Dennis Friedrich

Mesoporous thin film WO3 photoanode for photoelectrochemical water splitting: a sol–gel dip coating approach

A facile and cost-efficient method to fabricate a mesoporous structured WO3 photoanode was implemented for use in a tandem dual photosystem water splitting photoelectrochemical cell. Semi-transparent thin films of tungsten trioxide were fabricated by sol–gel process, incorporating a block co-polymer to induce a template-directed mesoporous structure. These thin films are deposited by dip coating onto transparent conducting oxide substrates and crystallized at a low temperature of 400 °C in air. These WO3 photoanodes exhibit a photocurrent of up to 0.6 mA cm−2 in potassium phosphate buffers of pH 2, 4, and 6 at 1.23 V vs. RHE under 300 mW cm−2 visible (400–900 nm) light irradiation with a faradaic efficiency of up to 75%. Furthermore, we have demonstrated that corrosion occurs in electrolytes of pH > 4. The faradaic efficiencies in varying pH solutions suggest that parasitic redox reactions occurs in acidic conditions, limiting the O2 production and demonstrating the need for stable surface co-catalysts to increase faradaic efficiencies. In neutral conditions, protective layers and/or co-catalysts are needed for increasing WO3 photoanode stability.

Gradient Self-Doped CuBi2O4 with Highly Improved Charge Separation Efficiency

A new strategy of using forward gradient self-doping to improve the charge separation efficiency in metal oxide photoelectrodes is proposed. Gradient self-doped CuBi2O4 photocathodes are prepared with forward and reverse gradients in copper vacancies using a two-step, diffusion-assisted spray pyrolysis process. Decreasing the Cu/Bi ratio of the CuBi2O4 photocathodes introduces Cu vacancies that increase the carrier (hole) concentration and lowers the Fermi level, as evidenced by a shift in the flat band toward more positive potentials. Thus, a gradient in Cu vacancies leads to an internal electric field within CuBi2O4, which can facilitate charge separation. Compared to homogeneous CuBi2O4 photocathodes, CuBi2O4 photocathodes with a forward gradient show highly improved charge separation efficiency and enhanced photoelectrochemical performance for reduction reactions, while CuBi2O4 photocathodes with a reverse gradient show significantly reduced charge separation efficiency and photoelectrochemical performance. The CuBi2O4 photocathodes with a forward gradient produce record AM 1.5 photocurrent densities for CuBi2O4 up to -2.5 mA/cm2 at 0.6 V vs RHE with H2O2 as an electron scavenger, and they show a charge separation efficiency of 34% for 550 nm light. The gradient self-doping accomplishes this without the introduction of external dopants, and therefore the tetragonal crystal structure and carrier mobility of CuBi2O4 are maintained. Lastly, forward gradient self-doped CuBi2O4 photocathodes are protected with a CdS/TiO2 heterojunction and coated with Pt as an electrocatalyst. These photocathodes demonstrate photocurrent densities on the order of -1.0 mA/cm2 at 0.0 V vs RHE and evolve hydrogen with a faradaic efficiency of ∼91%.

Bionic photovoltaic panels bio-inspired by green leaves

In strong solar light, silicon solar panels can heat up by 70°C and, thereby, loose approximately one third of their efficiency for electricity generation. Leaf structures of plants on the other hand, have developed a series of technological adaptations, which allow them to limit their temperature to 40–45°C in full sunlight, even if water evaporation is suppressed. This is accomplished by several strategies such as limitation of leaf size, optimization of aerodynamics in wind, limitation of absorbed solar energy only to the useful fraction of radiation and by efficient thermal emission. Optical and infrared thermographic measurements under a solar simulator and in a streaming channel were used to investigate the corresponding properties of leaves and to identify suitable bionic model systems. Experiments started with the serrated structure of ordinary green leaves distributed over typical twig structures and finally identified the Australian palm tree Licuala ramsayi as a more useful bionic model. It combines a large area for solar energy harvesting with optimized aerodynamic properties for cooling and is able to restructure itself as a protection against strong winds. The bionic models, which were constructed and built, are analyzed and discussed.

Solvent-induced surface state passivation reduces recombination in semisquarylium dye-sensitized solar cells

The semisquarylium dye SY1T that is strongly bound to the surface of nanocrystalline TiO2 experiences very fast back-electron transfer of injected electrons to the SY1T cation, when the TiO2/SY1T interface is surrounded by ultrahigh vacuum. However, when located in methoxypropionitrile (MPN), which is frequently used as electrolyte solvent in dye-sensitized solar cells, the back-electron transfer is significantly retarded. Results are obtained both for picosecond and microsecond time scales using transient absorption spectroscopy. As solvent-induced interfacial energy level shifts can be excluded as possible cause, the role of TiO2 surface states in the beneficial retardation process is investigated. Highly surface sensitive synchrotron-induced photoelectron spectroscopy exhibits high densities of surface states on the pristine nanocrystalline TiO2 (nc-TiO2) surfaces. While SY1T dye-sensitization from a SY1T solution in tetrahydrofuran saturates about 30% of the surface states, the subsequent in-situ adso...

