Black TiO
 2
 : What are exact functions of disorder layer

Funding information National Research Foundation of Korea, Grant/Award Numbers: 2015M1A2A2074663, NRF‐ 2019M1A2A2065612, 2019R1A4A1029237; Korea Institute of Energy Technology Evaluation and Planning, Grant/Award Number: 20163010012450 Abstract Among the substantial amount of photocatalyst materials, TiO2 has been enthusiastically studied for a few decades due to its outstanding photocatalytic activity and stability. Recently, black TiO2 consisting of approximately 2 nm of thin disorder layer around the surface showed surprisingly high solar hydrogen generation ability. The disorder layer of TiO2 can enhance its light absorption, charge separation, and surface reaction abilities, however exact fundamentals of photocatalytic water‐splitting pathways are still ambiguous. Herein, recent progress and investigations on exact functions of disorder layer and its application in photocatalytic CO2 reduction will be discussed. Throughout the comprehensive studies on disorder layer of TiO2, disorder engineering on photocatalyst materials will suggest the further extension of developing solar‐ fuel production technologies.


| INTRODUCTION
Since Fujishima and Honda's photoelectrochemical water splitting from TiO 2 , various kinds of metal oxides with p-type or n-type semiconductors have been focused for high solar-to-hydrogen conversion efficiency. 1 TiO 2 is the most popular photocatalyst in terms of not only outstanding photocatalytic activities including water splitting for hydrogen and removal of organic pollutants for environmental applications but also excellent photocorrosion stability. [2][3][4] However, the photoconversion efficiency of TiO 2 has been hampered by its large bandgap energy around 3.2 eV, which means that only~4% of solar light can be harvested. 4,5 To solve this drawback, doping strategies with transition metal ions (eg, Fe, Mn, Cr, V, and Cu) or nonmetal atoms (eg, N, C, and S) have been employed to extend light absorption to visible light. 4,6 More recently, disorder engineering of the TiO 2 surface was reported by Chen et al 7 in 2011. Since then, many follow-up research proved that very thin, approximately 2 nm, disorder layer (DL) on crystalline TiO 2 surface can induce strong visible light absorption and their absorption tail is approaching to near-infrared region. 7 The TiO 2 with DL surface absolutely looks very dark, sometimes black, and their photocatalytic activity for H 2 production is much higher than that of pure TiO 2 . The defective crystal, named DL, with high visible light absorption, has been described as the main reason for high solar hydrogen conversion, but visible light harvesting from the DL could not contribute to hydrogen production seriously so far. Nevertheless, metal oxide photoanodes or photocatalyst powders with various DLs not only from hydrogenated TiO 2 but also reduced by chemicals (eg, Li-ethylenediamine) or metals (eg, Mg) have shown superior hydrogen evolution from solar light. [8][9][10] Various methodologies of preparing black TiO 2 and its applications are summarized in Table 1.
In general, solar water splitting or CO 2 conversion from photocatalyst or photoelectrochemical cell consists of three consecutive steps: (a) solar light absorption, (b) hole-electron separation and transport, (c) surface reaction. Because visible light harvesting by DL could not contribute to hydrogen evolution, the main positive effects of DL might be from enhanced hole-electron separation and transport and/or enhanced surface oxidation or reduction reaction. Herein, we will discuss current progress and investigations on functionalities of black TiO 2 and its applications in photocatalytic CO 2 reduction to give thorough understanding and insights on the DL and further extend beyond the intrinsic properties.

