Background

The efficient conversion of solar energy into fuel via photochemical reactions is of great importance for the next-generation energy source for its cleanable, renewable, and abundant properties [1, 2]. Solar-hydrogen, the conversion of solar energy into hydrogen as chemical energy carrier, has been regarded as one of the most desirable ways in considering energy consumption, resource sustainability, and environmental issues [3, 4].

Since the pioneering work of Fujishima and Honda in 1972 [5], tremendous research on semiconductor-based photocatalysis and photoelectrolysis has yielded a better understanding of the mechanisms involved in photocatalytic and photoelectrochemical water splitting [6–9]. However, most of semiconductor photocatalysts can only absorb ultraviolet light due to their wide gap. As it is well known, ultraviolet light occupies only 3% ~ 5% of the solar spectrum; so, the energy conversion efficiency is usually very low [10–12]. Thus, exploiting of highly active visible-light-responsive photocatalysts to make the best use of solar energy in visible light region, which accounts for about 43% of the solar spectrum, is particularly important [13, 14]. In the past, developing and understanding of semicondutor electrodes or photocatalysts for photoelectrochemical or photocatalytic water splitting were mainly performed on simple binary systems (e.g., binary oxides [15, 16] and chalcogenides [17, 18]) and their composite structure [19]. Recently, the ternary system as potentially excellent photoelectrode or photocatalyst material has attracted more and more attention [20–22] because ternary system can offer more possibilities for bandgap and band position tuning.

Cadmium sulfide is an important visible-light response photocatalytic material, in which sulfide ions serve as electron donors. However, the sulfide ion is readily oxidized to sulfate by the photo-generated holes, with Cd2+ ions escaping into the solution. A feasible way for enhancing the photocatalytic activity and stability of cadmium sulfide is to develop CdS-based composite materials. Zinc sulfide has the similar crystal structure as cadmium sulfide. It is a good host material for the development of a visible-light-driven photocatalyst without adding noble metals by forming Cd1−xZn x S solid solutions with a narrow bandgap semiconductor, CdS [22, 23]. The bandgap of the solid solutions formed between ZnS and CdS can be regulated by changing the compositions and therefore the photocatalytic properties can be varied [24, 25].

In this article, we reported a highly efficient three-dimensional (3D) visible-light-active Cd1−xZn x S photocatalysts synthesized via one-step solvothermal pathway. The obtained photocatalysts had good crystallinity and ordered structure and showed excellent photocatalytic activity under the irradiation of visible light.

Methods

Synthesis of photocatalyst

Three-dimensional Cd1−xZn x S nanowires were synthesized in a Teflon-lined stainless steel cylindrical closed chamber with a 100-mL capacity. All the chemicals were of analytical grade. Ethylenediamine (en; 60 ml) and H2O (20 ml) were used as solvent. Thiourea [NH2CSNH2] (15 mmol) was added into the solvent as sulfur source, then 5-mmol mixture of cadmium acetate [(CH3COO)2Cd·2H2O] and zinc acetate [(CH3COO)2Zn·2H2O] was added into the mixed solution. After stirring for a few minutes, the closed chamber was placed inside a preheated oven at 160°C for 10 h and then cooled to room temperature. The obtained precipitates were filtered off and washed several times with water and ethanol, respectively. The final products were dried in vacuum at 45°C for a few hours.

Characterization

The morphology of the as-synthesized powder products were observed by field-emission scanning electron microscopy (Philips Sirion 200, Philips, Netherlands). The crystallographic structure was determined by X-ray diffraction (XRD, D8 DISCOVER X-ray diffractometer, Bruker, Karlsruhe, Germany) with Cu Kα radiation (1.54 Å). Surface composition of the sample was analyzed by X-ray photoelectron spectroscopy (XPS, AXIS ULTRA DLD, Kratos, Japan). The Raman spectrum was measured by the Jobin Yvon LabRam HR 800 UV system (Horiba, Kyoto, Japan) at room temperature. A laser wavelength of 514.5 nm was used as the excitation sources. Reflectance spectra of the obtained were collected using a UV/vis spectrometer (Lambda 20, Perkin Elmer, Inc., USA).

