Titanium Dioxide - TiO2 Pigment for Paints, Coatings & Inks

2 grades are available in varying particle sizes and can undergo a variety of post-treatment. However, the pigment is expensive, especially when the volume prices of systems are used.

Therefore, there always remains a need to develop a full-proof strategy to get the best results in terms of:

• scattering efficiency and cost/performance ratio
• optimizing the dispersion process
• considerations to select the right pigment type

In this guide, you will learn how to achieve the best possible white color strength and hiding power in your coatings and inks by selecting the right TiO2 pigment.

Titanium dioxide is by far the most suited white pigment because of its high refractive index and lack of visible light absorption. TiOgrades are available in varying particle sizes and can undergo a variety of post-treatment. However, the pigment is expensive, especially when the volume prices of systems are used.Therefore, there always remains a need to develop a full-proof strategy to get the best results in terms of:

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Abstract

TiO2 is a compound of great importance due to its remarkable catalytic and distinctive semiconducting properties. It is also a chemically stable, non-toxic and biocompatible material. Nano TiO2 is strong oxidizing agent with a large surface area and, hence, high photo-catalytic activities. With low production cost and a high dielectric constant, it is an inexpensive material. It can be prepared by diverse procedures such as solution and gas phase procedures. Nowadays, TiO2 is being used frequently for photo degradation of organic molecules and water splitting for hydrogen generation. Most important applications include purification, disinfection of waste water, self-cleaning coatings for buildings in urban areas and the production of the green currency of energy (hydrogen) by splitting water. The review describes the advances in the syntheses, properties and applications of TiO2 nano structures. Besides, efforts are also made to discuss the working mechanism and future challenges and perspectives.

TiO2 is a compound of great importance due to its remarkable catalytic and distinctive semiconducting properties.

1.&#;Introduction

Nowadays, nano-structured materials are an important area of research owing to their several unique characteristic features. Among all the transition metal oxides, TiO2 nano-structures are the most attractive materials in modern science and technology. TiO2 has been widely used commercially in doughnuts, cosmetics, pigments,1 catalysts, sunscreens,2,3 solar cells,4 water splitting etc. TiO2 is being used in plastics, paints, varnishes, papers, medicines, inks, medications, toothpaste, food products, and industries.5&#;10 In first of all, Fujishima and Honda4 reported photo-assisted water splitting under UV light on a TiO2 photo anode as a semiconductor.4,11,12 The diverse claims can be separated into &#;environmental&#; and &#;energy&#; groups, several of which depend on the TiO2 properties itself as well as on the changes of the TiO2 material host (e.g. with organic and inorganic dyes). In previous years, the research activity expansion has been observed in nanotechnology and nanoscience.13&#;17 On the modification, preparation and properties of nano-materials, a significant amount of research, reports and reviews have been published recently4,11&#;40 to know and précis the progress in this field. TiO2 nanostructures in various forms, among the unique characteristics of nanomaterials, are gaining broader applications due to their size-related characteristics. For nanometer scale TiO2 the energy band structure becomes discrete due to its surface, photochemical and photo physical properties. Consequently, several works have focused on nano crystalline TiO2 syntheses with a high surface area. As a photocatalyst,15,16,27 TiO2 nanostructures have drawn much attention and are projected to show a significant role in serving to resolve several pollution and environmental problems. Thus, using TiO2 for H2 production and photo-assisted water splitting devices offers a way for hygienic and low price production of hydrogen by solar energy.13,41

This review aims to give an inclusive data on the advances in TiO2 based nanostructure, recent investigation and the development efforts, which tack energy and environmental challenges in consideration. Besides, the crystal structure, optical, electrical/electronic and adsorption, surface area, porosity properties of TiO2 nanostructure are discussed. The procedures of preparations, fabrications (nanoparticles, nanorods, nanowires, and nanotubes), the conditions of syntheses and accountability for regulation of titanate nanostructures morphology are also discussed. TiO2 nanostructures applications in electrocatalysis, environment and energy challenges are also highlighted. Finally, the mechanism of action and future challenges are also highlighted.

