared according to the literature method.^35–39 In a typical synthesis of Zn(DDTC)₂, 10 mmol of Zn(Ac)₂, 20 mmol of Na(DDTC) were dissolved in 100 mL of distilled water, respectively. Then, the Na(DDTC) aqueous solution was added into the Zn(Ac)₂ aqueous solution. After constant stirring at ambient condition for 1 h, the mixture was allowed to stand for 3 h. The resulting white precipitate was filtered, washed with distilled water, and dried in air at 60 °C. Cd(DDTC)₂ was also prepared by the same way depicted as above, and only Zn(Ac)₂ was replaced by Cd(NO₃)₂.

Synthesis of Zn_xCd_1−xS nanorods and Zn_0.5Cd_0.5S/RGO

For the synthesis of Zn_xCd_1−xS nanorods, Zn(DDTC)₂ and Cd(DDTC)₂ in different Zn/Cd molar ratios were dissolved in 10.72 g of OM and loaded in a 100 mL three-neck flask clamped in a heating mantle. The mixture was heated from room temperature to 100 °C and degassed under vacuum to 120 °C. The reaction system was then filled with argon. Then the mixture was further heated to 240 °C within 30 min. Keeping the temperature for different hours and then we can obtain different samples.

Zn_0.5Cd_0.5S/RGO nanocomposites were synthesized according to the method that reported by our group.⁴⁰ In brief, 0.1 mmol of Zn(DDTC)₂ and 0.1 mmol of Cd(DDTC)₂ dissolved in OM were mixed with GO, which dispersed in DMSO solution. Then the Zn_xCd_1−xS/RGO was synthesized by using the same process as for synthesis of Zn_xCd_1−xS nanorods.

In order to use Zn_0.5Cd_0.5S/RGO as photocatalyst in the aqueous solution, the as-prepared Zn_0.5Cd_0.5S/RGO was washed using acetic acid to remove OM as previous reported method.^41,42 In a typical process, the as-prepared Zn_0.5Cd_0.5S/RGO composite was dispersed in 35 mL acetic acid under ultrasonic for 10 min. Then the slurry was put into a water bath at 70 °C for 10 h under stirring and then washed three times with ethanol.

Characterization

The size and morphology of the prepared samples were examined by a Tecnai G2 F20 STwin TEM (FEI, USA) operated at 200 kV. X-ray diffraction (XRD) patterns were measured using a Bruker D8 Advance X-ray diffractometer (with Cu Kα radiation at 0.15418 nm). The UV-vis absorption spectra were obtained on a Lambda 25 UV-vis-NIR spectrometer (Perkin-Elmer, USA). Photoluminescence (PL) spectra were measured in open-sized 1 cm path-length quartz by using a RF-5301 spectrophotometer. The photocurrent response of the samples was measured by using a semiconductor characterization system (Keithley 4200). The light source was a 300 W Xe lamp with a UV cutoff filter (λ > 420 nm) to allow only visible light to transmit.

Photocatalytic reactions

The photocatalytic activity of the as-prepared Zn_xCd_1−xS and Zn_xCd_1−xS/RGO was evaluated by the decomposition of methylene blue (MB) in an aqueous solution under visible light irradiation. Briefly, 35 mg of sample was suspended in 40 mL MB solution with the initial concentration of 10 ppm. The light source was a 300 W Xe lamp with a UV cutoff filter (λ > 420 nm) to allow only visible light to transmit. Prior to irradiation, the suspension was magnetically stirred in the dark for 12 h to ensure establishment of an adsorption–desorption equilibrium. During irradiation, the suspension was vigorously stirred and the temperature was kept at room temperature using a water cooling system. At a given irradiation time interval, 5 mL of the suspensions were collected and centrifuged to separate the photocatalyst. Photodegradation was monitored by measuring the absorption band of MB (664 nm) on a UV-visible spectrophotometer. The degradation efficiency was evaluated according to C/C₀ (C₀ is the density of dyes after adsorption equilibrium; C is the density of dyes after reaction).

