Omid Zandi

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.

Enhanced water splitting efficiency through selective surface state removal

Hematite (α-Fe2O3) thin film electrodes prepared by atomic layer deposition (ALD) were employed to photocatalytically oxidize water under 1 sun illumination. It was shown that annealing at 800 °C substantially improves the water oxidation efficiency of the ultrathin film hematite electrodes. The effect of high temperature treatment is shown to remove one of two surface states identified, which reduces recombination and Fermi level pinning. Further modification with Co-Pi water oxidation catalyst resulted in unprecedented photocurrent onset potential of ∼0.6 V versus reversible hydrogen electrode (RHE; slightly positive of the flat band potential).

Determination of photoelectrochemical water oxidation intermediates on haematite electrode surfaces using operando infrared spectroscopy

Semiconductor electrodes capable of using solar photons to drive water-splitting reactions, such as haematite (α-Fe2O3), have been the subject of tremendous interest over recent decades. The surface has been found to play a significant role in determining the efficiency of water oxidation with haematite; however, previous works have only allowed hypotheses to be formulated regarding the identity of relevant surface species. Here we investigate the water-oxidation reaction on haematite using infrared spectroscopy under photoelectrochemical (PEC) water-oxidation conditions. A potential- and light-dependent absorption peak at 898 cm(-1) is assigned to a Fe(IV)=O group, which is an intermediate in the PEC water-oxidation reaction. These results provide direct evidence of high-valent iron-oxo intermediates as the product of the first hole-transfer reaction on the haematite surface and represent an important step in establishing the mechanism of PEC water oxidation on semiconductor electrodes.

Titanium incorporation into hematite photoelectrodes: Theoretical considerations and experimental observations

A theoretical and experimental perspective on the role of titanium impurities in hematite (α-Fe2O3) nanostructured photoelectrodes for solar fuel synthesis devices is provided. Titanium incorporation is a known correlate to efficiency enhancement in α-Fe2O3 photoanodes for solar water oxidation; here the relevant literature and the latest advances are presented and various proposed mechanisms for enhancement are contrasted. Available experimental evidence suggests that Ti incorporation increases net electron carrier concentrations in electrodes, most likely to the extent that (synthesis-dependent) charge compensating cation vacancies are not present. However, electron conductivity increases alone cannot quantitatively account for the large associated photoelectrochemical performance enhancements. The magnitudes of the effects of Ti incorporation on electronic and magnetic properties appear to be highly synthesis-dependent, which has made difficult the development of consistent and general mechanisms explaining experimental and theoretical observations. In this context, we consider how the electronic structure correlates with Ti impurity incorporation in α-Fe2O3 from the perspective of synchrotron-based soft X-ray absorption spectroscopy measurements. Measurements are performed on sets of electrodes fabricated by five relevant and unrelated chemical and physical techniques. The effects of titanium impurities are reflected in the electronic structure through several universally observed spectral characteristics, irrespective of the synthesis techniques. Absorption spectra at the oxygen K-edge show that Ti incorporation is associated with new oxygen 2p-hybridized states, overlapping with and distorting the known unoccupied Fe 3d–O 2p band of α-Fe2O3. This is an indication of mixing of Ti s and d states in the conduction band of α-Fe2O3. A comparison of spectra obtained with electron and photon detection shows that the effects of Ti incorporation on the conduction band are more pronounced in the near-surface region. Titanium L2,3-edge absorption spectra show that titanium is incorporated into α-Fe2O3 as Ti4+ by all fabrication methods, with no long-range titania order detected. Iron L2,3-edge absorption spectra indicate that Ti incorporation is not associated with the formation of any significant concentrations of Fe2+, an observation common to many prior studies on this material system.

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.

The potential versus current state of water splitting with hematite

This review describes the potential of hematite as a photoanode material for photoelectrochemical (PEC) water splitting. The current understanding of key loss-mechanisms of hematite are introduced and correlated to performance enhancement strategies. The significant voltage loss associated with overcoming the competitive water oxidation and surface state recombination has recently been surmounted through a combination of high temperature annealing and surface modification with water oxidation catalysts. Substantial efforts have been made at nanostructuring electrodes to increase the charge separation efficiency without sacrificing light absorption. Even in optimized nanostructured electrodes, however, charge separation continues to be the primary barrier to achieving efficient water splitting with hematite. Specifically, significant depletion region recombination results in voltage dependant photocurrent which constrains the fill factor. Thus, future directions to enhance the efficiency of hematite electrodes are discussed with an emphasis on circumventing depletion region recombination.