Author Correction: Highly (001)-textured p-type WSe 2 Thin Films as Efficient Large-Area Photocathodes for Solar Hydrogen Evolution

A correction to this article has been published and is linked from the HTML and PDF versions of this paper. The error has been fixed in the paper.

Elucidation of the opto-electronic and photoelectrochemical properties of FeVO4 photoanodes for solar water oxidation

Triclinic iron vanadate (n-type FeVO4) thin films were fabricated for the first time by spray pyrolysis and elucidated as a potential photoanode material for solar water oxidation. FeVO4 has an ideal band gap for a photoanode of ∼2.0 eV, which corresponds to a potential solar-to-hydrogen (STH) efficiency of 16%. However, our findings show that the photoelectrochemical performance of FeVO4 is limited by very poor charge carrier separation efficiency in the bulk. Time-resolved microwave conductivity (TRMC) measurements revealed that the low mobility (∼5 × 10−5 cm2 V−1 s−1) and short diffusion length (∼2 nm) of undoped FeVO4 are the main reason for its fast bulk recombination. To overcome the poor charge separation efficiency in the bulk, molybdenum doping was used to enhance its mobility, lifetime, and carrier concentration. Doping with 2% Mo increased the photocurrent density by more than 45% at 1.6 V vs. RHE. Finally, we show that the near-ideal band gap of FeVO4 can be combined with the favorable carrier mobility of BiVO4 in a mixed phase compound, Fe1−xBixVO4, a new photoanode candidate for solar water splitting.

Formation and suppression of defects during heat treatment of BiVO4 photoanodes for solar water splitting

Metal oxide photoelectrodes typically suffer from poor carrier transport properties and extensive carrier recombination, which is caused by the presence of intrinsic or extrinsic defects in the material. Here, the influence of annealing temperature and atmosphere on the formation and suppression of defects in BiVO4—one of the best performing metal oxide photoanodes—is elucidated. Annealing in argon has little or no effect on the photoelectrochemical performance due to the competing effects of an increase in grain size (i.e., reduction of grain boundaries) and the unfavorable formation of oxygen vacancies. When annealing in air, the formation of oxygen vacancies is suppressed, resulting in up to ∼1.5-fold enhancement of the photocurrent and an order of magnitude increase of the charge carrier mobility. However, vanadium leaves the BiVO4 lattice above 500 °C, which leads to a decrease in carrier lifetime and photocurrent. This vanadium loss can be avoided by supplying excess vanadium in the gas phase during annealing. This leads to enhanced charge carrier mobility and lifetime, resulting in improved photocurrents. Overall, this strategy offers a general approach to prevent unfavorable changes of cation stoichiometry during high-temperature treatment of complex metal oxide photoelectrodes.

CVD-grown copper tungstate thin films for solar water splitting

In this paper, a direct chemical vapor deposition (CVD) approach is applied for the first time to synthesize high quality copper oxide (CuO), copper tungstate (CuWO4) and tungsten oxide (WO3) on F:SnO2 (FTO) substrates for photocatalytic water splitting. Variation of process parameters enables us to tune the stoichiometry of the deposits to obtain stoichiometric, W-rich, and Cu-rich deposits. It is found that the presence of Cu in WO3 thin films reduces the bandgap and enhances the absorption properties of the material in the visible range. The photoelectrocatalytic performance of stoichiometric CuWO4 was found to be superior to that of WO3 oxide under frontside illumination when thin films were used. However, detailed photoelectrochemical investigations of both thin and thicker CuWO4 films reveal that the incorporation of copper also decreases the mobility of both electrons and holes, the latter being the performance-limiting factor. These results are in line with our first-principles calculations of the electronic structure of CuWO4. A charge carrier mobility and diffusion length of ∼6× 10−3 cm2 V−1 s−1 and 30 nm were determined by time-resolved microwave conductivity measurements, values comparable to those of undoped bismuth vanadate (BiVO4). Our findings establish new insights into the advantages and limits of CuWO4-based photoanodes, and suggest a possibility of using very thin CuWO4 films on top of highly absorbing semiconductors with optimal electronic properties.

Charge Carrier Lifetimes in Cr–Fe–Al–O Thin Films

The effect of compositional variation on charge carrier lifetimes of Cr1Fe0.84Al0.16O3, a promising material for solar water splitting recently identified using combinatorial materials science, is explored using ultrafast time-resolved optical reflectance. The transient signal can be described by a biexponential decay, where the shorter time constant varies over 1 order of magnitude with changing Cr content while the longer one stays constant. Intrinsic performance limitations such as a low charge carrier mobility on the order of 10-3 cm2/(Vs) are identified. Charge carrier lifetime and mobility are discussed as screening criteria for solar water splitting materials.