Hydrothermal synthesis of a uniform sub-micrometer-spherical Zn_0.83Cd_0.17S photocatalyst with high activity for photocatalytic hydrogen production

Abstract

In this work, a series of Zn_0.83Cd_0.17S with high photocatalytic activity were hydrothermally synthesized and the effects of the hydrothermal temperature on the structural, chemical, morphological properties of the samples were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), particle size analyzer and X-ray photoelectron spectroscopy (XPS), respectively. The photoabsorption properties were measured using a UV-vis diffused reflectance spectrophotometer and the photocatalytic activities of the samples for hydrogen production were evaluated under 300 W Xe lamp irradiation. The results show that the sub-micro sized spherical Zn_0.83Cd_0.17S particles are uniform and mainly composed of cubic zinc-blende phase. An increase of temperature improves the crystallinity of the samples and the ratio of Cd and Zn in the solid solution and consequently makes the absorption edges gradually shift monotonically to longer wavelengths. Also, the hydrothermal temperature influences the particle size and distribution of sulfide solid solution. The sample synthesized at 160 °C exhibits the best photocatalytic activity for H₂ evolution with a hydrogen production rate of 45.97 mmol h⁻¹ g⁻¹ when the amount of the sample is 0.03 g in a 200 ml aqueous solution containing 0.35 M Na₂S and 0.25 M Na₂SO₃.

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1. Introduction

As a clean and environmentally-friendly energy, hydrogen energy seems to be one of the most promising ways to solve the sustainable energy requirements and environmental contamination problems. Solar driven photocatalysis on semiconductors to produce hydrogen is widely considered to be a potential route and therefore, the development of visible-light driven photocatalysts has attracted more and more interest in recent years.^1–4 Metal sulfides are well-known semiconductor photocatalysts for hydrogen production from aqueous solutions containing various sacrificial reagents. CdS is one of the most frequently studied metal sulfide photocatalysts with a narrow band gap of 2.4 eV that allows the absorption of visible light.^5,6 Unfortunately, CdS is apt to photocorrosion in the process of photocatalytic reactions.^7,8 ZnS is another important semiconductor material with a wide band gap, which is too large for a visible light response, but has excellent stability. Integrating the advantages of these two sulfides, Zn_xCd_1−xS solid solution is, therefore, expected to have both the enhanced photocatalytic activities for hydrogen production due to the controlled band gap^9–12 and improved stability. Many efforts have been made to synthesize Zn_xCd_1−xS solid solution and even further to improve the photocatalytic activity of the solid solution by loading noble metals,^13,14 constructing the heterojunction^15,16 and sensitization¹⁷ as well.

A variety of methods have been established to synthesize Zn_xCd_1−xS solid solution recently, including solid state reactions,¹⁸ cation-exchange reactions,¹⁹ microwave syntheses,²⁰ self-assembly approach,²¹ solvothermal method.²² Among the various fabrication methods, solvothermal synthesis was regarded as a promising route for its simplicity, low cost, high efficiency and good crystallization of the products.^23,24 In the solvothermal process, different solvents have been reported to synthesize Zn_xCd_1−xS solid solution, such as dimethyl sulfoxide (DMSO),²⁵ ethylene glycol,²⁶ and ethanol.²⁷ However, the above organic solvents are not better choices due to their toxicity and great harm to the environment. On the contrary, the hydrothermal synthesis with the water as the solvent has attracted more and more attention in these years.⁴

In our previous study, Zn_xCd_1−xS has been synthesized in the glycol solvent under 140 °C and among them, Zn_0.83Cd_0.17S showed the best photocatalytic hydrogen production activity with the rate of 4.02 mmol h⁻¹ g⁻¹.^28,29 However, the samples presented the poor crystallization in composition and two absorption edges in UV-vis diffuse reflectance spectrum. In this paper, we selected water as solvent to synthesize a series of Zn_0.83Cd_0.17S photocatalysts. The effects of temperature on the structural, chemical, morphological and optical properties of the Zn_0.83Cd_0.17S solid solution were investigated in detail. Also, the photocatalytic activities of the samples for the hydrogen production from water splitting were evaluated, the results show that Zn_0.83Cd_0.17S photocatalysts synthesized at 160 °C in this work exhibits the best photocatalytic activity for H₂ evolution with an initial rate of 45.97 mmol h⁻¹ g⁻¹, which is almost 11.4 times higher than that in the glycol in our previous work.³⁰

