Coleman X. Kronawitter

A perspective on solar-driven water splitting with all-oxide hetero-nanostructures

A perspective on the design of all-oxide heterostructures for application in photoelectrochemical cells for solar water splitting is provided. Particular attention is paid to those structures which possess nanoscale feature dimensionality, as structures of this type are most likely to utilize the benefits afforded by the formation of oxide heterojunctions and likely to show functional behavior relating to the interfacial region. In the context of this discussion, a novel hetero-nanostructure array, based on quantum-confined and visible light-active iron(III) oxide nanostructures and their surface modification with tungsten(VI) oxide, is introduced. The heterostructure architecture is designed to combine the functionality of the consituent phases to address the primary requirements for electrodes enabling the efficient generation of hydrogen using solar energy: visible light activity, chemical stability, appropriate bandedge characteristics, and potential for low-cost fabrication. Photoelectrochemical characterization for solar hydrogen/oxygen generation indicates the presence of unexpected minority carrier transfer dynamics within the oxide hetero-nanostructures, as observed additionally by ultrafast transient absorption spectroscopy.

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.

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

AbstractHematite (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.

Electron enrichment in 3d transition metal oxide hetero-nanostructures.

Direct experimental observation of spontaneous electron enrichment of metal d orbitals in a new transition metal oxide heterostructure with nanoscale dimensionality is reported. Aqueous chemical synthesis and vapor phase deposition are combined to fabricate oriented arrays of high-interfacial-area hetero-nanostructures comprised of titanium oxide and iron oxide nanomaterials. Synchrotron-based soft X-ray spectroscopy techniques with high spectral resolution are utilized to directly probe the titanium and oxygen orbital character of the interfacial regions occupied and unoccupied densities of states. These data demonstrate the interface to possess electrons in Ti 3d bands and an emergent degree of orbital hybridization that is absent in parent oxide reference crystals. The carrier dynamics of the hetero-nanostructures are studied by ultrafast transient absorption spectroscopy, which reveals the presence of a dense manifold of states, the relaxations from which exhibit multiple exponential decays whose magnitudes depend on their energetic positions within the electronic structure.

Physical and photoelectrochemical characterization of Ti-doped hematite photoanodes prepared by solution growth

We present the fabrication and characterization of Ti-doped hematite (α-Fe2O3) films for application as photoanodes in photoelectrochemical (PEC) cells for water splitting. It is demonstrated that Ti doping significantly improves the PEC activity as the photocurrent at 1.0 V vs. Ag/AgCl electrode for a 400 nm thick Ti-doped film (0.66 mA cm−2) was found to be ∼14 times higher than that of an undoped film (0.045 mA cm−2). The films were characterized by X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and ultrafast transient absorption spectroscopy to obtain information about their structural, electronic, and charge carrier dynamic properties. Based on characterization of the chemical states of the involved elements as well as the charge carrier dynamics of the films with Ti doping, it appears that the photocurrent enhancement is related to an increase in charge carrier density or reduced electron–hole recombination. The highest incident photon conversion efficiency (IPCE) measured for this system was 27.0% at 360 nm at a potential of 1.23 V vs. reversible hydrogen electrode (RHE), which was obtained on a 400 nm thick Ti-doped α-Fe2O3 film.

Solar light-driven photocatalytic hydrogen evolution over ZnIn2S4 loaded with transition-metal sulfides

A series of Pt-loaded MS/ZnIn2S4 (MS = transition-metal sulfide: Ag2S, SnS, CoS, CuS, NiS, and MnS) photocatalysts was investigated to show various photocatalytic activities depending on different transition-metal sulfides. Thereinto, CoS, NiS, or MnS-loading lowered down the photocatalytic activity of ZnIn2S4, while Ag2S, SnS, or CuS loading enhanced the photocatalytic activity. After loading 1.0 wt.% CuS together with 1.0 wt.% Pt on ZnIn2S4, the activity for H2 evolution was increased by up to 1.6 times, compared to the ZnIn2S4 only loaded with 1.0 wt.% Pt. Here, transition-metal sulfides such as CuS, together with Pt, acted as the dual co-catalysts for the improved photocatalytic performance. This study indicated that the application of transition-metal sulfides as effective co-catalysts opened up a new way to design and prepare high-efficiency and low-cost photocatalysts for solar-hydrogen conversion.