Results and discussion

The morphology of the synthesized Zn_xCd_1−xS (from x = 0.125 to x = 0.875), CdS and ZnS has been investigated using TEM. As shown in Fig. 1a, when x = 0, the CdS nanocrystals have a bullet-like shape with a diameter of 7.5 nm and length of 16 nm. For ZCS nanocrystals with a composition of x = 0.125, the bullet-like shape almost unchanged but their average diameter becomes smaller (Fig. 1b). With the increase of Zn content (x), the aspect ratio of ZCS nanorods increases from 1.5 for x = 0 to the maximum value of 7.8 for x = 0.5 and then slightly decreases to 6.6 for x = 0.6 and almost unchanged with further increase of x (Table 1). When x = 1, the morphology of ZnS exhibits a nanorod-shape with uniform and relatively smaller size. As a general trend, a decrease in the average size of the nanocrystals is observed with increasing of Zn content (Fig. 1b–h). This trend is different from the reported results. In the reports, Li and co-workers demonstrated that the shape of ZCS nanocrystals prepared in hexadecylamine/trioctylphosphine solution changed from rod shape to dot shape with the increase of Zn content.¹² The morphology variations with different Zn content are likely caused by differences in the binding strength of the respective cations with surfactants. In this study, the weaker binding of Cd²⁺with OM results in a broader size distribution and larger diameter of CdS nanocrystals as has been observed in Fig. 1a. The strong binding of OM with Zn²⁺ as well as selective adsorption of OM on [100] crystal planes of ZnS lead to the formation of ZCS nanorods and ZnS nanorods. The well resolved lattice fringes demonstrate the highly crystalline nature of the nanocrystals.

Fig. 1 TEM images of CdS (x = 0) (a), Zn_xCd_1−xS nanorods with different Zn content (b) x = 0.125, (c) x = 0.25, (d) x = 0.4, (e) x = 0.5, (f) x = 0.6, (g) x = 0.75, (h) x = 0.875 and ZnS (x = 1) (i).

Table 1 The physical properties of ZCS samples

Zn content (x)	Aspect ratio	a	c	Band gaps^a (eV)
a The band gaps were calculated according to the Kubelka–Munk (KM) method.
0	1.5	4.14	6.72	2.41
0.125	1.8	4.09	6.68	2.48
0.25	3.5	4.02	6.58	2.59
0.4	4.7	4.01	6.57	2.67
0.5	7.8	3.96	6.46	2.73
0.6	6.6	3.93	6.42	2.95
0.75	5.7	3.88	6.33	3.25
0.875	5.6	3.83	6.30	3.42
1	5.3	3.82	6.26	3.78

---

Fig. 2a shows the powder XRD patterns of ZCS nanocrystals, standard diffraction patterns of hexagonal wurtzite ZnS (JCPDS Card no. 36-1450) and CdS (JCPDS Card no. 41-1049). The diffraction peaks broadening at high Zn content reveals smaller diameter of the nanorods as observed in Fig. 1. The patterns of ZCS nanocrystals show a hexagonal wurtzite conformation throughout the range of compositions, in contrast to the phase transition from cubic zinc-blende to hexagonal wurtzite structure with the increase of Cd content in ZCS nanocrystals, which synthesized at high temperature.¹⁷ The characteristic XRD patterns of the alloyed ZCS nanocrystals exhibit seven major diffraction peaks, which can be indexed to the (100), (002), (101), (102), (110), (103), and (112) planes of hexagonal wurtzite, respectively. It is found that these diffraction peaks of the ZCS nanocrystals systematically shift to higher angles with the increase of Zn content. The continuous peak shifting of the nanocrystals indicates the formation of alloyed ZCS nanocrystals rather than the mixture of ZnS and CdS or separated nucleation of ZnS or CdS in the ZCS nanocrystals. Because if the nanocrystals were a mixture of ZnS and CdS instead of pure alloyed ZCS nanocrystals, the resultant XRD spectra would be the superposition of the spectra of pure ZnS and pure CdS. According to the XRD data, the a-axis and c-axis values of ZCS nanocrystals were calculated. As shown in Fig. 2b, a linear compression in both the a and c axes is observed throughout the range of compositions (from x = 0.125 to x = 0.875), consistent with the expectations for Vegard's law behavior.^43,44 It can be seen that with the increase of Zn contents, the lattice parameters decrease due to the smaller Zn²⁺ cationic radius than that of Cd²⁺, which further confirms the formation of alloyed ZCS nanocrystals with homogeneous structure.

Fig. 2 (a) XRD patterns of ZCS nanorods.(b) The a and c values of the ZCS nanorods as the function of Zn content (x).

