Recent progress in metal halide perovskite photocatalysts for hydrogen evolution

Abstract

Photocatalytic hydrogen production, which directly converts solar energy into green chemical fuel, has received widespread attention. However, despite significant efforts, the efficiency of conventional photocatalytic materials remains below industrial requirements, owing to the intrinsic limitations such as insufficient light absorption and poor carrier transport capability. Metal halide perovskite (MHP) materials feature superior optoelectronic properties and structural flexibility, rendering them highly attractive candidates for photocatalysis. This review provides a concise introduction to the structural characteristics of MHPs and summarizes their recent progress in the field of photocatalytic hydrogen evolution, including single-component MHPs and MHP-based composites. The review also discuss the current challenges and prospects of MHP photocatalysts, which hold promise for advancing photocatalytic solar-to-hydrogen technology.

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Xing Wang

Xing Wang received his BS degree from East China University of Science and Technology (ECUST) in 2022. He is currently a PhD candidate at the ECUST under the supervision of Prof. Yu Hou. His research focuses on the design and fabrication of stable perovskite for effective photocatalytic hydrogen evolution.

Yu Peng

Yu Peng completed her PhD from Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences in 2021. She is currently a postdoctoral research fellow at ECUST. Her research focuses on the rational design of photoactive materials for photocatalysis.

Shuang Yang

Shuang Yang obtained his BS degree from Tsingtao University of Science and Technology in 2011 and completed his PhD in 2016 from ECUST. He then conducted postdoctoral research at Nanyang Technological University and University of Nebraska-Lincoln. Currently, he is a professor at ECUST. His research focuses on the chemistry and physics of semiconducting materials and optoelectrical devices.

Hua Gui Yang

Hua Gui Yang obtained his PhD degree from the National University of Singapore in 2005. He joined the University of Queensland in 2007 as a postdoctoral research fellow. After finishing his postdoctoral training, he came back to China and took up a professor position at ECUST at the end of 2008. His research focuses on the rational design and fabrication of functional materials for solar energy conversion.

Yu Hou

Yu Hou completed his BS and PhD from ECUST in 2010 and 2015, respectively. From 2015 to 2017, he worked as a postdoctoral research fellow at ECUST and then joined the Griffith University as a visiting scholar. Currently, he is a professor at ECUST. His research focuses on novel functional materials of thin film-based solar devices and aims at developing new semiconductor absorbers and cell structures towards next-generation photovoltaic technology.

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1. Introduction

Solar energy is a clean and renewable energy source that is abundant, almost infinite, and widely distributed on earth. However, only 1% of the solar energy received by earth has been effectively used,^1,2 highlighting the importance of finding a suitable solar energy carrier to use this valuable resource more efficiently.^3–5 Hydrogen (H₂) is a clean, non-toxic, colorless, and environmentally friendly fuel without carbon dioxide emissions. Additionally, H₂ offers a broad range of energy applications, including H₂ fuel cells,^6–8 combustible fuels,^9–11 and synthetic chemical materials.^12–14 Thus, H₂ serves as an ideal carrier for solar energy. Among the various solar-to-H₂ conversion methods (e.g. photoelectrochemical H₂ evolution, photothermochemical H₂ evolution, and photocatalytic H₂ evolution), photocatalytic H₂ evolution technology, which converts solar energy to H₂ resembling natural photosynthesis, has garnered significant research interests.^15–18

Photocatalytic H₂ evolution reaction (HER) involves three main steps: (1) absorption of light by a photocatalyst to generate excitons; (2) separation and migration of carriers from the bulk to the surface of the photocatalyst; and (3) reduction and oxidation reactions on the photocatalyst surface. For achieving high-efficiency photocatalytic H₂ evolution, photocatalysts need to meet several requirements. First, appropriate energy levels are thermodynamically necessary for suitable photocatalysts. As exemplified by photocatalytic water splitting, the conduction band minimum (CBM) of photocatalysts should locate at a more negative potential than the redox potential of H⁺/H₂ (0 V at pH = 0 with respect to the normal hydrogen electrode (NHE)), the valence band maximum (VBM) of photocatalysts should locate at a more positive potential than the oxide potential of O₂/H₂O (1.23 V versus NHE), and a minimum bandgap of 1.23 eV is required. In addition, the bandgap of photocatalysts should be narrow enough to make full use of sunlight while satisfying the sufficient overpotential, together with sufficient carrier separation and transport ability. Since the original research of Fujishima and Honda in 1972, a rich variety of semiconductors have been exploited in the field of photocatalytic H₂ evolution, such as TiO₂,^19–21 CdS,^22–24 SrTiO₃,^25–27 and g-C₃N₄.^28–30 However, state-of-the-art solar-to-hydrogen (STH) efficiencies of most reported photocatalysts are still around 1%, far below the commercialization requirements, owing to the intrinsic limitations such as insufficient light absorption, poor carrier transport capability, and prevailing electron–hole recombination. In this context, it is highly urgent to develop a new class of photocatalysts with suitable optical and electronic properties for efficient photocatalytic H₂ evolution.

In the past few years, metal halide perovskites (MHPs) have achieved noteworthy breakthroughs in the field of optoelectronics,^31–33 including solar cells,^34–37 light emitting diodes,^38–40 and photodetectors.^41–43 Particularly, the light conversion efficiency (PCE) of MHP-based solar cells reached a record of 25.73%,⁴⁴ suggesting the highly efficient light absorption and charge transport abilities of MHP materials. Emphatically, MHP materials have plentiful superiorities for photocatalysts, such as high light absorption coefficient,^45,46 long carrier diffusion lengths, low binding energy,^47,48 tunable bandgaps,^49,50 and low-cost fabrication, rendering MHPs as viable candidates for photocatalytic H₂ evolution.

To date, despite several reviews on MHPs in photocatalysis,^51–53 comprehensive assessments focusing on elucidating the crucial relationship between the structure of MHPs (including the intrinsic structure of MHPs and the composites based on MHPs) and their performance in photocatalytic hydrogen evolution have been scarce. In this review, a thorough overview of recent advancements in the field of photocatalytic H₂ evolution is provided, including component engineering, energy band adjustment (e.g., extra halogen element doping), and composite structure (e.g., MHPs/conductor, MHPs/semiconductor). A specific emphasis is placed on elucidating the correlation between the structure of MHP-based photocatalysts and their performance in hydrogen evolution (Fig. 1). Additionally, the review highlights the current challenges and provides valuable perspectives for the further development of halide perovskites in photocatalytic H₂ evolution. This comprehensive review serves as a valuable reference for the design and development of highly efficient and stable MHP-based photocatalysts for H₂ evolution.

Fig. 1 Schematic overview of metal halide perovskite for photocatalytic hydrogen evolution.

2. Structure and properties of metal halide perovskites

MHPs have a general formula ABX₃, where A is a univalent organic or inorganic cation, such as methylamine cation (MA⁺), formamidine cation (FA⁺), and Cs⁺; B site is a divalent metal ion, such as Pb²⁺ and Sn²⁺; and X is a halogen ion, such as Cl⁻, Br⁻, and I⁻.^54,55 As shown in Fig. 2a, the B site and X site form a [BX₆]⁴⁻ octahedron, with B at the center and X at the vertices of the octahedron. Then, [BX₆]⁴⁻ octahedra are connected in three-dimensional (3D) space by sharing vertices to form a 3D network structure. The formability and structural stability of these perovskite-type metal halide materials can be evaluated by tolerance factor t, which is calculated as follows:
where R_A, R_B, and R_X are the effective ionic radii of ion A, ion B, and ion X in perovskite respectively. In general, to form a stable perovskite structure at room temperature, the t values need to be 0.81–1.11. An ideal cubic perovskite will be formed when t is 0.9 to 1.0, while the orthorhombic or tetragonal (Fig. 2b and c) may be formed when t is larger or smaller.⁵⁶

Fig. 2 The different crystal structures of MHPs: (a) cubic structure, (b) tetragonal structure, and (c) orthorhombic structure.

The introduction of large-sized organic cations at A site can break down the 3D framework, giving rise to low-dimensional perovskites including two-dimensional (2D), one-dimensional (1D), and zero-dimensional (0D) perovskites (Fig. 3).⁵⁷ 2D perovskites consist of alternating arrangements of inorganic [BX₆]⁴⁻ slabs and the interlayer organic molecules. Notably, the formation of 2D perovskites is not affected by the Goldschmidt tolerance factor, allowing for the utilization of an extensive library of organic cations for their preparation. Based on their crystal structure, 2D perovskites can be divided into Ruddlesden–Popper (RP) phase, Dion–Jacobson (DJ) phase, and the alternating cations in the interlayer space (ACI) phase. The structural formula of RP phase 2D perovskite is A¹₂A_n−1B_nX_3n+1, where A¹ is a long-chain univalent organic cation such as butylamine cation (BA⁺) or phenethylamine cation (PEA⁺), and A is a short-chain organic cation such as MA⁺ or FA⁺. n represents the number of inorganic skeleton layers wrapped by long-chain organic cations. When n = 1, it represents a single-layer perovskite at the molecular scale, and when n ≥ 2, it is a multilayer quasi-2D perovskite structure. The structural formula of the DJ phase 2D perovskite is A²A_n−1B_nX_3n+1, where A² represents a long-chain divalent organic cation, such as ethylenediamine cation (EDA²⁺) or 1,3-propanediamine cation (PDA²⁺). The structural formula of the ACI phase 2D perovskite is A³A_nB_nX_3n+1, where A³ is limited to some specific univalent organic cations, such as guanidine cation (GA⁺) and methylphenethylammonium cation (MPA⁺).^58,59

Fig. 3 The crystal structures of 2D, 1D, and 0D perovskite.

Further increase in the size of A-site organic cations will lead to a reduction in the structural dimension of MHPs to 1D and 0D. The structure of 1D perovskites consists of [BX₆]^4- octahedra chains with shells of organic cations, such as [DMEDA]PbBr₄ (DMEDA = N,N′-dimethylethylenediamine),⁶⁰ and [TDMP]PbBr₄ (TDMP = trans-2,5-dimethylpiperazine).⁶¹ 0D perovskites are bulk assemblies of individual [BX₆]⁴⁻ octahedra at the molecular level, where the [BX₆]⁴⁻ octahedra are separated by organic molecules, such as (C₉NH₂₀)₂SnBr₄,⁶² (C₄N₂H₁₄Br)₄SnBr₆.⁶³

On the one hand, the electronic dimension of MHPs decreases as the structural dimension decreases from 3D to 0D.⁶⁴ Thus, the bandgaps and quantum confinement effect of MHPs increase with decreasing structural dimension, leading to narrower absorption ranges and larger excitation binding energies for low-dimensional MHPs.^31,65 On the other hand, in contrast to 3D MHPs, where the A-site cation is limited to a few cations (including MA⁺, FA⁺, and Cs⁺), low-dimensional MHPs are less limited by the Goldschmidt tolerance factor and compatible with an abundance of functional organic cations such as chiral cations,^66,67 π-conjugated cations,^68,69 and flexible asymmetric cations.^70,71 This facilitates the separation and transportation of photogenerated carriers by reducing the quantum confinement effect and forming a polarization-induced electric field. Furthermore, owing to the hydrophobicity of bulky organic cations, low-dimensional MHPs exhibit much better stability than 3D MHPs, which has been confirmed in a lot of previously reported works.^72–74

3. Single-component MHP photocatalysts

Driven by their outstanding performance in optoelectronic devices, MHPs have captured significant attention in the field of photocatalysis. However, their susceptibility to moisture poses challenges for their application in photocatalytic water-splitting. Recently, Park et al. have found that MHPs can be stabilized in saturated HI solutions, leading to a surge of interest in photocatalytic HI splitting for H₂ production (Fig. 4).^75,76 In comparison to water splitting, which involves four electrons, photocatalytic HI splitting utilizes only two electrons,⁷⁷ resulting in an extremely low overpotential.^78,79 Additionally, this reaction presents a feasible alternative to the thermodynamic unfavourable HI thermal decomposition and holds promise for facilitating the industrial iodine-sulphur cycle for H₂ generation.⁸⁰ Consequently, there is considerable potential for designing efficient and stable MHPs for photocatalytic HER. In this section, we discuss the progress of single-component MHPs, classified into Pb-based and lead-free MHPs, for photocatalytic H₂ production (Table 1).

Fig. 4 The schematic diagram of HI splitting with different MHP photocatalysts.^81,83,84

Table 1 Summary of works on photocatalytic application of MHPs. R = H₂ evolution reaction rate, S = STH efficiency, A = AQE

MHP catalysts	Test conditions	Cocatalysts	MHP usage (mg)	Performance	Year^Ref.
MAPbI3	Xe lamp (λ ≥ 475 nm)	Pt	200	R = 11.4 μmol h^−1	2016^75
	100 mW cm^−2
	HI solution			S = 0.81%
MAPbBr3−xIx	Xe lamp (300 W, λ ≥ 420 nm)	Pt	250	R = 651.2 μmol h^−1	2018^85
	100 mW cm^−2
	HI solution			S = 0.81%
DMA3BiI6	LED (λ = 465 nm, 9 mW)	Pt	500	R = 47 μmol h^−1	2018^86
	HI solution			A = 82.8% at 465 nm
DMASnI3	Xe lamp (300 W)	Pt	200	R = 0.64 μmol h^−1	2018^87
	DI water
MAPb(I0.9Br0.1)3	Xe lamp (300 W, λ ≥ 420 nm)	Pt	20	R = 66.69 μmol h^−1	2019^88
	100 mW cm^−2
	The mixed HI/HBr solution			S = 1.42%
CsPbBr3−xIx	Xe lamp (λ ≥ 420 nm)	Pt	200	R = 44.8 μmol h^−1	2019^89
	120 mW cm^−2
	HI solution			A = 2.15% at 450 nm
MAPbI3	Xe lamp	Pt	50	R = 11.2 μmol h^−1	2019^90
	120 mW cm^−2
	HI solution
	Ultrasonication (70 W)
2-AMPSbI5 (Sb-precursor is SbI3)	ABET 103 solar simulator (150 W, λ = 250-1900 nm)		10	R = 1.86 μmol h^−1	2019^91
	100 mW cm^−2
	Methanol solution (10% vol)
MA3Bi2I9	Xe lamp (300 W, λ ≥ 400 nm)	Pt	40	R = 6.77 μmol h^−1	2019^92
	HI solution			S = 0.48%
Cs3Bi0.6Sb1.4I9	Simulated sunlight (AM 1.5 G)	Pt	100	R = 92.6 μmol h^−1	2020^93
	100 mW cm^−2
	HI solution
MAPbI3 with MA^+ film	Xe lamp (300 W)		50	R = 15.65 μmol h^−1	2021^94
	HI solution
(C6H5CH2NH3)2PbI4	Xe lamp (300 W, λ ≥ 420 nm)	Pt	150	R = 333 μmol h^−1	2021^81
	HI solution			S = 1.57%
Cs2Pt0.09Sn0.95Cl6	Xe lamp (300 W)	Pt		R = 16.11 μmol h^−1	2021^95
	TEOA solution (10% vol)
DMA3BiI6	LED lamp (100 W)	Pt	50	R = 5.7 μmol h^−1	2021^96
	76.4 mW cm^−2
	HI solution			A = 0.83% at 535 nm
Defect-rich Cs2AgBiBr6	Xe lamp (300 W, λ ≥ 420 nm)		500	R = 0.406 μmol h^−1	2021^97
	332.5 mW cm^−2
	HBr solution
Cs2SnI6	Xe lamp (300 W, λ ≥ 420 nm)	PtSA	10	R = 4.30 μmol h^−1	2021^98
	100 mW cm^−2
	HI solution
FAPbBr3−xIx	Xe lamp with an AM1.5G filter (300 W, λ ≥ 420 nm)	PtSA	100	R = 682.6 μmol h^−1	2022^99
	100 mW cm^−2			S = 4.50%
	The mixed HI/HBr solution			A = 33.4% at 530 nm
OA0.15MA0.85PbI3	PL-MW2000 light source in the full visible range	Pt	5	R = 2.68 μmol h^−1	2022^82
	HI solution			A = 1.67% at 420 nm
MA3Bi2Cl9−xIx	Xe lamp (300 W, λ ≥ 420 nm)	Pt	500	R = 341 ± 61.7 μmol h^−1	2022^100
	100 mW cm^−2
	Saturated HCl (or HCl/HI) solution			A = 1.25% at 435 nm
Cs3Bi2I9 PNs	Xe lamp (300 W, λ = 350–780 nm, 790 mW cm^−2)		50	R = 10.79 μmol h^−1	2022^101
	Ethanol solution

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3.1 Pb-based perovskites for photocatalytic hydrogen evolution

In 2016, Park et al. successfully demonstrated the first photocatalytic HER of MHP by stabilizing MAPbI₃ in its saturated HI solution using the dynamic equilibrium between dissolution and precipitation (as shown in Fig. 5a and b). Under visible light excitation, the photogenerated electrons in MAPbI₃ reduced H⁺ to H₂ and the photogenerated holes oxidized I⁻ to I₃⁻. The oxidized I₃⁻ ions were subsequently reduced back to I⁻ by H₃PO₂, achieving sustainable photocatalytic H₂ production. With Pt as co-catalyst, the HER rate and STH efficiency of MAPbI₃ (200 mg) were measured to be 11.4 μmol h⁻¹ and 0.81%, respectively. This result provided a new solution for the photocatalytic application of MHPs.

