Recent Advances in Cadmium Sulfide-Based Photocatalysts for Photocatalytic Hydrogen Evolution

Open AccessRenewablesREVIEWS2 Jan 2023Recent Advances in Cadmium Sulfide-Based Photocatalysts for Photocatalytic Hydrogen Evolution Xinlong Zheng, Yuhao Liu, Yuqi Yang, Yiming Song, Peilin Deng, Jing Li, Weifeng Liu, Yijun Shen and Xinlong Tian Xinlong Zheng State Key Laboratory of Marine Resource Utilization in South China Sea, Hainan Provincial Key Lab of Fine Chemistry, School of Science, Hainan University, Haikou 570228 Mechanical and Electrical Engineering College, Hainan University, Haikou 570228 Google Scholar More articles by this author , Yuhao Liu *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Marine Resource Utilization in South China Sea, Hainan Provincial Key Lab of Fine Chemistry, School of Science, Hainan University, Haikou 570228 Google Scholar More articles by this author , Yuqi Yang State Key Laboratory of Marine Resource Utilization in South China Sea, Hainan Provincial Key Lab of Fine Chemistry, School of Science, Hainan University, Haikou 570228 Mechanical and Electrical Engineering College, Hainan University, Haikou 570228 Google Scholar More articles by this author , Yiming Song State Key Laboratory of Marine Resource Utilization in South China Sea, Hainan Provincial Key Lab of Fine Chemistry, School of Science, Hainan University, Haikou 570228 Mechanical and Electrical Engineering College, Hainan University, Haikou 570228 Google Scholar More articles by this author , Peilin Deng State Key Laboratory of Marine Resource Utilization in South China Sea, Hainan Provincial Key Lab of Fine Chemistry, School of Science, Hainan University, Haikou 570228 Google Scholar More articles by this author , Jing Li State Key Laboratory of Marine Resource Utilization in South China Sea, Hainan Provincial Key Lab of Fine Chemistry, School of Science, Hainan University, Haikou 570228 Google Scholar More articles by this author , Weifeng Liu Mechanical and Electrical Engineering College, Hainan University, Haikou 570228 Google Scholar More articles by this author , Yijun Shen State Key Laboratory of Marine Resource Utilization in South China Sea, Hainan Provincial Key Lab of Fine Chemistry, School of Science, Hainan University, Haikou 570228 Google Scholar More articles by this author and Xinlong Tian *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Marine Resource Utilization in South China Sea, Hainan Provincial Key Lab of Fine Chemistry, School of Science, Hainan University, Haikou 570228 Google Scholar More articles by this author https://doi.org/10.31635/renewables.022.202200001 SectionsAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail High-efficiency photocatalytic hydrogen evolution (PHE) relies on the development of inexpensive, stable, and efficient photocatalysts. Cadmium sulfide (CdS), as a typical binary metal sulfide, has attracted considerable research attention due to its negative conduction band position, narrow band gap for visible-light response, and strong driving force for PHE. However, the construction of CdS-based photocatalysts and the PHE rate still require improvement for practical applications. In this review, recent advances in CdS-based photocatalysts for PHE via water splitting are systematically summarized. First, the semiconductor properties of CdS, including the crystal and band structures, are briefly introduced. Afterward, the fundamental mechanisms of PHE using semiconductor photocatalysts via water splitting are discussed. Subsequently, the photoactivity of bare CdS with different morphologies and structures, CdS with cocatalyst loading, and CdS-based heterojunction photocatalysts are reviewed and discussed in detail. Finally, the challenges and prospects for exploring advanced CdS-based photocatalysts are provided. Download figure Download PowerPoint Introduction The continuous consumption of nonrenewable fossil energy exposes human society to the risks of energy exhaustion and environmental pollution, making it critical to develop clean and renewable energy to address the energy crisis.1–9 Non-polluting hydrogen has a high energy density and is regarded as the most promising clean energy alternative for traditional fossil fuels.10–17 Among the various production techniques, photocatalytic hydrogen evolution (PHE) using solar energy via water splitting is an ideal path for hydrogen production, in which the development of semiconductor photocatalysts is key for the realization of high PHE rates.18–20 Fujishima and Honda21 first reported that photoelectrochemical water splitting could realize hydrogen evolution on the TiO2 electrode, guiding the development of