As expect, the alloyed ZCS nanocrystals with tunable composition display tunable optical properties. Fig. 3a clearly displays that the absorption edges of ZCS nanocrystals shift successively between those of CdS (514 nm) and ZnS (328 nm) with the change of Zn content. Such a large shift in the absorption edge should attribute to the change of composition rather than the quantum size effect, because the size of ZCS nanocrystals is larger than their Bohr radius (2.4–2.9 nm).¹² The peak wavelength of the PL spectra of ZCS nanocrystals was blue-shifted from 506 to 388 nm with increase of Zn content as shown in Fig. 3b, which is similar phenomenon to the absorption spectra. The observed shift of the absorption onset and emission maximum to shorter wavelength with the increase of Zn content further confirms the formation of ZCS solid solution via intermixing ZnS with CdS nanocrystals, rather than forming separate CdS and ZnS nanoparticles or core–shell structure CdS/ZnS, which is in agreement with the results of the XRD measurements.

Fig. 3 (a) UV-vis absorption spectra and (b) PL spectra of ZCS, ZnS and CdS.

The optical band gaps of ZCS nanorods calculated according to the Kubelka–Munk (KM) method⁴⁵ through the absorption spectra are plotted vs. Zn content x and displayed in Fig. 4. The calculated band gaps of the hexagonal CdS and ZnS nanocrystals are 2.41 and 3.78 eV, respectively, which are in agreement with those of their corresponding bulk materials (2.4 and 3.68 eV). The alloyed ZCS nanorods exhibit tunable band gaps between those of CdS and ZnS. But it is observed that variation of the band gaps deviates slightly from linear dependence, which has also been reported as a general character for many alloy nanocrystals, such as ZnS_xSe_1−x,⁴⁶ Zn_xCd_1−xSe,⁴⁷ and (CuInS₂)_x(ZnS)_1−x.²⁷ The band gaps of the alloy nanocrystals E_g(x) vs. Zn content x can be expressed by the equation:

E_g(x) = xE^ZnS_g + (1 − x)E^CdS_g − bx(1 − x) (1)

where E^ZnS_g and E^CdS_g are the band gaps of ZnS and CdS, respectively; x is molar fraction of the ZnS in the alloyed nanocrystals; and b is bowing parameter that characterizes the deviation from the linear relation between band gap and composition. The red line in Fig. 4 shows the fit to the experimental data points derived from the absorption data, which is a simple quadratic form:

E_g(x) = 2.45 + 0.31x + 0.98x² (2)

Fig. 4 Band gap energy as a function of Zn content (x) for ZCS, CdS and ZnS.

The coefficient of the quadratic term of the eqn (2) is the bowing parameter b in eqn (1) and has a value of 0.98 eV. The bowing parameter b is smaller than other solid solution such as (CuInS₂)_x(ZnS)_1−x with larger b than 2 eV,²⁷ because the cationic elements of Zn and Cd are from the same group and the charge-transfer effect as well as the volume deformation contribution are relatively small.

The transient photocurrent response of ZCS nanorods with different band gaps were investigated for five ON–OFF cycles under visible-light irradiation. Fig. 5a indicates that all samples show obvious photoresponse when the light turns on, which decrease rapidly to nearly zero as soon as the light turns off. The switch in these two states is quick and reversible, exhibiting a good reproducibility. Initially, with the increase of Zn content (x), the photocurrent density of ZCS nanorods increases from 0.23 mA cm⁻² for Zn_0.125Cd_0.875S to the maximum value of 1.01 mA cm⁻² for Zn_0.5Cd_0.5S and then sharply decreases to 0.04 mA cm⁻² for Zn_0.875Cd_0.125S, indicating that ZCS nanocrystals with different Zn content (x) have different photoresponse capacity. It is well known that when light with energy higher than the band gap is absorbed, the semiconductor will create photoinduced electron–hole pairs. The photocurrent density depends on absorption of the incident light and the charge transportation and separation.⁴⁸ The effect of Zn content is mainly attributed to the effects of Zn/Cd ratio on the band gaps and shape of ZCS, essentially the efficiency of incident light absorption and separation of the photogenerated electron–hole pairs. At lower x value, the visible light harvesting capacity of ZCS nanorods is stronger but the aspect ratio of ZCS nanorods is lower (Table 1). Many reports have showed that photocurrent and photocatalytic activity were improved by increasing the aspect ratio of 1D structure semiconductor.^49,50 High aspect ratio can enhance the separation of photogenerated electrons and holes by increasing delocalization of electrons in the 1D nanostructure. At larger x value, although the aspect ratio become higher than lower x value but ZCS with larger band gaps induces weaker visible light harvesting capacity as shown in Fig. 3a. As a result, the photocurrent densities of visible light decrease. Zn_0.5Cd_0.5S nanorods with suitable band gap and aspect ratio display the highest photoresponse. Impressively, the photocurrent density of Zn_0.5Cd_0.5S is even 25 times as high as that of Zn_0.875Cd_0.125S (Fig. 5b), suggesting a more efficient visible light harvesting and separation of photoinduced electron–hole pairs for the Zn_0.5Cd_0.5S nanorods. The high photoresponse capacity makes the ZCS nanorods promising applications in photodetector device, photovoltaic solar cells, and photocatalysis.