Fig. 5 (a) Photographs of MAPbI₃ in different concentrations of HI solution. (b) Diagram of equilibrium dissolution of MAPbI₃ powder in saturated HI solution.⁷⁵ Copyright 2016 Springer Nature. (c) The funnel bandgap of MAPbBr_3−xI_x. Copyright 2018 American Chemical Society. The schematic diagram of the migration of carriers for 2D perovskite in (d) solar cell and (e) photocatalyst. (f) The photocatalytic HER rates of (C₆H₅CH₂NH₃)₂PbI₄ (PMPI), (C₆H₅(CH₂)₂NH₃)₂PbI₄ (PEPI), and (C₆H₅(CH₂)₃NH₃)₂PbI₄ (PPPI).⁸¹ Copyright 2021 Wiley-VCH GmbH. (g) The scanning transmission election microscope-high angle annular dark field (STEM-HAADF) images of PtSA/FAPbBr_3−xI_x. Copyright 2022 Wiley-VCH GmbH. (h) Schematic diagram of the growth of organic passivation membrane on the surface of MAPbI₃.⁹⁴ Copyright 2021 American Chemical Society.

Following this approach, halide perovskites with different structures have been explored for photocatalytic HER. For example, Wang et al. synthesized three 2D RP perovskites with different lengths of phenylalkylammonium ions (C₆H₅(CH₂)_nNH₃⁺; n = 1, 2, and 3).⁸¹ In 2D MHP-based solar cells, the 2D MHP layer is consistently oriented parallel to the charge extraction layer, necessitating the passage of carriers through the poorly conductive organic layer for utilization, ultimately resulting in decreased overall charge transport capacity. (Fig. 5d). However, in 2D MHP-based powder photocatalytic systems, carriers traveling along the more conductive inorganic layer can be effectively utilized (Fig. 5e), thus rendering 2D MHPs as promising candidates for photocatalytic HER. Moreover, the excellent moisture resistance of 2D RP MHPs allows the photocatalytic measurement to be carried out in aqueous salt solutions, reducing reliance on strongly acidic HI-saturated solutions. This improvement significantly reduced the acidity of the solution, making the experimental conditions milder, and improved the reproducibility of the measurement due to the lower solubility of 2D perovskite in the aqueous salt solution. Further analysis demonstrated that the length of organic cations could significantly affect the dissolution behaviour, morphology, and photoelectric properties, such as bandgap and band edge position of 2D MHP crystals. Among the investigated 2D MHPs, (C₆H₅CH₂NH₃)₂PbI₄ with the shortest length of organic cations showed the best photocatalytic performance with an HER rate of 333 μmol h⁻¹ (150 mg) and an STH efficiency of 1.57% when using Pt as cocatalyst (Fig. 5f).

In addition, Li et al. combined the 3D perovskite MAPbI₃ with the 2D octadecylammonium lead iodide perovskite (OA₂PbI₄) to create multidimensional perovskites, denoted as OA_xMA_1−xPbI₃.⁸² These materials exhibited superior charge transfer abilities compared to traditional 3D perovskites and better photocatalytic performance than 2D perovskites. Consequently, OA_xMA_1−xPbI₃ samples showed an optimal photocatalytic HER rate of 2.68 μmol h⁻¹ (5 mg), outperforming bare MAPbI₃ (1.38 μmol h⁻¹, 5 mg). This excellent performance was ascribed to their superior absorption properties throughout the visible region compared to other OA_xMA_1−xPbI₃ samples.

Notably, the structural flexibility of MHPs imparts them great potential to tune their energy levels by adjusting halide composition. Huang et al. fabricated a mixed halide perovskite MAPbBr_3−xI_x (I-doped MAPbBr₃) with I gradient distribution in a mixed HI/HBr solution, via a light-assisted halide exchange method. In this material, the concentration of I⁻ increases gradually from the interior to the surface, forming a suitable bandgap funnel structure (Fig. 5c). As a result, MAPbBr_3−xI_x exhibited an HER rate of 652.1 μmol h⁻¹ (250 mg) and an STH efficiency of 1.05% with the use of Pt as co-catalyst, owing to the bandgap funnel facilitated transfer of photogenerated carriers. Furthermore, Zhao et al. tunned the energy levels of MAPbI₃ by one-pot crystallization.⁸⁸ It should be noticed that there was no bandgap funnel in obtained MAPb(I_1−xBr_x)₃ (Br-doped MAPbI₃) by this method. The synthesized mixed halide perovskite showed a championing HER rate of 66.96 μmol h⁻¹ (20 mg) and an outstanding STH efficiency of 1.42% without the use of co-catalysts when x = 0.1. This material exhibited great stability without significant decrease in photocatalytic activity over 252 h (63 runs, 4 h per run).

Similarly, Guan et al. synthesized an all-inorganic perovskite CsPbBr_3−xI_x with a funnel bandgap for photocatalytic HER.⁸⁹ The CsPbBr_3−xI_x/Pt system showed an optimized HER rate of 224 μmol h⁻¹. Moreover, the CsPbBr_3−xI_x/Pt system also displayed an apparent quantum efficiency (AQE) of 2.15% at 450 nm without obvious decrease in 50 h continuous HER. Such all-inorganic MHPs exhibit better stability compared with organic–inorganic hybrid MHPs, according to the previously reported works in MHPs-based solar cells.^102,103

Halide perovskites with FA⁺ ion as the A cation exhibit significantly longer carrier lifetimes and diffusion lengths compared to those based on MA⁺ ion,¹⁰⁴ making them promising candidates for photocatalytic HER. Wu et al. investigated the photocatalytic HER performance of Pt single atoms (PtSA) loaded FAPbBr_3−xI_x in mixed HBr/HI solution, whose crystal structure and morphology are depicted in Fig. 5g.⁹⁹ The PtSAs, which were uniformly distributed on the surface of FAPbBr_3−xI_x by coordinating with Br/I ions, decreased the free energy of H* adsorption for Pt/FAPbBr_3−xI_x to almost 0 (around 0.04 eV), leading to an enhanced photocatalytic HER rate of 682.6 μmol h⁻¹ (100 mg) and an impressive STH efficiency of 4.50%.

In addition, Wang et al. demonstrated that ultrasonication could improve the photocatalytic HER performance of MAPbI₃, resulting in a 7 times higher photocatalytic HER rate under ultrasonic conditions (23.30 μmol h⁻¹, ultrasonic power = 70 W) than under visible-light conditions alone (3.42 μmol h⁻¹).⁹⁰ This enhancement in the HER rate can be attributed to the establishment of a build-in polarized electric field triggered by dipole moments under ultrasonication, which can efficiently separate photogenerated carriers and thus enhance catalytic efficiency.

Though great progress has been made, the water-stability of MHPs remains a critical issue that needs to be addressed. To improve their moisture stability, Liu et al. created an organic membrane that effectively passivates MAPbI₃, using a light- and solution-based treatment (Fig. 5h).⁹⁴ The HIO₃ molecule on the surface of MAPbI₃ formed a hydrogen bond with another HIO₃ molecule on the neighbouring MAPbI₃ crystal, enhancing the crystallinity of MAPbI₃. Moreover, MA⁺ was anchored to the surface of the HIO₃ layer by electrostatic interaction with IO₃⁻, further enhancing the stability of MAPbI₃. In consequence, the photocatalytic HER rate was 2.24-fold enhancement than the bare MAPbI₃ without a passivating layer.

3.2 Lead-free perovskites for photocatalytic hydrogen evolution

Despite the impressive achievements of Pb-based perovskites in photocatalytic H₂ production, the toxicity of Pb limits their further application. To address this issue, lead-free perovskites have been explored, involving the replacement of Pb²⁺ with Sn²⁺, Bi³⁺, Sb³⁺, or the substitution of two Pb²⁺ with one univalent metal cation (e.g. K⁺, Ag⁺, and Cu⁺) and one trivalent metal cation (e.g. Sb³⁺, Bi³⁺).^105–107 In this section, the recent advances in lead-free perovskites for photocatalytic HER are discussed.

3.2.1 Sn-based metal halide perovskites. Sn, a non-toxic element possessing a similar electron configuration to Pb, is regarded as a promising substitute for Pb. Ju et al. reported the fabrication of a Sn-based halide perovskite, DMASnI₃ (DMA = dimethylammonium cation, CH₃NH₂CH₃⁺) (Fig. 6a), which displayed excellent water stability with no decomposition observed after 16 h of immersion in deionized (DI) water (Fig. 6b).⁸⁷ They observed that DMASnI₃ crystals exhibited invertible bandgap narrowing behaviour and long carrier lifetimes, indicating enormous potential for optoelectronic applications. However, the HER rate of DMASnI₃ crystals was measured to be only 0.64 μmol h⁻¹ (Fig. 6c), which could be ascribed to the 1D structure-induced broad bandgap, large effective masses of electrons/holes, and Sn-vacancies induced deep defect states.

Fig. 6 (a) The crystal structure of DMASnI₃. (b) The XRD pattern of black DMASnI₃ after treatment of DI water. (c) The photocatalytic HER rate of DMASnI₃ in DI water.⁸⁷ Copyright 2021 Wiley-VCH GmbH. (d) The schematic diagram of PtSA/Cs₂SnI₆ preparation process. (e) The photocatalytic HER rates of Cs₂SnI₆, PtNP/Cs₂SnI₆, and PtSA/Cs₂SnI₆.⁹⁸ Copyright 2021 Springer Nature. (f) The crystal structure of Cs₂SnCl₆ with different Pt doping concentrations. (g) The photocatalytic HER rates and average photoluminescence quantum yields of Cs₂Pt_xSn_1−xCl₆ (0 < x < 1).⁹⁵ Copyright 2021 Wiley-VCH GmbH.

Furthermore, Zhuo et al. synthesized Cs₂SnI₆ perovskite and loaded PtSA on its surface using an impregnation-heat activation method (Fig. 6d).⁹⁸ The unique coordination structure and electronic properties of Pt–I₃ sites led to a strong metal–support interaction effect and promoted the photogenerated electron transfer from Cs₂SnI₆ to the PtSA, reducing the Gibbs free energy and accelerating the H₂ generation kinetics. The 0.12 wt% PtSA/Cs₂SnI₆ demonstrated the highest activity for photocatalytic H₂ production, with a rate of 4.30 μmol h⁻¹ (10 mg), which was 17.2- and 5.8-fold enhancement than that of pure Cs₂SnI₆ and optimized 3.88 wt% Pt nanoparticle/Cs₂SnI₆, respectively (Fig. 6e).

Yin et al. employed a hydrothermal method to partially replace Sn⁴⁺ in Cs₂SnI₆ with Pt⁴⁺ to obtain a range of highly stable Cs₂Pt_xSn_1−xCl₆ perovskites, where Cs⁺, Cl⁻, Sn⁴⁺, and Pt⁴⁺ ions were uniformly distributed (Fig. 6f).⁹⁵ Among them, Cs₂Pt_0.05Sn_0.95Cl₆ demonstrated remarkable phase stability in water for 25 days and exhibited an excellent photocatalytic HER rate (16.11 μmol h⁻¹). Interestingly, the photocatalytic function of Cs₂Pt_xSn_1−xCl₆ could be fine-tuned by controlling the amount of Pt⁴⁺ (Fig. 6g). As the amount of Pt⁴⁺ increased, the photocatalytic HER performance of Cs₂Pt_xSn_1−xCl₆ decreased.

3.2.2 Bi- and Sb-based metal halide perovskites. Bi³⁺ with lone-pair 6s² exhibits a similar outer electronic configuration as Pb²⁺. Owing to their shallow carrier traps and excellent transport properties, Bi-based MHPs have emerged as promising candidates for lead-free halide perovskite photocatalysts. In this context, Zhao et al. synthesized a crystalline photocatalyst DMA₃BiI₆via a solvothermal method (Fig. 7a).⁸⁶ Photocatalytic measurements were carried out in its well-dispersed HI-H₃PO₂ system containing separated Pt ions and [BiI₆]³⁻ ions. When irradiated at 465 nm, the Pt/DMA₃BiI₆ photocatalytic system exhibited a superior AQE of ∼82.8% for photocatalytic H₂ production, achieving an impressive photocatalytic HER rate of 47 μmol h⁻¹ (500 mg) and showing stable activity without obvious decrease over 100 h. This high activity could be attributed to the efficient separation of carriers at the interface between [BiI₆]³⁻ and Pt ions.¹⁰⁸ In a subsequent study, they found that DMA₃BiI₆ could be stabilized in the aqueous DMAI solution for at least two weeks without additional acids or coatings, through a dissolution–recrystallization process of DMA₃BiI₆ surface molecules.⁹⁶ However, the photocatalytic performance of DMA₃BiI₆ in the DMAI solution was unsatisfactory, showing an HER rate of 4.55 μmol h⁻¹ (50 mg) and an AQE of 0.83% at 535 nm with Pt as the co-catalyst.

Fig. 7 (a) The crystal structure of DMA₃BiI₆.⁸⁶ Copyright 2018 Elsevier. (b)The SEM image (left) and EDS mapping (right) for I of MA₃Bi₂Cl_9−xI_x. (c) The funnel bandgap of MA₃Bi₂Cl_9−xI_x.¹⁰⁰ Copyright 2022 Wiley-VCH GmbH. (d) ultraviolet-visible diffuse reflection spectra (UV-vis DRS) of Cs₃Bi₂X₉ (X = Cl, Br, I) (the inset are physical photographs of them). (e) The photocatalytic HER rates of Cs₃Bi₂X₉ (X = Cl, Br, I).¹⁰¹ Copyright 2022 Wiley-VCH GmbH. (f) The UV-vis DRS of Cs₃Bi_2xSb_2−2xI₉ with different x. (g) Optical absorptions of Cs₃Bi₂I₉, Cs₃Sb₂I₉, and Cs₃Bi_0.6Sb_1.4I₉. (h) The steady-state photoluminescence spectra of C_s3Bi₂I₉, Cs₃Sb₂I₉, and Cs₃Bi_0.6Sb_1.4I₉.⁹³ Copyright 2020 Wiley-VCH GmbH. (i) The schematic diagram of photocatalytic HER on defect-rich Cs₂AgBiBr₆.⁹⁷ Copyright 2021 American Chemical Society.

Furthermore, Guo et al. synthesized a Bi-based perovskite MA₃Bi₂I₉ using a facile hydrothermal method, which displayed a photocatalytic HER rate of 6.77 μmol h⁻¹ (40 mg) and an STH efficiency of 0.48%.⁹² Additionally, it displayed no obvious decrease in HER rate after a 70 h test. In the later study, Tang et al. developed a mixed halide perovskite MA₃Bi₂Cl_9−xI_x in which the iodine element exhibited a gradient distribution (Fig. 7b). The MA₃Bi₂Cl_9−xI_x possessed a funnel bandgap (Fig. 7c), which could efficiently promote the migration of photogenerated carriers from the bulk to the surface to participate in the photocatalytic redox reaction.¹⁰⁰ Consequently, the HER rate of MA₃Bi₂Cl_9−xI_x (500 mg) was significantly improved to 341 ± 61.7 μmol h⁻¹ with Pt as the co-catalyst.

