The crystal structure of CdS influences the energy band gap; therefore, its suitability as a semiconductor photocatalyst for solar-to-hydrogen energy conversion. By simply adjusting the temperature of the hydrothermal reaction, CdS samples with different crystal phases were prepared, and their photocatalytic hydrogen production performance was tested. XRD confirmed the successful preparation of the two catalysts. TEM results revealed that the (111) and (100) crystal planes corresponded to the cubic and hexagonal CdS structures of the two catalysts. The band gap values of cubic phase and hexagonal phase CdS were 2.24 eV and 2.17 eV, respectively. Cubic CdS exhibited excellent activity, which was considerably higher than that of hexagonal CdS, for the photocatalytic hydrogen evolution reaction. Electrochemical impedance spectroscopy results showed that cubic CdS exhibits a smaller arc radius and lower resistance. The conduction potential of the two CdS phases was further calculated based on the Mott–Schottky plots, revealing that the conduction potential of cubic CdS is more negative than that of hexagonal CdS. Therefore, cubic CdS exhibits higher carrier migration rate and charge separation efficiency than hexagonal CdS, resulting in higher photocatalytic activity.
Research Article
CdS with tunable crystalline phase structures: controllable preparation and enhanced photocatalytic properties
https://doi.org/10.21203/rs.3.rs-3226329/v1
This work is licensed under a CC BY 4.0 License
Journal Publication
published 21 Sep, 2023
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CdS
hydrogen
crystal phases
photocatalysis
The rapid growth of industrialization and urbanization has led to a surge in fossil fuel consumption, resulting in global energy shortage and environmental pollution. To mitigate these issues, hydrogen has gained significant attention as a potential substitute for fossil fuels as a clean, renewable, and highly combustible energy source [1–10].
In recent years, photocatalysis technology has become a research hotspot for hydrogen production technology through solar-driven water decomposition [11–13]. In the study of photocatalytic hydrogen evolution, many semiconductor photocatalysts have been developed and utilized for the photocatalytic decomposition of water [14]. Among these, CdS has garnered considerable interest due to its favorable bandgap width, sufficiently negative band conductance potential, high carrier mobility, and efficient visible light absorption capacity [15–22]. Because the structure and size of CdS semiconductor materials have an extensive impact on their properties, many preparation methods have been studied, such as the template method [23], colloidal method [24], and hydrothermal/solvothermal method [15, 25]. For instance, Zhou et al. prepared hierarchical nanostructures through a simple and effective hydrothermal route at mild temperatures [26]. In addition, CdS nanochains [27], mesoporous tubular structures [28], and double-crystal CdS nanoribbons [29] have been synthesized.
The performance of photocatalytic hydrogen evolution is influenced by factors such as the type of CdS semiconductor material, carrier mobility, crystalline phase, interfacial charge transfer rate, and charge separation ability are the main factors that influence the photocatalytic performance toward the hydrogen evolution [30–33]. Under visible light irradiation, electrons in the valence band (VB) of CdS are excited to the conduction band (CB), generating holes in the VB [34, 35]. The resulting electron–hole pairs can promote the redox reaction of water [14]. However, the recombination of electrons and holes reduces the charge separation efficiency and carrier mobility, leading to a decline in the photocatalytic activity of CdS [36]. In addition, the different band structures of CdS semiconductor materials differ greatly in their visible light response and photogenerated carrier redox capabilities, which directly impact the catalyst performance [37, 38]. Consequently, there is a pressing need to develop CdS with high electron–hole separation efficiency and suitable band structures.
CdS exists in two crystalline phases, namely, hexagonal and cubic phases [39, 40]. It is generally believed that hexagonal CdS usually have high photocatalytic hydrogen production performance [41]. While literature reports have identified cubic CdS as an effective visible light catalyst [42–44], research on cubic CdS remains limited. Moreover, the underlying reasons for the superior photocatalytic properties of cubic CdS are still remain unclear. Therefore, further investigation into the photocatalytic mechanism of cubic CdS is necessary.
In this work, CdS semiconductor photocatalysts with cubic and hexagonal crystal structures were prepared via a simple hydrothermal method by modifying the synthesis conditions. Cubic CdS promoted the maximum hydrogen production after illumination under visible light conditions, showing superior photocatalytic performance to the hexagonal CdS. This result can be attributed to the enhanced visible light absorption capacity and a more suitable band structure of cubic CdS compared to those of hexagonal CdS. Through the study of photocatalytic hydrogen evolution activity of CdS with different crystal phases, it is hoped that the current work could inspire growing interest in the fabrication of other high-performance CdS/semiconductor composites.
2.1. Materials
All materials used in this work were acquired from commercial sources and utilized without further purification. Deionized water was obtained locally.
