Introduction to Photocatalysis

Author: Atir Naeem Qurashi – Photocatalysis Types, Mechanism and Applications:

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Photocatalysis is the process or activity that occurs by the interaction of a light source to the surface of a material (i.e., semiconductor materials). When this activity occurs, it is necessary that at least two reactions should take place simultaneously, oxidation reaction at with the help of photo-generated holes, and reduction reaction with the help of photo-generated electrons. The photocatalyst specie itself should not undergo any kind of change during the process and thus a precise synchronisation of the two aforementioned processes needs to take place at same time. Fujishima and Honda are termed as the scientists who first achieved an electrochemical photocatalysis of water at a semiconductor electrode in the year 1972. Later on it was discovered that TiO2 (Titanium dioxide, also known as titania) helps in decomposing cyanide in the water, and this fact ultimately became the reason of rising interest towards the material’s environmental applications. Titanium dioxide (TiO2) is appropriate for photocatalysis for many reasons, a few of the reasons are its common and timely availability, being relatively low cost chemical, and possessing high chemical stability.

The process of Photocatalysis can be used with a practical and successful approach in a real earth environment in order to decompose different types of pollutants, and it can enhance the quality of air in the atmosphere. Therefore the process of Photocatalysis can be used in the building and construction industry for the purpose of improving indoor air quality.

In area of semiconductor photo-catalysis, in general it is used to design, characterize, and implement potential photocatalytical applications with the help of doped or undoped Nano-structural materials, largely considering the particle sizes as well as their shapes in such processes. Nanoparticles, for example, porous metallic oxides, metallic nanoparticles, porous carbons, and their synthetic combinations are studied significantly for their potential applications in energy conversion/storage devices, gas storage, and in the processes such as photocatalysis and electro-catalysis.

Photocatalysis is the phenomena, which occurs in catalysis by the impact of photons. In general an efficient photocatalyst material is supposed to be a conductive nanomaterial, which directly absorbs incident light that brings it up to the higher energy states, and then it provides such energy to a reacting substance, so a chemical reaction takes place.

Advancements in Photocatalysis Research

Research in photocatalysis techniques was reduced for more than 25 years because of lack of interest in the subject as well as absence of practical applications. Eventually, in the year 1964, V.N. Filimonov made an investigation on isopropanol photooxidation from Zinc Oxide (ZnO) and Titanium dioxide (TiO₂₎. At around the same time, Kato and Mashio, Doerffler and Hauffe, and Ikekawa et al. (1965) made the explorations on photooxidation of carbon dioxide (CO₂) and organic solvents from Zinc Oxide (ZnO) radiance. After a few years; in 1970, Formenti et al. and Tanaka and Blyholde made observations on the oxidation of various alkenes and the photocatalytic decay of nitrous oxide (N₂O), respectively.

Finally, a successful discovery in photocatalysis research took place in 1972, when Akira Fujishima and Kenichi Honda found electrochemical photolysis of water occurring between connected TiO₂ and platinum (Pt) electrodes, where ultraviolet (UV) light was absorbed by the TiO₂ electrode. The electrons used to flow from the TiO₂ electrode (termed as anode; a site of oxidation reaction) to the platinum electrode (termed as cathode; the site of reduction reaction). Additionally the process of hydrogen production occurred at the cathode. This incident was one of the first cases where production of hydrogen came from a clean, convenient and economical source. While back then mostly the production of hydrogen came – and still today comes – from natural gas reforming and gasification process. These findings of Fujishima’s and Honda led to other advanced developments in photocatalysis technique; in the year 1977, Nozik found out that the usage of noble metals in the electrochemical photolysis process, such as platinum (Pt) and gold (Au), among the others, could make it possible to increase photoactivity, and even that an external energy source was not required. The researches afterwards conducted by Wagner and Somorjai (1980) and Sakata and Kawai (1981) presented the hydrogen production on the surface of strontium titanate (SrTiO₃) through photogeneration process, and the generation of hydrogen and methane from the illumination of TiO₂ and PtO₂ in ethanol, sequentially.

The R&D (Research and Development) in photocatalysis technique, principally in electrochemical photolysis of water, is still continued today, however, not a significant developed has been made for commercial purposes. In the year 2017, Chu et al. evaluated the future and scope of electrochemical photolysis of water, by discussing its major challenges like, developing an easy to perform, cost-effective and energy-efficient photo-electrochemical (PEC) multi-junction cell, which would, “imitate natural photosynthesis.

