Query: Photocatalysis on TiOn Surfaces: Principles, Mechanisms

Candidate papers:

abstract: In 1972, Fujishima and Honda discovered the photocatalytic splitting of water on TiO{sub 2} electrodes. This event marked the beginning of a new era in heterogeneous photocatalysis. Since then, research efforts in understanding the fundamental processes and in enhancing the photocatalytic efficiency of TiO{sub 2} have come from extensive research performed by chemists, physicists, and chemical engineers. Such studies are often related to energy renewal and energy storage. In recent years, applications to environmental cleanup have been one of the most active areas in heterogeneous photocatalysis. This is inspired by the potential application of TiO{sub 2} based photocatalysts for the total destruction of organic compounds in polluted air and wastewaters. There exists a vast body of literature dealing with the electron transfer and energy transfer processes in photocatalytic reactions. A detailed description of these processes is beyond the scope of this review. Here, the authors tend to focus on interfacial processes and to summarize some of the operating principles of heterogeneous photocatalysis. In section 2, the authors first look at the electronic excitation processes in a molecule and in a semiconductor substrate. The electronic interaction between the adsorbate molecule and the catalyst substrate is discussed in terms of the catalyzed ormore sensitized photoreactions. In section 3, thermal and photocatalytic studies on TiO{sub 2} are summarized with emphasis on the common characteristics and fundamental principles of the TiO{sub 2} based photocatalysis systems. In section 4, they address the research effort in the electronic modification of the semiconductor catalysts and its effect on the photocatalytic efficiency. Several representative examples will be presented including the Schottky barrier formation and modification at metal semiconductor interfaces. Some concluding remarks and future research directions will be given in the final section. 160 refs. less
author_names: A L Linsebigler

Guang Lu

John T Yates
title: Photocatalysis on TiO2 Surfaces: Principles, Mechanisms, and Selected Results
year: 1995

abstract: Photocatalysis has been widely applied in various areas, such as solar cells, water splitting, and pollutant degradation. Therefore, the photochemical mechanisms and basic principles of photocatalysis, especially TiO2 photocatalysis, have been extensively investigated by various surface science methods in the last decade, aiming to provide important information for TiO2 photocatalysis under real environmental conditions. Recent progress that provides fundamental insights into TiO2 photocatalysis at a molecular level is highlighted. Insights into the structures of TiO2 and the basic principles of TiO2 photocatalysis are discussed first, which provides the basic concepts of TiO2 photocatalysis. Following this, details of the photochemistry of three important molecules (oxygen, water, methanol) on the model TiO2 surfaces are presented, in an attempt to unravel the relationship between charge/energy transfer and bond breaking/forming in TiO2 photocatalysis. Lastly, challenges and opportunities of the mechanistic studies of TiO2 photocatalysis at the molecular level are discussed briefly, as well as possible photocatalysis models.
author_names: Qing Guo

Chuanyao Zhou

Zhibo Ma

Xueming Yang
title: Fundamentals of TiO2 Photocatalysis: Concepts, Mechanisms, and Challenges.
venue: Advanced materials
year: 2019

abstract: The use of sunlight to drive chemical reactions via photocatalysis is of paramount importance towards a sustainable future. Among several photocatalysts, earth abundant polymeric carbon nitride (PCN, often wrongly named g C3N4) has emerged as an attractive candidate due to its ability to absorb light efficiently in the visible and near infrared ranges, chemical stability, non toxicity, straightforward synthesis, and versatility as a platform for constructing hybrid materials. Especially, hybrids with metal nanoparticles offer the unique possibility of combining the catalytic, electronic, and optical properties of metal nanoparticles with PCN. Here, we provide a comprehensive overview of PCN materials and their hybrids, emphasizing heterostructures with metal nanoparticles. We focus on recent advances encompassing synthetic strategies, design principles, photocatalytic applications, and charge transfer mechanisms. We also discuss how the localized surface plasmon resonance (LSPR) effect of some noble metals NPs (e.g. Au, Ag, and Cu) bimetallic compositions, and even non noble metals NPs (e.g. Bi) synergistically contribute with PCN in light driven transformations. Finally, we provide a perspective on the field, in which the understanding of the enhancement mechanisms combined with truly controlled synthesis can act as a powerful tool to the establishment of the design principles needed to take the field of photocatalysis with PCN to a new level, where the desired properties and performances can be planned in advance, and the target material synthesized accordingly.
author_names: Ivo Freitas Teixeira