2. Experimental details

2.1 Synthesis of the samples

The chemicals used in this study were all analytical reagent grade and used without further purification. The Zn_0.83Cd_0.17S crystals were synthesized by a hydrothermal process (Zn_0.83Cd_0.17S is not the real chemical formula, which only presents the ratio of Zn²⁺ and Cd²⁺ during the synthesis process for convenience). Zn(Ac)₂·2H₂O and Cd(Ac)₂·2H₂O (total amount of Cd²⁺ and Zn²⁺ = 6 mmol) with the molar ratio of 5 : 1 were dissolved in 25 ml deionized water. After the complete dissolution, the mixture dripped to the thiourea aqueous solution (60 mmol thiourea dissolved in 25 ml deionized water) under continuous stirring and then the mixed solution was added into the Teflon-lined stainless steel autoclave. Before the hydrothermal reaction, the mixed solution was purged with N₂ for 5 min in order to evacuate the air in the solution, and then the autoclave was sealed and maintained in an oven at different temperatures (120, 140, 160, 180 °C) for 12 h with an average heating rate of hydrothermal of 3.5 °C min⁻¹ and cooled down to room temperature. After the hydrothermal reaction, the samples were centrifugally separated, rinsed with deionized water and washed with ethanol several times and dried at 70 °C for 12 h.

2.2 Characterization of the samples

The X-ray diffraction (XRD) patterns were obtained on an X-ray diffractometer (D8 A25ADVANCE, Bruker, Germany) using Cu Kα radiation to determine the crystal phase of the samples. Morphologies of the samples were observed using scanning electron microscopy (SEM; S-4700, Hitachi, Japan) and transmission electron microscopy (TEM; Tecnai G2 F30, FEI, U.S.A). Particle sizes were determined by a particle size analyzer using Malvern Masterizer 2000 laser diffraction equipment (Masterizer 2000, Malvern Instruments, Ltd., Malvern, UK). X-ray photoelectron spectroscopy (XPS) were measured by a Phi5400 spectroscopy (ESCA system, U.S.A.) using a monochromatic Al Kα X-ray source (1486.6 eV) operating at 15 kV. The binding energy was calibrated by taking C1s peak at 284.6 eV as reference. The samples were analyzed in a nitrogen adsorption apparatus (3H-2000PSI, Beishide, China.). The UV-vis diffused reflectance spectra of the samples were recorded from a spectrophotometer (UV-2450; Shimadzu, Japan) with an integrating sphere attachment ranging from 240 to 800 nm, and BaSO₄ was used as the reflectance standard.

2.3 Photocatalytic H₂ production on the samples

The photocatalytic reaction was conducted in a closed glass circulation system, the schematic diagram of the experimental set-up is shown in ESI Fig. S1.† The entire reaction process was irradiated using a 300 W Xe lamp (PLS-SXE300, Perfectlight, China). The emission spectrum of the Xe lamp in ESI Fig. S1† illustrates that the main wavelength range with the strongest irradiation is between 250 and 800 nm. The Zn_0.83Cd_0.17S photocatalyst with different amounts was dispersed in a 200 ml aqueous solution containing 0.35 M Na₂S and 0.25 M Na₂SO₃ in a 250 ml quartz reaction cell which was 5 cm away from the Xe lamp. The solution was continuously stirred with a magnetic stirrer. During the irradiation, the amount of H₂ was analyzed using gas chromatography (SP2100A, Beifen instrument, China) equipped with thermal conductivity detection (TCD).

