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• Reported by the International Energy Agency (IEA), hydrogen (H₂) serves as an important innovation requirement for achieving the goal of net-zero global carbon dioxide emissions by 2050, which not only makes an important contribution to decarbonization across a wide range of sectors such as long-distance transport, shipping, heavy industry and power generation, but also plays a key role in the overarching strategy to combat climate change [1–3]. Solar-driven photocatalytic H₂ production technology based on the reduction of protons in water molecules to generate H₂ through redox reactions with semiconductor materials provides a new pathway for energy production for sustainable human development [4,5]. To date, the limitations of the intrinsic energy band structure and the low-energy resonance effect of near-infrared (NIR) light itself have resulted in the inability of most semiconductor photocatalysts to respond within the NIR light region (about 49% of the full solar spectrum), rendering the efficiency of solar H₂ production unsatisfactory [6–8].

  A specific photothermal effect can be induced by the thermal energy (heat) generated by low-frequency NIR photons, adding another dimension of versatility to the photocatalytic process [9–11]. Specifically, in thermodynamics, the photothermal effect can increase the temperature during the photocatalytic reaction, accelerate the migration of light-induced carriers, reduce the reaction activation energy, and prompt the reaction to proceed in a positive direction; and in kinetics, the adsorption and desorption rates of the photocatalytic reactants also change with the reaction temperature, promoting the photocatalytic activity [12,13]. Therefore, designing a photothermally assisted composite photocatalytic system based on NIR light-driven light becomes an important way to expand the NIR light absorption and enhance the solar light conversion efficiency. Another key factor to improve the efficiency of solar H₂ production is the effect of the state of photo-induced carriers and the mass transfer process, which is known from the Boltzmann transport equation that the electric field distribution has a significant effect on the state of the carriers [14]. Within this context, researchers have used various modifications in order to further design the electric field in internal catalysts; such as band gap engineering, construction of heterojunctions, deposition of noble metals and heterogeneous morphology modulation [15–17]. More importantly, it should be noted that the photocatalytic reaction mainly consists of the mass transfer process and reaction environment of reactants and products, and the catalytic reaction can be improved by introducing additional physical fields that can enhance the mass flow and energy flow [18].

  At present, the piezoelectric field effect can induce polarization of materials under mechanical stress, thus enhancing the charge separation and transfer process, and is an effective means to enhance the mass transfer process in chemical production processes [19,20]. Meanwhile, the application of piezoelectric field can affect the spin state of charge carriers, reduce the recombination rate, and facilitate the promotion of photo-induced carrier separation and transfer to enhance the photocatalytic reaction activity [21,22]. Hence, the construction of advanced composites with piezoelectric as well as photothermal functions provides a novel and effective strategy to promote photocatalytic hydrogen production. SnFe₂O₄ (SFO) possesses the advantages of non-toxicity, low cost and excellent piezoelectric effect, thus introducing the piezoelectric effect into the photocatalytic system [23]. Furthermore, SFO as a metal-like material with broad-spectrum absorbance becomes an important component for photothermal conversion due to its high carrier density allowing it to absorb NIR light [24,25]. As a ternary metal sulfide, ZnIn₂S₄ (ZIS) nanosheets have attracted much attention for their suitable band gap, high specific surface area and excellent stability, and their layered structure facilitates efficient charge transport, making them ideal candidates for the synthesis of composite materials. As an idea, by integrating ZIS and SFO into a composite system, the H₂ production efficiency of NIR photocatalysis may be significantly improved by exploiting the synergistic effects of piezoelectricity and photo-thermal conversion to address the limitations of conventional photocatalytic systems [26,27].