semiconductor photocatalysts for PHE. Nevertheless, the corresponding PHE process can only be performed under UV-light absorption (∼4% in sunlight) due to the wide band gap of TiO2 (3.2 eV). Therefore, it is essential to develop semiconductor photocatalysts with narrower band gaps for the realization of PHE under visible light. For efficient visible-light PHE, visible-light absorption must be enhanced and the recombination of photogenerated electron–hole pairs should be hindered, which primarily depends on the band gap and crystal structure of the selected semiconductor photocatalysts.22 Metal sulfides (MSs) have the potential to realize efficient PHE under visible-light irradiation due to their excellent semiconductor properties, such as suitable conduction band (CB) positions, narrower band gaps, sufficient active sites, and appropriate crystal and band structures.19,23–26 In particular, binary MSs (BMSs) have simple and stable phases and have great potential for achieving high PHE rates.27,28 Among them, cadmium sulfide (CdS), as a typical II–VI BMS semiconductor, has a band gap of ∼2.4 eV and exhibits promising visible light photoelectric properties.21,29 Moreover, CdS also exhibits good charge capacity, which can greatly enhance the transfer of photogenerated electron–hole pairs and extend their lifetime, delivering a high PHE performance. Based on the unique semiconductor properties, great progress has been made on the investigation of CdS-based photocatalysts for PHE via water splitting30–32 and several reviews of CdS-based photocatalysts for PHE have been reported.29,33,34 Approaches such as synthesis methods, morphology engineering, structure design, cocatalyst loading, and heterojunction construction have been proposed to improve the PHE performance of CdS-based photocatalyst. Very recently, more advanced CdS-based photocatalysts have been reported with impressive PHE performance.35–39 In addition, new understanding of the mechanisms of the PHE enhancement have been also proposed and discussed. As a result, it is essential to provide a systematic and timely review of CdS-based photocatalysts for PHE, aiming to achieve a higher performance for practical applications. Therefore, our review focuses on CdS-based photocatalysts for PHE. The basic principles of PHE are first illustrated, and the unique semiconductor properties of CdS are discussed in detail. Subsequently, recent progress in CdS-based photocatalysts, including the optimization of morphology and structure, cocatalyst loading, and heterojunction construction for PHE, is summarized and discussed. Finally, challenges and future research directions for advanced CdS-based photocatalysts are provided. Basic Principles of PHE via Water Splitting PHE via water splitting consists of the semiconductor photocatalyst and reaction solution (Figure 1a).40 In particular, during the water-splitting process, the conversion of solar energy to hydrogen is accompanied by a positive Gibbs free energy change (uphill reactions, Figure 1b).40 From this perspective, PHE through water splitting could be defined as artificial photosynthesis. Figure 1c,d shows the role of the semiconductor photocatalyst and the photogenerated charge-transfer routes in the PHE process.41,42 The entire PHE process includes the following steps: (1) light irradiation of the semiconductor photocatalyst, (2) generation of electron–hole pairs (photogenerated charges), (3) photogenerated charge separation and bulk recombination, and (4) transfer of photogenerated electrons and holes to water. The final PHE rate is directly determined by the photogenerated electron density, which is affected by the other four PHE processes.40–42 To facilitate the PHE process under visible-light irradiation, the band gap of the selected semiconductor photocatalyst should be in the range of 1.23 to 3.0 eV.18 Apart from the band gap, the surface area, active sites, charge-diffusion distance, bulk recombination, and possibilities for commercial application of the photocatalysts should also be considered (Figure 1e). Therefore, the development and selection of approximate semiconductors is necessary for realizing efficient PHE under visible light. Figure 1 | (a) Schematic diagram of PHE in a single reactor. (b) Photosynthesis by photocatalytic water splitting. Reprinted with permission from ref 40. Copyright 2009 RSC. (c) Schematic illustration of PHE via water splitting by semiconductor photocatalyst. Reprinted with permission from ref 41. Copyright 2016 ACS. (d) Charge-transfer process during PHE process. Reprinted with permission from ref 42. Copyright 2020 ACS. (e) Web chart of comprehensive evaluation for particulate