Single-atom photocatalysts have shown their compelling potential and arguably become the most active research direction in photocatalysis due to their fascinating strengths in enhancing light-harvesting, charge transfer dynamics, and surface reactions of a photocatalytic system. While numerous comprehensions about the single-atom photocatalysts have recently been amassed, advanced characterization techniques and vital theoretical studies are strengthening our understanding on these fascinating materials, allowing us to forecast their working mechanisms and applications in photocatalysis. In this review, we begin by describing the general background and definition of the single-atom photocatalysts. A brief discussion of the metal-support interactions on the single-atom photocatalysts is then provided. Thereafter, the current available characterization techniques for single-atom photocatalysts are summarized. After having some fundamental understanding on the single-atom photocatalysts, their advantages and applications in photocatalysis are discussed. Finally, we end this review with a look into the remaining challenges and future perspectives of single-atom photocatalysts. We anticipate that this review will provide some inspiration for the future discovery of the single-atom photocatalysts, manifestly stimulating the development in this emerging research area.

Heterogeneous Single-Atom Photocatalysts: Fundamentals and Applications.

Discharging dye contaminants into water is a major concern around the world. Among a variety of methods to treat dye-contaminated water, photocatalytic degradation has gained attention as a tool for treating the colored water. Herein, we review the recent advancements in photocatalysis for dye degradation in industrial effluents by categorizing photocatalyst materials into three generations. First generation photocatalysts are composed of single-component materials (e.g., TiO2, ZnO, and CdS), while second generation photocatalysts are composed of multiple components in a suspension (e.g., WO3/NiWO4, BiOI/ZnTiO3, and C3N4/Ag3VO4). Photocatalysts immobilized on solid substrates are regarded as third generation materials (e.g., FTO/WO3-ZnO, Steel/TiO2-WO3, and Glass/P-TiO2). Photocatalytic degradation mechanisms, factors affecting the dye degradation, and the lesser-debated uncertainties related to the photocatalysis are also discussed to offer better insights into environmental applications. Furthermore, quantum yields of different photocatalysts are calculated, and a performance evaluation method is proposed to compare photocatalyst systems for dye degradation. Finally, we discuss the present limitations of photocatalytic dye degradation for field applications and the future of the technology.

https://link.springer.com/content/pdf/10.1007/s12274-019-2287-0.pdf

Photocatalysts for degradation of dyes in industrial effluents: Opportunities and challenges

Volatile organic compounds (VOCs) with the properties of volatility, toxicity and diffusivity pose a serious threat to human health and eco-environment. Catalytic oxidation technology has been considered as a highly efficient option for the treatment of VOCs. This review systematically summarizes the recent processes and advances on catalytic elimination of VOCs over nanoparticles catalysts. Firstly, catalytic performances of catalysts for VOC degradation are evaluated and compared on the basis of unified performance metrics. Secondly, catalytic mechanisms of VOC oxidation, based on experimental and theoretical studies, are systematically introduced. Then, catalytic reactors employed in VOC elimination processes are summarized. In particular, photothermocatalysis by integrating (thermo)catalysis with photocatalysis is also elucidated. Lastly, the perspectives to the scientific issues and challenges faced, as well as the future outlooks are proposed. Collectively, this review will provide theoretical and experimental foundation for rational fabrication and application of nanoparticles catalysts toward VOC elimination in future.

Recent advances in VOC elimination by catalytic oxidation technology onto various nanoparticles catalysts: a critical review

Rapid industrialization has provided comforts to mankind but has also impacted the environment harmfully. There has been severe increase in the pollution due to several industries, in particular due to dye industry, which generate huge quantities of wastewater containing hazardous chemicals. Although tremendous developments have taken place for the treatment and management of such wastewater through chemical or biological processes, there is an emerging shift in the approach, with focus shifting on resource recovery from such wastewater and also their management in sustainable manner. This review article aims to present and discuss the most advanced and state-of-art technical and scientific developments about the treatment of dye industry wastewater, which include advanced oxidation process, membrane filtration technique, microbial technologies, bio-electrochemical degradation, photocatalytic degradation, etc. Among these technologies, microbial degradation seems highly promising for resource recovery and sustainability and has been discussed in detail as a promising approach. This paper also covers the challenges and future perspectives in this field.

https://www.tandfonline.com/doi/pdf/10.1080/21655979.2020.1863034

A critical review on advances in the practices and perspectives for the treatment of dye industry wastewater.

