Lead-free perovskite Cs3Bi2Br9/FeS2 hollow core-shell Z-scheme heterojunctions toward optimized photothermal-photocatalytic H2 production
Yongmei Xia , Zuming He , Gang He , Lianxiang Chen , Juan Zhang , Jiangbin Su , Muhammad Saboor Siddique , Xiaofei Fu , Guihua Chen , Wei Zhou
Hydrogen energy is one of the most promising candidates for the energy sources because of its high density and abundant availability, together with the zero pollution [1]. Photocatalysis technology to convert solar energy into clean chemical energy has been regarded as a sustainable strategy to alleviate energy crisis and environmental pollution [2]. However, the fast and strong recombination of photogenerated electron-hole pairs usually ends-up in low efficiency of the photocatalytic hydrogen evolution, slow migration kinetics of photo-induced carriers, and poor light availability, limiting its further practical applications. In order to inhibit the recombination of electron-hole pairs, numerous studies have been conducted on the structural regulation of the photocatalysts to understand the efficient charge separation process in catalytic reaction. Nevertheless, the majority of the semiconductor photocatalysts (such as TiO2, g-C3N4), only utilize the ultraviolet and visible light for the photocatalytic reactions, resulting in underutilization of about 54% of near-infrared light (NIR) in solar energy [3]. Moreover, NIR can also elevate the temperature of the reaction system via photothermal effect, leading to the enhanced photocatalytic activity [4]. In parallel, the temperature of the photocatalytic system is usually controlled by the cooling system, resulting into the wastage of energy [5]. Therefore, the application of near-infrared light for heat-assisted photocatalysis can significantly enhance solar energy usage and elevate the temperature of the active catalytic site [6]. Moreover, the integration of morphological design and photothermal-assisted photocatalytic Z-scheme heterojunction is also expected to significantly enhance the photocatalytic activity to a greater extent.
Among various photocatalytic studies, FeS2 (FS) has gain high attention owing to its excellent light absorption capacity, suitable band structure, low material cost and low toxicity [7]. However, the pure FS photocatalyst has high concerns of low charge separation efficiency, which greatly limits its photocatalytic H2 production rate. To overcome these problems, it is necessary to compound FS with a semiconductor having suitable band structure to construct the Z-scheme heterojunctions, thereby preventing the photogenerated charge recombination. As a result, it not only achieved the efficient separation of the photogenerated carriers, but also maintained the redox ability of residual electrons, as of optimizing the photocatalytic system. For instance, Kuo et al. synthesized FeS2-TiO2 heterostructure under NIR irradiation for highly active photocatalytic hydrogen production [8]. Li and his collaborators first employed the NIR-response photothermal effect of FeS2 to develop a photothermal/pyroelectric system for enhancing the photocatalytic H2 production activity [9]. On the other hands, the application of metal halide perovskites as hydrogen evolution photocatalysts have also attracted a considerable attention due to their high long carrier mobility, wide-ranged light absorption, and excellent band-gap tunable photoelectric properties. In particular, non-toxic lead-free perovskite Cs3Bi2Br9 (CBB) quantum dots have been found as the potential candidates for photocatalytic hydrogen production [10]. This is because CBB has a sufficient potential of negative conduction band to give it a strong reducing power. Unfortunately, the single CBB exhibited an unsatisfactory photocatalytic performance due to the rapid recombination of photogenerated carriers and insufficient active sites [11]. Therefore, combining CBB with suitable semiconductors to construct a CBB-based heterostructure has been proved to be an effective strategy for improving the photocatalytic performance of CBB. Inspired by the aforementioned strategy, we have constructed the CBB nanoparticles coupled with hollow spherical FS photothermal materials to synthesize the Z-scheme core-shell heterojunction. Among, FS acts as a "heat island", supplying heat to the catalyst after adsorption for NIR light-assisted photocatalysis system [12]. Moreover, the staggered band arrangement between CBB and FS also has the potential to create Z-scheme heterojunction photocatalysts, promoting the photoinduced charge separation, together with the retention of strong photogenerated electrons for the reduction of H+, which is highly expected to achieve efficient photothermal conversion and charge separation, thereby increasing the H2 production. However, to best of our knowledge, no literature has been reported till date on the construction of efficient photocatalysts using CBB nanoparticles and FS hollow nanospheres.