Soft X-ray characterization of Zn1−xSnxOy electronic structure for thin film photovoltaics

Zinc tin oxide (Zn(1-x)Sn(x)O(y)) has been proposed as an alternative buffer layer material to the toxic, and light narrow-bandgap CdS layer in CuIn(1-x),Ga(x)Se(2) thin film solar cell modules. In this present study, synchrotron-based soft X-ray absorption and emission spectroscopies have been employed to probe the densities of states of intrinsic ZnO, Zn(1-x)Sn(x)O(y) and SnO(x) thin films grown by atomic layer deposition. A distinct variation in the bandgap is observed with increasing Sn concentration, which has been confirmed independently by combined ellipsometry-reflectometry measurements. These data correlate directly to the open circuit potentials of corresponding solar cells, indicating that the buffer layer composition is associated with a modification of the band discontinuity at the CIGS interface. Resonantly excited emission spectra, which express the admixture of unoccupied O 2p with Zn 3d, 4s, and 4p states, reveal a strong suppression in the hybridization between the O 2p conduction band and the Zn 3d valence band with increasing Sn concentration.

Activity of pure and transition metal-modified CoOOH for the oxygen evolution reaction in an alkaline medium

A new electrode structure enabling low overpotentials for the oxidation of water, based on three-dimensional arrays of CoOOH nanowires, is presented. The electrocatalytic activities of pure and M-modified cobalt oxyhydroxides (M = Ni or Mn) nanowires have been investigated in detail for the oxygen evolution reaction (OER) in an alkaline environment. The pure, Ni-, and Mn-modified nanowires, with preferentially exposed low-index surfaces, were fabricated directly on stainless steel mesh current collectors using an inexpensive and scalable chemical synthesis procedure. The unique electrode structure ensures excellent substrate–catalyst electrical contact and increases the surface area accessible to the electrolyte. The OER activity of CoOOH nanowires is shown to be significantly improved through incorporation of Ni. Specifically, optimal OER activity is obtained for CoOOH nanowires with 9.7% surface Ni content, which corresponds to four-times greater current density compared to pure CoOOH. In contrast, Mn modification of the CoOOH nanowires did not improve the OER activity. Tafel analysis suggests Ni incorporation leads to change in the OER rate-determining step based on an observed decrease in the Tafel slope. Electrochemical impedance spectroscopy reveals that Ni incorporation improves the ability of the catalysts to stabilize surface intermediates, whereas Mn incorporation impedes intermediate stabilization. This study provides new insights regarding the influence of transition metal impurities on the OER activity of CoOOH and provides a clear strategy for the optimization of CoOOH-based OER catalysts in alkaline electrolytes.

Effect of Noble Metal in CdS/M/TiO2 for Photocatalytic Degradation of Methylene Blue under Visible Light

ABSTRACT The CdS/M/TiO2 (M = Ag, Ru, Au, Pd, and Pt) three-component nanojunction systems were constructed using a two-step photodeposition method, and evaluated for their photocatalytic activities through the degradation of methylene blue in aqueous solution under visible light irradiation. The authors found that the photocatalytic activity of CdS/M/TiO2 (M = Ag, Ru, Au, Pd, Pt) three-component nanojunctions was superior to that of CdS/TiO2 two-component system. Moreover, the photocatalytic activity of the three-component nanojunction system was found to be dependent significantly on the type of the noble metals. The results can be explained by the influence of charge transfer on the basis of the work functions of different noble metals.

Doped, porous iron oxide films and their optical functions and anodic photocurrents for solar water splitting

The fabrication and morphological, optical, and photoelectrochemical characterization of doped iron oxide films is presented. The complex index of refraction and absorption coefficient of polycrystalline films are determined through measurement and modeling of spectral transmission and reflection data using appropriate dispersion relations. Photoelectrochemical characterization for water photo-oxidation reveals that the conversion efficiencies of electrodes are strongly influenced by substrate temperature during their oblique-angle physical vapor deposition. These results are discussed in terms of the films’ morphological features and the known optoelectronic limitations of iron oxide films for application in solar water splitting devices.