3. Results and discussion

3.1 Crystal structure and morphology

In general, all the samples are yellow in color. However, the sample's luminance is suddenly improved when the temperature is 180 °C, which is becoming luminous yellow. Fig. 1 shows that all the samples have three distinct diffraction peaks at 2θ angles of 28.6, 47.6 and 56.3, which matches perfectly with the (111), (220) and (311) of crystalline planes of cubic zinc-blende phase.^31,32 Increasing the temperature, the peaks become sharper and narrower indicating an improvement of crystallization of the samples. It also can been seen that the diffraction peaks of samples corresponding to the sulfide solid solution, were gradually shifted to small angles with the increase of temperature. According to the Bragg equation, 2d sin θ = nλ, the decrease of 2θ means the decrease of sin θ and accordingly the increase of interplanar crystal spacing (d). Due to the radii of Zn²⁺ (0.74 Å) is smaller than that of Cd²⁺ (0.97 Å), it is believed that Cd²⁺ insert into the ZnS lattice and formed the sulfide solid solution, which, meantime, leads to the increase of the d value and the lattice expansion of the materials. Besides, the increase of the d value also illustrates the increase of Cd content in the solid solution, which is bound to influence the photoabsorpion and photocatalytic property of the materials.

Fig. 1 XRD patterns of the samples synthesized at different temperatures for 12 h. (a) 120 °C (b) 140 °C (c) 160 °C and (d) 180 °C.

Moreover, a small peak (★) appearing at 27.13° matches with JCPDS card number (89-2171) is indexed to ZnS, which means that the sample synthesized at 180 °C is composed of sulfide solid solution and zinc sulfide. This may be due to the quicker crystal nucleus formation rate of ZnS than that of sulfide solid solution under the higher temperature. Therefore, the sample synthesized at 180 °C is not homogeneous. Quite oppositely, the homogeneous sulfide solid solutions can be formed when the hydrothermal temperature is no more than 160 °C.

To further understand the properties of the samples, the elemental composition and valence states of the samples were probed by XPS. The spectra of the overall survey scan of the samples indicates the presence of Zn, Cd, and S, along with O and C (seen in ESI Fig. S2†). The presence of very small amounts of C and O₂ in the spectrum is due to the carbon tape used for measurement and the gaseous molecules such as CO₂ and H₂O in the environment, respectively. Fig. 2 shows the high-resolution XPS spectra for Zn 2p, Cd 3d and S 2p of the samples synthesized at different temperatures. In the Zn 2p spectra, two peaks of Zn 2p_3/2 and Zn 2p_1/2 at approximately 1022.3 eV and 1045.4 eV (Fig. 2a) are attributed to the existence of Zn²⁺, which exactly matches the binding energy of Zn–S.^33,34 The peaks of Cd 3d_5/2 and Cd 3d_3/2 (Fig. 2b) are centered at 405 eV and 411.8 eV, indicating the presence of the Cd–S bonding.³⁵ The Zn 2p and Cd 3d peaks at binding energies are attributed to Zn_xCd_1−xS molecular environment, illustrating that the samples are the sulfide solid solution.²² The binding energies of S 2p_3/2 and S 2p_1/2 (Fig. 2c) are best fitted with two peaks at around 161.5 eV (monosulfide) and 162.5 eV (predominant disulfide anion).^35,36 Moreover, the binding energies of Zn 2p and Cd 3d are decreased with the hydrothermal temperature increasing from 120 to 160 °C, and increased when the temperature is 180 °C. These slight changes are probably related to the increase of the content of Cd in the solid solution and the formation of the second phase ZnS in the materials. Meanwhile, a regular decrease of the binding energy in the XPS spectra of S 2p with the increase of temperature is observed.

Fig. 2 High-resolution XPS spectra of the samples synthesized at different temperatures: (a) Zn 2p, (b) Cd 3d and (c) S 2p.

The morphology of the samples synthesized at different hydrothermal temperatures was investigated through SEM and TEM. ESI Fig. S3† and 3 are the SEM and TEM analysis of the samples synthesized at different temperatures, which shows that all the samples are uniform, spherical and sub-micro sized. TEM image in Fig. 3c presents that the synthesized sub-micrometer particles are about 180–200 nm in diameter and the HRTEM image in Fig. 3d shows clearly lattice fringes, suggesting a well-defined crystal structure. The fringes with lattice spacing of 0.322 and 0.198 nm correspond to the cubic (111) and (220) plane of the sulfide solid solution in XRD analysis. The associated selected electron diffraction (SAED; the inset in Fig. 3d) shows ring patterns, which is typical of polycrystalline material.

