Titanium dioxide (TiO2) possesses a higher refractive index than diamond, does not absorb visible rays, and is highly chemically stable. Therefore it is widely used in paints and cosmetics as a white pigment and ultraviolet (UV) absorbing agent. There are three types of crystal structures of titanium dioxide-anatase, rutile and brookite. Industrially used ones are anatase and rutile. Rutile is the most stable, and anatase converts to rutile at temperatures in excess of 700 °C.
Micronparticle titanium dioxide (TiO2) and nanoparticle Titanium Dioxide (Nano-TiO2) are two totally different materials. Micron particle size TiO2 is mainly used as white pigment in the paint and cosmetic industry. Nano-TiO2 has a much greater surface area of a given mass or volume of nanoparticles compared to an equivalent mass or volume of conventional TiO2 particles, resulting in enhanced catalytic activity and UV absorption at certain wavelengths.
Nano-TiO2 is available in pure anatase, pure rutile, and mixtures of anatase and rutile. It exhibits UV shielding effects, and rutile is widely used in the cosmetics sector, especially in sunscreens. Anatase displays photocatalytic functions (more so than rutile) and offers self-cleaning capabilities under sunlight, air cleaning, water quality improvement and anti-microbial and anti-mould functions for application in numerous paints and coatings sectors.
Commercially available brands of nano-TiO2 vary in particle size, surface area, purity (e.g., due to doping, coating, or quality control), surface characteristics, crystalline form, chemical reactivity, and other properties. Applications at varying degrees of commercial development are listed below.
Showa Denko K.K. produces nanoparticulate Super-TitaniaTM, which is used in the production of multilayer ceramic capacitors (MLCCs). They are mainly used in consumer electronic devices.
Cleaning and sanitary
Photocatalytic TiO2 coatings have been demonstrated to be more long-lasting and effective than standard anti-bacterial cleansers. Photocatalyst nanocoatings have been proven to combat Bird Flu and SARS infections in high risk areas. Hospitals and buildings in Asia, and more recently in Europe, have been coated with nano-TiO2 coatings to protect property against deadly infections and environmental pollution damage. Gens-Nano-TiO2 coating from MCH Nanosolutions (www.mchnanosolutions.com) turns a treated surface into an antibacterial, anti-fungal, mould free surface, while also acting as an air filter. Self-cleaning and other functions work in the presence of light to protect a building’s interior and exterior from environmental contamination. NanoCotzTM Eco-Refresh from Inspriraz (www.inspiraz.com.sg) is a nano-TiO2 and Titanium Phosphate catalytic coating that decomposes organic sources or odour and bacteria for walls & ceilings of building/home interior, upholsteries, curtains/blinds, air conditioning filters etc.
Indoor air quality control
Indoor air quality (IAQ) has become an important regulatory concern due to the increased time people spend in indoor environments, such as hotels, restaurants, commercial business facilities, university laboratories, hospitals and residences.
Nano-TiO2 photocatalyst coatings have been developed to improve indoor air quality by reducing the amount of volatile organic compounds (VOCs) and other toxic chemicals people are exposed to in these environments.
The nanomaterials have been commercialized in air purifier filters and coated onto walls and windows to control Sick Building Syndrome in Asia, where there is a fast-growing market.
Construction & exterior protection
In recent years, self-cleaning coatings using photocatalytic nanoparticle TiO2 have gained considerable attention in the construction industry. When exposed to solar radiation nano-TiO2 coated onto buildings acts as catalyst for photodecomposition of pollutant molecules adsorbed on its surface resulting in transformation into non-toxic compounds. The organic compounds neutralized by nano-TiO2 photocatalytic coatings include dirt (soot, grime, oil and particulates) biological organisms (mould, algae, bacteria and allergens), air-borne pollutants (VOCs including formaldehyde and benzene; tobacco smoke; and the nitrous oxides (NOx) and sulfuric oxides (SOx) that are significant factors in smog), and chemicals that cause odours. The catalyzed compounds break down into oxygen, carbon dioxide, water, sulfate, nitrate and other molecules that are either beneficial to or at have a relatively benign impact on the environment. Most inorganic pollutants and stains, including rust, are not catalyzed.
Figure 1: The Jubilee Church in Rome incorporates photocatalytic concrete additives from the Italcementi Group.
Nano-TiO2 self-cleaning coatings greatly benefit building maintenance, especially for skyscrapers, as they reduce the need for costly surface cleaning.
