We report a hierarchically branched TiO(2) nanorod structure that serves as a model architecture for efficient photoelectrochemical devices as it simultaneously offers a large contact area with the electrolyte, excellent light-trapping characteristics, and a highly conductive pathway for charge carrier collection. Under Xenon lamp illumination (UV spectrum matched to AM 1.5G, 88 mW/cm(2) total power density), the branched TiO(2) nanorod array produces a photocurrent density of 0.83 mA/cm(2) at 0.8 V versus reversible hydrogen electrode (RHE). The incident photon-to-current conversion efficiency reaches 67% at 380 nm with an applied bias of 0.6 V versus RHE, nearly two times higher than the bare nanorods without branches. The branches improve efficiency by means of (i) improved charge separation and transport within the branches due to their small diameters, and (ii) a 4-fold increase in surface area which facilitates the hole transfer at the TiO(2)/electrolyte interface.

We report a scalably synthesized WO3/BiVO4 core/shell nanowire photoanode in which BiVO4 is the primary light-absorber and WO3 acts as an electron conductor. These core/shell nanowires achieve the highest product of light absorption and charge separation efficiencies among BiVO4-based photoanodes to date and, even without an added catalyst, produce a photocurrent of 3.1 mA/cm(2) under simulated sunlight and an incident photon-to-current conversion efficiency of ∼ 60% at 300-450 nm, both at a potential of 1.23 V versus RHE.

A BiVO4 with a preferred [001] orientation and exposed {001} facets were grown epitaxially on FTO via a laser ablation, achieving the state-of-the-art photoelectrochemical performance for solar water-oxidation.

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...

Novel SnWO4 visible-light active photocatalysts with two polymorphs (orthorhombic α and cubic β phases) were prepared by a conventional solid-state reaction method, and their optical properties, electronic band structure, and photocatalytic activities were investigated. It was found that the low-temperature phase, α-SnWO4 with corner-shared WO6 octahedra, exhibited a dark-red color and indirect band gap of 1.64 eV, whereas the high-temperature phase, β-SnWO4 with unshared WO4 tetrahedra, exhibited a light-yellow color and direct band gap of 2.68 eV. The Mott−Schottky plots obtained using a thick film electrode in 1 M NaCl electrolyte revealed the n-type semiconductive properties of the SnWO4 polymorphs; i.e., the flat-band potential values of α- and β-SnWO4 were −0.61 and −0.66 V (SCE), respectively. From the electronic band structure calculations performed using density functional theory, the Sn 5p and O 2p orbitals were hybridized to construct the valence band in both SnWO4 polymorphs. However, the constructions of the conduction band were quite different. β-SnWO4 with its shorter W−O bond lengths in the WO4 tetrahedra has a higher conduction-band potential than α-SnWO4 phase, which has larger W−O bond lengths in the WO6 octahedra and, thus, was able to produce H2 from an aqueous methanol solution under visible-light irradiation (>400 nm). Both SnWO4 polymorphs also exhibited good photocatalytic activity for the degradation of rhodamine B dye solution under visible-light irradiation (>420 nm). The photocatalytic activity of these SnWO4 polymorphs was higher than that of other visible-light active photocatalysts with much smaller particle sizes, such as nanosized WO3 (9.72 m2/g) and TiON x (112.13 m2/g). This higher photocatalytic activity of the SnWO4 polymorphs is mainly attributed to their smaller band gaps and unique band structures, resulting from their different bonding nature.

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.

We report an efficient hybrid Si microwire (radial junction) and planar solar cell with a maximum efficiency of 11.0% under AM 1.5G illumination. The maximum efficiency of the hybrid cell is improved from 7.2% to 11.0% by passivating the top surface and p-n junction with thin a-SiN:H and intrinsic poly-Si films, respectively, and is higher than that of planar cells of the identical layers due to increased light absorption and improved charge-carrier collections in both wires and planar components.