Benjamin M. Klahr

Water Oxidation at Hematite Photoelectrodes: The Role of Surface States

Hematite (α-Fe(2)O(3)) constitutes one of the most promising semiconductor materials for the conversion of sunlight into chemical fuels by water splitting. Its inherent drawbacks related to the long penetration depth of light and poor charge carrier conductivity are being progressively overcome by employing nanostructuring strategies and improved catalysts. However, the physical-chemical mechanisms responsible for the photoelectrochemical performance of this material (J(V) response) are still poorly understood. In the present study we prepared thin film hematite electrodes by atomic layer deposition to study the photoelectrochemical properties of this material under water-splitting conditions. We employed impedance spectroscopy to determine the main steps involved in photocurrent production at different conditions of voltage, light intensity, and electrolyte pH. A general physical model is proposed, which includes the existence of a surface state at the semiconductor/liquid interface where holes accumulate. The strong correlation between the charging of this state with the charge transfer resistance and the photocurrent onset provides new evidence of the accumulation of holes in surface states at the semiconductor/electrolyte interface, which are responsible for water oxidation. The charging of this surface state under illumination is also related to the shift of the measured flat-band potential. These findings demonstrate the utility of impedance spectroscopy in investigations of hematite electrodes to provide key parameters of photoelectrodes with a relatively simple measurement.

Electrochemical and photoelectrochemical investigation of water oxidation with hematite electrodes

Atomic layer deposition (ALD) was utilized to deposit uniform thin films of hematite (α-Fe2O3) on transparent conductive substrates for photocatalytic water oxidation studies. Comparison of the oxidation of water to the oxidation of a fast redox shuttle allowed for new insight in determining the rate limiting processes of water oxidation at hematite electrodes. It was found that an additional overpotential is needed to initiate water oxidation compared to the fast redox shuttle. A combination of electrochemical impedance spectroscopy, photoelectrochemical and electrochemical measurements were employed to determine the cause of the additional overpotential. It was found that photogenerated holes initially oxidize the electrode surface under water oxidation conditions, which is attributed to the first step in water oxidation. A critical number of these surface intermediates need to be generated in order for the subsequent hole-transfer steps to proceed. At higher applied potentials, the behavior of the electrode is virtually identical while oxidizing either water or the fast redox shuttle; the slight discrepancy is attributed to a shift in potential associated with Fermi level pinning by the surface states in the absence of a redox shuttle. A water oxidation mechanism is proposed to interpret these results.

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.

Highly photoactive Ti-doped α-Fe2O3 thin film electrodes: resurrection of the dead layer

Uniform thin films of hematite and Ti-doped hematite (α-Fe2O3) were deposited on transparent conductive substrates using atomic layer deposition (ALD). ALDs epitaxial growth mechanism allowed the control of the morphology and thickness of the hematite films as well as the concentration and distribution of Ti atoms. The photoelectrochemical performances of Ti-doped and undoped hematite electrodes were examined and compared under water oxidation conditions. The incorporation of Ti atoms into hematite electrodes was found to dramatically enhance the water oxidation performance, with much greater enhancement found for the thinnest films. An optimum concentration ∼3 atomic% of Ti atoms was also determined. A series of electrochemical, photoelectrochemical and impedance spectroscopy measurements were employed to elucidate the cause of the improved photoactivity of the Ti-doped hematite thin films. This performance enhancement was a combination of improved bulk properties (hole collection length) and surface properties (water oxidation efficiency). The improvement in both bulk and surface properties is attributed to the resurrection of a dead layer by the Ti dopant atoms.

Photoelectrochemical investigation of ultrathin film iron oxide solar cells prepared by atomic layer deposition.

Atomic layer deposition was used to grow conformal thin films of hematite with controlled thickness on transparent conductive oxide substrates. The hematite films were incorporated as photoelectrodes in regenerative photoelectrochemical cells employing an aqueous [Fe(CN)(6)](3-/4-) electrolyte. Steady state current density versus applied potential measurements under monochromatic and simulated solar illumination were used to probe the photoelectrochemical properties of the hematite electrodes as a function of film thickness. Combining the photoelectrochemical results with careful optical measurements allowed us to determine an optimal thickness for a hematite electrode of ∼20 nm. Mott-Schottky analysis of differential capacitance measurements indicated a depletion region of ∼17 nm. Thus, only charge carriers generated in the depletion region were found to contribute to the photocurrent.

Directed Growth of Electroactive Metal-Organic Framework Thin Films Using Electrophoretic Deposition

Electrophoretic deposition (EPD) is used to assemble metal-organic framework (MOF) materials in nano- and micro-particulate, thin-film form. The flexibility of the method is demonstrated by the successful deposition of 4 types of MOFs: NU-1000, UiO-66, HKUST-1, and Al-MIL-53. Additionally, EPD is used to pattern the growth of NU-1000 thin films that exhibit full electrochemical activity.

Photocatalytic water oxidation with hematite electrodes

Hematites favorable 2.1 eV band gap, valence band position, stability, abundance, and light absorption properties make it a promising semiconductor material for solar-driven water oxidation. While a mechanism for water oxidation at the surface of hematite has not yet been experimentally established, it is widely agreed upon that surface-state mediated charge recombination at the electrode–electrolyte interface competes with water oxidation. This kinetic competition ultimately limits the water splitting efficiency. The identity and role of these surface states in the water oxidation reaction is still unclear. This perspective presents recent results in probing photocatalytic water oxidation with hematite electrodes and the role of surface states. In addition, the function of surface coatings on the hematite surface, and their role as catalysts or surface passivation materials, are discussed.

Metal–Organic Framework Thin Films as Platforms for Atomic Layer Deposition of Cobalt Ions To Enable Electrocatalytic Water Oxidation

Thin films of the metal-organic framework (MOF) NU-1000 were grown on conducting glass substrates. The films uniformly cover the conducting glass substrates and are composed of free-standing sub-micrometer rods. Subsequently, atomic layer deposition (ALD) was utilized to deposit Co(2+) ions throughout the entire MOF film via self-limiting surface-mediated reaction chemistry. The Co ions bind at aqua and hydroxo sites lining the channels of NU-1000, resulting in three-dimensional arrays of separated Co ions in the MOF thin film. The Co-modified MOF thin films demonstrate promising electrocatalytic activity for water oxidation.

Synthesis of nanocrystals of Zr-based metal–organic frameworks with csq-net: significant enhancement in the degradation of a nerve agent simulant

The synthesis of nano-sized particles of NU-1000 (length from 75 nm to 1200 nm) and PCN-222/MOF-545 (length from 350 nm to 900 nm) is reported. The catalytic hydrolysis of methyl paraoxon was investigated as a function of NU-1000 crystallite size and a significant enhancement in the rate was observed for the nano-sized crystals compared to microcrystals.

Voltage dependent photocurrent of thin film hematite electrodes

The current density (J) vs. applied voltage (V) curves of thin-film hematite electrodes under illumination exhibit non-ideal behavior; as a result, very poor fill factors and photocurrent densities are generally observed. A simple model is presented to describe the photocurrent density behavior of hematite photoelectrodes, which assumes only drift collection of holes in a uniform electric field. Excellent agreement is found between the model and experimental results. Use of this model provides important insight into the limitations of hematite electrodes as well as strategies to achieve improved efficiency.