Enhanced Photoelectrochemical Performance Research Articles

A facile two-step method for fabrication of plate-like WO3 photoanode under mild conditions.

Fabrication of photoelectrodes on a large-scale, with low-cost and high efficiency is a challenge for their practical application in photoelectrochemical (PEC) water splitting. In this work, a typical plate-like WO(3) photoanode was fabricated with chemical etching of the as-prepared mixed tungsten-metal oxides (W-M-O, M = Cu, Zn or Al) by a reactive magnetron co-sputtering technique, which results in a greatly enhanced PEC performance for water oxidation in comparison with that obtained from a conventional magnetron sputtering method. The current approach is applicable for the fabrication of some other semiconductor photoelectrodes and is promising for the scaling up of applications for highly efficient solar energy conversion systems.

Microwave-Assisted Self-Doping of TiO2 Photonic Crystals for Efficient Photoelectrochemical Water Splitting

In this article, we report that the combination of microwave heating and ethylene glycol, a mild reducing agent, can induce Ti(3+) self-doping in TiO2. A hierarchical TiO2 nanotube array with the top layer serving as TiO2 photonic crystals (TiO2 NTPCs) was selected as the base photoelectrode. The self-doped TiO2 NTPCs demonstrated a 10-fold increase in visible-light photocurrent density compared to the nondoped one, and the optimized saturation photocurrent density under simulated AM 1.5G illumination was identified to be 2.5 mA cm(-2) at 1.23 V versus reversible hydrogen electrode, which is comparable to the highest values ever reported for TiO2-based photoelectrodes. The significant enhancement of photoelectrochemical performance can be ascribed to the rational coupling of morphological and electronic features of the self-doped TiO2 NTPCs: (1) the periodically morphological structure of the photonic crystal layer traps broadband visible light, (2) the electronic interband state induced from self-doping of Ti(3+) can be excited in the visible-light region, and (3) the captured light by the photonic crystal layer is absorbed by the self-doped interbands.

ZnO@TiO2 core–shell nanorod arrays with enhanced photoelectrochemical performance

ZnO@TiO2 core–shell nanorod arrays (NRAs) were fabricated by radio frequency magnetron sputtering of a Ti layer on ZnO NRAs followed by post-oxidation treatment. Both the photoluminescence spectra and open circuit voltage decay measurement show that the thin TiO2 shell can effectively passivate the surface trap states of ZnO NRAs. Compared with the bare ZnO, ZnO@TiO2 core–shell nanostructure sensitized with CdS quantum dots (QDs) using as the photoelectrodes for photoelectrochemical (PEC) cells shows enhanced PEC performance. It is supposed that this enhanced PEC performance is attributed primarily to two effects. On one hand, the TiO2 shell layer suppresses the recombination of photogenerated carriers by passivating the surface recombination sites in ZnO NRAs and forming an energy barrier between the ZnO core and TiO2 shell. On the other hand, the rough surface of the TiO2 shell on ZnO NRAs facilitates the deposition of CdS QDs, which contributes to enhance light absorption in visible light region. However, the experiment results suggest that an optimal thickness of about 15nm for the TiO2 (or pre-deposited Ti) layer is highly desired, the further incremental thickness of the layer will restrain the PEC performance of photoelectrode due to several shortages, such as reduced light harvesting due to the shadow effect of the incomplete oxidized Ti layer, the introduction of additional leakage current, and the blocking effect for the penetration of the redox couples ions (S2−/Sn2−) into ZnO NRAs.

Optical absorption and photoelectrochemical performance enhancement in Si tube array for solar energy harvesting application

We studied large-scale fabrication and photovoltaic properties of Si tube array (SiTA) for solar energy harvesting application. The SiTA is fabricated with photolithography by using Poisson spot effect, followed by investigating its light absorption properties and photoelectrochemical performance to compare its solar energy harvesting property with the well studied Si hole and Si pillar array. It was found that SiTA has the highest onset potential and photocurrent among the three Si micro/nanostructures because of its “hole-in-pillar” hierarchal structures. The result suggests an alternative strategy in enhancing the efficiency of Si photovoltaic devices using this hierarchal structure.

