Scholarly article on topic 'Black TiO2 for solar hydrogen conversion'

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Abstract of research paper on Nano-technology, author of scientific article — Bin Wang, Shaohua Shen, Samuel S. Mao

Abstract Titanium dioxide (TiO2) has been widely investigated for photocatalytic H2 evolution and photoelectrochemical (PEC) water splitting since 1972. However, its wide bandgap (3.0–3.2 eV) limits the optical absorption of TiO2 for sufficient utilization of solar energy. Blackening TiO2 has been proposed as an effective strategy to enhance its solar absorption and thus the photocatalytic and PEC activities, and aroused widespread research interest. In this article, we reviewed the recent progress of black TiO2 for photocatalytic H2 evolution and PEC water splitting, along with detailed introduction to its unique structural features, optical property, charge carrier transfer property and related theoretical calculations. As summarized in this review article, black TiO2 could be a promising candidate for photoelectrocatalytic hydrogen generation via water splitting, and continuous efforts are deserved for improving its solar hydrogen efficiency.

Academic research paper on topic "Black TiO2 for solar hydrogen conversion"

Accepted Manuscript

Black TiO2 for solar hydrogen conversion Bin Wang, Shaohua Shen, Samuel S. Mao

PII: S2352-8478(16)30134-4

DOI: 10.1016/j.jmat.2017.02.001

Reference: JMAT 91

To appear in: Journal of Materiomics

Received Date: 1 December 2016 Revised Date: 8 February 2017 Accepted Date: 12 February 2017

Please cite this article as: Wang B, Shen S, Mao SS, Black TiO2 for solar hydrogen conversion, Journal of Materiomics (2017), doi: 10.1016/j.jmat.2017.02.001.

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Graphical Abstract

We review the recent advances of black TiO2 for photocatalytic H2 evolution and PEC water splitting, along with detailed introduction to its unique structural features, optical property, charge carrier transfer property and related theoretical calculations.

[Title Page]

Black TiO2 for Solar Hydrogen Conversion

Bin Wang1, Shaohua Shen2,3*, Samuel S. Mao4,5*

1. School of Science, MOE Key Laboratory for Non-equilibrium Synthesis and Modulation of Condensed Matter, State Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University, Xi'an, 710049, People's Republic of China.

2. International Research Centre for Renewable Energy, State Key Laboratory of Multiphase Flow in Power Engineering, Xi'an Jiaotong University, Shaanxi 710049,

China. Email: shshen_xjtu@mail.xjtu.edu.cn (S. Shen)

3. Xi'an Jiaotong University Suzhou Academy, Suzhou, Jiangsu 215123, China

4. Department of Mechanical Engineering, University of California at Berkeley, Berkeley, CA 94720, United States. Email: ssmao@berkeley.edu (S. S. Mao)

5. Samuel Mao Institute of New Energy, Science Hall, 1003 Shangbu Road, Shenzhen

518031, China

Abstract

Titanium dioxide (TiO2) has been widely investigated for photocatalytic H2 evolution and photoelectrochemical (PEC) water splitting since 1972. However, its wide bandgap (3.0-3.2 eV) limits the optical absorption of TiO2 for sufficient utilization of solar energy. Blackening TiO2 has been proposed as an effective strategy to enhance its solar absorption and thus the photocatalytic and PEC activities, and aroused widespread research interest since Chen's pioneering work in TiO2 hydrogenation. In this article, we reviewed the recent progress of black TiO2 for photocatalytic H2 evolution and PEC water splitting, along with detailed introduction to its unique structural features, optical property, charge carrier transfer property and related theoretical calculations. As summarized in this review article, black TiO2 could be a promising candidate for photoelectrocatalytic hydrogen generation via water splitting, and continuous efforts are deserved for improving its solar hydrogen efficiency.

Keywords: Black TiO2; Photoelectrochemical water splitting; Photocatalytic; Hydrogen evolution

1. Introduction

Solar water splitting for hydrogen evolution could convert solar energy to chemical energy in the form of clean and renewable hydrogen fuel.[1,2] One challenge of this strategy has relied on the development of low-cost, stable and highly active semiconductor as photocatalysts or photoelectrodes for solar water splitting. There are three main steps in the photoelectrocatalytic reaction process occurring on the photocatalysts or photoelectrodes: 1) semiconducting photocatalysts or photoelectrodes absorb solar energy to generate photoinduced electron-hole pairs. 2) the electron-hole pairs are separated in the bulk of photocatalysts or photoelectrodes and migrate to the surface active sites of photocatalysts or the surface of photoelectrodes and counter electrode. 3) the photoinduced electrons and holes on photocatalysts (counter electrode and photoelectrodes) participate in water reduction and oxidation reactions, respectively. Given the very small part of ultraviolet (UV) light in solar spectrum, it's desirable for semiconductors to possess appropriate energy band structure to utilise visible light, which accounts for about 43% of solar energy, as well as high efficiency of charge separation for the photoelectrocatalytic redox reactions.

Due to a couple of advantages including nontoxicity, low cost, earth abundance and excellent chemical stability, TiO2 has been widely investigated for photocatalytic [3,4] and photoelectrochemical (PEC) water splitting [5], CO2 reduction [6] and dye-sensitized solar cells [7]. TiO2 have four types of crystal phases, tetragonal rutile, tetragonal anatase, orthorhombic brookite, and monoclinic TiO2(B), with large

electronic bandgaps of 3.0 - 3.2 eV.[2] This wide gap restricts the optical absorption of TiO2 in the ultraviolet (UV) region, which only accounts for less than 5% of solar energy, leading to the insufficient utilization of solar energy. Therefore, it is of crucial importance to improve the optical absorption of TiO2 in visible region for enhanced photocatalytic and PEC activities. To this end, numerous strategies have been attempted to regulate the electronic energy band structure of TiO2 for the expansion of its optical response from UV to visible and infrared region.[8-11] For example, metal and nonmetal elemental doping has been extensively used to create impurity levels in the band gap of TiO2 to expand its optical response in visible light response.[12-15] Nevertheless, the impurity energy levels created by doping in the forbidden band of TiO2 could always serve as recombination centers for photo-excited charges, which gave rise to greatly decreased photoactivities of doped TiO2.[12]

Different from the traditional strategy of element doping, Chen et al. developed a novel hydrogenation approach to synthesize surface disordered black TiO2. It was revealed that the disordered surface could yield mid-gaps states and up-shift the valence band edge of TiO2 nanocrystals, boosting the visible and infrared absorptions and then the photocatalytic activity.[16] Motivated by Chen's initial study on black TiO2, increasing attentions have been paid on black TiO2 and its various applications in energy conversion and storage.[17,18] Moreover, this hydrogenation concept has been applied to different metal oxides, such as V2O5,[19] TiO2,[20-23] WO3,[24] Fe2Os,[25] and SrTiO3[26] etc, for improved performances in photocatalysis,[20,24-26] lithium-ion battery,[19] fuel cell,[21] CO2 reduction[23] and supercapacitor,[22] etc.

In this article, we will review the recent progress achieved in black TiO2 for photocatalytic hydrogen evolution and photoelectrochemical (PEC) water splitting with the unique crystal structures, optical, electronic and charge carrier transfer properties of black TiO2 discussed in detail, which could provide insightful understanding of mechanisms responsible for the improved photocatalytic and PEC performances.