Eduardo C M Barbosa

Shik Chi Edman Tsang

Pedro H C Camargo
title: Carbon nitrides and metal nanoparticles: from controlled synthesis to design principles for improved photocatalysis.
venue: Chemical Society reviews
year: 2018

abstract: Catalysts are involved at some stage in the manufacture process of virtually all commercially produced chemical product. Among the materials used as catalysts, metal oxides are one of the most used due to their versatility and wide range of physical properties. Identifying the principles of surface to adsorbate charge transfer is key to a better understanding of metal oxide materials as both catalysts and gas sensors. Using density functional theory (DFT) we modeled the adsorption of small molecules over stoichiometric and reduced metal oxide surfaces of group IV metals and quantify the effect of electron transfer upon adsorption. We found that charge transfer only occurs during the adsorption process of an adsorbate more electronegative than the surface. We also found a correlation between the work function of the metal oxide, and the ionic adsorption of the oxygen molecule. Mixed phase rutile/anatase catalysts show increased reactivity compared with the pure phases alone. However, the mechanism causing this effect is not fully understood. Using DFT and the +U correction we calculated the bands offsets between the phases taking into account the effect of the interface. We found rutile to have both higher conduction and valence band offsets than anatase, leading to an accumulation of electrons in the anatase phase accompanied by hole accumulation in the rutile phase. We also probed the electronic structure of our heterostructure and found a gap state caused by electrons localized in undercoordinated Ti atoms which were present within the interfacial region. Interfaces between bulk materials and between exposed surfaces both showed electron trapping at undercoordinated sites. Finally, we studied the effect of the size of gold nanoparticles in the catalytic properties of gold decorated TiO2 surfaces. We found that the adsorption energy of several intermediates reactives in the CO oxidation and water gas shift reaction does not change with the size of the nanoparticles. In conclusion, the factor that affects the reactivity of the system is the density of undercoodinated gold atoms on the interface perimeter.
author_names: Juan Carlos Abanades Garcia
title: Catalysis and Photocatalysis over TiO2 Surfaces Detailed from First Principles
year: 2014

abstract: Abstract An abundant and largely available natural energy resource is sunlight, which possesses great potential in driving environment friendly photochemical/photocatalytic reactions. The conversion of solar energy to chemical energy has been considered as one of the most prospective long term solutions to solve global energy and environmental issues. The photocatalyst is the key to realize such a conversion. Common photocatalytic materials include metal free organic compounds and semiconductors, in which most of the interest has been focused on the semiconductor system. A photocatalyst employs the catalyst for speeding up chemical reactions in the presence of light. Significant features of the photocatalytic systems are proper bandgap, the morphology of the material, more exposed surface area, stability, and its reusability. Photocatalysis is widely being practised for the degradation and mineralization of hazardous organic compounds to CO2 and H2O. Thus, leads to the reduction of toxic metal ions into nontoxic states, deactivate and destruct all the waterborne microorganisms, decomposes the air pollutants like NO2, CO2, and NH3, degraded the waste plastics and green synthesis of industrially important chemicals. The term chalcogen applies to a few elements (O, S, Se, Te, and Po) the compounds formed by these elements are known as chalcogenides and also some chalcogenide metals (ZnS, CdS, and CdSe) can be used as photocatalysts, which is an area of present day intensive research. ZnO and TiO2 are the most commonly used metal oxides for photocatalytic and sensing purposes, due to their outstanding properties and potential electronic communicator. Semiconducting nano metal oxides (NMOs) play an important role in developing smart materials that are highly efficient for sensing and simultaneously decontaminate harmful chemicals in our environment. As the material loading plays an important role in photocatalytic performances, physical mixture through grounding is the most straightforward approach to realize transition metal oxide and chalcogenide (TMO&Cs) based heterojunctions. In this chapter, the authors discussed the unique properties, significance of chalcogenide based nanomaterials, principles and fundamentals as photocatalysts. Methods of synthesis like sol gel method, ultrasonic method, hydro (solvo) thermal method, surfactant thermal method, and in situ photo assisted deposition method, and other nonconventional synthesis methodologies, and mechanisms of chalcogenide based nanomaterials as photocatalysts were also discussed along with their applications.
author_names: G V S S Sarma

Murthy Chavali

Maria Nikolova

Manoj Kumar Enamala

Chandrasekar Kuppan
title: Basic principles, fundamentals, and mechanisms of chalcogenide based nanomaterials in photocatalytic reactions
year: 2021

abstract: Photocatalysis method for environmental applications has been using for a long time. This review article traces back the origin of catalysis, its classification and journey of development to heterogeneous photocatalysis and the article's novelty is in the simplicity, and easily understandable language, designed for the beginners. These heterogeneous photocatalysts are grouped into eleven different categories. As the paper is focused on photocatalysis, an insight on fundamental principles and mechanisms of photocatalysis are explained systematically with schematic illustrations and reactions that take place during redox oxidation and reduction reactions in photocatalysis. With an approach towards utilizing green energy and expanding the photocatalyst' absorption wavelength range towards the visible regime, bandgap engineering techniques by adopting doping and hetero structures are explained with examples of different materials. In addition, dominating factors of photocatalysis reaction viz. composition of a heterogeneous photocatalyst, doping, hetero structures, pH, surface defects on photocatalysis reaction are explored, focussing on variable charge transfer mechanisms. The main influencing factor in generating reactive oxygen species is pH of the photocatalysis reaction and are studied indetail. The effect of alkalinity or acidity in catalyst surfaces and molecular interaction depending upon the point zero charges of the photocatalyst are discussed. For the better study of catalyst properties, careful analysis and study is a much needed field as a scope for further improvement. Hence, this article will guide a beginner to understand the photocatalysis topic with ease.
author_names: Debika Devi Thongam