5.&#;Mechanism of water splitting

On surface modification of TiO2, a number of efforts have been made to expand the photocatalytic activities as simple change can eagerly adjust the mechanisms and accelerates the kinetics of photocatalysis.258,259 In the UV regime, the band gap for TiO2 lies. Besides, the latter has lower photocatalytic activities due to the lower ECB of ritual by &#;0.2 eV. For modification of TiO2 nanomaterials, the goals are to improve their optical activities.213,214 There are several methods to recover the performance of TiO2 nanomaterials. In visible light, other colorful compounds sensitizing TiO2 might be improved optical activity. Second, doping with other elements can also change the optical properties of TiO2 nanomaterials. At the atomic level, Hussain et al.260 explored the interfacial structure between a rutile TiO2(110) surface pre-characterized and liquid water. The mechanism of splitting of water occurring in the occurrence of semiconductor photocatalyst was explained by Salvador in 1&#;4 equations as follows.261Semiconductor + 2hν &#; 2h+ + 2e&#;

2H+ + 2e&#; &#; H2(EH2O/H2)

EtOH serves as a sacrificial reagent which also helps to increase the entire process efficiency.64 Water splitting mechanism on Ag/TiO2 was explained by Liu et al.262

According to Salvador et al.261 during the process of water splitting, formation of H+ and OH&#; can take place through the reaction of H2O molecules with holes in TiO2 valence band. The consumption of ·OH can occur through chemical reaction with CH3CH2OH. With enhanced energy, the electrons react with H+ at the surface of Ag NPs where H+ can take an electron through SPR (surface plasmon resonance) to generate an H atom, and then 2H atoms associate with each other for the formation of an H2 molecule.TiO2 + hν &#; hvb+ + ecb&#;ecb&#; &#; eAg&#;eAg&#; + hν(visible) &#; eSPR&#;hvb+ + H2O &#; ·OH + H+CH3CH2OH + ·OH &#; CH3COOH + H+eSPR&#; + H+ &#; HH + H &#; H2

With an I&#;/IO3&#; (iodide/iodate) redox-mediator system, water splitting mechanism on TiO2 was explained by Nishijima et al.263 Besides, they studied the photoassisted oxidation of water continued with impartially high efficiency when Fe3+ ions were used as electron acceptors. This whole process of splitting of water on doped titanium dioxide; containing a rutile phase; was compared to that on doped titanium dioxide with an anatase phase. After all, they proposed the mechanism of water splitting in following equations:TiO2 + hν &#; e&#; + h+2H2O + 4h+ &#; O2 + 4H+Fe3+ + e&#; &#; Fe2+C2H5OH + 3H2O + 6h+ &#; 2CO2 + 6H+2H+ + 2e&#; &#; H24OH&#; + 4h+ &#; O2 + 2H2OIO3&#; + 3H2O + 6e&#; &#; I&#; + 6OH&#;2H2O + 2e&#; &#; H2 + 2OH&#;I&#; + 6OH&#; + 6h+ &#; IO3&#; + 3H2O

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Mechanism for water splitting on TiO2 photosensitized by glutathione capped metal nanoclusters was explained by Chen and Kamat.264 In aqueous buffer solution (pH = 7), a mesoscopic TiO2 film photosensitized by glutathione capped metal nanoclusters (Aux-GSH NCs) was utilized as the photoanode with platinum counter electrode. After all, they also proposed the water splitting mechanism as follows:

Aux &#; GSH*&#;TiO2 &#; Aux &#; GSH+&#;TiO2(e)TiO2(e) + Pt &#; TiO2 + Pt(e)Pt(e) + H+ &#; 1/2H2Aux &#; GSH+ + 1/2H2O &#; Aux &#; GSH + 1/4O2 + H+