2.2. Preparation of CdS Photocatalysts
CdS photocatalysts were synthesized via a hydrothermal method. A total of 5 mmol CdCl2·2.5H2O and 5 mmol Na2S·9H2O were added to a beaker with 70 mL deionized water under ontinuous stirring for 30 min. The resulting mixture was then transferred to a Teflon-lined autoclave and heated at 160°C for 12 h. After cooling, the resulting product was filtered, washed several times with deionized water and ethanol, and dried under vacuum at 60°C for 10 h to obtain cubic CdS. Hexagonal CdS was obtained using the same method, modifying the reaction temperature in the Teflon autoclave to 180°C.
2.3. Characterization
X-ray diffraction (XRD) was conducted using a Bruker D8 Advance X-ray diffractometer with Cu Kα radiation. Absorption spectra were recorded on a Cary 500 UV-vis spectrophotometer with BaSO4 as a reference in the 400–800 nm range. Transmission electron microscopy (TEM; JEM-2010, FEI, Tecnai G2 F20 FEG) and high-resolution TEM (HRTEM) were performed to characterize the micromorphologies of the samples. The nitrogen adsorption–desorption isotherms and pore size distribution were measured using a Micromeritics 3Flex apparatus. The photoelectric properties of the samples were determined using a conventional three-electrode cell with a Zennium electrochemical workstation (Zahner, Germany).
2.4. Photocatalytic Activity Measurement
A 300 W Xe lamp (PLS-SXE300C, Perfectlight Co., Beijing) equipped with an optical cut-off filter (λ ≥ 400 nm) was used as the light source. A total of 80 mg of the prepared samples were suspended in 75 ml H2O solution containing 5 mL triethanolamine (TEOA) as a sacrificial agent and H2PtCl6·6H2O as a Pt source (1 wt% Pt). The amount of evolved hydrogen was determined using an online gas chromatograph (Fuli, GC 7890, MS-5 A column, TCD, Ar carrier gas). The amount of volved H2 was detected with an online gas chromatography. The apparent quantum efficiency (AQE) at 400 nm was conducted.
The AQE was calculated by the following equation:
3.1. Characterization
The crystallinity of the CdS samples was analyzed using XRD, as shown in Fig. 1. Cubic CdS exhibits a typical cubic phase structure (JCPDS 075-1546), with diffraction peaks at 26.50°, 30.70°, 43.97°, and 52.07°, corresponding to the (111), (200), (220), and (311) crystal plane, respectively [45]. Notably, no impurity peaks were observed, suggesting that cubic CdS maintained its highly crystalline structure after the hydrothermal treatment. In addition, the narrow diffraction peaks confirmed the high crystallinity of the resultant product. For hexagonal CdS, the diffraction peaks located at 24.81°, 26.51°, 28.18°,36.62°, 43.68°, 47.84°, and 51.82°, corresponding to the (100), (002), (101), (102), (110), (103), and (112), respectively [46, 47]. The pattern was identified as hexagonal phase CdS (JCPDS 041-1049). The narrow diffraction peaks of hexagonal CdS indicated its high crystallinity. Overall, the XRD results confirmed the successful preparation of cubic and hexagonal CdS with high crystallinity, making them ideal materials for photocatalysts.
To obtain the detailed microstructure information of CdS, the samples were observed through TEM and HRTEM. Figure 2(a) and 2(b) show that the two types of CdS are composed of nanoparticles. The HRTEM image in Fig. 2(c) shows clear fringes with a lattice spacing of 0.336 nm, which corresponds to the (111) plane of cubic CdS [15]. This is consistent with the results of the XRD experiments. Similarly, as shown in Fig. 2(d), CdS has a lattice fringe spacing of 0.358 nm, which can be indexed to the (100) crystal planes of hexagonal CdS [34]. Therefore, the successful synthesis of cubic CdS and hexagonal CdS can therefore also be confirmed through TEM. The difference in the crystal structure between the two CdS phases is caused by the adjustment of different hydrothermal temperatures.
The light utilization of catalysts is closely related to their light absorption capacity. The optical properties of the prepared CdS samples were analyzed by UV-vis diffuse reflectance spectroscopy. The visible light absorption spectra of the two samples are shown in Fig. 3. The cubic CdS sample exhibited an absorption edge at ~ 554 nm (corresponding to a band gap of 2.24 eV), while the hexagonal CdS had an absorption edge at 571 nm (corresponding to a band gap of 2.17 eV) [48]. The difference in the absorption range of the two samples shows that using different preparation temperatures can influence the band gap of materials.
The nitrogen adsorption–desorption isotherms and pore size distribution of the as-prepared CdS samples are shown in Fig. S1. The Brunauer–Emmett–Teller specific surface area of cubic CdS (63.0 cm2 g− 1) was higher than that of hexagonal CdS (44.5 cm2 g− 1), suggesting that cubic CdS can provide more active sites for reactions. The pore size distribution indicates the mesoporous characteristics of the as-prepared samples.