photocatalysts. Reprinted with permission from ref 18. Copyright 2018 ACS. Download figure Download PowerPoint Semiconductor Properties of CdS In general, CdS has two types of crystal structures: cubic CdS (c-CdS) and hexagonal CdS (h-CdS).43 As shown in Figure 2a, c-CdS (zinc blende structure), with a tetrahedral structure, possesses an equiaxed system with a facial structure. Unlike c-CdS, h-CdS with a hexagonal cone structure is always defined to the wurtzite phase, which is similar to wurtzite Cu2ZnSnS4 (Figure 2b). Due to the difference in crystal structures, c-CdS and h-CdS generally exhibit different stabilities and properties. The calculated band structures of c-CdS and h-CdS are shown in Figure 2c,d, respectively,44 and both are direct band gaps and exhibit similar band structure shapes and bandwidths. Moreover, the dispersions along Γ-L in c-CdS and Γ-A-Γ in h-CdS are almost identical. Figure 2 | Crystal structures of (a) c-CdS and (b) h-CdS. Band structures of (c) c-CdS and (d) h-CdS. Reprinted with permission from ref 44. Copyright 1983 APS. (e) Theoretical simulation of band structure alignment between c-CdS and h-CdS. Ec, potential of conduction band edge; Ev, potential of valence band edge. Reprinted with permission from ref 43. Copyright 2018 Elsevier. Download figure Download PowerPoint The direct band gap of CdS is determined by a combination of experimental and theoretical methods. Experimentally, the band gap of CdS can be calculated using the Kubelka–Munk formula according to the UV–vis spectrum: α h ν = A ( h ν − E g ) 1 / 2 (1)where α is the absorption coefficient of CdS, hν is the photon energy, Eg is the band gap of CdS, and A is a constant.23,45,46 Based on this equation, the experimental band gap of CdS was calculated to be ∼2.4 eV, indicating the possibility of the PHE process under visible light. Theoretically, the density functional theory (DFT)-calculated band gap of CdS was ∼1.146 eV,44 which is lower than the experimental value due to the limitations of DFT calculations.47 Compared to the pioneer semiconductor photocatalyst TiO2, the narrower band gap of CdS not only absorbs more photons in sunlight but also facilitates the excitation of photogenerated electrons from the valence band (VB) to the CB, resulting in a high PHE performance under the visible-light irradiation. According to the different phases and band gaps of the CdS, Ai et al.43 reported the phase junctions of c-CdS and h-CdS to enhance the charge transfer efficiency, which provides an effective path to improve the PHE performance of CdS-based photocatalysts. As shown in Figure 2e, the recombination of photogenerated electron–hole pairs is rapid due to their own band structure. After the combination of c-CdS and h-CdS, photogenerated electrons in the CB of h-CdS were rapidly transferred to the condition band of c-CdS, along with photogenerated hole transfer by an opposite route in the VB. The formation of the bonding region width could significantly decrease the energy loss caused by the different conduction levels between c-CdS and h-CdS and act as a shelter to hinder the recombination of electron–hole pairs. Therefore, apart from the crystal and band structure, the construction of an appropriate phase junction band structure is an important research subject for the development of CdS-based photocatalysts with a high PHE rate. CdS-Based Photocatalysts for PHE Due to the excellent semiconductor properties of CdS, researchers have made great efforts to develop CdS-based photocatalysts to realize visible-light PHE. In this section, the progress of CdS-based photocatalysts for PHE will be systematically discussed, including bare CdS, cocatalyst loading, and heterojunction construction. Morphology and structure control For the development of CdS photocatalysts, morphology and structure are two important factors that strongly influence the final PHE rate. Morphology and structure engineering could increase the specific area, provide more active sites for the hydrogen evolution reaction (HER), accelerate charge transfer, enhance light harvesting, and hinder electron–hole pair recombination, which are beneficial for the improvement of PHE rates.29,48 Accordingly, researchers have made great effort to fabricate CdS photocatalysts with morphology and structure engineering, aiming to increase the PHE rate. As shown in Figure 3, several representative morphologies and structures are nanoflowers,49 nanosheets,50 nanowires,51 and nanoparticles.52 Figure 3 | Several representative CdS photocatalysts with different morphologies and structures. Schematic diagram and scanning electron microscopy (SEM) image of flower-shape CdS with (002) facet. Reprinted with permission from ref 49. Copyright 2018 Elsevier. Schematic diagram and SEM image of ultrathin CdS nanosheet. Reprinted with permission from ref 50. Copyright 2019 Wiley. Schematic diagram and SEM image of core-shell CdS nanowires. Reprinted with permission from ref 51. Copyright 2017 Elsevier. Schematic diagram and SEM image of core-Au CdS with plasmonic satellites nanostructure. Reprinted with permission from ref 52. Copyright 2018 Elsevier. Download figure Download PowerPoint As mentioned in Section “Semiconductor Properties of CdS,” CdS semiconductors can crystalize in the zinc blende phase and wurtzite phase. The (002) facet of wurtzite CdS has the highest energy,53 which is worth researching in PHE. Liu et al.49 studied the PHE performance of CdS nanoflowers with (002) facet exposure. In this study, the authors synthesized a series of CdS photocatalysts with different morphologies and degrees of (002) facet orientation. Under visible-light irradiation, the CdS nanoflowers reached the highest PHE rate of 5.98 mmol g−1 h−1 (Figure 4a), which was far higher than the morphologies of the porous nanoflowers, nanobelts, nanonets, and nanoparticles with a decrease in the exposed (002) facet.49 This work not only concluded that the (002) facet of wurtzite CdS plays a critical role in PHE rate enhancement but also suggests that the nanoflower structure can easily construct a tight surface with sufficient active sites, which is similar to the ZnxCd1−xS solid solution photocatalysts with nanoflower morphology (Figure 4b).48 Figure 4 | (a) PHE rate of CdS photocatalysts with different morphologies: nanoflower, porous nanoflower, nanobelt, nanonet, and commercial CdS. Reprinted with permission from ref 49. Copyright 2018 Elsevier. (b) Charge-transfer mechanism of ZnxCd1−xS nanoflowers. Reprinted with permission from ref 48. Copyright 2020 Elsevier. Schematics of (c) charge transfer and (d) PHE mechanism of ultrathin CdS nanosheets. Reprinted with permission from ref 50. Copyright 2019 Wiley. PHE mechanism of (e and f) CdS nanowires and (g and h) 1D core–shell [email protected]2. Reprinted with permission from ref 51. Copyright 2017 Elsevier. (i) PHE process on core–shell CdS/g–C3N4 nanowires. Reprinted with permission from ref 60. Copyright 2013 ACS. (j) Synthesis and PHE mechanism of CdS core-Au plasmonic satellites nanostructure. Reprinted with permission from ref 52. Copyright 2018 Elsevier. Download figure Download PowerPoint The crystallinity and specific surface area of semiconductor photocatalysts are two important factors that affect their final PHE performance.22 In general, for the acquisition of a high specific surface area, the crystallinity of semiconductor photocatalysts must be decreased.29 Therefore, it is essential to balance the crystallinity and specific surface area of CdS photocatalysts to achieve a high PHE rate. Typically, two-dimensional (2D) ultrathin nanosheet photocatalysts have the unique properties of large surface areas and abundant exposed sites.54–58 However, it is still difficult to synthesize the 2D ultrathin CdS nanosheets with high crystallinity. Cheng et al.50 fabricated highly crystalline 2D ultrathin CdS nanosheets by a simple one-step solid-phase strategy and achieved a PHE rate of 5.99 mmol g−1 h−1. As shown in Figure 4c,d, during the PHE process, the separation of photogenerated electron–hole pairs could be efficiently accelerated by the CdS photocatalyst with 2D ultrathin nanosheets due to its high crystallinity, short interface distances, enrichment of active adsorption sites, and high surface-to-volume ratio. The photocorrosion inhibition performance was also improved to achieve an enhanced PHE rate and photostability. This study not only emphasizes the balance of crystallinity and specific surface area of CdS photocatalysts but also pioneers an effective path to hinder the photocorrosion phenomenon, which is beneficial for photostability enhancement and the realization of overall water splitting of CdS-based photocatalysts. Compared to other morphologies, CdS semiconductor photocatalysts with core–shell nanowire morphologies have unique properties, especially fast carrier relaxation time, hindered charge recombination, high photoluminescence quantum efficiency, and desirable optical properties.22,59 Han et al.51 fabricated the one-dimensional (1D) [email protected]2 core–shell nanowires by employing CdS nanowires as nano building blocks via a facile hydrothermal method and achieved the PHE rate of 24.66 mmol g−1 h−1. The large and intimate coaxial interfacial contact between the MoS2 thin shell and the 1D CdS core formed a synergistic interaction, which