Aux &#; GSH+ + EDTA &#; Aux &#; GSH + oxidized EDTA

6.&#;Lithium batteries

As the most promising energy storage technologies, lithium ion batteries (LIBs) are measured as the best for renewable energy, electric vehicles and mobile electronics. In recent years, as a possible anode, TiO2 (B) has received growing interest for Li ion batteries. As compared to commercialized Li4Ti5O12, it offers higher energy storage. From the micron to the nanoscale, it is better than rutile and anatase. The important factors playing an important role and greatly affect the lithium ion batteries (LIB) performance are particle size and crystallographic alignment of the nanostructured.265&#;267 The crystallographic orientations [such as TiO2(B)] and the theoretical studies268,269 confirmed that lithium ion mobility favors the direction in the order of b > c &#; a-axis channel.270

7.&#;Gas sensors

The other application of TiO2 nanocrystalline is as gas sensor because gas adsorption causes a change in electrical conductivity like ZnO semiconductors.155,270&#;272 Thus, TiO2 is usually used as an oxygen gas instrument, e.g. to gauge the burning process of fuel in car engines to control fuel environmental pollution and consumption. Grimes et al.155 considered the use of titanium dioxide as sensor.

8.&#;White pigments

The pigment being the most abundant component in the coating affects the properties of the coating materials. Because of its highest refractive index of stability, colorless, and relatively low and uniform absorption of visible light, titanium dioxide has newly become a core white pigment source of scattering of light. By two procedures i.e. hydrothermal and hydrolysis, Samya El-Sherbiny et al.273 created anatase and rutile. In paper coating, the usage of the prepared nano-pigments disclosed that a slight amount of titanium dioxide was satisfactory to attain important improvements in brightness and opacity. It is due to ability of scattering light.

