Oxygen vacancy (VO) engineering is an effective method to tune the photoelectrochemical (PEC) performance, but the influence of VO on photoelectrodes is not well understood. Using hematite as a prototype, we herein report that VO functions in a more complicated way in PEC process than previously reported. Through a comprehensive analysis of the key charge transfer and surface reaction steps in PEC processes on a hematite photoanode, we clarify that VO can facilitate surface electrocatalytic processes while leading to severe interfacial recombination at the semiconductor/electrolyte (S‐E) interface, in addition to the well‐reported improvements in bulk conductivity. The improved bulk conductivity and surface catalysis are beneficial for bulk charge transfer and surface charge consumption while interfacial charge transfer deteriorates because of recombination through VO‐induced trap states at the S‐E interface.
Oxygen vacancy (V O )e ngineering is an effective method to tune the photoelectrochemical (PEC) performance, but the influence of V O on photoelectrodes is not well understood. Using hematite as ap rototype,w eh erein report that V O functions in am ore complicated way in PEC process than previously reported. Through ac omprehensive analysis of the key charge transfer and surface reaction steps in PEC processes on ah ematite photoanode,w ec larify that V O can facilitate surface electrocatalytic processes while leading to severe interfacial recombination at the semiconductor/electrolyte (S-E) interface,i na ddition to the well-reported improvements in bulk conductivity.T he improved bulk conductivity and surface catalysis are beneficial for bulk charge transfer and surface charge consumption while interfacial charge transfer deteriorates because of recombination through V O -induced trap states at the S-E interface.Oxygen vacancies (V O )w idely exist in metal oxide based materials as akind of intrinsic defect. [1] They can significantly change the bulk (e.g.,conductivity,energy level) and surface (e.g.,m olecular adsorption, surface component) properties, and V O engineering is thus an important method in tuning the properties of supercapacitors, [2] electrocatalysts, [3] and photo-(electro)catalysts. [4] Photoelectrochemical (PEC) water splitting is apromising approach to produce hydrogen from solar energy in ac arbon-neutral way. [5] V O has been reported to promote the PEC process in many systems (such as BiVO 4 , [6] In 2 O 3 , [7] WO 3 , [8] and Fe 2 O 3[9] ), and it is widely accepted to contribute to improving the bulk carrier concentration. However,i ti ss till ambiguous to what extent the carrier concentration can lead to improved solar conversion processes,that is,does ahigher carrier concentration lead to abetter PEC performance?M oreover,V O can work as defects and change the surface properties of semiconductors, [7] thus it is not reasonable to ignore these impacts when analysing the role of V O in PEC processes.Fundamentally,aP EC process consists of three pivotal steps,n amely sequential light harvesting, charge separation and transfer,a nd surface reaction. [10] Thec harge separation and surface reaction during aPEC process mainly occur at the semiconductor-electrolyte (S-E) interface, [11] while the charge transfer is largely determined by the bulk conductivity. Therefore,i no rder to better understand the role of V O in aP EC process,i ti si mportant to determine the influence of V O not only on the widely reported bulk conductivity,but also on the S-E interfacial charge transfer,w hich has rarely been investigated. Herein, we used ah ematite photoanode as ap rototype to comprehensively understand how V O influences the photocurrent density (j)interms of bulk conductivity, surface electrocatalytic (EC) capability,a nd interfacial recombination. Interestingly, j does not show alinear dependence on carrier concentration (N d ). Thesurface EC capability and S-E interfacial charge transfer are ...
Hydrogen-rich organic molecules such as alcohols are widely used as hydrogen donors in transfer hydrogenation. Nevertheless, water as a more abundant and eco-friendly hydrogen source has hardly been used due to the high difficulty in splitting water molecules. Herein, we designed a photocatalytic water-donating transfer hydrogenation (PWDTH) technique, in which hydrogen was extracted from water under light illumination and then in situ added to different unsaturated bonds (C=C, C=O, N=O) for chemical synthesis. Platinum loaded carbon nitride (Pt/CN) was used as the model catalyst for this cascade reaction, which is beyond its normal applications for water splitting. This approach was highly accessible to efficiency optimization, either by modifying CN for extended light absorption and enhanced charge transfer, or by alloying Pt with another metal for better catalytic activities. Remarkably, a quantum efficiency up to 21.8% was achieved for nitrobenzene hydrogenation under 380 nm irradiation which is 3 times higher than that obtained in a single water splitting reaction, indicating the PWDTH can be more rewarding than hydrogen evolution for solar energy harvesting. Deep insights into the underlying mechanism was provided by detailed measurements and interpretations of femtosecond transient absorption spectra, action spectra (quantum efficiency as a function of excitation wavelength) and reaction kinetic profiles under varied conditions including the variation of light intensities, temperatures and water isotopes. The mild Page 1 of 22 ACS Paragon Plus Environment ACS Catalysis 2 reaction conditions, simple processing and broad substituent group tolerance endow this approach a high potential toward a general solar to chemical conversion technique.
A water-THF biphasic system containing N-methyl-2-pyrrolidone (NMP) was found to enable the efficient synthesis of 5-hydroxymethylfurfural (HMF) from a variety of sugars (simple to complex) using phosphated TiO2 as a catalyst. Fructose and glucose were selectively converted to HMF resulting in 98 % and 90 % yield, respectively, at 175 °C. Cellobiose and sucrose also gave rise to high HMF yields of 94 % and 98 %, respectively, at 180 °C. Other sugar variants such as starch (potato and rice) and cellulose were also investigated. The yields of HMF from starch (80-85 %) were high, whereas cellulose resulted in a modest yield of 33 %. Direct transformation of cellulose to HMF in significant yield (86 %) was assisted by mechanocatalytic depolymerization-ball milling of acid-impregnated cellulose. This effectively reduced cellulose crystallinity and particle size, forming soluble cello-oligomers; this is responsible for the enhanced substrate-catalytic sites contact and subsequent rate of HMF formation. During catalyst recyclability, P-TiO2 was observed to be reusable for four cycles without any loss in activity. We also investigated the conversion of the cello-oligomers to HMF in a continuous flow reactor. Good HMF yield (53 %) was achieved using a water-methyl isobutyl ketone+NMP biphasic system.
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