Photocatalytic hydrogen evolution

The photocatalytic performance of the synthesized 3D Cd1−xZn x S photocatalysts were investigated in a gas-closed circulation system (Labsolar-III, Beijing Perfactlight Technology Co. Ltd., Beijing, China) with a top-window Pyrex cell. A 300-W Xe lamp (SOLAREDGE700, Beijing Perfactlight Technology Co. Ltd., Beijing, China) was used as the light source, and UV light was removed by a cut-off filter (λ > 420 nm). Luminous power of the light source is about 40 W. The amount of H2 evolved was analyzed by an online gas chromatography (GC7900, Techcomp Ltd., Beijing, China) equipped with a thermal conductivity detector, MS-5A column, and N2 was used as carrier. In all experiments, 100 mL deionized water containing the mixed sacrificial agent which composed of 0.25 M Na2SO3 and 0.35 M Na2S were added into the reaction cell. Then, these photocatalysts were directly placed into the electrolyte solution. The whole system was vacuumized with a vacuum pump before reaction to remove the dissolved air. The temperature for all photocatalytic reactions was kept at about 20°C.

Results and discussions

The surface morphologies of the obtained Cd1−xZn x S are shown in Figure 1. Figure 1a is the scanning electron microscopy (SEM) image of CdS; it presents porous flower-like 3D structure clearly, shorter nanowires appear at the periphery. As the value of x increases, nanosheet emerges gradually, that is, the secondary structure builds up slowly. Figure 2 shows the XRD patterns of the as-prepared photocatalysts. CdS exhibits a Greenockite structure, while ZnS presents a Wurtzite polycrystalline structure, respectively. The diffraction peaks of the photocatalysts shift to a higher angle side as the value of x increases. The successive shift of the XRD patterns means that the crystals obtained are Cd1−xZn x S solid solution, not a simple mixture of ZnS and CdS [26].

Figure 1
figure 1

Typical SEM images of the obtained Cd 1− x Zn x S photocatalysts. (a) Cd0.98S, (b) Cd0.9Zn0.1S, (c) Cd0.72Zn0.26S, and (d) Cd0.24Zn0.75S.

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Figure 2
figure 2

XRD patterns of the as-prepared Cd 1− x Zn x S photocatalysts with different x values. (curve a) Cd0.98S, (curve b) Cd0.9Zn0.1S, (curve c) Cd0.72Zn0.26S, (curve d) Cd0.24Zn0.75S, and (curve e) Zn0.96S.

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The surface information is collected by XPS of the sample Cd0.72Zn0.26S (Figure 3). The survey scan spectrum (Figure 3a) indicates the existence of Cd, Zn, and S in the Cd0.72Zn0.26S sample. The two sharp peaks (Figure 3b) located at 404.3 and 411.2 eV are corresponding to the Cd 3d5/2 and Cd 3d3/2 level, respectively. The peaks of 1,020.8 and 1,043.7 eV can be assigned to the Zn 2p3/2 and 2p1/2 levels, respectively (Figure 3c). The single S 2p peak at 161.1 eV (Figure 3d) demonstrates that sulfur exists as a sulfur ion.

Figure 3
figure 3

Representative XPS spectra of typical sample Cd 0.72 Zn 0.26 S. (a) survey spectrum, (b) Cd 3d XPS spectrum, (c) Zn 2p XPS spectrum, and (d) S 2p XPS spectrum.

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Raman scattering is a nondestructive technique for structural study of the material and a powerful probe to obtain the vibrational states of a solid. It is an inelastic process in which incoming photons exchange energy with the crystal vibrational mode. Figure 4 reveals the Raman spectrum of the as-obtained Cd0.72Zn0.26S sample. Bulk CdS has two characteristics of longitudinal-optical (LO) phonon peaks: (1) 1-LO (first harmonic (at 300/cm)) and (2) 2-LO (second harmonic (at 600/cm)) vibrations [27]. The two phonon peaks are also observed in the as-obtained Cd0.72Zn0.26S; they are located at 306.5 and 608.1/cm, respectively, and shift toward the higher energy side compared with that of the pure CdS. This can be ascribed to the Cd → Zn substitution in the obtained nanophotocatalysts. In addition, from Figure 4, the Raman intensities of 1-LO and 2-LO are both relatively strong and narrow, which implies its good crystallinity and ordered structure [28].

Figure 4
figure 4

Raman spectrum of the typical sample Cd 0.72 Zn 0.26 S.