3.2. Photocatalytic Hydrogen Evolution Activity
The photocatalytic hydrogen evolution activity of the prepared samples was analyzed under visible light irradiation (λ ≥ 400 nm) by using 5 mL TEOA as the sacrificial agent and H2PtCl6·6H2O as the Pt source. As presented in Fig. 4, cubic CdS realized the highest hydrogen production (~ 680 µL) after 180 min of hydrogen evolution. The corresponding apparent quantum efficiency of cubic CdS was determined to be 1.02%. Meanwhile, according to the diagram of hydrogen production over time, the hydrogen evolution rate of cubic CdS showed a gradual increase over time. These results indicate that cubic CdS exhibits high photocatalytic hydrogen evolution activity. For hexagonal CdS, however, the hydrogen evolution rate was negligible. This result suggests that regulating the crystal structure of CdS is a promising strategy for the design of efficient CdS-based photocatalysts. As comparison, the photocatalytic activity of P25 powders has been collected and added under the same conditions. As shown in Fig. S2, the hydrogen evolution rate of P25 is significantly lower than cubic CdS.
3.3. Eelectrochemical Properties and Mechanism
According to the Mott-Schottky plots in Fig. 5(a), the flat band potential of cubic CdS was approximately − 1.15 V vs. Ag/AgCl (pH = 6.8). Namely, the CB position of CdS was calculated to − 0.95 V vs. NHE (pH = 6.8). The flat band potential of hexagonal CdS was approximately − 0.86 V vs. Ag/AgCl (pH = 6.8) (Fig. 5(b)). The CB position of hexagonal CdS was calculated as − 0.66 V vs. NHE (pH = 6.8). The CB potential of the control cubic CdS was more negative. The accumulated electrons on CdS can reduce the H+ to H2, while the holes on the CdS surfaces can oxidize the electron donor (TEOA).
Electrochemical-impedance is an effective tool for understanding photogenerated carrier migration and photogenerated charge–hole pairs separation processes. Cubic CdS showed a lower arc radius than hexagonal CdS in the electrochemical impedance spectroscopy Nyquist plots (Fig. 6), indicating its lower electrical resistance [49]. Thus, cubic CdS possesses fast channels for the efficient transport and efficient separation of photoexcited charges. The reduced electrical resistance of cubic CdS is presumably attributed to its high electrical conductivity and excellent carrier mobility.
Based on the aforementioned results, the potential mechanism underlying the photocatalytic hydrogen evolution of cubic CdS is proposed. Under visible light irradiation, CdS is excited to generate photo-generated electrons and holes. The holes are consumed by sacrificial agents. And, the photo-generated electrons of CdS are sufficient to reduce [PtCl6]2− to produce Pt. The resulting Pt will act as an excellent hydrogen evolution cocatalyst, which can rapidly capture the photoinduced electrons on CdS and promote the interfacial hydrogen generation reaction, similar to the widely reported cocatalyst modified photocatalysts [41, 50–52].
CdS catalysts with cubic and hexagonal crystal structures were prepared by a simple hydrothermal method. The successful synthesis of these two CdS catalysts with different crystal structures was further confirmed through XRD and TEM. UV-vis reflectance spectra revealed that both catalysts possessed good absorbability of visible light. The photocatalytic hydrogen evolution yield of cubic CdS was much higher than that of hexagonal CdS, and the hydrogen production reaches 680 µL in 180 min, exhibiting higher photocatalytic hydrogen evolution performance. This can be attributed to the smaller electrochemical impedance and more negative conduction potential of cubic CdS, which makes it more effective in photoexcited charge separation and transfer, and improves the photocatalytic activity. The photocatalytic hydrogen evolution performance of two different crystal phases of CdS was analyzed, confirming that the potential of CdS as a promising photocatalyst for visible-light-driven water splitting.
Supplementary Information
The online version contains supplementary material available at https://doi.org/10.1007/xxxxxx-xxx-xxxxx-x.
Data availability
All data generated or analyzed during this work are included in this article.
Ethical approval
This paper is our original unpublished work, and it has not been submitted to any other journal for review. All authors voluntarily agree to participate in this research study and publication of the paper.
Competing interests
There has no competing interest of a financial or personal nature.
Authors' contributions
N. Qin: conceptualization, supervision, writing–review & editing, funding acquisition; L. Li: investigation, methodology, writing–original draft; H. Zheng: investigation, formal analysis; Q. Cui: investigation, formal analysis.
Funding This work was supported by the independent innovation application research project of Zhongyuan University of Technology (No. K2020YY001), and Young Backbone Teacher of Zhongyuan University of Technology.
Availability of data and materials
All data generated or analyzed during this work are included in this article.
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No competing interests reported.