Competitive Photoelectrochemical Methanol and Water Oxidation with Hematite Electrodes

Photocatalytic water and methanol oxidation were studied at thin film hematite electrodes synthesized by atomic layer deposition (ALD). Systematic photoelectrochemical characterization along with O2 evolution measurements were carried out in order to better understand the mechanisms of both water and methanol oxidation at hematite electrodes. When both water and methanol are present in the solution, they are oxidized competitively with each other, allowing the detection and assignment of distinct surface states characteristic to each process. The measurement of different surface states for methanol and water oxidation, along with the absence of measurable surface states in an inert acetonitrile electrolyte, clearly shows that the detected surface states are chemically distinct reaction intermediates of water or methanol oxidation.

Atomic layer stack deposition-annealing synthesis of CuWO4

A stack deposition-annealing (SDA) approach was demonstrated for the synthesis of CuWO4via atomic layer deposition (ALD). The resulted CuWO4 showed promising activity as a photoanode for photoelectrochemical (PEC) water splitting. This methodology can, in principal, be utilized for the ALD of many ternary and quaternary oxides for various applications.

Localized Surface Plasmon Resonance in Semiconductor Nanocrystals

Localized surface plasmon resonance (LSPR) in semiconductor nanocrystals (NCs) that results in resonant absorption, scattering, and near field enhancement around the NC can be tuned across a wide optical spectral range from visible to far-infrared by synthetically varying doping level, and post synthetically via chemical oxidation and reduction, photochemical control, and electrochemical control. In this review, we will discuss the fundamental electromagnetic dynamics governing light matter interaction in plasmonic semiconductor NCs and the realization of various distinctive physical properties made possible by the advancement of colloidal synthesis routes to such NCs. Here, we will illustrate how free carrier dielectric properties are induced in various semiconductor materials including metal oxides, metal chalcogenides, metal nitrides, silicon, and other materials. We will highlight the applicability and limitations of the Drude model as applied to semiconductors considering the complex band structures and crystal structures that predominate and quantum effects that emerge at nonclassical sizes. We will also emphasize the impact of dopant hybridization with bands of the host lattice as well as the interplay of shape and crystal structure in determining the LSPR characteristics of semiconductor NCs. To illustrate the discussion regarding both physical and synthetic aspects of LSPR-active NCs, we will focus on metal oxides with substantial consideration also of copper chalcogenide NCs, with select examples drawn from the literature on other doped semiconductor materials. Furthermore, we will discuss the promise that LSPR in doped semiconductor NCs holds for a wide range of applications such as infrared spectroscopy, energy-saving technologies like smart windows and waste heat management, biomedical applications including therapy and imaging, and optical applications like two photon upconversion, enhanced luminesence, and infrared metasurfaces.

Impacts of surface depletion on the plasmonic properties of doped semiconductor nanocrystals

Degenerately doped semiconductor nanocrystals (NCs) exhibit a localized surface plasmon resonance (LSPR) in the infrared range of the electromagnetic spectrum. Unlike metals, semiconductor NCs offer tunable LSPR characteristics enabled by doping, or via electrochemical or photochemical charging. Tuning plasmonic properties through carrier density modulation suggests potential applications in smart optoelectronics, catalysis and sensing. Here, we elucidate fundamental aspects of LSPR modulation through dynamic carrier density tuning in Sn-doped In2O3 (Sn:In2O3) NCs. Monodisperse Sn:In2O3 NCs with various doping levels and sizes were synthesized and assembled in uniform films. NC films were then charged in an in situ electrochemical cell and the LSPR modulation spectra were monitored. Based on spectral shifts and intensity modulation of the LSPR, combined with optical modelling, it was found that often-neglected semiconductor properties, specifically band structure modification due to doping and surface states, strongly affect LSPR modulation. Fermi level pinning by surface defect states creates a surface depletion layer that alters the LSPR properties; it determines the extent of LSPR frequency modulation, diminishes the expected near-field enhancement, and strongly reduces sensitivity of the LSPR to the surroundings.Degenerately doped semiconductor nanocrystals exhibit localized surface plasmon resonance in the infrared. Semiconducting properties such as band structure modification due to doping and surface states are now shown to strongly affect plasmonic modulation.