In situ construction of Cs₃Bi₂I₉/WO₃ 0D/1D Z-scheme heterojunction photocatalyst for photochemical CO₂ reduction under visible light

Abstract

The photocatalytic efficiency of lead-free Bi-based halide perovskites, such as Cs₃Bi₂X₉ (X = Br, I) for CO₂ reduction is often hindered by self-aggregation and insufficient oxidation ability. In this work, a visible-light-driven (λ > 420 nm) Z-scheme heterojunction photocatalyst composed of 0D Cs₃Bi₂I₉ nanoparticles on 1D WO₃ nanorods for photocatalytic CO₂ reduction and water oxidation is synthesized using an in situ growing approach. The resulting 0D/1D Cs₃Bi₂I₉/WO₃ Z-scheme heterojunction photocatalyst exhibits a visible-light-driven photocatalytic CO₂ reduction performance for selective CO production, achieving a selectivity of 98.7% and a high rate of 16.5 μmol/(g·h), approximately three times that of pristine Cs₃Bi₂I₉. Furthermore, it demonstrates decent stability in the gas-solid photocatalytic CO₂ reduction system. The improved performance of Cs₃Bi₂I₉/WO₃ is attributed to the formation of the 0D/1D Z-scheme heterojunction, which facilitates charge transfer, reduces charge recombination, and maintains the active sites of both 0D Cs₃Bi₂I₉ for CO₂ reduction and 1D WO₃ for water oxidation. This work provides valuable insights into the potential of morphological engineering and the design of simultaneous Z-scheme heterojunction for lead-free halide perovskites.

Graphical abstract

Keywords

lead-free halide perovskites / 0D/1D structure / Z-scheme heterojunction / photocatalytic CO₂ reduction / morphological engineering

1 Introduction

The increasing global demand for energy has raised concerns about the depletion and environmental pollution associated with primary energy sources [1–4]. One promising solution is the conversion of CO₂ into valuable chemical products [5,6]. Photocatalytic CO₂ reduction stands out as an innovative technology that offers a sustainable way to capture and reuse CO₂ by leveraging solar energy, reducing carbon emissions, and converting CO₂ into valuable chemicals [7–9]. As a central component of photocatalysis, numerous photocatalysts have been developed for photocatalytic CO₂ reduction [5,10,11]. However, these materials still face limitations in light absorption, charge separation, CO₂ adsorption, and the high energy barrier required to break the C=O bond (~750 kJ/mol) [12–14]. Consequently, further research is essential to optimize the performance of photocatalysts to promote their practical application.

Metal halide perovskites (ABX₃) have emerged as promising candidates for photocatalysis due to their excellent light absorption, charge transport properties, and the ability to tune their band structures via compositional adjustments [15]. However, lead-containing ABX₃ materials still face significant challenges, such as degradation susceptibility and environmental toxicity [16,17], which severely limit their practical application in photocatalysis. Therefore, researchers have begun to explore lead-free alternatives using elements such as bismuth (Bi), antimony (Sb), and tin (Sn) [18–21]. Among these, Bi³⁺ is particularly appealing due to its similar electronic configuration and ionic radius to Pb²⁺, which helps retain the high optoelectronic performance of halide perovskites [22]. The dimer phase Cs₃Bi₂I₉, in particular, has been developed for a wide range of photocatalytic applications, including CO₂ reduction [12], hydrogen evolution [23], and pollutant degradation [24]. However, its performance in photocatalysis is still limited by the relatively lower charge carrier mobility, the incapability to oxidize water, and a tendency to aggregate during the photocatalytic reactions [25,26].