efficiently hindered the recombination of photogenerated electron–hole pairs. As shown in Figure 4e,f, the CdS nanowires were excited by visible-light irradiation to generate electron–hole pairs. The photogenerated electron–hole pairs were easily recombined in the CdS nanowires without the addition of MoS2, resulting in a low carrier density. After loading the MoS2 shell on the CdS nanowire surface, forming a core–shell nanowire morphology, a comparable energy difference formed between the CBs of CdS and MoS2, leading to a strong thermodynamic driving force to facilitate the transfer of photogenerated electrons from the CdS nanowires to the MoS2 shell (Figure 4g,h). Moreover, the large and intimate coaxial interfacial contact in the [email protected]2 core–shell nanowires was favorable for the migration of photogenerated electrons, thereby enhancing the PHE rate. Zhang et al.60 synthesized core–shell CdS/g–C3N4 nanowires using a combined solvothermal and chemisorption method. As shown in Figure 4i, under visible-light irradiation, the photogenerated holes on the VB of CdS could be directly transferred to g–C3N4, whereas the photogenerated electrons on g-C3N4 were injected into the CB of CdS. Therefore, after the formation of core–shell CdS/g–C3N4 nanowires, the excited photogenerated electrons with high energy, injected from g–C3N4 to CdS, directly participated in the PHE reaction with an enhanced PHE rate. Moreover, the photogenerated holes in CdS can rapidly migrate to g-C3N4, resulting in a weaker photocorrosion of CdS. This work not only provides an important scientific route for morphology and structure engineering but also hinders the photocorrosion of CdS, which is beneficial for further PHE via overall water splitting. Similarly, Xu et al.52 designed a photocatalyst for CdS core-Au plasmonic satellite nanostructures and realized a PHE rate of 6.385 mmol g−1 h−1 under visible-light irradiation (Figure 4j). In this study, the authors confirmed that the surface plasmon resonance effect and particle size of core-Au plasmonic satellites efficiently enhanced the PHE rate of CdS, which provides other important experimental guidance for further PHE enhancement of CdS photocatalysts. Apart from the different morphologies and structures of CdS photocatalysts mentioned above, researchers have made great efforts to synthesize various kinds of CdS photocatalysts to enhance the PHE rate. Several representative examples are WS2–CdS nanohybrids,61 CdS nanorods,62 CdS/graphene nanoribbon composites,63 and CdS embedded in metal–organic frameworks (MOFs-CdS).64 In future research, the PHE performance enhancement of CdS photocatalysts could be realized by combining the advantages of various morphologies and structures of CdS photocatalysts while hindering the photocorrosion phenomenon. CdS photocatalyst with cocatalysts Many studies have proven that the photocatalytic potential of MS photocatalysts can be optimized by cocatalyst loading, especially with noble metals such as Pt, Au, and Ru.24,41 A schematic of the corresponding mechanism is shown in Figure 5a. Due to the lower Fermi energy level (EF), the photogenerated electron–hole pairs were separated more efficiently, and the photogenerated electrons were easily transferred to the noble metal.41 As shown in Figure 1d, the photogenerated electron–hole pairs are prone to bulk and surface charge recombination without the loading of cocatalysts.42 On the contrary, after loading the noble metal cocatalyst on the surface of the MS photocatalyst, both bulk and surface charge recombination were reduced due to the more efficient separation of photogenerated electron–hole pairs, resulting in an improved PHE rate.41 Figure 5 | (a) Schematic illustration of PHE using cocatalyst-loaded semiconductor photocatalyst. Reprinted with permission from ref 41. Copyright 2016 ACS. (b) Charge-transfer mechanism in GO-CdS-Pt photocatalyst. Reprinted with permission from ref 65. Copyright 2012 RSC. (c) Schematic diagram of photogenerated charges in Pt3Co-CdS photocatalyst. Reprinted with permission from ref 66. Copyright 2013 RSC. (d) Graphical representation of CdS-Pt photocatalysts with different Pt contents. Reprinted with permission from ref 67. Copyright 2015 RSC. (e) Fabrication process of GQDs, CdS nanoparticles, and the final CdS/GQDs photocatalysts. (f and g) PHE mechanism of CdS/GQDs under different wavelength range. Reprinted with permission from ref 70. Copyright 2017 Elsevier. (h) Photogenerated electron-transfer mechanism in Bi-carbon QDs/CdS photocatalyst. Reprinted with permission from ref 72. Copyright 2019 RSC. Download figure Download PowerPoint Based on the basic principle of noble