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Curves a, b, c, d, and e of Figure 5 show the UV-vis absorption spectra of the as-prepared Cd0.98S, Cd0.9Zn0.1S, Cd0.72Zn0.26S, Cd0.24Zn0.75S, and Zn0.96S, respectively. The absorption edge of Cd1−xZn x S solid solutions are red-shifted relative to ZnS (Figure 5a), which can be attributed to the incorporation of Zn into the lattice of CdS or entered its interstitial sites (the radii of Zn2+ ion (0.74 Å) is smaller than that of Cd2+ (0.97 Å)). The bandgap of Cd1−xZn x S can be acquired from plots of (αEphoton)2 versus the energy (Ephoton) of absorbed light (α and Ephoton are the absorption coefficient and the discrete photon energy, respectively). The extrapolated value (a straight line to the x-axis) of Ephoton at α = 0 gives absorption edge energies corresponding to Eg. From Figure 5b, the bandgap of the synthesized Cd1−xZn x S are 2.37 eV (curve a), 2.48 eV (curve b), 2.60 eV (curve c), 2.86 eV (curve d), and 3.67 eV (curve e), respectively. The bandgaps of Cd1−xZn x S are beneficial to absorbing solar light to drive the water splitting reaction.

Figure 5
figure 5

UV-vis absorption spectra (a) and bandgap evaluation (b) from the plots of (αE photon )2vs. E photon. (curve a) Cd0.98S, (curve b) Cd0.9Zn0.1S, (curve c) Cd0.72Zn0.26S, (curve d) Cd0.24Zn0.75S, and (curve e) Zn0.96S, respectively.

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The photocatalytic hydrogen evolution of the obtained 3D Cd1−xZnxS photocatalysts under the irradiation of visible light is given in Figure 6. All of the Cd1−xZn x S photocatalysts show much higher photocatalytic H2 evolution capacity than that of the sole CdS at visible light irradiation (λ > 420 nm). In addition, the photocatalytic activity of the Cd1−xZn x S solid solutions is strongly dependent on the composition of the solid solutions. It is improved obviously with the increase of Zn content (x value). When the x value increases to 0.75, the 3D solid solutions photocatalyst has the highest photocatalytic activity. This is because ZnS has a high energy conversion efficiency, it is a good host material for the development of a visible-light-driven photocatalyst by forming solid solutions with a narrow bandgap semiconductor, CdS. The more negative reduction potential of the conduction band of solid solutions would allow for more efficient hydrogen generation than CdS. In addition, the large bandgap and wide valence bandwidth benefit the separation of the photo-generated electrons and holes, and the photocorrosion of the photocatalysts can be reduced effectively. The highest activity probably means that Cd0.24Zn0.75S has an optimum bandgap and a moderate position of the conduction band, beneficial for visible light absorption and photo-generated electron-hole pair separation. Moreover, the 3D structure offers a larger surface area, thus supplying more surface reaction sites and better charge transport environment. Therefore, the efficiency of water splitting is improved further. It is worth noting that no H2 was detected for ZnS photocatalyst because its bandgap is too large to absorb the visible light.

Figure 6
figure 6

Photocatalytic H 2 evolution of the obtained Cd1−xZn x S photocatalysts. (curve a) Cd0.98S, (curve b) Cd0.9Zn0.1S, (curve c) Cd0.72Zn0.26S, and (curve d) Cd0.24Zn0.75S.

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Conclusions

We reported highly efficient three-dimensional Cd1−xZn x S photocatalysts synthesized via one-step solvothermal pathway for photocatalytic H2 evolution under the irradiation of visible light. The Raman spectrum implied the obtained Cd1−xZn x S had good crystallinity and ordered structure. The XPS demonstrated that sulfur existed as a sulfur ion, while Cd and Zn are in 3d and 2p state, respectively. The bandgap of the synthesized Cd1−xZn x S varied from 2.37 to 2.86 eV, which were suitable for the absorption of visible light. The photocatalytic activity of the obtained Cd1−xZn x S photocatalysts were improved markedly compared with that of the sole CdS. This can be attributed to their appropriate bandgap and position of the conduction band that is beneficial for visible light absorption and photo-generated electron-hole pair separation, as well as 3D structure that offered a larger surface area, thus supplying more surface reaction sites and better charge transport environment.