天津大学于涛团队文章Advanced Functional Materials：同质结 CdS 上负载 Cu 亚纳米团簇光热促进海水析氢
First Author:Yuan Tang (Tianjin University),Yan Sun (Tianjin University)
Contact: Tao YU (Tianjin University), Zhuofeng HU (Sun Yat-sen University)
DOI:10.1002/adfm.202405527
Graphic abstract
Brief introduction of the results
近日，天津大学于涛老师和中山大学胡卓锋老师合作在Advanced Functional Materials上发表了题为“Photothermal effect of Cu NCs on CdS homojunction boosting hydrogen evolution in alkaline seawater”的研究论文（DOI: 10.1002/adfm.202405527），该研究通过原位光还原技术将铜纳米团簇（NCs）锚定在具有不饱和边缘S的CdS均相结（CdS-H）表面，从而在碱性海水制氢过程中提高了光催化剂的稳定性和制氢能力，为合理设计在碱性海水中具有高稳定性和优异性能的光催化剂提供了新的视角。
Quick facts about the full article
The development of hydrogen evolution reaction (HER) photocatalysts with ideal activity and stability in alkaline seawater is of great significance, but there are still great challenges. This study demonstrates a photocatalytic system that promotes hydrogen production from alkaline seawater by anchoring Cu NCs on CdS-H. The study has fully confirmed that the unsaturated edge S not only effectively anchors Cu NCs, but also promotes the adsorption of Cl- ions, inhibits the oxidation of lattice S, and regulates the stability of photocatalysts in alkaline seawater. The photothermal effects induced by anchored Cu NCs accelerated the reaction kinetics and demonstrated the directional migration of photocarriers at the metal-semiconductor interface by an in-situ approach. This work provides the possibility for CdS-based photocatalysts to achieve high stability and high performance hydrogen evolution in alkaline seawater.
introduction
Seawater resources are abundant on Earth (97%) and freshwater resources are scarce (only 3%), but most of the hydrogen production experiments reported to date have been carried out in ideal deionized water. Photocatalytic seawater hydrogen production is considered more sustainable than electrolysis of seawater to produce hydrogen. However, the complex ionic composition of seawater, especially the high Cl- content, is prone to the formation of oxidizing free radicals and destabilizing the catalyst. In addition, the directional transfer and effective utilization of photocarriers in seawater have also been neglected, which hinders the industrial application of seawater hydrogen production. Different from the previous Cl-repulsion strategies, the adsorption site of chloride ions was designed on the CdS model catalyst in this study, which solved the inherent defects of the CdS model photocatalyst. More importantly, the anchored Cu NCs promote the directional transfer of photocarriers at the metal-semiconductor interface, while the photothermal effect induced by Cu NCs accelerates the reaction kinetics, thereby comprehensively improving the activity and stability of the CdS model photocatalyst.
Illustrated reading
Performance testing
Figure 1. Photocatalytic H2 evolution performance. (a) H2 evolution amount of pristine CdS, CdS-H and Cu0.5/CdS-H in alkaline seawater at different irradiation time, the insets in a is the histogram of hydrogen evolution rate, (b) Apparent quantum yield of Cu0.5/CdS-H in alkaline seawater, (c) Cycle experimental performance of Cu0.5/CdS-H for 10 h in alkaline seawater, (d) Photocatalytic hydrogen evolution performance of Cu0.5/CdS-H in alkaline seawater under different conditions (where the temperature control system is shown in Figure S4, Supporting Information).
The superiority of the material design was highlighted by a systematic evaluation of the photocatalytic hydrogen evolution of the original CdS and Cu0.5/CdS-H in alkaline seawater. CdS-H exhibits a higher photocatalytic hydrogen evolution efficiency (5.8 mmol/g/h) in alkaline seawater (1.0 M KOH and 0.5 M NaCl) compared to the original CdS, which is attributed to the efficient separation of electron-hole pairs due to the difference in Fermi levels in the homojunction. The photocatalytic hydrogen evolution rate of Cu0.5/CdS-H in alkaline seawater is significantly increased by about 9.0 times compared with the original CdS, which is better than most of the reported photocatalysts. The apparent quantum yield (AQY) of Cu0.5/CdS-H is in good agreement with its UV-Vis absorption spectra, indicating that the hydrogen evolution of Cu0.5/CdS-H in alkaline seawater is a light-driven process.
