Titanium dioxide (Ti02) is a semiconductor oxide with a wide and direct bandgap, having excellent electrical, electrochemical, and optical properties (Kumar et al., 2017; Longoni et al., 2017). The combination of high transparency in the visible range and electrical conductivity expands its applications further than the traditional catalysis implementation of Ti02. In recent years, the potential of this material has been studied in various fields such as solar cells (Shogh, Mohammadpour, Zad, & Taghavinia, 2015), gas sensors (Tshabalala, Motaung. Mhlongo, & Ntwaeaborwa, 2016), and photocatalysis

Bright field (left) and dark field (right) images from (a) tubes, (b) rods, and (c) plates. Figures reprinted from Garcia-Tcccdor et al. (2017)

FIGURE 5.12 Bright field (left) and dark field (right) images from (a) tubes, (b) rods, and (c) plates. Figures reprinted from Garcia-Tcccdor et al. (2017).

(Kaplan et al., 2016), among others, where the dimensions, morphology, and structural defects play an essential role.

Ti02 presents three different crystalline structures: a rutile (tetragonal) stable phase and two metastable phases, known as anatase (tetragonal) and brookite (orthorhombic). Owing

PL spectra acquired at T = 300 К corresponding to

FIGURE 5.13 PL spectra acquired at T = 300 К corresponding to (a) composites with undoped Sn02 nanoparticles and (b) composites with Li. PL spectrum from GO is also included as a reference, (c) and (d) CIE 1031 chromaticity coordinates of the undoped and Li-doped PL spectra from samples in (a) and (b). respectively. The black triangle denotes the RGB gamut with pure red, green, and blue in the vertices. Pure white (W) is also shown for comparison.

Reprinted from Del Prado ct al. (2019), with permission from Elsevier.

to their physicochemical properties, stability at room temperature, and more developed synthesis methods so far, the most common and extensively used phases are the tetragonal rutile and anatase phases, being the rutile phase the most stable phase at high temperature, with a melting point at around 1840 °C. This means that any other ТЮ2 phase will transform irreversibly to the rutile phase at certain conditions. Both anatase and rutile phases posses wide bandgaps (3-3.2 eV), are transparent to visible radiation, and exhibit good electrical conductive and high refractive index (n > 2), which make them appealing materials for electronic and optoelectronic applications such as solar cells, solar fuels, LIBs, and photocatalysis. ТЮ2 also demonstrated excellent photocatalytic activity, which can be improved by using nanostructures with appropriate doping. Nowadays, Ti02 is a go-to material for water purification or contaminant gas reduction due to its low cost and its high chemical stability; therefore, great efforts have been invested during the last decades to tune their physical and chemical properties using both doping and downscaling approaches. The selection of proper Ti02 synthesis methods in order to control the chemical composition, crystal phase, crystal size, and morphology, as well as its structural defects is essential as these aspects are required in the development and improvement of different TiCb-based applications (Liu et al., 2017; Usui, Yoshioka, Wasada, Shimizu, & Sakaguchi, 2015).

In this section, some of the most promising applications of Ti02 low-dimensional structures in photovoltaics, optoelectronics, photocatalysis, gas sensing, and phase-control-based devices are described.

Photovoltaic Applications

One of the main applications of ТЮ2 in the photovoltaic field is as active material of the Gratzel solar cells. The structure of one of these cells, also known as DSSCs, is shown in Figure 5.14. These Ti02-based photovoltaic devices, which involve low cost and easy fabrication, are based on a photosensitized anode formed by a dye solution and Ti02 nanostructures, a cathode and an electrolyte.

