Surface Localization of Defects in Black TiO2: Enhancing Photoactivity

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Surface Localization of Defects in Black TiO2: Enhancing Photoactivity or Reactivity Kan Zhang, and Jong Hyeok Park J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b02289 • Publication Date (Web): 08 Dec 2016 Downloaded from http://pubs.acs.org on December 9, 2016

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The Journal of Physical Chemistry Letters

Surface Localization of Defects in Black TiO2: Enhancing Photoactivity or Reactivity Kan Zhang*, Jong Hyeok Park* †

Department of Chemical and Biomolecular Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-

gu, Seoul 120-749, Republic of Korea

Corresponding Author

K.Z ([email protected]), J. H. Park ([email protected])

ACS Paragon Plus Environment

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ABSTRACT In the past several years, surface-disordered TiO2, which is referred to as black TiO2 and can absorb both visible and near-infrared solar light, has triggered an explosion of interest for many important applications. Despite the excellent optical and electrical features of black TiO2 for various photoelectrochemical (PEC) and photochemical reactions, the current understanding of the photocatalytic mechanism is unsatisfactory and incomplete. Based on previous studies, we present new insight into the surface localization of defects and perspectives on the liquid/solid interface. The future prospects for understanding black TiO2 from this perspective suggest that defect engineering at the liquid/solid interface is a potential method of guiding nanomaterial design.

TOC GRAPHIC

KEYWORDS: Black TiO2, disorder, photoactivity, reactivity, interface engineering


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Titanium dioxide (TiO2), which is also called titanium pigment, is widely used as an additive in sunscreen cream because it can efficiently absorb the ultraviolet (UV) light in sunlight. In 1972, a pioneering work by Fujishima and Honda demonstrated that crystalline TiO2 can convert absorbed UV light into a charge flow to split water [1], opening up a wide range of potential eco-friendly applications for solar in chemical fuel conversion and environmental purification [2-5]. In particular, the most stable TiO2 has an excellent band structure that can produce electron/hole pairs with a high redox ability to drive numerous photocatalytic reactions. It is not, however, suitable for practical applications because of several disadvantages. The main challenges are the limited charge flow under sunlight illumination because of its large band gap (3.2 eV) and its rapid charge recombination. Because only ~4% of the solar spectrum lies in the UV region, the solar energy conversion efficiency of TiO2 remains impractical. Therefore, tremendous efforts have been expended in the past decade to harvest visible photons because visible light accounts for ~46% of sunlight illumination [6-8]. It is commonly believed that the electronic structure of a semiconductor is intimately associated with its band gap—specifically, its light-harvesting region—and its subsequent photocatalytic performance [9,10]. In many cases, however, although a suitable value of the band gap is beneficial for light absorption, the reactivity is closely related to several other bulk and surface properties. Indeed, the controlling bulk (charge transport) and surface (chemical reaction) recombination processes are critical for photocatalytic performance (Figure 1).[11] Relatively far-sighted investigations have indicated that these properties of TiO2, including its reactivity and photoreactivity, are strongly related to its defect disorder.[12,13] This is especially true in the defect engineering of TiO2, in which crystal and surface defects are produced by oxygen vacancy, Ti3+ doping or atomic arrangement disorder and can profoundly influence the photoreactivity in terms of its electronic structure,


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charge transport, and surface properties.[14, 15]

Figure 1. Schematics of bulk and surface charge recombination during photocatalysis.

In 2011, a black TiO2 produced by hydrogenation was reported [16]. Unsurprisingly, the dramatic color change from white to black involves near-infrared light absorption (Figure 2a and b). In contrast to cases of doping TiO2 that exhibited various colors because of the introduction of new band levels, the band gap narrowing of black TiO2 was ascribed to a thin disordered layer surrounding ordered TiO2, as shown in Figure 2c and d [17, 18]. However, as noted by Dr. Diebold, the finding that introducing a disordered surface layer in a TiO2 crystal yields better photocatalytic ability is contrary to conventional wisdom [19]. To date, despite various efforts to study defective TiO2 through reduction under various conditions, controversy persists regarding the real role of disordered TiO2 in photocatalysis. On the one hand, the presence of the disordered part narrows the bandgap of TiO2, allowing near-infrared light absorption; however, the photoactivity of black TiO2 does not match its attractive absorption spectra [20, 21]. On the other hand, another issue—whether the disordered part modifies the surface properties of black TiO2, tuning its reactivity reactive with water or other substances—remains unaddressed.