Ji et al. fabricated a number of Cs₃Bi₂X₉ perovskites, where X represents Cl, Br, or I.¹⁰¹ They observed that the absorption range of Cs₃Bi₂X₉ increases with the atomic number of halogens (Fig. 7d). Additionally, the Bi–Bi distance in the octahedral units gradually decreases from Cs₃Bi₂Cl₉ to Cs₃Bi₂I₉, alleviating the strong confinement effect of electron–hole pairs and accelerating the separation and transfer of photogenerated charges. Consequently, Cs₃Bi₂I₉ with the higher light absorption exhibited the most outstanding photocatalytic activity, with an HER rate of 107.89 μmol h⁻¹ (50 mg) in ethanol solution (Fig. 7e).

In contrast, under I-rich and Bi-poor conditions, Bi³⁺ vacancies readily appear in Bi-based perovskites with the formation energy of 1.34 eV.⁹³ These vacancies form deep energy levels within the band gap, impeding the separation of photogenerated carriers and serving as sites for recombination. To confront this challenge, Chen et al. introduced Sb into Cs₃Bi₂I₉ and obtained Cs₃Bi_2xSb_2−2xI₉.⁹³ The theoretical calculation confirmed that the addition of Sb resulted in a lower participation of Bi³⁺ in the conduction band, which weakened the impact of Bi vacancies on the band structure. This effect was reflected in the narrower band gap and fewer intermediate band of Cs₃Bi_2xSb_2−2xI₉ than pure Cs₃Bi₂I₉ and Cs₃Sb₂I₉ (Fig. 7f–h), which significantly improved its performance of H₂ evolution reaction, with an optimal HER rate of 92.6 μmol h⁻¹ (100 mg) among all reported lead-free perovskites.

Additionally, He et al. prepared a defect-rich Cs₂AgBiBr₆ by inducing surface defects by visible light, which exhibited better photocatalytic efficiency than Cs₂AgBiBr₆.⁹⁷ The defect-rich Cs₂AgBiBr₆ exhibited an HER rate of 0.406 μmol h⁻¹ (500 mg), surpassing that of pure Cs₂AgBiBr₆ (0.04 μmol h⁻¹, 500 mg), and exhibited no performance degradation after an 80 h photocatalytic HER.

Similarly, the Sb element, which is adjacent to Pb, also contains ns² lone pair electrons and exhibits stable positive trivalent state, making it a promising alternative to Pb.¹⁰⁹ Rokesh et al. synthesized a new organic–inorganic hybrid perovskite, named 2-(aminomethyl pyridine)SbI₅ (2-AMPSbI₅), via sol–gel method.⁹¹ The 2-AMPSbI₅, using Sb-iodide precursors, displayed a promising HER rate (1.86 μmol h⁻¹, 10 mg) in ethanol solution.

4. MHP-based composite photocatalysts

4.1 Band structure of composite photocatalysts

To enhance the separation of photogenerated carriers, composite photocatalysts with heterogeneous structures have been investigated. These composite photocatalysts often consist of a combination of photocatalysts with other materials, including conductors, semiconductors, and insulators. Among the various composites, heterojunctions formed between semiconductors have received significant attention, primarily categorized into I-type, II-type, Z-type, and S-type heterojunctions.

The band arrangement of two semiconductors in a I-type heterojunction is straddling. When exposed to light, photogenerated carriers are transferred from the wide-bandgap semiconductor to the semiconductor with lower conduction and higher valence bands to participate in the photocatalytic reaction (Fig. 8a).¹¹⁰ In contrast, the band arrangement of two semiconductors in a II-type heterojunction is staggered. Under illumination, electrons transfer to the lower conduction band, while holes transfer to the higher valence band, both actively participating in the photocatalytic reaction (Fig. 8b).¹¹¹ Although the II-type heterojunction enhances charge carrier separation, it comes at the expense of the oxidation–reduction capacity of the two semiconductors. Z-type heterojunctions, named for their photogenerated carrier transfer process resembling the letter “Z”, share a similar structure with II-type heterojunctions.¹¹² However, oxidation and reduction reactions occur in semiconductors with higher conduction band and lower valence band, respectively. The remaining electrons and holes are recombined indirectly or interactively (Fig. 8c). The S-type heterojunction originates from the band bending that occurs at the interface of two semiconductors, which resembles the shape of the letter “S”, and its internal charge transfer is akin to the Z-type heterojunction (Fig. 8d).¹³⁹ Both S-type and Z-type heterostructures effectively promote charge transfer without compromising the respective oxidation and reduction capabilities of the two semiconductors, garnering considerable research interest in recent years.^140–143 Meanwhile, the redox reaction is separated in space, and it is therefore expected to achieve an efficient overall water splitting through the appropriate medium and test system.⁷⁷

Fig. 8 Schematic diagrams of semiconductor heterojunctions. (a) I-type heterojunction. (b) II-type heterojunction. (c) Z-type heterojunction. (d) S-type heterojunction.

In order to improve the hydrogen production performance of MHP-based photocatalysts, various composite structures have been constructed, such as black phosphorus/MAPbI₃ I heterojunction,¹¹⁶ MAPbI₃/MoS₂ II heterojunction,¹³¹etc. (Table 2). In the following section, we mainly introduce the applications of MHP/conductor or MHP/semiconductor composites in the photocatalytic HER.

Table 2 Summary of works on photocatalytic application of MHP-based composite catalysts. R = H₂ evolution reaction rate, S = STH efficiency, A = AQE

Composite catalysts	Test conditions	Total catalyst usage (mg)	Performance	Year^Ref.
Pt(0.75 wt%)@TiO2/MAPbI3	Xe lamp (300 W, λ ≥ 420 nm)	15	R = 79.4 μmol h^−1	2018^113
	200 mW cm^−2		S = 0.86%
	HI solution		A = 70% at 420 nm
CsPbBr3/Ru@TiO2	Continuous wave (CW) laser (445 nm, 50 mW)	3.2	R = 30.72 μmol h^−1	2018^114
	Trifluoroacetic acid/triethylamine		A = 0.4 ± 0.1% at 445 nm
5% rGO/MAPbI3	Xe lamp (300 W, λ ≥ 420 nm)	100	R = 93.9 μmol h^−1	2018^115
	120 mW cm^−2		A = 1.5% at 450 nm
	HI solution
1.2% BP/MAPbI3	Xe lamp (300 mW cm^−2, λ ≥ 420 nm)	30	R = 112.26 μmol h^−1	2019^116
	25.3 mW cm^−2		S = 0.93%
	HI solution		A = 1.5% at 450 nm
15% Ni3C/MAPbI3	Xe lamp (300 mW cm^−2, λ ≥ 420 nm)	50	R = 118.1 μmol h^−1	2019^117
	100 mW cm^−2		S = 0.91%
	HI solution		A = 16.6% at 420 nm
MAPbBr3@Pt/Ta2O5/PEDOT:PSS	Xe lamp (300 W, λ ≥ 420 nm)	157.5	R = 105 μmol h^−1	2019^118
	150 mW cm^−2		A = 16.4% at 420 nm
	HBr solution
2.5% rGO/Cs2AgBiBr6	Xe lamp (300 W, λ ≥ 420 nm)	200	R = 9.78 μmol h^−1	2020^119
	HBr solution		A = 0.16% at 450 nm
g-C3N4/Cs3Bi2I9 (10 : 0.1 wt%)	Xe lamp (450 W) equipped with a water-filter and a WG320-filter	6	R = 5.52 μmol h^−1	2020^120
	Methanol solution (10% vol)
Pt(1.25 wt%)@TiO2/EtbtBi2I10/rGO	Xe lamp (300 W)	61	R = 16.76 μmol h^−1	2020^121
	HF solution
CPD(1 wt%)/MAPbI3/Pt(2 wt%)	Xe lamp (300 W, λ ≥ 420 nm, 300 mW cm^−2)	103	R = 1184.2 μmol h^−1	2020^122
	100 mW cm^−2		S = 2.15%
	HI solution		A = 53.6% at 420 nm
MoS2 NSs/MAPbI3	White-light LED lamps (10 W × 9, 380 nm ≤ λ ≤ 780 nm, 450 mW cm^−2)	105	R = 206.1 μmol h^−1	2020^123
	HI solution
MA3Bi2I9/DMA3BiI6	Xe lamp (300 W, λ ≥ 420 nm)	200	R = 39.64 μmol h^−1	2020^124
	HI solution
MAPbI3/CoP	Xe lamp (150 W, λ ≥ 420 nm)	ca. 3.12	R = 6.5 μmol h^−1	2020^125
	HF solution
MAPbI3-MC/ML-MoS2	Xe lamp (300 W, λ ≥ 420 nm)	100	R = 1360 μmol h^−1	2020^126
	100 mW cm^−2		S = 1.09%
	HI solution
MoS2(20 wt%)/MA0.6FA0.4PbI3	Xe lamp (λ ≥ 420 nm, 300 mW cm^−2)	50	R = 66.4 μmol h^−1	2020^127
	35 mW cm^−2		S = 0.87%
	HI solution		A = 14.5% at 550 nm
DMASnBr3(33 wt%)/g-C3N4@Pt(3 wt%)	Xe lamp (1500 W, 300–800 nm, 500 W m^−2) equipped with UV outdoor filter	21	R = 36.33 μmol h^−1	2021^128
	TEOA solution (10% vol)
NiCoB/MAPbI3	Xe lamp (300 W, λ ≥ 420 nm)	65	R = 170.7 μmol h^−1	2021^129
	HI solution		A = 1.96% at 450 nm
Cs3Bi2Br9/g-C3N4	Xe lamp (1500 W, 300-800 nm, 500 W m^−2) equipped with UV outdoor filter	21	R = 22.05 μmol h^−1	2021^130
	TEOA solution (10% vol)
MAPbI3/MoS2 nanoflowers	Xe lamp (280 W, λ ≥ 420 nm, 410 mW cm^−2)	50	R = 1469.45 μmol h^−1	2021^131
	100 mW cm^−2		S = 7.35%
	HI solution		A = 22.1% at 500 nm
Cs2AgBiBr6/N-C	Xe lamp (300 W, λ ≥ 420 nm)	10	R = 3.8 μmol h^−1	2021^132
	100 mW cm^−2		A = 0.59% at 420 nm
	HBr solution
MAPbI3/PANI NWs	White-light LED lamps (10 W, 380 nm ≤ λ ≤ 780 nm)	107	R = 38.8 μmol h^−1	2021^133
	HI solution		A = 0.3% at 420 nm
Cs2AgBiBr6/MoS2(20 wt%)	Xe lamp (300 W, λ ≥ 420 nm, 300 mW cm^−2)	50	R = 4.375 μmol h^−1	2022^134
	41 mW cm^−2		A = 0.2% at 450 nm
	HBr solution
CsPbBr3(30 wt%)/ZIF-8	Xe lamp (300 W)	300	R = 2.36 μmol h^−1	2022^135
	Na2SO3 DI water solution
g-C3N4/Cs2AgBiBr6 (1 : 10 wt%)	Xe lamp (300 W, λ ≥ 420 nm)	50	R = 3 μmol h^−1	2022^136
	HBr solution
NiCoP(12.5% wt%)/Cs2AgBiBr6	Xe lamp (300 W, λ ≥ 420 nm)	100	R = 37.32 μmol h^−1	2022^137
	HBr solution
CsPbBr3/GO-Pt	Xe lamp (300 W, λ ≥ 400 nm)	10	R = 10.6 μmol h^−1	2022^138
	Aromatic alcohol

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4.2 Metal halide perovskite/0D nanoparticles composites

4.2.1 Metal oxide nanoparticles. TiO₂ is often used as an electron transport layer in perovskite solar cells due to its good energy level alignment with MHP materials,^144,145 making MHPs/TiO₂ composite systems highly promising for photocatalytic HER. Pavliuk et al. first prepared a Pt@TiO₂/CsPbBr₃ composite photocatalyst in which CsPbBr₃ and Pt@TiO₂ acted as solar-light harvesters and photocatalysts (Fig. 9a), respectively. Triethylamine and trifluoroacetic acid were employed as a sacrificial agent and an electron donor, respectively, to stabilize CsPbBr₃. As a result, the Pt@TiO₂/CsPbBr₃ system demonstrated an HER rate of approximately 30.72 μmol h⁻¹ (3.2 mg) and an STH efficiency of about 0.4%. Similarly, Wang et al. established a TiO₂ electron-transporting channel between Pt and MAPbI₃.¹¹³ This combination resulted in a significant elevation in the photocatalytic HER rate of 79.4 μmol h⁻¹ (15 mg), an approximately 89-fold increase compared to the HER rate of Pt/MAPbI₃. Additionally, the AQE of HER was about 70% at 420 nm, and the STH efficiency was approximately 0.86%. Furthermore, Ta₂O₅ nanoparticles were loaded with Pt and then combined with MAPbI₃ as Ta₂O₅/MAPbI₃/Pt composite photocatalyst, which exhibited an HER rate of 39.9 μmol h⁻¹ (15 mg).¹¹³

Fig. 9 (a) The schematic diagram of photocatalytic HER on Pt@TiO₂/CsPbBr₃ in TEOA solution.¹¹⁴ Copyright 2018 Elsevier. (b) The schematic diagram of photocatalytic HER on Pt@Ta₂O₅/MAPbBr₃/PEDOT:PSS in HBr solution.¹¹⁸ Copyright 2019 American Chemical Society. (c) SEM images of Ni₃C/MAPbI₃. (d) The mechanism of photocatalytic HER of Ni₃C/MAPbI₃.¹¹⁷ Copyright 2019 American Chemical Society.

In a subsequent study, Wang et al. prepared a MAPbBr₃@Pt/Ta₂O₅/PEDOT:PSS composite photocatalyst by simultaneously introducing a photogenerated electron extractor (Ta₂O₅) and a photogenerated hole extractor (PEDOT:PSS) (Fig. 9b).¹¹⁸ By combining MAPbBr₃ with Pt/Ta₂O₅ and PEDOT:PSS, electron and hole transport channels were established, which greatly improved the charge transport efficiency and realized efficient catalytic reactions. Consequently, the HER rate of the MAPbBr₃@Pt/Ta₂O₅/PEDOT:PSS composite photocatalyst (105 μmol h⁻¹, 157.5 mg) was 52-fold higher than that of pristine MAPbBr₃, emphasizing the outstanding advantages of establishing a dual charge transport highway for improving the photocatalytic performance of MHP materials.

4.2.2 Transition metal carbide nanoparticles. Ni₃C, as an earth-abundant and fascinating iron group transition metal carbide, has demonstrated exceptional photocatalytic activity for H₂ production in acidic conditions.^146–148 Through a simple surface charge-promoted self-assembly technique, Zhao et al. synthesized a MAPbI₃/Ni₃C composite photocatalyst.¹¹⁷ Scanning electron microscope (SEM) images showed that Ni₃C was loaded on the surface of MAPbI₃ (Fig. 9c). The photocatalytic process of this composite is shown in the Fig. 9d. The HER rate of 15% Ni₃C/MAPbI₃ reached 118.1 μmol h⁻¹ (50 mg), which was about 55-fold enhancement than that of MAPbI₃ (2.15 μmol h⁻¹, 50 mg) and far higher than that of Pt/MAPbI₃ (26.7 μmol h⁻¹, 50 mg). Notably, the photocatalytic HER rate of Ni₃C/MAPbI₃ showed no obvious decrease after a designed photoreaction process involving 10 cycles, one-month storage, and an additional 10 cycles, highlighting the superb stability of this composite photocatalyst. The excellent photocatalytic HER activity and stability of Ni₃C/MAPbI₃ were believed to originate from the improved carrier separation and transfer ability, the increased number of reaction sites on the surface of MAPbI₃, and the intrinsic stability of the composite photocatalyst.