  In this work, ZIS nanosheets were encapsulated on the surface of SFO octahedrons by a low-temperature water bath method to form a core-shell SnFe₂O₄@ZnIn₂S₄ (SFO@ZIS) composite photocatalyst for highly efficient photothermal-assisted NIR-response photocatalytic H₂ production. A series of characterization analyses show that the localized surface plasmon resonance (LSPR) effect of SFO extends the NIR light absorption range of ZIS, meanwhile, its unique piezoelectric properties contribute to the enhancement of the activity of NIR photothermally-assisted photocatalytic reactions driven by utilizing piezoelectric fields. This study contributes to the broader goal of sustainable and efficient H₂ production by constructing an efficient NIR-driven photothermal-assisted photocatalytic hydrogen production system using materials with piezoelectric effect.

  The SFO@ZIS composites with core-shell structure were successfully prepared by growing ZIS nanosheets in situ on the surface of SFO octahedron through a straightforward low-temperature water bath method (Fig. 1a). Scanning electron microscopy (SEM) characterization and analysis showed that the pristine ZIS formed in the shape of microspheres stacked with multilayered nanosheets (Fig. 1b), while the SFO was in the shape of octahedra with a uniform size distribution of about 200 nm (Fig. 1c) [28–30]. As depicted in Fig. 1d, ZIS nanosheets are tightly wrapped around the SFO octahedrons in the SFO@ZIS composite. By further observing the transmission electron microscopy (TEM) images of Figs. 1e and f, it reveals that the ZIS nanosheets with a thickness of about 10–13 nm are uniformly wrapped around the surface of the octahedral SFO, forming a core-shell structure. In addition, the high-resolution transmission electron microscopy (HR-TEM) image shown in Fig. 1g reveals distinct lattice fringes with plane spacings of 0.32 nm and 0.48 nm, corresponding to the (102) crystallographic plane of ZIS and the (111) crystallographic plane of SFO, respectively [28,31–33]. Correspondingly, energy dispersive X-ray spectroscopy (EDX, Fig. S1 in Supporting information) analyses, high-angle annular dark field scanning transmission electron microscope (HAADF-STEM) and corresponding elemental mapping images (Fig. 1h) demonstrated that the elements of Zn, In, S, Sn, Fe, and O were uniformly distributed in the composite, which further indicated that the ZIS nanosheets were dispersed on the surfaces of the SFO octahedrons, leading to the formation of successful SFO@ZIS composites with a core-shell structure.

  Figure 1

  

  Figure 1.  (a) Schematic diagram for the synthesis of the core-shell SFO@ZIS composite. SEM images of (b) ZIS, (c) SFO and (d) SFO@ZIS-7. (e, f) TEM, (g) HR-TEM and (h) element mapping images of SFO@ZIS-7 composite.

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  With the use of X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), and X-ray photoelectron spectroscopy (XPS), the prepared samples were comprehensively analyzed in terms of phase purity, functional group structure, and elemental chemical states. As illustrated in Fig. S2a (Supporting information), the characteristic diffraction peaks of pristine ZIS at 21.59°, 27.69°, and 47.18° correspond to the (006), (102), and (110) crystal planes of hexagonal-phase ZnIn₂S₄ (PDF #65–2023) [34,35], whereas the diffraction peaks of pure SFO at 30.1°, 35.5°, 43.1°, 57.0°, and 63.6° are ascribed to the (220), (311), (400), (511) and (440) crystal planes of SnFe₂O₄ (PDF #11–0614), respectively [32,36,37]. Significantly, all SFO@ZIS composites display diffraction peaks from the pristine ZIS and there is a characteristic peak at 35.5° belonging to the SFO (311) crystal plane. Moreover, the peak intensities of the (220), (400), (511), and (440) crystal planes were gradually enhanced with the increase of SFO content, which fully demonstrated the successful construction of the SFO@ZIS composite.