Fig. 5 (a) Transient photocurrent response of ZCS nanorods and (b) photocurrent density as a function of Zn content (x).

Based on the above photoresponse results, Zn_0.5Cd_0.5S nanorods were chosen as visible-light photocatalyst for the following photocatalysis study. In order to improve the stability of 1D Zn_0.5Cd_0.5S nanorods during photocatalytic reaction as well as improve separation rate of photoinduced electrons and holes, graphene was chosen as co-catalyst due to it 2D structure and excellent conductivity. The morphology of the Zn_0.5Cd_0.5S/RGO nanocomposites treated by acetic acid was analyzed by TEM. Lower magnification TEM images as shown in Fig. S1† indicate that Zn_0.5Cd_0.5S nanorods are prone to flock together during the evaporation of solvent. But Zn_0.5Cd_0.5S nanorods in Zn_0.5Cd_0.5S/RGO nanocomposites can keep good dispersity, because they are anchored on the RGO sheets tightly. The deep black parts in Fig. S1b† are caused by the overlap of RGO sheets embedded with Zn_0.5Cd_0.5S nanorods. The diameter and length distribution of Zn_0.5Cd_0.5S nanorods almost unchanged after formation Zn_0.5Cd_0.5S/RGO nanocomposites (Fig. S1†). Fig. 6a further shows that after the acid treated ZCS nanorods still maintain the monodisperse 1D structure on RGO nanosheets. Energy dispersive spectroscopy (EDS) analysis of this region, given in the inset of Fig. 6a confirms the existence of Zn and Cd with approximately 1 : 1 ratio implying its stoichiometry, Zn_0.5Cd_0.5S. HRTEM image of the typical Zn_0.5Cd_0.5S nanorods presents clearly lattice fringe, indicating the single crystal nature of Zn_0.5Cd_0.5S nanorods in Zn_0.5Cd_0.5S/RGO nanocomposites (Fig. 6b).

Fig. 6 (a) TEM and (b) HRTEM images of Zn_0.5Cd_0.5S/RGO nanocomposites. Inset in image (a) is the EDX spectrum.

The photocatalytic activities of the resulting Zn_0.5Cd_0.5S/RGO nanocomposites were evaluated by degradation of MB aqueous solution under visible light irradiation. Temporal changes in the concentration of MB were monitored by examining the variations in maximal absorption in UV-vis spectra at 664 nm. Fig. 7a shows the result of MB degradation in the presence of different samples. The Zn_0.5Cd_0.5S/2 wt% RGO sample has the best photocatalytic activity. After 60 min irradiation, the solution was nearly colorless and the value of C/C₀ was near to zero. Less or more RGO in the composite induce lower photocatalytic efficiency, which indicates that the concentration of RGO had a significant effect on the dispersion of Zn_0.5Cd_0.5S nanorods, absorbing photons, recombination rate of photogenerated electrons and holes. With the increase of RGO in the composite, more free Zn_0.5Cd_0.5S nanorods were loaded uniformly on RGO nanosheets and more line-to-line interface formed, which enhanced the separation of photogenerated electrons and holes. An excess amount of RGO in the composite can shield the incident light, thus preventing the generation of electrons from the photocatalyst below. As a result, the photodegradation rate decreases. Fig. 7b clearly shows the decomposition of MB molecular under visible light irradiation by using Zn_0.5Cd_0.5S/2 wt% RGO as photocatalyst. After exposure to visible light for only 10 min, 64% of MB had decomposed, and after 90 min that value had increased to larger than 98%. That is to say, less than 90 mg of Zn_0.5Cd_0.5S/2 wt% RGO photocatalyst can result in almost completely decomposing of 1 mg MB in 90 min under visible light irradiation. In order to investigate the stability of the Zn_0.5Cd_0.5S/2 wt% RGO in the process of photocatalysis, five successive cyclic MB degradation tests were carried out. From the results shown in Fig. 8, MB degradation efficiency of this nanocomposites material does not show noticeable decease after five successive cycles, indicating the high stability of Zn_0.5Cd_0.5S/2 wt% RGO photocatalyst.