Cu0.5/CdS-H, on the other hand, has an AQY of 11.0% (λ = 450.0 nm), confirming its excellent light absorption and utilization capabilities (Fig. 1b). Due to the photothermal effect induced by Cu NCs, the photocatalytic hydrogen evolution efficiency showed an upward trend in the near-infrared region (650.0 ~ 750.0 nm). It is worth noting that Cu0.5/CdS-H can maintain high photocatalytic hydrogen production performance in alkaline seawater for 10.0 hours, indicating that the photocatalyst also has good stability in alkaline seawater. When the wavelength of incident light is reduced to λ > 500 nm, the photocatalytic performance of Cu0.5/CdS-H is greatly reduced to only 4.9 mmol/g/h, which is due to the weakening of the interband transition with the decrease of photon energy. Compared with the simulated sunlight conditions, the hydrogen evolution rate decreased to 5.8 mmol/g/h (67.4%) under the condition that the condensate isolated the heat, which further proved that the photothermal effect can increase the reaction rate. At the incident wavelength λ > 600 nm, the hydrogen evolution rate decreases to 15.5 mmol/g/h (12. 9% ), further illustrating the significance of the photothermal effect induced by infrared light radiation on the evolution of photocatalytic hydrogen on Cu0.5/CdS-H surface.
Catalyst structure characterization
Figure 2. Morphological structure and physical phase characterization of Cu0.5/CdS-H and CdS (a) HRTEM images of Cu0.5/CdS-H, (b, c) Locally enlarged HRTEM images of Cu0.5/CdS-H, (d) HAADF-STEM images of Cu0.5/CdS-H and elements mapping of Cu, Cd, S, (e) XRD spectrum, (f) UV–vis-NIR diffuse reflectance spectroscopy, inset in the figure f is the band gap width of pristine CdS and Cu0.5/CdS-H, (g) Cd 3d spectra of CdS and Cu0.5/CdS-H, (h) S 2p spectra of CdS and Cu0.5/CdS-H, (i) Atomic model diagram and Cu 2p spectra of CdS and Cu0.5/CdS-H.
The topography of Cu0.5/CdS-H was characterized by high-resolution transmission electron microscopy (HRTEM). Transmission electron microscopy images show that the Cu0.5/CdS-H grains are regularly distributed with a dense striated structure (Fig. 2a). The 0.32 nm lattice fringe is the (101) plane of WZ-CdS-H, while the 0.29 nm lattice fringe is the (200) plane of ZB-CdS-H (Fig. 2b and c). The (111) crystal plane of Cu NCs was well scanned, and CdS-H particles with a diameter of about 30 nm were uniformly loaded with Cu NCs with a diameter of 2-3 nm (Figure 2b). The elemental map shows that cadmium, copper, and S are evenly distributed throughout the structure (Figure 2d).
The X-ray diffraction (XRD) plots for all samples correspond to the (100), (111), and (101) planes of the typical hexagonal phase of CdS (PDF 41-1049) and the (200) planes of the cubic phase (PDF 10-0454) (Figure 2e). According to UV-Vis diffuse reflectance spectroscopy (DRS), Cu0.5/CdS-H showed a wider range of light absorption compared to the original CdS (Figure 2f). X-ray photoelectron spectroscopy (XPS) further determined the surface chemical composition and valence state of the synthetic sample. The S2p spectra of Cu0.5/CdS-H are located at 162.9 eV and 164 eV, indicating the presence of unsaturated S ligands and S2- in the crystal lattice (Figure 2h). The Cu2p spectra of Cu0.5/CdS-H are located at 932.1 eV and 951.8 eV, corresponding to the Cu-Cu bond. The Cu 2p peaks at 933.0 eV and 953.0 eV confirm the formation of Cu-S bonds, which provides direct evidence for the anchoring of Cu NCs.
Photophysical and charge transfer properties
Figure 3. Directed migration of photocarriers in Cu0.5/CdS-H (a) Contact potential difference of CdS-H subtracting the potential under darkness from that under irradiation, the surface photovoltage distribution of CdS-H under darkness (b) and visible-light irradiation conditions (c), (d) Surface photovoltage of Cu0.5/CdS-H under darkness and visible light irradiation, surface photovoltage distribution of Cu0.5/CdS-H under darkness (e) and visible-light irradiation conditions (f), (ΔCPD = CPDlight - CPDdark), In situ XPS spectra of pristine CdS and Cu0.5/CdS-H: (g) Cu 2p, (h) S 2p, (i) Cd 3d.
Kelvin probe force microscopy (KPFM) was used to study the separation and transport mechanisms of photogenerated electrons at the interface. The change of potential values at positions A and B with Cu0.5/CdS-H in the dark and under light irradiation (λ > 420 nm) indicates that photogenerated electrons are transferred from the surface of CdS-H to Cu NCs, thus promoting the extraction of photogenerated electrons and the reduction of hydrogen protons. In-situ XPS analysis provides a more precise analysis of the migration direction of photogenerated electrons under visible light irradiation. During light irradiation, the binding energy changes corresponding to S2p and Cu2p in Cu0.5/CdS-H indicate that photogenerated electrons are transferred from lattice S to Cu NCs by Cu-S bonds as transfer intermediates under visible irradiation.