Types of Photocatalysis

There are two main types of Photocatalysis:

a.    Homogeneous Photocatalysis

b.    Heterogeneous Photocatalysis

A.   Homogeneous Photocatalysis

The homogeneous photocatalysis, involves the existence of reactants and the photocatalysts in the same phase, i.e., both can be in the form of gases. One of the very common examples of homogeneous photocatalysts used is ozone and photo-Fenton systems (Fe⁺ and Fe⁺/H₂O₂). Here the reactive species is to be the hydroxyl radical (•OH) which tends to be used for various purposes and objective. This mechanism of producing hydroxyl radical (•OH) by ozone can follow these two paths mentioned below.

O₃ + hν → O₂ + O(1D)

O(1D) + H₂O → •OH + •OH

O(1D) + H₂O → H₂O₂

H₂O₂ + hν → •OH + •OH

In the same way, the Fenton system (Fe⁺) produce hydroxyl radical (•OH) by the mechanism shown below.

Fe²⁺ + H₂O₂→ HO• + Fe³⁺ + OH⁻

Fe³⁺ + H₂O₂→ Fe²⁺ + HO•2 + H⁺

Fe²⁺ + HO• → Fe³⁺ + OH⁻

In the photo-Fenton systems (Fe⁺ and Fe⁺/H₂O₂) type processes, supplemental sources of OH radicals are also considered: via photolysis of H₂O₂, and by the reduction of Fe³⁺ ions under UV light as follows:

H₂O₂ + hν → HO• + HO•

Fe³⁺ + H₂O + hν → Fe²⁺ + HO• + H⁺

The productivity and effectiveness of Fenton type processes is influenced by various operating parameters/variables such as concentration of hydrogen peroxide (H₂O₂), pH as well as intensity of the UV light. The leading edge of this process is the capability of using sunlight with light sensitivity up to 450 nm, hence; saving the process from use of high costs of UV lamps and electrical energy. These type of reactions in homogeneous photocatalysis are proved to be more convenient and economical than the other photocatalysis types, however the major downsides of this process are the low pH values, which are needed, because iron (Fe) precipitates at some higher pH values, furthermore the fact that iron (Fe) has to be eliminated after the treatment.

B.   Heterogeneous Photocatalysis

It is obvious from the definition that “Heterogeneous catalysis” involves the catalysts and reactants being in a different phases. Heterogeneous photocatalysis is a subject matter that involves a relatively large variety of reactions, that include but not limited to; mild or total oxidation reactions, dehydrogenation process, hydrogen transfer reaction, ¹⁸O₂–¹⁶O₂ and deuterium-alkane isotopic exchange reaction, metal deposition, water detoxification, gaseous pollutant removal process, etc.

In general and commonly used heterogeneous photocatalysts include transition metal oxides and semiconductors, which undergo unique characteristics. Contrary to the metals that possess a continuum of electronic states, semiconductors usually have ineffective energy regions where no such energy levels are available to develop rejoining of an electron and a hole produced by photo-activation in the solid substance. The void/ineffective region, which outstretches from the top of the filled valence band to the bottom of the vacant conduction band, is termed as the band gap.

In this process a photon with energy equal to or greater than that of the materials band gap is absorbed by the semiconductor used, then an electron excites from the valence band to the conduction band, so it generates a positive (+ve) hole in the valence band. This phenomena of photo-generated electron-hole pair is called the exciton. The excited electron and hole can recombine or rejoin and release the energy that they gained when the electron was excited as heat. This Exciton and recombination is not desirable and the higher levels of it take the process to an ineffective photocatalyst. Because of this reason endeavors to flourish and develop functional photocatalysts frequently put an emphasize on extending the exciton lifetime, so that improving electron-hole separation using different approaches that usually depend upon structural characteristics such as phase hetero-junctions (for example; anatase-rutile interfaces), noble-metal nanoparticles, silicon nanowires and substitutional cation doping etc. The main objective of photocatalyst designing is to facilitate and pave the way for reactions between the excited electrons with oxidants to produce reduced products. Additionally in order to make reactions between the holes generated long with the reductants to produce oxidized products. Because of this reason of the generation of positive (+ve) holes and electrons, redox reactions occur at the surface of semiconductors.