Fig. 7 (a) Photocatalytic degradation of MB solution in the presence of Zn_0.5Cd_0.5S/RGO with different amount of RGO. (b) Absorption spectra of MB solution at different time under visible light irradiation using Zn_0.5Cd_0.5S/2% RGO as photocatalyst.

Fig. 8 Recycling performances of the Zn_0.5Cd_0.5S/2% RGO as photocatalyst for 5 cycles.

For visible light-driven photocatalysis, the efficiency of visible light harvest and separation of the photogenerated electron–hole pairs are crucial factors. Comparing the UV-vis spectra of Zn_0.5Cd_0.5S/RGO with Zn_0.5Cd_0.5S nanorods, it can be found that the visible light adsorption intensity increase after the addition of RGO, indicating the enhanced efficiency of visible light harvest of Zn_0.5Cd_0.5S/RGO nanocomposites (Fig. S2†). Furthermore, the electrochemical impedance spectroscopy (EIS) technique was performed to evaluate the migration resistance of the photogenerated electrons in the material (Fig. S3†). The results of the electrodes prepared by Zn_0.5Cd_0.5S nanorods and Zn_0.5Cd_0.5S/RGO exhibit a semicircle in the high-frequency region and a straight line in the low-frequency region. The presence of RGO in Zn_0.5Cd_0.5S/RGO nanocomposites obviously shortened the semicircle compared to Zn_0.5Cd_0.5S nanorods, suggesting a drastic decrease of the electron migration resistance within the Zn_0.5Cd_0.5S/RGO nanocomposites, which will effectively avoid the recombination of the photogenerated electrons and holes. Based on the above results, a tentative photocatalytic mechanism has been proposed and schematically illustrated in Fig. 9. Firstly, the uniform dispersion of Zn_0.5Cd_0.5S nanorods on RGO prevents the aggregation of Zn_0.5Cd_0.5S nanorods during photocatalytic reaction. When the well dispersed Zn_0.5Cd_0.5S nanorods are irradiated by visible light, electrons and holes generate from the VB of Zn_0.5Cd_0.5S. An enhanced separation rate of photogenerated electrons and holes is achieved by the fast transfer of the photogenerated electrons from the CB of Zn_0.5Cd_0.5S to RGO through the line-to-line interface between 1D Zn_0.5Cd_0.5S nanorods and 2D RGO nanosheets. The photogenerated electrons on the surface of RGO nanosheets and holes on the surface of Zn_0.5Cd_0.5S nanorods can react with H₂O and O₂, respectively, which generate highly reactive hydroxyl radicals (˙OH) and superoxide anion radicals (˙O₂⁻) that can degrade MB molecules into CO₂ and H₂O. Above results demonstrate that tailoring the morphologies of photocatalyst, choosing proper co-catalyst and rational design interface between photocatalyst and co-catalyst are important ways for the rational design of novel high efficient photocatalysts.

Fig. 9 A tentative photocatalytic mechanism for MB degradation by photocatalysis using the Zn_0.5Cd_0.5S/RGO nanocomposites.

Conclusions

Hexagonal ZCS ultrafine nanorods were successfully synthesized by a facile one-pot method. The lattice parameter, aspect ratio, and band gap of ZCS nanorods could be adjusted simply by controlling the molar ratio of precursors. Zn_0.5Cd_0.5S nanorods with suitable band gap and aspect ratio display the highest photoresponse, 25 times higher than that of Zn_0.875Cd_0.125S. The Zn_0.5Cd_0.5S/RGO nanocomposites with a RGO content of 2.0 wt% showed the highest photocatalytic activity for the degradation of methylene blue (MB), due to the uniform dispersion of Zn_0.5Cd_0.5S nanorods on RGO and the enhanced separation rate of photoinduced electrons and holes by fast transfer of the photogenerated electrons through the line-to-line interface between 1D ZCS nanorods and 2D RGO nanosheets. The high photoresponse capacity and high photocatalytic activity for the degradation of dyes make the ZCS nanorods promising for applications in photodetector device, photovoltaic devices, and photocatalysis.

