ReviewState-of-the-art progress in the rational design of layered double hydroxide based photocatalysts for photocatalytic and photoelectrochemical H2/O2 production
Graphical abstract
Fig. 1. Schematic representation of typical synthetic methods and typical LDH based photocatalysts for PC/PEC water splitting.
Introduction
The increasing energy crisis and growing environmental pollution issues have promoted scientists to develop clean and sustainable energy solutions [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12]. Water splitting, based on photocatalytic (PC) or photoelectrochemical (PEC) technology, have been regarded as a capable and effective way for the future environmental remediation and energy conversion as the used semiconductors can directly harvest and convert renewable solar light into high-energy–density chemical energy [13], [14], [15], [16], [17], [18], [19], [20]. The entire photo(electro)chemical process is progressed under ambient temperatures and atmospheric pressures with relatively stable photocatalytic semiconductors to dissociate two H2O molecules into two H2 molecules one O2 molecule. Generally, photocatalytic H2/O2 production can be achieved by two routes: suspension photocatalyst systems and photoelectrode (i.e., photocatalyst immobilized on a conducting substrate) [16], [21], [22], [23]. Hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are two important half reactions of water splitting (Fig. 1). The exploration and development of H2-evolving and O2-evolving photocatalysts with high photocatalytic activity and stability are extremely needed and important for boosting the water-splitting efficiency. Recently, various semiconductor photocatalysts, such as oxides [24], [25], sulfides [26], [27], and oxynitrides [28] have been widely developed and extensively studied based on photocatalytic principles. Currently, with the rapid growth of nanotechnology and various advanced characterization methods, 2D lamellar nanomaterials have aroused considerable attention, which is resulted from their unique lamellar structures, electronic properties and intrinsic activity of active sites [29], [30], [31].
Layered double hydroxides (LDHs), also known as hydrotalcite-like materials, are composed of positively charged brucite-like host layers containing edge-sharing M(OH)6 octahedra, with intercalated anions confined in the interlayer hydrated galleries (Fig. 2). Briefly, the general molecular formula can be described as [M1−x2+Mx3+(OH)2(An−)x/n]x+·mH2O, where M2+ and M3+ are bivalent metal cation (e.g., Mg2+, Ni2+, Zn2+, Cu2+, Co2+) and trivalent metal cation (e.g., Al3+, Fe3+, Ga3+, Mn3+) respectively, and An- are the counter ion (e.g., CO32−, Cl−, NO3−, SO42−, PO43−) [32], [33], [34]. The metal octahedra in LDHs are bonded to each other through a metal–oxygen-metal oxo bridging linkage [35], [36]. The presence of oxo-bridges are beneficial for the metal-to-metal charge migration which in turn act as a vital role for visible light redox reactions by mitigating undesirable electron-hole recombination [31], [37], [38]. In addition, the generated O–H bond perpendicular to the brucite layers in the coordination octahedron can react with holes of valence bands to produce OH, which is an important intermediate in photocatalysis oxidation reaction [39]. Moreover, the well dispersed positive charge of the octahedron is blocked within the brucite layers, which makes it an ideal host to implement electrostatic interactions with the intercalating anions. Those anions have been proved as the positive influence for enlarging specific surface area and different spaces among layers of LDHs and ultimately improve the photocatalytic performance by facilitating the essential reaction between the reactants and the photoinduced charge carriers [40], [41]. Hence, LDH has been regarded as one of the most promising photocatalyst materials. In addition, other semiconductors (e.g. TiO2, BiVO4) coated with high redox activity LDH photo(electro)catalysts layers are crucial components for boosting the water oxidation/reduction performance of semiconductor-based photocatalysts and photoelectrocatalysts [42], [43]. The transitional metal elements (such as Fe, Co, and Ni) in LDH layers serve as the surface catalytic sites for oxygen generation process. The engineering of LDH-cocatalysts can give rise to the reduced photocurrent onset potential, enriched active sites, accelerated reaction kinetics, and suppressed corrosion [44].
