Rong Chen, Kui Li, Xiao-Shu Zhu, Shuai-Lei Xie, Long-Zhang Dong, Shun-Li  Li* and Ya-Qian Lan*

The in-situ synthesis of ZnO-embedded Zn1-xCdxS/CdS heterostructure nanocrystals were  achieved using a simple surfactant-free hydrothermal route. The morphology and the content  of exposed active sites of Zn1-xCdxS/CdS could be facilely modulated by varying the particle  size of ZnS precursors, and the heterostructure with the smallest particle size showed the  largest photocatalytic activity of 22.99 mmol/h/g in Na2S + Na2SO3 solution. Furthermore, the  in-situ embedding of ZnO cocatalyst on the Zn1-xCdxS/CdS heterostructure dramatically  improved its photocatalytic activity to 84.17 mmol/h/g, which is 765 times higher than that of  CdS prepared by hydrothermal method. More significantly, the ZnO-embedded Zn1-xCdxS/CdS  heterostructure gave considerable H2 -production rate even in the absence of hole scavenger,  although the bare Zn1-xCdxS/CdS exhibited no hydrogen evolution, and it showed very  competitive photocatalytic activity in methanol in comparison with that in Na2S + Na2SO3 solution under the similarly alkaline circumstance. These investigations indicate that the  existence of ZnO cocatalyst in the heterostructure could not only effectively improve the  photocatalytic activity by suppressing the recombination of charge carriers and acting as  photocatalytic reaction sites, but also make it possible to adopt methanol as the sacrificial  reagent for sulfide photocatalyst.

Introduction  

Semiconductor nanocrystals have attracted much attention in  the areas of photocatalysis,1 and photoelectrochemistry (PEC)2 owing to their excellent photocatalytic and photoelectronic  properties.3 However, the limited visible light absorption and  rapid recombination of photo-generated charge carriers in  semiconductor photocatalysts restrict their practical  applications.4 Fabricating the sulfide heterostructure not only  facilitates the separation of charge carriers,5 but also improves  the visible light absorption due to their narrow band gap.6 However, the preparation of sulfide heterostructures usually  involves complex routes by loading one semiconductor on the  other one, which needs precise control of synthetic parameters  in each step.7 This method not only limits the scale-up synthesis of heterostructure,5 but also induces the formation of interfacial  problems.8 Moreover, the surface based characteristic of  photocatalytic water splitting reaction makes the content of  exposed active sites an important factor in the design of highperformance heterostructure photocatalyst. The organic  surfactants were extensively adopted to modulate the  morphology of the nanomaterials. Whereas, the surfactants  inevitably contaminate the heterostructures, and may  substantially impair the performance of the heterostructures.9 Therefore, searching for an efficient surfactant-free  hydrothermal method for the in-situ synthesis of sulfide  heteorstructure with improved amount of active sites is an  important subject from both theoretical and practical  viewpoints.

Cocatalysts, such as graphene, Pt, NiO and MoS2 could  facilitate the separation of photo-exited holes/electrons pairs,  and/or act as H2 -production sites.10 ZnO was widely used as  photocatalyst due to its high electron mobility and long photoexited lifetime.11 Notably, it was extensively demonstrated that  the oxygen vacancies in ZnO could work as electron traps to  accept electrons and inhibit the recombination of charge  carriers.12 The oxygen vacancies in ZnO could also act as the  favored sites for catalyzing the N-formylation reaction as  reported by Zhu’s group.13 Furthermore, it was demonstrated in  Zong’s investigation that the ZnO could function as cocatalyst  for improving the catalytic activity of combined reforminghygrogenolysis of glycerol.14 These unique features of ZnO  make it an excellent cocatalyst for improving the photocatalytic  activity of sulfide photocatalysts. On the other hand, it is well  known that the sulfide photocatalysts show an excellent photocatalytic activity only in the Na2S + Na2SO3 sacrificial  solution, which is toxic and not stable in the air condition.15 Unfortunately, the sulfide photocatalysts give the negligible  photocatalytic activity in the widely used hole scavenger, such  as methanol.16 If we could fabricate the hetero-nanomaterial  composed by ZnO and sulfide heterostructure, we can simultaneously take the advantages of the two components for  photocatalytic water splitting.17 This material maybe not only  improve the photocatalytic activity of sulfide photocatalysts in  the traditional Na2S + Na2SO3 solution, but also make it  possible for the utilization of methanol solution as sacrificial  reagent for sulfide photocatalysts. Up to date, both the  fabrication of fine grained ZnO-embedded sulfide  heterostructure and efficient method for improving the  photocatalytic activity in methanol sacrificial solution were  scarely reported.

