Construction of a 0D/2D S‑Scheme Heterojunction Based on Ni-Doped CsPbBr₃ and ZnO for Enhanced Photocatalytic CO₂ Reduction

Abstract

Doping with transition metals and constructing S-scheme heterojunctions are well-recognized strategies for boosting photocatalytic CO₂ reduction. Herein, we adopt a cascade design approach to fabricate a Ni-doped CsPbBr₃/ZnO S-scheme heterojunction, aiming to further improve the material’s photocatalytic activity. First, Ni-doped CsPbBr₃ nanocrystals were prepared by a hot-injection method. Subsequently, they were loaded onto ZnO nanosheets through a simple electrostatic self-assembly approach, successfully yielding a 0D/2D Ni-CsPbBr₃/ZnO S-scheme heterojunction photocatalyst. In this system, the introduction of transition metal Ni, which possesses a relatively high d-band center position, into CsPbBr₃ effectively extends the light response region and enhances the light-harvesting capacity. Meanwhile, the dimensional effects of the 0D/2D heterostructure shorten the transport path of photogenerated carriers, while the internal electric field formed at the Ni-CsPbBr₃/ZnO heterojunction interface further promotes carrier separation and transport, resulting in an increased local electron density. Under UV-Vis illumination for 4 h, the CO and CH₄ yields over 4CsPbBr₃-3Ni/ZnO reached 155.06 and 125.87 μmol/g, respectively, which are 4.38-fold and 5.43-fold those of CsPbBr₃-3Ni, 4.33-fold and 60.22-fold those of pure ZnO, and 1.40-fold and 4.32-fold those of 4CsPbBr₃/ZnO. The excellent photocatalytic CO₂ reduction performance is attributed to the synergistic interplay between d-band center engineering and S-scheme heterojunction formation. This synergy concurrently strengthens light harvesting, facilitates directional charge migration, maintains a high redox potential, and optimizes the CO₂ adsorption and activation. This study offers meaningful guidance for the rational design of high-efficiency photocatalysts.

1. Introduction

With fossil fuels being consumed at an alarming rate and CO₂ levels climbing unabated, the world faces worsening energy deficits and environmental harm. , Solar-driven photocatalytic reduction of CO₂ into high-value chemicals or renewable energy carriers is widely regarded as a viable strategy for mitigating the energy crisis and facilitating the transition toward carbon neutrality. However, current photocatalytic CO₂ reduction still faces bottlenecks such as low conversion efficiency and limited light absorption range. Therefore, a central scientific challenge in this field is to design efficient photocatalysts for enhancing CO₂ adsorption and activation while also facilitating the separation and migration of photogenerated carriers. ,

CsPbBr₃, serving as a reduction-type halide perovskite, has garnered considerable attention in photocatalysis owing to its sufficiently negative conduction band position, high photoluminescence quantum yield, and excellent optoelectronic properties. However, pure CsPbBr₃ nanocrystals tend to aggregate, exhibit insufficient adsorption and activation capability toward CO₂ molecules, and suffer from a high photogenerated carrier recombination rate. These factors severely limit their photocatalytic CO₂ reduction performance. To overcome these drawbacks, ion doping presents an effective solution. In particular, the incorporation of transition metal elements can modulate the electronic structure of the catalyst without destroying its crystal lattice, thereby enhancing its surface affinity toward CO₂ molecules. Among them, the high activity of Ni in catalysis and electrochemistry is well recognized. , Therefore, Ni doping, which possesses a relatively high d-band center, not only introduces additional active sites (Ni atoms can serve as surface centers to facilitate CO₂ adsorption and activation, and lower the energy barrier for CO bond cleavage), but also improves the lattice stability of the host. ,

ZnO, as a non-toxic, low-cost, and easily prepared oxidation-type semiconductor, has long been investigated in photocatalysis owing to its strong oxidation capability. In particular, two-dimensional (2D) ZnO nanomaterials have shown great promise, owing to their large specific surface area and outstanding electrical conductivity. Zhao et al., through first-principles calculations, revealed the evolution of photocatalytic performance of ZnO nanosheets with different layer numbers (from 1 to 8 layers), and found that ultrathin ZnO nanosheets not only possess suitable band edge positions and good light absorption, but also provide rich active sites for CO₂, thus significantly improving the photoreduction CO₂ performance.

In recent years, S-scheme heterojunctions have attracted extensive research interest owing to their superior carrier separation capability and strong redox properties. Typically composed of an oxidation-type semiconductor (OP) and a reduction-type semiconductor (RP), this heterojunction operates via a charge-transfer pathway that promotes the recombination of less active photogenerated electrons and holes, while preserving the highly reactive carriers in the RP conduction band and the OP valence band, thus enabling highly efficient photocatalysis. ,

The staggered band alignment and work function difference between CsPbBr₃ and ZnO provide a thermodynamic basis for constructing an S-scheme heterojunction. Meanwhile, loading zero-dimensional (0D) nanocrystals onto the surface of 2D nanosheets can not only effectively suppress their agglomeration but also expose more adsorption and active sites. In addition, the dimensional effects of both components help to reduce the interfacial barrier, shorten the migration path of photogenerated carriers, and thus enhance charge carrier transport efficiency, leading to improved overall photocatalytic activity. To date, various 0D/2D S-scheme heterojunction systems, such as CsPbBr₃/BiOBr, CsPbBr₃/WO₃, CsPbBr₃/BiOCl, and CsPbBr₃/Ag_i-TiO₂, have been successfully constructed, and their unique advantages in photocatalytic CO₂ reduction have been widely demonstrated.