To enhance photocatalyst performance, the Z-scheme strategy, which integrates oxidation photocatalyst and reduction photocatalyst, has proven the most effective due to its intrinsic advantages combining broad solar spectrum absorption, enhanced charge separation, and sufficient redox potentials [24,27–29]. For example, Jiang et al. [30] reported that a Cs₃Bi₂I₉/BiVO₄ Z-scheme heterojunction enhanced photocatalytic CO₂ reduction performance as compared to the pristine Cs₃Bi₂I₉ due to the enhanced charge separation induced by the Z-scheme band alignment. Moreover, the tunability of the dimensionality of the two counterparts in a Z-scheme heterojunction can further promote photocatalytic performance. For example, the heterojunction with a point-on-line structure formed by 1D nanowires and 0D nanoparticles not only enhances charge separation but also alleviates aggregation, ensuring better exposure of active sites. Previous studies have shown that Z-scheme heterojunctions, created from materials with different dimensionalities, have exhibited excellent performance in photocatalytic CO₂ reduction and hydrogen production. A typical example is the CdS/WO₃ Z-scheme photocatalyst, where 0D CdS nanoparticles were loaded onto the 1D WO₃ nanorods, promoting the photocatalytic hydrogen evolution by virtue of the 0D/1D structure with Z-scheme band alignment [31]. Interestingly, 1D WO₃ nanorod is an ideal support and oxidation photocatalyst for Z-scheme heterojunction due to its low cost, ease of access, good stability, and strong water oxidizing properties [31–33]. Loading 0D Cs₃Bi₂I₉ onto 1D WO₃ therefore may enable the full exploitation of the reduction capability and light-responsive activity, while simultaneously preventing self-aggregation of Cs₃Bi₂I₉, thereby exposing more active sites. Moreover, if a Z-scheme heterojunction can be formed between 0D Cs₃Bi₂I₉ and 1D WO₃, the superior reducing ability of Cs₃Bi₂I₉ and the oxidizing power of WO₃ can be combined for completing the photocatalytic redox reactions of CO₂ reduction and water oxidation [34]. However, to the best of the authors’ knowledge, no previous study has attempted to integrate 0D Cs₃Bi₂I₉ nanoparticles with 1D WO₃ nanorods in a Z-scheme band alignment to enhance photocatalytic CO₂ reduction reaction and water oxidation.

This work proposes the construction of a 0D/1D Z-scheme heterojunction photocatalyst by in situ loading of Cs₃Bi₂I₉ onto WO₃ nanorods for photocatalytic CO₂ reduction. The resulting Z-scheme heterojunction, Cs₃Bi₂I₉/WO₃ (CBI/WO₃), achieved a CO production rate of 16.5 μmol/(g·h) under visible light irradiation (λ≥ 420 nm), demonstrating a threefold enhancement as compared to the pristine Cs₃Bi₂I₉. Additionally, the CO selectivity reached up to 98.7%, with only a trace amount of CH₄ detected. This high CO₂-to-CO rate and CO selectivity stands among the best photocatalysts composed of lead-free halide perovskites (Table S1). The highly enhanced performance is attributed to the formation of 0D/1D Z-scheme heterojunction, which facilitates charge transfer, reduces charge recombination, and preserves and the active sites of 0D Cs₃Bi₂I₉ for CO₂ reduction along with the active sites of 1D WO₃ for water oxidation.

2 Results and discussion

2.1 Morphological analysis of CBI/WO₃

As illustrated in Fig.1(a), WO₃ nanorods were prepared via a hydrothermal method [31], while Cs₃Bi₂I₉ nanoparticles were prepared via a hot-injection method [35]. The CBI/WO₃ heterojunction photocatalysts were then synthesized by in situ growth of Cs₃Bi₂I₉ onto WO₃ (for experimental details, see the Electronic Supplementary Material). In this work, a series of CBI/WO₃ heterojunctions with varying CBI to WO₃ mass ratios were synthesized, including CBI/WO₃-5%, CBI/WO₃-8%, CBI/WO₃-10%, CBI/WO₃-15%, and CBI/WO₃-20%. Digital photographs of the gray WO₃, bright light Cs₃Bi₂I₉, and dark brown CBI/WO₃-15% powder samples are shown in Figure S1. For comparison, a CBI/WO₃ mixture (CBI/WO₃-mix) was prepared by physically mixing Cs₃Bi₂I₉ and WO₃.

The scanning electron microscopy (SEM) image in Fig.1(b) demonstrates that the 0D Cs₃Bi₂I₉ nanoparticle tends to aggregate. In contrast, Fig.1(c) shows that the as-synthesized WO₃ forms 1D rod-like structures. The SEM image of CBI/WO₃-15% in Fig.1(d) reveals that 0D Cs₃Bi₂I₉ nanoparticles are uniformly dispersed on the 1D rod-like WO₃, forming a 0D/1D structure. Energy-dispersive X-ray spectroscopy (EDX) mapping of CBI/WO₃-15% confirms the uniform distribution of all elements, validating the even dispersion of 0D Cs₃Bi₂I₉ on the 1D WO₃ support. The Zeta potentials of Cs₃Bi₂I₉ and WO₃ were measured to support the formation of the 0D/1D structure. As shown in Fig. S2, Cs₃Bi₂I₉ has a Zeta potential of −47.06 mV, while WO₃ has a Zeta potential of 23.26 mV. These opposite polarities create an interfacial potential difference, which facilitates electrostatic attraction between Cs₃Bi₂I₉ and WO₃ [36], favoring the formation of the heterojunction.