Atomic Layer Deposition of a Submonolayer Catalyst for the Enhanced Photoelectrochemical Performance of Water Oxidation with Hematite

Hematite photoanodes were coated with an ultrathin cobalt oxide layer by atomic layer deposition (ALD). The optimal coating-1 ALD cycle, which amounts to <1 monolayer of Co(OH)2/Co3O4-resulted in significantly enhanced photoelectrochemical water oxidation performance. A stable, 100-200 mV cathodic shift in the photocurrent onset potential was observed that is correlated to an order of magnitude reduction in the resistance to charge transfer at the Fe2O3/H2O interface. Furthermore, the optical transparency of the ultrathin Co(OH)2/Co3O4 coating establishes it as a particularly advantageous treatment for nanostructured water oxidation photoanodes. The photocurrent of catalyst-coated nanostructured inverse opal scaffold hematite photoanodes reached 0.81 and 2.1 mA/cm(2) at 1.23 and 1.53 V, respectively.

Growth of β-Ga2O3 and GaN nanowires on GaN for photoelectrochemical hydrogen generation

Enhanced photoelectrochemical (PEC) performances of Ga2O3 and GaN nanowires (NWs) grown in situ from GaN were demonstrated. The PEC conversion efficiencies of Ga2O3 and GaN NWs have been shown to be 0.906% and 1.09% respectively, in contrast to their 0.581% GaN thin film counterpart under similar experimental conditions. A low crystallinity buffer layer between the grown NWs and the substrate was found to be detrimental to the PEC performance, but the layer can be avoided at suitable growth conditions. A band bending at the surface of the GaN NWs generates an electric field that drives the photogenerated electrons and holes away from each other, preventing recombination, and was found to be responsible for the enhanced PEC performance. The enhanced PEC efficiency of the Ga2O3 NWs is aided by the optical absorption through a defect band centered 3.3 eV above the valence band of Ga2O3. These findings are believed to have opened up possibilities for enabling visible absorption, either by tailoring ion doping into wide bandgap Ga2O3 NWs, or by incorporation of indium to form InGaN NWs.

Facile preparation of nanostructured α-Fe2O3 thin films with enhanced photoelectrochemical water splitting activity

We report on the use of a facile electrospray technique for the synthesis of α-Fe2O3 thin films on a FTO substrate for photoelectrochemical (PEC) water splitting. The effect of synthesis parameters such as substrate temperature, discharge potential and post-heat treatment on morphology, particle size and PEC performance of α-Fe2O3 films were investigated. With an increase in substrate temperature, the surface morphology of the α-Fe2O3 film was altered from a packed worm-like surface to highly porous nanostructures. XRD analysis revealed that the (110) grain orientation of the film was transformed to the (104) grain orientation at 300 °C, due to the oxidation of the precursor at the surface of the substrate. Raman spectroscopy and XPS analysis indicated the presence of highly pure α-Fe2O3 in the film. By changing the discharge potential, the size of particles in the film was reduced to a minimum of 23 nm. Under optimized conditions the nanostructured α-Fe2O3 films showed a water splitting photocurrent of ∼0.6 mA cm−2 at 1.23 V versus RHE under standard illumination conditions (AM 1.5 G 100 mW cm−2), and an incident photon to current efficiency (IPCE) of 13% at 350 nm (at 1.4 V versus RHE) which are among the best results obtained for undoped α-Fe2O3 photoanodes. This enhanced PEC performance can be attributed to the efficient charge separation at the α-Fe2O3-electrolyte interface due to the larger interfacial area of small-sized particles in the film. This study thus provides a simple route for the synthesis of highly active α-Fe2O3 thin films that can be extended to metal doped films such as Ti-doped α-Fe2O3.

Coincorporation of N and Ta into TiO2 Nanowires for Visible Light Driven Photoelectrochemical Water Oxidation

We report a synthesis of N- and Ta-coincorporated TiO2 (N,Ta:TiO2) and Ta-incorporated TiO2 (Ta:TiO2) nanowire (NW) arrays and their application as photoanodes for water photooxidation. Tantalum is incorporated into TiO2 NWs with concentrations ranging from 0.11 to 3.47 atomic % by a simple solvothermal synthesis. N,Ta:TiO2 nanowires are prepared via nitridation of Ta:TiO2 nanowires in NH3 flow at a relatively low temperature (500 °C). N,Ta:TiO2 NWs with the optimum Ta concentration of 0.29 atomic % also demonstrate significant enhancement in photoelectrochemical performance with the photocurrent reaching 0.52 and 0.18 mA/cm2 under AM 1.5 G and visible light (>420 nm) illumination, compared with 0.26 and 0.13 mA/cm2 for that of N:TiO2 NWs, although the active spectrum of the N,Ta:TiO2 NW sample only extends to ∼520 nm (2.38 eV), compared to ∼540 nm (2.30 eV) for N:TiO2 NWs. We believe that the enhancement shown by the N,Ta-coincorporated sample is due to fewer recombination centers from charge compensation effects and suppression of the formation of an amorphous layer on the nanowires during the nitridation process.