In this study, CBB/FS hollow nanospheres were synthesized with CBB QDs anchored on raspberry-like FeS2 hollow microsphere via in-situ growth method. The photocatalytic activity of CBB/FS was evaluated under simulated solar radiation (AM 1.5G). Moreover, the contribution of photothermal effect and Z-scheme heterojunction to the enhancement of CBB/FS photocatalytic performance was further analyzed by a series of characterizations and hydrogen production experiments, respectively. Finally, based on various experimental characterizations and theoretical simulations, the photothermal-assisted photocatalytic mechanism for the improved performance was unveiled.
Detailed experiments can be found in Supporting information. The 3D CBB/FS heterojunctions were synthesized by the electrostatic self-assembly strategy, as shown in Fig. 1a and Fig. S1 (Supporting information). The morphologies of the as-synthesized samples were characterized by scanning electron microscopy (SEM, Hitachi S-4800), transmission electron microscope (TEM), and high-resolution TEM (HRTEM). In this aspect, pure FS micro/nanosphere illustrated the typical concave hollow structure microspheres with approximately diameter 3–5 µm (Fig. 1b). As shown in Fig. 1c, after decorating with CBB, the hollow part and surface of the FS microsphere remain intact, anchoring the CBB nanoparticles to form core-shell structure microspheres, inducing the multiple light reflection interfaces that further enhanced the absorption and photothermal efficiency. Furthermore, the TEM (Fig. 1d) and HRTEM images of CBB/FS-5 (Fig. 1e) distinctly visualized the characteristic lattice spacing of 0.28, 0.34, and 0.24 nm, corresponding to the (202), (102) plane of CBB, and the (210) plane of FS, respectively [13,14]. Additionally, the energy dispersive X-ray (EDX) spectra verified the presence of Cs, Bi, Br, Fe, and S in CBB/FS-5 composite (Fig. S2 in Supporting information), indicating the successful development of the composite. The distinct streaks at the (210) and (111) planes of FS, and (103) planes of CBB were also observed in the selected area electron diffraction (SAED, Fig. 1f) image of the polycrystalline of CBB/FS composite. The high annular dark-field scanning TEM (HAADF-STEM, Fig. 1g) and elemental mapping (Figs. S3a-e in Supporting information) analysis further reveals that the CBB nanosheets were uniformly distributed on the surface of the FS hollow microspheres, illustrating the successfully synthesis of the CBB/FS heterojunctions, which was also consistent with the EDX findings.
The powder X-ray diffraction (XRD) measurements were employed to further unveil the crystallinity of the as-prepared samples, and the major characteristic diffraction peaks were observed at 32.97°, 37.00°, 40.68°, 47.32°, and 56.15° correspondence to the (200), (210), (211), (220), and (311), respectively (Fig. 1h), which also found to be aligned with the cubic crystal system planes of the FeS2 (JCPDS No. 71–0053) [15]. Moreover, the main characteristic peaks at 2θ = 22.2°, 27.1°, 31.6°, 38.8°, and 38.8° were mainly attributes to the (102), (003), (202), (104), and (204) planes of the hexagonal phase CBB (JCPDS No. 44–0714), respectively [16]. The characteristic diffraction peaks of the CBB and FS in the CBB/FS heterojunctions further confirmed the successful construction of CBB/FS heterojunctions, as consistent with that of the EDX findings. The XPS survey also unveil the presence of Cs, Bi, Br, S and Fe elements in CBB/FS-5 (Fig. S4 in Supporting information), which was also found consistent with the findings of the EDX, elemental mapping, and XRD. The high-resolution Cs 3d spectrum of CBB can be convolved into two peaks at 724.5 and 738.08 eV, which were indexed to Cs 3d5/2 and 3d3/2, respectively (Fig. 1i) [17]. Furthermore, the Bi 4f peaks in the pure CBB located at 158.93 and 164.25 eV, corresponds to the Bi 4f7/2 and 4f5/2 [18], and the peaks with binding energies of 68.23 and 69.15 eV were assigned to the Br 3d5/2 and Br 3d3/2, respectively (Figs. 1j and k) [19]. Likewise, two short peaks at 161.65 and 162.9 eV were assigned to the S 2p3/2 and S 2p1/2, corresponding to the S2- species of the FS, respectively (Fig. 1l) [20]. Additionally, the Fe 2p spectra of the FS exhibited the presence of Fe 2p3/2 and Fe 2p1/2 peaks at 706.35 and 719.20 eV, originated from Fe-S 2p3/2 and Fe-S 2p1/2, respectively (Fig. 1m) [21]. Noticeably, the binding energy values of the Bi 4f, Cs 3d, and Br 3d in CBB/FS were found to be more negative than that of the pristine CBB by about 0.1–0.30 eV. In contrast, the binding energy values of the Fe 2p and S 2p in CBB/FS exhibited the positive shift of roughly 0.20–0.30 eV as compared to the pristine FS. This phenomenon indicates the flow of electrons from FS to CBB at intimate interfacial interaction in the CBB/FS heterojunctions.