In view of the aforementioned ideas, we select Zn1- xCdxS/CdS as the sulfide heterostructure to synthesize ZnOembedded fine-grained Zn1-xCdxS/CdS heterostructure using a  surfactant-free hydrothermal method based on the following  points: i) Zn1-xCdxS/CdS shows improved structure matching  and photocatalytic performance due to the tunable parameters,  and the excellent photocatalytic activity of Zn1-xCdxS;18 ii) ZnS  precursors with different morphologies were prepared by  changing the concentrations of NaOH, and the content of  exposed active sites of the Zn1-xCdxS/CdS heterostructure could  be facilely modulated by the varied particle sizes of ZnS  precursors; iii) the in-situ embedding of the well-dispersed ZnO  nanocrystals on the Zn1-xCdxS/CdS heterostructure was  obtained by the reaction between OH- and the residual Zn2+ derived from the cation exchange of ZnS with Cd2+ .

Herein, the Zn1-xCdxS/CdS heterostructures with modulated  morphologies were facilely fabricated through varying the  particle sizes of ZnS precursors by the implantation of OH- on  their surfaces. The photocatalytic activity of Zn1-xCdxS/CdS  heterostructure was found closely dependent on the exposed  active sites, and the heterostructure with the smallest particle  size shows the maximal H2 -evolution activity of 22.99  mmol/h/g. The in-situ synthesis of ZnO-embedded Zn1- xCdxS/CdS heterostructure was obtained by introducing  different contents of NaOH. The ZnO nanocrystal, serving as  cocatalyst to separate the charge carriers and acting as H2- production sites, shows tremendous enhancement on the  photocatalytic activity of Zn1-xCdxS/CdS heterostructure to  84.17 mmol/h/g, especially in the methanol solution. Notably,  the ZnO-embedded Zn1-xCdxS/CdS shows distinct  photocatalytic H2 -production of 0.18 mmol/h/g even in the  absence of sacrificial agent, although the Zn1-xCdxS/CdS shows  no hydrogen production. The content of exposed active sites  and the loading of ZnO cocatalyst on the heterojunction show  synergistic effect on the photocatalytic H2 -evolution activity.  

Experimental section

Materials

Cadmium nitrate tetrahydrate (Cd(NO3)2 ·4H2O), zinc chloride  (ZnCl2 ), thiourea (CH4N2S) and sodium hydroxide (NaOH) are  analytical grade, and used as received without further  purification.

Synthesis of ZnS precursors with different particle sizes  

The ZnS precursors were prepared using ZnCl2 , CH4N2S as  Zn, S sources via a hydrothermal method. NaOH was adopted  to modulate the particle size of ZnS. In a typical process, 3.0  mmol ZnCl2 and 6.0 mmol CH4N2S were dissolved in 25 mL  distilled water (solution A). Different contents of NaOH with  the molar ratios of NaOH/Zn equaling to 0, 1, 2, 4, 8, 10, 20  were dissolved in 5 mL distilled water (solution B). Then the  solution B was added into solution A slowly under ultrasound  for 0.5 h. The resultant solution was subsequently transferred  into 50 mL teflon-lined autoclave and maintained 180 oC for 21  h. The final white products were rinsed three times with  distilled water and ethanol respectively, and dried at 60 oC for  overnight in a vacuum oven to evaporate the solvent. The  obtained ZnS nanocrystals with different particle size were used  as precursor for the fabrication of the heterostructure samples.