Fig. 3 SEM of the samples synthesized at (a) 140 °C, (b) 160 °C, TEM (c), HRTEM image and the associated SEAD pattern (d and its inset) of the sample synthesized at 140 °C for 12 h.

3.2 Sub-micrometer particle size and distribution

The effects of the hydrothermal temperature on particle size and distribution of the samples were obtained by the laser particle size analyzer, with the results shown in Fig. 4 and Table 1. Number weighted particle size distributions of powders were measured and the diameters corresponding to the cumulative number under 10, 50 and 90% are reported as the D₁₀, D₅₀ and D₉₀, respectively. Agglomeration ratio D₅₀ is considered to be the average median diameter which is representative of the degree of powder cohesiveness. The value of D₅₀ firstly increase and then decrease with the increase of hydrothermal temperature and there is a dramatical reduction of D₅₀ when the temperature reaches 180 °C. Also, D₁₀ and D₉₀ present the similar regularities. This may be related to the growth rate and the formation rate of solid solution crystal nuclei. At the low temperature, the former is larger than the latter which lead to the increase of the particle size. At the high temperature, the former is smaller than the latter, which gives rise to the formation of the large amount of small particles.

Fig. 4 Size distribution by number of Zn_0.83Cd_0.17S samples synthesized at different temperatures.

Table 1 The D and span value of each particle size distribution in Fig. 4

Number diameter (nm)	120 °C	140 °C	160 °C	180 °C
D10	105.707	220.194	105.709	43.821
D50	164.182	255.002	164.183	58.771
D90	255.004	824.992	255.002	122.42
Span	0.91	2.37	0.91	1.34

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The width of particle size distributions was measured by span (span is determined by the equation: span = (D₉₀ − D₁₀)/D₅₀) according to a British Standards. A smaller span value indicates a narrower particle size distribution and more uniform size. The span values of the samples synthesized at 120 °C and 160 °C are much lower than the other samples, which means that samples obtained at 120 °C and 160 °C are more homogeneous. Furthermore, the sample synthesized at 160 °C is a little more uniform than that at 120 °C and possesses an average pore diameter of 2–9 nm with the BET specific surface area of 47.36 m² g⁻¹ (seen in ESI Fig. S4†). Besides, there is a protuberance (◆) appeared in Fig. 4b, which is probably due to that the measured sample is not dispersed well during the preparation. In Fig. 4d, the formation of the tailing part marked (#) shows the severe agglomeration of the particles, which may be related to the following reasons: (1) the smallest size of the particles and (2) the effect of high temperature during the synthesis, which is consistent with the SEM images.

3.3 UV-vis diffuse reflectance spectrum

The UV-vis absorption spectra of the synthesized Zn_0.83Cd_0.17S samples are shown in the Fig. 5. All the samples exhibit intense absorption bands with steep edges corresponding to wavelength from 448 to 471 nm, which is in the visible light region. The samples synthesized at 120 °C and 140 °C have the similar absorption, with the absorption edge at around 450 nm. In addition, the absorption edges of the samples gradually red-shift as the temperature increases, the absorption edges of the samples synthesized at 160 °C and 180 °C are 460 nm and 471 nm, respectively. Many reports have proved that the absorption edge of the Zn_xCd_1−xS solid solution would red shift with the increase of Cd content.^20,32 The absorption edges of the solid solutions gradually red shift with the increase of temperature is due to the increase of Cd content in the solid solution which was in agreement with the results of the XRD patterns.

Fig. 5 The UV-vis spectrum for the samples synthesized at (a) 120 °C (b) 140 °C (c) 160 °C and (d) 180 °C for 12 h.

Surprisingly, the samples synthesized at 180 °C presents a different absorption curve in shape from the other samples, whose absorption feature is similar with the results given in ref. 26 and 31. Based on the above references, it can be inferred that the sample synthesized at 180 °C is a composite of ZnS and solid solution rather than homogeneous solid solution which discussed in XRD. Generally, ZnS has a wider energy gap than the sulfide solid solution, which leads to the appearance of shape – different absorption curve.