Control of morphology and crystal phase among other parameters is essential in order to achieve a good efficiency in these devices. DSSCs are usually fabricated using nanocrystalline Ti02, mesoporous Ti02, ТЮ2 nanotubes, or hybrid anatase-rutile Ti02 nanocrystalline material. However, recent studies have demonstrated a better efficiency for DSSCs using doped ТЮ2. For example, the application of photoanodes prepared with N-doped Ti02 nanotubes, improves the DSSC performance (Tran et al., 2017). Figure 5.15 shows Ti02 nanotubes synthesized by a hydrothermal method and the corresponding J-V curves. The photoinduced current due to a xenon lamp used in the experiment increases around ЗО^Ю % using N-doped Ti02 as compared to the behavior of undoped Ti02 electrodes due to the enhancement of the electron injection efficiency and the decrease of the dark current.

Doping Ti02 has been a deeply explored field leading to enhanced photovoltaic behavior. Shogh et al. (2015) demonstrated a 26 % improvement in cell performance, obtained from Nd- Ti02 photoelectrode compared to a pure one, due to the injection and transport enhancement in the doped photoelectrode. Most of the patents based on DSSC are focused on the achievement of better efficiency and improved efficiency by using different nanocrystalline Ti02 and Ti02-based composites (Fakharuddin, Jose, Brown, Fabregat-Santiago, & Bisquert, 2014).

Scheme of a DSSC based on Ti0 nanostructures

FIGURE 5.14 Scheme of a DSSC based on Ti02 nanostructures.

ТЕМ images showing (a) undoped and (b) N-doped (0.5 M) Ti0 nanotubes, and (c) the corresponding J-Vcurves

FIGURE 5.15 ТЕМ images showing (a) undoped and (b) N-doped (0.5 M) Ti02 nanotubes, and (c) the corresponding J-Vcurves.

Reprinted from Tran ct al. (2017), with permission from Elsevier.

Photocatalytic Applications

ТЮ2 has been studied as one of the most efficient environmental photocatalyst, being a good candidate for photodegradation of several pollutants or water splitting. Kaplan et al. (2016) synthesized a uniform anatase/rutile/brookite nanocomposite with excellent photocatalytic properties under UV light. It was determined that after 60 min of reaction, 94% conversion of Bisphenol A was achieved. On the other hand, doping Ti02 usually can improve the response of this material. Demirci et al. (2016) studied the effect of Ag on the photodegradation of methylene blue under UV light. The results showed that Ag-doped Ti02 have a degradation efficiency of 55% while undoped Ti02 present a degradation efficiency around 36%.

The conversion of solar energy to hydrogen by means of water splitting is one of the most promising ways to achieve clean and renewable energy. Since the discovery of photocatalytic splitting of water on a Ti02 electrode in 1972, there are numerous works focused on the study of the properties of this material under light illumination. Wang et al. (2016) fabricated rutile Ti02 nanorods grown on the inner surface of arrayed ana- tase Ti02 nanobowls as a new type of photoanodes for photoelectrochemical water splitting. The rutile/anatase junction improves charge separation; under solar light irradiation this hierarchical structure has a photocurrent density of 1.24 mA cm-2 at 1.23 eV, which is almost two times higher than pure Ti02. Besides, a tuned heterojunction between Ti02 and other compounds has been also exploited in the photocatalysis field. Figure 5.16 shows a sandwich-structure CdS/Au nanoparticles/Ti02 nanorod array in which the presence of CdS and Au leads to enhanced photocurrent and light absorption. In this case, the interface plays a paramount role in the photocatalytic behavior, as reported by Li et al. (2014). Appropriate doping and tailored heterojunctions are among the most employed strategies to improve the photocatalytics response in Ti02- based devices.

Recent efforts are invested in the analysis of single-molecule photocatalysis on Ti02 surfaces in order to assess the involved physico-chemical mechanisms and design improved photocatalytic response. As an example, Yang et al. (2016) investigated photoin- duced water dissociation under UV irradiation on rutile-Ti02 surfaces and demonstrated the role played by hydrogen bond network in the H20 molecule dissociation by H atom transfer.

(a) Sandwich-microstructure formed by CdS/Au/Ti0 nanorods,

FIGURE 5.16 (a) Sandwich-microstructure formed by CdS/Au/Ti02 nanorods, (b) SEM image of a top view from the nanorod array, (c) Photocurrent-applied potential (J-У) curves obtained by different irradiation conditions (full-spectrum, visible light (>430 nm), and ultraviolet (275-375 nm)).