Acknowledgements

The authors gratefully acknowledge the financial support of NSFC (21101166, 51272157, 51472160), Key Basic Research Program of Shanghai Municipal Science and Technology Commission (13NM1401102), Innovation Program of Shanghai Municipal Education Commission (14YZ084), the Hujiang Foundation of China (B14006), and the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning.

Notes and references

1. J. O. Winter, T. Y. Liu, B. A. Korgel and C. E. Schmidt, Adv. Mater., 2001, 13, 1673–1677.
2. S. Santra, H. Yang, J. T. Stanley, P. H. Holloway, B. M. Moudgil, G. Walterd and R. A. Mericle, Chem. Commun., 2005, 3144–3146.
3. W.-S. Song and H. Yang, Chem. Mater., 2012, 24, 1961–1967.
4. M. Shalom, S. Ruhle, I. Hod, S. Yahav and A. Zaban, J. Am. Chem. Soc., 2009, 131, 9876–9877.
5. X. Chen, S. Shen, L. Guo and S. S. Mao, Chem. Rev., 2010, 110, 6503–6570.
6. M. G. Bawendi, M. L. Steigerwald and L. E. Brus, Annu. Rev. Phys. Chem., 1990, 41, 477–496.
7. A. P. Alivisatos, Science, 1996, 271, 933–937.
8. Z. Zhuang, Q. Peng and Y. Li, Chem. Soc. Rev., 2011, 40, 5492–5513.
9. J. Yang, J. H. Zeng, S. H. Yu, L. Yang, G. E. Zhou and Y. T. Qian, Chem. Mater., 2000, 12, 3259–3263.
10. H. Wei, S. Chen, X. Ren, B. Qian, Y. Su, Z. Yang and Y. Zhang, CrystEngComm, 2012, 14, 7408–7414.
11. X. Zhong, Y. Feng, W. Knoll and M. Han, J. Am. Chem. Soc., 2003, 125, 13559–13563.
12. Y. Li, M. Ye, C. Yang, X. Li and Y. Li, Adv. Funct. Mater., 2005, 15, 433–441.
13. Y. Yu, J. Zhang, X. Wu, W. Zhao and B. Zhang, Angew. Chem., Int. Ed., 2012, 51, 897–900.
14. W. Zhang, X. Zhou and X. Zhong, Inorg. Chem., 2012, 51, 3579–3587.
15. S. Sadhu and A. Patra, J. Phys. Chem. C, 2012, 116, 15167–15173.
16. J. Wang, P. Yang, J. Zhao and Z. Zhu, Appl. Surf. Sci., 2013, 282, 930–936.
17. Q. Li, H. Meng, P. Zhou, Y. Zheng, J. Wang, J. Yu and J. Gong, ACS Catal., 2013, 3, 882–889.
18. H. Wei, X. Ren, Z. Han, T. Li, Y. Su, L. Wei, F. Cong and Y. Zhang, Mater. Lett., 2013, 102–103, 94–97.
19. S. Bhandari, R. Begum and A. Chattopadhyay, RSC Adv., 2013, 3, 2885.
20. H. Wei, Y. Su, Z. Han, T. Li, X. Ren, Z. Yang, L. Wei, F. Cong and Y. Zhang, Nanotechnology, 2013, 24, 235706–235707.
21. C. Sun, J. S. Gardner, G. Long, C. Bajracharya, A. Thurber, A. Punnoose, R. G. Rodriguez and J. J. Pak, Chem. Mater., 2010, 22, 2699–2701.
22. Y. H. A. Wang, X. Zhang, N. Bao, B. Lin and A. Gupta, J. Am. Chem. Soc., 2011, 133, 11072–11075.
23. I. Tsuji, H. Kato, H. Kobayashi and A. Kudo, J. Am. Chem. Soc., 2004, 126, 13406–13413.
24. T. Torimoto, S. Ogawa, T. Adachi, T. Kameyama, K. Okazaki, T. Shibayama, A. Kudo and S. Kuwabata, Chem. Commun., 2010, 46, 2082–2084.