Enhancement of Photoelectrochemical Performance of AACVD‐produced TiO2 Electrodes by Microwave Irradiation while Preserving the Nanostructure

AbstractTiO2 electrodes are deposited on FTO‐glass substrates at 350 and 400 °C by aerosol‐assisted chemical vapour deposition (AACVD) and the deposited TiO2 electrodes are irradiated with microwave radiation (2.45 GHz) at various percentages (10, 25, 50, and 100%). X‐ray diffraction (XRD) pattern shows that the deposited electrodes have anatase phase TiO2 oriented in the (101) direction, and the crystallinity of these electrodes increases after microwave treatment. Field emission gun scanning electron microscopy (FEG‐SEM) surface topography analysis proves the preservation of the nanostructure after exposure to various percentages of microwave radiation. The photoelectrochemical (PEC) studies prove a threefold enhancement of photocurrent density of AACVD‐produced TiO2 electrodes after 100% microwave irradiation. This improved performance of PEC properties is attributed to improvements in the crystallinity and the particle‐necking properties. The results presented demonstrate that microwave processing is a promising alternative method to conventional sintering for TiO2 photoanodes.

Surface tuning for promoted charge transfer in hematite nanorod arrays as water-splitting photoanodes

Hematite (α-Fe2O3) nanorod films with their surface tuned by W6+ doping have been investigated as oxygen-evolving photoanodes in photoelectrochemical cells. X-ray diffraction, field emission scanning electron microscopy, UV-visible absorption spectroscopy, and photoelectrochemical (PEC) measurements have been performed on the undoped and W6+-doped α-Fe2O3 nanorod films. W6+ doping is found to primarily affect the photoluminescence properties of α-Fe2O3 nanorod films. Comparisons are drawn between undoped and W6+-doped α-Fe2O3 nanorod films, WO3 films, and α-Fe2O3-modified WO3 composite electrodes. A close correlation between dopant concentration, photoluminescence intensity, and anodic photocurrent was observed. It is suggested that W6+ surface doping promotes charge transfer in α-Fe2O3 nanorods, giving rise to the enhanced PEC performance. These results suggest surface tuning via ion doping should represent a viable strategy to further improve the efficiency of α-Fe2O3 photoanodes. Open image in new window

Synthesis and characterization of TiO2/CdS core–shell nanorod arrays and their photoelectrochemical property

TiO2/CdS core–shell nanorod arrays have been fabricated via a two-step method. Vertically aligned TiO2 nanorod arrays (NRs) were synthesized by a facile hydrothermal method, and followed by depositing CdS nanoparticles on TiO2 NRs by spin-coating successive ion layer adsorption and reaction (spin-SILAR) method. The surface morphology, structure, optical and photoelectrochemical behaviors of the core–shell NRs films are considered. The UV–vis absorption spectrum results suggested that the absorption peak of the TiO2/CdS core–shell NRs shifts from the ultraviolet region to the visible region in comparison to that of the pure TiO2 NRs. The obviously enhanced photoelectrochemical (PEC) performances of the heterojunction NRs were found under illumination of the simulated sunlight in comparison with that of the TiO2 NRs. The enhanced PEC performance and formation mechanism of TiO2/CdS core–shell NRs were discussed in detail.

Enhanced photoelectrochemical activity of an excitonic staircase in CdS@TiO2 and CdS@anatase@rutile TiO2 heterostructures

TiO2 nanorod arrays grown on conductive substrates were converted using chemical strategies into CdS@TiO2 and CdS@anatase@rutile TiO2 heterostructures to fabricate visible-light harvesting assemblies. Compared to pure TiO2 nanorods, CdS@TiO2 heterostructures evidently extended the absorption edge and exhibited enhanced photoelectrochemical (PEC) response in the visible region. Further enhancement of PEC performance was achieved by introducing an intermediate anatase TiO2 layer in the CdS@rutile TiO2 heterostructures. An excitonic cascade of band alignment (CdS, anatase-TiO2 and rutile-TiO2) was constituted by arranging different semiconductors in order to align the edges of their conducting band, which improved charge separation and suppressed the recombination processes by facilitating the transfer of forward electrons and limiting the reverse processes due to spatial separation of the electron and hole in different material regions.

