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Solar illumination is a promising source of primary energy to reduce global warming and to clean polluted waters, thus fostering research of the design of efficient photocatalysts for hydrogen production by water splitting and for contaminant degradation. In particular, photocatalysis by indium sulfide (In2S3) is drawing attention due to its suitable narrow bandgap of 2.0–2.3 eV for visible light harnessing, yet large-scale application of unmodified In2S3 is limited. Here we review the photocatalyst criteria for water splitting, the synthesis and morphological manipulations of In2S3, the synthesis of heterojunctions by coupling semiconductors to increase performance, and doping In2S3. In2S3-based heterojunctions, i.e., traditional type II, all-solid-state, and direct Z-scheme photocatalytic systems show benefits such as larger charge separation, broad solar spectrum absorption, and amended conduction band and valence band edge potentials for maximum pollutant removal and H2 production. The effect of dopant incorporation on electronic modulations of In2S3 is explained by the density functional theory.

In recent years, photocatalysis has gained particular attention due to its crucial potential applications in addressing many essential energy and environmental challenges. Considerable efforts have been devoted to developing photocatalysts to understand the fundamental processes and enhance photocatalytic efficiencies. The rate of photoinduced e − –h + reassembly is one of the difficulties encountered in semiconductor photocatalysis. Various alternative photosystems were designed to overcome this problem and thereby improve the efficiency of the heterojunction photocatalyst. Among the explored methods, the charge carrier separation using a built-in electric field attracts considerable attention as a new concept. The present review highlights the development of p–n heterojunctions to overcome the existing challenges in rigorously explored type-I, II, and III heterojunctions. Herein, reports on widely explored TiO 2 , ZnO, and various other transition metal oxides based p–n heterojunctions are extensively deliberated. This review pinpoints the benefits of constructing p–n junctions, including their impact on optical absorption, physical, and chemical properties over other n–n and p–p heterojunctions. The mechanistic route followed to construct effective p–n heterojunction and practical work carried out by generated internal electric field in isolating the charge carriers is also highlighted. Transition-metal-oxides based p–n heterojunction shows promising practical applications in various fields, including H 2 evolution, CO 2 reduction, overall water splitting, photo-reforming, and photodegradation of harmful pollutants. The various challenges and future perspectives for developing metal oxides-based p–n heterojunction materials are also summarized. Graphical abstract

The COVID-19 pandemic has caused unprecedented global health and economic crises. The emergence of long COVID-19 has raised concerns about the interplay between SARS-CoV-2 infections, climate change, and the environment. In this context, a concise analysis of the potential long-term effects of the COVID-19 epidemic along with the awareness aboutenvironmental issues are realized. While COVID-19 effects in the short-term have reduced environmental air pollutants and pressures, CO 2 emissions are projected to increase as the economy recovers and growth rates return to pre-COVID-19 levels. This review discusses the systematic effects of both the short-term and long-term effects of the pandemic on the clean energy revolution and environmental issues. This article also discusses opportunities to achieve long-term environmental benefits and emphasizes the importance of future policies in promoting global environmental sustainability. Future directions for growth and recovery are presented to cope with long COVID-19 epidemic along with the critical findings focussing on various aspects: waste management, air quality improvement.