2. Preparation of Black TiO2

There have been various synthetic methods developed to prepare black TiO2, mainly including reduction of TiO2, oxidation of low valence Ti compounds and some other unique methods[17,18] Generally, during these synthetic processes for obtaining black

TiO2 nanomaterials, oxygen vacancies, Ti3+ or H atom dopants were introduced into the TiO2 lattice structures by reduction or oxidation reactions at various atmospheres. In the reduction methods, hydrogen (H2) gas is the most common reducing agent/atmosphere for the hydrogenation of TiO2. Chen et al. initially synthesized black TiO2 by calcining pure white TiO2 nanoparticles under a 20.0-bar pure H2 atmosphere at about 200 °C for 5 days.[16] Later, different reduction atmospheres, such as H2/Ar, H2/N2, etc., were used to obtain black TiO2 at high-pressure, low-pressure, and even ambient pressure atmospheres.[17] Myung et al. prepared black TiO2 nanoparticles by annealing a yellow TiO2 gel in Ar gas at 400 - 600 oC for 5 h. [27] Tian et al. fabricated black rutile TiO2 by post-annealing ultra-small amorphous TiO2 nanoparticles (smaller than 5 nm) at 970 ±20 K for 2 h in Ar gas.[28] Specially,

Zhu et al. reported facile hydrogenation of TiO2 in the presence of a small amount of Pt at 200-700 oC in 8% H2/N2 with atmospheric pressure, as shown in Figure 1a. During the hydrogenation process, H2 was dissociated into H atoms by Pt and then interrupted the surface lattice O and Ti of P25 when the temperature was above 240 oC, forming Ti-OH groups and oxygen vacancies. As the temperature was increased to 540 oC or higher, atomic H species would diffuse from the surface into bulk, generating localized Ti-O(H)-Ti species. This H2 spillover promoted the formation of TiO2/TiO2-x with a crystalline-disordered core-shell structure. [29] Similarly, Xu et al. developed a facile Pd-catalyzed methodology for instant hydrogenation of rutile TiO2 at room temperature under non-pressurised hydrogen gas flow within several minutes.[30] It was revealed that introduced H2 molecules would spontaneously dissociate on Pd nanoparticles and transform into highly active atomic hydrogen [H] species, which could further migrate along the Pd surface to the TiO2 nanoparticles in the vicinity to reduce the lattice Ti4+ and create point defects such as oxygen vacancies in the treated TiO2 (figure 1b).[30] Inspired by the idea of reduction methods for black TiO2, different metals (Al,[31] Mg,[32] Zn[33]), imidazole,[34] ascorbic acid,[35] NaBH4,[36] and CaH2,[37] etc., were also evidenced as effective reducing agents to prepare black TiO2. Other physical and electrochemical reduction methods, such as H2 plasma,[38] electron beam treatment,[39] proton implantation[40] and electrochemical reduction[41] were also reported to synthesize black TiO2. In these oxidation methods for synthesis of black TiO2, different compounds containing low valence Ti species, such as TiH2,[42] metallic Ti,[43] TiO,[44] Ti2O3,[45] TiCl3,[46] TiN,[47] were incompletely

oxidized to TiO2_x with high-concentration oxygen vacancy or Ti3+, which appeared in

black and presented strong absorption in visible and infrared light.

Figure 1. a) Strategies for the hydrogenation of Pt promoted TiO2 to obtain the TiO2@TiO2-x with a crystalline-disordered core-shell structure. Route 1: hydrogenation of TiO2 followed by the loading of Pt; route 2: hydrogen spillover involved simultaneous hydrogenation of Pt/TiO2. Reprinted with permission from ref. 29. Copyright 2014, The Royal Society of Chemistry. b) Schematics of the approach for facile hydrogenation of TiO2 in the presence of Pd. Reprinted with permission from ref. 30. Copyright 2016, The Royal Society of Chemistry.

Except for the reduction and oxidation methods, there were some other unique methods to prepare black TiO2. For example, Fan et al. reported that the white amorphous TiO2 could turn to black, on account of the incorporation of hydroxyl groups into TiO2 matrix as induced by powerful ultrasonic treatment.'48' Ullattil and

Periyat synthesized black anatase TiO2 with rich oxygen vacancies via sol-gel

assisted microwave strategy by incorporating Mn into TiO2 lattice. The obtained black TiO2 with narrowed bandgap of 1.72 eV showed enhanced absorption in visible light region. This is because the Mn2+ with a larger size (0.83 A) than anatase Ti4+ (0.74 A) could force to displace the lattice oxygen, leading to the formation of oxygen

vacancies.[49] Chen et al. discovered that the color of TiO2 nanoparticles suspension

changed from white to black after laser irradiation for 120 min, because Ti species and disordered structure were induced at the near-surface region of TiO2 nanospheres under high-energy laser irradiation.[50] Other methods, such as microwave-induced plasma,[51] vacuum treatment,[52] and microwave-assisted ionic liquid synthesis,[53] etc., were also applied to prepare black TiO2. For these different synthesis methods of black TiO2, more details could be found in the review written by Liu et al. and Chen et al. and related literatures[17,18]. The intrinsic properties of black TiO2 fabricated by various methods will be introduced in the following sections.

3. Properties of Black TiO2

3.1 Structural properties of black TiO2

As widely reported, black TiO2 has presented excellent photocatalytic and PEC performances, as resulted from structural and compositional modification, such as disordered surface structure, Ti3+/oxygen vacancies, Ti-OH and Ti-H groups, which will benefit optical absorption as well as charge carrier separation.

3.1.1 Disordered surface structure

Chen et al. reported a unique black TiO2 nanocrystals with disordered surface structure by hydrogenation for the first time.[16] The pristine TiO2 nanocrystals displayed well-resolved lattice fringes, as shown in Figure 2A. After hydrogenation, the black TiO2 possessed a crystalline TiO2 core and a highly disordered surface

overlayer with ca. 1 nm in thickness (Figure 2B). In such disordered-engineered black TiO2 nanoparticles with a crystalline-disordered core-shell structure, as shown in Figure 2C, D, a structural deviation from the standard crystalline anatase was readily seen at the outer layer, where the straight lattice line was bent at the edge of the nanoparticle, and the plane distance was no longer uniform (Figure 2D).[54]

Distance I nm

Figure 2. HRTEM images of TiO2 nanocrystals (A) before and (B) after hydrogenation, respectively. Reprinted with permission from ref. 16. Copyright 2011, American Association for the Advancement of Science. (C) HRTEM image and (D) line analysis of black hydrogenated TiO2 nanocrystal prepared by high pressure H2 treatment. Reprinted with

permission from ref. 54. Copyright 2013, Nature Publication Group.

In other independent studies, the disordered structures were also successfully

introduced into the near-surface region of hydrogenated TiO2 nanoparticles, forming a

crystalline-disordered core-shell structure, by different methods. For examples, Wang

et al. prepared black TiO2 nanoparticles by H2 plasma (200 W) in a thermal plasma furnace at 500 oC for various times. It was demonstrated that the H-doped black TiO2 had a hydrogen-stabilized amorphous outlayer surrounding a crystalline core, forming an amorphous shell-crystalline core structure (TiO2@TiO2-xHx).[38] The same group also reported an aluminium reduction approach for mass production of black TiO2 with a crystalline core-amorphous shell structure (TiO2@TiO2-x).[31] In these studies, however, the chemical composition and structure of the disordered shell in black TiO2 remain poorly understood. In 2015, Tian et al. studied the chemical composition and the atomic and electronic structure of the disordered shell in black TiO2 using advanced transmission electron microscopy methods.[28] They found that the outermost layer of black TiO2 nanoparticles consisted of a disordered Ti2O3 shell. As shown in Figure 3, a transition region, which consisted first of four to five monolayers of defective rutile (marked by the white dashed rectangle), containing Ti interstitial atoms (marked by yellow arrows), and then capped off by an ordered reconstruction layer of interstitial Ti atoms (marked by the solid white rectangle), connected the disordered Ti2O3 shell to the perfect rutile core. This reconstructed layer served as a template for the disordered Ti2O3 layers formed by interstitial diffusion of Ti ions.