2.2 Structural and chemical states analysis of CBI/WO₃

X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), and Fourier transform infrared spectroscopy (FTIR) were conducted to comprehensive analyze the structure and chemical states of the CBI/WO₃-15% heterojunction. As illustrated in Fig.2(a), the characteristic XRD peaks of WO₃ align with those of the monoclinic WO₃ (JCPDS No. 33-1387) [31], while the characteristic peaks of Cs₃Bi₂I₉ correspond to the hexagonal phase of Cs₃Bi₂I₉ (JCPDS No. 23-0847) [37]. Notably, both XRD peaks of WO₃ and Cs₃Bi₂I₉ can be found in the XRD pattern of CBI/WO₃-15%, providing evidence for the co-existence of WO₃ and Cs₃Bi₂I₉ in CBI/WO₃-15%.

Additionally, X-ray photoelectron spectroscopy (XPS) analysis shows peaks corresponding to Cs 3d, Bi 4f, I 3d, W 4f, and O 1s in the full XPS spectrum of the CBI/WO₃-15% heterojunction (Fig. S3), further supporting the co-existence of Cs₃Bi₂I₉ and WO₃ in the CBI/WO₃-15% heterojunction. The HRTEM images of CBI/WO₃-15% presented in Fig.2(b)-Fig.2(d) reveal the formation of CBI/WO₃ heterojunction, with lattice spacings of 0.364 nm belonging to the (200) crystal plane of WO₃, and lattice spacing of 0.42 nm belonging to the (110) plane of Cs₃Bi₂I₉ being observed in the same CBI/WO₃-15% sample. Those observations confirm that Cs₃Bi₂I₉ has been successfully grown on the surface of WO₃, aligning with the SEM results (Fig.1(d) and Fig.1(e)).

Furthermore, the FTIR spectrum in Fig. S4 shows that the structure of WO₃ is preserved in the CBI/WO₃-15% heterojunction, with prominent transmittance peaks in the range of 650-1000 cm⁻¹ and around 3400 cm⁻¹, which can be attributed to the O-W-O and W-OH stretching vibrations [38], respectively.

2.3 Photocatalytic CO₂ reduction performance

A 300 W Xe lamp equipped with a 420 nm cutoff filter (light intensity of 100 mW/cm²) was used as the visible light source to evaluate the photocatalytic CO₂ reduction performance in a semi-closed gas cycle reactor. As shown in Fig.3(a), no CO₂ reduction product was observed in the presence of pristine WO₃, and the rate of CO₂-to-CO production for pristine Cs₃Bi₂I₉ was only 5.3 μmol/(g·h). Detailed CO₂-to-CO yield data, along with their error values from three independent tests, are provided in Table S2. It is speculated that the conduction band potential of a single WO₃ photocatalyst is not negative enough to drive the CO₂ reduction reaction [39]. While a single Cs₃Bi₂I₉ photocatalyst can reduce CO₂, its efficiency is limited due to its inability to oxidize water, charge carrier recombination, and self-aggregation.

In contrast, the 0D/1D heterostructure of CBI/WO₃ demonstrated significantly enhanced CO₂ reduction performance, with the highest CO₂-to-CO rate reaching up to 16.5 μmol/(g·h) in the presence of the CBI/WO₃-15% photocatalyst, approximately 3 times that of the pristine Cs₃Bi₂I₉. Additionally, the CO selectivity was as high as 98.7%, with only a trace amount of CH₄ detected (Fig.3(b)). The CBI/WO₃-15% photocatalyst, with its high CO₂-to-CO rate and CO selectivity, ranks among the best photocatalysts composed of lead-free halide perovskites using environmentally friendly materials (Table S1). For comparison, a physical mixture of Cs₃Bi₂I₉ and WO₃ (CBI/WO₃-mix) was also tested, yielding a CO production rate of 7.2 μmol/(g·h), which was significantly lower than that of CBI/WO₃-15%, highlighting the importance of interfacial charge transfer in the intimate heterojunction formed by in situ growth of 0D Cs₃Bi₂I₉ on 1D WO₃.