The optical properties of the as-prepared samples were also analyzed by UV–vis-NIR absorbance spectrum, which further suggested that the absorption edges of pristine CBB located at about 480, whereas the FS samples present a broadband absorption in the whole UV–vis-NIR scope (Fig. S5a in Supporting information). The CBB/FS heterojunctions also showed the intense absorption capability in the vis-NIR scope, suggesting the potential photothermal properties of the FS. Similarly, the band gaps for CBB and FS were further examined by the Kubelka-Munk function at ~2.48 eV and 1.0 eV, respectively (Fig. S5b in Supporting information), which were also found consistent with their calculated band structures (Fig. S6 in Supporting information). The positive slopes of the Mott-Schottky (M-S) curves indicate that the materials are n-type semiconductors, with the conduction band (CB) value at −0.2 V relative to the Fermi level [22]. In this aspect, the flat-band potentials (Ef) of the FS and CBB were determined to be −0.38 and −0.20 V vs. normal hydrogen electrode (NHE), resulting in CB of 0.58 and -0.40 V vs. NHE for FS and CBB, respectively (Figs. S5c and d in Supporting information). Therefore, according to the equation ECB = EVB - Eg, the valence bands (VB) of FS and CBB were found to be 0.42 and 2.0 8 eV vs. NHE, respectively [23,24], as consistent with the VB-XPS of CBB and FS (Fig. S7 in Supporting information). The surface area of CBB, FS and CBB/FS-5 composites is summarized in Table S1 (Supporting information). CBB/FS-5 exhibited the largest surface area, which was far superior to those of FS and CBB. In other words, CBB/FS-5 had more active sites than pure photocatalysts for photocatalytic H2 production.
The photothermal catalytic performance of the as-prepared samples was studied without using the condensed water circulation to control the temperature of the system (Pt was not used as the cocatalyst in all experiments). In this aspect, the pure CBB and FS exhibited a relatively lower H2 production rate, however, the CBB/FS-5 composites remarkably displayed the highest H2 production rate (31.5 mmol g-1 h-1), i.e., 112.6 and 77.1 times higher than that of the pure FS (0.28 mmol g-1 h-1) and CBB (0.41 mmol g-1 h-1), respectively, which was also found superior to the previously reported FS-based catalysts (Figs. 2a and b, Fig. S8 and Table S2 in Supporting information). Moreover, the apparent quantum yield (AQY) of the CBB/FS-5 (Fig. S9a and Table S3 in Supporting information) was also examined at different wavelengths, and results were summarized as 26.2% (400 nm), 29.5% (420 nm), 24.3% (450 nm), 12.4% (500 nm), 5.1% (550 nm), and 3.2% (600 nm), respectively. In addition, a negligible decrease was observed in the photocatalytic stability of the CBB/FS-5 catalyst for H2 production, even after five consecutive cycles (Fig. S9b in Supporting information). Subsequently, the characterization of the relevant structure of CBB/FS-5 before and after irradiation by XRD and XPS analysis, further confirming the robust stability of the CBB/FS-5 (Fig. S10 in Supporting information).
To unveil the intrinsic mechanism for the improved photothermal catalytic performance of the CBB/FS, the H2 evolution performance over the pure CBB and CBB/FS-5 photocatalysts were carried out at different temperatures of 10, 15, and 25 ℃, with using the condensed water circulation to control the temperature. The reaction temperature also showed the substantial influence on the hydrogen evolution rate of CBB and CBB/FS-5, suggesting that the catalytic activity increase with the increase of temperature (Figs. 2c and d). Noticeably, a significant increase in the photocatalytic H2 production rate at room temperature further illustrates that the FS-based photocatalysts exhibited a photothermal synergy in enhancing the hydrogen production. Moreover, to explore the impact of photothermal properties during photocatalysis, infrared (IR) thermal imaging was utilized to record the temperature fluctuations of the photocatalytic systems (the temperature of the reaction solution is observed at 5 min intervals). In this aspect, temperature of the pure CBB photocatalytic system after 20 min of illumination raised from 12.6 ℃ to 27.5 ℃, which was only 4.1 ℃ higher than that of the pure water (Figs. 2e and f, and Fig. S11 in Supporting information). However, the temperature of the CBB/FS-5 photocatalytic system rose from 12.6 ℃ to 54.8 ℃, and the increase temperature was as high as 42.2 ℃. These finding of the photothermal catalysis were mainly attributed to the high temperature-induced higher transport rate of the photogenerated carriers, and the lower resistance. Subsequently the rapid photogenerated charge separation and transfer, and the enhanced photocatalytic activity were observed, as consistent with the basic principles of semiconductor physics.