A Ga2O3underlayer as an isomorphic template for ultrathin hematite films toward efficient photoelectrochemical water splitting

Hematite photoanodes for photoelectrochemical (PEC) water splitting are often fabricated as extremely-thin films to minimize charge recombination because of the short diffusion lengths of photoexcited carriers. However, poor crystallinity caused by structural interaction with a substrate negates the potential of ultrathin hematite photoanodes. This study demonstrates that ultrathin Ga2O3 underlayers, which were deposited on conducting substrates prior to hematite layers by atomic layer deposition, served as an isomorphic (corundum-type) structural template for ultrathin hematite and improved the photocurrent onset of PEC water splitting by 0.2 V. The benefit from Ga2O3 underlayers was most pronounced when the thickness of the underlayer was approximately 2 nm. Thinner underlayers did not work effectively as a template presumably because of insufficient crystallinity of the underlayer, while thicker ones diminished the PEC performance of hematite because the underlayer prevented electron injection from hematite to a conductive substrate due to the large conduction band offset. The enhancement of PEC performance by a Ga2O3 underlayer was more significant for thinner hematite layers owing to greater margins for improving the crystallinity of ultrathin hematite. It was confirmed that a Ga2O3 underlayer was applicable to a rough conducting substrate loaded with Sb-doped SnO2 nanoparticles, improving the photocurrent by a factor of 1.4. Accordingly, a Ga2O3 underlayer could push forward the development of host-guest-type nanocomposites consisting of highly-rough substrates and extremely-thin hematite absorbers.

Highly stable copper oxide composite as an effective photocathode for water splitting via a facile electrochemical synthesis strategy

Hydrogen generation through photoelectrochemical (PEC) water splitting using solar light as an energy resource is believed to be a clean and efficient way to overcome the global energy and environmental problems. Extensive research effort has been focused on n-type metal oxide semiconductors as photoanodes, whereas studies of p-type metal oxide semiconductors as photocathodes where hydrogen is generated are scarce. In this paper, highly efficient and stable copper oxide composite photocathode materials were successfully fabricated by a facile two-step electrochemical strategy, which consists of electrodeposition of a Cu film on an ITO glass substrate followed by anodization of the Cu film under a suitable current density and then calcination to form a Cu2O/CuO composite. The synthesized Cu2O/CuO composite was composed of a thin layer of Cu2O with a thin film of CuO on its top as a protecting coating. The rational control of chemical composition and crystalline orientation of the composite materials was easily achieved by varying the electrochemical parameters, including electrodeposition potential and anodization current density, to achieve an enhanced PEC performance. The best photocathode material among all materials prepared was the Cu2O/CuO composite with Cu2O in (220) orientation, which showed a highly stable photocurrent of −1.54 mA cm−2 at a potential of 0 V vs reversible hydrogen electrode at a mild pH under illumination of AM 1.5G. This photocurrent density was more than 2 times that generated by the bare Cu2O electrode (−0.65 mAcm−2) and the stability was considerably enhanced to 74.4% from 30.1% on the bare Cu2O electrode. The results of this study showed that the top layer of CuO in the Cu2O/CuO composite not only minimized the Cu2O photocorrosion but also served as a recombination inhibitor for the photogenerated electrons and holes from Cu2O, which collectively explained much enhanced stability and PEC activity of the Cu2O/CuO composite. Thus, the electrochemical strategy proposed in this study for the synthesis of the Cu2O/CuO composite opens a new way to use copper oxides as photocathode materials in PEC cells for a highly stable and effective water splitting.

Deposition of CdS nanoparticles within free-standing both-side-open stretched TiO 2 nanotube-array films for the enhancement of photoelectrochemical performance

A heterojunction CdS/TiO 2 photoelectrode was prepared using pre-crystallized annealing, followed by solution-based methods, to deposit CdS within the tubes of a free-standing both-side-open TiO 2 nanotube-array (TiNT) film. Such open-ended structures make the nanochannels more amenable to CdS deposition compared with close-ended structures, thus preventing entry clogging. In the experiment on photoelectrochemical (PEC) cells, the use of CdS/both-side-open TiNT film allows easy access of the outer light and the electrolyte to the nanochannels because of the uniform CdS deposition within the tubes. As compared to the one-side-close TiNT-based PEC cell, the maximum incident-photon-to-current conversion efficiency at 455 nm is increased from 41% to 68% in the both-side-open TiNT-based device measured at a potential of − 0.4 V versus Ag/AgCl under front-side illumination.