Figure 3. Atomic-resolution high-angle annular dark-field (HAADF) image (left) and the schematic of the surface of black rutile TiO2 nanocrystal (right), which was derived from the amorphous TiO2 nanoparticle annealed in Ar atmosphere. Reprinted with permission from ref. 28. Copyright 2015, American Chemical Society

3.1.2 Ti3+ and oxygen vacancies

Ti3+ and oxygen vacancies have been observed in black TiO2 prepared by different methods.'43,55-58' Some advanced techniques, such as X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR) and Raman spectroscopy

were employed to characterize the existence of Ti and oxygen vacancies. Wang et al. hydrogenated TiO2 nanosheets with exposed {001} facets by calcining the pristine TiO2 under a high pressure (10 bar) H2 atmosphere at 400 °C for 2 h.'59' The Raman spectra (Figure 4a, b) showed that the peak intensities of hydrogenated TiO2 nanosheets was lower than that of TiO2, and its Raman modes at 144 cm-1 and 636 cm-1 were shifted to a higher frequency, indicating that the formation of oxygen

vacancies by hydrogen treatment. Liu et al. found that Ti and oxygen vacancies were introduced in the bulk and surface of TiO2-x nanoparticles synthesized by a simple interface ion diffusion-redox reaction using TiH2 and H2O2 as precursors.'60' The XPS data showed that additional peaks of Ti 2p3/2 (457.4 eV) and 2pm (463.2 eV)

located at lower binding energies (Figure 4c), revealing the existence of Ti3+ in the black TiO2 nanoparticles. Grabstanowicz et al. prepared black TiO2 by oxidizing TiH2 in H2O2 oxidation followed by calcinations in Ar at 630 °C.'57' The low temperature

(4.2 K) electron paramagnetic resonance (EPR) spectra was conducted to characterize the presence of Ti . As shown in Figure 4d, two paramagnetic signals with g = 1.975 and g = 1.943, which were consistent with the perpendicular and parallel components of the axially symmetric lattice Ti centers, were detected, suggesting the existence of Ti3+ in the black Ti02.

3250 3300 3350 3400 3450 3500 3550

454 456 458 460 462 464 466 468 B lGaussl

Binding energy (eV)

Figure 4. a), b) Raman spectra and c)Ti 2p XPS of Ti02 sheets and hydrogenated Ti02 nanosheets with exposed {001} facets. Reprinted with permission from ref. 59. Copyright

2012, The Royal Society of Chemistry, c) High-resolution XPS spectra of Ti 2p for black Ti02 prepared by oxidation of TiH2 in H202. Reprinted with permission from ref. 60 Copyright 2014, Elsevier, d) EPR spectra of black Ti02 synthesized by oxidation of TiH2 following calcination in Ar at 630 °C. Reprinted with permission from ref. 57. Copyright

2013, American Chemical Society.

3.1.3 Ti-OH and Ti-H groups

Ti-OH and Ti-H groups were often reported in the black TiO2 nanomaterials.[16, 21, 38, 61, 62] Chen et al. found a shoulder peak of Ti-OH in the O 1s XPS spectra in the hydrogenated black TiO2 nanoparticles synthesized at 200 oC for 5 day.[16] Similar results have also been reported in the studies of hydrogenated TiO2 nanowires[61] and hydrogenated TiO2 nanotubes.[22] Wang et al. found an additional broad Ti 2p peak centered at 457.1 eV and a slightly broader O 1s peak at about 531.8 eV in black TiO2-xHx, which were attributed to the surface Ti-H bonds and Ti-OH bonds, respectively. They also characterized the Ti-OH groups by Fourier transform infrared (FTIR) spectra, and observed extra peaks at 3685, 3670, 3645, and 3710 cm-1 in the FTIR spectra of hydrogenated TiO2 prepared by H2-plasma treatment.[38] The former three peaks were characteristic of the tetrahedral coordinated vacancies, and the last peak at 3710 cm-1 corresponded to the terminal OH groups, which could be attributed to the H atoms embedded in the TiO2 network.[38] They further confirmed the appearance of Ti-OH groups by 1H nuclear magnetic resonance (NMR) spectra, and claimed that the additional signals at 5 = 0.4 and 0.01 ppm could be assigned to the terminal and internal hydroxyl groups of anatase, respectively, which were associated with H atom located in the disordered surface layer of the black TiO2.[38] 3.2 Optical properties

The color of hydrogenated TiO2 could change from white to pale yellow,[63] yellowish green,[61] blue,[33,42,64,65] gray,[33,44,55,56,61,64"67] brown,[62,67] or black,[16,31,32,4°,43,51,61,67"71] depending on the preparation methods and conditions. Chen et al. reported that the onset of optical absorption of the black hydrogenated TiO2 nanocrystals was

redshifted to 1200 nm corresponding to the band gap of about 1.0 eV, as shown in Figure 5A.[16] The abrupt change in the absorbance spectra at approximately 806.8 nm suggested that the optical gap of the black TiO2 nanocrystals was substantially narrowed by hydrogenation. It was further revealed that the valence band maximum energy of black TiO2 nanocrystals shifted toward the vacuum level (Figure 5B), and the optical transition from the blue-shifted valence band edge to the disorder-induced conduction band tail states was presumably responsible for much narrowed band gap of black TiO2, as shown in Figure 5C. Similar shift in valence band was also observed in other studies on the black TiO2 nanomaterials obtained by various methods.[36,39,67,68]

(a) \<b)

750 950 1150

l VÍb) black TÍ02

(a) white TiOz

........

B — black Ti02

— white TiC>2

VB XPS

i . i (1.26 eVf ' i.i.

t 3.3 eV 2.18 eV i

conduction band

valence band

500 700 900 1100 1300 20 15 10 5 0

Wavelength / nm Binding Energy / eV

Black Ti02 white Ti02

Figure 5. A) Spectral absorbance and B) Valence-band XPS spectra of the white and black TiO2 nanocrystals. The inset in A enlarges the absorption spectrum in the range from approximately 750 to 1200 nm. C) Schematic illustration of the DOS of disorder-engineered black TiO2 nanocrystals and pristine TiO2 nanocrystals. Reprinted with permission from ref. 16. Copyright 2011, American Association for the Advancement of Science.

However, there has not been a consistent mechanistic understanding of the enhancement in optical absorption of black TiO2. Fan et al. revealed that the color

change of black TiO2 prepared by ultrasonic treatment was attributed to the hydroxyls groups introduced on amorphous TiO2, which altered the electronic structure and induced the localized band bending, resulting in improved optical absorption as well as narrowed band gap.[48] Liu et al. demonstrated that the occupied gap states induced

by oxygen vacancies and Ti3+ led to the visible-light absorption of hydrogenated TiO2 synthesized by NaBH4 reduction.[23] Tian et al. claimed that the disordered Ti2O3 shell, which was a narrow-band-gap semiconductor, accounted for the optical absorption in visible region for the black TiO2 prepared by pulsed laser vaporization.[28]

3.3 Theoretical calculation

It was frequently reported that hydrogenation could introduce mid-gap states or shallow dopant levels in the forbidden band of pristine TiO2, resulting in reduced bandgap and enhanced optical absorption properties.[16'36'39'68'69] Theoretical calculation revealed that one reason for the reduced bandgap of TiO2 is the introduce of mid-gap states. Chen et al. revealed that H atoms were bonded to O and Ti atoms. Two groups of mid-gap states were created at about 1.8 and 3.0 eV, respectively (Figure 6a).[16] The higher-energy group was derived from the Ti 3d orbitals, while the lower-energy group was hybridized from O 2p and Ti 3d orbitals, which accounted for the blue shift of valence band edge (Figure 6b). The introduced H atoms might passivate the dangling bonds and stabilize the lattice disorder, and make a substantial contribution to either state. Similar results have also been reported in many independent studies by other groups. Based on periodic density functional