To probe the charge transfer process and dynamics of the CBB/FS heterostructure, photoluminescence (PL) and time resolved photoluminescence (TRPL) decay spectroscopy were performed, and found that the pure FS displayed a higher intensity of the PL that the CBB, attributing to its low catalytic activity (Fig. 3a). However, after embedding CBB into the concave hollow FS microspheres, the CBB/FS-5 composite exhibited the lowest PL intensity in comparison with all the other catalysts, indicating the significant hinderance of the charge recombination for CBB/FS heterostructure [25]. Moreover, the CBB/FS-5 composite also revealed a longer average fast decay lifetime (τave = 35.88 ns) than that of FS (τave = 2.84 ns) and CBB (τave = 4.07 ns), demonstrating the enhanced migration of electron-hole pairs (Fig. 3b and Table S4 in Supporting information). Afterwards, the photoelectrochemical experiments for the as-prepared samples were also conducted, and the photocurrent densities of the CBB/FS composites were found to be enhanced compared to those of the pure CBB and FS (Fig. 3c). Noticeably, the CBB/FS-5 composite exhibited a 3.4 and 3.2-fold higher current intensities than those of the FS and CBB, respectively. These finding further implies that the effective separation of the carriers within the aforementioned range also provides the favorable conditions for photocatalytic hydrogen production. Furthermore, a substantially lower arc radius of the CBB/FS heterostructures as compared to the pristine CBB and FS also validate the excellent carrier migration performance of the CBB/FS catalyst (Fig. 3d) [26]. The linear sweep voltammetry (LSV) curves of the as-prepared samples further, showed excellent charge of the FS, CBB, and CBB/FS composites. Transfer kinetics of CBB/FS-5 photocatalyst (Fig. 3e). In addition, the overpotentials of H2 evolution reached up to 10 µA/cm2 for FS and CBB are −170 and −150 mV, respectively, while CBB/FS-5 composite only required up to 0.80 mV of overpotential, indicating its minimum reduction ability as consistent with the findings of the H2 production [27]. Besides, the electrochemically active surface area (ECSA) of the as-synthesized photocatalysts were also investigated. The ECSA of the photocatalysts was compared by electrochemical double-layer capacitance (Cdl) measurement [28,29]. The ECSA based on the cyclic voltammetry (CV) curves at different sweep rates for each sample were also examined as show in Fig. 3f and Fig. S12 (Supporting information). It can be seen that the electrochemical Cdl of the CBB/FS-5 composite was 81 mF/cm2 with 163.5 cm-2 ECSA, which is 20.25 and 11.2 times higher than those of the FS (4 mF/cm2 with 7.38 cm-2 ECSA) and CBB (7.23 mF/cm2 with 18.2 cm-2 ECSA) nanospheres, respectively, suggesting that the formation of the heterojunction interfaces increased the number of catalytic active sites. Collectively, the aforementioned findings verified the enhanced migration of electron-hole pairs, increased active sites, and the inhibited recombination of photocarriers in the heterostructure.
To verify the Z-scheme charge transfer mechanism in the CBB/FS-5 heterojunctions, the in situ XPS analysis showed that under the light irradiation conditions, the binding energies of Bi 4f, Cs 3d and Br 3d of CCB/FS-5 were shifted 0.2–0.3 eV to higher energy levels compared to the one tested in the dark (Figs. 1i-k). In contrast, the Fe 2p and S 2p peaks shift of 0.1–0.2 eV towards the lower binding energies in CBB/FS-5, indicating the charge transfer from FS to CBB (Figs. 1l and m). However, the back flow of the photoinduced electrons from CBB to FS after irradiation validates the proposed Z-scheme charge transportation mechanism, which was also found consistent with TRPL spectra and DFT calculations. To further explore the electron transfer and separation routes at the interface between FS and CBB, electron paramagnetic resonance (EPR) experiment was conducted, which displayed the typical characteristic peaks assigned to DMPO-•O2− (Fig. 4a). In comparison, the CCB did not observed any •O2− radicals during the photocatalytic process under the light irradiation. However, the FS and CBB/FS produced significantly strengthened •O2− signals, illustration the Z-scheme charge transfer mechanism of the CBB/FS heterostructure.