4.2.3 Transition metal phosphide nanoparticles. Transition metal phosphides are hopeful substitutes for noble metal cocatalysts to enhance the outstanding carriers’ separation and migration capacity and enable low-cost production.^149–151 Using a simple in situ photosynthesis method, Cai et al. first synthesized a MAPbI₃/cobalt phosphide (CoP) composite structure.¹²⁵ CoP nanoparticles could be used as cocatalysts as they can not only effectively extract photoinduced electrons from MAPbI₃ and enhance the separation of carriers but also promote interfacial photocatalytic reactions. Based on these advantages, the MAPbI₃/CoP composite photocatalyst exhibited an excellent HER rate of ca. 2.45 μmol h⁻¹ (ca. 3.12 mg) in HI solution within 3 h, which was about 8-fold enhancement than that of bare MAPbI₃. Moreover, when the photocatalytic reaction time increased to 27 h, its HER rate reached ca. 6.5 μmol h⁻¹ (ca. 3.12 mg), which could be attributed to the ameliorative crystallinity of MAPbI₃ and the increased reaction active sites of CoP co-catalyst during the incessant photocatalytic reaction. Furthermore, Huang et al. developed a NiCoP/Cs₂AgBiBr₆ composite photocatalyst via the electrostatic coupling method.¹³⁷ With visible-light irradiation, NiCoP acted as an electron extractor to improve the charge separation efficiency of Cs₂AgBiBr₆, thereby boosting the photocatalytic H₂ production ability. Specifically, the composite photocatalyst displayed a maximum HER rate of 37.32 μmol h⁻¹ (100 mg) at a loading amount of 12.5% NiCoP, with approximately 88-fold enhancement over that of pristine Cs₂AgBiBr₆. In addition, transition metal phosphides are economical and environmentally friendly, favouring their further development.

4.2.4 Carbon-based nanomaterials. Being part of the carbon-based nanomaterial family, carbonized polymer dots (CPDs) exhibit exceptional potential in enhancing the photocatalytic HER activity of MHP owing to their high photochemical stability, outstanding photophysical, and chemical properties.^152,153 Zhao et al. developed a hybrid photocatalytic system by combining Pt/MAPbI₃ with CPDs synthesized from citric acid (CA) and p-aminosalicylic acid (PASA), in which the CPDs acted as efficient photogenerated hole extractors. Benefiting from the well-matched energy levels, the optimized Pt/MAPbI₃/CA-PASA CPD hybrid photocatalytic system exhibited an outstanding HER rate of 1184.2 μmol h⁻¹ (103 mg), an STH efficiency of 2.15%, and an AQE of 53.6% at 420 nm.

4.3 Metal halide perovskite/2D vdW material composites

In recent years, 2D van der Waals (vdW) materials have demonstrated promising potential in the field of photocatalysis. The advantages of using 2D vdW materials can be summarized as follows: (1) the 2D nature of these materials leads to unique electronic properties that can enhance electron transport and improve charge transfer capacity; (2) the atomic-scale thickness can effectively shorten the carrier diffusion distance and improve carrier utilization; (3) the inherently large specific surface area is beneficial for the adsorption and activation of reactive molecules. Thus, combining MHPs with 2D vdW materials provides great potential for photocatalytic HER.

4.3.1 Graphene derivatives. Graphene derivatives, such as graphene oxide (GO) and reduced graphene oxide (rGO), which hold good electron transfer properties and acid stability,^154–156 have been utilized in MHP-based composites to improve the HER performance of MHP. For instance, Wu et al. synthesized a MAPbI₃/rGO composite using a photoreduction method.¹¹⁵ SEM images showed that rGO was loaded on the surface of MAPbI₃. During photocatalytic HER, photoinduced electrons in MAPbI₃ were transferred to rGO via the Pb–O–C bond to participate in the reduction of H⁺ at the rGO position, and photogenerated holes oxidized I⁻ to I₃⁻ (Fig. 10a). Consequently, the MAPbI₃/rGO composite displayed an enhanced HER rate of 93.9 μmol h⁻¹ (100 mg), which was 67-fold enhancement than that of bare MAPbI₃. In another study, Chen et al. first demonstrated the selective oxidation of aromatic alcohol to the aromatic aldehyde using CsPbBr₃/GO-Pt composite, accompanied by HER.¹³⁸ GO with abundant functional groups induced the nucleation process of CsPbBr₃, leading to the smaller size perovskite nanocrystals. Additionally, GO-Pt acted as photogenerated electron extractors, inhibiting the recombination of electron–hole pairs. Consequently, the optimized CsPbBr₃/GO-Pt exhibited an HER rate of 10.6 μmol h⁻¹ (10 mg) with high selectivity (>99%) for benzyl aldehyde generation (10.5 μmol h⁻¹, 10 mg) under visible light, which was about 5-fold enhancement than that of the CsPbBr₃-Pt sample.

Fig. 10 (a) The schematic diagram of photocatalytic HER on MAPbI3/rGO.¹¹⁵ Copyright 2018 Wiley-VCH GmbH. (b) The schematic diagram of photocatalytic HER on ML-MoS₂/MAPbI₃-MCs. (c) The schematic illustration of structure of ML-MoS₂/MAPbI₃-MCs.¹²⁶ Copyright 2020 Elsevier. (d) The schematic illustration of structure of MoS₂ nanoflowers/MAPbI₃ microcrystals.¹³¹ Copyright 2021 American Chemical Society. (e) The schematic mechanism of photocatalytic HER on g-C₃N₄@DMASnBr₃/Pt in triethanolamine solution.¹²⁸ Copyright 2018 Wiley-VCH GmbH. (f) The schematic illustration of structure of CsPbX₃@ZIF-8 (X = Br, Cl, I).¹³⁵ Copyright 2022 American Chemical Society.

Liu et al. reported a 0D lead-free perovskite (Etbt)₄Bi₂I₁₀ (Etbt = 3-ethylbenzo[d]thiazol-3-ium) and constructed a (Etbt)₄Bi₂I₁₀-based composite photocatalyst by introducing Pt/TiO₂ and rGO as an electron extractor and a hole extractor, respectively.¹²¹ Unlike conventional protonated cations, Etbt does not contain conventional hydrogen-bond donors, which helps prevent hydrogen-bonding interactions from eroding hybrid perovskites. Consequently, (Etbt)₄Bi₂I₁₀ showed outstanding stability in DI water, acid solution, and ultraviolet light, achiving a potential HER rate of 0.736 μmol h⁻¹ (130 mg) with 0.5 wt% Pt nanoparticles loading. After incorporating Pt/TiO₂ and rGO to achieve a dual-charge transport-modulated composite photocatalyst, the photocatalytic system showed a significantly improved HER rate of 16.76 μmol h⁻¹ (61 mg). In a later study, Wang et al. combined Cs₂AgBiBr₆ with rGO by a photo-reduction process and applied it for HER in a saturated HBr aqueous solution.¹¹⁹ The Cs₂AgBiBr₆/rGO with optimal rGO loading displayed improved photocatalytic performance compared to that of pure Cs₂AgBiBr₆, which could be attributed to the promoted separation and migration of carriers by rGO. The HER rate reached 9.78 μmol h⁻¹ (200 mg) under visible light irradiation, without significant decrease for 120 h continuous photocatalytic HER.

4.3.2 Molybdenum disulphide. MoS₂ is a layered material that contains unsaturated sulphur, providing plentiful active sites for H₂ at the edges.^157–160 Owing to its specific structure and properties, MoS₂ has been combined with various semiconductors to improve their catalytic performance in HER.^161,162 Wang et al. demonstrated the in situ coupling of MAPbI₃ with MoS₂ nanosheets (MoS₂ NSs), resulting in a hybrid photocatalyst with high efficiency for visible-light-driven photocatalytic HER.¹²³ The composite exhibited excellent HER performance with an HER rate of 206.1 μmol h⁻¹ (105 mg), which was a 121-fold enhancement compared to that of bare MAPbI₃. In addition, Zhang et al. fabricated a series of MA_1−xFA_xPbI₃ by partially replacing MA⁺ with FA⁺, which was then coupled with MoS₂ cocatalyst for photocatalytic HER. The optimized MoS₂/MA_0.6FA_0.4PbI₃ composite (i.e., 20 wt% MoS₂/MA_0.6FA_0.4PbI₃) exhibited a superior 66.4 μmol h⁻¹ (50 mg) visible HER rate, which was approximately 88-fold enhancement compared to that of pristine MAPbI₃ (1.2 μmol h⁻¹, 50 mg). The outstanding photocatalytic HER performances of MoS₂/MA_0.6FA_0.4PbI₃ could be attributed to the prolonged lifetime of photoinduced carriers inside MA_0.6FA_0.4PbI₃ due to the partial replacement of MA⁺ with FA⁺, and the enhanced carrier transfer and separation capabilities by MoS₂ decoration.

Subsequently, Zhao et al. developed a novel II-type heterojunction composite photocatalyst (Fig. 10b), consisting of monolayer MoS₂ nanosheets (ML-MoS₂) and MAPbI₃ microcrystals (MAPbI₃-MCs) (Fig. 10c), which could effectively inhibit carriers recombination for high-efficiency photocatalytic HER.¹²⁶ This discovery developed new possibilities for enhancing the HER performance of perovskite-based photocatalysts, achieving a remarkable STH efficiency of 1.09% and an HER rate of 1360 μmol h⁻¹ (100 mg) under visible light. Furthermore, when other MHP microcrystals such as δ-FAPbI₃-MCs, MASnI₃-MCs, and MA₃Bi₂I₉-MCs were combined with ML-MoS₂ nanosheets, their HER rates increased by 8.4, 14.4, and 26 times, respectively, compared to their respective bare perovskite microcrystals. This indicates a hopeful approach for promoting the photocatalytic HER performance of halide perovskites. To further increase the active area of MoS₂, Guan et al. fabricated a MoS₂ nanoflowers/MAPbI₃ microcrystals heterostructure (Fig. 10d),¹³¹ The MoS₂ edges offered large amounts of unsaturated S₂²⁻ that could be considered as active sites for HER.^163,164 Specifically, MAPbI₃/MoS₂ (3 : 7) exhibited a drastic enhancement in the HER rate, reaching up to 1469.45 μmol h⁻¹ (50 mg) and a champion STH efficiency of 7.35%. Based on available research, this is one of the highest STHs in photocatalytic HER. Furthermore, Zhang et al. fabricated an I-type heterostructured MoS₂/Cs₂AgBiBr₆ for efficient and enduring photocatalytic HER via loading MoS₂ on Cs₂AgBiBr₆ using a dissolution–recrystallization method.¹³⁴ The optimized MoS₂/Cs₂AgBiBr₆ composite achieved an HER rate of 4.375 μmol h⁻¹ (50 mg) in aqueous HBr solution, approximately 20-fold enhancement compared to that of bare Cs₂AgBiBr₆ (0.215 μmol h⁻¹, 50 mg), and presented an intermittent 500 h photocatalytic HER stability without obvious decrease. The enhanced activity of MoS₂/Cs₂AgBiBr₆ could be attributed to the kinetics-facilitated heterostructure consisting of stable Cs₂AgBiBr₆ and MoS₂.

4.3.3 Graphitic carbon nitride. g-C₃N₄ is a promising photocatalyst due to its high surface area, excellent interfacial charge separation and transport capability, and easy preparation, which is expected to integrate with MHPs with high light absorption ability.^165–168 Bresolin et al. first fabricated a Cs₃Bi₂I₉/g-C₃N₄ composite photocatalyst via anchoring Cs₃Bi₂I₉ perovskite on g-C₃N₄ nanosheets.¹²⁰ This composite was tested in a methanol solution (10 vol% in water) with an HER rate of 5.52 μmol h⁻¹ (6 mg), which was approximately 46% higher than that of bare g-C₃N₄. The improved photocatalytic performance was attributed to the enhanced light absorption and electrochemical properties of the newly designed composite. Romani et al. synthesized water-insoluble DMASnBr₃/g-C₃N₄ heterostructures for photocatalytic HER in a 10 vol% triethanolamine (TEOA) solution (Fig. 10e).¹²⁸ The HER rate of the optimized g-C₃N₄@DMASnBr₃/Pt heterostructure (36.33 μmol h⁻¹, 21 mg) was more than 288-fold higher than that of Pt/DMASnBr₃ and 100-fold better than that of bare g-C₃N₄. This excellent photocatalytic HER performance could be attributed to the good band alignment of g-C₃N₄/DMASnBr₃, which effectively enhance the separation of photogenerated carriers. The researchers also developed a heterostructured photocatalyst by combining Cs₃Bi₂Br₉ with g-C₃N₄.¹³⁰ Photocatalytic HER was performed in a 10 vol% aqueous TEOA with Pt as the cocatalyst. The optimal photocatalytic HER rate was achieved when the loading of Cs₃Bi₂Br₉ was 2.5%, reaching approximately 22.05 μmol h⁻¹ (21 mg), which was about an order of magnitude better than that of bare g-C₃N₄ (1.701 μmol h⁻¹, 21 mg). However, when the Cs₃Bi₂Br₉ loading amount exceeded 5%, the HER capacity of the composite photocatalyst decreased, probably due to the self-trapping of the photogenerated electrons.

Additionally, Song et al. synthesized mesoporous g-C₃N₄ and then loaded it onto the surface of Cs₂AgBiBr₆.¹³⁶ High-resolution transmission electron microscope (HRTEM) images clearly showed the secure contact between Cs₂AgBiBr₆ and g-C₃N₄. Spectroscopy analysis indicated the establishment of a II-type heterojunction structure between Cs₂AgBiBr₆ and g-C₃N₄. The optimized g-C₃N₄/Cs₂AgBiBr₆-10 (with a mass ratio of g-C₃N₄ and Cs₂AgBiBr₆ being 1 : 10) exhibited an excellent photocatalytic HER rate of 3 μmol h⁻¹ (50 mg) without using any noble metal cocatalyst, achieving a 2.5-fold enhancement compared to that of the original Cs₂AgBiBr₆. Moreover, its photocatalytic HER efficiency showed no obvious decrease after storage for 40 days in air condition, indicating excellent stability and photocatalytic recyclability. Jiang et al. utilized a nitrogen-doped carbon (N-C) material based on g-C₃N₄ and coupled it with Cs₂AgBiBr₆via a facile hydrothermal method.^132,169 The photocatalytic HER performance test found that N-C itself had no photocatalytic HER ability. After combining with Cs₂AgBiBr₆, the composite material exhibited good photocatalytic HER activity. Particularly, when tested a hydrothermal temperature of 140 °C, Cs₂AgBiBr₆/N-C-140 exhibited the best HER rate of 3.8 μmol h⁻¹ (10 mg), about 19 times that of the bare Cs₂AgBiBr₆. This improvement was ascribed to the large surface area of N-C and the presence of nitrogen atoms in N-C, which enhanced electron conductivity and promoted interfacial charge separation between Cs₂AgBiBr₆ and N-C.

4.3.4 Black phosphorous. Black phosphorus (BP) with high charge carrier mobility and abundant active sites has also been introduced to improve light absorption and photoinduced carriers separation of MHPs.^170,171 For example, Li et al. reported a BP/MAPbI₃ heterostructure.¹¹⁶ Under solar-light irradiation, the photogenerated electrons could transfer from perovskite MAPbI₃ to BP to participate in the HER. Interestingly, the HER rate of the BP/MAPbI₃ heterostructure (112.26 μmol h⁻¹, 30 mg) was more than 100-fold enhancement compared to that of bare MAPbI₃. This outstanding photocatalytic HER activity could be ascribed to the construction of a type I heterojunction, which could broaden light harvesting and enhance the separation and migration of carriers at the interface of BP/MAPbI₃.

4.3.5 Lamellar transition metal borides. Jiang et al. developed a NiCoB/MAPbI₃ composite photocatalyst by combining amorphous NiCoB with a nanoscale lamellar structure, and MAPbI₃ using electrostatic self-assembly.¹²⁹ The photogenerated electrons from MAPbI₃ were able to transfer to NiCoB and participate in the reduction of H⁺, thereby promoting the separation of photoinduced carriers. At a NiCoB loading of 30%, the photocatalytic HER rate of MAPbI₃ reached a maximum of 170.7 μmol h⁻¹ (65 mg) under visible light irradiation. Additionally, the composite showed good stability in a 24-hour cycling photocatalytic HER.