  From the FT-IR spectra of Fig. S2b (Supporting information), it can be seen that pristine ZIS presents several characteristic peaks, of which the absorption peaks at 3439 and 1628 cm^-1 are both caused by the hydroxyl (-OH) stretching vibration, while the absorption peak at 1106 cm^-1 is attributed to the stretching vibration of the In-S bond [38,39]. As for the SFO@ZIS composites, in addition to the characteristic peaks of ZIS, another specific peaks with corresponding to the tensile vibrations of Sn-O and Fe-O were also observed near 578 and 456 cm^-1, respectively, which further verified the successful construction of the SFO@ZIS composite [40,41]. Simultaneously, the chemical compositions and bonding states of the surface elements of the as-prepared samples were investigated by XPS. As depicted in the XPS survey of Fig. S2c (Supporting information), six elements, Zn, In, S, Sn, Fe, and O, can be found in SFO@ZIS-7, consistent with the EDS and elemental mapping results described above. Fig. S2d (Supporting information) shows the Zn 2p spectrum of SFO@ZIS-7 with two binding energy peaks at 1044.8 and 1021.9 eV attributed to Zn 2p_1/2 and Zn 2p_3/2, respectively [42]. In addition, the high-resolution In 3d spectrum in Fig. S2e (Supporting information) shows that SFO@ZIS-7 presents two peaks at 452.4 and 444.9 eV respectively, corresponding to In 3d_3/2 and 3d_5/2, respectively [43,44]. Additionally, the peaks of SFO@ZIS-7 (Fig. S2f in Supporting information) at 162.8 and 161.5 eV belong to the S 2p_1/2 and S 2p_3/2 orbitals of the S^2- anion, respectively [45]. For the Sn 3d spectrum of SFO@ZIS-7 (Fig. S2g in Supporting information), there are two characteristic peaks at 494.5 and 486.0 eV which belong to Sn 3d_3/2 and Sn 3d_5/2, respectively [46]. Fig. S2h (Supporting information) reveals that the Fe 2p spectrum of SFO@ZIS-7 shows characteristic peaks at 723.8 and 711.2 eV, corresponding to Fe 2p_1/2 and Fe 2p_3/2, respectively [47]. Meanwhile, the high-resolution O 1s spectrum of SFO@ZIS-7 (Fig. S2i in Supporting information) can be divided into two characteristic peaks of 533.5 and 531.9 eV, corresponding to surface oxygen and lattice oxygen, respectively [48,49]. Remarkably, the change in binding energy reveals a significant change in the chemical environment of the elements in the SFO@ZIS-7 composite, further confirming the successful synthesis of the SFO@ZIS composite.