Reproduced with permission (Li ct al., 2014). Copyright 2014, The American Chemical Society.

Toward Ti02 Phase-Control-Based Devices

In addition to the defect structure, doping, and morphology, most of the ТЮ2 applications also depend on the crystallographic phase. Despite the fact that the band gap energy (.Eg) for rutile and anatase phases is similar (Eg ~3.05 and ~3.2 eV at 300 K, respectively), their electrical, optical, and chemical properties are different due to different atomic arrangements and symmetries of the Ti and О atoms within the crystalline lattice. For example, rutile is the most thermally stable Ti02 phase and highly resistant to chemical agents so it is suitable for operation in harsh environments such as in water splitting or photoelectro- chemical applications, while anatase is well known because of the higher photocatalytic activity. However, bulk anatase transforms into the rutile phase at temperatures above ~700 °C, hindering its use in devices that operate at higher temperatures. So, great efforts have been invested to understand and manipulate the anatase to rutile phase transition (ART) temperature as well as developing methods to control precisely the anatase/rutile phase composition. Indeed, mixed anatase/rutile phase demonstrates advantages compared to their individual forms leading to a synergetic effect between the two phases that improves, for example, the photocatalytic activity, the efficiency of DSSCs or the hydrogen production in TiCb-based solar fuels. It is well known that the synthesis method, sample morphology, and dimensions have critical impact on the ART. but doping can also promote/accelerate or inhibit/delay efficiently the ART in a wide range of temperatures (Hanaor, & Sorrell, 2011). Aluminum and iron doping are examples of such effects as shown in Figure 5.17.

Figure 5.17a and b show thermo-XRD patterns of undoped and 10 cat.% Al-doped nanoparticles as the temperature increases, where the asterisk marks the temperature when the rutile phase starts to appear. That temperature increases from 800 °C for undoped material to 920 °C for Al-doped samples. A similar experiment was performed for different concentrations of A1 and Fe, decreasing the temperature down to 600 °C for 20 cat.% Fe-doped nanoparticles as shown in Figure 5.17c (Vasquez et ah, 2014). In addition to thermal annealing, the ART can also be driven by laser irradiation. In this regard, it was observed that Fe doping accelerates the laser-induced ART from several hours (for undoped anatase) to few seconds, contrary to A1 doping that inhibits the phase transformation (Vasquez et ah, 2015). This ability in combination with advanced focused laser sources and precise sample positioning open up new fields for

Thermo-XRD patterns on

FIGURE 5.17 Thermo-XRD patterns on (a) undoped and (b) Al-doped Ti02 nanoparticles, (c) Temperature at which the rutile phase starts to appear in XRD as a function of the concentration and A1 and Fe dopants. Scheme of the laser patterning of rutile phase on 20 cat.% Fe-doped anatase. Raman spectra acquired on (e) laser irradiated and (f) unirradiated areas.

Reprinted with permission from Vasquez et al. (2014) and Vasquez et al. (2015). Copyright (2014) and

(2015) American Chemical Society.

device fabrication, processing, and characterization, as shown in the scheme of Figure 5.17d for a laser-printed pattern (micropatterning) on Fe-doped anatase nanoparticles. The characteristic Raman spectra of rutile and anatase phases acquired on irradiated and nonirradiated areas are shown in Figures 5.17e and f, respectively, with the characteristic phonon lines marked by solid squares. This concept has potential applications in device processing since the phase composition could be spatially controlled at room temperature using appropriate laser power and dopant concentration (Benavides, Trudeau, Gerlein, & Cloutier, 2018; Vasquez et al., 2014; Wilkes, Deng, Choi, & Gupta, 2018). These results have also lead to two patents based on anatase to rutile phase transition (Patent ES2525393B2 (W02016038230A1) (Vasquez et al., 2014) and Patent ES2525737B2 (WO2016046426A1)).

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