4.4 Metal halide perovskite/1D semiconductor composites

Conductive 1D polyaniline nanowires (PANI NWs) with high electrical conductivity and excellent chemical stability have been coupled with MHP to create high-performance composite photocatalysts. For example, Li et al. fabricated a high-performance 3D/1D heterostructured composite photocatalyst by combining conductive 1D polyaniline nanowires (PANI NWs) with MAPbI₃ using an in situ doping-induced assembly method.¹³³ SEM images obtained by Li et al. showed that PANI NWs were firmly embedded on the surface of MAPbI₃. The MAPbI₃/PANI NWs composite photocatalyst showed a remarkable 29-fold increase in the photocatalytic HER rate (38.8 μmol h⁻¹, 107 mg) compared to that of pristine MAPbI₃ microcrystals. Additionally, this composite photocatalyst demonstrated good stability during 30 h of irradiation, with each cycle lasting 6 h.

4.5 Metal halide perovskite/other material composites

In addition to the aforementioned materials, other semiconductors have also been used to couple with MHPs to create efficient photocatalytic systems. One such material is metal–organic frameworks (MOFs), which possess a large specific surface area, adjustable morphology, and diverse structure.^172,173 By coupling MOFs with MHPs, the photocatalytic HER performance of MHPs can be enhanced. For instance, Feng et al. fabricated a CsPbBr₃/zeolitic imidazolate framework (ZIF-8) composite using the mechanical milling method. This fabrication method was expected to enable large-scale synthesis of ZIF-8 composites (Fig. 10f).¹³⁵ SEM images showed that ZIF-8 was loaded on the surface of CsPbBr₃. The HER rate of this composite was measured to be ∼11 times higher (2.36 μmol·h⁻¹, 300 mg, 2.5 h) than that of ZIF-8 alone (0.205 μmol·h⁻¹, 300 mg, 2.5 h), benefiting from the ZIF-8 promoted photogenerated carrier separation. Moreover, ZIF-8 was discerned to enhance the stability of CsPbBr₃ in aqueous solution, with only ∼8% decrease in H₂ productivity after 4 weeks.

In addition to the above-mentioned strategies, the formation of MHP-MHPs heterojunctions has also been demonstrated as an effective approach to obtaining high-performance photocatalytic systems. For example, Tang et al. developed a lead-free Bi-based hybrid perovskite heterojunction, MA₃Bi₂I₉/DMA₃BiI₆, through an in situ growth method.¹²⁴ The VB and CB positions of MA₃Bi₂I₉ and DMA₃BiI₆ showed a well-matched II-type heterostructure, which facilitates the formation of an interfacial carriers transfer pathway at the interface of MA₃Bi₂I₉ and DMA₃BiI₆. This resulted in an HER rate of 39.64 μmol h⁻¹ (200 mg) without the addition of Pt co-catalysts.

5. Summary and prospects

MHPs have garnered significant interest in the field of photocatalytic H₂ evolution due to their outstanding photoelectric properties and tunable crystal structure. This review comprehensively discusses the structure and photocatalytic HER performance of different kinds of MHPs, including both lead-based perovskites and lead-free perovskites. Additionally, we summarize the developed strategies used to improve the photocatalytic performance of MHPs, including energy band structure adjustment (e.g., extra halogen element doping), and composite structures (e.g., MHPs/conductor, MHPs/semiconductor). However, despite their potential, the current photocatalytic performance of MHPs remains unsatisfactory for practical application, due to energy loss during charge generation, separation, and transfer, as well as concerns about the instability and toxicity of MHPs. In light of current MHP research, we propose several possible methods to further enhance the photocatalytic performance of MHP-based photocatalysts.

(1) Designing MHPs with optimal intrinsic crystalline structures at the molecular level. Photocatalysts play a crucial role in photocatalytic reactions, encompassing light absorption, photogenerated carriers generation, separation, transport, and surface redox reactions. To achieve efficient H₂ evolution, photocatalysts require appropriate electronic structures, wide light absorption range, large absorption coefficients, and excellent carrier transport performance.¹⁷⁴ The strong structural diversity and designability of MHPs make them highly advantageous for the design of photocatalysts. Emphatically, incorporating bulky organic cations at the A site can improve the intrinsic moisture resistance of MHPs, owing to the hydrophobic nature of the organic cations. However, introducing bulky organic cations may cause the breakdown of the 3D perovskite framework, leading to the formation of low-dimensional perovskites such as 2D, 1D, and 0D perovskites. These low-dimensional perovskites, especially 1D and 0D perovskites, usually exhibit stronger quantum confinement effects and wider bandgaps compared to their 3D analogues, which may impair their optoelectronic properties.⁶⁵ Thus, striking a balance between optoelectronic performance and stability in MHPs design is of utmost importance. For instance, quasi-2D MHPs with a general formula of A₂A_n−1′Pb_nI_3n+1, (where A′ is a bulky organic cation and A is MA or FA) have demonstrated higher environmental stability compared with MAPbI₃, due to the protection of the hydrophobic organic spacers. In particular, solar cells based on quasi-2D MHPs have recently achieved a remarkable power conversion efficiency (PCE) of 22.26%, indicating the preserved excellent semiconducting properties of these materials.¹⁷⁵ Further theoretical calculations and molecular engineering hold promise in promoting the development of MHPs for efficient and stable photocatalytic HER.

(2) Constructing band-matched heterostructured MHP composites. Although some strategies have achieved improved carrier separation efficiency, they often come at the expense of redox ability, ultimately leading to unsatisfactory photocatalytic HER performance. Additionally, the existence of photoinduced electron–hole pairs in the pristine photocatalyst may hinder the efficient interfacial transfer of electron–hole pairs in other catalysts. To overcome these limitations and further improve the photocatalytic HER performance of MHPs, novel forms of heterostructures, such as Z- and S-scheme heterojunctions, are expected.^139,176 By selectively consuming or recombining a certain portion of photogenerated electrons and holes, electrons and holes with stronger redox ability can be retained, thus enhancing the photocatalytic activity of these heterojunctions. Moreover, it is important to consider the condition under which most MHPs-based photocatalytic HERs are realized, often using a HI solution. Thus, selecting a suitable acid-resistant photocatalyst for oxygen evolution based on the I⁻/I₃⁻ redox pair is expected to improve the efficiency of the photocatalytic HER process and enable overall water splitting.

(3) Developing encapsulation layers. Encapsulation is a crucial step in improving the stability of photocatalytic plate devices, as the water-instability of MHPs presents a significant challenge in the context of photocatalytic H₂ production. An ideal encapsulation material should possess excellent stability, good electrical conductivity, and light transmission. There are generally two types of encapsulations: full coverage and edge encapsulation. Full coverage encapsulation involves preparing the encapsulation layer on top of the module, while edge encapsulation involves placing a sealant around the module.^177–179 Edge encapsulation offers advantages such as reduced impact on the contact layer, lowered light transmission requirements, and minimized potential side reactions between the encapsulating materials and the MHPs. However, the encapsulation effect may be slightly reduced compared to full coverage encapsulation. To further increase the barrier effect, desiccant can be introduced into the edge encapsulation process. Fully covered encapsulation provides better protection, but it has a greater impact on the MHP layer and has higher light transmission requirements, as it directly contacts the MHP functional layer. An alternative approach involves multilayer encapsulation, which combines the benefits of both full coverage and edge encapsulation methods to improve the water stability of MHPs while maintaining their photocatalytic activity.¹⁸⁰ However, it is important to note that this method is may be challenging to fabricate and could incur higher expensive.

(4) Developing lead-free MHP photocatalysts. Among the candidates, Sn-based perovskites have shown promise due to the similarity of Sn²⁺ to Pb²⁺ in terms of electron configuration and ionic radius. However, the rapid oxidation of Sn²⁺ to Sn⁴⁺ remains a challenge. Various stabilization methods have been explored, such as adding reducing agents (e.g. H₃PO₂),¹⁸¹ but their stability remains inferior to Pb-based perovskites. A deeper understanding of Sn²⁺ bonding interactions in Sn-based perovskites could offer insights into addressing this issue. Similarly, Ge-based perovskites also face the challenge of Ge²⁺ oxidation. Bi-, Sb-based perovskites and double perovskites have demonstrated good stability under various conditions. However, their photocatalytic performance is generally limited by poor optoelectronic properties arising from their low electronic dimensional.¹⁸² Therefore, a promising direction for MHP-based photocatalytic H₂ evolution is the rational design of Bi-, Sb-based perovskites and double perovskites with high electronic dimensional using theoretical calculations and molecular engineering.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by National Ten Thousand Talent Program for Young Top-notch Talent, National Natural Science Fund for Excellent Young Scholars (52022030), National Natural Science Foundation of China (51972111, 52203330), the Science and Technology Innovation Plan of Shanghai Science and Technology Commission (22YF1410000), Shanghai Pilot Program for Basic Research (22TQ1400100-5), “Dawn” Program of Shanghai Education Commission (22SG28), Shanghai Municipal Natural Science Foundation (22ZR1418000), Postdoctoral Research Foundation of China (2021M701190) and Shanghai Engineering Research Center of Hierarchical Nanomaterials (18DZ2252400).