T-)1 I r 11' \ n C 1 TT J

theory calculations, Raghunath et al. claimed that the interstitial H atoms could induce mid-gap at 1.2 eV below the CB, which were mainly formed by the H s and Ti 3d, and acted as extra electron donors, leading to the shift of fermi level of TiO2 toward its conduction band.[72] Deng et al. revealed that real-hydrogen passivation of anatase TiO2 could create an occupied gap state at about 0.7 eV above the host VBM, which was mainly derived from Ti-H bond with H s and Ti s and d characters, effectively reducing the band gap.[73] Moreover, Pan et al. revealed that the H doping in TiO2 is phase-dependent from DFT and PBE-GGA calculations. The interstitial H atom could narrow the bandgap of three TiO2-anatase, rutile and brookite, while the substitutional hydrogen could only narrow the bandgap of brookite and rutile TiO2.[74]

Figure 6. a) Calculated DOS of TiO2 in the form of a disorder-engineered nanocrystal, an unmodified nanocrystal, and a bulk crystal. (b) Decomposition of the total DOS of disorder-engineered black TiO2 nanocrystals into partial DOS of the Ti, O, and H orbitals. Reprinted with permission from ref. 16. Copyright 2011, American Association for the Advancement of Science. c) model A structure of crystalline Ti210O420H12. and model B

structure of disordered Ti218O436H70. d) The calculated electronic DOS of models A and B. Reprinted with permission from ref. 54. Copyright 2013, Nature Publication Group.

First-principles calculations suggested that the adatoms (H atom) in the surface of nanophase anatase TiO2 not only interacted strongly with the Ti 3d and O 2p states but also induced the lattice disorders, resulting in a considerable contribution to the mid-gap states.[75] Chen et al. further conducted ab initio DFT calculations on large TiO2 clusters with models of Ti210O420H12 and Ti218O436H70 representing a normal white TiO2 nanocrystal of 2.5 nm diameter and a hydrogenated black TiO2 nanoparticle with crystalline-disorder core-shell structure, respectively, as shown in Figure 6c.[54] Compared to Ti210O420H12, the disordered Ti218O436H70 introduced new states extending from the valence band to more than 1.20 eV above the valence band maximum (VBM) and a small number of mid-gap states centered at about 1.80 eV above the VBM (Figure 6d).[54] Based on DFT-PBE calculation, Liu et al. demonstrated that the O distortion gave rise to the blueshift of VBM with conduction band minimum (CBM) unchanged, and Ti distortion led to both the redshift of CBM and blueshift of the VBM.[76] Given more Ti-O bonds fixed with Ti atom than O atom, much more energy was required to deform the Ti sublattice than the O sublattice. Thus, the lattice disorder of anatase was mainly triggered from the O sublattice, which was consistent with the experimental observation that the band gap in black TiO2 was reduced mainly from the large blueshift of VBM.

3.4. Charge carrier transfer property in black TiO2

It is widely accepted that the improved separation and transportation ability of photoinduced charge carriers, related to the introduced oxygen vacancies'36, 55], disordered surface layer'16, 76, 77] and increased donor density'61, 69], was account for the enhanced photoelectrocatalytic activity of black TiO2. Jiang et al. found that the PL emission intensity of hydrogenated P25 TiO2 (H-P25) was only one-third of that of the pristine P25 (Figure 7a), indicating that the recombination rate of photogenerated electrons and holes had been inhibited considerably in H-P25. It was supposed that the oxygen vacancies could serve as electron capture traps and significantly enhance the separation of the charge carriers.'55' As calculated from the slopes of Mott-Schottky plots (Figure 7b), Wang et al. demonstrated that the carrier density of

hydrogenated TiO2 nanowires (2.1 X 10 cm- ) was 4 orders of magnitude higher than

that of the pristine TiO2 (5.3X10 cm- ), due to the increased oxygen vacancies acting electron donors in n-type TiO2.'61' The increased donor density could improve the charge transport in TiO2 and the electron transfer at the interface of TiO2 and conductive substrate. Cui et al. also reported that the donor density of TiO2 could be increased after aluminum reduction due to the introduction of substantial oxygen vacancy, which effectively improved the charge transport in black TiO2.'69' Based on the DFT-PBE calculation, Liu et al. suggested that the surface lattice disorder would blueshift the VBM of anatase by introducing midgap states with its CBM almost unchanged '76' The highly localized nature of the midgap states could enhance the spatial separation of photoexcited electrons and holes in black TiO2. Meanwhile, the

photoexcited holes generated in the anatase core of black TiO2 nanoparticles would be driven to the surface disordered layers due to its higher VBM energy, and then immediately take part in the photocatalytic oxidation reactions.'761 Yan et al. found that the surface disordered layer created on TiO2 nanorod arrays by electrochemical reduction could improve the charge separation in bulk as well as suppress the charge recombination at the electrode/electrolyte interface.'781

Although black TiO2 exhibited prior photoelectrochemical performance than the pristine TiO2, its visible-light photocatalytic activity is still unsatisfactory. Damon et al. conducted a systematic investigation into the ultrafast charge carrier relaxation dynamics of hydrogen-treated TiO2 (H:TiO2) nanowire arrays using time-resolved fluorescence (TRF) and femtosecond transient absorption (TA) pump-probe spectroscopies with both UV and visible excitation.'791 It was demonstrated that hydrogenation created some deep trapping states (oxygen vacancy states, VO), approximately 0.75 eV below the bottom of CB, in the forbidden band of TiO2 by reducing some Ti4+ into Ti3+, as shown in Figure 7c. Under UV light irradiation, the excited charge carrier relaxation to the VO state was relatively slower as compared to that of untreated TiO2 Nanowires. Upon excitation with visible light, however, the charge carrier recombination of the electron-hole pair was very fast at the trap states below the VO. The overall short lifetime of the charge carriers was unfavorable for PEC water splitting, which was consistent with the lack of photocurrent under visible light irradiation.

3.0 H:Ti02 350 «C/

rJ 1 2.0 oC F

-1 0 -0.9 -0.8 -0.7 -0.6 -0 5

Potential (V) vs. Ag/AgCI

H:Ti02 (350 °C)

Potential (V) vs. Ag/AgCI

3.0 eV

Non-fluorescent ~ Trap States

Hydrogen Treated

Vis Pump

Non-fluorescont — States

Hydrogen Treated

Figure 7. a) PL spectra of P25 and H-P25. Reprinted with permission from ref. 55. Copyright 2012, American Chemical Society. b) Mott Schottky plots of the pristine TiO2 and the H:TiO2 nanowires annealed at 350 oC, and the inset in b) is the Mott Schottky plots of H:TiO2 nanowires prepared at 350, 400, and 450 oC. Reprinted with permission from ref. 61. Copyright 2011, American Chemical Society. c) Proposed model for energy levels related to the optical properties and dynamics studies. CB and VB represent the conduction band and valence band, respectively. Reprinted with permission from ref. 79. Copyright 2013, American Chemical Society.