Photoelectrochemical degradation of methyl orange by TiO2 nanopore arrays electrode and its comparison with TiO2 nanotube arrays electrode

Highly ordered TiO(2) nanopore arrays (TNPs) electrode was applied as an electrode material for photoelectrochemical (PEC) degradation of methylic orange (MO). As a comparison, the self-organized TiO(2) nanotube arrays (TNAs) electrode about 500 nm in length was fabricated by Ti anodization in 0.5 wt% HF-H(2)O solution. The COD removal rate and color removal rate for PEC degradation of MO using the TNPs electrode was found to be 9 and 7%, respectively, as high as that obtained for TNAs electrode when biased at 0.5 V. The results indicate that the fast electron separation and transport properties of TNPs electrode makes it possesses enhanced PEC performance for the degradation of MO. In addition, the color removal rate of MO by TNPs PEC process increased with increasing bias potential and electrolyte concentration, but decreased with the increasing pH value and initial concentration of the reaction solution.

Microwave-assisted low temperature fabrication of nanostructured α-Fe 2O 3 electrodes for solar-driven hydrogen generation

It is demonstrated for the first time that significant enhancement of photoelectrochemical performance could be achieved by using microwave-assisted annealing for the fabrication of α-Fe 2O 3 thin films. The process can also lead to significant energy savings (>60% when compared with conventional methods). Different types of Fe thin films were oxidized using both microwave and conventional heating techniques. The photoelectrochemical performance of electrodeposited, undoped and Si-doped iron oxide samples showed that microwave-annealing resulted in superior structural and performance enhancements. The photocurrent densities obtained from microwave annealed samples are among the highest values reported for α-Fe 2O 3 photoelectrodes fabricated at low temperatures and short times; the highest photocurrent density at 0.55 V vs. V Ag/AgCl, before the dark current onset, was 450 μA cm −2 for the Si-doped films annealed at 270 °C for 15 min using microwave irradiation (and 180 μA cm −2 at 0.23 V vs. V Ag/AgCl) while conventional annealing at the same temperature resulted in samples with negligible (3 μA cm −2) photoactivity. In contrast, a 450 °C/15 min conventional heat treatment only resulted in a film with 25% lower photocurrent density than that of the microwave annealed sample. The improved performance is attributed to the lower processing temperatures and rapidity of the microwave method that help to retain the nanostructure of the thin films whilst restricting the grain growth to a minimum. The lower processing temperature requirements of the microwave process can also open up the possibility of fabricating hematite thin films on conducting, flexible, plastic electronic substrates.

Highly efficient photoelectrochemical hydrogen generation using a ZnO nanowire array and a CdSe/CdS co-sensitizer

Highly efficient photoelectrochemical (PEC) hydrogen generation was achieved by fabricating CdSe deposited ZnO/CdS core/shell nanowire (NW) array photoanodes by a facile three-step solution-based method. Well-defined electrical pathways in 1-dimensional (1D) NW structures allowed efficient charge carrier collection, and CdSe/CdS co-sensitization enabled utilization of the visible region in the solar spectrum. PEC devices using CdSe/CdS/ZnO NW arrays showed improved absorption spectra, and they demonstrated a remarkable enhancement in PEC performance. Our proposed structure is a promising candidate photoanode for solar energy-to-hydrogen conversion devices.

Electrochemical Preparation and Characterization of Surface-Fluorinated TiO2 Nanoporous Film and Its Enhanced Photoelectrochemical and Photocatalytic Properties

In recent years, surface fluorination of TiO2 has been demonstrated to be an efficient method for improving its photocatalytic (PC) reactivity toward some pollutants. In this paper, a new and simple method for surface fluorination and porosity-creating of TiO2 is presented. The sample was prepared by anodization of TiO2 in HF aqueous solutions formed by direct thermal oxidization of titanium sheet. The preparation conditions were optimized. The prepared samples were characterized by scanning electron microscopy (SEM), Raman spectroscopy, UV–vis diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS), and photoluminescence (PL). The photoelectrochemical (PEC) and photocatalytic (PC) properties of the samples were also investigated. SEM and Raman showed that the anodization made the TiO2 nanoporous, which consequently increased markedly the specific surface area, but did not change the bulk crystal structure under a certain preparation condition. DRS analysis revealed that the light absorption was decreased after electrochemical etching. The band gap energy narrowed from 3.15 to 3.04 eV. The surface fluorination of the etched-TiO2 was evidently supported by XPS. Highly enhanced PEC performance and PC activities for the degradation of target pollutants, phenol, methylene blue, and reactive brilliant red on the etched-TiO2 were observed. The possible reasons for such an improvement were studied in detail by a combination of the above-mentioned methods and (photo) electrochemical techniques. It is mainly attributed to the enhanced specific surface area, negative-shifted appearing energy band edges, decreased surface recombination centers, and/or favorable charge transfer rate. The F-containing TiO2 electrode has an excellent stability against fluoride desorption.