Sean P. Berglund

Enhancing visible light photo-oxidation of water with TiO2 nanowire arrays via cotreatment with H2 and NH3: synergistic effects between Ti3+ and N.

We report a synergistic effect involving hydrogenation and nitridation cotreatment of TiO(2) nanowire (NW) arrays that improves the water photo-oxidation performance under visible light illumination. The visible light (>420 nm) photocurrent of the cotreated TiO(2) is 0.16 mA/cm(2) and accounts for 41% of the total photocurrent under simulated AM 1.5 G illumination. Electron paramagnetic resonance (EPR) spectroscopy reveals that the concentration of Ti(3+) species in the bulk of the TiO(2) following hydrogenation and nitridation cotreatment is significantly higher than that of the sample treated solely with ammonia. It is believed that the interaction between the N-dopant and Ti(3+) is the key to the extension of the active spectrum and the superior visible light water photo-oxidation activity of the hydrogenation and nitridation cotreated TiO(2) NW arrays.

Synthesis of BiVO4 nanoflake array films for photoelectrochemical water oxidation

Because of the potential for application in photoelectrochemical cells for water splitting, the synthesis of nanostructured BiVO4 is receiving increasing attention. Here we report a simple new drop-casting method for the first time to synthesize un-doped and doped bismuth vanadate (BiVO4) nanoflake array films. Synthesis parameters such as the amount of polyethylene glycol 600 (PEG-600) and the precursor solution drying time are investigated to optimize the films for photoelectrochemical water oxidation. The BiVO4 films consisting of nanoflakes with an average thickness of 20 nm and length of 2 μm were synthesized from a precursor solution containing Bi3+, V3+ and PEG-600 with a Bi:V: PEG-600 volume ratio of 2:2:1, dried at 135 °C for 55 min. Photoelectrochemical measurements show that the BiVO4 nanoflake array films have higher photoelectrochemical activity than the BiVO4 nanoparticle films. Additionally, the nanoflake arrays were tested after incorporating W and Mo to enhance the photoelectrochemical activity. The 2% W, 6% Mo co-doped BiVO4 nanoflake array films demonstrate the best photoelectrochemical activity with photocurrent densities about 2 times higher than the un-doped BiVO4 nanoflake films and greater than the photocurrents of individually Mo doped or W doped BiVO4 films. The origin of enhanced photoelectrochemical activity for the co-doped film may be due to the improved conductivity through the BiVO4 or slightly enhanced water oxidation kinetics.

Nanostructured Bi2S3/WO3 heterojunction films exhibiting enhanced photoelectrochemical performance

To improve the photoelectrochemical activity of WO3, Bi2S3/WO3 heterojunction films were designed by coupling WO3 films with varying amounts of urchin-like Bi2S3 nanospheres. The WO3 films were composed of WO3 nanoprism arrays, which were synthesized using a solvothermal method. After coating a single layer of Bi2S3 on top of the WO3 film, the resulting Bi2S3/WO3 heterojunction film showed enhanced photoelectrochemical activity. At 1.2 V vs. Ag/AgCl, the initial photocurrent density of the Bi2S3/WO3 heterojunction film with one layer of Bi2S3 was 1.33 mA cm−2 in 0.1 M Na2SO4 and 1.19 mA cm−2 in a 0.2 M NaCl mixed water–ethanol solution, which was 40% and 32% higher than the bare WO3 film under the same conditions, respectively. The optimal number of Bi2S3 layers for coupling with the WO3 film was found to be 3 layers, which had the highest photocurrent density and IPCE values. The photoelectrochemical activity of Bi2S3/WO3 heterojunction film was not stable for water oxidation due to photocorrosion in aqueous electrolyte, but it was stable in the NaCl mixed water–ethanol solution and a non-aqueous solution containing iodide/triiodide as a redox couple. The origin of enhanced photoelectrochemical activity of the Bi2S3/WO3 heterojunction film was primarily ascribed to the band potential matching between WO3 and Bi2S3, which is advantageous for charge separation.

p-Si/W2C and p-Si/W2C/Pt Photocathodes for the Hydrogen Evolution Reaction

p-Si/W2C photocathodes are synthesized by evaporating tungsten metal in an ambient of ethylene gas to form tungsten semicarbide (W2C) thin films on top of p-type silicon (p-Si) substrates. As deposited the thin films contain crystalline W2C with a bulk W:C atomic ratio of approximately 2:1. The W2C films demonstrate catalytic activity for the hydrogen evolution reaction (HER), and p-Si/W2C photocathodes produce cathodic photocurrent at potentials more positive than 0.0 V vs RHE while bare p-Si photocathodes do not. The W2C films are an effective support for Pt nanoparticles allowing for a considerable reduction in Pt loading. p-Si/W2C/Pt photocathodes with Pt nanoparticles achieve photocurrent onset potentials and limiting photocurrent densities that are comparable to p-Si/Pt photocathodes with Pt loading nine times higher. This makes W2C an earth abundant alternative to pure Pt for use as an electrocatalyst on photocathodes for the HER.