A review on heterogeneous photocatalysis for environmental remediation: From semiconductors to modification strategies

Synthesis and characterization of a heterojunction rGO/ZrO2/Ag3PO4 nanocomposite for degradation of organic contaminants.

The effective use of sunlight by photocatalyst systems is challenging. Here, we report an active-core@inert-shell nanohybrid composite that converts incompatible near-infrared (NIR) radiation to functional ultraviolet-visible (UV–vis) photons. The active core is composed of NaYF4: Yb3+, Gd3+, and Tm3+ upconverting nanoparticles (UCNPs). An inert bismuth tungstate (Bi2WO6) shell with a narrow band gap (2.65 eV) was grown over the core via ethylenediaminetetraacetic acid-metal complex formation. Solar infrared photons were successfully converted to UV–vis photons, which activated the Bi2WO6 shell via Forster resonance energy transfer (FRET). The high UV–vis-NIR response of the UCNP@Bi2WO6 composite was demonstrated by 94% degradation of Bisphenol A in 180 min. Integrating UCNPs with Bi2WO6 resulted in a high quantum yield of 3.16 × 10−5 molecules/photon, which is superior to the yield of the photocatalysts currently used. High photocurrent density (0.78 mA/cm2) confirmed the excellent photoelectrochemical potential for water splitting. Hydroxyl radicals generated at the valence band and superoxide radicals at defect sites in the Bi2WO6 explain the excellent catalytic activity of UCNP@Bi2WO6. A figure of merit (FOM) reflecting important operational parameters was calculated and compared with previous reports for catalytic performance evaluation. The high FOM value of the UCNP@Bi2WO6 hybrid composite indicates its excellent potential for practical applications.

Near-infrared to visible photon transition by upconverting NaYF4: Yb3+, Gd3+, Tm3+@Bi2WO6 core@shell composite for bisphenol A degradation in solar light

Here, we propose an approach to inhibit interfacial charge recombination by an electron transport layer (ETL) that selectively transports electrons and simultaneously restricts the passage of holes. The ETL is embedded as a junction between a photocatalyst (NiCo2S4/CdO) and an electron collector (carbon cloth (CC)). The electron density gradient, band gap offset, and p–orbital states above the fermi level in the ETL regulate the directional flow of electrons from NiCo2S4/CdO to CC. The distinct locations of electrons and holes were confirmed based on the oxidation state and preferential deposition of platinum and lead ions on NiCo2S4/CdO@CC. The molecular orbitals and density of states and their relation to ETL’s performance were explained using first principles calculations. Reduction in recombination energy loss was supported by the Nyquist and Bode phase analyses. ETL’s inclusion resulted in a remarkable 77 % increase in H2 yield, suggesting that they have excellent potential for practical applications.

Hassan Anwer

Papers

Photocatalysts for degradation of dyes in industrial effluents: Opportunities and challenges

Synthesis and characterization of a heterojunction rGO/ZrO2/Ag3PO4 nanocomposite for degradation of organic contaminants.

Near-infrared to visible photon transition by upconverting NaYF4: Yb3+, Gd3+, Tm3+@Bi2WO6 core@shell composite for bisphenol A degradation in solar light

Selective transport and separation of charge–carriers by an electron transport layer in NiCo2S4/CdO@CC for excellent water splitting

Graphene quantum dots on stainless-steel nanotubes for enhanced photocatalytic degradation of phenanthrene under visible light.