Moreover, as shown in Fig.3(c), no CO or other products were detected in the absence of photocatalyst, light illumination, or CO₂, confirming that CO₂ was reduced through a photocatalytic process. A small amount of O₂ was also detected (Table S3). The 0D/1D Cs₃Bi₂I₉/WO₃ Z-scheme heterojunction photocatalyst exhibited a visible-light-driven photocatalytic H₂O oxidation performance for O₂ production, with a rate of approximately 8.0 μmol/(g·h).

The stability of the CBI/WO₃-15% heterojunction photocatalyst was evaluated by conducting a cyclic catalytic test, with a duration of 3 h per cycle, as shown in Fig.3(d). The results demonstrated that the CBI/WO₃-15% photocatalyst maintained a stable CO production rate after three cycles of 3 h reaction. XRD and FTIR analyses before and after the cyclic catalytic tests showed no significant changes in the structure of the photocatalyst (Figs. S5(a) and S5(b)), indicating its decent stability.

2.4 Charge transfer direction in CBI/WO₃ heterojunction

Different directions of charge transfer can lead to different types of heterojunctions, significantly influencing the photocatalytic performance. Previous studies have revealed that when Cs₃Bi₂I₉ is combined with other materials to form heterojunctions, charge transfer primarily occurs through the Bi and I atoms, specifically their associated 3p and 5p orbitals [40]. This is due to the conduction band minimum of Cs₃Bi₂I₉ being derived from the 3p and 5p orbitals of Bi and I [40,41]. In the case of WO₃, charge transfer predominantly occurs through the 4d orbitals of W, as these orbitals contribute to the conduction band [42].

To investigate the interfacial charge transfer direction between 0D Cs₃Bi₂I₉ and 1D WO₃ in CBI/WO₃-15%, in situ XPS and electron spin resonance (ESR) measurements were conducted. As shown in Fig.4(a)-Fig.4(c), both pristine Cs₃Bi₂I₉ and CBI/WO₃-15% exhibit distinct XPS peaks for Cs 3d, Bi 4f, and I 3d. Specifically, the XPS peaks at 738.40 and 724.41 eV correspond to the Cs 3d_3/2 and Cs 3d_5/2 of Cs₃Bi₂I₉ [43] (Fig.4(a)). The peaks at 164.31 and 158.98 eV correspond to Bi 4f_5/2 and Bi 4f_7/2 (Fig.4(b)), while the peaks at 630.14 and 618.68 eV correspond to I 3d_3/2 and I 3d_5/2 of Cs₃Bi₂I₉ [4], respectively (Fig.4(c)). Notably, under dark conditions, the Cs 3d, Bi 4f, and I 4f peaks of CBI/WO₃-15% shift to higher binding energies compared to those of pure 0D Cs₃Bi₂I₉ (Fig.4(a)-Fig.4(c)).

Furthermore, as depicted in Fig.4(d), both pure WO₃ and CBI/WO₃-15% exhibit W 4f characteristic peaks at 38.01 and 35.84 eV, corresponding to W 4f_5/2 and W 4f_7/2 [44], respectively. Under dark conditions, the W 4f peaks of CBI/WO₃-15% shift to lower binding energies compared to those of pure WO₃. These findings suggest that electrons transfer from Cs₃Bi₂I₉ to WO₃ in the dark, indicating the formation of an intimate heterojunction between Cs₃Bi₂I₉ and WO₃.

Interestingly, under light illumination, the Cs 3d, Bi 4f, and I 4f peaks of CBI/WO₃-15% shift to lower binding energies, while the W 4f peaks shift to higher binding energies, compared to their pristine counterparts, indicating that electrons transfer from WO₃ to Cs₃Bi₂I₉ under illumination. The different directions of electron transfer under dark and light conditions can be explained by the fact that, in the dark, electrons from Cs₃Bi₂I₉ tend to migrate to WO₃ upon contact to align their Fermi levels. Once the Fermi levels of Cs₃Bi₂I₉ and WO₃ align, the energy bands of Cs₃Bi₂I₉ and WO₃ bend upwards and downwards at the interface [45,46], respectively, creating an internal electric filed pointing from Cs₃Bi₂I₉ to WO₃. Under light illumination, photons excite the semiconductors, generating electron-hole pairs and increasing the electron concentration in the 4d orbitals of W in WO₃. Thereafter, as driven by the IEF, the electrons from the 4d orbitals of W will be forced to transfer toward Cs₃Bi₂I₉, where they recombine with the holes of Cs₃Bi₂I₉.