The mechanism of the oxidative reaction shown below, depicts the positive holes that react with the moisture present on the material’s (metal oxide) surface and produce a hydroxyl radical (•OH). This reaction begins with photo-induced exciton-generation in the metal oxide surface (where; MO stands for metal oxide):

MO + hν → MO (h⁺ + e⁻)

Oxidative reactions due to photocatalytic effect are as below:

h⁺ + H₂O → H⁺ + •OH

2 h⁺ + 2 H₂O → 2 H⁺ + H₂O₂

H₂O₂→ 2 •OH

Reductive reactions due to photocatalytic effect are as below:

e⁻ + O₂ → •O₂⁻

O₂⁻+ H₂O + H⁺→ H₂O₂ + O₂

H₂O₂ → 2 •OH

Finally, the generation of hydroxyl radical (•OH) takes place in both reactions mentioned above. Such hydroxyl radical (•OH) are extremely oxidative in nature and non-selective with redox potential of (E₀ = +3.06 V) as well.

Factors 

In general terms the oxidation rates and productivity of the photocatalytic systems highly rely on various operational parameters that control the photo-degradation of organic molecules. A number of case studies have reported the significance of these operational parameters. The photo-degradation depends upon some of the basic parameters mentioned below:

a. Concentration of substrate

b. Amount of photocatalyst

c. The pH of solution

d. Temperature of reaction medium

e. Time of irradiation of light

f. The intensity of light

g. Surface area of photocatalyst

h. Dissolve oxygen in the reaction medium

i. Nature of the photocatalyst

j. Nature of the substrate

k. Doping of metal ions and non-metal

l. Structure of photocatalyst and substrate

Hence, the photo-degradation of organic compounds have been studied by a number of scientists and researchers; so a final conclusion is made that the optimum conditions for the photo-degradation of organic compounds much rely upon above parameters.

Mechanism of Photocatalysis

With the passage of time new and different strategies have been applied. For example surface and interface modification by managing particle size and shape, composite or coupling materials, doping of transition metal, doping of nonmetal, application of co-doping (like; metal–metal, metal–nonmetal, nonmetal–nonmetal), deposition of noble metal, and by the use of organic dye and metal complexes sensitization of surface, in order to enhance and boost the photocatalytic properties.

a) Doping

The application of doping is known as the additions of impurities to a pure substance. Doping is divided into two sub categories that are; (1) Cationic doping and (2) Anionic doping.

Cationic doping involves the doping of cations to the semiconductors, such as the metals like Al, Cu, V, Cr, Fe, Ni, Co, Mn, etc. On the other hand anionic doping involves the use of anions, such as nonmetals like N, S, F, C, etc. The crystal lattice of a photocatalyst receives a new and unique impact from each different dopant. The doping of metal as well as nonmetal ion increases the photo-responsiveness on the surface of a photocatalyst to make it to the visible region by building new energy levels (or impurity state) between the Valence band (VB) and Conduction band (CB) to decrease its band gap. The electrons that are excited by light are the shifted from the impurity state to the Conduction band (CB).

Various nanoparticles like Fe-doped TiO2, WO3/ZnO, and Fe-doped CeO2 whose photocatalytic activity was investigated by Siriwong et al. (2012). Whereas Metal-doped Strontium titanate (SrTiO3) photocatalyst was developed by Chen et al. (2012) for the use of water splitting, on the other hand Zhang et al. (2013) studied and explained that the effect of dopants of nonmetal like B, C, N, F, P, and S as anions on electronic structures of Strontium titanate (SrTiO3). The photocatalytic activity of titania (TiO2) and Zinc oxide (ZnO) after doping by rare-earth metal La was explored by Anandan et al. (2012). On the other hand doping of Cu, Al, and Fe with TiO2 semiconductor was reported by Maeda and Yamada (2007).

The photocatalytic activity of the photocatalysts is increased by Co-doping of metals and nonmetals. Studies of various researchers have been reported about the Co-doping of Cr + N in ZnO and/or Cu + Al co-doped ZnO, on the other hand Ga + N co-doped TiO2, and W + C co-doped TiO2 nanowires etc.

b) Composites/Coupling

Another viable technique to make photocatalysts efficient in the visible light for various applications is coupling of semiconductors or composite. Such that, a large band gap a small band gap semiconductors are coupled together so, they have a more negative Conduction band (CB) level. So, the result will be; the electrons of Conduction band (CB) can be injected from the small band gap semiconductor to the large band gap semiconductor. This technique and the dye sensitization method are alike, however the only contrast being that the electrons will move from one semiconductor to another. The production of Hydrogen via coupled SnO2, CdS, CdS/ Pt–TiO2, and NiS/ZnxCd1–xS/reduced graphene oxide has been examined.