Based on the above background, in this work we select 2D ZnO as the oxidation-type semiconductor and support, and load Ni-doped 0D CsPbBr₃ as the reduction-type semiconductor onto its surface to construct a 0D/2D Ni-CsPbBr₃/ZnO S-scheme heterojunction material system for photoreduction CO₂. Ni doping introduces new energy levels in the vicinity of the CsPbBr₃ Fermi level, extending the spectral response range. Meanwhile, the electron-rich Ni sites can act as centers for CO₂ adsorption and activation, synergistically with the S-scheme heterojunction to promote the separation and migration of photogenerated charges. A comprehensive investigation of the structure, morphology, optoelectronic properties, and charge transfer pathway is carried out using multiple characterization techniques, including XRD, TEM, XPS, and ESR, alongside evaluation of the CO₂ reduction performance under UV-Vis illumination.

2. Experimental Section

2.1. Chemical Reagents

All reagents were commercially available and used without further purification. Lead bromide (PbBr₂), cesium carbonate (Cs₂CO₃), oleylamine (C₁₈H₃₅NH₂, OAm), 1-octadecene (C₁₈H₃₆, ODE), oleic acid (C₁₈H₃₄O₂, OA), zinc nitrate hexahydrate (Zn­(NO₃)₂·6H₂O), sodium hydroxide (NaOH), nickel­(II) bromide (NiBr₂), and anhydrous ethanol were purchased from Sinopharm Chemical Reagent Co., Ltd. High-purity CO₂ gas (99.999%) was supplied by Changchun Juyang Gas Co., Ltd.

2.2. Synthesis of 0D Ni-Doped CsPbBr₃ Nanocrystals (NCs)

CsPbBr₃ (CPB) NCs were prepared via a simple hot-injection route. The detailed procedure is as follows:

Cs₂CO₃ (0.44 g) was dissolved in a mixture of ODE (20 mL) and OA (10 mL). The resulting solution was introduced into a three-neck flask, which was then evacuated and gradually heated to 120 °C under vacuum with continuous stirring for 1 h. Subsequently, the temperature was increased to 150 °C and maintained for 10 min to afford the cesium oleate solution.

PbBr₂ (0.07 g) was dissolved in a mixed solvent consisting of ODE (5 mL), OAm (0.5 mL), and OA (0.5 mL). This solution was transferred to a three-neck flask, evacuated, and gradually heated to 120 °C. After holding at this temperature for 1 h, N₂ gas was introduced. The mixture was then slowly heated to 170 °C, at which point the prepared cesium oleate solution (0.4 mL) was rapidly injected. Following 10 s of stirring, the mixture was promptly quenched in an ice-water bath to ambient temperature. The crude product was centrifuged at 8500 rpm for 5 min, the supernatant was removed, and the pale yellow-green CPB nanocrystals were collected as the final product.

Ni-doped CPB was synthesized using the same route. The Ni²⁺ content was adjusted by varying the molar ratio of NiBr₂ to PbBr₂ (1%, 3%, and 5%), and the corresponding samples were denoted as CPB-1Ni, CPB-3Ni, and CPB-5Ni, respectively. All other steps remained unchanged.

2.3. Synthesis of 2D ZnO Nanosheets (NSs)

Zn­(NO₃)₂·6H₂O (2.1567 g) and NaOH (0.9 g) were each dissolved in deionized water (15 mL). Subsequently, the Zn­(NO₃)₂·6H₂O solution was slowly dropped into the NaOH solution, stirred for 30 min, and then sealed in a 50 mL autoclave at 120 °C for 24 h. Upon cooling, the supernatant was discarded and the precipitate was collected, washed sequentially with deionized water and anhydrous ethanol (three times each), and finally dried at 70 °C for 8 h in an oven to yield 2D ZnO NSs.

2.4. Synthesis of 0D/2D Ni-Doped CsPbBr₃/ZnO S-Scheme Heterojunction

2, 4, and 8 times the amount of CPB-3Ni prepared in 2.2 was respectively dispersed together with ZnO (200 mg) in n-hexane solution (20 mL). The resultant mixture was subjected to vigorous stirring at ambient temperature for 3 h to obtain Ni-CsPbBr₃/ZnO nanocomposites (denoted as xCPB-3Ni/ZnO, where x = 2, 4, and 8). Subsequently, the suspension was rapidly transferred to a centrifuge tube, washed by centrifugation, and the final product was collected. A schematic diagram of the fabrication process is presented in Figure .

1.

Schematic diagram of the fabrication process for Ni-CsPbBr₃/ZnO nanocomposites.

Detailed information on instrument models, manufacturers, and testing procedures is given in the Supporting Information (SI).