4. Black TiO2 for Photocatalytic Hydrogen Evolution and PEC Water Splitting

Early in 1951, Cronemeyer and Gilleo found that rutile single-crystals reduced by hydrogen had long wavelength absorption as well as increased electrical conductivity due to the ionization trapped electrons in oxygen vacancies.'80, 811 In 1980, Harris and Schumacher studied the photoelectrochemical properties of TiO2 treated in hydrogen at high temperature, and found that hydrogen treatment could reduce recombination centers and increase the lifetime of the holes in TiO2, which were ascribed to the introduction of oxygen vacancies, Ti3+ species, and hydroxyl groups.'821 In this section, the significant advances in photocatalytic hydrogen evolution and PEC water splitting achieved with black TiO2 were discussed in detail. Table 1 and table 2 summarize the recent reports on the black TiO2 for photocatalytic hydrogen evolution and PEC water splitting, respectively. It's obvious that the performances of black TiO2 largely depend on the preparation methods, indicating that more attention should be paid to the experimental procedures and details for the desired properties that we are targeting.

4.1 Black TiO2 for photocatalytic hydrogen evolution

In 2011, Chen et al. synthesized the black TiO2 nanocrystals by hydrogenation treatment at 200 oC under 20 bar H2.'161 The black TiO2 nanocrystals exhibited substantial full-spectrum solar-driven photocatalytic activities for hydrogen evolution in water-methanol solution at a rate of 10 mmol h-1 g-1 with good long-term stability (22-day testing period). As inspired by Chen's pioneering work on black TiO2 for

photocatalysis, significant processes have been achieved on black TiO2 for photocatalytic hydrogen evolution in past five years.'29-32,35,38,42,44,62,66,67,70,83-881

Lu et al. treated the commercial Degussa P25 under 35 bar hydrogen at room temperature for 20 days, and the obtained black TiO2 nanocrystals displayed improved photocatalytic activity with a high H2 generation rate of 3.94 mmol g-1 h-1, while the original P25 only had a H2 generation rate of 0.19 mmol g-1 h-1.'661 The authors thought that the higher photocatalytic activities were correlated with the Uv-vis absorption properties and the surface disordered structures of black TiO2. Rather than high pressure hydrogenation, Tian et al. obtained hydrogenated TiO2 nanobelts by annealing the TiO2 nanobelts under ambient-pressure H2/Ar flow atmospheres at various temperature.'891 The oxygen vacancies and Ti3+ species created by hydrogenation could reduce the band gap of black TiO2 nanobelts and improve the separation of photogenerated charges, leading to the higher photocatalytic activity of the black TiO2 nanobelts (6.32 mmol h-1 g-1) than that of untreated TiO2 nanobelts (0.65 mmol h-1 g-1). Specially, Zhu et al. synthesized black TiO2 by calcining P25 in the presence of a small amount of Pt at 200-700 oC in 8% H2/N2 with atmospheric pressure. The obtained TiO2/TiO2-x with a crystalline-disordered core-shell structure showed the best activity with a production of 1.936 mmol H2 after 5 h irradiation, which is 3 times and 38 times to that of Pt/P25 and bare P25, respectively (figure 8a).'291 It was demonstrated that the introduction of oxygen vacancies and the band tail states during hydrogenation were responsible for its enhanced catalytic activity due to the suppression of electron-hole recombination. Similarly, Xu et al. developed a

facile Pd-catalyzed methodology for instant hydrogénation of rutile TiO2 at room temperature under non-pressurised hydrogen gas flow within several minutes.[30] The prepared black rutile TiO2 treated in O2 (2 vol%)-containing H2/Ar flow exhibited a hydrogen generation rate of 3.32 mmol h-1 g-1 and 130 ^mol h-1 g-1 under AM 1.5 G irradiation and visible light irradiation (X > 400 nm), which were ca. 2.1 times and ca. 11.8 times to that of TiO2 nanoparticles, respectively (figure 8b, c). The enhanced performance of black TiO2 could be ascribed to the higher separation of photo-excited charges and subsequently the stronger redox capability towards the absorbed molecules during the photocatalytic reactions, which could be related with the induced crystalline core/disordered shell structure. Moreover, the black rutile TiO2 also showed well-sustained long-term stabilities in the cyclic photocatalytic hydrogen evolution tests for 10 days, as shown in figure 8d.[30]

Figure 8. a) Time-dependent H2 production over Degussa P25, Pt/P25 and black TiO2 obtained at different temperature (200, 400, 500 and 700 oC) under simulated solar light.

Reprinted with permission from ref. 29. Copyright 2014, The Royal Society of Chemistry. Photocatalytic hydrogen production on different hydrogenated TiO2 under b) AM 1.5 G irradiation and c) visible light irradiation (X > 400 nm). The inset plots in b) and c) are the normalized time-course record of hydrogen generation. d) Cycling measurements of photocatalytic hydrogen evolution by O&H-R/Pd and H-R/Pd under AM 1.5 G irradiation. Reprinted with permission from ref. 30. Copyright 2016, The Royal Society of Chemistry.

Black TiO2 prepared by other reduction methods, such as Al reduction, [31,67] Zn reduction[33], imidazole reduction[34] et al., also exhibited excellent photocatalytic activities. For example, black TiO2 obtained by Al reduction at 500 oC steadily produced hydrogen gas at rates of about 6.4 mmol h-1 g-1 under UV light irradiation. It was revealed that the oxygen-deficient disordered shell were responsible for the enhanced optical absorption and separation of photoexcited charge carriers, leading to the improved performance of black TiO2.[31] Sinhamahapatra et al. reported a new controlled magnesiothermic reduction approach to synthesize reduced black TiO2 under a 5% H2/Ar atmosphere.[32] The maximum hydrogen production rates of the black TiO2 were 43 mmol h-1 g-1 under the full solar wavelength range of light, which were superior to those of previously reported black TiO2 materials. The authors

believed that the presence of Ti3+ and oxygen vacancies could reduce the recombination of electron-hole pairs and enhance the visible light absorption by upward shift the VB top and downward shift the CB bottom, respectively, resulting in the outstanding photocatlytic activity of the black TiO2. Wang et al. demonstrated that

H doping in the amorphous shell of the hydrogenated TiO2 synthesized by H2 plasma could enhance light absorption and reduce charge carriers recombination centers, leading to a high photocatalytic H2 evolution rate of 8.2 mmol h-1 g-1, which was about 13.5 fold higher than that of the pristine TiO2.'38' Similar results have also been reported on black TiO2 prepared by oxidation methods'44,86,90'. For example, Liu et al. reported that black TiO2 nanoparticles obtained by mild hydrothermal treatment of TiH2 in H2O2 aqueous solution showed enhanced hydrogen evolution under visible-light irradiation with a rate of 25 pmol/h, which was 12.5 times higher than that of P25 TiO2.'42' The higher activity of black TiO2 was ascribed to the presence of oxygen vacancies and Ti3+ species, which could suppress the recombination of electron-hole pairs and promote catalytic activity.

Although most black TiO2 boosted visible light absorption, however, its photocatalytic activities under visible light were still unsatisfactory due to the short lifetime of visible-light-excited electrons and holes. In order to overcome this bottleneck, Huang's group synthesized core-shell nanostructured black rutile TiO2 by Al reduction following sulfidation in H2S. The obtained Al-reduced and sulfidized TiO2 showed enhanced photocatalytic H2 production with a rate of 0.258 mmol h-1 g-1.'67' Furthermore, the same group reported that the black P25 nanoparticles with a series of non-metal dopants (H, S, N and I) showed increased photocatalytic activities for hydrogen generation under both UV and visible-light irradiations.'70' The N doped black TiO2 synthesized by Al reduction following nitridation in NH3/Ar showed enhanced photocatalytic H2 evolution from 20 vol% methanol aqueous solution, with

the rate of 15.0 mmol h-1 g-1 and 200 pmol h-1 g-1 under full-sunlight irradiation and visible-light, respectively. The authors claimed that non-metal dopants and oxygen-deficient amorphous shells could reduce the recombination centers and improve the solar absorption, respectively, giving rise to the enhanced activity of black TiO2.

Table 1 The photocatalytic activities for H2 evolution over black TiO2 photocatalysts.