Harsh Chaturvedi
title: Advances in nanomaterials for heterogeneous photocatalysis
year: 2021

abstract: Preceding work on photoelectrochemistry at semiconductor single crystal electrodes has formed the basis for the tremendous growth in the three last decades in the field of photocatalysis at semiconductor powders. The reason for this is the unique ability of inorganic semiconductor surfaces to photocatalyze concerted reduction and oxidation reactions of a large variety of electron donor and acceptor substrates. Whereas great attention was paid to water splitting and the exhaustive aerobic degradation of pollutants, only a small amount of research also explored synthetic aspects. After introducing the basic mechanistic principles, standard experiments for the preparation and characterization of visible light active photocatalysts as well as the investigation of reaction mechanisms are discussed. Novel atom economic C C and C N coupling reactions illustrate the relevance of semiconductor photocatalysis for organic synthesis, and demonstrate that the multidisciplinary field combines classical photochemistry with electrochemistry, solid state chemistry, and heterogeneous catalysis.
author_names: Horst Kisch
title: Semiconductor photocatalysis mechanistic and synthetic aspects.
venue: Angewandte Chemie
year: 2013

abstract: Photochemical reactions induced by TiO2 nanoparticles share common mechanistic features where electron and hole pairs are formed, migrate to the surface, and their recombination competes with their reaction with various substrates. The main interest in TiO2 photocatalysis is related to its potential application for decontamination of water and air. However, the absorption of TiO2, which is limited to UV light, does not enable the use of natural or cheap light sources, and therefore tremendous effort has been invested in inducing visible light activity via modification of TiO2 including doping with nonmetals and metals, surface coating, and bi and multicomponent assembling. In addition, much research has been carried out to inhibit the electron hole recombination and enhance the reactions of holes and electrons with substrates. The basic mechanism of bare and modified TiO2 and the main principles of the photocatalytic processes remain similar, although the excitation energy is different and the energies of the electrons and holes and their reaction kinetic parameters may vary. These photocatalytic processes are reviewed and discussed.
author_names: Joseph Rabani

Sara Goldstein
title: Mechanisms of Reactions Induced by Photocatalysis of Titanium Dioxide Nanoparticles
year: 2013

abstract: Abstract In recent years, although several two dimensional materials such as graphene, graphitic carbon nitride, and transition metal dichalcogenides have been explored in energy and environmental applications, MXenes have obtained more importance because of their excellent stability and cost effectiveness and they allow the functionalization of surface moieties. This chapter discusses various methods used for the synthesis of MXenes and MXenes based photocatalysts for hydrogen evolution and degradation of organic pollutants with recent findings and advancements in this field. After a brief introduction to the types of MXenes and their structure, general principles of photocatalysis, mechanism involved in hydrogen production, and organic pollutant degradation processes have been explained in detail. The focus is mainly on the idea behind the design and development of MXenes based photocatalysts along with enhancement in photocatalytic activity. Finally, a summary is given to the recent findings and development of MXenes based photocatalysts in order to utilize the visible light region of sunlight and future perspective of this material is discussed at the end.
author_names: Vishal Sharma

Ashish Kumar

Venkata Krishnan
title: Two dimensional MXene based heterostructures for photocatalysis
year: 2020

abstract: Solar driven water splitting provides a leading approach to store the abundant yet intermittent solar energy and produce hydrogen as a clean and sustainable energy carrier. A straightforward route to light driven water splitting is to apply self supported particulate photocatalysts, which is expected to allow solar hydrogen to be competitive with fossil fuel derived hydrogen on a levelized cost basis. More importantly, the powder based systems can lend themselves to making functional panels on a large scale while retaining the intrinsic activity of the photocatalyst. However, all attempts to generate hydrogen via powder based solar water splitting systems to date have unfortunately fallen short of the efficiency values required for practical applications. Photocatalysis on photocatalyst particles involves three sequential steps: (i) absorption of photons with higher energies than the bandgap of the photocatalysts, leading to the excitation of electron hole pairs in the particles, (ii) charge separation and migration of these photoexcited carriers, and (iii) surface chemical reactions based on these carriers. In this review, we focus on the challenges of each step and summarize material design strategies to overcome the obstacles and limitations. This review illustrates that it is possible to employ the fundamental principles underlying photosynthesis and the tools of chemical and materials science to design and prepare photocatalysts for overall water splitting.
author_names: Qingming Wang

Kazunari Domen
title: Particulate Photocatalysts for Light Driven Water Splitting: Mechanisms, Challenges, and Design Strategies.
venue: Chemical reviews
year: 2019