  The optical absorption capacity of the as-prepared photocatalysts were characterized by UV–vis-NIR diffuse reflectance spectroscopy (DRS) in the absorption wavelength range of 350–1500 nm (Fig. 2a). The pristine ZIS, as a visible-responsive semiconductor, exhibits an absorption edge of about 490 nm, whereas the pure SFO has a broad absorption band in the range of 350–1500 nm. Significantly, the light absorption of the SFO@ZIS composites was gradually enhanced with the incorporation of SFO content, especially in the NIR region, which was mainly attributed to the LSPR effect of SFO [23]. Furthermore, the band gap energy (E_g) of the catalyst material was calculated based on the Kubelka-Munk function (αhν² = A(hν − E_g)), where α is the absorption coefficient, h is the Planck constant and ν is the photon frequency [50]. Fig. S3 (Supporting information) shows that the band gap of SFO@ZIS composites gradually decreases with the increase of the SFO doping ratio, indicating that the introduction of SFO effectively extends the optical absorption range of SFO@ZIS composites. Meanwhile, the valence band (VB) values of ZIS, SFO, and ZIS@SFO-7 were determined from the VB-XPS spectra (Fig. S4 in Supporting information), and their conduction band (CB) values were calculated by using the formula E_VB = E_CB + E_g, which further summarized their energy band structure diagrams (Fig. S5 in Supporting information). Besides, Fig. S6 (Supporting information) evident that the color of the SFO@ZIS composites progressively deepens as the SFO doping ratio increases, which coincides with the result of its enhanced light absorption ability. Interestingly, due to the LSPR effect and the excellent phonon transport capability of the SFO, it can capture solar radiation and convert it into heat more efficiently [51]. Specifically, firstly, photo-excitation and non-radiative decay of plasma excitations generate energetic electrons (hot electrons), then the excited energetic electrons release heat through electron-electron scattering, then this heat is transferred to the lattice through the electron-phonon coupling mechanism leading to the thermalization of the lattice, and ultimately the heat is diffused to the surroundings through the phonon-phonon interactions, resulting in the photothermal effect (Fig. 2b) [52–54]. Although the above experimental results indicate that the powder of the prepared material has a photothermal effect, its specific effect on the temperature of the photocatalyst solution still needs to be further explored. Figs. 2c and d indicate that the temperature of the pure ZIS reaction system only reached up to 35.7 ℃ after 8 min of irradiation with NIR light. In contrast, the surface temperature that of SFO@ZIS-7 rises significantly to 61.4 ℃, indicating that due to the introduction of SFO, the SFO@ZIS composites have a superior photothermal effect. Meanwhile, to investigate the effect of having SFO introduction on the carrier behavior, the photocurrent density plots and Mott-Schottky plots of ZIS and SFO@ZIS-7 composites were tested under light and dark conditions. As presented in Fig. 2e, as ZIS only responds in the visible region, there is almost no photocurrent signal under NIR light irradiation, while SFO@ZIS-7 displayed significant photocurrent density under NIR light irradiation. Furthermore, in the Mott-Schottky plot, SFO@ZIS has a less linear slope under NIR light irradiation (Fig. 2f), and the N_D of SFO@ZIS-7 in darkness and light is obtained as 4.32 × 10¹⁹ and 6.15 × 10¹⁹ cm^-3 by the carrier density formula (Fig. 2g), which suggests that light can effectively enhance the carrier concentration of the composite [55]. Meanwhile, the relationship between semiconductor temperature (T) and carrier mobility (µ) was further investigated using Eq. 1, where ɷ is the angular frequency, h is Planck constant, and k₀ is Boltzmann constant [56].

  μ∝[exp(hω2πk0T)−1]

  (1)

  Figure 2

  

  Figure 2.  (a) UV–vis-NIR diffuse reflectance spectra of as-synthesized photocatalysts. (b) Schematic representation of the LSPR effect leading to photothermal conversion. (c) Infrared thermograms of ZIS, SFO and SFO@ZIS-7 powders under NIR irradiation conditions for temperature variation with time. (d) Temperature versus time profiles of the prepared catalysts under NIR light illumination for 8 min. (e) Transient photocurrent response curves of ZIS and SFO@ZIS-7 under NIR light illumination. (f) Mott-Schottky plots of ZIS and SFO@ZIS-7 under light and dark conditions and (g) corresponding calculated values of carrier concentration (N_D).

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  Combining Eq. 1 with the experimental results, it is evident that higher temperatures lead to higher carriers' concentrations and transport rates, which effectively demonstrates the positive influence of the photothermal effect on carrier dynamics. Based on the above results, by introducing the SFO of the LSPR effect, it can be observed that the SFO@ZIS not only realizes the NIR photo-response, but also promotes the separation and migration of photo-generated carriers through the photothermal effect.