Cocatalyst/Mass(g) Synthesis method Lamp(filter)

Reactant solution

H2 evolution under

UV light/visible Reference light (mmol h-1 g-1)

1 wt% Pt/0.05

0.5 wt% Pt/0.1

0.5 wt% Pt/0.1

1 wt % Pt/0.02

0.5 wt% Pt/0.1

1 wt % Pt/0.05

0.5 wt% Pt/0.1 ~1 wt% Pt/0.025

Hydrogenation in 8 vol % H2/N2 atmosphere

Hydrogen plasma

CaH2 reduction

Hydrogenation in 8 vol % H2/N2 atmosphere

Aluminium reduction

TiN oxidation and NaBH4 reduction

Al reduction following sulfidation in H2S Magnesiothermic

300 W Xe lamp(AM 1.5 G filter)

AM 1.5 solar power system

300 W Xe lamp

300 W Xe lamp(AM 1.5 G filter) 300 W Hg lamp and an AM 1.5G solar light simulator (400 nm cut off filter) AM 1.5 simulated solar power system and 300 W Xe lamp(420 nm cutoff filter) AM 1.5 simulated solar power system 400 W Xe

100 mL 50

vol.% methanol 120 mL 25

vol.% methanol 200 mL 20

vol.% methanol 100 mL 50

vol.% methanol

120 mL 25

vol.% methanol

120 mL 30

vol.% methanol

120 mL 25

vol.% methanol 50 mL 20

7.744[a]/—

8.2/—

5.2/—

10.635[a]/-

6.4/0.14

2.1396[a]/~0.56[a]

0.258/— 43/0.44

reduction under lamp (IR filter, vol.%

H2/Ar atmosphere 400 to 780 nm passed) methanol

-[b] Hydrogenation in H2 or H2/Ar AM 1.5 solar simulator illumination 50 vol.% methanol 7 ^mol h-1 cm-2

Al reduction 300 W Xe 200 mL 20

0.5 wt% Pt/0.1 following lamp(400 nm vol.% 15/~0.2

nitridation in NH3 cutoff filter) 300 W Xe methanol 100 mL 20

1 wt % Pt/0.1 TiH2 oxidation lamp(410 nm cutoff filter) vol.% methanol 1.88/0.1384

1 wt% Pt/0.1 Hydrogenation in H2 atmosphere AM 1.5 solar simulator illumination 300 W Xe 100 mL 20 vol.% methanol 100 mL 10 2.41/—

0.6 wt% Pt/0.025 TiCl3 oxidation lamp(420 nm cutoff filter) 300 W Xe vol.% methanol 100 mL 20 —/0.1167

1 wt% Pt/0.1 TiO oxidation lamp(400 nm cutoff filter) vol.% methanol —/0.0225

1 wt% Pt/0.1 Imidazole reduction 300 W Xe lamp(400 nm cutoff filter) 300 W Xe 120 mL 25 vol.% methanol 100 mL 20 —/0.115

1 wt% Pt/0.1 TiH2 oxidation lamp(400 nm cutoff filter) vol.% methanol —/0.25

1 wt% Pt/0.006 Hydrogenation in H2 atmosphere 150 W Xe lamp (AM 1.5G filter) 10 mL 30 vol.% methanol 571/—

0.5 wt% Pt/0.1 TiH2 assisted reduction method 300 W Xe lamp 200 mL 20 vol.% methanol 5.8/—

1 wt% Pt/0.05 Hydrogenation in H2 atmosphere 300 W Xe lamp 100 mL 20 vol.% methanol 6.32/—

1 wt% Pt/0.1 Hydrogenation in H2 atmosphere 300 W Xe lamp 100 mL 20 vol.% methanol 2.15/—

1 wt% Pt/0.02 Hydrogenation in H2 atmosphere 300 W Xe lamp(AM 1.5 G filter) 100 mL 50 vol.% methanol 29[a]/—

0.6 wt% Pt/0.02 Hydrogenation in H2 atmosphere A Newport Oriel full 50 vol.% methanol(no 10/0.1

0.5 wt% Pt/0.05

1 wt% Pt/0.1

1 wt% Pt/0.5

1 wt% Pt/0.025

0.6 wt% Pt/0.02

Biotemplate method combined with an ethanediamine encircling process

Solvothermal method(TiCl3 and Zn powder as precursors)

Hydrogenation in H2 atmosphere

Hydrogenation in H2 atmosphere

Vacuum heat treatment

Proton Implantation

spectrum solar simulator(AM 1.5 G filter and 400 nm cutoff

filter) A xenon lamp

with a illumination current of 15 A (AM 1.5 G filter)

300 W Xe lamp(420 nm cutoff filter)

300 W Xe lamp

300 W Xe lamp(AM 1.5 G filter) 150 W Xe lamp(AM 1.5

G filter) AM 1.5 solar simulator

volume data)

100 mL 20

vol.% methanol

120 mL 30

vol.% methanol

200 mL 20

vol.% methanol 100 mL 50

vol.% methanol 100 mL 50

vol.% methanol 50 vol.% methanol

3.3[a]/-

6[a]/0.08[a]

3.94/—

8.66[a]/-

-1.23[c]/—

-15^L h-1 cm"2/-

[a] The unit of photocatalytic activities in the original references ware normalized to "mmol

h-1 g-1".

[b] The sample used for photocatalytic H2 evolution test was film, and the unit of the H2 evolution over this sample was "^mol h-1 cm-2" or "^L h-1 cm-2".

[c] The photocurrent density was estimated from the figure in the reference.

4.2 Black TiOi for PEC water splitting

Black TiO2 has also been evidenced as a promising candidate for PEC water splitting, due to its outstanding charge transfer properties and appropriate band structure.131,36,67,69,71,77,91,92,98"1071 Wang et al. hydrogenated TiO2 nanowires by

annealing the pristine TiO2 nanowires in hydrogen atmosphere at various temperatures in a range of 300-550 oC, with color changing from white to yellowish green and finally to black.'61' The hydrogenated TiO2 nanowire (H:TiO2) obtained at 350 oC showed substantially enhanced PEC water-splitting performance with a saturated current of 0.6 mA/cm (Figure 9a) and a highest photoconversion efficiency of ~1.63% at -0.6 V vs. Ag/AgCl (Figure 9b), respectively. It was found that the oxygen vacancies created by hydrogenation could increase the electron densities of black TiO2 nanowires, improving the charge transport in TiO2 and the interface between the semiconductor and the FTO substrate. Moreover, the increased electron density would shift the Fermi level of TiO2 toward the conduction band, leading to the increase of the degree of band bending at the TiO2 surface and the improved charge separation at the semiconductor/electrolyte interface. The authors attributed the improved charge separation and transportation to the much higher PEC performances of the black TiO2 nanowires in the UV region.'61' The improvement in charge separation and transfer abilities have been frequently also reported to be responsible for the superior PEC performances of black TiO2 photoanode synthesized by electrochemical reduction,'108' Al reduction,'31,67,69' CaH2 reduction'91', H2 plasma,'101,104' N2H4 Reduction'103' and NaBH reduction'36' et al. For example, Cui et

al. synthesized black TiO2 nanotube by Al reduction, and the obtained sample showed

-2 -2 remarkably enhanced photocurrent densities of 3.65 mA cm- and 0.065 mA cm- at