3. Results and Discussion

3.1. Structural and Morphological Characterization

XRD analysis was first performed to determine the phase composition and purity of the products. Figure (a) shows that the diffraction pattern of CPB matches well with JCPDS No. 18-0364, with no extraneous peaks detected, confirming the successful preparation of high-purity single-phase CPB crystals. The XRD patterns of the Ni-doped samples (CPB-1Ni, CPB-3Ni, and CPB-5Ni) exhibit the same set of diffraction peaks as pure CPB, with no indication of secondary phases, demonstrating that Ni doping does not disrupt the main structure of CPB. As clearly shown in the local magnified XRD patterns (Figure S1), the diffraction peaks of the CPB-xNi samples are shifted to higher 2θ angles relative to pure CPB. The amplitude of the shift increases with the rise in Ni doping content, thereby confirming Ni as a substitutional doping in this study. , The ionic radii of Cs⁺, Pb²⁺, and Ni²⁺ are 167, 119, and 69 pm, respectively. Evidently, the ionic radius of Ni²⁺ is closer to that of Pb²⁺, and both share the same chemical valence state. Therefore, Ni²⁺ preferentially substitutes for Pb²⁺ sites in the lattice without disrupting the host structure. , Owing to the considerably smaller ionic radius of Ni²⁺ relative to Pb²⁺, the substitution of Pb by Ni gives rise to lattice contraction, which in turn causes the XRD peaks of CPB-Ni to shift toward larger 2θ angles. Similarly, all diffraction peaks of ZnO shown in Figure (b) match well with the standard pattern (JCPDS No. 36-1451). In the patterns of xCPB-3Ni/ZnO, characteristic diffraction peaks of both CPB-3Ni and ZnO are observed. With increasing amounts of CPB-3Ni, the characteristic diffraction peaks of CPB gradually intensify, while those of ZnO diminish correspondingly. No additional diffraction peaks are observed throughout the series, indicating that CPB-3Ni has been successfully loaded onto ZnO. To further corroborate the above conclusion, we performed FTIR analysis. As displayed in Figure S2, the FTIR spectrum of 4CPB-3Ni/ZnO encompasses all the characteristic peaks of CPB-3Ni and ZnO, , fully demonstrating the successful construction of heterojunction.

2.

XRD patterns of pure CPB, ZnO, CPB-xNi and xCPB-3Ni/ZnO.

We characterized the morphology of the samples using SEM and TEM. As presented in Figure (a), the ZnO nanosheets (NSs) are irregular in shape and possess a smooth surface. TEM image further reveals that the ZnO NSs are rich in pores (Figure (b)). Figure (c) shows that CPB-3Ni possesses a cubic shape with high crystallinity and good monodispersity, and each roughly 10 nm in diameter. Clear and regularly arranged lattice fringes can be observed, with interplanar spacings of 0.41 and 0.29 nm, which are assigned to the (110) and (200) planes of CPB-3Ni, respectively. , Compared with pure CPB, the lattice fringe spacing is slightly reduced, which is in good agreement with the earlier XRD analysis and further corroborates the effective substitution of Pb²⁺ ions by Ni²⁺ ions. As can be observed in Figure (d), CPB-3Ni NCs are uniformly loaded onto the surface of ZnO NSs, forming a CPB-3Ni/ZnO heterojunction. The effective combination of the two components significantly increases the exposure ratio of active sites, which is favorable for the adsorption, activation, and reduction of CO₂. The SEM images of samples with different CPB-Ni loadings (Figure S3) indicate that excessive loading can cause the aggregation of nanocrystals, which may not only cover some active sites but also prevent light from reaching the heterojunction interface, thereby reducing the photocatalytic efficiency. Further observation of the morphology of 4CPB-3Ni/ZnO via TEM (Figure (e)) clearly reveals that the cubic 0D CPB-3Ni NCs are uniformly dispersed and cover the 2D ZnO NSs surface. This face-to-face architecture is beneficial in two respects. On one hand, it accelerates charge transfer across the interface and minimizes the traveling distance of photogenerated carriers that participate in the reaction. On the other hand, it provides a large number of sites for CO₂ adsorption and activation. , Moreover, the HRTEM image (Figure (f)) shows close contact and a clear interface between the two components. The lattice fringe with a spacing of 0.26 nm is assigned to the (002) plane of ZnO, while the fringes of 0.41 and 0.29 nm are assigned to the (110) and (200) planes of CPB-3Ni, respectively. In the elemental mapping images of 4CPB-3Ni/ZnO (Figure (g)), the brightness/darkness of each element reflects its relative content, while also visually displaying the spatial distribution of Zn, O, Cs, Pb, Br, and Ni within the sample. Taken together, these findings unequivocally demonstrate the successful fabrication of a 0D/2D Ni-doped CsPbBr₃/ZnO heterostructure.

3.

(a, b) SEM and TEM images of ZnO NSs; (c) HRTEM image of CPB-3Ni NCs; (d–g) SEM, TEM, HRTEM, and elemental mapping images of 4CPB-3Ni/ZnO.