  To evaluate the influence of SFO introduction on the hydrogen production performance of the SFO@ZIS composite system, photocatalytic hydrogen production experiments were carried out under NIR irradiation conditions. As demonstrated in Fig. 3a and Fig. S7 (Supporting information), the NIR-driven H₂ production rates of pristine ZIS and SFO are relatively low, while SFO@ZIS composites exhibit significantly improved. It is noteworthy that the SFO@ZIS-7 photocatalyst exhibits a hydrogen production rate of 14.7 µmol g^-1 h^-1, which was 38 and 23 times higher than that of pure ZIS and SFO, respectively. This performance exceeds that of other NIR-responsive photocatalytic systems for hydrogen production (Fig. S8 and Table S1 in Supporting information). In general, materials with a NIR response absorb NIR photon energy and efficiently convert it into heat [57]. Thus, the influence of the photothermal effect on the NIR hydrogen-producing activity of the reinforced composites was investigated by employing a condensate recirculation system to regulate the temperature of the reaction system (Fig. S9 in Supporting information). As depicted in Fig. 3b, the photocatalytic H₂ production yields of the photocatalyst showed an enhanced trend with increasing ambient temperature. Specifically, at room temperature (RT), the NIR-responsive photocatalytic H₂ production performance of SFO@ZIS-7 was improved by about 1.32 times compared to that at 5 ℃ (Fig. S10 in Supporting information) and was significantly higher than that of ZIS (0.105 times), which can be attributed to the fact that the photothermal effect of SFO effectively improves the transport and separation properties of photogenerated carriers. Based on the results of photocatalytic H₂ production tests at different temperatures and the Arrhenius equation, the activation energy (E_a) values of ZIS and SFO@ZIS-7 under NIR irradiation were calculated [58]. As observed in Fig. 3c, the E_a of SFO@ZIS-7 was significantly reduced to 27.9 kJ/mol compared with the pure ZIS (34.7 kJ/mol), providing effective evidence that the photothermal effect promotes the photocatalytic H₂ production from the composite. Meanwhile, to further explore the role of the photothermal effect, we monitored the temperature change of the containing samples with pure water solution under NIR illumination via an infrared camera (Fig. S11 in Supporting information). The experimental results showed that the temperature of the pure aqueous solution reached 16.7 ℃ after 2 h of NIR light irradiation without the addition of photocatalyst (Fig. 3d), and the temperature of the ZIS solution reached 18.1 ℃ (Fig. 3e). Remarkably, the final temperature of the reaction solution of the SFO@ZIS-7 composite reached 21.6 ℃ under the same conditions (Fig. 3f), which was increased by 8.4 ℃ compared with the initial value (Fig. S12 in Supporting information), indicating that the excellent photothermal effect of SFO plays an important role in raising the temperature of the reaction environment. Furthermore, in combination with Eq. S3 (see Supporting information for specific calculations), the NIR photothermal conversion efficiency (η) of SFO@ZIS-7 was calculated. As indicated in Fig. 3g, the SFO@ZIS-7 composite has a time constant (τ_s) of 46.39 min, and its corresponding η is 52.12%, which significantly exceeds the performance of most NIR photocatalytic-photothermal systems in the previous literature (Fig. S13 and Table S2 in Supporting information) and further verifies that the excellent photothermal conversion effect of the composite. To further investigate the mechanism of photothermal-assisted NIR photocatalytic H₂ production, the photoelectrochemical characterization of SFO@ZIS-7 was tested at different temperatures to investigate the effect of photothermal effect on the carrier dynamics of the composite. Figs. 3h and i present the transient photocurrent response curves and electrochemical impedance spectra (EIS) Nyquist plots of SFO@ZIS-7 at different temperatures, respectively. The results reveal that the photocurrent intensity of SFO@ZIS-7 is enhanced and the Nyquist arc radius decreases with increasing temperature. The above result indicated that the increase in temperature can effectively improve the separation efficiency and mobility of photogenerated carriers and reduce their carrier transport resistance, thus enhancing the NIR-driven H₂ production rate of the SFO@ZIS composite. Moreover, the response overpotential of SFO@ZIS-7 at different temperatures was measured by linear scanning voltammetry (Fig. S14 in Supporting information), which revealed that the response overpotential of the composite was significantly reduced to −1.94 V with increasing temperature, which could effectively promote the proton reduction reaction, which was consistent with the results of the photocatalytic H₂ production experiments. Meanwhile, recognizing that catalyst stability is an important influence on the performance of photocatalytic hydrogen production, we tested the recycling performance of SFO@ZIS-7 composites. The performance of the SFO@ZIS-7 composite did not degrade significantly after four cyclic tests (Fig. S15a in Supporting information). Furthermore, both XRD pattern (Fig. S15b in Supporting information) and SEM image (Fig. S15c in Supporting information) diagrams confirmed that the crystal structure and morphology of SFO@ZIS-7 were well-preserved after the reaction, which indicated the superior sustainability of SFO@ZIS composite.