9.&#;TiO2-based nanocomposites as catalysts

TiO2 photo catalysts have been widely studied for the uptake of organic contaminants in water and air.274 The present article also concentrated on hybrid nanocrystals based on TiO2, describing different examples of synthetic methods and deliberating their applications in water treatment. Some papers are reported in the area of hybrid nanocrystals preparation. The papers on the photocatalytic features of nanocatalysts indicated their huge measure applications as challenges. However, these nanomaterials hold good assurance for the dilapidation of organic and inorganic contaminants in water or gas phase. The main features of nano sized TiO2 materials are enormously high surface to volume ratio and turning into a great density of catalytically surface sites.275 The another fact is the size reliant band gap of nano sized semiconductors. It is likely to finely adjust the redox potentials of photogenerated electron hole couples to selectively control photochemical reactions. Besides, the catalyst surface is occupied easily by charges photogenerated in nano-catalysts.276 The hybrid nanocrystals formed by two or more components hold great promise for the development of multifunctional nanocatalysts among the embarrassment of methods proposed in the literature so far.277&#;281 Indeed, the opportunity is offered by hybrid nanocrystals to merge in one material resulting in countless possible combinations.282&#;285 Under visible light irradiation, spatial separation of e&#;/h+ could also be provide by the photoactivity hybrid nanocrystals. Thus, improving is an occasion to magnetically recuperate the photocatalysts or stimulate biocidal utility even in the dark. The surface properties, the particle size of the catalysts, their morphology, the composition, organization of the metal and TiO2 are the factors, which also affect the photocatalytic efficiency.286 Therefore, for the preparation of metal nanoparticle hybrid hetero-structures, numerous synthetic methods have been described, which include impregnation,287 UV irradiation,288 electrodeposition,289 sonochemistry,290 hydrothermal,291 sol&#;gel,292 and flame-spray synthesis.293 Wenqing et al.294 prepared nanocomposites of titanium dioxide (P25) and reduced graphene oxide (RGO) utilizing numerous methods such as hydrazine reduction, UV-assisted photocatalytic reduction and hydrothermal. Those nanocomposites were studied as photocatalysts for the progress of hydrogen from alcohol solution under UV-vis irradiation. It was found that the assimilation of RGO into P25 considerably increased the photocatalytic activity for H2 evolution, and the P25&#;RGO composite synthesized by the hydrothermal method showed the excellent presentation. Deepa et al.295 prepared Au/TiO2 nanocomposites for photocatalytic hydrogen generation in the company of a sacrificial electron donor like ethanol or methanol under UV-visible and visible light irradiation. These nanocomposites exhibited excellent photocatalytic activity for hydrogen generation under UV-visible conditions. Amount of hydrogen evolved using Au/TiO2 nanocomposites under dark condition was zero. Anna and Jerzy296 studied the hydrogen generation by water splitting on Pt&#;TiO2 catalyst. Besides, they also examine the influence of numerous sacrificial chemicals like MeOH, Na2S, and ethylene diamine tetraacetic acid (EDTA), I&#; and IO3&#; ions on the photocatalytic efficacy in water splitting reactions in ultraviolet (UV) illumination. Photocatalytic water splitting was achieved at EDTA and Na2S utilization as the sacrificial reagents. Yatskiv et al.297 reported that the photocatalytic systems having mesoporous TiO2 and Pd/SiO2, produced extra quantities of molecular hydrogen at keeping in dark after the ending of irradiation. Václav and Daniela298 prepared TiO2/ZnS/CdS composites by homogeneous hydrolysis of aqueous solutions mixture of TiO2SO4, ZnSO4, and CdSO4 with thioacetamide for photocatalytic hydrogen production from water. Hydrogen generation was seen in the company of Pd and Pt nanoparticles deposited on TiO2/ZnS/CdS composites. The excellent photocatalytic activity for H2 evolution indicated with TiZnCd7 on surface deposited with palladium, which had 78.5% ZnS, 20.21% TiO2, and 1.29% CdS·TiO2/ZnS/CdS. Quanjun et al.299 reported new composite material having of TiO2 nanocrystals via two steps hydrothermal process utilizing thiourea, sodium molybdate and graphene oxide as precursors with tetrabutylorthotitanate and MoS2/graphene hybrid as titanium precursors. TiO2/MoS2/graphene composite attained high H2 generation rate of 165.3 μmol h&#;1 without a noble metal co-catalyst. Anna et al.300 interpreted the development of the quantum yield of H2 generation using TiO2/Ag0 to TiO2/Ni0 to TiO2/Cu0 in terms of alterations in the electronic interface between the semiconductor surface and metal nanoparticles. They observed a controlled metal amount range for maximum quantum yield of hydrogen generation. Ke et al.301 examined Pt-loaded nanocomposites, pristine MWNTs and TiO2 for their photocatalytic activities for water splitting with triethanolamine as an electron donor. Hydrogen was effectively generated on Pt/MWNT&#;TiO2 in visible light illumination (λ > 420 nm). Hydrogen production rate up to 8 mmol g&#;1 h&#;1 under full spectral irradiation of a Xe-lamp, or more was attained. Takuya et al.302 investigated the photocatalytic hydrogen generation in MeOHn with CuO/TiO2, SnO/TiO2, ZnO/TiO2, CuO/Al2O3/TiO2 and Al2O3/TiO2 nanocomposites. The maximum hydrogen generation was achieved with CuO/Al2O3/TiO2 nanocomposites having 0.3 wt% Al2O3/TiO2/0.2 wt% CuO. Nitish et al.303 demonstrated the higher hydrogen evolution reaction (HER) activity of Pt nanoparticles (Pt NPs) reinforced on titanium dioxide (TiO2) nanocrystals (Pt&#;TiO2) and nitrogen doped reduced graphene oxide (N-rGO) constructed TiO2 nanocomposite (Pt&#;TiO2&#;N-rGO). Lu et al.304 prepared TiO2 photocatalysts with nickel sulfide co-catalyst by stocking nickel sulfide on TiO2 with solvothermal synthesis method. It was reported that NiS was used as a co-catalyst with TiO2 for the photocatalytic generation of H2. High hydrogen generation was achieved with NiS as hexagonal structure with content in the composite of 7 at% in relation to TiO2. The rate of H2 generation was augmented by 30 times than that of TiO2 alone. Fuyun et al.305 prepared nanocomposite of N-doped TiO2 with graphene oxide (NTG) to improve the photocatalytic efficiency. NTG exhibited high photocatalytic efficiency hydrogen evolution. It was of 716.0 or 112.0 μmol h&#;1 g&#;1 at high pressure Hg or Xe lamp, which was around 9.2 or 13.6 times higher than P25 photocatalyst.

