Photocatalytic degradation of methylene blue in aqueous solution by using ZnO-SnO2 nanocomposites

ZnO hierarchical microsphere for enhanced photocatalytic activity

Solar radiation is a sustainable, unlimited source of energy for electricity and chemical reactions, yet the conversion efficiency of actual processes is limited and controlled by photocarriers migration and separation. Enhancing the conversion efficiency would require to suppress the recombination of photogenerated electron–hole pairs and improve the low redox potentials. This can be done during the growth of step-scheme (S-scheme) heterojunctions. Here we review the charge transfer of S-scheme heterojunctions involving a reduction and oxidation photocatalyst in staggered band arrangement with Fermi level differences. We present factors determining the validation of the S-scheme mechanism with respective characterization techniques, including in situ and ex situ experiments, and theoretical studies. We show mechanistic drawbacks of traditional photocatalytic systems to highlight the advantages of S-scheme photocatalysts. We describe co-catalyst loading, bandgap tuning, and interfacial optimization that ultimately achieve highly efficient photocatalysis. Last, application for water splitting, CO2 conversion, pollutant degradation, bacterial inactivation and others is discussed.

Step-scheme heterojunction photocatalysts for solar energy, water splitting, CO2 conversion, and bacterial inactivation: a review

This article presents an overview of the reports on the doping of TiO2 with carbon, nitrogen, and sulfur, including single, co-, and tri-doping. A comparison of the properties of the photocatalysts synthesized from various precursors of TiO2 and C, N, or S dopants is summarized. Selected methods of synthesis of the non-metal doped TiO2 are also described. Furthermore, the influence of the preparation conditions on the doping mode (interstitial or substitutional) with reference to various types of the modified TiO2 is summarized. The mechanisms of photocatalysis for the different modes of the non-metal doping are also discussed. Moreover, selected applications of the non-metal doped TiO2 photocatalysts are shown, including the removal of organic compounds from water/wastewater, air purification, production of hydrogen, lithium storage, inactivation of bacteria, or carbon dioxide reduction.

C-,N- and S-Doped TiO2 Photocatalysts: A Review

The use of ZnO for the functionalization of textile substrates is growing rapidly, since it can provide unique multifunctional properties, such as photocatalytic self-cleaning, antimicrobial activity, UV protection, flame retardancy, thermal insulation and moisture management, hydrophobicity, and electrical conductivity. This paper aims to review the recent progress in the fabrication of ZnO-functionalized textiles, with an emphasis on understanding the specificity and mechanisms of ZnO action that impart individual properties to the textile fibers. The most common synthesis and application processes of ZnO to textile substrates are summarized. The influence of ZnO concentration, particle size and shape on ZnO functionality is presented. The importance of doping and coupling procedures to enhance ZnO performance is highlighted. The need to use binding and seeding agents to increase the durability of ZnO coatings is expressed. In addition to functional properties, the cytotoxicity of ZnO coatings is also discussed. Future directions in the use of ZnO for textile functionalization are identified as well.

Zinc Oxide for Functional Textile Coatings: Recent Advances

Zinc oxide photocatalyst with multifaceted and tunable structure-electronic properties has garnered foremost relevance in the wastewater purification. The drawbacks supplemented with ZnO such as wide band gap, immense charge carrier recombination and photo-corrosion (deactivation) during the photocatalytic reactions obfuscate for large scale applications. Accordingly, research is swiftly inclining to spotlight the ZnO as an appropriate semiconductor to many competitive metal oxides (TiO2, Bi2O3 and WO3) in the area of heterogeneous photocatalysis. In this focused review, several strategies like synthesizing with hierarchical morphology and exposed facets, impurity doping, metal deposition, integrating with other semiconductors and modifying with carbon materials which induces multifunctional properties to boost the performance of ZnO are articulated. It is presumed that the photocatalytic activity of the modified ZnO arises from the collective contribution of effective charge carrier separation, generation of surplus active free radicals coupled with striking structure-electronic properties and nature of the photocatalytic reactions. The overall findings enunciate that such amendments to ZnO are beneficial to improve structural stability and also to obtain desired features driving for various energy applications.

Advancements in the zinc oxide nanomaterials for efficient photocatalysis

The bulk heterostructuring of TiO2via codoping with nonmetals still holds possibilities for the potential development of a visible light photocatalyst and for overcoming the obstacles bracketed with metal ion substitution. In particular, tridoping of C–N–S into TiO2 was spotlighted with due consideration of its mesmerizing features like simultaneous and relatively low energy substitution of carbon, nitrogen and sulfur from the available precursors (thiourea, L-cystine and L-cysteine), stabilization of the anatase crystal structure, red shift in the band gap response towards the solar spectrum, cooperative interactions with these codopants, and the absence of any impure phase formation even at elevated temperature and with high doping density during the substitution process. Despite these flexibilities, accessible reports on C–N–S-TiO2 are not extensive and the discussions presented are far from the relevant aspects of the doping mechanism. With the intention of shedding light on the pros and cons of C–N–S-TiO2, this review is framed with the following viewpoints: (i) underscoring their beneficial effects in photocatalysis; (ii) underlining the doping mode of each dopant in the codoped system with respect to the reaction conditions; (iii) contradictions about the doping states of each dopant in the codoped system with reference to the previous literature; (iv) tentative discussion concepts like modifications of defect structures, dopant distribution, doping mode, mutual interferences among the dopants and crystallization kinetics in the course of codoping. The results emphasize that the codoping process involving carbon, nitrogen and sulfur is quite obfuscated as several doping modes are witnessed for each dopant, which are coupled to other factors like dopant diffusivity and solubility, extent of doping, dopant segregation at the surface, nature of the dopant precursor, unpredictable interactions of the dopant states, and interactive reactions between the dopant and titania precursor together with the annealing conditions. With critical analysis with reference to TiO2, it is envisaged that thiourea is the best functional precursor to attain diverse doping states for nitrogen, and cationic and anionic doping states for carbon and sulfur, together with the formation of adsorbed sulfate anions. Future research must shed light on the dopant–dopant and dopant–lattice interactions followed by their synergism at the structure-electronic level to uncover the doping mechanism in the codoped systems.

C–N–S tridoping into TiO2 matrix for photocatalytic applications: observations, speculations and contradictions in the codoping process

Doping zinc oxide (ZnO) nanomaterials with foreign ions is a suitable approach to overcome the poor visible light response and inefficient charge carrier separation of pure ZnO, which is of significant importance in ZnO-based photocatalysis. This chapter discusses the effects of various doping strategies (including nonmetal doping, metal doping, and codoping) on the optical response, defect chemistry, morphology, crystallite size, and photocatalytic performance of ZnO nanomaterials. It is tentatively proposed that the doping content, preparation conditions, and structural–morphological properties of ZnO are the important factors to obtain efficient doped ZnO nanomaterials for photocatalytic applications. Other factors that can affect the photocatalytic activity of doped ZnO nanomaterials are also briefly summarized.

C. Sushma

Papers

Advancements in the zinc oxide nanomaterials for efficient photocatalysis

C–N–S tridoping into TiO2 matrix for photocatalytic applications: observations, speculations and contradictions in the codoping process

Doped zinc oxide nanomaterials: structure–electronic properties and photocatalytic applications

Plasmonic Ag-ZnO: Charge Carrier Mechanisms and Photocatalytic Applications*