25. T. Torimoto, T. Adachi, K.-i. Okazaki, M. Sakuraoka, T. Shibayama, B. Ohtani, A. Kudo and S. Kuwabata, J. Am. Chem. Soc., 2007, 129, 12388–12389.
26. B. Chen, H. Zhong, W. Zhang, Z. Tan, Y. Li, C. Yu, T. Zhai, Y. Bando, S. Yang and B. Zou, Adv. Funct. Mater., 2012, 22, 2081–2088.
27. D. Pan, D. Weng, X. Wang, Q. Xiao, W. Chen, C. Xu, Z. Yang and Y. Lu, Chem. Commun., 2009, 4221–4223.
28. W. Zhang and X. Zhong, Inorg. Chem., 2011, 50, 4065–4072.
29. T. Pons, E. Pic, N. Lequeux, E. Cassette, L. Bezdetnaya, F. Guillemin, F. Marchal and B. Dubertret, ACS Nano, 2010, 4, 2531–2538.
30. H. Yan, J. Yang, G. Ma, G. Wu, X. Zong, Z. Lei, J. Shi and C. Li, J. Catal., 2009, 266, 165–168.
31. D. Lu, T. Takata, N. Saito, Y. Inoue and K. Domen, Nature, 2006, 440, 295.
32. H. Bai, C. Li and G. Shi, Adv. Mater., 2011, 23, 1089–1115.
33. X. Huang, X. Qi, F. Boey and H. Zhang, Chem. Soc. Rev., 2012, 41, 666–686.
34. J. Zhang, J. Yu, M. Jaroniec and J. R. Gong, Nano Lett., 2012, 12, 4584–4589.
35. Y. Zhang, H. Xu and Q. Wang, Chem. Commun., 2010, 46, 8941–8943.
36. Y. Zhang, Y. Du, H. Xu and Q. Wang, CrystEngComm, 2010, 12, 3658–3663.
37. S. Shen, Y. Zhang, L. Peng, B. Xu, Y. Du, M. Deng, H. Xu and Q. Wang, CrystEngComm, 2011, 13, 4572–4579.
38. S. Shen, Y. Zhang, L. Peng, Y. Du and Q. Wang, Angew. Chem., Int. Ed., 2011, 50, 7115–7118.
39. S. Shen, Y. Zhang, Y. Liu, L. Peng, X. Chen and Q. Wang, Chem. Mater., 2012, 24, 2407–2413.
40. S. Shen, A. Ma, Z. Tang, Z. Han, M. Wang, Z. Wang, L. Zhi and J. Yang, ChemCatChem, 2015, 7, 609–615.
41. T. Jin, S. Guo, J. L. Zuo and S. Sun, Nanoscale, 2013, 5, 160–163.
42. Y. Lee, G. Lee, J. Shim, S. Hwang, J. Kwak, K. Lee, H. Song and J. Park, Chem. Mater., 2006, 18, 4209–4211.
43. X. Zhong, M. Han, Z. Dong, T. J. White and W. Knoll, J. Am. Chem. Soc., 2003, 125, 8589–8594.
44. T. Ji, W. B. Jian and J. Fang, J. Am. Chem. Soc., 2003, 125, 8448–8449.
45. A. Hagfeldtt and M. Gratzel, Chem. Rev., 1995, 95, 49–68.
46. D. Mesri, Z. Dridi and A. Tadjer, Comput. Mater. Sci., 2007, 39, 453–456.
47. R. Venugopal, P. I. Lin and Y. T. Chen, J. Phys. Chem. B, 2006, 110, 11691–11696.
48. T. Zhai, L. Li, Y. Ma, M. Liao, X. Wang, X. Fang, J. Yao, Y. Bando and D. Golberg, Chem. Soc. Rev., 2011, 40, 2986–3004.
49. H. J. Yun, H. Lee, J. B. Joo, W. Kim and J. Yi, J. Phys. Chem. C, 2009, 113, 3050–3055.
50. J. Liu, O. Margeat, W. Dachraoui, X. Liu, M. Fahlman and J. Ackermann, Adv. Funct. Mater., 2014, 24, 6029–6037.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra01846c

---