Photoinduced reactivity of titanium dioxide

https://doi.org/10.1016/j.progsolidstchem.2004.08.001Get rights and content

Abstract

The utilization of solar irradiation to supply energy or to initiate chemical reactions is already an established idea. If a wide-band gap semiconductor like titanium dioxide (TiO2) is irradiated with light, excited electron–hole pairs result that can be applied in solar cells to generate electricity or in chemical processes to create or degrade specific compounds. Recently, a new process used on the surface of TiO2 films, namely, photoinduced superhydrophilicity, is described. All three appearances of the photoreactivity of TiO2 are discussed in detail in this review, but the main focus is on the photocatalytic activity towards environmentally hazardous compounds (organic, inorganic, and biological materials), which are found in wastewater or in air. Besides information on the mechanistical aspects and applications of these kinds of reactions, a description of the attempts and possibilities to improve the reactivity is also provided. This paper would like to assist the reader in getting an overview of this exciting, but also complicated, field.

Introduction

Photoinduced processes are studied in a manifold ways and various applications have been developed since their first description. Despite the differences in character and utilization, all these processes have the same origin. Semiconductors can be excited by light with higher energy than the band gap and an energy-rich electron–hole pair is formed. This energy can be used electrically (solar cells), chemically (photochemical catalysis), or to change the catalyst surface itself (superhydrophilicity). Several excellent reviews [1], [2] have been written in this field, especially on the topic of photocatalysis for pollutant degradation, but recent literature has not been reviewed yet. Here, we give an overview of the recent literature concerning these photoinduced phenomena. We concentrate on titanium dioxide, as it is one of the most important and most widely used compounds in all application areas mentioned above. The first part of this article will be devoted to the introduction of titanium dioxide and its photoinduced processes (2 Titanium dioxide, 3 Photoinduced processes), after which we will treat photocatalytic reactions and mechanisms (4 Mechanistical aspects, 5 Improving photocatalytic reactions) in detail. The last part will describe research, performed on the application of titanium dioxide as photoactive material, in which emphasis is placed on the photocatalytic purification/disinfection of water and air. In conclusion, a critical evaluation of the work performed will be given, in which we will emphasize the questions that remained open until now and what kind of research is desired to further develop this field of science.

Titanium, the world’s fourth most abundant metal (exceeded only by aluminium, iron, and magnesium) and the ninth most abundant element (constituting about 0.63% of the Earth’s crust), was discovered in 1791 in England by Reverend William Gregor, who recognized the presence of a new element in ilmenite. The element was rediscovered several years later by the German chemist Heinrich Klaporth in rutile ore who named it after Titans, mythological first sons of the goddess Ge (Earth in Greek mythology).

Titanium metal is not found unbound to other elements that are present in various igneous rocks and sediments. It occurs primarily in minerals like rutile, ilmenite, leucoxene, anatase, brookite, perovskite, and sphene, and it is found in titanates and many iron ores. The metal was also found in meteorites and has been detected in Sun and M-type stars. Rocks brought back from moon during the Apollo 17 mission have 12.1% TiO2. Titanium is also found in coal, ash, plants, and even in the human body.

Mineral sources are rutile, ilmenite, and leucoxene (a weathering product of ilmenite). Ninety-three to 96% of rutile consists of titanium dioxide, ilmenite may contain between 44% and 70% TiO2 and leucoxene concentrates may contain up to 90% TiO2. In addition, a high-TiO2 slag is produced from ilmenite that contains 75–85% TiO2. About 98% of the world’s production is used to make white pigments, and only the remaining 2% is used for making titanium metal, welding rod coatings, fluxes, and other products [3].

