Recent density-functional theory calculations suggest that codoping TiO 2 with donoracceptor pairs is more effective than monodoping for improving photoelectrochemical water-splitting performance because codoping can reduce charge recombination, improve material quality, enhance light absorption and increase solubility limits of dopants. Here we report a novel ex-situ method to codope TiO 2 with tungsten and carbon (W, C) by sequentially annealing W-precursor-coated TiO 2 nanowires in flame and carbon monoxide gas. The unique advantages of flame annealing are that the high temperature (41,000°C) and fast heating rate of flame enable rapid diffusion of W into TiO 2 without damaging the nanowire morphology and crystallinity. This is the first experimental demonstration that codoped TiO 2 :(W, C) nanowires outperform monodoped TiO 2 :W and TiO 2 :C and double the saturation photocurrent of undoped TiO 2 for photoelectrochemical water splitting. Such significant performance enhancement originates from a greatly improved electrical conductivity and activity for oxygen-evolution reaction due to the synergistic effects of codoping.

However, the PEC performance of hematite, especially at low bias, is still severely hindered by three main electron/hole recombination pathways that occur in the bulk, interfaces, and surfaces. [ 1,3 ] As schematically illustrated in Figure 1 a, hematite nanorods (NRs), one of the representative nanostructures adopted for photoanodes, exhibit large bulk recombination owing to its poor majority carrier conductivity, via small polaron hopping conduction with a low electron mobility of 10 −2 cm 2 V −1 s −1 , [ 4 ] and short hole collection depth (≈12 nm = hole diffusion length (≈5 nm) + space charge layer width (≈7 nm). [ 5 ] The low intrinsic conductivity and short hole collection depth greatly limit the charge transport/ separation effi ciency of hematite for PEC water oxidation. In addition to bulk recombination, there are interfacial recombinations between the hematite NRs and the conductive substrate, frequently fl uorinedoped SnO 2 (FTO), and recombination losses due to the electron back-injection into the electrolyte on the exposed areas of FTO. [ 6,7 ] Finally, there are signifi cant surface recombination losses due to the presence of surface states and sluggish oxygen evolution reaction (OER) kinetics of hematite. [ 8 ] As a result, these recombination losses lead to low photocurrent density and large overpotential for hematite based PEC water oxidation.Extensive amount of work has been done to reduce those recombination losses, and most of them normally focuses on reducing one or two recombination losses. For the reduction of bulk recombination, great efforts have been devoted to nanostructuring and/or doping of hematite. To date, a number of nanostructures including nanowires, [ 9 ] nanorods, [10][11][12] nanotubes, [ 13 ] nanosheet, [ 14 ] caulifl ower [ 15 ] and porous structures, [ 16,17 ] and diverse metal dopants including Si, Ti, Sn, Zr, Nb, Ag, Pt, Mn, and Al [ 12,[17][18][19] have been studied to shorten the hole transport distance and to increase the electrical conductivity respectively. Separately, it was shown that adding an under-layer of SiO x , TiO 2 , Nb 2 O 5 , or Ga 2 O 3 suppresses the back electron injection and reduces the interface recombination of hematite. [ 7,20,21 ] For the surface recombination, various approaches including For a hematite (α-Fe 2 O 3 ) photoanode, multiple electron/hole recombination pathways occurring in the bulk, interfaces, and surfaces largely limit its low-bias performance (low photocurrent density at low-bias potential) for photoelectrochemical water splitting. Here, a facile and rapid three-step approach is reported to simultaneously reduce these recombinations for hematite nanorods (NRs) array photoanode, leading to a greatly improved photocurrent density at low bias potential. First, fl ame-doping enables high concentration of Ti doping without hampering the morphology and surface properties of the hematite NRs, which reduces both the bulk and surface recombinations effectively. Second, the addition of a dense-layer between the hematite NRs and fl uo...

We report a new flame reduction method to generate controllable amount of oxygen vacancies in TiO2 nanowires that leads to nearly three times improvement in the photoelectrochemical (PEC) water-splitting performance. The flame reduction method has unique advantages of a high temperature (>1000 °C), ultrafast heating rate, tunable reduction environment, and open-atmosphere operation, so it enables rapid formation of oxygen vacancies (less than one minute) without damaging the nanowire morphology and crystallinity and is even applicable to various metal oxides. Significantly, we show that flame reduction greatly improves the saturation photocurrent densities of TiO2 nanowires (2.7 times higher), α-Fe2O3 nanowires (9.4 times higher), ZnO nanowires (2.0 times higher), and BiVO4 thin film (4.3 times higher) in comparison to untreated control samples for PEC water-splitting applications.

Self-assembled multilayer thin films, fabricated by the sequential adsorption of appositively charged polyelectrolytes, were used as a template for the in situ nucleation and growth of silver (Ag) nanoparticles. Since weak polyelectrolytes have a pH-dependent degree of dissociation, the multilayer film formed of polyallylamine (PAH) and polyacryllic acid (PAA) at low pH values contains free carboxylic acid groups that do not electrostatically bind to the positive groups of PAH chains during the film assembly. These free acid groups in the polyelectrolyte multilayer film were used to bind the silver ions, which were subsequently reduced with a NaBH4 solution to form Ag nanoparticles. Electron-microscopy examination of the films and UV–vis spectroscopy showed that the concentration and size of the Ag nanoparticles are determined by the processing conditions of the multilayer assembly. Furthermore, due to the regeneration of the acid groups upon reduction, the synthesis process can be repeatedly cycled to increase the volume fraction of Ag in the polyelectrolyte multilayer films. After three reaction cycles, Ag nanoparticles with an average diameter of 7 nm at a volume fraction of 65% were obtained in the polyelectrolyte multilayers (PEMs).