Clarifying the chemical interaction and electron-transfer behavior between ZnO and CPB-3Ni is of great significance for probing the photocatalytic performance of the 4CPB-3Ni/ZnO heterojunction. The XPS survey spectra in Figure S4 show that, besides the Zn and O signals, characteristic peaks of Cs, Pb, and Br are also observed in 4CPB-3Ni/ZnO. The absence of an obvious Ni peak in the survey spectrum may be due to its low content. The high-resolution Ni 2p spectrum of CPB-3Ni (Figure (a)) exhibits two characteristic peaks at 870.27 and 855.65 eV, which are assigned to Ni 2p_1/2 and Ni 2p_3/2, respectively, indicating that Ni²⁺ has been successfully substituted for Pb²⁺ sites in the CPB lattice, consistent with literature reports. ,, For ZnO, the Zn 2p spectrum (Figure (b)) displays the characteristic Zn 2p_3/2 and Zn 2p_1/2 peaks at 1021.64 and 1044.83 eV, with an energy separation of 23.19 eV, indicating that Zn exists in an ionic state (Zn²⁺). The asymmetric O 1s peak in Figure (c) can be resolved into two components at 530.28 and 531.92 eV, which are assigned to the lattice oxygen (Zn–O) of ZnO and surface hydroxyl or chemisorbed oxygen, respectively. , The Cs 3d spectrum for CPB-3Ni (Figure (d)) shows two peaks at 724.34 and 738.26 eV, which are indexed to Cs 3d_5/2 and Cs 3d_3/2. Similarly, the Pb 4f peaks at 138.28 and 143.14 eV (Figure (e)) are identified as Pb 4f_7/2 and Pb 4f_5/2, while the Br 3d peaks at 68.24 and 69.19 eV (Figure (f)) are assigned to Br 3d_5/2 and Br 3d_3/2, respectively. Notably, the Ni 2p, Cs 3d, Pb 4f, and Br 3d signal peaks of 4CPB-3Ni/ZnO shift to higher binding energies relative to those of CPB-3Ni, indicating a decreased electron density surrounding the CPB-3Ni species in the heterojunction. In contrast, the Zn 2p and O 1s peaks exhibit a shift to lower binding energies compared with pure ZnO, which serves to compensate for the interfacial electron redistribution. These peak shifts reveal that electrons are spontaneously transferred from CPB-3Ni to ZnO within the 4CPB-3Ni/ZnO heterojunction, demonstrating a strong interfacial interaction between ZnO and CPB-3Ni. This result constitutes compelling experimental proof for the establishment of an S-scheme heterojunction between CPB-3Ni and ZnO.

4.

High-resolution XPS spectra of (a) Ni 2p, (b) Zn 2p, (c) O 1s, (d) Cs 3d, (e) Pb 4f, and (f) Br 3d.

3.2. CO₂ Photoreduction Performance Analysis

The photocatalytic CO₂ reduction performance of the samples was systematically assessed via photoreduction experiments under UV-Vis light irradiation. The reactions were performed in acetonitrile, and a trace amount of water was dropped onto the inner wall of the reactor. Acetonitrile was selected as the primary solvent for two main reasons: (i) CsPbBr₃ nanocrystals are more stable in acetonitrile than in water; (ii) CO₂ exhibits appreciable solubility in acetonitrile. − As displayed in Figure (a), the dominant carbonaceous products are CO and CH₄. After 4 h of continuous illumination, the CO and CH₄ yields over CPB were 30.79 and 9.48 μmol/g, respectively. Ni doping significantly enhanced the production of both CO and CH₄, with CPB-3Ni exhibiting the best performance, achieving CO and CH₄ yields of 35.40 and 23.17 μmol/g, respectively, and the CH₄ yield was approximately 2.44 times that of CPB. Studies have shown that Ni doping can elevate the d-band center of the material, enhance the catalyst-reactant interaction, and prolong the residence time of carbon-containing intermediates on the surface. These effects collectively provide kinetic advantages for multi-step hydrogenation reactions, leading to a marked improvement in CH₄ selectivity. , However, further increasing the Ni content led to a decrease in photocatalytic activity. This deterioration can be ascribed to the introduction of additional defects at excessively high Ni doping concentrations, serving as recombination centers for photogenerated charge carriers and thus inhibiting electron-hole separation.

5.

(a, b) CO and CH₄ yields, (c) electron utilization efficiency, (d) control experiments under different conditions, (e) N₂ adsorption–desorption isotherms, and (f) CO₂ adsorption isotherms of the samples.

The CO and CH₄ yields over ZnO were 35.80 and 2.09 μmol/g, respectively. Upon coupling ZnO with CPB-3Ni, the photocatalytic activity was significantly enhanced (Figure (b)). Compared with CPB-3Ni, the CO yield (155.06 μmol/g) and CH₄ yield (125.87 μmol/g) over 4CPB-3Ni/ZnO increased by 4.38 and 5.43 times, respectively. Compared with ZnO, the increases were 4.33 and 60.22 times, respectively. With increasing CPB-3Ni loading, the activity of the composite photocatalysts first increased and then decreased, and 4CPB-3Ni/ZnO exhibited the optimal catalytic performance. In addition, the product yields over 4CPB-3Ni/ZnO were measured at different illumination times (Figure S5), and the results showed a steady increase in both CO and CH₄ yields with time.

The utilization efficiency of photogenerated electrons (R), which is a direct indicator for evaluating photocatalytic performance of catalyst, is presented in Figure (c). The significantly elevated R value observed for 4CPB-3Ni/ZnO relative to the other samples further substantiates its superior photoreduction capability. Control experiments for 4CPB-3Ni/ZnO (Figure (d)) showed that no products were detected under catalyst-free or dark conditions. Moreover, when Ar was used instead of CO₂, no gaseous products were generated, verifying that the carbon source was CO₂. As shown in Figure S6, the CO and CH₄ yields over 4CPB-3Ni/ZnO remained about 90% after five cycling runs, indicating that the composite exhibits excellent photostability. In addition, the XRD and SEM characterization results reveal that no obvious changes occur in the crystal structure or morphology of the sample after the reaction.