Synthesis of the heterostructure samples with  modulated morphology by the different particle size of ZnS precursors

30 mg ZnS precursors (prepared with the molar ratios of  NaOH/Zn = 0, 1, 2, 4, 8, 10, 20) were dissolved in 10 mL  deionized water under ultrasound for a few minutes. Suitable  content of Cd(NO3)2 ·4H2O (Cd/(Zn+Cd) = 30 at%) were  dissolved in deionized water (2.8 mL) and then drop into the  aforementioned solution quickly under mild stirring. After  several minutes, the obtained solution was transferred into 15  mL autoclave and maintained 140 oC for 12 h. The final  products were rinsed with distilled deionized water and ethanol  for three times, respectively, and dried at 60 oC overnight in the  vacuum oven to evaporate the solvent.

In-situ synthesis of the ZnO cocatalyst

30 mg ZnS with the smallest particle size (prepared with the  NaOH/Zn molar ratio of 4) was dissolved in 8 mL deionized  water under ultrasound for a few minutes (Solution A). Suitable  content of Cd(NO3)2 ·4H2O (Cd/(Zn+Cd) = 30 at%) was  dissolved in deionized water (3 mL) (Solution B). Different  contents of NaOH with the ratio of NaOH/Cd equaling to 1, 5,  10, 15 and 20 were dissolved in 1 mL distilled water and then  dropped into the mixture solutions of A+B quickly under mild  stirring. After several minutes, the obtained solution was  transferred into 15 mL autoclave and maintained 140 oC for 12  h. The final yellow products were rinsed with distilled  deionized water and ethanol for three times respectively, and  dried at 60 oC overnight in the vacuum oven to evaporate the  solvent.

Characterization

The powder X-ray diffraction (XRD) patterns were recorded on  a D/max 2500 VL/PC diffractometer (Japan) equipped with  graphite monochromatized Cu Kα radiation (λ = 1.54060 Å).  Corresponding work voltage and current is 40 kV and 100 mA,  respectively. The transmission electron microscopy (TEM) and  high-resolution TEM (HRTEM) images were recorded on JEOL-2100F apparatus at an accelerating voltage of 200 kV.  Surface morphologies of the heterojunction materials were  examined by a scanning electron microscope (SEM, JSM- 7600F) at an acceleration voltage of 10 kV. The energydispersive X-ray spectroscopy (EDX) was taken on JSM- 5160LV-Vantage typed energy spectrometer. UV−visible  diffused reflectance spectra was recorded using a Cary 5000  UV-Vis spectrometer (Viarian, USA) with BaSO4 as a  reflectance standard. The Brunauer−Emmett−Teller (BET)  specific surface area (SBET), nitrogen adsorption of the  heterojunction samples were analyzed by an Autosorb-iQ  adsorption apparatus (Quantachrome instruments, USA). All of  the samples were degassed at 90 °C for 3 hours prior to  nitrogen adsorption measurements. The SBET was determined  by a multipoint BET method using adsorption data in the  relative pressure (P/P0 ) range of 0.05 − 0.3. Electrochemical  impedance spectra (EIS) measurements were carried out in  three-electrode system and recorded over a frequency range of  500 kHz-200 MHz with ac amplitude of 10 mV at 0.5 V. Na2S  (0.1 M) and Na2SO3 (0.02 M) mixture solution was used as the  supporting electrolyte. EIS data were recorded using  Electrochemical workstation (EC-lab, SP-150, VMP3-based  instruments, America) under a surface power density of about  0.1 mW/cm2 . Raman spectra of powder samples were obtained  on Lab-RAM HR800 with a laser excitation wavelength of  514.5 nm.