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However, these properties are not strongly supported by this material’s photocatalytic activity. In the past five years, studies on defective TiO2 have continued, and progress has been made on elucidating its chemical composition, electronic structure, and photoactivity mechanisms and in developing new applications. This perspective is intended to bring together the photocatalytic and photoelectrochemical (PEC) aspects to understand the superior reactivity and photoactivity of black TiO2.

Figure 2. a) Pictures of white and black TiO2 nanocrystals. b) Optical absorption spectra of white and black TiO2. Reproduced with permission.[16] Copyright 2011, American Association for the Advancement of Science. High-resolution transmission electron microscopy images: (c) white TiO2 and (d) black TiO2 nanocrystal. Reproduced with permission.[17] Copyright 2013, Nature Publishing Group.


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Do the atomic defects in black TiO2 significantly improve the photocatalytic performance? Atomic defects, which are mainly caused by oxygen vacancies or Ti3+ species, were previously believed to enhance the photocatalytic performance through bandgap narrowing or charge separation. These atomic defects are generally induced via two approaches: inherent oxygen deficiency resulting from the pre-formed synthesis conditions and post-treated TiO2 under reducing conditions. The black TiO2 obtained by high-pressure hydrogen treatment is naturally associated with such atomic defects [22]. However, the formation of oxygen vacancies or Ti3+ species upon TiO2 reduction is supported by several pieces of experimental evidence. (1) The color is tunable between yellow and blue, which is ascribed to d-d transitions.[23] (2) The conduction band (CB) minimum is decreased by ∼1 eV, and the downward CB position usually acts as an electron acceptor because of localized electronic states (Figure 3a) [24,25]. These two characteristics associated with atomic defects were inconsistent with black TiO2, which shows a maximum valance band (VB) upward shift of 2.18 eV [16]. Generally, the upward VB position for the formation of the mid-gap state results from a band of delocalized states (Figure 3b) [26, 27] through which multiphoton excitation might be achieved. However, black TiO2 has only a certain degree of photoactivity under visible light that is similar to the photocatalytic performance gain contributed by oxygen vacancies or Ti3+ species in TiO2 [20, 28]. However, no direct evidence regarding the long-wavelength effects on black TiO2 was reported until recently. Irrespective of light absorption, black TiO2 exhibits much higher photoactivity than oxygen vacancy-containing or Ti3+-defective TiO2. Subsequent investigations involving black TiO2 revealed that the main contributor to its photoactivity is still likely to be oxygen vacancies, which improve charge separation and whole carrier density [29-31]. However, this conclusion does not fully explain why the hydrogenated TiO2 is black or the reason underlying its impressive


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photoactivity beyond the effects of oxygen vacancies or Ti3+ species. Naldoni and co-workers proposed that black TiO2 results from a synergistic effect of the oxygen vacancies on core and surface disorder [32]. Conversely, Tian et al. attributed black TiO2’s characteristics to the surface formation of a Ti2O3 shell accompanied by lattice distortion in the core [33]. Although both explanations for the formation of black TiO2 were consistent with the electronic structure, whether its high photoactivity also originates from these synergistic effects remains unclear. The photocatalytic performance and electronic structure of defective TiO2 relies strongly on the concentration and distribution of oxygen vacancies or Ti3+ [34]. For example, Tan et al. performed a detailed bandgap alteration of atomically defective TiO2 with a gradually increasing concentration of oxygen vacancies or Ti3+ based on first-principles density functional theory (DFT) calculations, demonstrating a downward band shift and the development of multiple bands with widths 0.5-1.5 eV below the CB minimum [35]. Kong et al. observed that decreasing the relative concentration ratio of bulk defects to surface defects in TiO2 nanocrystals significantly improves the charge separation efficiency and, thus, enhances the photoactivity.[13] As a result, the superior photoactivity of black TiO2 can be tentatively attributed to the surface localization of defects, leading users to investigate trap states and reactivity. Leshuk et al. reported the hydrogenation of TiO2 at temperatures ranging from 250 to 450 °C under atmospheric pressure. All the samples showed worse photoactivity than the unhydrogenated one, allowing conclusions to be drawn regarding the excessive formation of bulk defects.[36] Yu et al. used similar synthesis conditions for the hydrogenation of TiO2 and found that bulk defects diffuse to the surface at temperatures above 600 °C, thereby greatly improving the photoactivity [37].