Parallel Screening of Electrocatalyst Candidates Using Bipolar Electrochemistry

Here we report simultaneous screening of bimetallic electrocatalyst candidates for the oxygen reduction reaction (ORR) using bipolar electrochemistry. The analysis is carried out by dispensing different bimetallic precursor compositions onto the cathodic poles of an array of bipolar electrodes (BPEs) and then heating them in a reducing atmosphere to yield the catalyst candidates. Because BPEs do not require a direct electrical connection for activation, up to 33 electrocatalysts can be screened simultaneously by applying a voltage to the electrolyte solution in which the BPE array is immersed. The screening of the electrocatalyst candidates can be achieved in about 10 min. The current required to drive the ORR arises from oxidation of Cr microbands present at the anodic poles of the BPEs. Therefore, the most effective electrocatalysts result in oxidation (dissolution) of the most microbands, and simply counting the microbands remaining at the end of the screen provides information about the onset potential required to reduce oxygen. Here, we evaluated three Pd-M (M = Au, Co, W) bimetallic electrocatalysts. In principle, arbitrarily large libraries of electrocatalysts can be screened using this approach.

Antimony-Doped Tin Oxide Nanorods as a Transparent Conducting Electrode for Enhancing Photoelectrochemical Oxidation of Water by Hematite

We report the growth of well-defined antimony-doped tin oxide (ATO) nanorods as a conductive scaffold to improve hematites photoelectrochemical water oxidation performance. The hematite grown on ATO exhibits greatly improved performance for photoelectrochemical water oxidation compared to hematite grown on flat fluorine-doped tin oxide (FTO). The optimized photocurrent density of hematite on ATO is 0.67 mA/cm(2) (0.6 V vs Ag/AgCl), which is much larger than the photocurrent density of hematite on flat FTO (0.03 mA/cm(2)). Using H2O2 as a hole scavenger, it is shown that the ATO nanorods indeed act as a useful scaffold and enhanced the bulk charge separation efficiency of hematite from 2.5% to 18% at 0.4 V vs Ag/AgCl.

Screening of transition and post-transition metals to incorporate into copper oxide and copper bismuth oxide for photoelectrochemical hydrogen evolution

A new dispenser and scanner system is used to create and screen Bi-M-Cu oxide arrays for cathodic photoactivity, where M represents 1 of 22 different transition and post-transition metals. Over 3000 unique Bi : M : Cu atomic ratios are screened. Of the 22 metals tested, 10 show a M-Cu oxide with higher photoactivity than CuO and 10 show a Bi-M-Cu oxide with higher photoactivity than CuBi2O4. Cd, Zn, Sn, and Co produce the most photoactive M-Cu oxides, all showing a 200-300% improvement in photocurrent over CuO. Ag, Cd, and Zn produce the highest photoactivity Bi-M-Cu oxides with a 200-400% improvement over CuBi2O4. Most notable is a Bi-Ag-Cu oxide (Bi : Ag : Cu atomic ratio of 22 : 3 : 11) which shows 4 times higher photocurrent than CuBi2O4. This material is capable of evolving hydrogen under illumination in neutral electrolyte solutions at 0.6 V vs. RHE when Pt is added to the surface as an electrocatalyst.

Reactive Ballistic Deposition of Nanostructured Model Materials for Electrochemical Energy Conversion and Storage

Porous, high surface area materials have critical roles in applications including catalysis, photochemistry, and energy storage. In these fields, researchers have demonstrated that the nanometer-scale structure modifies mechanical, optical, and electrical properties of the material, greatly influencing its behavior and performance. Such complex chemical systems can involve several distinct processes occurring in series or parallel. Understanding the influence of size and structure on the properties of these materials requires techniques for producing clean, simple model systems. In the fields of photoelectrochemistry and lithium storage, for example, researchers need to evaluate the effects of changing the electrode structure of a single material or producing electrodes of many different candidate materials while maintaining a distinctly favorable morphology. In this Account, we introduce our studies of the formation and characterization of high surface area, porous thin films synthesized by a process called reactive ballistic deposition (RBD). RBD is a simple method that provides control of the morphology, porosity, and surface area of thin films by manipulating the angle at which a metal-vapor flux impinges on the substrate during deposition. This approach is largely independent of the identity of the deposited material and relies upon limited surface diffusion during synthesis, which enables the formation of kinetically trapped structures. Here, we review our results for the deposition of films from a number of semiconductive materials that are important for applications such as photoelectrochemical water oxidation and lithium ion storage. The use of RBD has enabled us to systematically control individual aspects of both the structure and composition of thin film electrodes in order to probe the effects of each on the performance of the material. We have evaluated the performance of several materials for potential use in these applications and have identified processes that limit their performance. Use of model systems, such as these, for fundamental studies or materials screening processes likely will prove useful in developing new high-performance electrodes.

Chemical bath deposition of vertically aligned TiO2 nanoplatelet arrays for solar energy conversion applications

We report a facile, scalable, and low cost chemical bath deposition of vertically aligned TiO2 nanoplatelet arrays on various substrates including fluorine-doped tin oxide coated glass substrates and their applications for photoelectrochemical (PEC) water splitting and dye sensitized solar cells. The TiO2 arrays consisting of single crystal rutile nanoplatelets with heights (film thicknesses) of up to 1 μm, lengths of up to 130 nm, and widths of ∼5 nm were grown via controlling oxidation and hydrolysis of TiCl3 at low pH (0.71–0.85) and low TiCl3 concentration (8–40 mM). As a photoanode for water oxidation in a PEC water splitting cell, the TiO2 nanoplatelets show excellent charge separation characteristics with a saturated photocurrent in 1 M KOH electrolyte under AM 1.5 G illumination of ∼0.4 mA cm−2 reached at an exceptionally low bias of −0.6 V vs. Ag/AgCl (0.4 V vs. reversible hydrogen electrode). Dye sensitized solar cells assembled using N719 dye sensitized-TiO2 nanoplatelet arrays also show promising performance with photoconversion efficiencies of 1.28% for as-synthesized (no thermal post-treatment) and 3.7% for annealed TiO2 nanoplatelets.

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%.