c)  Metallization

In order to improve the photocatalytic activity of a semiconductor various noble metals like Pt, Au, Ag, Ni, Cu, Rh, Pd, etc, have been used. The likelihood of electron–hole recombination / rejoining is decreased by this process, and this results an effective charge separation as well as higher rates of photocatalytic reaction. Because of these properties of noble metals electron transfer can be assisted, that leads to the higher photocatalytic activity.

d) Dye Sensitization

Dye sensitization is an auspicious technique for surface development and modification of photocatalysts for the utilization of visible light for the sake of energy conversion. Dyes possess the oxidation-reduction characteristics as well as visible light sensitivity that can be useful for solar cells and in photocatalytic systems. A catalytic reaction can be started because when the dyes are brought under the exposure of visible light they inject electrons to the Conduction band (CB) of semiconductors. A quick and fast electron injection and slow backward reaction are the prime conditions in order to convert absorbed light directly into electrical energy with higher efficiency in solar cells or via production of hydrogen.

Applications of Photocatalysis

Various semiconductors are being utilized in the purification of water and air, for self-cleaning, self-sterilization process, antifogging and antimicrobial activity, and many others. In these domains, the Titanium dioxide (TiO2) photocatalyst has attained so much recognition due to its higher catalytic efficiency, its chemical stability, being economical, having low toxicity, and reasonable adjustment with conventional building materials. TiO2 is also applicable for destructing microorganisms, such as bacteria and viruses. It is even useful in inactivating some cancer cells, and for the photo-splitting of water for hydrogen gas production; that is termed as the fuel of the future.

a) Water Treatment

In wastewater treatment processes various binary as well as ternary semiconductors are being utilized as photocatalysts. The titanium dioxide (TiO2) and zinc oxide (ZnO) photocatalysts are often being utilized in the purification of wastewater. Zinc oxide photocataly is an excellent oxidation substance that is largely used in the treatment of wastewater in industries like in pharmaceuticals, printing press and dyeing, paper and pulp industry, etc. The titanium dioxide (TiO2) nanotubes also known as (TNTs) are very favorable photocatalysts for the photocatalytic decontaminating of water. Benjwal et al. (2015) studies present that the graphene oxide–TiO2/Fe3O4- based ternary nanocomposites are of prospective implementations in the treatment of wastewater.

b) Removing Trace Metals

Some of the trace elements like mercury (Hg), chromium (Cr), and lead (Pb), as well as other metals, are exceedingly hazardous to human health. By utilizing the heterogeneous photocatalysis for the purpose of maintaining the quality of water as well as human health, such toxicities of metals can successfully be removed, even at the lower level of concentrations like parts per million (ppm).

c) Water Splitting

For the reaction of water splitting various species such as sulfides, oxides and selenides have been produced as the photocatalysts. Titanium dioxide (TiO₂) nanoparticles, several semiconductors (coupled) like CaFe204/TiO₂, heterojunction WO3/BiVO4, as well as core or shell nanofibers like CdS/Zno, and many more, provide very useful ways for hydrogen production from water.

d) Self-Cleaning Functions

The Titanium dioxide (TiO₂) photocatalyst has attained a lot of recognition as a useful photofunctional substance, the reason is that the cleaning of glass and tile surfaces require chemical detergents, depletion with high energy, and it’s expensive too. The self-cleaning surface based on titanium dioxide makes the inorganic as well as organic molecules to remain absorbed and degraded on it effortlessly. Afterwards, it becomes easy to wash with water because of high hydrophilicity of TiO₂ film. The said outcome of TiO₂ becomes functional on this condition; when the rate of absorbed organic pollutants on the surface of material is lesser than that of incident solar photons per unit time. The coating, paint materials for the walls of buildings and construction processes are much exposed to bad weather conditions such as natural rainfalls and harsh sunlight, so titanium dioxide (TiO₂) is the best utilisation for self-cleaning for above mentioned processes.

e) Antifogging

Antifogging being one of the superhydrophilic technologies that possess an extraordinary wide range of applications. When the air with higher level of humidity comes under contact to the surfaces of materials such as glasses and mirrors, the phenomena of fogging occurs with the formation of a lot of tiny water droplets, which in general results the scattering of light on these surfaces. Contrary to this, when there is a highly hydrophilic surface, it prevents the formation of water droplets, whereas a thin and uniform water film is formed, resulting the prevention of fogging. When the surface develops a highly hydrophilic state, it stays in that condition unchanged for many days to come, at least for a week. This new and economical technology has been used with easy and convenient processing to produce several types of eyeglasses, mirrors and other glass products. Now a days antifogging superhydrophilic side view mirrors are being installed in various Japanese made cars.