0.23 vs. Ag/AgCl with full spectrum and visible light irradiation, respectively, which

were ~7 times and ~1.85 times higher than that of unreduced TiO2 nanotube, as

shown in figure 9c, d.[69] The efficient photoelectrochemical performance of the black TiO2 nanotube were attributed to the improved charge transport and separation, together with the extended visible light response. Zhu et al. claimed that the improved charge transport combined with the facilitated charge separation was responsible for the higher PEC activity of CaH2 reduced TiO2 (1.99 mA cm-2 at 0.23 V vs. Ag/AgCl), which was 4.6 times higher than that of P25 (0.43 mA cm-2).[91] Zhang et al. reported

that the reduced black TiO2 nanotubes obtained by an electrochemical reduction

method showed an improved saturation photocurrent density of 2.8 mA cm at 1.23 V vs. RHE and a photoconversion efficiency of 1.27% under simulated AM 1.5G illumination due to the accelerated electron transfer and reduced recombination of electrons and holes.[108]

Potential (V) vs. Ag/AgCl Potential (V) vs. Ag/AgCl

Figure 9. a) Linear sweeps voltammogram curves and b) calculated photoconversion

efficiencies of pristine TiO2 and black TiO2 nanowires (H:TiO2) annealed in hydrogen at

various temperatures. Reprinted with permission from ref. 61. Copyright 2011, American

Chemical Society. Linear sweep voltammograms curves of TiO2 (TNTs) and black TiO2

nanotube (B-TNTs) synthesized by Al reduction collected under c) full-spectrum light and d) visible light (X > 420 nm) irradiation. Reprinted with permission from ref. 69. Copyright 2014, The Royal Society of Chemistry.

Despite of remarkable enhancement in the visible light absorption, these developed black TiO2 cannot fully exploit visible light for PEC water splitting,'61, 69' due to the rapid recombination of excited charges by visible light. To this end, various strategies, such as non-metallic element doping '67,98,109' and heterojunction,'101,102,105' have been attempted to improve the charge transfer and separation and hence the PEC performance of black TiO2 under visible light irradiation. Li et al. synthesized a unique crystalline core/amorphous shell TiO2@TiO2-x nanosheets by non-thermal dielectric barrier discharge (DBD) plasma in different atmospheres of H2, Ar and NH3.

The photocurrent of TiO2@TiO2-x nanosheet prepared by NH3 plasma produced a

photocurrent density of ~ 0.4 pA cm- at 0.6 V vs. Ag/AgCl under visible light irradiation (X > 400 nm), which was 8 times that of TiO2 nanosheet (~ 0.05 pA cm-2).'109' The authors revealed that the nitriding species and excited hydrogen

contained in NH3 plasma could result in the nitrogen doping and creation of Ti3+ into the TiO2 crystal lattice, which give rise to the enhanced wide-spectrum light absorption. Furthermore, the synergistic effect of reduction and nitridation in NH3 plasma could suppress the recombination of photoexcited charges and improve the photocatalytic activity of the TiO2 synthesized by NH3 plasma under visible light. Similarly, Hoang et al. investigated the synergistic effect of hydrogenation and

nitridation cotreatment on TiO2 nanowire (NW) arrays for improving the water photooxidation performance under visible light illumination.'98' The H,N-cotreated TiO2 sample showed remarkable PEC water oxidation performance with a current of 0.159 mA/cm2 at 1.23 V vs. RHE under visible light (X > 420 nm), which accounted for ~41% of the full AM 1.5 G photocurrent (Figure 10a). The authors believed that

the interaction between the N-dopant and Ti could reduce the band gap of TiO2,

extending the working spectrum of the H,N-TiO2 sample to ~570 nm compared with

~550 nm for the N-TiO2 and ~420 nm for pristine TiO2 (figure 10b). Different form

the previous studies, Siuzdak et al. developed a novel photoelectrode composed of

n-type hydrogenated TiO2 nanotubes infiltrated with a p-type conducting polymer

(pEDOT:PSS) for PEC water splitting. The composite material showed a highly

enhanced anodic photocurrent density of 106 pA cm- under visible irradiation, as

-2 -2 compared with hydrogenated TiO2 (54 pA cm- ) or pure polymer film (2 pA cm- ),

resulting from the efficient charges separation at the p-n junctions.'101' Specially, Luo

et al. synthesized a ternary composite of Au/reduced graphene oxide/hydrogenated

TiO2 nanotube arrays (Au/RGO/H-TNTs) for visible-light-driven PEC water splitting,

in which the Au nanoparticles acted as photosensitizer due to the localized surface

plasmon resonance effect (LSPR) and the RGO middle layers served as electron

mediator and transporter. The Au/RGO/H-TNTs composites showed a highest

visible-light-driven photocurrent density and IPCE value at 580 nm of were 224

pA/cm and 5.8%, respectively, when compared with its pure counterparts (figure 10c,

d), which should be attributed to the LSPR induced strong visible light absorption and rapid transfer of hot electrons in the deposited RGO layers from Au to H-TNTs.[102]

Figure 10. a) Linear sweep voltametry (5 mV/s) of the H,N-TiO2 sample at 1.23 V vs. RHE, b) IPCE profiles at 1.23 V vs. RHE. Reprinted with permission from ref. 98. Copyright 2012, American Chemical Society. c) Transient photocurrent response and d) IPCE plots in the range of 400 - 700 nm of TiO2 nanotubes (TNTs), hydrogenated TiO2 nanotubes (H-TNTs), reduced graphene oxide/hydrogenated TiO2 nanotubes (RGO/H-TNEs) and Au/reduced graphene oxide/hydrogenated TiO2 nanotubes (Au/RGO/H-TNTs) at an applied potential of 1.23 vs. RHE under illumination of visible light (>400 nm), respectively. Reprinted with permission from ref. 102. Copyright 2016, Elsevier.

Table 2 Literature survey of photoelectrochemical studies of black TiO2 photoanodes.

Synthesis method Lamp Photocurrent density under UV light/visible light(mA cm-2) IPCE(X) Applied bias potential Refer Electrode ence

CaH2 reduction 150 W Xe lamp 1.99/~0.2 — 0.23 V vs. Ag/AgCl 1 M NaOH [91]

Hydrogenation in H2 atmosphere Solar simulator (AM 1.5 G filter) 31.4 — 1.2 V vs. Ag/AgCl 0.1 M NaOH [77]

Electrochemical reduction 300 W Xe lamp 2.8/0.008 92% (360 nm) 1.23 V vs. RHE 1 M KOH [108]

vapor-fed aerosol flame synthesis 300 W Xe lamp (400 nm cutoff filter) —/0.001090 — — 0.5 M Na2SO4 [106]

Al reduction 150W Xe lamp 3.65/0.065 ~80% (300 to 370 nm) 0.23 V vs. Ag/AgCl 1 M NaOH [69]

Hydrogenation in 10 vol % H2/N2 atmosphere Hg-Xe lamp 7.0/— — 0.22 V vs. Ag/AgCl 1 M KOH [105]

Hydrogenation in H2 150 W Xe lamp (AM 1.5 G filter) 0.78/— — 0.2 V vs. Ag/AgCl 1 M KOH [100]

N2H4 reduction 150 W Xe lamp (AM 1.5 G filter) 0.69/0.053 — 1.23 V vs. RHE 1 M KOH [103]

Hydrogenation in H2 atmosphere 300 W Xe lamp (400 nm cutoff filter) —/0.082 — 1.23 V vs. RHE 1 M KOH [102]

TiN oxidation and NaBH4 reduction AM 1.5 simulated solar power system 0.0082/— — 0.3 V vs. Ag/AgCl 1 M KOH [92]

Hydrogenation in H2 and nitridation in NH3 150 W Xe lamp (AM 1.5 G filter and 420 nm cutoff filter) 0.388/0.159 — 1.23 V vs. RHE 1 M KOH [98]

Electrochemical reduction Xe lamp (AM 1.5 G filter) 0.65/— 82 % (360 nm) 0 V vs. Ag/AgCl 1 M KOH [107]