Figure (e) presents the N₂ adsorption-desorption isotherms of pure ZnO and 4CPB-3Ni/ZnO, both of which exhibit type IV isotherms with distinct H3 hysteresis loops. The BET surface area of 4CPB-3Ni/ZnO is determined to be 17.7 m²/g, lower than that of pure ZnO (26.7 m²/g), which may be attributed to the coverage or blockage of some pores on the ZnO surface by the loading of CPB-3Ni. Based on these results, it can be inferred that, under the present experimental conditions, the specific surface area is not the dominant factor determining the photocatalytic performance. The pore size distribution of the samples was analyzed by the Barrett-Joyner-Halenda (BJH) method (Figure S7). The results show that pure ZnO possesses a typical mesoporous structure. Compared with pure ZnO, 4CPB-3Ni/ZnO exhibits a relatively larger pore size, which may be due to the accumulation of CPB-3Ni nanocrystals on the ZnO surface, forming additional interstitial pores or gaps. After loading CPB-3Ni, the mesoporous structure of 4CPB-3Ni/ZnO is retained, and such abundant surface structure and porous characteristics are beneficial for enhancing CO₂ adsorption capacity. ,

Figure (f) displays the CO₂ adsorption isotherms of the various samples, it can be seen that 4CPB-3Ni/ZnO exhibits superior CO₂ adsorption capacity compared with ZnO and CPB-3Ni. This anomalous enhancement can be ascribed to the introduction of Ni and the formation of the S-scheme heterojunction, with the specific mechanisms involving the following two aspects: (i) the relatively high d-band center and the unfilled d-orbital electrons of Ni favor the adsorption of H-/O-containing species, endowing the material with strong CO₂ adsorption ability and high catalytic activity; (ii) the S-scheme heterojunction generates an internal electric field at the interface, increasing the electron density in the interfacial region and promoting the chemisorption of polar CO₂ molecules. Under this synergistic effect, the CO₂ adsorption capacity of the 4CPB-3Ni/ZnO composite is significantly enhanced. As shown in Figure S8, the photocatalytic CO₂ reduction performance of pure ZnO, CPB, CPB-3Ni, 4CPB/ZnO, and 4CPB-3Ni/ZnO exhibits significant differences, which further confirms the existence of this synergistic enhancement effect.

3.3. Photoelectrochemical Characterization

The carrier separation dynamics of the samples were probed by PL and TRPL. The emission peak intensity can effectively reflect the recombination behavior of photogenerated carriers. As shown in Figure (a), the PL peak of CPB exhibits a slight red shift as the Ni doping concentration rises, and this red shift can be traced to the band orbital hybridization caused by the partial substitution of Pb²⁺ by Ni²⁺. Meanwhile, the PL peak intensity initially declines and then rises with increasing Ni content, reaching its minimum at CPB-3Ni. This trend indicates that an appropriate Ni doping level can effectively inhibit the recombination of photogenerated electrons and holes while promoting their separation. Nevertheless, when the Ni doping level is too high, the lattice contraction becomes more severe and more defects are introduced. These defects function as recombination sites, accelerating carrier recombination, which is consistent with the photocatalytic CO₂ activity data.

6.

(a, b) PL, (c) TPR, and (d) EIS spectra of pure CPB, ZnO, CPB-xNi and xCPB-3Ni/ZnO.

Figure (b) reveals that pure ZnO has a typical ultraviolet emission peak near 380 nm. After modification with CPB-3Ni, the ultraviolet emission peak intensity of the xCPB-3Ni/ZnO composite samples is markedly reduced, indicating that the construction of the heterojunction effectively inhibits photogenerated electron-hole recombination. Among them, 4CPB-3Ni/ZnO shows the lowest peak intensity, implying the highest interfacial charge separation efficiency. The photos in Figure S9 reveal that CPB-3Ni/ZnO retains its photoluminescence without quenching and is well dispersed. Under sunlight it shows a clear yellow-green colour, while UV excitation gives a bright green fluorescence, demonstrating both excellent light-absorption performance and a pronounced capacity for generating photogenerated charge carriers. As revealed by the TRPL measurements of the transient fluorescence decay curves (Figure S10) and the average decay time τ (Table S1), the average carrier lifetime (τ _average) of xCPB-3Ni/ZnO is significantly extended. This extension can be ascribed to the effective combination of ZnO and CPB-3Ni, which suppresses non-radiative recombination. , An extended carrier lifetime implies a higher probability of carriers taking part in the photocatalytic reaction, which is advantageous for CO₂ photoreduction. Thus, the synergistic effect of Ni doping and heterojunction formation promotes charge separation in both the bulk and surface regions, extends the carrier lifetime, and consequently enhances the CO₂ reduction activity.