Notes and references

1. J. Sun, D. K. Zhong and D. R. Gamelin, Composite photoanodes for photoelectrochemical solar water splitting, Energy Environ. Sci., 2010, 3, 1252–1261.
2. J. Song, T. L. Kim, J. Lee, S. Y. Cho, J. Cha, S. Y. Jeong, H. An, W. S. Kim, Y.-S. Jung, J. Park, G. Y. Jung, D.-Y. Kim, J. Y. Jo, S. D. Bu, H. W. Jang and S. Lee, Domain-engineered BiFeO₃ thin-film photoanodes for highly enhanced ferroelectric solar water splitting, Nano Res., 2018, 11, 642–655.
3. J. Liu, Y. Liu, N. Y. Liu, Y. Z. Han, X. Zhang, H. Huang, Y. Lifshitz, S. T. Lee, J. Zhong and Z. H. Kang, Metal-free efficient photocatalyst for stable visible water splitting via a two-electron pathway, Science, 2015, 347, 970–974.
4. Y. O. Wang, A. Vogel, M. Sachs, R. S. Sprick, L. Wilbraham, S. J. A. Moniz, R. Godin, M. A. Zwijnenburg, J. R. Durrant, A. I. Cooper and J. W. Tang, Current understanding and challenges of solar-driven hydrogen generation using polymeric photocatalysts, Nat. Energy, 2020, 5, 633.
5. W. Bi, X. Li, L. Zhang, T. Jin, L. Zhang, Q. Zhang, Y. Luo, C. Wu and Y. Xie, Molecular co-catalyst accelerating hole transfer for enhanced photocatalytic H₂ evolution, Nat. Commun., 2015, 6, 8647.
6. M. Z. Jacobson, W. G. Colella and D. M. Golden, Cleaning the air and improving health with hydrogen fuel-cell vehicles, Science, 2005, 308, 1901–1905.
7. J. Pan, Y. Sun, W. Li, J. Knight and A. Manthiram, A green lead hydrometallurgical process based on a hydrogen-lead oxide fuel cell, Nat. Commun., 2013, 4, 2178.
8. Y. Yang, X. He, P. Zhang, Y. H. Andaloussi, H. Zhang, Z. Jiang, Y. Chen, S. Ma, P. Cheng and Z. Zhang, Combined intrinsic and extrinsic proton conduction in robust covalent organic frameworks for hydrogen fuel cell applications, Angew. Chem., Int. Ed., 2020, 59, 3678–3684.
9. S. Manigandan, R. Sarweswaran, P. B. Devi, Y. Sohret, A. Kondratiev, S. Venkatesh, M. R. Vimal and J. J. Joshua, Comparative study of nanoadditives TiO₂, CNT, Al₂O₃, CuO and CeO₂ on reduction of diesel engine emission operating on hydrogen fuel blends, Fuel, 2020, 262, 116336.
10. C. Gong, Z. Li, J. Sun and F. Liu, Evaluation on combustion and lean-burn limit of a medium compression ratio hydrogen/methanol dual-injection spark-ignition engine under methanol late-injection, Appl. Energy, 2020, 277, 115622.
11. C. Acar and I. Dincer, The potential role of hydrogen as a sustainable transportation fuel to combat global warming, Int. J. Hydrogen Energy, 2020, 45, 3396–3406.
12. J. Zhong, X. Yang, Z. Wu, B. Liang, Y. Huang and T. Zhang, State of the art and perspectives in heterogeneous catalysis of CO₂ hydrogenation to methanol, Chem. Soc. Rev., 2020, 49, 1385–1413.
13. C. Mao, J. Wang, Y. Zou, G. Qi, J. Y. Yang Loh, T. Zhang, M. Xia, J. Xu, F. Deng, M. Ghoussoub, N. P. Kherani, L. Wang, H. Shang, M. Li, J. Li, X. Liu, Z. Ai, G. A. Ozin, J. Zhao and L. Zhang, Hydrogen spillover to oxygen vacancy of TiO_2–xH_y/Fe: Breaking the scaling relationship of ammonia synthesis, J. Am. Chem. Soc., 2020, 142, 17403–17412.
14. D. Wu, K. Deng, B. Hu, Q. Lu, G. Liu and X. Hong, Plasmon-assisted photothermal catalysis of low-pressure CO₂ hydrogenation to methanol over Pd/ZnO catalyst, ChemCatChem, 2019, 11, 1598–1601.
15. Q. Xiang, J. Yu and M. Jaroniec, Synergetic effect of MoS₂ and graphene as cocatalysts for enhanced photocatalytic H₂ production activity of TiO₂ nanoparticles, J. Am. Chem. Soc., 2012, 134, 6575–6578.
16. J. Ran, J. Zhang, J. Yu, M. Jaroniec and S. Z. Qiao, Earth-abundant cocatalysts for semiconductor-based photocatalytic water splitting, Chem. Soc. Rev., 2014, 43, 7787–7812.
17. Q. Lu, Y. Yu, Q. Ma, B. Chen and H. Zhang, 2D transition-metal-dichalcogenide-nanosheet-based composites for photocatalytic and electrocatalytic hydrogen evolution reactions, Adv. Mater., 2016, 28, 1917–1933.
18. M. Zhu, Z. Sun, M. Fujitsuka and T. Majima, Z-scheme photocatalytic water splitting on a 2D heterostructure of black phosphorus/bismuth vanadate using visible light, Angew. Chem., Int. Ed., 2018, 57, 2160–2164.
19. Q. Guo, Z. Ma, C. Zhou, Z. Ren and X. Yang, Single molecule photocatalysis on TiO₂ surfaces, Chem. Rev., 2019, 119, 11020–11041.
20. S. Jayachitra, D. Mahendiran, P. Ravi, P. Murugan and M. Sathish, Highly conductive NiSe₂ nanoparticle as a co-catalyst over TiO₂ for enhanced photocatalytic hydrogen production, Appl. Catal., B, 2022, 307, 121159.
21. H. G. Yang, C. H. Sun, S. Z. Qiao, J. Zou, G. Liu, S. C. Smith, H. M. Cheng and G. Q. Lu, Anatase TiO₂ single crystals with a large percentage of reactive facets, Nature, 2008, 453, 638–641.
22. Q. Sun, N. Wang, J. Yu and J. C. Yu, A hollow porous CdS photocatalyst, Adv. Mater., 2018, 30, 1804368.
23. H. Ren, J.-L. Yang, W.-M. Yang, H.-L. Zhong, J.-S. Lin, P. M. Radjenovic, L. Sun, H. Zhang, J. Xu, Z.-Q. Tian and J.-F. Li, Core–shell–satellite plasmonic photocatalyst for broad-spectrum photocatalytic water splitting, ACS Mater. Lett., 2021, 3, 69–76.
24. C.-Q. Li, X. Du, S. Jiang, Y. Liu, Z.-L. Niu, Z.-Y. Liu, S.-S. Yi and X.-Z. Yue, Constructing direct Z-scheme heterostructure by enwrapping ZnIn₂S₄ on CdS hollow cube for efficient photocatalytic H₂ generation, Adv. Sci., 2022, 9, 2201773.
25. L. Mu, Y. Zhao, A. Li, S. Wang, Z. Wang, J. Yang, Y. Wang, T. Liu, R. Chen, J. Zhu, F. Fan, R. Li and C. Li, Enhancing charge separation on high symmetry SrTiO₃ exposed with anisotropic facets for photocatalytic water splitting, Energy Environ. Sci., 2016, 9, 2463–2469.
26. C. M. Pelicano, M. Saruyama, R. Takahata, R. Sato, Y. Kitahama, H. Matsuzaki, T. Yamada, T. Hisatomi, K. Domen and T. Teranishi, Bimetallic synergy in ultrafine cocatalyst alloy nanoparticles for efficient photocatalytic water splitting, Adv. Funct. Mater., 2022, 32, 2202987.
27. J. Pan, Z. Chen, P. Wang, P. Wang, Q. Yu, W. Zhao, J. Wang, M. Zhu, Y. Zheng and C. Li, The overall water splitting of CdS/Ti³⁺-SrTiO₃ core–shell heterojunction via OER enhancement of MnO_x nanoparticles, Chem. Eng. J., 2021, 424, 130357.
28. Q. Liu, J. Shen, X. Yu, X. Yang, W. Liu, J. Yang, H. Tang, H. Xu, H. Li, Y. Li and J. Xu, Unveiling the origin of boosted photocatalytic hydrogen evolution in simultaneously (S, P, O)-codoped and exfoliated ultrathin g-C₃N₄ nanosheets, Appl. Catal., B, 2019, 248, 84–94.
29. C. Hu, F. Chen, Y. Wang, N. Tian, T. Ma, Y. Zhang and H. Huang, Exceptional cocatalyst-free photo-enhanced piezocatalytic hydrogen evolution of carbon nitride nanosheets from strong in-plane polarization, Adv. Mater., 2021, 33, 2101751.
30. Z. Wang, Z. Wang, X. Zhu, C. Ai, Y. Zeng, W. Shi, X. Zhang, H. Zhang, H. Si, J. Li, C.-Z. Wang and S. Lin, Photodepositing CdS on the active cyano groups decorated g-C₃N₄ in Z-scheme manner promotes visible-light-driven hydrogen evolution, Small, 2021, 17, 2102699.
31. Y. Liu, B. Dong, A. Hagfeldt, J. Luo and M. Graetzel, Chemically tailored molecular surface modifiers for efficient and stable perovskite photovoltaics, SmartMat, 2021, 2, 33–37.
32. Y. Peng, X. Wang, L. Li, H. Ye, S. Yang, H. G. Yang, J. Luo and Y. Hou, Moisture-resistant chiral perovskites for white-light circularly polarized photoluminescence, Adv. Opt. Mater., 2023, 11, 2201888.
33. C. Zou, Z. Zhou, X. Liu, F. Zhang, J. Xie, Y. Su, S. Yang and Y. Hou, Multication tin-lead perovskite photodiodes with engineered lattice strain for ultrasensitive broadband photodetection, Adv. Opt. Mater., 2022, 10, 2201769.
34. Y. Yin, Z. Guo, G. Chen, H. Zhang and W.-J. Yin, Recent progress in defect tolerance and defect passivation in halide perovskite solar cells, Acta Phys. Chim. Sin., 2021, 4, 2008048.
35. Y.-H. Zhang and Y. Li, Interface materials for perovskite solar cells, Rare Met., 2021, 40, 2993–3018.
36. Q. Li, Y. Zheng, Z. Wei, J. Xie, C. Zou, X. Liu, D. Liu, Z. Zhou, H. G. Yang, S. Yang and Y. Hou, Halide diffusion equilibrium and its impact on efficiency evolution of perovskite solar cells, Adv. Energy Mater., 2022, 12, 2202982.
37. Y. Shi, J. He, H. Lian, X. Liu, H. Yuan, Y. Hou, S. Yang and H. G. Yang, Cooperative adsorption of metal-organic complexes on CsPbI₂Br perovskite surface for photovoltaic efficiency exceeding 17%, ChemSusChem, 2022, 15, e202201394.
38. Z. Yuan, Z. Hu, I. Persson, C. Wang, X. Liu, C. Kuang, W. Xu, S. Bai and F. Gao, Interface-assisted cation exchange enables high-performance perovskiteLEDs with tunable near-infrared emissions, Joule, 2022, 6, 2423–2436.
39. C. Zhang, S. Wang, X. Li, M. Yuan, L. Turyanska and X. Yang, Core/shell perovskite nanocrystals: Synthesis of highly efficient and environmentally stable FAPbBr₃/CsPbBr₃ for LED applications, Adv. Funct. Mater., 2020, 30, 1910582.
40. E.-P. Yao, Z. Yang, L. Meng, P. Sun, S. Dong, Y. Yang and Y. Yang, High-brightness blue and white LEDs based on inorganic perovskite nanocrystals and their composites, Adv. Mater., 2017, 29, 1606859.
41. D. H. Chun, Y. J. Choi, Y. In, J. K. Nam, Y. J. Choi, S. Yun, W. Kim, D. Choi, D. Kim, H. Shin, J. H. Cho and J. H. Park, Halide perovskite nanopillar photodetector, ACS Nano, 2018, 12, 8564–8571.
42. L. Martínez-Goyeneche, L. Gil-Escrig, I. Susic, D. Tordera, H. J. Bolink and M. Sessolo, Narrowband monolithic perovskite–perovskite tandem photodetectors, Adv. Opt. Mater., 2022, 10, 2201047.
43. W. Tian, H. Zhou and L. Li, Hybrid organic–inorganic perovskite photodetectors, Small, 2017, 13, 1702107.
44. J. Park, J. Kim, H.-S. Yun, M. J. Paik, E. Noh, H. J. Mun, M. G. Kim, T. J. Shin and S. I. Seok, Controlled growth of perovskite layers with volatile alkylammonium chlorides, Nature, 2023, 616, 724–730.
45. S. Sun, T. Salim, N. Mathews, M. Duchamp, C. Boothroyd, G. Xing, T. C. Sum and Y. M. Lam, The origin of high efficiency in low-temperature solution-processable bilayer organometal halide hybrid solar cells, Energy Environ. Sci., 2014, 7, 399–407.
46. N.-G. Park, Perovskite solar cells: an emerging photovoltaic technology, Mater. Today, 2015, 18, 65–72.
47. W. Zhang, M. Saliba, S. D. Stranks, Y. Sun, X. Shi, U. Wiesner and H. J. Snaith, Enhancement of perovskite-based solar cells employing core–shell metal nanoparticles, Nano Lett., 2013, 13, 4505–4510.
48. V. D’Innocenzo, G. Grancini, M. J. P. Alcocer, A. R. S. Kandada, S. D. Stranks, M. M. Lee, G. Lanzani, H. J. Snaith and A. Petrozza, Excitons versus free charges in organo-lead tri-halide perovskites, Nat. Commun., 2014, 5, 3586.
49. C. Zhou, Q. Ou, W. Chen, Z. Gan, J. Wang, Q. Bao, X. Wen and B. Jia, Illumination-induced halide segregation in gradient bandgap mixed-halide perovskite nanoplatelets, Adv. Opt. Mater., 2018, 6, 1801107.
50. W. Zhou, P. Han, X. Zhang, D. Zheng, S. Yang, Y. Yang, C. Luo, B. Yang, F. Hong, D. Wei, R. Lu and K. Han, Lead-free small-bandgap Cs₂CuSbCl₆ double perovskite nanocrystals, J. Phys. Chem. Lett., 2020, 11, 6463–6467.
51. S. Purohit, K. L. Yadav and S. Satapathi, Metal halide perovskite heterojunction for photocatalytic hydrogen generation: Progress and future opportunities, Adv. Mater. Interfaces, 2022, 9, 2200058.
52. K. Ren, S. Yue, C. Li, Z. Fang, K. A. M. Gasem, J. Leszczynski, S. Qu, Z. Wang and M. Fan, Metal halide perovskites for photocatalysis applications, J. Mater. Chem. A, 2022, 10, 407–429.
53. B.-M. Bresolin, Y. Park and D. W. Bahnemann, Recent progresses on metal halide perovskite-based material as potential photocatalyst, Catalysts, 2020, 10, 709.
54. G. E. Eperon, G. M. Paternò, R. J. Sutton, A. Zampetti, A. A. Haghighirad, F. Cacialli and H. J. Snaith, Inorganic caesium lead iodide perovskite solar cells, J. Mater. Chem. A, 2015, 3, 19688–19695.
55. Y. Zhao and K. Zhu, Organic–inorganic hybrid lead halide perovskites for optoelectronic and electronic applications, Chem. Soc. Rev., 2016, 45, 655–689.
56. Y. Liu, Z. Yang and S. Liu, Recent progress in single-crystalline perovskite research including crystal preparation, property evaluation, and applications, Adv. Sci., 2018, 5, 1700471.
57. C. Zhou, H. Lin, S. Lee, M. Chaaban and B. Ma, Organic–inorganic metal halide hybrids beyond perovskites, Mater. Res. Lett., 2018, 6, 552–569.
58. W. Wu, X. Shang, Z. Xu, H. Ye, Y. Yao, X. Chen, M. Hong, J. Luo and L. Li, Toward efficient two-photon circularly polarized light detection through cooperative strategies in chiral quasi-2D perovskites, Adv. Sci., 2023, 10, 2206070.
59. C. M. M. Soe, C. C. Stoumpos, M. Kepenekian, B. Traoré, H. Tsai, W. Nie, B. Wang, C. Katan, R. Seshadri, A. D. Mohite, J. Even, T. J. Marks and M. G. Kanatzidis, New type of 2D perovskites with alternating cations in the interlayer space, (C(NH₂)₃)(CH₃NH₃)_nPb_nI_3n+1: Structure, properties, and photovoltaic performance, J. Am. Chem. Soc., 2017, 139, 16297–16309.
60. Z. Yuan, C. Zhou, Y. Tian, Y. Shu, J. Messier, J. C. Wang, L. J. van de Burgt, K. Kountouriotis, Y. Xin, E. Holt, K. Schanze, R. Clark, T. Siegrist and B. Ma, One-dimensional organic lead halide perovskites with efficient bluish white-light emission, Nat. Commun., 2017, 8, 14051.
61. L.-J. Feng, Y.-Y. Zhao, R.-Y. Song and X.-W. Lei, Three homologous 1D lead halide perovskites with broadband white-light emissions, Inorg. Chem. Commun., 2022, 136, 109146.
62. C. Zhou, H. Lin, H. Shi, Y. Tian, C. Pak, M. Shatruk, Y. Zhou, P. Djurovich, M.-H. Du and B. Ma, A Zero-dimensional organic seesaw-shaped tin bromide with highly efficient strongly Stokes-shifted deep-red emission, Angew. Chem., Int. Ed., 2018, 57, 1021–1024.
63. C. Zhou, Y. Tian, M. Wang, A. Rose, T. Besara, N. K. Doyle, Z. Yuan, J. C. Wang, R. Clark, Y. Hu, T. Siegrist, S. Lin and B. Ma, Low-dimensional organic tin bromide perovskites and their photoinduced structural transformation, Angew. Chem., Int. Ed., 2017, 56, 9018–9022.
64. Z. Xiao, W. Meng, J. Wang, D. B. Mitzi and Y. Yan, Searching for promising new perovskite-based photovoltaic absorbers: the importance of electronic dimensionality, Mater. Horizons, 2017, 4, 206–216.