Photocatalytic Hydrogen Production

The photocatalytic H2 -production experiments were performed  via a photocatalytic H2 -production activity evaluation system  (CEL-SPH2N, CEAULight, China) in a 300 mL Pyrex flask,  and the openings of the flask were sealed with silicone rubber  septum. A 300 W xenon arc lamp through a UV-cutoff filter  with a wavelength range of 420−1000 nm, which was  positioned 13 cm away from the reaction solution, was used as  a visible light source to trigger the photocatalytic reaction. The  focused intensity on the flask was ∼200 mW·cm−2, which was  measured by a FZ-A visible-light radiometer (CEL-SPH2N,  CEAULight, China). In a typical photocatalytic H2 -production  experiment, 5 mg of the as-prepared photocatalyst was  suspended in 100 mL of mixed aqueous solution containing  Na2S (0.35 M) and Na2SO3 (0.25 M). Before irradiation, the  system was vacuumed for 5 min via the vacuum pump to  completely remove the dissolved oxygen and ensure the reactor  was in an anaerobic condition. A continuous magnetic stirrer  was applied at the bottom of the reactor to keep the  photocatalyst particles in suspension during the experiments. H2 content was analyzed by gas chromatography (GC-7900,  CEAULight, China). All glasswares were carefully rinsed with  distilled water prior to usage. 

Results and discussion  

Zn1-xCdxS/CdS heterostructures with adjustable  morphologies were accessible by varying the particle sizes of  ZnS precursors. 19 Scheme 1 illustrates the fabrication route for  modulating the morphologies of ZnS precursors, the Zn1- xCdxS/CdS heterostructure and ZnO-embedded Zn1-xCdxS/CdS  heterostructure. The ZnS precursors possessing different  particle sizes were firstly fabricated, where the hydroxide plays  an indispensable role in the formation of the fine-grained ZnS  precursor by the repulsive interaction among ZnS  nanoparticles.20

The morphologies of Zn1-xCdxS/CdS heterostructures with  the optimal content of Cd could be facilely modulated by the  different particle sizes of ZnS precursors (Fig. S1).21 The Zn2+ derived from the cation exchange between ZnS and Cd2+ reacted with the superfluous hydroxyl ion and produced  [Zn(OH)4] 2-, then immediately transformed into the ZnO,22 and  the ZnO-embedded Zn1-xCdxS/CdS heterostructure was formed.  The Zn1-xCdxS/CdS heterostructure samples derived from the  ZnS precursors with different particle sizes were abbreviated as  ZACm (m =0, 1, 2, 4, 10, 20, m equals to the molar ratio of  NaOH/Zn in preparation of ZnS). As can be observed from  SEM images, the adoption of NaOH leads to the decrease of  average diameter of ZnS nanoparticles from 1.5 µm to around  20 nm (Fig. 1a, b). Significantly, the morphologies of Zn1- xCdxS/CdS heterostructure samples were closely dependent on  that of ZnS precursors: the heterostructure derived from the  ZnS with smaller particle sizes shows the higher porous  morphology. (Fig. 1c, d and Fig. S2).

Characterization of the phase structure via XRD patterns (Fig.  1e) indicates that the modulated morphology of heterostructure  samples resulting from the different particle sizes of ZnS show  tremendous effect on the crystallinity of the heterostructure.  Compared with ZAC0, ZAC4 possesses much stronger  diffraction peaks corresponding to CdS and Zn1-xCdxS phases  due to its smaller particle size, and larger content of heterostructure active sites confirmed by the largest amount of  Cd from the electron dispersive X-ray spectrum (EDX) results  (Fig. S3). The S, Zn and Cd distribute uniformly in the whole  matrix of the nanoparticles (Fig. S4). Effect of the as-prepared  heterostructure samples on their UV-Vis diffuse reflection  spectra and band structure is displayed in Fig. 1f. Notably, the  spectra of the heterostructure samples show two band edges,  and the entire diffuse reflection spectra could be divided into  region I and II, which are corresponding to absorption in the  UV and visible regions, respectively.18 Moreover, the  heterostructure samples with smaller particle size possess the  larger bandgap (Eg ) in the region II (inset of Fig. 1f), which  may be attributed to the quantum effect of CdS.18