| CHARGE SEPARATIONS IN BLACK TiO 2
Within the limited range and amount of light absorption, the separated charge utilization is a critical factor in the solar water-splitting process. 31 Efficient charge separation is promoted by an internal electromagnetic field formed within the electronic structure of a material. Typically, well-matched band alignment between unidentical semiconductors such as metal oxides or chalcogenides, so-called heterojunction, provides an energetically favorable structure for charge separation. [32][33][34] Black TiO 2 has been reported to possess significantly altered electronic structure and surface properties, even it originates from crystalline polymorph of itself. 7 could largely blueshift the valence band maximum (VBM), leading to bandgap reduction of black TiO 2 . As illustrated in Figure 1A,B, Ti and O sublattice distortion of anatase TiO 2 supercell showed blueshifted VBM, accompanied by redshift or no change in conduction band minium (CBM) level. Along with the VBM shift, additionally created intermediate mid-gap energy levels serve as trapping or sinking sites for photogenerated charge carriers. 8,37 Zhang et al also reported significantly blueshifted valence band energy state ( Figure 1C) along with additionally generated intermediate defect states by Perdew-Burke-Ernzerhof calculation ( Figure 1D,E). In this regard, when black TiO 2 or DL is formed on the surface of pristine TiO 2 by disorder engineering (core-shell structure or linearjunctioned), the well-matched heterojunction can drive the efficient separation of electron-hole pairs through type-II band alignment. 8,40 In other words, the photogenerated holes will be more favorably driven to the surface DL due to higher VBM energy to participate in the photocatalytic surface reaction. Cho et al 9 have successfully localized DL within a commercial P25 (composed of both anatase and rutile TiO 2 ) and formed order/disorder multiple heterojunctions within a single TiO 2 nanoparticle. As shown in Figure 2A-D, THE potential and charge distribution across the order/disorder multiple heterojunctions exhibited interfacial polarization across the region, where it can form energy band cascade ( Figure 2E), exceeding H 2 production rate of commercial Pt/TiO 2 system without novel metal cocatalyst. 9 Also, Xia et al 41 proposed that due to collective movements of interfacial dipoles present at the crystalline/disordered interfaces of TiO 2 , the built-up charge at the boundary induces interfacial polarization.
Furthermore, since charge recombination is also influenced by surface adsorption (between absorbate and associated derivate), oxygen vacancy can serve as carrier scavenger. At the disordered surface, adsorbed molecular oxygens (O 2− or O − ) to the defect sites can induce hole trapping at the surface. 42,43 In addition, the DL provides a lower energy barrier for both adsorption and dissociation required under photocatalytic reaction, allowing efficient charge transport at the TiO 2 surface. 31,44 Apart from TiO 2 -based photocatalyst materials, some metal oxide semiconductors such as WO 3 and BiVO 4 are also reported to form similar DL and exhibit enhancements in charge separation performance. The disordered crystal of WO 3 , which can be expressed as W 1−x O 3−y , was confirmed to have altered local electronic structure at the atomic scale, dramatically enhancing charge transfer efficiency of DL/WO 3 . 45 Also, a thin (2 nm thick) surface DL formed on monoclinic bismuth vanadate (BiVO 4 ) could improve charge separation and transfer efficiencies, leading to 2.1 times higher photocurrent than bare BiVO 4 photoelectrode. 46

