Progress in Black Titania: A New Material for Advanced Photocatalysis

Abstract

excited charges should separate from each other and migrate to the surface to perform photocatalytic reactions, before they are annihilated in the recombination process. [ 3,17,18 ] Nevertheless, TiO 2 has a wide bandgap of 3.0−3.2 eV for the three common natural polymorphs -anatase, rutile and brookite. [ 8 ] That limits the optical absorption of TiO 2 in the ultraviolet (UV) region of the solar spectrum, resulting in insuffi cient utilization of solar energy (less than 5%). Another hurdle for TiO 2 material in the photo-chemical applications is its low quantum effi ciency that results from its high recombination of photo-generated electron-hole pairs. [ 16,18 ] Therefore, improving the optical absorption properties and reducing electron-hole recombination of TiO 2 are expected to be signifi cant for superior photoactivity.Various strategies have been adopted to tune TiO 2 's optical and electronic properties to improve its visible-light photoactivity. For example, metal ions (Co, Ni, Mn, Fe, Cr, …) or non-metal ions (N, S, C, I, …) are doped in the titania matrix to induce mid-gap states or to narrow the bandgap of TiO 2 , aiming to enhance its visible-light response. [ 3,17 ] Doping with open-shell metals is claimed to be benefi cial for red-shifting TiO 2 photochemistry into the visible, while closed shell cation dopants have no photochemical benefi t. Some d -block transition metal doping often generates deeply localized d -states in the forbidden bandgap of TiO 2 and results in recombination centers for carriers. [ 3,19 ] For the incorporation of non-metal ions, heavy doping is always diffi cult due to the large differences in chemistry between the alien ions and O 2− in titania, thus the visible light absorption is not signifi cantly enhanced. [ 20 ] Dye-sensitized or noble metal nanodots decorated TiO 2 nanostructures have been largely designed to enhanced its optical properties and to improve the charge separation effi ciency. [ 6,9,15,17,21 ] Additionally, charge separation in TiO 2 is believed to be effi ciently improved by forming heterojunctions with other semiconductors, such as metal oxides and chalcogenides. [ 6,22 ] In 2011, a hydrogenated black titania with a narrowed bandgap of ≈1.5 eV was reported to boost the full spectrum sunlight absorption and the photocatalytic activity. [ 23 ] This discovery has triggered world-wide scientifi c interests in black TiO 2 nanomaterials. [ 8 ] Black TiO 2 is featured by selfstructural modifi cations, involving self-doped Ti 3+ /oxygen vacancy, or incorporation of H-doping. [ 5,8 ] Owing to these modifi cations, their crystal and electronic structures as well as the surface property are signifi cantly changed, and the most marked effect is the color changing. The intrinsic TiO 2The photocatalytic activity of TiO 2 has aroused a broad range of research effort since 1972. Although TiO 2 has a very high effi ciency in utilizing ultraviolet light, its overall solar activity is very limited due to its wide bandgap (≈3.0−3.2 eV). This is a bottleneck for TiO 2 to...

Black Magic in Gray Titania: Noble‐Metal‐Free Photocatalytic H2 Evolution from Hydrogenated Anatase

“…While the material effects visible light absorption (the blacker,t he better), black titaniap roduced by high pressure hydrogenation was recently reported to show another highly interesting feature;n oble-metal-free photocatalytic H 2 generation. [9][10][11] In general, the functiono ft he noble metal cocatalyst has been described in terms of i) providing an electron acceptor to adjustt he energy levels and thus mediatee lectron transfer to the electrolyte, ii)forming solid-state junctions (metal/semiconductor), and/ori ii) acting as ah ydrogen recombination center that strongly promotes H 2 formation. The materials' intrinsic activity for photocatalytic hydrogen evolution is coupledn either with their visible light absorption behavior nor the formation of amorphousm aterial, but rather must be ascribed to optimized and specific defect formation (gray is better than black).…”

“…The materials' intrinsic activity for photocatalytic hydrogen evolution is coupledn either with their visible light absorption behavior nor the formation of amorphousm aterial, but rather must be ascribed to optimized and specific defect formation (gray is better than black). [3,[9][10][11][12] For TiO 2 -based photocatalysts, significant efforts have been dedicated not only to accelerating reaction rates bute ven more so to extendingt he light absorption of TiO 2 (anatase: E g = 3.2 eV;r utile: E g = 3.0 eV [7] )i nto the visible range of the spectrum,t oa llow for am ore efficient use of solar light. In photocatalysis, the ulti-mate challenge remains the efficient splitting of H 2 Oi nto H 2 and O 2 ,t hereby generating the fuel of the future (H 2 )f rom water and sunlight.…”

“…In photocatalysis, the ulti-mate challenge remains the efficient splitting of H 2 Oi nto H 2 and O 2 ,t hereby generating the fuel of the future (H 2 )f rom water and sunlight. [9][10][11] In general, the functiono ft he noble metal cocatalyst has been described in terms of i) providing an electron acceptor to adjustt he energy levels and thus mediatee lectron transfer to the electrolyte, ii)forming solid-state junctions (metal/semiconductor), and/ori ii) acting as ah ydrogen recombination center that strongly promotes H 2 formation. [2,3] However,T iO 2 has inherent limitations in thermodynamics (band gap that absorbso nly UV light) and kinetics( slow electron transfer from the conduction band to form H 2 ).…”

“…Ever since the pioneering work of Fujishima and Honda on aqueous photocatalysis, [1] the topic has attracted tremendous scientific interestw ith more than 10 000 papers published on the photocatalytic productiono fh ydrogen (H 2 )a sas torable and clean energy carrier.T he underlying concept to photocatalysis is strikingly simple:l ight (preferably sunlight) is used to generate electron-hole pairs in as uitable semiconductor and these excited carriers migrate to the semiconductor surface where they react with an adjacent aqueous electrolyte to form O 2 and H 2 . [9][10][11] Therefore, photocatalytic M/TiO 2 systems have been widely investigated with av iew to optimizing their efficiencyt owards H 2 generation from water (with or withouts acrificial agents such as ethanol). As aconsequence, al arge number of alternative semiconductor materials have been investigated.…”

“…[2,3] However,T iO 2 has inherent limitations in thermodynamics (band gap that absorbso nly UV light) and kinetics( slow electron transfer from the conduction band to form H 2 ). [3,[9][10][11][12] For TiO 2 -based photocatalysts, significant efforts have been dedicated not only to accelerating reaction rates bute ven more so to extendingt he light absorption of TiO 2 (anatase: E g = 3.2 eV;r utile: E g = 3.0 eV [7] )i nto the visible range of the spectrum,t oa llow for am ore efficient use of solar light. Nevertheless, owing to its chemical and photochemical stabilitya nd for ecological and economic reasons, TiO 2 remains the most investigated photocatalytic platform.T iO 2 ,i navast variety of morphologies and modifications, has been used to trigger aw ide range of photoinduced reactions( see, for example, refs.…”