  Figure 3

  

  Figure 3.  (a) Yields of photocatalytic hydrogen precipitation with different combinations of catalysts under NIR illumination conditions. (b) Photocatalytic hydrogen precipitation yields of different combinations of catalysts under NIR illumination at different temperatures. (c) Apparent activation energies of ZIS and SFO@ZIS-7 at different temperatures. Photothermal IR images of the reactor under NIR illumination corresponding to (d) H₂O, (e) ZIS and (f) SFO@ZIS-7 catalyst material, respectively. (g) The photothermal conversion efficiency of SFO@ZIS-7 calculated by linear fitting method. (h) Photocurrent response curves and (i) EIS plots of SFO@ZIS-7 under different temperature conditions.

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  Piezoelectric photocatalysis can combine photocatalysis with piezoelectric catalysis, accelerating charge separation and generating an irradiation polarization field through mechanical vibration, which can effectively improve the photocatalytic performance of the materials [59,60]. In this study, the SFO is an excellent piezoelectric material, which is used to couple with ZIS to construct the photothermal-assisted piezo-photocatalytic system for promoting the synergistic effect of near-infrared driven photocatalytic H₂ production performance. As we know, piezoelectric response is one of the critical factors driving the piezoelectric catalytic reaction, thus piezo-response force microscopy (PFM) was used to investigate the piezoelectric properties of the prepared materials. The piezoelectric morphology images (Figs. S16a and b in Supporting information) exhibited significant piezoelectric response signals for SFO and SFO@ZIS-7. Meanwhile, the bright contrast of the amplitude and phase images demonstrates the pressure potential and phase difference, indicating that SFO and SFO@ZIS-7 display excellent piezoelectricity in the corresponding directions (Figs. S16c-f in Supporting information). Additionally, to further evaluate the piezoelectric properties of the as-prepared materials, the displacement-voltage curves and phase curves of SFO and SFO@ZIS-7 were tested. As depicted in Figs. S16g and h (Supporting information), typical butterfly-shaped amplitude hysteresis loops and phase hysteresis loops can be observed, indicating that piezoelectric properties and polarization response behaviors of SFO and SFO@ZIS-7. Remarkably, the SFO and SFO@ZIS-7 disclose a phase inversion close to 200° when the applied voltage is reversed from −10 V to +10 V. These above results effectively demonstrate the excellent piezoelectric performance of the SFO and SFO@ZIS-7, which helps to utilize the piezoelectric field to boost NIR driven photothermal-assisted photocatalytic H₂ production.