| SURFACE REACTIVITY ON DISORDER LAYER
Along with the fast electron-hole separation by DL, outermost defect at the disordered TiO 2 surface can improve the surface reactivity. Wang et al 11 reported that the hydrogenated black rutile TiO 2 nanowire array grown on FTO glass can generate four times higher photocurrent density than the pristine rutile TiO 2 . 11 The enhancement of surface reactivity by DL is also found in other photocatalytic materials such as WO 3 and BiVO 4 . 45,46 Meanwhile, Yan et al 47 announced that the improved photoactivity of disordered rutile TiO 2 is not only owing to its increased charge separation efficiency but also due to the enhanced charge injection efficiency. The defects in DL provides the shallow states energy level at the surface, which results in facilitating transfer of minority charge carrier from TiO 2 to water. On the other hand, the previous investigation of Park's group regarding the electrochemical water splitting ability of TiO 2 with DL is shown in Figure 3A,B. 9 In the case of disordered TiO 2 , the overpotential of both oxygen and hydrogen evolution reaction is improved, which implies that the enhanced watersplitting reactivity certainly contributes to higher photocatalytic H 2 production performance.
Although there have been many reports that present enhanced reactivity of TiO 2 by the formation of a defect in DL, the role of a defect in hydrogen or oxygen evolution reaction mechanism is still ambiguous. In fact, atomic defects in TiO 2 are divided into oxygen and titanium vacancies and these vacancies are randomly distributed along DL of TiO 2 . Moreover, TiO 2 has various crystal phases with many facets, each of which has different surface energy state, adsorption energy, and reactivity. 51 Thus, the relationship among the defect, crystal phase, and exposed facet makes a complex understanding on reaction mechanism and the uncertainty of defect cannot suggest a clear answer of why disordered TiO 2 has higher reactivity. Nevertheless, the defect of TiO 2 obviously plays an important role in watersplitting reaction, following that there have been many efforts to discover how defect influences photocatalytic and photoelectrochemical reaction. Valdés et al 48 investigated the water oxidation and photo-oxidation reaction mechanism on the rutile TiO 2 (110) surface by using the density functional theory calculation ( Figure 3C). The result showed that the rate-limiting step during the water oxidation reaction was step A, which is the formation of the adsorbed hydroxyl group at coordinatively unsaturated site. Adsorption of the hydroxyl group on the clean rutile (110) surface is difficult because water molecules rarely dissociate without any point defect. On the other hands, Li et al 49 investigated photocatalytic oxygen evolution on anatase TiO 2 surface ( Figure 3D). In the case of anatase (101), (102), and (001) surfaces, the first proton removal step to form an adsorbed hydroxyl group requires high overpotential, same as (110) surface of rutile TiO 2 . Although the result from Valdés and Li could not suggest the direct correlation of defects on the reactivity of anatase and rutile TiO 2 by using a clean surface model, the rate-limiting step of either water oxidation or photooxidation reaction was confirmed to be the formation of the adsorbed hydroxyl group at the active site.

→
Step A: 2H O + * H O + HO*+H + e . Apart from the mechanistic study on the water oxidation reaction of TiO 2 , the behavior of water molecules nearby the defect on the TiO 2 surface has been widely studied. Interestingly, many reports announced that oxygen vacancy at the bridging oxygen site on both anatase (101) and rutile (110) surface can strongly induce the hydroxyl group adsorption with much smaller freeenergy change of water dissociation than the clean surface ( Figure 3E). 50,52,53 In other words, the absence of oxygen atom at the bridging oxygen site is energetically very unstable, so the oxygen vacancy prefers to be filled with hydroxyl group from the dissociation of the nearby water. Considering that the rate-limiting step of watersplitting reaction on rutile TiO 2 is the formation of adsorbed hydroxyl group, as reported by Valdés et al, 48 the water dissociation ability of defect may help to provide hydroxyl group, which in turn reducing overpotential for the water oxidation reaction. However, the significant deviation arises that the water oxidation reaction starts with the adsorption of the hydroxyl group at fivecoordinated titanium ion, while the hydroxyl group adsorption induced by defect occurs at the bridging oxygen site. 48,49 Moreover, defect in TiO 2 is not only located at the surface but also able to exist at the subsurface, resulting in increasing huge complexity of expecting the role of defect. [54][55][56] Thus, intensive and extensive mechanism studies on defects in DL of TiO 2 are necessary to elucidate how defects improve photocatalytic or photoelectrochemical water-splitting reaction.