Mechanism of reaction
Figure 4. Reaction mechanisms (a) The plane-integrated electron density difference along the vertical direction for the CdS-H, the inset of figure 5a shows the Bader charge of Cu0.5/CdS-H, (b) ESR spectra of the prepared samples at 5 min (TEMPO-e-), (c) XPS spectra of S 2p after the reaction, inset shows the Adsorption sites of Cl- ions on CdS-H (d) Apparent activation energy of pristine CdS, CdS-H and Cu0.5/CdS-H at different temperatures in alkaline seawater, (e) Relationships between the hydrogen evolution rate on pristine CdS and Cu0.5/CdS-H and light intensity on a double logarithmic scale in alkaline seawater (here S stands for the number of reaction stages), (f) ΔG*H of Cu0.5/CdS-H systems, (g) Infrared thermography of different catalysts over time in illumination. (h) Photocatalytic mechanism of Cu0.5/CdS-H.
In order to clarify the effect of halogens in alkaline seawater solution on the hydrogen evolution process of Cu0.5/CdS-H catalyst, an adsorption model of Cl- ions on the catalyst was constructed. The adsorption energy of Cl- ions on CdS-H was calculated to be -2.79 eV, and the results showed that Cl- ions were more favorable for adsorption at the S site of CdS-H WZ phase. Logically, this will promote the stabilization of the lattice S atoms in CdS-H, thereby inhibiting the catalyst photocorrosion process.
The surface temperature of the synthetic photocatalyst was measured by infrared thermal imager, and the surface temperature of Cu0.5/CdS-H was significantly higher than that of the original CdS and increased with the increase of copper deposition, indicating that Cu NCs have the potential to be photothermally driven to catalyze the kinetics of the reaction. Cu0.5/CdS-H exhibits stronger active hydrogen adsorption capacity compared to the original CdS due to the introduction of Cu NCs. Evidence of Cl immobilization on the catalyst was detected in XPS surveys of Cu0.5/CdS-H after the reaction, and the 164.1eV position corresponds to the S-Cl bond, which favors the inhibition of photocorrosion of CdS while hindering the oxidative corrosion of the catalyst by Cl-. The homojunction Fermi levels are different, and bending bands and internal electrostatic fields are formed at the alternating phase interface.
Under visible light irradiation, photogenerated electrons are transferred from the valence band (VB) to the conduction band (CB) of staggered CdS-H. Driven by an internal electrostatic field, photogenerated electrons on the CB of WZ-CdS-H migrate to the CB of ZB-CdS-H, while photogenerated holes on the VB of ZB-CdS-H migrate to VB of WZ-CdS-H. The edge-attached unsaturated S atoms in ZB-CdS-H form Cu-S bonds with a moderate amount of Cu atoms, which become electron transport channels. Cu NCs are both capture points for photogenerated electrons and adsorption points for *H species, reducing *H species to H2. The photothermal effect accelerates the reaction kinetics in the system, prompting a more directional migration of electrons from CdS-H to Cu NCs.
brief summary
In summary, the adsorption sites of chloride ions were designed on the CdS model catalyst to solve the inherent defects of the stability of the CdS model photocatalyst. More importantly, the anchored Cu NCs promote the directional transfer of photocarriers at the metal-semiconductor interface, while the photothermal effect induced by Cu NCs accelerates the reaction kinetics, thereby comprehensively improving the activity and stability of seawater hydrogen production. The optimized Cu0.5/CdS-H has a hydrogen production activity of 15.5 mmol/g/h and a quantum yield of 11.0% at 450 nm. Cu NCs are dispersed and anchored to the surface of CdS-H by coordination with marginal unsaturated S atoms through an in-situ photoinduction process, driving directed migration of photocarriers. In addition, the action of Cl- ions and Cu NCs not only expands the photoreaction range, but also promotes the reaction kinetics and accelerates the directional migration of electrons through the photothermal effect. This study provides a new perspective for the rational design of low-cost, high-performance photocatalysts in alkaline seawater.
参考文献：Y. Tang, Y. Sun, Y. F. Li, Y. C. Guo, B. X. Liu, X. Tan, Z. F. Hu, D. C. Zhong, J. H. Ye, and T. Yu*, Photothermal Effect of Cu NCs on CdS Homojunction Boosting Hydrogen Evolution in Alkaline Seawater. Adv. Funct. Mater. 2024, 2405527.
Article link: https://doi.org/10.1002/adfm.202405527