  The effect of the introduction of piezoelectric materials on the H₂ production properties of the prepared materials was investigated using triethanolamine (TEOA) as a sacrificial agent and Pt as a co-catalyst. As demonstrated in Fig. 4a, the H₂ production rate of SFO@ZIS-7 under NIR irradiation displays a significant enhancement compared to the pure ZIS and SFO. Fig. 4b presents the H₂ production performance of the prepared photocatalysts under sonication-only conditions, and the results reveal that ZIS exhibits a weak signal of hydrogen production, at the same time, pure SFO produces 0.32 µmol g^-1 h^-1 of hydrogen under ultrasound due to its excellent piezoelectric properties. Noticeably, the H₂ production performance of SFO@ZIS-7 under ultrasonic vibration was also significantly enhanced, but the enhancement was lower than that of the light condition. Furthermore, the piezoelectric photocatalytic hydrogen generation ability of the SFO@ZIS catalyst material under NIR illumination was further investigated in combination with external mechanical vibrations generated by an ultrasonic cleaner (Fig. S17 in Supporting information). As can be seen in Fig. 4c, the hydrogen production performance of ZIS, SFO, and SFO@ZIS-7 are all enhanced. Among them, the NIR-driven piezoelectric photocatalytic H₂ production generation yield of SFO@ZIS-7 can achieve a rate of 17.9 µmol g^-1 h^-1, which outperforms other existing NIR-based piezoelectric photocatalytic hydrogen production systems (Table S3 in Supporting information). Remarkably, the NIR-driven piezoelectric photocatalytic H₂ production performance of SFO@ZIS-7 was higher than the sum of the H₂ production values of only light irradiation and only ultrasonic vibration, which indicated that the SFO@ZIS-7 composite further enhanced photocatalytic hydrogen generation of the system due to the presence of the synergistic coupling of piezoelectric and plasmonic effects (Fig. 4d). Moreover, the variation of mechanical force promotes the carrier separation to different degrees, which influences the photocatalytic performance, therefore, the hydrogen production performance of SFO@ZIS-7 was further evaluated under different vibrational powers. As provided in Figs. 4e and f, the hydrogen production performance of SFO@ZIS-7 increased with the increase in the power of ultrasonic vibration, which may be attributed to the higher power of ultrasonic vibration leading to a more pronounced deformation of the catalyst, thus contributing to the hydrogen production performance [21,61,62]. Meanwhile, a series of electrochemical characterizations were carried out under different conditions (only light, only ultrasound, and a combination of light and ultrasound) to further investigate the influence of the piezoelectric effect on the carrier behavior of the prepared photocatalysts. Figs. 4g and h indicate the photocurrent density curves and EIS Nyquist plots of SFO@ZIS-7 under different test conditions, respectively. The results reveal that the SFO@ZIS-7 photocurrent intensity is strongest and the impedance is weakest under simultaneous light and ultrasound compared to only light and ultrasound vibration conditions. In addition, linear scanning voltammetry (LSV) curves also revealed that SFO@ZIS-7 had the lowest overpotential under simultaneous light and sonication (Fig. 4i). These above results can demonstrate that the piezoelectric effect further promotes the carrier dynamics process of SFO@ZIS composite, which enhances the photocatalytic hydrogen production rate of the reaction system.

  Figure 4

  

  Figure 4.  Photocatalytic H₂ evolution performance of the as-prepared catalysts under (a) NIR light irradiation, (b) ultrasonication and (c) simultaneous irradiation of ultrasonication and NIR light. (d) H₂ production rate of the as-prepared photocatalysts under different conditions. (e) Photocatalytic piezoelectric H₂ production curves and (f) H₂ precipitation rates of the prepared materials at different ultrasonic powers. (g) Transient photocurrent response, (h) EIS plots and (i) LSV curves of SFO@ZIS-7 under different conditions.

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  Based on the above characterization tests and experimental results, a possible hydrogen production mechanism for NIR-driven photothermal-assisted piezoelectric photocatalysis of SFO@ZIS composite was proposed (Fig. S18 and the corresponding mechanism description can be found in Supporting information).

  To sum up, an efficient and stabilized NIR-driven photothermal-assisted piezoelectric photocatalytic system was proposed for in situ growth of ZIS nanosheets on the surface of SFO octahedrons to form core-shell structured SFO@ZIS composite via a simple low-temperature water-bath method. Under ultrasonic vibration and NIR (λ > 800 nm) irradiation, the optimal H₂ production rate of the SFO@ZIS composite is up to 17.9 µmol g^-1 h^-1. The improvement in the photocatalytic hydrogen generation rate of SFO@ZIS composites can be ascribed to several reasons: (ⅰ) The LSPR effect generated by NIR light excitation of SFO effectively extends the NIR response of composite; (ⅱ) The photothermal effect of SFO effectively increases the temperature of the reaction system, which further promotes the separation and migration rate of photo-generated carriers and reduces the reaction potential; (ⅲ) Under light irradiation and ultrasonic vibration, the SFO produces energy band bending due to mechanical pressure, which transfers high-energy hot electrons generated by the LSPR effect to the CB of the ZIS and effectively facilitates the separation efficiency of photogenerated charge. This study presents a design idea for a piezoelectric photocatalytic synergistic system and a meaningful strategy and systematic approach to realize NIR-driven photothermal-assisted piezoelectric photocatalytic H₂ production.