We employed transient photocurrent response (TPR) and electrochemical impedance spectroscopy (EIS) to further evaluate the photogenerated carrier dynamics. Figure (c) presents the photocurrent-time curves of the samples. All samples show a rapid increase in photocurrent upon illumination and a prompt return to baseline upon light interruption. The photocurrent intensity of CPB-3Ni is higher than that of CPB, indicating its superior capability for photogenerated carrier generation, separation, and migration. The EIS spectra (Figure S11) further reveal that CPB-3Ni exhibits the smallest arc radius, demonstrating its optimal interfacial charge-transfer ability and effective suppression of charge recombination. After coupling ZnO with CPB-3Ni, the photocurrent intensity is further enhanced, suggesting that the close contact between the two components effectively promotes charge separation and migration. , Among them, 4CPB-3Ni/ZnO exhibits the highest photocurrent intensity. The EIS spectra (Figure (d)) indicate that 4CPB-3Ni/ZnO has the smallest semicircle, which corresponds to the lowest charge-transfer resistance at the interface and the highest efficiency for carrier separation and migration. , The TPR and EIS results together confirm that the charge transport kinetics of the 4CPB-3Ni/ZnO heterojunction is significantly enhanced.

Ultraviolet-visible diffuse reflectance spectroscopy (UV-Vis DRS) was utilized to assess the band gaps and absorption behavior of the samples. As shown in Figure (a), the CPB-Ni samples exhibit a marked red shift in their absorption edges relative to pure CPB, indicating that Ni doping effectively broadens the light response range of CPB. Meanwhile, no new absorption bands are observed, suggesting that Ni doping has a negligible impact on the electronic structure of CPB. Figure (b) presents the Tauc plots of pure CPB and CPB-Ni, and the data reveal that Ni doping gradually narrows the band gap (E_g) from 2.35 to 2.30 eV. Figure (c) displays the absorption spectra of xCPB-3Ni/ZnO composites, where, in addition to the ZnO absorption edge, an edge attributable to CPB-3Ni can also be clearly discerned. The coupling of ZnO with CPB-3Ni leads to a further optimization of the optical band gap (from 3.12 eV for pure ZnO to a lower value, as shown in Figure (d)), which simultaneously enhances light harvesting and promotes photogenerated carrier production, indicating a marked expansion of the light absorption range.

7.

(a, c) UV-Vis DRS spectra and (b, d) Tauc plots of pure CPB, ZnO, CPB-xNi and xCPB-3Ni/ZnO.

To construct the band alignment of ZnO and CPB-3Ni, it is necessary to determine the conduction band (E_CB), valence band (E_VB), and Fermi level (E_f) positions. The E_CB values of ZnO and CPB-3Ni can be obtained by Mott-Schottky curves (Figure (a, b)). The flat-band potentials (E_FB) values are −0.56 and −1.56 V (vs. Ag/AgCl) for ZnO and CPB-3Ni, respectively, which are equivalent to −0.36 and −1.36 V vs. NHE. Both samples exhibit positive slopes in their Mott-Schottky curves, indicating n-type semiconducting behavior, and their E_CB values are approximately equal to their E_FB. , Based on the relationship E_VB = E_CB + E_g, the E_VB values of ZnO and CPB-3Ni are calculated to be +2.76 and +0.94 V, respectively, once their E_CB values are set at −0.36 and −1.36 V (as determined from the Mott-Schottky analysis). XPS valence band spectra can reflect the energy differences between the Fermi level and the valence band maximum. , The spectra in Figure (b) and (d) give the energy differences between E_f and E_VB for ZnO and CPB-3Ni are 2.90 and 1.27 eV, respectively, from which the E_f values are deduced to be −0.14 and −0.33 V. On the basis of the foregoing analyses, the band structure diagrams of ZnO and CPB-3Ni are plotted (Figure S12). The diagrams show a distinct Fermi level difference between ZnO and CPB-3Ni, which provides a thermodynamic basis for the formation of an S-scheme heterojunction.

8.

(a, b) Mott-Schottky curves and (c, d) XPS valence band spectra of pure ZnO and CPB-3Ni.

3.4. CO₂ Photoreduction Mechanism Analysis

To both verify the S-scheme heterojunction construction in 4CPB-3Ni/ZnO and elucidate the origin of its enhanced CO₂ photoreduction activity, we performed electron spin resonance (ESR) experiments with 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) as a spin trap to capture ·O₂ ^– and ·OH radical species. As displayed in Figure (a), under simulated light irradiation, the ·O₂ ^– signal intensity of the 4CPB-3Ni/ZnO heterojunction is significantly enhanced relative to the single components, indicating a higher accumulation of photogenerated electrons in the CB of CPB-3Ni for the heterojunction. This enhanced electron accumulation imparts a stronger reduction capability to convert O₂ into ·O₂ ^– (−0.33 V vs. NHE, O₂/·O₂ ^–). Figure (b) reveals that no DMPO-·OH signal is detected for CPB-3Ni, whereas both ZnO and the 4CPB-3Ni/ZnO heterojunction exhibit the typical four-peak DMPO-·OH signal (intensity ratio 1:2:2:1), and the composite shows the strongest ·OH signal. Based on the band structures of ZnO and CPB-3Ni, it is known that the VB potential of CPB-3Ni is insufficient for oxidizing H₂O to generate ·OH (+2.37 V vs. NHE, H₂O/·OH), which directly rules out the type-II heterojunction mechanism. Instead, the S-scheme pathway involves electron transfer from the CB of ZnO to the VB of CPB-3Ni. This dynamic equilibrium leaves a high concentration of holes on the ZnO surface, enabling continuous oxidation of H₂O to ·OH, perfectly matching the ESR results.