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Figure 3. Excitation state of oxygen vacancy-containing or Ti3+-defective TiO2 (a) and black TiO2 (b). Furthermore, in addition to the photochemical reactions of black TiO2, this material’s PEC performance demonstrates significantly enhanced conductivity and carrier density [38-40]. The characterized properties are exactly the same as those of atomically defective TiO2 (oxygen vacancies or Ti3+ doping) in which the conductivity and carrier density are also increased [4146]. However, another electrochemical investigation revealed that the surface disordered layer can serve as a passivation layer that enhances the surface water oxidation kinetics [47, 48] and stability [49]. In our recent work, we systemically investigated the surface properties of black TiO2 in light of its PEC properties and demonstrated the remarkable narrowing of the depletion layer and increasing electron diffusion length, which facilitated unlimited electron collection in a 20-µm length of TiO2 (Figure 4) [50]. Because the photocatalytic reaction is included among the surface thermodynamics reactions, the findings regarding the solid/liquid interface demonstrate the unusual photoactivity and reactivity of black TiO2, which affect the overall photocatalytic performance in a different manner than simple atomic defects in the bulk material. On the other


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hand, if the disorder layer could generate electron/hole pairs under light illumination, the open circuit voltage of PEC device should be changed. Unfortunately, the classic phenomenon regarding bandgap narrow in form of heterojunction could not be reported as far as we known [50].

Figure 4. (a) Schematic of the configuration of black TiO2 wire arrays. (b) Plots of the photocurrent densities (1.23 V vs reversible hydrogen electrode [RHE]) and enhancement factors as a function of the length of the wire arrays. Energy diagram for the charge-transfer equilibrium with the electrolyte: (c) TiO2 and (d) black TiO2. Reproduced with permission.[50] Copyright 2014, Wiley.


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Does surface disorder in black TiO2 significantly improve its photocatalytic performance? The photocatalytic process involves three crucial steps: light absorption, charge separation in the bulk region, and charge recombination on the surface. The first two processes have received the most attention, with much less attention being focused on the third. Presumably, this step is taken for granted after loading the co-catalyst. Our recent work developed a Janus crystalline/disordered TiO2 nanoparticle with an open-ended interface that exhibited impressive photocatalytic H2 activity with no co-catalyst (Figure 5a). The time-correlated single-photon counting (TCSPC) results indicated that the Janus crystalline/disordered TiO2 nanoparticle efficiently suppresses indirect electron/hole pair recombination at surface traps or defects [51]. The Schmuki group also developed black TiO2 via hydrogenation or high-energy proton implantation and claimed that the disordered region of this material acts as a catalytic center for co-catalyst-free H2 evolution (Figure 5b) [52,53]. In principle, from a catalytic perspective, the presence of structural defects helps to decrease the coordination number of the neighboring sites on the material surface, thereby tailoring the electronic structure and catalytic activities [54]. Therefore, both of the above findings seem to allow co-catalyst-free H2 evolution from black TiO2 because both the surface trap state and the catalytic center are crucial for the evolution of H2 from water on the photocatalyst surface. In the previous two decades, surface defects (i.e., both Ti3+ and oxygen vacancies) were demonstrated to spontaneously cause water dissociation by forming a metastable surface active complex, resulting in high reactivity [55-57]. In contrast, the surface disordered layer efficiently covered the intrinsic trapping states of TiO2, creating an opportunity for water redox outside of other intermediaries [58, 59]. However, the co-catalytic activity of the disorder part has not yet been confirmed.


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Figure 5. (a) Comparison of the co-catalyst-free H2 generation and cycling performance of P25 and blue P25 after 1 day of continuous reaction using methanol as a sacrificial agent. (b) Photocatalytic H2 production under open-circuit conditions in methanol/water (50/50 vol %) with TiO2 nanotubes and nanorods treated in different atmospheres under AM 1.5 (100 mW/cm2) illumination. Reproduced with permission.[51] Copyright 2016, The Royal Society of Chemistry. Reproduced with permission.[52] Copyright 2015, American Chemical Society. Similar performance gains in terms of both PEC water splitting and photocatalytic H2 generation have been achieved using a distinct thin amorphous TiO2 layer [60-63]. The surface amorphous layer can passivate surface states at the electrode/electrolyte interface or affect the space-charge layer width to decrease electron-hole recombination pathways. Generally, amorphous materials exhibit a set of universal properties that are distinct from those of their crystalline counterparts. A crystal can evolve away from perfect crystalline order and eventually develop into an amorphous state when disorder is introduced [64, 65]. However, how disorder causes such deviation in the properties of a solid from those of a crystal remains unclear; indeed, no standardized model exists for theoretical simulations of the disordered structure [19]. In contrast to the thinner amorphous layer that usually shields light penetration from reaching the