Moreover, to further unveil the charge transfer mechanism in the Z-scheme CCB/FS heterojunction, DFT calculation were performed to obtain the work functions (Φ) of these two components, as shown in Figs. 4b and c. In this aspect, the Φ values of the FS (200) and CBB (001) were estimated to be ~2.56 and 5.60 eV (vs. vacuum level), respectively, indicating the lower Ef of CBB than FS [30]. However, upon their contact with each other, the electrons of the FS could spontaneously slide into the CBB across the interface of CBB/FS heterojunction, which was also confirmed by the aforementioned analysis of in situ-XPS and EPR. In addition, the sum of atomic charges of the FS and CBB in CBB/FS were calculated by Mulliken population analyses (Table 1) [31,32], which showed that the negative charge of about −0.96 eV from FeS2 surface was transferred to the CBB surface, confirming the transfer of electrons through a Z-scheme pathway.
DownLoad:
CSV
| Samples | CBB | FS | |||
| Atom | Cs | Bi | Br | Fe | S |
| Mulliken charges (eV) | −0.88 | −0.78 | 0.70 | 2.96 | −2.00 |
| Total charges (eV) | −0.96 | 0.96 | |||
The analysis for the charge density difference and electronic location functions over CBB/FS heterojunctions are shown in Figs. 4d and e, and Table S5 (Supporting information). Upon successful formation of the CBB/FS heterojunction, the charge density difference clearly displayed the charge redistribution of the Fe and Bi sites, indicating the electron donating nature of the FS. However, the CBB showed the gain of the electrons (yellow areas represent charge accumulation, blue areas represent depletion (Fig. 4d) and the isosurface is set to 0.002 e/Bohr3), respectively. These finding further indicated the migration of electrons from FS to CBB via a Z-scheme rather than a Type-Ⅱ heterojunction scheme pathway [33], validating the formation of a built electric field (IEF) for the heterojunction. Additionally, the hydrogen adsorption Gibbs free energy (ΔGH*) was also calculated to assess the hydrogen evolution activity at different active sites (Fig. 4f). Generally, a catalyst with ΔGH* approaching zero is desirable [34,35]. In this aspect, the Cs, Bi, and Fe sites in FS and CCO’s yields the ΔGH* value of −0.87, −0.615, and 0.47 eV, respectively. However, the ΔGH* values for Cs, Fe and Bi sites in CBB/FS were noticed as 0.259, −0.08 (near zero), and 0.24 eV, respectively, indicating the confirmation of the Fe sites as the catalytic active sites for the composites [36,37].
In light of the above experimental and theoretical analysis, the proposed mechanism of the photothermal-assisted synergistically enhance photocatalytic H2 production over CBB/FS is shown in Fig. 4g. The energy band structure of CBB and FS exhibited a surprising band structure configuration. Upon interaction between CBB and FS, the free electrons within the higher Fermi energy level of FS will diffuse into CBB until an equilibrium was achieved under the IEF and interfacial band bending effects [38,39]. Under the light irradiated, the photogenerated electrons (e-) on CBB’s CB could rapidly transported to the FS’s VB and recouple with FS’s photoinduced holes (h+). This electron transfer preserved the photogenerated e- and h+ on the FS’s CB and CBB’s VB, respectively, facilitating the electron-hole pair separation and retained their high redox ability concurrently at the heterojunction interface [40]. The photo-generated h+ in the VB of CBB are consumed by TEOA and the large number of photo-induced e- retained on the FS could reduce the protons to form H2 [41,42]. Meanwhile, the photothermal effect can effectively promote the migration, adsorption, and reaction of reactant molecules by increasing the temperature of the system [43]. In addition, the cavity structure of the FS also allows the light to be reflected several times in the interior, promoting the absorption and utilization of the light. Overall, the synergistic coupling of Z-scheme heterojunction and photothermal effect significantly enhanced the photocatalytic water splitting into H2 production.In this work, a promising photothermal catalyst of CBB/FS heterojunctions systems had been developed for the photocatalytic hydrogen evolution reaction. The CBB/FS Z-scheme heterojunctions displayed strong stability and super photothermal catalytic