Dielectric

barrier-generated plasma discharge Xe lamp (400 nm cutoff filter) —/0.0004 — — 0.01 M Na2SO4 [109]

in Ar, H2, or NH3

Al reduction following AM 1.5 simulated solar ~ 4.5[a]/~ 0.36[a] 74.3% ~ 84.0% (300 - 1.23 V vs. RHE 1 M NaOH [67]

sulfidation in H2S power system 150 W Xe lamp 380 nm)

H2 plasma (AM 1.5 G filter and 420 nm cutoff filter) 0.054/0.0009 — 0.5 V vs. Ag/AgCl 0.5 M K2SO4 [101]

H2 plasma 150 W Xe lamp 1.08/— — 0.23 V vs. Ag/AgCl 1 M NaOH [104]

H2 plasma 150 W Xe lamp (AM 1.5 G filter) ~0.2/— — 1.23 V vs. RHE 1 M NaOH [71]

Hydrogenation in H2/Ar atmosphere White light source (k > 380 nm) ~0.56/— — 1.0 V vs. Ag/AgCl 1 M NaOH [99]

Hydrogenation in H2 atmosphere 150 W Xe lamp (AM 1.5 G filter) ~1. 97/— Higher than 95 % (300 -370 nm) -0.6 V vs Ag/AgCl 1 M NaOH [61]

NaBH4 reduction 500 W Xe lamp 300 W Xe lamp 5.64/— 68.7 % (330 nm) 1.23 V vs. RHE 1 M NaOH [36]

Hydrogenation in H2 atmosphere with a cold mirror and a cold filter 300 W Xe higher than 0.2[a]/— 0.6 V vs Ag/AgCl 0.1 M H2SO4 [93]

TiH2 oxidation lamp(410 nm cutoff filter) —/-3.8X10"7 0.2 M Na2SO4 [86]

Hydrogenation in H2 atmosphere AM 1.5 solar simulator illumination ~0.215[a]/— — 0.6 V vs Ag/AgCl 1 M KOH [87]

TiCl3 oxidation 300 W Xe lamp(420 nm cutoff filter) 300 W Xe —/~7X10"4[a] — 0 V vs Ag/AgCl 0.5 M Na2SO4 [83]

TiO oxidation lamp(400 nm cutoff filter) —/~5X10"4[a] 0.2 M Na2SO4 [44]

Imidazole reduction 300 W Xe lamp(400 nm cutoff filter) —/2X10"4 — 0 V vs Ag/AgCl 1.1 V(for Uv light 1 M Na2SO4 [34]

Hydrogenation in H2/Ar atmosphere 150 W Xe lamp(400 nm cutoff filter) 2.5006/0.3 — irradiation) and 0.4 V (for visible light irradiation) vs Ag/AgCl 0.1 M NaNO3 [110]

TiH2 assisted reduction method 150 W Xe lamp(400 nm cutoff filter) 1.08/~0.09[a] — 0.7 V vs. SCE 1 M NaOH [88]

Biotemplate method combined with an A xenon lamp with 15 A of illumination 0.292/— — 0.4 V vs. Ag/AgCl 1 M KOH [96]

ethanediamine encircling process

Electrochemical reduction

Carbothermal reduction reaction

current(AM 1.5 G filter) 150 W Xe lamp(AM 1.5 G filter)

150 W Xe lamp

1.18/—

30/—

> 60%(350 -380 nm)

-1.8 V vs. SCE

0.4 V vs. Ag/AgCl

0.5 M Na2SO4 [78] 1 M NaOH [111]

'a' The photocurrent density was estimated from the figure in the reference.

5. Summary and Perspective

In this article, we have reviewed recent processes on black TiO2 for photocatalytic hydrogen evolution and photoelectrochemical (PEC) water splitting. Generally, the approaches for the synthesis of black TiO2 could be mainly divide three categories: i) reduction of TiO2, ii) oxidation of low valence Ti compounds and iii) some other unique methods. Some unique structural or chemical properties, such as surface disordered overlayer, Ti3+ and oxygen vacancies, Ti-OH and Ti-H groups could be created or introduced into black TiO2 depending on the various synthetic processes. These structural or chemical modifications substantially determined the enhanced optical properties of black TiO2 by inducing midgap states or narrowing its band gap. Furthermore, oxygen vacancies in the black TiO2 could increase the donor density, thus improving the charge transport in TiO2. Moreover, the midgap states or the

structural and chemical alterations, such as disordered overlayer, Ti3+ and oxygen

vacancies, could improve the separation of photoexcited charges and significantly

promote the photocatalytic performance of black TiO2 originated from various studies

of different research groups. However, it was mainly in the ultraviolet region that the

improved photoelectrocatalysis performances of black TiO2 were achieved. The solar

hydrogen conversion efficiencies of black TiO2 were still limited, especially in visible light region, due to the rapid charge carrier recombination of the visible-light excited charges.

To achieve high photocatalytic and PEC efficiencies for practical solar hydrogen conversion, various strategies, including chemical codoping with different elements, construction of heterojunctions etc., should be adopted to further enhance its optical absorption property and promote the charges separation under visible light. Systematic investigations into photoelectrocatalytic processes and reactions, from charge carrier transport and separation to surface catalysis under visible light irradiation, are necessary to understand the underlying mystery of the unsatisfactory activities of black TiO2 under visible light. Advanced characterization analysis, such ex-situ/in-situ X-ray absorption spectroscopies, transient absorption spectroscopy, surface photovoltaic spectrum etc., should also be conducted to directly probe the dynamics of charge carriers, providing deeper insight into the mechanism of the recombination of photoexcited electron-hole pair. Overall, through experimental and theoretical research, it could be expected that the developed black TiO2 could fully exploit visible light for solar hydrogen conversion.

Acknowledgements

The authors gratefully acknowledge the financial supports form the National Natural Science Foundation of China (No. 51672210, No.51323011, No. 51236007), the Natural Science Foundation of Shaanxi Province (2014KW07-02), the Program for

New Century Excellent Talents in University (NCET-13-0455), the Nano Research Program of Suzhou City (ZXG201442) and the Natural Science Foundation of Jiangsu Province (BK 20141212). S. Shen was supported by the Foundation for the Author of National Excellent Doctoral Dissertation of China (No. 201335), the National Program for Support of Top-notch Young Professionals, and the Fundamental Research Funds for the Central Universities.

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Bin Wang

Bin Wang obtained his PhD degree in Thermal Engineering from Xi'an Jiaotong University in 2016. He is currently a lecturer in School of Science, Xi'an Jiaotong University, China. His research interests focus on synthesis of metal oxide nanomaterials for photocatalytic solar energy conversion.

Shaohua Shen

Shaohua Shen obtained his PhD degree in Thermal Engineering from Xi'an Jiaotong University in 2010. During 2008-2009 and 2011-2012, he worked as a guest researcher at Lawrence Berkeley National Laboratory and a postdoctoral researcher at the University of California at Berkeley. He is currently a full professor at Xi'an Jiaotong University, China. His research interests include synthesis of nanomaterials and development of devices for photocatalytic and photoelectrochemical solar energy conversion.

Samuel S. Mao

Samuel S. Mao is Director of Clean Energy Engineering Laboratory of the University of California at Berkeley. After receiving his Ph.D. degree from the University of California at Berkeley in 2000, Prof. Mao started his career at Lawrence Berkeley National Laboratory, where he was a career staff scientist until 2013. He returned to U.C. Berkeley campus as an adjunct professor in 2004, when he also established the Clean Energy Engineering Laboratory that has spun off an international technology development and commercialization institution, the Institute of New Energy, launched in 2013. Having published 130 research articles that have received more than 22,000 citations, Prof. Mao is also an inventor of 30 patents in the U.S. and abroad.