The rapid development on LDH-based catalysts could be regarded as a “breakthrough” for renewable energy production and storage. Some excellent reviews related to the application of LDH-based advanced materials for water spilling, CO2 reduction, supercapacitor, and battery have been published. However, early reviews have mainly focused on the electrocatalytic water spilling using LDH materials [32], [45], [46], [47], [48], [49], [50], [51], [52], [53], [54], [55]. Researches on indexed journals concerning the keywords “layered double hydroxide” and “water splitting” are in a great amount and increasing rapidly year by year between 2010 and 2020 (Fig. 3). In 2016, Zhao et al [56] published the extensive “Layered Double Hydroxide Nanostructured Photocatalysts for Renewable Energy Production” review covering 96 references, which comprehensively summarized the utilization of LDH derived materials for water splitting, CO2 reduction and alcohol photo reforming. Recently, the use of LDH-based materials for photo(electro-) catalytic applications has been reviewed by Jiao et al [57]. But the content of PC/PEC water splitting is thin and concise. Seen from Fig. 3, the ever-growing researches on the use of LDH catalysts in water splitting require the timely updates of recent research, especially in the direction of photo-catalytical water splitting. Hence, it is of great importance for us to summarize the advanced progress about using LDHs based catalyst for photo/photoelectro-catalytic water splitting in a more accessible and detailed way. Herein, this review contains the fundamental aspects of PC and PEC water splitting under UV–Vis and special focus is given to LDH-based efficient and affordable catalysts. The quite disperse literatures regarding LDH photocatalysts or photoelectrocatalysts have been classified as Bi/ter-nary LDH, α-Fe2O3/LDH, BiVO4/LDH, TiO2/LDH, WO3/LDH, CdS/LDH, MoSx/LDH, C3N4/LDH, and GO/LDH (Fig. 4, and Table 2, Table 3). We will focus on the advantages as well as limitations of the most frequently used LDH based water-splitting photocatalysts and photoelectrocatalysts, major strategies applied to overcome these shortcomings. Special attention is placed on the most important design issues, the correlation among nanostructuring, composition engineering, physical/chemical properties, approaches, computational breakthroughs and functional mechanism that enable outstanding PC/PEC catalytic behaviors through rational engineering of nanoscale LDH photo-active materials. Finally, by integrating our systematic overview and analyses of all the representative related studies surveyed from 2015 to 2020, we will provide our insights for constructing robust LDH-based photocatalysts. The understanding and discussion presented in this review are believed not only beneficial for the rational design of efficient LDH based catalysts for PC/PEC water splitting system but also meaningful for other types of PC/PEC system used for solar energy conversion (e.g.CO2 reduction, N2 fixation) and water remediation (e.g. photocatalytic degradation, photoelectrochemical oxidation).
Section snippets
Strategies to prepare LDHs
In order to give the reader a comparative overview on the synthetic methods of LDH based semiconductors, the most-recent LDH catalysts that are applied as water splitting photocatalysts and photoelectrocatalysts are well summarized in Table 2, Table 3. By summarizing the methods provided in these literatures, the most frequently used methods are mainly Co-precipitation, hydrothermal method, sol–gel method, solvothermal method, and electrodeposition method (Table 1). The former methods
Basics of photocatalytic and photoelectrochemical water splitting
Hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are two important half reactions of water splitting. For the HER and the OER reaction, the energy CB energy level has to be more negative than the hydrogen evolution potential (0 V vs. NHE) and the VB energy level needs to be more positive than the water oxidation potential (1.23 V vs. NHE), respectively. Photocatalytic water splitting mechanism can be elaborated as following steps (Fig. 6)[82]: First, the electrons of LDHs
Various LDH based materials towards light-driven water splitting
We have searched all the PC/PEC water splitting studies conducted mainly between 2015 and 2020 on the application of various types of LDH photocatalysts with controllable composition. Considering the similar basic reaction steps and the pivotal role of catalysts in PC/PEC water splitting process, we clustered our discussion about these novel LDH based materials into four basic categories: Bi/ter-nary LDH, Metal oxides/LDH (α-Fe2O3/LDH, BiVO4/LDH, TiO2/LDH, WO3/LDH), Metal sulfide/LDH (CdS/LDH,
Summary and outlook
Designing and preparing highly active and stable photocatalysts are crucial to enhance the efficiency of OER and HER. Recently, Layered double hydroxide derived materials have shown prospective potential to be the candidate for photoanode/photocatalyst, which is resulted from their changeable 2D lamellar structure, various component of electrochemical active transition metal ions, visible light response, and tunable intrinsic electronic structure. The recent reports of the synthetic methods and
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
We thank the National Natural Science Foundation of China (51179068, 51679086, 51521006, 51879102, and 51809089). Research and Development Projects in Key Areas of Hunan Province, China (2019NK2062). The authors also thank Cao sisi for her help in the English writing.
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