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active core materials, this disordered layer unexpectedly showed gradually improved light absorption as its thickness increased [66-71]. These studies indicate that the disordered surface in black TiO2 may have unusual properties that affect the bulk material. Charge transport between the ordered and disordered parts in black TiO2 Although increasing interest has developed regarding the photochemical or PEC properties of black TiO2 over the past five years, these properties have been attributed to the prominent charge separation efficiency in the bulk region. By performing calculations based on DFT and the Perdew-Burke-Ernzerhof (PBE) functional, Liu et al. demonstrated that the charge separation mechanism in black TiO2 is based on the localization of holes accompanied by the delocalization of electrons [18]. The wave function of itinerant conduction electrons spreads throughout both the crystalline and disordered regions, but the holes are localized in the midgap state of the disordered part. The photo-induced charge generation and separation in black TiO2 are treated as integral behaviors, regardless of their respective occurrence in the crystalline or disordered part. Our recent work revealed spatially separated crystalline and disordered regions in Janus TiO2 and determined the deteriorated charge separation in fully disordered TiO2. However, rapid charge separation could be confirmed when the disordered TiO2 was placed in heterogeneous contact with crystalline TiO2 [50]. The results clearly showed the strong synergistic effect on charge transfer between the crystalline and disordered parts. The Xu and Jia group focused on interfacial charge transfer at the crystalline and disorder junction and confirmed that unparalleled metallic conduction is possible at their interface [72]. To date, however, the alignment of charge transfer routes with type II or the movement of these routes from the crystalline region to the disordered region has not been observed. In addition, whether the electronic states of the Ti3+ species are localized or delocalized remains unknown, whereas the conductivity and charge


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carrier density of black TiO2 have been shown to be orders of magnitude higher than those of crystalline TiO2 [35, 73]. Consequently, once a charge is transferred to the disordered part, its superior conductivity will accelerate charge transport. Contradicting concerns Because of its unprecedented texture and unusual electrochemical and optical properties, black TiO2 has been the subject of substantial research activities investigating its application in photocatalytic or PEC water splitting. However, its formation and photocatalytic mechanism remain somewhat controversial. Chen et al. attributed the formation of black TiO2 to H doped into the TiO2 lattice with H-Ti bonds. Danon et al. suggested that the black color of hydrogenated TiO2 originates from the doping of Cr ion derived from the stainless steel reactor [74]. Indeed, hydrogenated TiO2 was found to be blue when created in a quartz reactor instead of a stainless steel reactor. Other explanations proposed for the black color include the formation of titanium sub-oxides, such as Ti2O3 [33], Ti4O7 [75], or other Magnéli phase crystals [76] via strong reduction. Another issue is the photocatalytic mechanism of black TiO2 with a core/shell structure. Photocatalysis mainly occurs at the surface, which requires not only efficient electron/hole separation in the bulk but also charge transport to surface redox sites. Zhu et al. demonstrated that the photocatalytic H2 performance of platinized black TiO2 is much worse than that of hydrogenated platinized TiO2 [77]. These results suggest that the surface disorder involves confined itinerant electrons migrating to active sites of the TiO2 surface to some extent, although the disordered surface is subsequently loaded with Pt. Nandasiri et al. demonstrated that the H implantation into (110) rutile TiO2 lattice is detrimental to its stability because of the outward diffusion of H atoms, in addition to desorption and surface reduction [78], which contradicts