| APPLICATIONS IN PHOTOCATALYTIC CO 2 REDUCTION
The utilization of carbon dioxide into valuable products has been recently spotlighted due to the worldwide environmental crisis. It is notable that one of the fascinating strategies to remove carbon dioxide is the photocatalytic reduction of CO 2 into C 1 products, such as methane, methanol, carbon monoxide, and so forth. [57][58][59] However, photocatalytic CO 2 reduction is not energetically favorable because it is generally operated in aqueous media containing carbonate ions. In detail, hydrogen production reaction and CO 2 conversion reaction compete together at the same catalyst surface because the standard reduction potential of CO 2 into C 1 products is closely located at the reduction potential of water ( Figure 4A). 60 Moreover, the negative energy level of the standard reduction potentials of CO 2 makes it harder to chose appropriate photocatalyst materials. Among the limited candidates for photocatalytic CO 2 reduction, TiO 2 is known as the best photocatalyst for CO 2 reduction because of its large bandgap energy with highly negative energy levels of CBM, long charge carrier lifetime, and fast water oxidation kinetics. 57,61 Interestingly, it is reported several times that DL of black TiO 2 can improve photocatalytic CO 2 reduction. 62,63 As mentioned above, fast charge separation and good surface reactivity of DL may utilize photoexcited electron-hole pairs, which results in a higher photocatalytic CO 2 conversion rate. In this chapter, recent reports in photocatalytic CO 2 reduction by disordered TiO 2 are introduced in detail.
F I G U R E 4 A, Schematic illustration of photocatalytic CO 2 reduction mechanism and the standard reduction potentials of CO 2 reduction reactions. Reproduced with permission from Reference, 60  Energetic study of photocatalytic CO 2 reduction on defective TiO 2 was reported by Ji et al. 67 They found that oxygen vacancy at the anatase TiO 2 (101) surface has much higher activity on CO 2 reduction than Ti atom in the perfect surface. Also, Liu et al 68 found that oxygen vacancy can either improve the binding of CO 2 , activation, and dissociation or stabilize the reaction intermediates. In line with the theoretical investigations, Yin et al 64 announced that disordered hydrogenated blue TiO 2 prepared by lowtemperature lithium-ethylenediamine solvothermal reaction outperforms pristine TiO 2 in CH 4 formation rate and the selectivity. In Figure 4B, in situ diffuse reflectance infrared Fourier transform spectroscopy results of blue TiO 2 during photocatalytic CO 2 reduction showed a proportional relationship between the amount of CO 2 − band and the  Figure 4C). Even though the results from Yin et al 64 and Gao et al 65 showed different C 1 products from each TiO 2 , it is notable that black TiO 2 greatly increased its photocatalytic reduction performance from its oxygen vacancy around the surface. The photocatalytic performance of black TiO 2 can be further enhanced by introducing cocatalyst or active facet exposure. Ye et al 28 synthesized inverse opal structured Niloaded black TiO 2 and applied in photocatalytic CO 2 reduction and the introduction of nickel on black titania improved twice of its photocatalytic performance ( Figure 4D). Moreover, Fang et al 66 prepared a high Ti 3+ concentration of reduced TiO 2 with active (001) facet exposure. Interestingly, reduced TiO 2 with (001) facet exposure showed higher CH 4 and CO production rate than normally reduced TiO 2 , while the selectivity of methane production, reached 83.4% ( Figure 4E). The author suggested that the pristine anatase (101) facet cannot overcome electron-hole recombination at the adjacent trapping sites while the coexposure of (001) and (101) facets can separate electron-hole pairs into each facet thereby enhancing photocatalytic performance ( Figure 4F).

| CONCLUSION
Although the true functionalities of black TiO 2 still have somewhat counterintuitive explanations, it is clear that black TiO 2 outperforms its crystalline polymorph with significantly modified photoelectrochemical properties beyond their intrinsic states. However, regardless of disorder-engineering methods, the position of DL must be selectively localized on the surface or within the heterogeneous interface, since recombination process in black TiO 2 is dominantly governed by trap-assisted nonradiative charge recombination. As mentioned throughout the text; (a) the suggested VBM blueshift of black TiO 2 reinforcing charge transfer and transport at the order/disorder interface cannot fully attribute for overall enhancements in separation efficiencies and its recombination pathways, (b) the kinetic properties of DL during water-splitting process is still unclear whether the actual redox reaction takes places on the very surface or the subsurface between the DL and the core. More advanced theoretical studies and experiments to define the nature of black TiO 2 will further complete the overall comprehension of designing semiconductor materials to a selective and optimal state. Such thorough investigations will boost photocatalytic and photoelectrochemical efficiencies without the employment of novel metal cocatalysts, thereby contributing practical applications in overall energy and environmental technologies.