f) Antibacterial and Cancer Treatment

Matsunaga et al. (1985) made an observation for the antibacterial effects of titanium dioxide (TiO₂) based photocatalysis. They gave this novel concept of utilization of photochemical sterilization with titanium dioxide (TiO₂) semiconductor using the irradiation of metal halide lamp. Titanium dioxide (TiO₂) inactivated various microbial toxins (like microcystins, brevotoxins, lipopolysaccharide endotoxin, etc.) as well as killing a broad range of organisms that include viruses, bacteria, algae, fungi and even to the extent of cancer cells. The coatings of simple titanium dioxide (TiO₂) and doped TiO₂ are under utilization for inhibition of not only the bacteria’s reproduction, but also to decompose the cells of bacteria under mild conditions at the same time.  For antimicrobial applications using UV light, precise composite of TiO2 and Ag- TiO2 nanofilms have been prepared, because Sliver (Ag) is one of the good antibacterial materials.

In order to add antimicrobial properties in interior paints in building and constructions the photocatalytic material TiO₂ is commonly used as one of the best applications for this purpose. Moreover a nanosized photocatalytic TiO₂ and Zno mixture gives very useful antimicrobial effects. A Japanese Arc-Flash industry utilized technology of photocatalyst fixation that directly sprayed the photocatalysts on the surface. The titanium dioxide (TiO₂) nanoparticles were the prime component of those coatings; such coatings are extremely effective and efficient in sterilizing as well as sanitizing the hospitals, kitchens, schools, floors, etc. and killing bacteria with a 98% efficiency, ultimately enhancing the hygiene standards around.

g) Self-Sterilization

Light energy activates chemically to the titanium dioxide (TiO₂), and this TiO₂ becomes able to decompose dirt, various kinds of pathogens, like protozoa, fungi, bacteria, viruses and others, from different surfaces as well as from the air, water and other biological hosts, all this with the help of Ultra Violet Irradiation.

TiO₂ is being utilized as an effective additive for cosmetic and food products, because of the fact that it is a self-sterilizing process and also TiO₂ being chemically as well as physically a safe and stable material. For clinical purpose, animal as well as in cultured cell experiments, TiO₂ coated silicon catheters are safely utilized because they are conveniently sterilized with the help of light sources. A potential application for diabetic treatments in monitoring of blood glucose, a TiO₂ photocatalytic monolayer coated self-sterilizing lancet has also been developed.

Future scope 

Photocatalysis is characterized as the alteration of a process’ reaction rate in the presence of light and a photocatalyst. Various applications are developed, such as treatment of water, disinfection system, removal of volatile organic compounds (VOCs) and nitrogen oxides (NOx), production of hydrogen gas H2, and conversion of CO2, , even with some of them approaching to commercial levels. In order to generate homogeneous as well as heterogeneous photocatalysis processes, various types of light sources that range from ultraviolet to solar (sources) are being utilized. The applications and utilizations of photocatalysis processes at ambient temperature and pressure, and the ability to activate the catalyst using sunlight, makes it appealing for low-cost and convenient usage, additionally for the usage in passive situations. In contrast to traditional catalysis, photocatalysts usually do not have active sites in a conventional sense, and rate is also determined by exposure or intensity of irradiation light.

Photocatalysis is a rapidly evolving technology with a wide range of applications. It can be implemented at room temperature and pressure, as well as using renewable energy sources. In recent years, a lot of research has gone into developing more efficient photocatalytic systems, and several applications are now available on a commercial level. Although, designing a reactor in which these processes occur is still challenging, because not only are phase contact, turbulence, and mixing are the critical measures to be taken (as they are in traditional reactors), but the light contact, either it is solar or artificial, adds a new dimension to consider for the instrumentation. Yet an additional application for photocatalysis process is in the construction sectors or coatings, where research into surface characteristics and active phase anchoring is still underway.

So it can be said that the technique of photocatalysis has a bright future and there are high hopes attached to it, which will be achieved through proper research and development.

Other Links:

1- https://www3.nd.edu/~kamatlab/research_photocatalysis.html

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