In this review, we mainly summarize the important progress in the development of organic–inorganic hybrid photocatalysts for the production of hydrogen using biomass and its derivatives. Recent progresses and challenges are also discussed.

An intense increase in non-biodegradable plastics and waste metals is an immediate threat to the world and needs to be addressed urgently. There are several strategies deployed to control, eliminate, and recycle these non-degradable solid wastes. Among them, the finest way in terms of sustainability and environmental safety is recycling these wastes into value-added products and applying the products to resolve other environmental problems such as water pollution, energy crisis, and greenhouse gas emission. Among the many value-added products that can be obtained from wastes, metal–organic frameworks (MOF) are of particular interest to researchers working on photocatalysis, sensing, gas separation, and degradation. This review deals with the synthesis of MOFs using solid wastes containing metals and organic linkers through recycling. In addition, the applications of synthesized MOFs in tackling environmental problems such as CO₂ adsorption, H₂ production, and photoreduction are discussed with critical opinions on each of these areas while highlighting the challenges, benefits, and future opportunities in this area of research.

The future fuel, hydrogen, is a clean, sustainable energy source with a substantial density of energy per unit volume/weight. Breakthroughs in hydrogen production, storage, and transportation are essential to meet the sustainable global energy demands. Solar-to-hydrogen conversion through water-splitting reactions (via photo/electro/photoelectro-processes) is a promising strategy for producing green hydrogen fuel. Specifically, the photocatalytic hydrogen generation reaction, mimicking artificial photosynthesis, is a simple and cost-effective method adopted for solar-hydrogen production. Various semiconductor photocatalysts and hybrid photocatalytic systems have been developed to address the sluggish kinetics and selectivity of pristine water/seawater splitting reactions. Recently, seawater has been used as feedstock for large-scale hydrogen production to advance the field and alleviate the scarcity of freshwater sources. This review article, therefore, aims to highlight the importance of seawater splitting reactions using different photocatalytic systems. A brief introduction to the fundamentals, historical progress, and mechanism of the seawater splitting reaction is presented. The impact of seawater components and accelerating reagents on the intrinsic performance of water splitting catalysts is discussed in detail, followed by an elaborate discussion of natural water and artificial seawater splitting with emphasis on onerous photocatalyst designs. Finally, the current challenges and opportunities of saltwater electrolysis for sustainable hydrogen fuel generation and applications are discussed.

Copper oxide (Cu₂O) is a potential material as a catalyst for CO₂ reduction. Cu₂O nanostructures have many advantages, including interfacial charge separation and transportation, enhanced surface area, quantum efficiency, and feasibility of modification via composite development or integration of the favorable surface functional groups. We cover the current advancements in the synthesis of Cu₂O nanomaterials in various morphological dimensions and their photochemical and electrochemical applications, which complies with the physical enrichment of their enhanced activity in every application they are employed in. The scope of fresh designs, namely composites or the hierarchy of copper oxide nanostructures, and various ways to improve CO₂ reduction performance are also discussed in this review. Photochemical and electrochemical CO₂ transformations have received tremendous attention in the last few years, thanks to the growing interest in renewable sources of energy and green facile chemistry. The current review provides an idea of current photochemical and electrochemical carbon dioxide fixing techniques by using Cu₂O-based materials. Carboxylation and carboxylative cyclization, yield valuable chemicals such as carboxylic acids and heterocyclic compounds. Radical ions, which are induced by photo- and electrochemical reactions, as well as other high-energy organic molecules, are regarded as essential mid-products in photochemical and electrochemical reactions with CO₂. It has also been claimed that CO₂ can be activated to form radical anions.

Transition metal quantum dots composites for an electrocatalytic hydrogen evolution reaction.

Using a p-n heterojunction is one of the efficient methods to increase charge transfer in photocatalysis applications. So, herein, p-type UiO-66 (NH₂) and n-type CuFe₂O₄ (CFO) are used to form an effective p-n heterojunction. Due to their poor charge separation in their pristine form, both UiO-66 (NH₂) and CFO materials cannot produce hydrogen; however, the composite p-n heterojunction formed between these materials makes fast charge separation and so hydrogen is efficiently produced. The optimized catalyst UCFO 25% produces a maximum of 62.5 µmol/g/h hydrogen in an aqueous methanol solution. The formation of a p-n heterojunction is confirmed by Mott–Schottky analysis and optical properties, crystallinity and the local atomic environment of the material was analyzed by various analytical tools like UV-Vis spectroscopy, XRD, and XANES.