Ilmenite also called titanic iron ore is a weakly magnetic iron-black or steel-grey mineral found in metamorphic and plutonic rocks. It is used as a source of titanium metal. Kupffer discovered it in 1827 and named it after the Ural Ilmen Mountain (Russia) where it was first found. It is found in primary massive ore deposits or as secondary alluvial deposits (sands) that contain heavy minerals. Manganese, magnesium, calcium, chromium, silicon, and vanadium are present as impurities. Two-third of the known ilmenite reserves that can economically be worked up are in China, Norway (both having massive deposits), and former Soviet Union (sands and massive deposits); but the countries with the largest outputs are Australia (sands), Canada (massive ore), and the Republic of South Africa (sands).

Rutile is the most stable form of titanium dioxide and the major ore of titanium was discovered in 1803 by Werner in Spain, probably in Cajuelo, Burgos. Its name is derived from the Latin rutilus, red, in reference to the deep red color observed in some specimen when the transmitted light is viewed. It is commonly reddish brown but also sometimes yellowish, bluish or violet, being transparent to opaque. Rutile may contain up to 10% iron, and also other impurities such as tantalum, niobium, chromium, vanadium, and tin. It is associated with minerals such as quartz, tourmaline, barite, hematite and silicates. Notable occurrences include Brazil, Swiss Alps, the USA and some African countries.

Brookite was named in honor of the English mineralogist, H.J. Brooke, and was discovered by A. Levy in 1825 at Snowen (Pays de Gales, England). Its crystals are dark brown to greenish black opaque. Crystal forms include the typical tabular to platy crystals with a pseudohexagonal outline. Associate minerals are anatase, rutile, quartz, feldspar, chalcopyrite, hematite, and sphene. Notable occurrences include those in the USA, Austria, Russia, and Switzerland.

Anatase, earlier called octahedrite, was named by R.J. Hauy in 1801 from the Greek word ‘anatasis’ meaning ‘extension’, due to its longer vertical axis compared to that of rutile. It is associated with rock crystal, feldspar, and axinite in crevices in granite, and mica schist in Dauphiné (France) or to the walls of crevices in the gneisses of the Swiss Alps.

TiO2 is characterized by the presence of photoinduced phenomena. These are depicted in Fig. 1.

All these photoinduced processes originate from the semiconductor band gap. When photons have a higher energy, than this band gap, they can be absorbed and an electron is promoted to the CB, leaving a hole in the VB. This excited electron can either be used directly to create electricity in photovoltaic solar cells or drive a chemical reaction, which is called photocatalysis. A special phenomenon was recently discovered: trapping of holes at the TiO2 surface causes a high wettability and is termed ‘photoinduced superhydrophilicity’ (PSH). All photoinduced phenomena involve surface bound redox reactions.

TiO2 mediated photocatalytic reactions are gaining nowadays more and more importance and this is reflected in the increasing number of publications that deal with theoretical aspects and practical applications of these reactions (Fig. 2).

By far, the most active field of TiO2 photocatalysis is the photodegeneration of organic compounds. TiO2 has become a photocatalyst in environmental decontamination for a large variety of organics, viruses, bacteria, fungi, algae, and cancer cells, which can be totally degraded and mineralized to CO2, H2O, and harmless inorganic anions. This performance is attributed to highly oxidizing holes and hydroxyl radicals (HOradical dot) that are known as indiscriminate oxidizing agents [4], [5]. The oxidizing potential of this radical is 2.80 V, being exceeded only by fluorine.

The photoconversion (reduction and oxidation) of inorganic compounds is another group of reactions in which TiO2 is applied. The photoreduction of metals, usually using hole trapping, is now redirected from a metalized semiconductor photocatalyst synthetic approach [6], [7] to a process that removes dissolved metal ions from wastewater [8]. Oxidation is used to isolate metal ions which cannot be reduced and for CN decontamination.

The possibility to induce selective, synthetically useful redox transformations in specific organic compounds has also become increasingly more attractive for organic synthesis [9], [10], [11], [12], [13], [14], [15].

The ability to control photocatalytic activity is important in many other applications including utilization of TiO2 in paint pigments [16], [17], [18], [19], [20], [21], [22] and cosmetics [23]. A low photoactivity is required for these applications, in order to prevent chalking (physical loss of pigments as the surface is degraded) and reduce UVC-induced pyrimide dimer formation (which can damage the DNA in cells).