metal loading, Gao et al.65 reported the fabrication of high-quality graphene oxide-CdS-Pt (GO-CdS-Pt) nanocomposites for efficient PHE. For photoactivity comparison, the PHE rate of the GO-CdS-Pt composite, loading of 0.5% of Pt cocatalyst, was 2.5 times higher than that of GO-CdS and 10.3 times higher than that of CdS. As shown in Figure 5b, the effective charge transfer and anti-recombination of GO-CdS-Pt resulted in an enhanced PHE rate and photostability. Hu and Jimmy66 designed a platinum cobalt (PtCo) alloy cocatalyst and synthesized Pt3Co-CdS for an efficient PHE (Figure 5c). The results showed that the PHE rate of Pt3Co-CdS (15.89 mmol g−1 h−1) was two times higher than Pt-CdS, which is due to the better accumulation of photogenerated electrons and higher conductivity of Pt3Co-CdS than that of Pt-CdS.66 Therefore, the loading of noble metal-based cocatalysts was an effective way to enhance the PHE rate of CdS photocatalyst due to their intrinsic band structure. However, it is desirable to minimize the content of costly noble metals on the premise of minimal loss of the PHE rate. Zhang et al.67 used a simple and convenient one-pot solvothermal process to fabricate Pt-CdS and proposed a criterion for the enhancement coefficient to identify the ideal loading amount of Pt (Figure 5d). Under visible-light irradiation, the PHE rate of CdS photocatalyst was remarkably improved from 2.10 to 10.29 mmol g−1 h−1 by loading with only 0.06% Pt (wt. %). The Pt loading amount was substantially lower than the reported optimal values, which would greatly reduce the fabrication cost. Moreover, the authors confirmed that the enhanced PHE rate and photostability of Pt-CdS photocatalyst were attributed to the hexagonal 1D structure of CdS and the high dispersion of Pt in the Pt(0) state. Therefore, the loading amount of noble metal cocatalysts could be significantly reduced without unduly sacrificing the PHE rate and photostability, which provides a facile and effective path to synthesize efficient and stable CdS-based photocatalysts for PHE. To further decrease the fabrication cost of the cocatalysts, researchers have also concentrated on the application of other low-cost cocatalysts without any noble metals to realize a higher PHE rate of CdS-based photocatalysts. Semiconductor nanocrystals, with a uniform distribution crystal size of 2–10 nm are defined as quantum dots (QDs),68,69 which have great potential to act as effective cocatalysts for CdS photocatalysts due to their size-dependent electronic performance, extended light-harvesting region, and quantum confinement effect. Lei et al.70 reported CdS/graphene QDs (CdS/GQD) nanohybrids synthesized using a facile hydrothermal method for efficient PHE and revealed the essential roles of GQDs. As shown in Figure 5e, the GQDs tightly decorated the CdS surface, forming the “dot-on-particle” heterodimer structure and rendering the nanohybrids with strong light absorption beyond the wavelength band edge of CdS. Under visible-light irradiation, the CdS/GQDs photocatalyst exhibited a PHE rate of 2.385 mmol g−1 h−1, which was 2.7 times higher than that of the bare CdS photocatalyst under the same experimental conditions. Figure 5f,g illustrates the charge-transfer mechanism of the CdS/GQDs photocatalyst during the PHE process. GQDs are types of graphene derivatives with smaller sizes and strong quantum effects and have good visible-light absorption properties.71 Although the GQDs could extend the absorption wavelength of CdS, the CdS/GQDs photocatalyst exhibited a low PHE rate at wavelengths greater than 560 nm, indicating that the long-wavelength photons absorbed by the GQDs did not participate in the production of excess electrons to induce the PHE process (Figure 5f). However, when the wavelength was below 560 nm, the PHE performance could have been improved by absorbing additional photons derived from the GQDs. For the GQDs in the photocatalyst, the graphene microdomains provided a fast electron transfer route and thus rendered a more efficient electron–hole pair separation, resulting in an enhanced PHE rate (Figure 5g).71 Wang et al.72 synthesized a series of low-cost metal atom-doped carbon QDs cocatalysts by pyrolysis, including Zn, Co, Bi, Cd, and Ti. In this study, the metal-doped carbon QDs were loaded onto CdS nanowires for efficient PHE. In particular, the Bi-doped carbon QDs/CdS exhibited the best interfacial charge separation and PHE rate of 1.77 mmol g−1 h−1, which was 4.2 times higher than that of bare CdS.72 As shown in Figure 5h, the Bi-carbon QDs act as electron acceptors due to the conjugated π structure. The Bi-carbon QDs and CdS formed intimate chemical inte