  Declaration of competing interests

  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.

  CRediT authorship contribution statement

  Zhouze Chen: Writing – original draft, Data curation, Conceptualization. Yujie Yan: Data curation. Jun Luo: Data curation. Pengnian Shan: Data curation. Changyu Lu: Writing – review & editing, Resources, Project administration, Funding acquisition. Feng Guo: Supervision, Resources, Project administration, Funding acquisition. Weilong Shi: Writing – review & editing, Writing – original draft, Supervision, Resources, Project administration, Funding acquisition, Data curation, Conceptualization.

  Acknowledgments

  This work was financially supported by the National Natural Science Foundation of China (Nos. 21906039 and 22006057), China Postdoctoral Science Foundation (No. 2023M743178), Jiangsu Province Industry-University-Research Cooperation Project (No. BY20231482) and the Open Fund of the Key Laboratory of Solar Cell electrode Materials in China Petroleum and Chemical Industry (No. 2024A093), Key Laboratory of Functional Inorganic Material Chemistry (Heilongjiang University) Ministry of Education, Excellent Youth Fund of Basic Research Project of Universities in Shijiazhuang (No. 241790627 A), Outstanding Youth Project of Hebei GEO University in 2024 (No. JQ202403), PhD Research Startup Foundation of Hebei GEO University in 2024 (No. BQ2024026) and the National Pre-research Funds of Hebei GEO University in 2024 (No. KY2024YB03).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2025.111302.

  
  
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  Figure 1  (a) Schematic diagram for the synthesis of the core-shell SFO@ZIS composite. SEM images of (b) ZIS, (c) SFO and (d) SFO@ZIS-7. (e, f) TEM, (g) HR-TEM and (h) element mapping images of SFO@ZIS-7 composite.

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  Figure 2  (a) UV–vis-NIR diffuse reflectance spectra of as-synthesized photocatalysts. (b) Schematic representation of the LSPR effect leading to photothermal conversion. (c) Infrared thermograms of ZIS, SFO and SFO@ZIS-7 powders under NIR irradiation conditions for temperature variation with time. (d) Temperature versus time profiles of the prepared catalysts under NIR light illumination for 8 min. (e) Transient photocurrent response curves of ZIS and SFO@ZIS-7 under NIR light illumination. (f) Mott-Schottky plots of ZIS and SFO@ZIS-7 under light and dark conditions and (g) corresponding calculated values of carrier concentration (N_D).

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  Figure 3  (a) Yields of photocatalytic hydrogen precipitation with different combinations of catalysts under NIR illumination conditions. (b) Photocatalytic hydrogen precipitation yields of different combinations of catalysts under NIR illumination at different temperatures. (c) Apparent activation energies of ZIS and SFO@ZIS-7 at different temperatures. Photothermal IR images of the reactor under NIR illumination corresponding to (d) H₂O, (e) ZIS and (f) SFO@ZIS-7 catalyst material, respectively. (g) The photothermal conversion efficiency of SFO@ZIS-7 calculated by linear fitting method. (h) Photocurrent response curves and (i) EIS plots of SFO@ZIS-7 under different temperature conditions.

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  Figure 4  Photocatalytic H₂ evolution performance of the as-prepared catalysts under (a) NIR light irradiation, (b) ultrasonication and (c) simultaneous irradiation of ultrasonication and NIR light. (d) H₂ production rate of the as-prepared photocatalysts under different conditions. (e) Photocatalytic piezoelectric H₂ production curves and (f) H₂ precipitation rates of the prepared materials at different ultrasonic powers. (g) Transient photocurrent response, (h) EIS plots and (i) LSV curves of SFO@ZIS-7 under different conditions.

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