Some major cornerstones in the development of TiO2 in photoactivated processes are:

  • 1972

    the first photoelectrochemical cell for water splitting (2H2O→2H2+O2) is reported by Fujishima and Honda [24] using a rutile TiO2 photoanode and Pt counter electrode;

  • 1977

    Frank and Bard [25], [26] examined the reduction of CN in water, which is the first implication of TiO2 in environmental purification;

  • 1977

    Schrauzer and Guth [27] reported the photocatalytic reduction of molecular nitrogen to ammonia over iron-doped TiO2.

  • 1978

    the first organic photosynthetic reaction is presented, an alternative photoinduced Kolbe reaction [7] (CH3COOH→CH4+CO2) that opens the field of organic photosynthesis;

  • 1983

    implementation by Ollis [28], [29] of semiconductor-sensitized reactions for organic pollutant oxidative mineralization;

  • 1985

    application of TiO2 as microbiocide [30], effective in photokilling of Lactobacillus acidophilus, Saccharomyces cerevisiae and Escherichia coli;

  • 1986

    Fujishima et al. [31] reported the first use of TiO2 in photokilling of tumor cells (HeLa cells);

  • 1991

    O’Regan and Grätzel [32] reported about an efficient solar cell using nanosized TiO2 particles;

  • 1998

    highly hydrophilic TiO2 surfaces with excellent anti-fogging and self-cleaning properties are obtained by Wang et al. [33].

Section snippets

General remarks

Titanium dioxide (TiO2) belongs to the family of transition metal oxides [34]. In the beginning of the 20th century, industrial production started with titanium dioxide replacing toxic lead oxides as pigments for white paint. At present, the annual production of TiO2 exceeds 4 million tons [35], [36], [37]. It is used as a white pigment in paints (51% of total production), plastic (19%), and paper (17%), which represent the major end-use sectors of TiO2. The consumption of TiO2 as a pigment

General remarks

All photoinduced phenomena are activated by an input of super-band gap energy to the semiconductor TiO2. Absorption of a photon with enough energy leads to a charge separation due to an electron promotion to the conduction band and a generation of a hole (h+) in the valence band. The subsequent mode of action of the photogenerated electron–hole pair (e–h+), determines which of the phenomena is the dominant process, because even if they are intrinsically different processes, they can and in

Present ideas and models

The main pathway of photomineralization (i.e., the breakdown of organic compounds) carried out in aerated solution may be easily summarized by the following reaction:Organic compoundhv≥EgTiO2CO2+H2O+mineral acid

A schematic representation of this process is displayed in Fig. 13.

The radical ions formed after the interfacial charge transfer reactions can participate in several pathways in the degradation process:

  • they may react chemically with themselves or surface-adsorbed compounds;

  • they may

General remarks

TiO2 has a photonic efficiency of less than 10% for most degradation processes. Furthermore, TiO2 photocatalyzed reactions are non-selective oxidations. Since they are governed by a free radical mechanism, the degradation rate of a large variety of molecules is found to be approximately the same. On one hand, this lack of sensitivity may be advantageous, but a poor selectivity also implies that the catalyst does not differentiate between highly hazardous contaminants and contaminants of low

Photocatalytic applications

Photocatalysis provides a number of attractive features:

  • a wide variety of compounds may undergo selective redox transformations, decompose, or be deposited;

  • it operates at near ambient temperature;

  • it utilizes solar energy.

A number of research topics in photocatalysis have emerged that offer potential for commercial development. Of particular promise are the following subjects:

  • selective synthesis of organic compounds;

  • removal of organic pollutants;

  • removal of inorganic pollutants;

  • photokilling of

Concluding remarks

Environmental contamination is a growing problem that cannot be neglected as it influences our world and daily life. Eliminating contaminated compounds costs energy, which increases the CO2 emission that causes global warming. A solution for this dilemma can be found in the field of semiconductor chemistry, which implies the use of an inert “environmentally harmonious” catalyst, non-hazardous oxidants (oxygen) and solar energy input. In this way the contaminated environment can be gently

References (1352)

  • A. Mills et al.