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other reports. Does other possibility of surface textures affect its photoactivity and reactivity? Actually, the surface reactivity of TiO2 was triggered a lot interest for a long time. The earlier reviews titled “photocatalysis on TiO2 surfaces: principles, mechanisms, and selected results” and “surface science studies of the photoactivation of TiO2-new photochemical processes” have emphasized the importance of TiO2 surface chemistry [79, 80]. In particular, the reactivity of TiO2 caused by the surface defects, such as bonding energies with molecule, chemisorbed energy and hydrophilicity are an element of coherence in understanding. Most of the surface reactivity on TiO2 have been concerned with various theory and practice, their photocatalytic and photochemical activities were proved to associate with sub-band-gap bulk and surface electronic states of TiO2 [81-87]. In consequence, the surface properties are closely related to TiO2 polymorphs. So, what is the surface reactivity of disorder layer when the disorder layer formed on the surface of different TiO2 polymorphs? Liu et. al investigated the co-catalytic activities of hydrogenated black TiO2 with different polymorphs (anatase, rutile, anatase/rutile). Interestingly, as shown in Figure 5a, the disorder layer formed on rutile TiO2 surface could not play cocatalytic center for H2 generation, whereas the disorder layer on anatase/rutile TiO2 junction can generate a strong synergistic effect for co-catalytic H2 generation [88]. Therefore, the defect configurations in disorder layer for different TiO2 polymorphs seems to be crucial. As Chen et al pointed out, the photocatalytic activity of black TiO2 relied on its preparation method, just like the co-catalytic H2 activity induced by oxygen vacancies, Ti3+ states, Ti interstitials, and surface reconstruction could not observed for black TiO2 reduced by other common methods [21]. Xia and Chen studied the structural properties of hydrogenated black TiO2 from the percentage of various crystalline facets and the ratio of the crystalline/amorphous phases aspects,


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demonstrating that the surface excess pressure and stress induced by disorder layer have a different exposed crystalline facet tendency from the thermodynamically stable order [89]. In line with other electrocatalysts [90, 91], the strain-texture can tune surface electronic potential, which is able to be responsible for catalytic activity. However, to draw an exact conclusion on the surface defect inducing active sites, some in-situ analysis tools targeting catalytic hotspots are expected, for example, scanning electrochemical microscopy, super-resolution fluorescence imaging et al. Liu’s report lefts behind another question that is how the black TiO2 comprising mixed anatase and rutile phases can produce high reactivity. According to theoretical simulation by Deskins’s group, the unpaired electrons are more stable within the interfacial region than in the pure phases alone (Figure 5b). As a result, the interfacial region is potentially more reactive, since localized electrons could be transferred to adsorbates on the surface near the interfaces [92]. For the black TiO2 with mixed anatase and rutile phases, the co-catalytic center located at their interface, namely a disorder layer at anatase and rutile interface, will produce a higher reactivity for co-catalyst free H2 generation of TiO2 or other applications. Xia et al reported that the black TiO2 nanocrystals with mixed anatase and rutile phase can induce excellent microwave absorption

performance,

due

to

the

collective-movement-of-interfacial-dipole

in

crystalline/disordered and anatase/rutile interfaces [93]. The results also suggested that the disorder layer could induce an interfacial polarization effect by the presence of defects (Figure 5c). The located defects in black TiO2 surface are therefore proposed to form interface dislocations or new grain boundaries, electronic defects or phase boundaries. As above mentioned, the disordered TiO2 might be different from amorphous one, a simple experiment can be employed to determine the fact. It is well-known that thermally converting


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amorphous TiO2 to anatase crystalline is more favourable than converting that to rutile TiO2 at low temperature. When a lower sintering process (200 °C) was performed to crystallize fully disordered rutile TiO2 in mixed anatase/rutile crystal, the obtained production still was rutile TiO2, rather than anatase TiO2 (Figure 5d), which is a direct evidence for the difference between amorphous and disorder crystal. So, the atomic structure of the disordered TiO2 needs to be taken into account which should have a special atomic arrangement differing from amorphous TiO2. Then, an elaborate structure characterization in atomic scale should be carried out to understand the ill-defined geometry. Scanning tunneling microscope (STM) or scanning transmission electron microscopy (STEM) could be considered as currently suitable surface analysis tools to define these unexplored atomic arrangements. Furthermore, the first-principles calculations on photoactivity and reactivity can be set up based on geometry model obtained by STM or STEM. An accurate theoretical model for the black TiO2 is being expected to give the right direction for surface engineered photocatalyst as well as recently pointing out electrocatalyst in the clean fuel generation.