Other applications are in lacquers and paints that resist UV degradation. De Cie GmbH (www.decie.de) produces nano-TiO2 glass, leather, metal, stone and wood coatings. Their anti-Fog coating incorporates nano-TiO2 and a hybrid polymer. The sol-gel process is used for producing the coating and the average thickness of the coating is 30-100 nm, depending on application.
UV-Titan from Merck Performance Materials (www.mer- ck-performance-materials.com) is a range of ultra-fine, surface treated, and transparent TiO2 nanoparticles with uses in automotive coatings, wood lacquers, and plas-tics. It is used to provide UV protection in transparent lacquers and plastic (UPVC window profiles) applica-tions. As an inorganic UV screen, it gives long lasting protection against the darkening of wood substrates and helps to maintain the wood’s natural look. UV-Titan can also be used in metallic effect automotive coatings, where it influences the flip/flop effect of the metallic pigment.
Incorporating nano-TiO2 in construction materials such as cement, plaster and concrete, etc. has also been developed in recent years. TioCem (www.heidelbergcement.com/NR/…/TioCem_Broschuere_englisch.pdf)from HeidelbergCement is one such product that is commercially available. Another major product is TX Active from Italcementi Group (www.italcementigroup.com) that has been applied to a number of bridges and buildings.
Figure 2: Schematic of photocatalytic action (Source: Nanoprotect)
Nanoporous TiO2 thin films have been widely used as the working electrodes in dye-sensitized solar cells (DSSCs). DSSCs consist of a sensitizing dye, a transpar-ent conducting substrate (F-doped tin oxide), a nanometre sized TiO2 film, iodide electrolyte, and a counter electrode (Pt or carbon). DSSCs are being developed as they offer high energy conversion efficiency and low fabrication cost. Nano-TiO2 is utilized for the for nanoporous thin film due to its appropriate energy levels, dye adsorption ability, low cost, and easy preparation.
Figure: 3 Schematic illustration of a generic dye-sensitized solar cell (Source: University of Santa Barbara).
Paper with superhydrophobic surfaces has been devel-oped by adding modified nano-TiO2 to cellulose pulp. Tissue paper incorporating nano-TiO2 is another appli-cation utilizing its anti-bacterial properties. Companies developing products in this area include Domtar Paper (www.domtar.com), Nippon Paper Crecia Co., Ltd. (www.nipponpapergroup.com), Ein and Coattec Industries.
Cosmetics and sunscreens
In sunscreens, nanoparticles of titanium dioxide are efficient UV-filters as they transmit, reflect and scatter the visible part of the solar radiation while they strongly absorb in the UV region. Compared to the available UV absorbers, ultrafine nano-TiO2 possesses effective UV filter properties over the entire ultraviolet spectrum (UVC + UVB + AVA). Other desirable properties include transparency.
Sakai Chemical, Japan
The company offers STR-100C-LP and STR-100A-LP. They also bought out BASF’s T-Lite range of nano-TiO2 products in 2011. The products are sold by Kowa in North America and in Europe by Kowa GmbH – in Europe. They also offer additional grades of ultrafine TiO2. Product details are available at http://chemical.kowa.com/download/Overview%20of%204%20basic%20grades%20of%20STR%20series%20TiO2.pdf
EMD Chemicals Performance Materials, Germany www.emd-performance-materials.com
The company is a subsidiary of Merck KGaA. TiO2 products are Eusolex T-2000, Eusolex T-45D and Eusolex T-AQUA.
Evonik Degussa, Germany
The company produces P-25 anatase TiO2 with primary particle size of 21nm.
Ishihara Sangyo Kaisha, Japan
The company produces the TTO S-4, TTO S-3 and TTO V-3 series of powders with primary particle size of 15nm. Product details are available at http://www.iskweb.co.jp/eng/products/functional02.html.
The company produces HOMBITEC Ultrafine transparent titanium dioxide with inorganic surface treatment and crystal lattice doping.
Product details are available at www.sachtleben.de/fileadmin/pdf_dateien/brochures/043_HOMBITEC.pdf
Showa Denka, Japan
The company produces Maxlight™ TS/F-TS (Silica coated titanium dioxide) for application in cosmetics and sunscreens.
The company produces a wide range of nanoparticle TiO2 products, including MT-100T, MT-500B, MT-100Z. Product details are available at www.tayca.co.jp/english/products/micro_titanium/spec.html
Titan Kogyo, Japan
The company produces Stt 65C-S ultrafine titanium dioxide for application in electrophotographic toner additives, UV protection cosmetics and UV protection film, material for flip-flop coatings and rubber and plastic heat stabilizers. Product details are available at www.titankogyo.co.jp/english/