To determine the band positions of Cs₃Bi₂I₉ and WO₃, ultraviolet visible (UV-Vis) absorption, Mott-Schottky, and valance band-XPS (VB-XPS) measurements [47] were performed. UV-Vis combined with Tauc plots were used to determine the band gaps (E_g), which were found to be 1.98 eV for Cs₃Bi₂I₉ [48,49] and 2.83 eV for WO₃ [50,51], both indicating indirect band gap semiconductors (Figs. S6, S7(a), and S7(d). The Mott-Schottky measurement determined the flat band potentials (E_fb) of Cs₃Bi₂I₉ and WO₃, which were −0.69 and −0.46 eV, respectively (Figs. S7(b) and S7(e)). Additionally, VB-XPS measurements provided the valence band positions (E_VB) for Cs₃Bi₂I₉ and WO₃, yielding values of 1.14 and 2.32 eV, respectively (Figs. S7(c) and S7(f)). Using the relationship of E_CB = E_g− E_VB, the conduction band positions (E_CB) were calculated to be −0.84 eV for Cs₃Bi₂I₉ and −0.51 eV for WO₃, which were in close agreement with the E_fb values obtained from the Mott-Schottky measurement, confirming the accuracy of the measurement of the conduction band positions. Hence, Cs₃Bi₂I₉ and WO₃ form a staggered band position relationship, as depicted in Fig. S8.

ESR measurements were also conducted to further validate the Z-scheme charge transfer path of CBI/WO₃. Under dark conditions, no ESR peak was observed in the presence of the photocatalysts (Fig. S9). However, under light illumination, distinct DMPO-·O₂^– characteristic peaks were observed for both CBI/WO₃-15% and Cs₃Bi₂I₉ displayed (Fig.4(e)), indicating that CBI/WO₃-15% preserved the reduction ability of Cs₃Bi₂I₉ to reduce oxygen into superoxide radicals. Furthermore, as shown in Fig.4(f), the appearance of DMPO-OH characteristic peaks with an equidistant pattern of 1:2:2:1 quadruplet in the presence of both CBI/WO₃-15% and WO₃ confirmed that CBI/WO₃-15% maintained the oxidation ability of WO₃. These preserved reduction ability of Cs₃Bi₂I₉ and oxidation ability of WO₃ are typical features of a Z-scheme heterojunction. Hence, based on the preserved redox capabilities and the staggered bandgap positions, it can be concluded that the CBI/WO₃ heterojunction follows a typical Z-scheme charge transfer pathway.

To determine the source of the product from the photocatalytic experiment, CO₂ reduction was performed using ¹⁸H₂O and ¹³CO₂. As shown in Fig.5(a), nearly all the O₂ detected originated from ¹⁸H₂O, with the content of ¹⁶O₂ content being negligible. The reduction products, ¹³CO and a trace amount of ¹³CH₄ were identified to be originated from ¹³CO₂ (Fig.5(b)). These results confirm that CO₂ was reduced to CO and a trace amount of CH₄, while water was oxidized to O₂ in the presence of the CBI/WO₃ heterojunction photocatalyst.

Based on these findings, the photocatalytic CO₂ reduction reaction mechanism for the CBI/WO₃ Z-scheme heterojunction is proposed, as depicted in Fig.5(c) and Fig.5(d). Upon light illumination, charge carriers are generated both in Cs₃Bi₂I₉ and WO₃. Following the Z-scheme charge transfer pathway, the low-energy holes from Cs₃Bi₂I₉ recombine with the low-energy electrons from WO₃, leaving the high-energy electron in the Cs₃Bi₂I₉ to reduce CO₂ adsorbed on the catalyst, forming CO and trace amounts of CH₄, while the holes generated in WO₃ oxidize H₂O to produce oxygen. The design of 0D/1D Z-scheme heterojunction allows the high-energy electrons to be retained in 0D Cs₃Bi₂I₉ to participate in the reduction of CO₂, producing CO and trace amounts of CH₄. Meanwhile, the 1D WO₃ nanorod provides a 1D charge carriers transfer channel, efficiently transferring photogenerated holes to react with H₂O to produce O₂ and photogenerated electrons to Cs₃Bi₂I₉, where they recombine with the low-energy holes in Cs₃Bi₂I₉.