activity. Noticeably, the optimized CBB/FS-5 achieved a photocatalytic hydrogen evolution rate of 31.5 mmol g-1 h-1, which was 112.6 and 77.1 times higher than that of pure FS and CBB under simulated solar illumination, respectively, together with the apparent quantum yield of 29.5% at 420 nm. The significantly enhanced the photocatalytic H2 evolution was mainly attributed to three factors. (1) FS with full-spectrum absorption could convert the absorbed visible and NIR light into heat, which elevated the temperature of catalyst and positively contributed to the photocatalytic H2 evolution. (2) The Z-scheme charge transfer accelerated the charge separation of CBB/FS. (3) The hollow structure increased the absorption area and scattering path of the light, and at the same time provided more catalytic active sites, enhancing the separation and transport ability of photogenerated carriers. This study provides an innovative pathway for designing the photothermal catalysts aimed at delivering the sustainable and efficient solutions to energy and environmental crisis.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Yongmei Xia: Writing – original draft, Visualization, Software, Methodology, Investigation, Funding acquisition, Formal analysis, Data curation. Zuming He: Writing – review & editing, Writing – original draft, Funding acquisition, Formal analysis. Gang He: Software, Resources, Formal analysis, Data curation. Lianxiang Chen: Software, Formal analysis. Juan Zhang: Software, Formal analysis. Jiangbin Su: Software, Formal analysis. Muhammad Saboor Siddique: Writing – review & editing, Software, Resources, Formal analysis, Data curation. Xiaofei Fu: Supervision, Software, Data curation. Guihua Chen: Software, Formal analysis, Data curation. Wei Zhou: Writing – review & editing, Supervision, Software, Formal analysis, Data curation.
This work was supported by the National Natural Science Foundation of China (No. 52172206) and the Project of Science & Technology Office of Jiangsu Province (No. KB20181043), and the Talent Research Projects of Qilu University of Technology (Shandong Academy of Sciences) (No. 2024RCKY018).
Supplementary material associated with this article can be found, in the online version, at doi:
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Figure 1 (a) Schematic diagram of the synthesis process of CBB/FS. SEM images of (b) FS and (c) hollow CBB/FS-5 nanospheres. (d) TEM, (e) HRTEM images, (f) SAED patterns, (g) HAADF-STEM images of the CBB/FS-5 composite. (h) XRD patterns of prepared samples. High-resolution XPS spectra of (i) Cs 3d, (j) Bi 4f, and (k) Br 3d of CCB and CCB/FS. (l) S 2p and (m) Fe 2p of FS and CCB/FS heterojunctions.
Figure 2 (a) Photocatalytic H2 generation and (b) H2 evolution rates of as-prepared photocatalysts under the AM 1.5G irradiation. (c) Photocatalytic H2 evolution curves and (d) H2 evolution rates of CBB and CBB/FS-5 under different temperatures. Photothermal-assisted infrared temperature distributions for (e) CBB and (f) CBB/FS-5 samples under different irradiation time intervals.
Figure 3 (a) PL spectra, (b) TR-PL profiles, (c) photocurrents, (d) EIS, (e) LSV curves (obtained in 0.1 mol/L Na2SO4 electrolyte solution), and (f) electrochemical Cdl of the FS, CBB, and CBB/FS composites.
Figure 4 (a) EPR spectra of DMPO-•O2− in the methanolic and aqueous dispersion of FS, CBB, and CBB/FS after 10 min of irradiation. The work functions of (b) FS (200) and (c) CBB (100) from DFT calculation. (d, e) Calculated charge density difference distribution (unit in e/Bohr) of CBB/FS (the yellow and blue areas represent electron depletion layer and accumulation layer, respectively). (f) The calculated ΔGH* on the different FS, CBB, and CBB/FS sites. (g) Schematic illustration of the band structures of CBB and FS (left), interfacial charge transfer and the formation of an IEF upon heterojunction (middle), and schematic of CBB/FS for photocatalytic H2 production and illustration of the charge transfer process of the photogenerated electrons (right).
Table 1. The Mulliken atomic charges of the samples.
| Samples | CBB | FS | |||
| Atom | Cs | Bi | Br | Fe | S |
| Mulliken charges (eV) | −0.88 | −0.78 | 0.70 | 2.96 | −2.00 |
| Total charges (eV) | −0.96 | 0.96 | |||
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