65. S. A. Kulkarni, N. Yantara, K. S. Tan, N. Mathews and S. G. Mhaisalkar, Perovskite nanostructures: Leveraging quantum effects to challenge optoelectronic limits, Mater. Today, 2020, 33, 122–140.
66. Y. Peng, X. Liu, L. Li, Y. Yao, H. Ye, X. Shang, X. Chen and J. Luo, Realization of vis–NIR dual-modal circularly polarized light detection in chiral perovskite bulk crystals, J. Am. Chem. Soc., 2021, 143, 14077–14082.
67. Y. Ai, X.-G. Chen, P.-P. Shi, Y.-Y. Tang, P.-F. Li, W.-Q. Liao and R.-G. Xiong, Fluorine substitution induced high T_c of enantiomeric perovskite ferroelectrics: (R)- and (S)-3-(Fluoropyrrolidinium)MnCl₃, J. Am. Chem. Soc., 2019, 141, 4474–4479.
68. Y. Gao, E. Shi, S. Deng, S. B. Shiring, J. M. Snaider, C. Liang, B. Yuan, R. Song, S. M. Janke, A. Liebman-Peláez, P. Yoo, M. Zeller, B. W. Boudouris, P. Liao, C. Zhu, V. Blum, Y. Yu, B. M. Savoie, L. Huang and L. Dou, Molecular engineering of organic–inorganic hybrid perovskites quantum wells, Nat. Chem., 2019, 11, 1151–1157.
69. Y. Gao, Z. Wei, S.-N. Hsu, B. W. Boudouris and L. Dou, Two-dimensional halide perovskites featuring semiconducting organic building blocks, Mater. Chem. Front., 2020, 4, 3400–3418.
70. Y. Peng, J. Bie, X. Liu, L. Li, S. Chen, W. Fa, S. Wang, Z. Sun and J. Luo, Acquiring high-T_C layered metal halide ferroelectrics via cage-confined ethylamine rotators, Angew. Chem., Int. Ed., 2021, 60, 2839–2843.
71. H.-Y. Liu, H.-Y. Zhang, X.-G. Chen and R.-G. Xiong, Molecular design principles for ferroelectrics: Ferroelectrochemistry, J. Am. Chem. Soc., 2020, 142, 15205–15218.
72. M. Shao, T. Bie, L. Yang, Y. Gao, X. Jin, F. He, N. Zheng, Y. Yu and X. Zhang, Over 21% efficiency stable 2D perovskite solar cells, Adv. Mater., 2022, 34, 2107211.
73. B. R. Wygant, A. Z. Ye, A. Dolocan, Q. Vu, D. M. Abbot and C. B. Mullins, Probing the degradation chemistry and enhanced stability of 2D organolead halide perovskites, J. Am. Chem. Soc., 2019, 141, 18170–18181.
74. R. K. Ulaganathan, R. C. Murugesan, C.-Y. Lin, A. Subramanian, W.-L. Chen, Y.-M. Chang, A. Rozhin and R. Sankar, Stable formamidinium-based centimeter long two-dimensional lead halide perovskite single-crystal for long-life optoelectronic applications, Adv. Funct. Mater., 2022, 32, 2112277.
75. S. Park, W. J. Chang, C. W. Lee, S. Park, H.-Y. Ahn and K. T. Nam, Photocatalytic hydrogen generation from hydriodic acid using methylammonium lead iodide in dynamic equilibrium with aqueous solution, Nat. Energy, 2016, 2, 16185.
76. P. C. K. Vesborg, Photocatalysis: HI-time for perovskites, Nat. Energy, 2017, 2, 16205.
77. C. Bie, L. Wang and J. Yu, Challenges for photocatalytic overall water splitting, Chem, 2022, 8, 1567–1574.
78. J. Schneidewind, M. A. Argüello Cordero, H. Junge, S. Lochbrunner and M. Beller, Two-photon, visible light water splitting at a molecular ruthenium complex, Energy Environ. Sci., 2021, 14, 4427–4436.
79. C. Levy-Clement, A. Heller, W. A. Bonner and B. A. Parkinson, Spontaneous photoelectrolysis of HBr and  HI, J. Electrochem. Soc., 1982, 129, 1701.
80. J. H. Norman, K. J. Mysels, R. Sharp and D. Williamson, Studies of the sulfur-iodine thermochemical water-splitting cycle, Int. J. Hydrogen Energy, 1982, 7, 545–556.
81. H. Wang, H. Zhang, J. Wang, Y. Gao, F. Fan, K. Wu, X. Zong and C. Li, Mechanistic understanding of efficient photocatalytic H₂ evolution on two-dimensional layered lead iodide hybrid perovskites, Angew. Chem., Int. Ed., 2021, 60, 7376–7381.
82. Y. Li, L. Li, S. She, S. Chen, Y. Liu, B. Xu, F.-W. Lee and X. Zhu, Multidimensional perovskite for visible light driven hydrogen production in aqueous HI solution, ACS Appl. Energy Mater., 2022, 5, 207–213.
83. H. Huang, D. Verhaeghe, B. Weng, B. Ghosh, H. Zhang, J. Hofkens, J. A. Steele and M. B. J. Roeffaers, Metal halide perovskite based heterojunction photocatalysts, Angew. Chem., Int. Ed., 2022, 61, e202203261.
84. V. Armenise, S. Colella, F. Fracassi and A. Listorti, Lead-free metal halide perovskites for hydrogen evolution from aqueous solutions, Nanomaterials, 2021, 11, 433.
85. Y. Wu, P. Wang, Z. Guan, J. Liu, Z. Wang, Z. Zheng, S. Jin, Y. Dai, M.-H. Whangbo and B. Huang, Enhancing the photocatalytic hydrogen evolution activity of mixed-halide perovskite CH₃NH₃PbBr_3–xI_x achieved by bandgap funneling of charge carriers, ACS Catal., 2018, 8, 10349–10357.
86. H. Zhao, Y. Li, B. Zhang, T. Xu and C. Wang, PtI_x/[(CH₃)₂NH₂]₃[BiI₆] as a well-dispersed photocatalyst for hydrogen production in hydroiodic acid, Nano Energy, 2018, 50, 665–674.
87. D. Ju, X. Zheng, J. Liu, Y. Chen, J. Zhang, B. Cao, H. Xiao, O. F. Mohammed, O. M. Bakr and X. Tao, Reversible band gap narrowing of Sn-based hybrid perovskite single crystal with excellent phase stability, Angew. Chem., Int. Ed., 2018, 57, 14868–14872.
88. Z. Zhao, J. Wu, Y.-Z. Zheng, N. Li, X. Li, Z. Ye, S. Lu, X. Tao and C. Chen, Stable hybrid perovskite MAPb(I_1−xBr_x)₃ for photocatalytic hydrogen evolution, Appl. Catal., B, 2019, 253, 41–48.
89. Z. Guan, Y. Wu, P. Wang, Q. Zhang, Z. Wang, Z. Zheng, Y. Liu, Y. Dai, M.-H. Whangbo and B. Huang, Perovskite photocatalyst CsPbBr_3-xI_x with a bandgap funnel structure for H₂ evolution under visible light, Appl. Catal., B, 2019, 245, 522–527.
90. M. Wang, Y. Zuo, J. Wang, Y. Wang, X. Shen, B. Qiu, L. Cai, F. Zhou, S. P. Lau and Y. Chai, Remarkably enhanced hydrogen generation of organolead halide perovskites via piezocatalysis and photocatalysis, Adv. Energy Mater., 2019, 9, 1901801.
91. K. Rokesh, M. Sakar and T.-O. Do, 2-(Aminomethyl pyridine)SbI₅: An emerging visible-light driven organic–inorganic hybrid perovskite for photoelectrochemical and photocatalytic applications, Mater. Lett., 2019, 242, 99–102.
92. Y. Guo, G. Liu, Z. Li, Y. Lou, J. Chen and Y. Zhao, Stable lead-free (CH₃NH₃)₃Bi₂I₉ perovskite for photocatalytic hydrogen generation, ACS Sustain. Chem. Eng., 2019, 7, 15080–15085.
93. G. Q. Chen, P. Wang, Y. Q. Wu, Q. Q. Zhang, Q. Wu, Z. Y. Wang, Z. K. Zheng, Y. Y. Liu, Y. Dai and B. B. Huang, Lead-free halide perovskite Cs₃Bi_2xSb_2-2xI₉(x ≈ 0.3) possessing the photocatalytic activity for hydrogen evolution comparable to that of (CH₃NH₃)PbI₃, Adv. Mater., 2020, 32, 2001344.
94. F. Liu, M. Wang, X. Liu, B. Wang, C. Li, C. Liu, Z. Lin and F. Huang, A rapid and robust light-and-solution-triggered in situ crafting of organic passivating membrane over metal halide perovskites for markedly improved stability and photocatalysis, Nano Lett., 2021, 21, 1643–1650.
95. H. Yin, J. Chen, P. Guan, D. Zheng, Q. Kong, S. Yang, P. Zhou, B. Yang, T. Pullerits and K. Han, Controlling photoluminescence and photocatalysis activities in lead-free Cs₂Pt_xSn_1-xCl₆ perovskites via ion substitution, Angew. Chem., Int. Ed., 2021, 60, 22693–22699.
96. H. Zhao, K. Chordiya, P. Leukkunen, A. Popov, M. Upadhyay Kahaly, K. Kordas and S. Ojala, Dimethylammonium iodide stabilized bismuth halide perovskite photocatalyst for hydrogen evolution, Nano Res., 2021, 14, 1116–1125.
97. Z. He, Q. Tang, X. Liu, X. Yan, K. Li and D. Yue, Lead-free Cs₂AgBiBr₆ perovskite with enriched surface defects for efficient photocatalytic hydrogen evolution, Energy Fuels, 2021, 35, 15005–15009.
98. P. Zhou, H. Chen, Y. G. Chao, Q. H. Zhang, W. Y. Zhang, F. Lv, L. Gu, Q. Zhao, N. Wang, J. S. Wang and S. J. Guo, Single-atom Pt-I3 sites on all-inorganic Cs2SnI6 perovskite for efficient photocatalytic hydrogen production, Nat. Commun., 2021, 12, 4412.
99. Y. Q. Wu, Q. Wu, Q. Q. Zhang, Z. Z. Lou, K. F. Liu, Y. D. Ma, Z. Y. Wang, Z. K. Zheng, H. F. Cheng, Y. Y. Liu, Y. Dai, B. B. A. Huang and P. Wang, An organometal halide perovskite supported Pt single-atom photocatalyst for H₂ evolution, Energy Environ. Sci., 2022, 15, 1271–1281.
100. Y. Q. Tang, C. H. Mak, C. Wang, Y. Fu, F. F. Li, G. H. Jia, C. W. Hsieh, H. H. Shen, J. C. Colmenares, H. S. Song, M. J. Yuan, Y. Chen and H. Y. Hsu, Bandgap funneling in bismuth-based hybrid perovskite photocatalyst with efficient visible-light-driven hydrogen evolution, Small Methods, 2022, 6, 2200326.
101. Y. L. Ji, M. Y. She, X. Bai, E. Z. Liu, W. H. Xue, Z. Zhang, K. R. Wan, P. Liu, S. Y. Zhang and J. L. Li, In-depth understanding of the effect of halogen-induced stable 2D bismuth-based perovskites for photocatalytic hydrogen evolution activity, Adv. Funct. Mater., 2022, 32, 2201721.
102. C. X. Bao, J. Yang, S. Bai, W. D. Xu, Z. B. Yan, Q. Y. Xu, J. M. Liu, W. J. Zhang and F. Gao, High performance and stable all-inorganic metal halide perovskite-based photodetectors for optical communication applications, Adv. Mater., 2018, 30, 1803422.
103. C. Liu, Y. Yang, O. A. Syzgantseva, Y. Ding, M. A. Syzgantseva, X. F. Zhang, A. M. Asiri, S. Y. Dai and M. K. Nazeeruddin, α-CsPbI₃ bilayers via one-step deposition for efficient and stable all-inorganic perovskite solar cells, Adv. Mater., 2020, 32, 2002632.
104. A. A. Zhumekenov, M. I. Saidaminov, M. A. Haque, E. Alarousu, S. P. Sarmah, B. Murali, I. Dursun, X.-H. Miao, A. L. Abdelhady, T. Wu, O. F. Mohammed and O. M. Bakr, Formamidinium lead halide perovskite crystals with unprecedented long carrier dynamics and diffusion length, ACS Energy Lett., 2016, 1, 32–37.
105. B.-W. Park, B. Philippe, X. Zhang, H. Rensmo, G. Boschloo and E. M. J. Johansson, Bismuth based hybrid perovskites A₃Bi₂I₉ (A: Methylammonium or cesium) for solar cell application, Adv. Mater., 2015, 27, 6806–6813.
106. W. Zhang, M. Hong and J. Luo, Halide double perovskite ferroelectrics, Angew. Chem., Int. Ed., 2020, 59, 9305–9308.
107. F. Jiang, D. Yang, Y. Jiang, T. Liu, X. Zhao, Y. Ming, B. Luo, F. Qin, J. Fan, H. Han, L. Zhang and Y. Zhou, Chlorine-incorporation-induced formation of the layered phase for antimony-based lead-Free perovskite solar cells, J. Am. Chem. Soc., 2018, 140, 1019–1027.
108. M. Liu, Y. Chen, J. Su, J. Shi, X. Wang and L. Guo, Photocatalytic hydrogen production using twinned nanocrystals and an unanchored NiS_x co-catalyst, Nat. Energy, 2016, 1, 16151.
109. Y. Yang, C. Liu, M. L. Cai, Y. J. Liao, Y. Ding, S. Ma, X. P. Liu, M. Guli, S. Y. Dai and M. K. Nazeeruddin, Dimension-controlled growth of antimony-based perovskite-like halides for lead-free and semitransparent photovoltaics, ACS Appl. Mater. Interfaces, 2020, 12, 17062–17069.
110. X. Meng, S. Wang, C. Zhang, C. Dong, R. Li, B. Li, Q. Wang and Y. Ding, Boosting hydrogen evolution performance of a CdS-based photocatalyst: In situ transition from type I to type II heterojunction during photocatalysis, ACS Catal., 2022, 12, 10115–10126.
111. H. Cai, B. Wang, L. Xiong, J. Bi, L. Yuan, G. Yang and S. Yang, Orienting the charge transfer path of type-II heterojunction for photocatalytic hydrogen evolution, Appl. Catal., B, 2019, 256, 117853.
112. B.-J. Ng, L. K. Putri, X. Y. Kong, Y. W. Teh, P. Pasbakhsh and S.-P. Chai, Z-Scheme photocatalytic systems for solar water splitting, Adv. Sci., 2020, 7, 1903171.
113. X. Wang, H. Wang, H. Zhang, W. Yu, X. Wang, Y. Zhao, X. Zong and C. Li, Dynamic interaction between methylammonium lead iodide and TiO₂ nanocrystals leads to enhanced photocatalytic H₂ evolution from HI splitting, ACS Energy Lett., 2018, 3, 1159–1164.
114. M. V. Pavliuk, M. Abdellah and J. Sá, Hydrogen evolution with CsPbBr₃ perovskite nanocrystals under visible light in solution, Mater. Today Commun., 2018, 16, 90–96.
115. Y. Wu, P. Wang, X. Zhu, Q. Zhang, Z. Wang, Y. Liu, G. Zou, Y. Dai, M.-H. Whangbo and B. Huang, Composite of CH₃NH₃PbI₃ with reduced graphene oxide as a highly efficient and stable visible-light photocatalyst for hydrogen evolution in aqueous HI solution, Adv. Mater., 2018, 30, 1704342.
116. R. Li, X. Li, J. Wu, X. Lv, Y.-Z. Zheng, Z. Zhao, X. Ding, X. Tao and J.-F. Chen, Few-layer black phosphorus-on-MAPbI₃ for superb visible-light photocatalytic hydrogen evolution from HI splitting, Appl. Catal., B, 2019, 259, 118075.
117. Z. Zhao, J. Wu, Y.-Z. Zheng, N. Li, X. Li and X. Tao, Ni₃C-decorated MAPbI₃ as visible-light photocatalyst for H₂ evolution from HI splitting, ACS Catal., 2019, 9, 8144–8152.
118. H. Wang, X. Wang, R. Chen, H. Zhang, X. Wang, J. Wang, J. Zhang, L. Mu, K. Wu, F. Fan, X. Zong and C. Li, Promoting photocatalytic H₂ evolution on organic–inorganic hybrid perovskite nanocrystals by simultaneous dual-charge transportation modulation, ACS Energy Lett., 2019, 4, 40–47.
119. T. Wang, D. Yue, X. Li and Y. Zhao, Lead-free double perovskite Cs₂AgBiBr₆/RGO composite for efficient visible light photocatalytic H₂ evolution, Appl. Catal., B, 2020, 268, 118399.
120. B. M. Bresolin, P. Sgarbossa, D. W. Bahnemann and M. Sillanpaa, Cs₃Bi₂I₉/g-C₃N₄ as a new binary photocatalyst for efficient visible-light photocatalytic processes, Sep. Purif. Technol., 2020, 251, 117320.
121. G. N. Liu, R. Y. Zhao, B. Xu, Y. Sun, X. M. Jiang, X. Hu and C. Li, Design, synthesis, and photocatalytic application of moisture-stable hybrid lead-free perovskite, ACS Appl. Mater. Interfaces, 2020, 12, 54694–54702.
122. Y. Zhao, Q. Zeng, Y. Yu, T. Feng, Y. Zhao, Z. Wang, Y. Li, C. Liu, J. Liu, H. Wei, S. Zhu, Z. Kang, H. Zhang and B. Yang, Enhanced charge separation and photocatalytic hydrogen evolution in carbonized-polymer-dot-coupled lead halide perovskites, Mater. Horizons, 2020, 7, 2719–2725.
123. F. Wang, X. Liu, Z. Zhang and S. Min, A noble-metal-free MoS₂ nanosheet-coupled MAPbI₃ photocatalyst for efficient and stable visible-light-driven hydrogen evolution, Chem. Commun., 2020, 56, 3281–3284.
124. Y. Tang, C. H. Mak, R. Liu, Z. Wang, L. Ji, H. Song, C. Tan, F. Barrière and H. Y. Hsu, In situ formation of bismuth-based perovskite heterostructures for high-performance cocatalyst-free photocatalytic hydrogen evolution, Adv. Funct. Mater., 2020, 30, 2006919.
