Among the ongoing research on photocatalysis under visible-light, it has been shown that doped or hybrid catalysts are more active than a single catalyst alone. However, problems including visible light absorption, a low quantity of energetic sites on surfaces, and rapid recombination of the photo-electron hole pair produced by light have prohibited photocatalysts from being used in a practical and widespread manner. To overcome these problems, synthesis of nanostructure hybrid catalyst using several methods has attracted much attention. Several procedures have been suggested for the preparation of photocatalysts with the desired structure and morphology. Preparation methods similar to partial modification may lead to diverse structures and qualities. In this regard, the development of efficient, low-cost photocatalysts and rapid synthesis is the most important issues that should be considered. This review discusses various methods and mechanisms that work with the modification of vanadium compounds as photocatalysts to progress their photocatalytic efficiency. In addition, the effects of synthesis temperature, solution pH and concentration on the photocatalytic performance are also described in detail.

Dimethyl ether (DME) is a synthetically produced alternative fuel to diesel-based fuel and could be used in ignition diesel engines due to increasing energy demand. DME is considered extremely clean transportation and green fuel because it has a high cetane number (around 60), low boiling point (−25 °C), and high oxygen amount (35 wt%) which allow fast evaporation and higher combustion quality (smoke-free operation and 90% fewer NOx emissions) than other alternative CO2-based fuels. DME can be synthesized from various routes such as coal, petroleum, and bio-based material (i.e., biomass and bio-oil). Dimethyl ether can be produced from CO2 to prevent greenhouse gas emissions. This review aims to summarize recent progress in the field of innovative catalysts for the direct synthesis of dimethyl ether from syngas (CO+H2) and operating conditions. The problems of this process have been raised based on the yield and selectivity of dimethyl ether. However, regardless of how syngas is produced, the estimated total capital and operating costs in the industrial process depend on the type of reactor and the separation method.

In recent decades, the use of photocatalysts in the evolution of hydrogen (H2) has received much attention. However, the use of the well-known titanium oxide and another photocatalyst as a base for noble metals is limited due to their major weakness in electron-hole pair separation. The use of cocatalysts can be a good way to overcome this problem and provide better performance for the evolution of hydrogen. In this review, suitable high-efficiency cocatalysts for solar hydrogen production have been thoroughly reviewed. New strategies and solutions were examined in terms of increasing the recombination of charge carriers, designing reactive sites, and enhancing the wavelengths of light absorption. Several new types of cocatalysts based on semiconductors in noble groups and dual metals have been evaluated. It is expected that these photocatalysts will be able to reduce the activation energy of reaction and charge separation. In this regard, the existing views and challenges in the field of photocatalysts are presented. The characteristics of monoatomic photocatalysts are reviewed in this manuscript and the latest advances in this field are summarized. Further, the future trends and upcoming research are also briefly discussed. Finally, this review presents noble metal-based photocatalysts for providing suitable photocatalysts on a larger scale and improving their applicability.

In recent years, g-C3N4@MXene photocatalysts have received much attention due to their special composition and excellent properties. MXenes consisting of transition metal carbides, nitrides, and carbonitrides derived from the MAX phase are used as cocatalysts or g-C3N4 (GCN) supporting composites in a variety of photocatalytic processes that accelerate the separation of charge carriers with their heterojunction structure. In addition to the high ability of g-C3N4@MXene nanocomposite to absorb light, it has high photocorrosion resistance in the processes of hydrogen evolution, wastewater treatment, nitrogen fixation, NO treatment, and oxidation and reduction photoreactions. In this review, the latest developments and new technologies for the manufacture and application of noble metal-free g-C3N4@MXene nanocomposite have been discussed and the future perspective has been drawn to deal with challenges related to energy and the environment.

At present, the composition and crystalline structure of transition metal nitrides or carbides (MXenes) and their derivatives are continuously expanding due to their unique physicochemical properties, especially in the photocatalytic field. Advances over the past four years have led to improved preparation of new MAX phases, resulting in new MXenes with excellent photo-thermal effect, considerable specific surface area, long-term stability and optimum activity. Since MXenes have good electrical conductivity and their bandgap is adjustable under the visible light range, this group is one of the best promising candidates for hydrogen production from photo-splitting of water as an environment-friendly method of converting sunlight to chemical energy. Progress in noble metal-free photocatalyst associated with more understanding of the fundamental mechanism of photocatalysis has enabled a proper choice of cocatalyst with better efficiency. In this study, the photocatalytic production of hydrogen through MXens as a support and co-catalyst on metal sulfide is summarized and discussed. Recent advances in the design and synthesis of MXenes-based metal sulfide nanocomposites to increase the efficiency of photocatalytic hydrogen production are then highlighted. Finally, the challenges and future prospects for the development of MXenes-based metal sulfide composites are outlined.