Recently, gold nanoparticles were reinforced on TiO2&#;C3N4 for CO oxidation in visible light illumination exploiting hydrothermal method; beginning with titanium glycolate and graphitic C as precursors. TiO2&#;C3N4 microspheres were prepared and then decorated with gold nanoparticles by letting HAuCl reaction at alkaline pH (pH = 10) in the attendance of sodium carbonate. After 2 h, the dried and washed powder was calcinated at 350 °C.306 Besides, visible light dynamic silver modified titania catalyst was described for the utilization in the decomposition of MeOH, CH3COOH, 2-PrOH and Escherichia coli.307 A plasmonic gold silver alloy on TiO2 photocatalyst was also described that indicated elevated degradation of stearic acid at 490 nm than reported using pristine TiO2.308. Qiu et al.309 have described that CuxO/TiO2 photocatalyst resulted into effective VOCs uptake. Furthermore, Wang et al.310 prepared and used TiO2 NRs/FexOy/Ag core shell nanostructures for photodegradation of rhodamine B in solar light illumination. Recently, a reviews described successful removal of benzene, methylene blue and carbamazepine by photodegradation using CNT/TiO2 nanostructured composites.311

10.&#;Future challenges and perspectives

Recently, the applications of TiO2 nanostructures have been exploited to clean environment and produce hydrogen. But there are certain limitations, which we have to overcome to make the structures applicable in real life problems. The efficiencies of these structures are not excellent; especially for decomposing of persistent organic volatile pollutants and production of hydrogen at large scale. Generally, doped TiO2 nanostructures result into poor photoactivity. Other challenges are to augment spectral sensitivity of these structures to visible and NIR regions and the bio-compatibility of TiO2 nanostructures. Therefore, there is a great need of future research focusing constant photoactivity in the long run. These can be achieved by modifying the synthetic routs. Nonmetal doped TiO2 nanostructures have low photocatalytic activity under visible light UV radiation. Therefore, some materials such as polymers, glass, ceramics, or metals may serve as magical identities in this area for economic and eco-friendly applications.

Future research needs the development of new synthetic procedures and nanostructures with higher surface states. It may be served by non-lithographic complementary metal oxide semiconductor compatible techniques. This technique may be applicable for new doping materials, dopant incorporation into TiO2 nanostructures and applications for environmental and alternate energy areas. Besides, visible to near infra-red activated titanium nanostructures should be designed. TiO2 nanostructures may serve as the ideal materials in biological and medicinal science. Therefore, there is a great need to study the bio-compatibility of these structures at supra molecular level. In a nut shell, the researchers have several challenges to tackle with them in near future. Therefore, there is a great need to improve the structures and properties of these materials. The basic knowledge of chemistry, physics and computer modeling may help to achieve the task.

11.&#;Conclusion

An inclusive review of the syntheses, properties and applications of TiO2 nanostructures is offered. These nanostructures can be prepared by different synthesis procedures as per the requirements. It was observed that the physico-chemical properties of titanium dioxide nanostructures are responsible for wide applications in several fields such as gas sensors, white pigments, lithium batteries, photocatalytic applications (photodegradation of organic compounds), photovoltaic applications and water splitting. TiO2 nanostructures play a great role in water purification by degrading biological and organic pollutants. The use in generating hydrogen as a green currency is the biggest asset of nano TiO2 structures. It was observed that the decomposing and water splitting properties of TiO2 nanostructures are not good enough, which can be exploited at a commercial level economically. It may be predicted that these materials may be the choice in water purification. Most interestingly, we believe that TiO2 nanostructures will achieve a great reputation in generation of hydrogen fuel &#; a need of the next century.

Conflicts of interest

There is no conflict of interest.

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