125. C. Cai, Y. Teng, J. H. Wu, J. Y. Li, H. Y. Chen, J. H. Chen and D. B. Kuang, In situ photosynthesis of an MAPbI₃/CoP hybrid heterojunction for efficient photocatalytic hydrogen evolution, Adv. Funct. Mater., 2020, 30, 2001478.
126. X. Zhao, S. Chen, H. Yin, S. Jiang, K. Zhao, J. Kang, P. F. Liu, L. Jiang, Z. Zhu, D. Cui, P. Liu, X. Han, H. G. Yang and H. Zhao, Perovskite microcrystals with intercalated monolayer MoS₂ nanosheets as advanced photocatalyst for solar-powered hydrogen generation, Matter, 2020, 3, 935–949.
127. Y. Zhang, J. Shi, X. Ding, J. Wu, Y.-Z. Zheng and X. Tao, Stable mixed-organic-cation perovskite MA_1–xFA_xPbI₃ integrated with MoS₂ for enhanced visible-light photocatalytic H₂ evolution, Ind. Eng. Chem. Res., 2020, 59, 20667–20675.
128. L. Romani, A. Speltini, F. Ambrosio, E. Mosconi, A. Profumo, M. Marelli, S. Margadonna, A. Milella, F. Fracassi, A. Listorti, F. De Angelis and L. Malavasi, Water-stable DMASnBr₃ lead-free perovskite for effective solar-driven photocatalysis, Angew. Chem., Int. Ed., 2021, 60, 3611–3618.
129. L. Jiang, Y. Guo, S. Qi, K. Zhang, J. Chen, Y. Lou and Y. Zhao, Amorphous NiCoB-coupled MAPbI₃ for efficient photocatalytic hydrogen evolution, Dalton Trans., 2021, 50, 17960–17966.
130. L. Romani, A. Speltini, C. N. Dibenedetto, A. Listorti, F. Ambrosio, E. Mosconi, A. Simbula, M. Saba, A. Profumo, P. Quadrelli, F. De Angelis and L. Malavasi, Experimental strategy and mechanistic view to boost the photocatalytic activity of Cs₃Bi₂Br₉ lead-free perovskite derivative by g-C₃N₄ composite engineering, Adv. Funct. Mater., 2021, 31, 2104428.
131. W. Guan, Y. Li, Q. Zhong, H. Liu, J. Chen, H. Hu, K. Lv, J. Gong, Y. Xu, Z. Kang, M. Cao and Q. Zhang, Fabricating MAPbI₃/MoS₂ composites for improved photocatalytic performance, Nano Lett., 2021, 21, 597–604.
132. Y. Jiang, K. Li, X. Wu, M. Zhu, H. Zhang, K. Zhang, Y. Wang, K. P. Loh, Y. Shi and Q. H. Xu, In situ synthesis of lead-free halide perovskite Cs₂AgBiBr₆ supported on nitrogen-doped carbon for efficient hydrogen evolution in aqueous HBr solution, ACS Appl. Mater. Interfaces, 2021, 13, 10037–10046.
133. W. B. Li, F. Wang, Z. G. Zhang, X. H. Ma and S. X. Min, Coupling of MAPbI₃ microcrystals with conductive polyaniline for efficient visible-light-driven H₂ evolution, Sustain, Energy Fuels, 2021, 6, 76–80.
134. Y. Zhang, Z. Sun, Z. Wang, Y. Zang and X. Tao, Efficient and long-term photocatalytic H₂ evolution stability enabled by Cs₂AgBiBr₆/MoS₂ in aqueous HBr solution, Int. J. Hydrogen Energy, 2022, 47, 8829–8840.
135. S. Feng, S. Ning, L. Wang, J. Zhao, J. Ou, Z. Wu, S. Luo, Z. Lin, K. Yan, C. Wu and Y. Xu, Modifying CsPbX₃ (X = Cl, Br, I) with a zeolitic imidazolate framework through mechanical milling for aqueous photocatalytic H₂ evolution, ACS Appl. Energy Mater., 2022, 5, 6248–6255.
136. K. Song, J. Gou, L. Yang and C. Zeng, Environmentally stable mesoporous g-C₃N₄ modified lead-free double perovskite Cs₂AgBiBr₆ for highly efficient photocatalytic hydrogen evolution, Catal. Lett., 2023, 153, 534–543.
137. Q. Huang, Y. M. Guo, J. X. Chen, Y. B. Lou and Y. X. Zhao, NiCoP modified lead-free double perovskite Cs₂AgBiBr₆ for efficient photocatalytic hydrogen generation, New J. Chem., 2022, 46, 7395–7402.
138. T. Chen, M. Li, L. Shen, M. B. J. Roeffaers, B. Weng, H. Zhu, Z. Chen, D. Yu, X. Pan, M. Q. Yang and Q. Qian, Photocatalytic anaerobic oxidation of aromatic alcohols coupled with H₂ production over CsPbBr₃/GO-Pt catalysts, Front. Chem., 2022, 10, 833784.
139. Q. Xu, L. Zhang, B. Cheng, J. Fan and J. Yu, S-scheme heterojunction photocatalyst, Chem, 2020, 6, 1543–1559.
140. H. Li, S. Tao, S. Wan, G. Qiu, Q. Long, J. Yu and S. Cao, S-scheme heterojunction of ZnCdS nanospheres and dibenzothiophene modified graphite carbon nitride for enhanced H₂ production, Chin. J. Catal., 2023, 46, 167–176.
141. C. Cheng, J. Zhang, B. Zhu, G. Liang, L. Zhang and J. Yu, Verifying the charge-transfer mechanism in S-scheme heterojunctions using femtosecond transient absorption spectroscopy, Angew. Chem., Int. Ed., 2023, 62, e202218688.
142. F. Zhao, Y. L. Law, N. Zhang, X. Wang, W. Wu, Z. Luo and Y. Wang, Constructing spatially separated cage-like Z-scheme heterojunction photocatalyst for enhancing photocatalytic H₂ evolution, Small, 2023, 19, 2208266.
143. Z. Long, X. Yang, X. Huo, X. Li, Q. Qi, X. Bian, Q. Wang, F. Yang, W. Yu and L. Jiang, Bioinspired Z-scheme In₂O₃/C₃N₄ heterojunctions with tunable nanorod lengths for enhanced photocatalytic hydrogen evolution, Chem. Eng. J., 2023, 461, 141893.
144. H. Zhou, Q. Chen, G. Li, S. Luo, T.-B. Song, H.-S. Duan, Z. Hong, J. You, Y. Liu and Y. Yang, Interface engineering of highly efficient perovskite solar cells, Science, 2014, 345, 542–546.
145. N. J. Jeon, H. G. Lee, Y. C. Kim, J. Seo, J. H. Noh, J. Lee and S. I. Seok, o-Methoxy substituents in spiro-OMeTAD for efficient inorganic–organic hybrid perovskite solar cells, J. Am. Chem. Soc., 2014, 136, 7837–7840.
146. M. B. Zakaria, C. Li, Q. Ji, B. Jiang, S. Tominaka, Y. Ide, J. P. Hill, K. Ariga and Y. Yamauchi, Self-construction from 2D to 3D: One-pot layer-by-layer assembly of graphene oxide sheets held together by coordination polymers, Angew. Chem., Int. Ed., 2016, 55, 8426–8430.
147. H. Zhang, Z. Luo, Y. Liu and Y. Jiang, Noble-metal-free Ni₃C as co-catalyst on LaNiO₃ with enhanced photocatalytic activity, Appl. Catal., B, 2020, 277, 119166.
148. R. C. Shen, K. L. He, A. P. Zhang, N. Li, Y. H. Ng, P. Zhang, J. Hu and X. Li, In-situ construction of metallic Ni₃C@Ni core-shell cocatalysts over g-C₃N₄ nanosheets for shell-thickness-dependent photocatalytic H₂ production, Appl. Catal., B, 2021, 291, 120104.
149. M. Sun, H. Liu, J. Qu and J. Li, Earth-Rich transition metal phosphide for energy conversion and storage, Adv. Energy Mater., 2016, 6, 1600087.
150. S. Cao, C.-J. Wang, W.-F. Fu and Y. Chen, Metal phosphides as co-catalysts for photocatalytic and photoelectrocatalytic water splitting, ChemSusChem, 2017, 10, 4306–4323.
151. F. Zhang, J. Zhang, J. Li, X. Jin, Y. Li, M. Wu, X. Kang, T. Hu, X. Wang, W. Ren and G. Zhang, Modulating charge transfer dynamics for g-C₃N₄ through a dimension and interface engineered transition metal phosphide co-catalyst for efficient visible-light photocatalytic hydrogen generation, J. Mater. Chem. A, 2019, 7, 6939–6945.
152. Y. Liu, X. Li, Q. Zhang, W. Li, Y. Xie, H. Liu, L. Shang, Z. Liu, Z. Chen, L. Gu, Z. Tang, T. Zhang and S. Lu, A general route to prepare low-ruthenium-content bimetallic electrocatalysts for pH-universal hydrogen evolution reaction by using carbon quantum dots, Angew. Chem., Int. Ed., 2020, 59, 1718–1726.
153. M. Han, C. Y. Kang, Z. X. Qu, S. J. Zhu and B. Yang, Surface molecule induced effective light absorption and charge transfer for H₂ production photocatalysis in a carbonized polymer dots-carbon nitride system, Appl. Catal., B, 2022, 305, 121064.
154. Z. H. Pan, T. Hisatomi, Q. Wang, S. S. Chen, A. Iwase, M. Nakabayashi, N. Shibata, T. Takata, M. Katayama, T. Minegishi, A. Kudo and K. Domen, Photoreduced graphene oxide as a conductive binder to improve the water splitting activity of photocatalyst sheets, Adv. Funct. Mater., 2016, 26, 7011–7019.
155. H. L. Tan, H. A. Tahini, X. Wen, R. J. Wong, X. Tan, A. Iwase, A. Kudo, R. Amal, S. C. Smith and Y. H. Ng, Interfacing BiVO₄ with reduced graphene oxide for enhanced photoactivity: A tale of facet dependence of electron shuttling, Small, 2016, 12, 5295–5302.
156. D. Mateo, I. Esteve-Adell, J. Albero, J. F. S. Royo, A. Primo and H. Garcia, 111 oriented gold nanoplatelets on multilayer graphene as visible light photocatalyst for overall water splitting, Nat. Commun., 2016, 7, 11819.
157. Y. Li, Z. Yin, G. Ji, Z. Liang, Y. Xue, Y. Guo, J. Tian, X. Wang and H. Cui, 2D/2D/2D heterojunction of Ti₃C₂ MXene/MoS₂ nanosheets/TiO₂ nanosheets with exposed (001) facets toward enhanced photocatalytic hydrogen production activity, Appl. Catal., B, 2019, 246, 12–20.
158. B. J. Sun, W. Zhou, H. Z. Li, L. P. Ren, P. Z. Qiao, W. Li and H. G. Fu, Synthesis of particulate hierarchical tandem heterojunctions toward optimized photocatalytic hydrogen production, Adv. Mater., 2018, 30, 1804282.
159. C. B. Bie, B. C. Zhu, L. X. Wang, H. G. Yu, C. H. Jiang, T. Chen and J. G. Yu, A bifunctional CdS/MoO₂/MoS₂ catalyst enhances photocatalytic H₂ evolution and pyruvic acid synthesis, Angew. Chem., Int. Ed., 2022, 61, e2022120.
160. Y. Lei, M. Yang, J. Hou, F. Wang, E. Cui, C. Kong and S. Min, Thiomolybdate [Mo₃S₁₃]²⁻ nanocluster: a molecular mimic of MoS₂ active sites for highly efficient photocatalytic hydrogen evolution, Chem. Commun., 2018, 54, 603–606.
161. J. Shi, R. Tong, X. Zhou, Y. Gong, Z. Zhang, Q. Ji, Y. Zhang, Q. Fang, L. Gu, X. Wang, Z. Liu and Y. Zhang, Temperature-mediated selective growth of MoS₂/WS₂ and WS₂/MoS₂ vertical stacks on Au foils for direct photocatalytic applications, Adv. Mater., 2016, 28, 10664–10672.
162. H. He, J. Lin, W. Fu, X. Wang, H. Wang, Q. Zeng, Q. Gu, Y. Li, C. Yan, B. K. Tay, C. Xue, X. Hu, S. T. Pantelides, W. Zhou and Z. Liu, MoS₂/TiO₂ edge-on heterostructure for efficient photocatalytic hydrogen evolution, Adv. Energy Mater., 2016, 6, 1600464.
163. T. F. Jaramillo, K. P. Jørgensen, J. Bonde, J. H. Nielsen, S. Horch and I. Chorkendorff, Identification of active edge sites for electrochemical H₂ evolution from MoS₂ nanocatalysts, Science, 2007, 317, 100–102.
164. H. I. Karunadasa, E. Montalvo, Y. Sun, M. Majda, J. R. Long and C. J. Chang, A molecular MoS₂ edge site mimic for catalytic hydrogen generation, Science, 2012, 335, 698–702.
165. J. S. Cai, J. Y. Huang, S. C. Wang, J. Iocozzia, Z. T. Sun, J. Y. Sun, Y. K. Yang, Y. K. Lai and Z. Q. Lin, Crafting mussel-inspired metal nanoparticle-decorated ultrathin graphitic carbon nitride for the degradation of chemical pollutants and production of chemical resources, Adv. Mater., 2019, 31, 1806314.
166. F. Xue, Y. T. Si, C. Cheng, W. L. Fu, X. Y. Chen, S. H. Shen, L. Z. Wang and M. C. Liu, Electron transfer via homogeneous phosphorus bridges enabling boosted photocatalytic generation of H₂ and H₂O₂ from pure water with stoichiometric ratio, Nano Energy, 2022, 103, 107799.
167. F. Lin, S. Zhou, G. H. Wang, J. Wang, T. Y. Gao, Y. R. Su and C. P. Wong, Electrostatic self-assembly combined with microwave hydrothermal strategy: construction of 1D/1D carbon nanofibers/crystalline g-C₃N₄ heterojunction for boosting photocatalytic hydrogen production, Nano Energy, 2022, 99, 107432.
168. Z. P. Wang, Z. L. Wang, X. D. Zhu, C. Z. Ai, Y. M. Zeng, W. Y. Shi, X. D. Zhang, H. R. Zhang, H. W. Si, J. Li, C. Z. Wang and S. W. Lin, Photodepositing CdS on the active cyano groups decorated g-C₃N₄ in Z-scheme manner promotes visible-light-driven hydrogen evolution, Small, 2021, 17, 2102699.
169. H. Yu, L. Shang, T. Bian, R. Shi, G. I. N. Waterhouse, Y. Zhao, C. Zhou, L.-Z. Wu, C.-H. Tung and T. Zhang, Nitrogen-doped porous carbon nanosheets templated from g-C₃N₄ as metal-free electrocatalysts for efficient oxygen reduction reaction, Adv. Mater., 2016, 28, 5080–5086.
170. Q. Zhang, S. Huang, J. Deng, D. T. Gangadharan, F. Yang, Z. Xu, G. Giorgi, M. Palummo, M. Chaker and D. Ma, Ice-assisted synthesis of black phosphorus nanosheets as a metal-free photocatalyst: 2D/2D heterostructure for broadband H₂ evolution, Adv. Funct. Mater., 2019, 29, 1902486.
171. M. Z. Rahman, C. W. Kwong, K. Davey and S. Z. Qiao, 2D phosphorene as a water splitting photocatalyst: fundamentals to applications, Energy Environ. Sci., 2016, 9, 709–728.
172. R. Yan, T. Ma, M. Cheng, X. Tao, Z. Yang, F. Ran, S. Li, B. Yin, C. Cheng and W. Yang, Metal–organic-framework-derived nanostructures as multifaceted electrodes in metal–sulfur batteries, Adv. Mater., 2021, 33, 2008784.
173. A. Dhakshinamoorthy, A. M. Asiri and H. Garcia, 2D Metal–organic frameworks as multifunctional materials in heterogeneous catalysis and electro/photocatalysis, Adv. Mater., 2019, 31, 1900617.
174. X. Tao, Y. Zhao, S. Wang, C. Li and R. Li, Recent advances and perspectives for solar-driven water splitting using particulate photocatalysts, Chem. Soc. Rev., 2022, 51, 3561–3608.
175. Y. Zhang and N.-G. Park, Quasi-two-dimensional perovskite solar cells with efficiency exceeding 22%, ACS Energy Lett., 2022, 7, 757–765.
176. G. Liao, C. Li, S.-Y. Liu, B. Fang and H. Yang, Z-scheme systems: From fundamental principles to characterization, synthesis, and photocatalytic fuel-conversion applications, Phys. Rep., 2022, 983, 1–41.
177. F. Bella, G. Griffini, J.-P. Correa-Baena, G. Saracco, M. Grätzel, A. Hagfeldt, S. Turri and C. Gerbaldi, Improving efficiency and stability of perovskite solar cells with photocurable fluoropolymers, Science, 2016, 354, 203–206.
178. Y. Kim, H. Kim, S. Graham, A. Dyer and J. R. Reynolds, Durable polyisobutylene edge sealants for organic electronics and electrochemical devices, Sol. Energy Mater. Sol. Cells, 2012, 100, 120–125.
179. J. Li, R. Xia, W. Qi, X. Zhou, J. Cheng, Y. Chen, G. Hou, Y. Ding, Y. Li, Y. Zhao and X. Zhang, Encapsulation of perovskite solar cells for enhanced stability: Structures, materials and characterization, J. Power Sources, 2021, 485, 229313.
180. Z. Huang, J. Long, R. Dai, X. Hu, L. Le, X. Meng, L. Tan and Y. Chen, Ultra-flexible and waterproof perovskite photovoltaics for washable power source applications, Chem. Commun., 2021, 57, 6320–6323.
181. Q. Zhang, S. Liu, M. He, W. Zheng, Q. Wan, M. Liu, X. Liao, W. Zhan, C. Yuan, J. Liu, H. Xie, X. Guo, L. Kong and L. Li, Stable lead-free tin halide perovskite with operational stability > 1200 h by suppressing Tin(II) oxidation, Angew. Chem., Int. Ed., 2022, 61, e202205463.
182. Z. Xiao, Z. Song and Y. Yan, From lead halide perovskites to lead-free metal halide perovskites and perovskite derivatives, Adv. Mater., 2019, 31, 1803792.

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