2.5 Photophysical and photoelectrochemical properties

The photophysical and photoelectrochemical properties of photocatalysts are crucial for their performance. Herein, a series of photophysical and photoelectrochemical characterizations were conducted on Cs₃Bi₂I₉, WO₃, and the Z-scheme photocatalyst CBI/WO₃-15%. The result of the surface photovoltage spectroscopy (SPV) is shown in Fig.6(a). It can be found that the surface photovoltage signals for CBI/WO₃-15%, Cs₃Bi₂I₉, and WO₃ are positive, with CBI/WO₃-15% exhibiting the most positive signal. The positive SPV signals indicate that the CBI/WO₃-15% heterojunction facilitates the most efficient transfer of photogenerated holes to the surface of the semiconductor [52], considering that both Cs₃Bi₂I₉ and WO₃ are n-type semiconductors as confirmed by Mott-Schottky plots (Figs. S7(b) and S7(e)) [41].

Electrochemical impedance spectroscopy (EIS) was further employed to assess the charge carrier transfer efficiency. As illustrated in Fig.6(b), CBI/WO₃-15% demonstrated the smallest electrochemical impedance, implying the highest charge transfer efficiency [53]. Furthermore, the equivalent resistances were fitted by adopting the Randles equivalent circuit model (inset of Fig.6(b)) with the fitting results listed in Table S4. The smaller charge transfer resistance (R_ct) of CBI/WO₃-15% compared to that of pristine Cs₃Bi₂I₉ indicates a rapid transfer of photogenerated charge carriers [54]. Moreover, the reduced Warburg impedance (Z_w) for CBI/WO₃-15% signifies a more rapid charge diffusion rate [55].

The photocurrent response is a critical indicator of the photoelectrochemical property of the material. As depicted in Fig.6(c), the current intensity for the Z-scheme heterojunction CBI/WO₃-15% was significantly enhanced, providing strong evidence that the charge transfer between Cs₃Bi₂I₉ and WO₃ was greatly improved. This improvement in charge transfer efficiency underscores the effectiveness of the Z-scheme configuration in facilitating electron-hole separation, thereby enhancing the overall photocatalytic performance of the heterojunction system. These findings suggest a substantial optimization of the interfacial interactions within the heterojunction, which could be pivotal for advancing the design of efficient photocatalytic materials.

TRPL measurements were conducted to quantitatively assess the charge separation of the Z-scheme heterojunction CBI/WO₃-15%, as shown in Fig.6(d), with the fitting parameters listed in Table S5. The average PL lifetimes for CBI/WO₃-15%, Cs₃Bi₂I₉, and WO₃ were determined to be 61.12, 62.40, and 66.79 ns, respectively. It is speculated that the construction of the Z-scheme heterojunction enhances charge transfer and separation, leading to a reduction in the PL lifetime [30]. The reduction in PL lifetime can be attributed to the efficient dissociation of photogenerated electron-hole pairs, which minimizes recombination events that would typically prolong the PL emission [43–45]. In conclusion, the photophysical and photoelectrochemical characterizations above demonstrate that the Z-scheme heterojunction effectively enhances separation and transfer of photogenerated charge carriers, contributing to improved photocatalytic CO₂ reduction performance.

3 Conclusions

This work introduces a Cs₃B₂I₉/WO₃ heterojunction photocatalyst with both 0D/1D structure and Z-scheme band alignment for photocatalytic CO₂ reduction in a gas–solid system. The 0D/1D Z-scheme photocatalyst exhibited an efficient photocatalytic CO₂ reduction performance under visible light irradiation (λ≥ 420 nm), achieving a CO generation rate of 16.5 μmol/(g·h), which is a threefold increase compared to pristine Cs₃Bi₂I₉, along with a high selectivity for CO of 98.7%. The effective conversion of CO₂-to-CO can be attributed to the morphological advantages of the 0D/1D structure, which exposes more active sites, as well as the successful formation of the Z-scheme heterojunction which preserves the strong reduction capability of Cs₃Bi₂I₉ and the robust oxidation ability of WO₃. This work provides valuable insights and strategies for the design of heterojunctions with diverse dimensionalities.

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