9.

ESR spectra of the samples: DMPO-·O₂® (a) and DMPO-·OH (b); AFM image (c) and surface potential along the marked line (d) of 4CPB-3Ni/ZnO.

To further verify that the photogenerated charges at the 4CPB-3Ni/ZnO heterojunction interface follow the S-scheme electron transfer mechanism, photoassisted AFM-KPFM was employed to probe the surface potential changes of the heterojunction under dark and light conditions. , As shown in the AFM image of Figure (c), the CPB-3Ni NCs are uniformly distributed on the surface of ZnO NSs with a clearly identifiable interface, which is consistent with the SEM and TEM results. Under dark conditions (Figure (d)), the surface potential difference between ZnO and CPB-3Ni is 32.5 mV, indicating the formation of a built-in electric field at their interface. Upon light irradiation, the surface potential of ZnO increases while that of CPB-3Ni decreases, suggesting that photogenerated electrons migrate from ZnO to CPB-3Ni and accumulate on its surface, further confirming the S-scheme charge transfer mechanism. Notably, in the 4CPB-3Ni/ZnO heterojunction, the surface potential difference of the CPB-3Ni component before and after illumination is 7.9 mV, which is significantly higher than that of pure CPB-3Ni (1.3 mV, Figure S13), further demonstrating that the construction of the heterojunction markedly improves the separation efficiency of photogenerated carriers.

In situ Fourier transform infrared spectroscopy (In situ FTIR) was employed to dynamically monitor the photocatalytic CO₂ reduction process over 4CPB-3Ni/ZnO. As shown in Figure , the peaks at 1094/1361/1410/1792 cm^–1, 1539/1637 cm^–1, 1399/1604/1618 cm^–1, and 1734 cm^–1 correspond to bicarbonate (HCO₃ ^–), monodentate carbonate (m-CO₃ ^2–), bidentate carbonate (b-CO₃ ^2–), and bridging carbonate (c-CO₃ ^2–), respectively. − The formation of these carbonate species may originate from the reaction of CO₂ with surface-adsorbed H₂O to generate H₂CO₃, which subsequently decomposes on the catalyst surface. The characteristic vibrational peaks attributed to the CO₂ adsorption intermediate (*CO₂) appear at 1242, 1339, 1534, and 1695 cm^–1, indicating that CO₂ is effectively adsorbed and activated on the surface of 4CPB-3Ni/ZnO. − With prolonged illumination time, characteristic peaks corresponding to *COOH gradually emerge at 1556, 1571, and 1715 cm^–1. *COOH is recognized as a critical intermediate in the reduction of CO₂ to CO and CH₄. Meanwhile, the *COO^– bands at 1298, 1375, and 1518 cm^–1 indicate the presence of a reduction pathway from *COO^– to *COOH and eventually to CO. The peak at 1654 cm^–1 corresponds to the bending vibration of adsorbed H₂O, implying that the *COOH intermediate could facilitate the pairing of electrons and protons, driving *CO and H₂O production via dehydrogenation on the active sites. , As shown in Figure S14, the absorption peak at 1992, 2030, 2061, 2092, and 2175 cm^–1 is assigned to *CO, confirming their capture as a key intermediate during CO₂ reduction. , Generally, *CO is prone to preferential desorption, leading to the formation of gaseous. In addition, characteristic peaks of *CHO (1055/1079/1137/1391 cm^–1), *CH₂O (1424 cm^–1), *CH₃O (1102/1131/1751 cm^–1), *CH₂ (1472 cm^–1), and *CH₃ (1436/1461 cm^–1) are also observed, indicating that *CO can undergo further deep hydrogenation on the 4CPB-3Ni/ZnO surface to gradually form CH₄. Overall, the 4CPB-3Ni/ZnO composite is capable of photocatalytically converting CO₂ into CO and CH₄.

10.

In situ FTIR spectra of photocatalytic CO₂ reduction over 4CPB-3Ni/ZnO in the regions of (a) 1225–1575 cm^–1 and (b) 1595–1800 cm^–1.

Based on the results of in situ FTIR, the pathways of photocatalytic CO₂ reduction can be inferred as follows: ,,,,

$${\mathrm{CO}}_2+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}_2{\mathrm{CO}}_3$$ 1

$${\mathrm{H}}_2{\mathrm{CO}}_3\to 2{\mathrm{H}}^{+}+{{\mathrm{CO}}_3}^{2-}$$ 2

$${\mathrm{CO}}_2\to {}^{*}{\mathrm{CO}}_2$$ 3

$${}^{*}{\mathrm{CO}}_2+{\mathrm{H}}^{+}+{\mathrm{e}}^{-}\to {}^{*}\mathrm{COOH}$$ 4

$${}^{*}{\mathrm{CO}}_2+{\mathrm{e}}^{-}\to {}^{*}{\mathrm{COO}}^{-}$$ 5

$${}^{*}{\mathrm{COO}}^{-}+{\mathrm{H}}_2\mathrm{O}\to {}^{*}\mathrm{COOH}+{\mathrm{OH}}^{-}$$ 6

$${}^{*}\mathrm{COOH}+{\mathrm{H}}^{+}+{\mathrm{e}}^{-}\to {}^{*}\mathrm{CO}+{\mathrm{H}}_2\mathrm{O}$$ 7

$${}^{*}\mathrm{CO}\to \mathrm{CO}$$ 8

$${}^{*}\mathrm{CO}+{\mathrm{H}}^{+}+{\mathrm{e}}^{-}\to {}^{*}\mathrm{CHO}$$ 9

$${}^{*}\mathrm{CHO}\to {}^{*}{\mathrm{CH}}_2\mathrm{O}\to {}^{*}{\mathrm{CH}}_3\mathrm{O}\to {}^{*}{\mathrm{CH}}_2\to {}^{*}{\mathrm{CH}}_3\to {\mathrm{CH}}_4$$ 10