    J Photochem Photobiol A: Chem

    (1997)
  • S.K. Lee et al.

    Platinum Metals Rev

    (2003)
  • D. Chen et al.

    Chem Eng Sci

    (2001)
  • P. Pichat

    Catal Today

    (1994)
  • A. Karthikeyan et al.

    J Non-Cryst Solids

    (2000)
  • T. Ohno et al.

    J Catal

    (1998)
  • J.H. Braun

    Prog Org Coat

    (1987)
  • R. Blakey

    Prog Org Coat

    (1985)
  • N.S. Allen et al.

    Eur Polym J

    (1992)
  • S.L. Pugh et al.

    Dyes Pigments

    (2002)
  • Y. Kubota et al.

    J Photochem Photobiol A: Chem

    (2001)
  • A.L. Pruden et al.

    J Catal

    (1983)
  • T. Matsunaga et al.

    FEMS Microbiol Lett

    (1985)
  • N. Kumazawa et al.

    J Electrochem Chem

    (1999)
  • N.O. Savage et al.

    Sensors Actuators B

    (2001)
  • N. Savage et al.

    Sensors Actuators B

    (2001)
  • Y.X. Leng et al.

    Surf Coat Technol

    (2002)
  • D.E. Mac Donald et al.

    Biomaterials

    (2002)
  • J.N. Armor

    Appl Catal B: Environ

    (1992)
  • N.Y. Topsøe et al.

    J Catal

    (1995)
  • H. Schneider et al.

    J Catal

    (1994)
  • S. Hu et al.

    J Catal

    (1996)
  • L.J. Alemany et al.

    Appl Catal B: Environ

    (1996)
  • E. Hums et al.

    Catal Today

    (1996)
  • C.J.G. van der Grift et al.

    Catal Today

    (1996)
  • R. Weber et al.

    Appl Catal B: Environ

    (1999)
  • S. Krishnamoorthy et al.

    Catal Today

    (1998)
  • F. Boccuzzi et al.

    Catal Today

    (2002)
  • J. Li et al.

    Appl Catal A: Gen

    (2002)
  • D.J. Duvenhage et al.

    Appl Catal A: Gen

    (2002)
  • D.J. Duvenhage et al.

    Appl Catal A: Gen

    (1997)
  • M.A. Vannice et al.

    J Catal

    (1979)
  • M. Bollinger et al.

    Appl Catal B: Environ

    (1996)
  • S. Tsubota et al.

    Stud Surf Sci Catal

    (1991)
  • L. Fan et al.

    Appl Catal A: Gen

    (2003)
  • H. Kim et al.

    Appl Catal B: Environ

    (1998)
  • J. Despres et al.

    Appl Catal B: Environ

    (2003)
  • W.D. Brown et al.

    Solid State Electron

    (1978)
  • C. Li et al.

    Nucl Instrum Meth Phys Res B

    (2000)
  • US Geological Survey Minerals Yearbook,...
  • L.M. Dorfman et al.

    NSRDS-NB

    (1973)
  • R.M. Alberci et al.

    Appl Catal B: Environ

    (1997)
  • J.M. Hermann et al.
  • B. Kreutler et al.

    J Am Chem Soc

    (1978)
  • M.A. Fox et al.

    Chem Rev

    (1993)
  • S. Yanagida et al.

    J Phys Chem

    (1989)
  • L. Cermanati et al.

    Chem Commun

    (1998)
  • S.R. Kumar et al.

    Mater Lett

    (1999)
  • T. Rentschler et al.

    ECJ

    (1999)
  • N.S. Allen

    Polym Degrad Stab

    (1990)
  • Cited by (4200)

    View all citing articles on Scopus
    View full text