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Interplay of chemistry and nanotechnology has effectively substantiated the ever-expanding horizons of materials chemistry, leading to a paradigm shift in (nano)materials synthesis. Current challenges of chemically processed materials include efforts to redesign synthetic procedures by using less hazardous starting materials, choosing milder reaction conditions, shortening time scale of chemical transformations and most importantly reduction of energy requirements. In this context, successful substitution of classical energy input by microwave radiation is a promising alternative in most fields of common chemical synthesis. This review highlights the latest developments in the synthesis of advanced inorganic materials by microwave-assisted chemical reactions. When compared to conventional convective and conductive heating techniques, microwave irradiation provides efficient internal volumetric heating through generation of localised high temperature zones in the reaction media by direct coupling of microwave energy to the molecules present in the reaction mixture, thereby enabling rapid synthesis with superior yield and both reduced reaction time as well as processing steps. Abbreviations: [BMIM]BF 4 : 1-butyl-3-methylimidazolium tetrafluoroborate; [C 2 mim][NTf 2 ]: 1ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide; [C 4 mim][NTf 2 ]: 1-butyl-3methylimidazolium bis(trifluoromethylsulfonyl)imide; [C 12 Py][ClO 4 ]:

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The magnitude of energy challenge not only calls for efficient devices but also for abundant, inexpensive, and stable photoactive materials that can enable efficient light harvesting, charge separation and collection, as well as chemical transformations. Photoelectrochemical systems based on semiconductor materials have the possibility to transform solar energy directly into chemical energy the so-called “solar hydrogen.” The current challenge lies in the harvesting of a larger fraction of electromagnetic spectrum by enhancing the absorbance of electrode materials. In this context, atomically precise thin films of metal oxide semiconductors and their multilayered junctions are promising candidates to integrate high surface areas with well-defined electrode–substrate interface. Given its self-limited growth mechanism, the atomic layer deposition (ALD) technique offers a wide range of capabilities to deposit and modify materials at the nanoscale. In addition, it opens new frontiers for developing precursor chemistry that is inevitable to design new processes. Herein, the authors review the properties and potential of metal oxide thin films deposited by ALD for their application in photoelectrochemical water splitting application. The first part of the review covers the basics of ALD processes followed by a brief discussion on the electrochemistry of water splitting reaction. The second part focuses on different MOx films deposited by atomic layer deposition for water splitting applications; in this section, The authors discuss the most explored MOx semiconductors, namely, Fe2O3, TiO2, WO3, and ZnO, as active materials and refer to their application as protective coatings, conductive scaffolds, or in heterojunctions. The third part deals with the current challenges and future prospects of ALD processed MOx thin films for water splitting reactions.