The S-scheme heterojunction charge transfer mechanism (Figure ) further elucidates the observed enhancement in both charge separation efficiency and photocatalytic activity of the composite. According to both the calculated and experimental data, CPB-3Ni exhibits higher E_f, E_CB, and E_VB levels than ZnO. The Fermi level difference drives electron diffusion from CPB-3Ni (higher E_f) to ZnO (lower E_f) upon their contact, which correlates well with the XPS shifts. This electron diffusion causes the Fermi levels of ZnO and CPB-3Ni to shift upward and downward, respectively, until they reach the same level. Upon light irradiation, the formation of an interfacial internal electric field (IEF, oriented from CPB-3Ni to ZnO) and the concurrent band bending, in conjunction with Coulombic attraction, collectively promote the photogenerated electron transfer from the CB of ZnO to the VB of CPB-3Ni, , which corresponds to the results of ESR and KPFM. This process effectively eliminates less active carriers, while preserving the strongly reductive electrons in the CB of CPB-3Ni and the highly oxidative holes in the VB of ZnO. Such a directed migration mechanism not only reduces the probability of carrier recombination but also ensures that photogenerated electron-hole pairs with high redox potentials can effectively participate in subsequent catalytic reactions, thus leading to a significant improvement in CO₂ photoreduction activity.

11.

Charge transfer process of CO₂ photoreduction over 4CPB-3Ni/ZnO.

4. Conclusions

In summary, we selected Ni-doped 0D CsPbBr₃ nanocrystals with a relatively high d-band center position as the reduction-type semiconductor, and used 2D ZnO nanosheets as the oxidation-type semiconductor and support. A 0D/2D Ni-CsPbBr₃/ZnO S-scheme heterojunction material system was constructed via electrostatic self-assembly and applied to the photoreduction of CO₂. Ni doping enables synergistic regulation of the band structure, active sites, and d-band center position of CsPbBr₃, significantly enhancing its CO₂ adsorption and activation capability. Based on the combined results of XPS, ESR, KPFM, and band structure analysis, the presence of an S-scheme charge transfer mechanism in Ni-CsPbBr₃/ZnO is corroborated. This mechanism effectively directs charge separation and migration, retains the strong redox capability of photogenerated carriers, and consequently significantly boosts the CO₂ photoreduction efficiency. After 4 h of UV-Vis illumination, the CO and CH₄ yields over CPB-3Ni are 1.15-fold and 2.44-fold higher than those over pure CsPbBr₃, respectively. While compared with CPB-3Ni, the CO and CH₄ yields over 4CPB-3Ni/ZnO are increased by 4.38-fold and 5.43-fold, respectively. This study provides a powerful strategy for enhancing CO₂ photoreduction activity by integrating doping modification with S-scheme heterojunction construction.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.6c07948.

• Characterization; photocatalytic CO₂ reduction test; photoelectrochemical test; (Figure S1) local magnified XRD patterns of pure CPB and CPB-xNi; (Figure S2) FTIR spectra of pure ZnO, CPB-3Ni, and 4CPB-3Ni/ZnO; (Figure S3) SEM images of 2CPB-3Ni/ZnO, 4CPB-3Ni/ZnO, and 8CPB-3Ni/ZnO; (Figure S4) XPS survey spectra of pure ZnO, CPB-3Ni, and 4CPB-3Ni/ZnO; (Figure S5) CO and CH₄ yields over 4CPB-3Ni/ZnO under different illumination times; (Figure S6) cycle tests and XRD pattern of 4CPB-3Ni/ZnO after photocatalytic CO₂ reduction test; (Figure S7) pore size distribution curves of pure ZnO and 4CPB-3Ni/ZnO; (Figure S8) CO and CH₄ yields of the samples; (Figure S9) photos of CPB-3Ni and 4CPB-3Ni/ZnO under sunlight and UV light; (Figure S10) TRPL spectra of CPB-3Ni and 4CPB-3Ni/ZnO; (Figure S11) EIS spectra of pure CPB and CPB-xNi; (Figure S12) band structure diagrams of ZnO and CPB-3Ni; (Figure S13) AFM image and surface potential of pure CPB-3Ni; (Figure S14) in situ FTIR spectra of photocatalytic CO₂ reduction over 4CPB-3Ni/ZnO in the regions of 1975–2225 cm^–1 and 1025–1225 cm^–1; (Table S1) average lifetime (τ _average) of the samples [τ _average = (A ₁ τ ₁ ² + A ₂ τ ₂ ²)/(A ₁τ₁ + A ₂τ₂)] (PDF)

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T. Zhang and S. Wang contributed equally to this work.

The authors declare no competing financial interest.