3.4 Photocatalytic activity for hydrogen production

The photocatalytic performance of the samples in H₂ production was measured in the system consisting of 0.35 M Na₂S and 0.25 M Na₂SO₃ aqueous solution. Fig. 6a shows the influence of temperature on the H₂ evolution rate over the samples. All the samples show a visible-light photocatlytic activity for hydrogen production. The H₂ evolution rate of the samples firstly increases with the temperature and reaches the highest value of 31.86 mmol h⁻¹ g⁻¹ when the hydrothermal temperature is 160 °C and then the H₂ evolution rate is reduced when further increasing the temperature.

Fig. 6 (a) Hydrogen evolution of different the samples synthesized at different temperatures (b) hydrogen evolution of different amounts the sample synthesized at 160 °C for 12 h in Na₂SO₃ and Na₂S aqueous solutions.

The H₂ evolution rate of the sample synthesized at 180 °C is lower than that synthesized at 160 °C, which is opposite from our expectation on the basis of the characterization as what mentioned before. The sample synthesized at 180 °C has a better crystallization and a large absorption edge than other samples. Meanwhile, the average diameter of the sample synthesized at 180 °C is less than other samples which means a higher specific surface area that is beneficial for the photoabsorption. All of these would indicate that the sample synthesized at 180 °C should possess a higher H₂ evolution rate than that other samples. However, it is just the contrary, which means that other factors should be considered. Firstly, although the particles have smallest size, but its severe agglomeration reduces the specific surface area greatly. Among other samples, the sample synthesized at 160 °C has the second smallest size and good dispersity and homogeneity, which would lead to a larger specific surface area than that at 180 °C. Secondly, based on the discussion in 3.1, the sample synthesized at 180 °C may have both ZnS and solid solution. ZnS is only responding to ultraviolet light, its existence in the sample surely reduces the absorption and utilization of the visible light from xenon lamp. And meantime the sample synthesized at 160 °C presents as obvious red shift than those at 140 °C and 120 °C. Consequently, the sample synthesized at 160 °C presents the best photocatalytic activity in the hydrogen production.

On the other side, the amounts of the photocatalyst is another important factor to influence photocatalytic property during the photocatalysis process. Increase the amount of the catalyst, the incident rays from the light source can be sufficiently used for the photocatalysis with the improved phoatocatalytic activity. On the contrary, the increase of the catalyst is also bound to lead to the shadowing effects among catalyst particles, which is not conductive to photocatalytic reaction. Therefore, the effect of the amount of catalyst on the H₂ evolution rate over the samples was investigated with the result shown in Fig. 6b. Clearly, the hydrogen production rate is first improved and then reduced with the increase of the catalyst amount. The highest H₂ generation rate (45.97 mmol h⁻¹ g⁻¹) is obtained when the amount of the sample is 0.03 g.

4. Conclusions

A series of Zn_0.83Cd_0.17S samples have been synthesized using hydrothermal method. The effect of the hydrothermal temperature on the structure, morphology and photocatalytic activities of the samples were investigated and the following conclusions can be drawn.

(1) All the spherical samples are of different sizes and composed of cubic zinc-blende phase. Temperature influences the structure and composition of the samples and the particle size and distribution as well. Increasing the temperature, the crystallization of the samples is improved and the content of Cd in the solid solution gradually increases.

(2) All the samples exhibit intense absorption bands with steep edges corresponding to wavelength from 448 to 471 nm. The absorption edges of the samples gradually red-shift with the hydrothermal temperature due to the increase of the content of Cd in the solid solution. The absorption curve of the sample synthesized at 180 °C is different from the others in shape, which is related to the heterogeneous composition of the sample.

(3) The Zn_0.83Cd_0.17S synthesized at 160 °C exhibits the best photocatalytic activity for H₂ evolution with an initial rate of 45.97 mmol h⁻¹ g⁻¹. The best amount of the sample catalyst is 0.03 g in a 200 ml aqueous solution containing 0.35 M Na₂S and 0.25 M Na₂SO₃ under 300 W Xe lamp irradiation.

Conflicts of interest

The authors have declared no conflicts of interest.

Acknowledgements

This work was supported by the State Key Laboratory of Urban Water Resource and Environment (Harbin Institute of Technology; No. 2015DX07).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra12556e

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