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Figure 6. (a) Rates of photocatalytic H2 evolution from various TiO2 particle suspensions under AM 1.5 (100 mWcm2) illumination. a) Comparison of different anatase (an, an1, an2), rutile (ru), and mixed-phase (m1, m2) powders before and after hydrogenation (Hy-) at 500 °C,20 bar (powder Ar/H2 exposed to Ar/H2 stream at 500 °C for 1h; powder Ar exposed Ar steam at 500 °C for 1h). Reproduced with permission.[80] Copyright 2014, Wiley. (b) Electron spin density plot (isovalue = 0.017e/A3) of the interfacial region for a slab with vacuum-exposed surfaces. The results are from a rutile (001)/anatase (100) interface with U = 8.4/6.3 eV for rutile/anatase. Reproduced with permission.[92] Copyright 2015, AIP Publishing LLC. (c) The collective movements of interfacial dipoles at the anatase/rutile and crystalline/disordered interfaces amplify the response to the incoming electromagnetic field and thus induce enhanced microwave absorption performance. Reproduced with permission.[93] Copyright 2013, Wiley. (d) XRD pattern of P25 (mixed anatase/rutile TiO2), blue P25 with disordered rutile TiO2 (prepared by ref. 51) and re-annealed blue P25.


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Perspective regarding the surface localization of defects Black TiO2 has been the subject of extensive scientific research seeking to understand its chemical composition and electronic structure and the origin of its photocatalytic activity. Although some issues that may affect its functions remain unaddressed, the high conductivity and narrow bandgap of black TiO2 favor numerous applications relating to electrochemistry, such as batteries [94-97], supercapacitors [98, 99], fuel cells [100] solar cells [101, 102] and PEC sensors [73]. However, the foundational principles of the above electrochemical devices regarding the photoactivity and reactivity at solid/liquid interface have been ignored. Black TiO2 containing enriched defects at the surface is essentially the same as black TiO2 with other surface modifications [103]. However, the disordered texture caused by the surface localization of defects clearly distinguished this material from its crystalline and amorphous counterparts, especially when it is in intimate contact with liquid electrolyte or reactant. An accurate structure characterization in atomic scale combined with relevant theoretical simulation are therefore expected to promote our in-depth fundamental understanding on surface localization of defects for black TiO2. The surface defect or several nearest subsurface layer could be qualitatively analyzed by X-ray absorption near edge structure (XANES) and X-ray absorption fine structure (XAFS) to probe local structural details around specific metal atoms and are sensitive to the oxidation states of the metal [104, 105]. An atomically layered nanomaterials with disordering can be proposed as an ideal platform for the surface analysis and electron microscopy and also can be realistic model for theoretical simulation. We therefore appeal the material scientist, surface chemist and physico-chemist to pay their attention to this respect. Further research on black TiO2 should involve interface characterization during various electrochemical reactions rather than focusing exclusively on optical and electrical properties. The final objective should


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be to modulate interfacial reactions by engineering disorder so that this material can be used in numerous energy-related applications.


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ASSOCIATED CONTENT AUTHOR INFORMATION Kan Zhang is currently a research professor in the Department of Chemical and Biomolecular Engineering, Yonsei University, Republic of Korea, and was a postdoctoral research fellow at the same institution. He obtained his Ph.D. degree from the SKKU Advanced Institute of Nanotechnology (SAINT) at Sungkyunkwan University, Republic of Korea, in 2015, and received the “Outstanding Self-Financed Students Abroad (Korea)” award in 2013 from the Chinese government. His research interests involve the synthesis and modification of metal oxides and sulfides for solar energy conversion and electrochemical energy storage. Jong Hyeok Park is an associate professor in the Department of Chemical and Biomolecular Engineering, Yonsei University, Republic of Korea. He received his Ph.D. in chemical engineering from KAIST, Republic of Korea, in August, 2004. He then joined the University of Texas at Austin, TX, USA, as a postdoctoral researcher in 2004 (under Prof. Allen J. Bard). From 2008 to 2014, he was an associate professor at Sungkyunkwan University, Republic of Korea. He is the author or co-author of 200 papers and 50 patents.

Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by the NRF of Korea Grant funded by the Ministry of Science, ICT, and

Future

Planning

(NRF-2013R1A2A1A09014038,

2015M1A2A2074663,


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2016M3D3A1A01913254 (C1 Gas Refinery Program)). This work was supported in part by the Yonsei University Future-leading Research Initiative of 2015(2015-22-0067).


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Surface Localization of Defects in Black TiO2: Enhancing Photoactivity

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