Self-Optimizing Photoelectrochemical Growth of Nanopatterned Se–Te Films in Response to the Spectral Distribution of Incident Illumination

Photoelectrochemical growth of Se–Te films spontaneously produces highly ordered, nanoscale lamellar morphologies with periodicities that can be tuned by varying the illumination wavelength during deposition. This phenomenon has been characterized further herein by determining the morphologies of photoelectrodeposited Se–Te films in response to tailored spectral illumination profiles. Se–Te films grown under illumination from four different sources, having similar average wavelengths but having spectral bandwidths that spanned several orders of magnitude, all nevertheless produced similar structures which had a single, common periodicity as quantitatively identified via Fourier analysis. Film deposition using simultaneous illumination from two narrowband sources, which differed in average wavelength by several hundred nanometers, resulted in a structure with only a single periodicity intermediate between the periods observed when either source alone was used. This single periodicity could be varied by manipulating the relative intensity of the two sources. An iterative model that combined full-wave electromagnetic effects with Monte Carlo growth simulations, and that considered only the fundamental light-material interactions during deposition, was in accord with the morphologies observed experimentally. Simulations of light absorption and concentration in idealized lamellar arrays, in conjunction with all of the available data, additionally indicated that a self-optimization of the periodicity of the nanoscale pattern, resulting in the maximization of the anisotropy of interfacial light absorption in the three-dimensional structure, is consistent with the observed growth process of such films

Optical and electrochemical effects of H₂ and O₂ bubbles at upward-facing Si photoelectrodes

The effects of the size, contact-angle, and coverage of gas bubbles on solar fuels devices were characterized at cm-scale, upward-facing planar and microwire-array Si photoelectrodes in stagnant electrolytes. Experimental measurements were supported by ray-tracing simulations of surface attached gas bubble films. A dilute, redox-active tracer allowed for the quantification of the mass-transport effects of bubble coverage during photoanodic O₂ (g) evolution at upward-facing photoanodes in 1.0 M KOH(aq.). Measurements of the gas coverage at upward-facing p-Si photocathodes in 0.50 M H₂SO₄ (aq.) allowed for the nucleation rate and contact angle of H₂ (g) bubbles to be evaluated for systems having various surface free energies. Under simulated solar illumination, the rapid departure of small O₂ (g) bubbles produced stable photocurrents at upward-facing oxygen-evolving Si photoanodes and yielded increased mass-transport velocities relative to a stagnant electrolyte, indicating that bubbles can provide a net benefit to the photoelectrochemical performance of an upward-facing photoanode in solar fuels devices

Inorganic Phototropism in Electrodeposition of Se-Te

Photoelectrochemical deposition of Se–Te on isolated Au islands using an unstructured, incoherent beam of light produces growth of Se–Te alloy toward the direction of the incident light beam. Full-wave electromagnetic simulations of light absorption indicated that the induced spatial growth anisotropy was a function of asymmetric absorption in the evolving deposit. Inorganic phototropic growth is analogous to biological systems such as palm trees that exhibit phototropic growth wherein physical extension of the plant guides the crown toward the time-averaged position of the sun, to maximize solar harvesting

Electrocatalysis of the hydrogen-evolution reaction by electrodeposited amorphous cobalt selenide films

Using an electrochemical method under ambient conditions, crystallographically amorphous films of cobalt selenide have been deposited from aqueous solution onto planar Ti supports. These films have been evaluated as electrocatalysts for the hydrogen-evolution reaction. In 0.500 M H_2SO_4, the cobalt selenide films required an overpotential of ~135 mV to drive the hydrogen-evolution reaction at a benchmark current density of −10 mA cm^(−2). Galvanostatic measurements indicated stability of the electrocatalytic films for >16 h of continuous operation at −10 mA cm^(−2). The facile preparation method, and the activity of the cobalt selenide films, suggest that electrodeposited metal chalcogenides are potentially attractive earth-abundant electrocatalysts for the hydrogen-evolution reaction

The Influence of Structure and Processing on the Behavior of TiO_2 Protective Layers for Stabilization of n-Si/TiO_2/Ni Photoanodes for Water Oxidation

Light absorbers with moderate band gaps (1–2 eV) are required for high-efficiency solar fuels devices, but most semiconducting photoanodes undergo photocorrosion or passivation in aqueous solution. Amorphous TiO_2 deposited by atomic-layer deposition (ALD) onto various n-type semiconductors (Si, GaAs, GaP, and CdTe) and coated with thin films or islands of Ni produces efficient, stable photoanodes for water oxidation, with the TiO_2 films protecting the underlying semiconductor from photocorrosion in pH = 14 KOH(aq). The links between the electronic properties of the TiO_2 in these electrodes and the structure and energetic defect states of the material are not yet well-elucidated. We show herein that TiO_2 films with a variety of crystal structures and midgap defect state distributions, deposited using both ALD and sputtering, form rectifying junctions with n-Si and are highly conductive toward photogenerated carriers in n-Si/TiO_2/Ni photoanodes. Moreover, the photovoltage of these electrodes can be modified by annealing the TiO_2 in reducing or oxidizing environments. All of the polycrystalline TiO_2 films with compact grain boundaries investigated herein protected the n-Si photoanodes against photocorrosion in pH = 14 KOH(aq). Hence, in these devices, conduction through the TiO_2 layer is neither specific to a particular amorphous or crystalline structure nor determined wholly by a particular extrinsic dopant impurity. The coupled structural and energetic properties of TiO_2, and potentially other protective oxides, can therefore be controlled to yield optimized photoelectrode performance

Optically tunable mesoscale CdSe morphologies via inorganic phototropic growth

Inorganic phototropic growth using only spatially conformal illumination generated Se–Cd films that exhibited precise light-defined mesoscale morphologies including highly ordered, anisotropic, and periodic ridge and trench nanotextures over entire macroscopic substrates. Growth was accomplished via a light-induced electrochemical method using an optically and chemically isotropic solution, an unpatterned substrate, and unstructured, incoherent, low-intensity illumination in the absence of chemical directing agents or physical templates and masks. The morphologies were defined by the illumination inputs: the nanotexture long axes aligned parallel to the optical E-field vector, and the feature sizes and periods scaled with the wavelength. Optically based modeling of the growth closely reproduced the experimental results, confirming the film morphologies were fully determined by the light–matter interactions during growth. Solution processing of the Se–Cd films resulted in stoichiometric, crystalline CdSe films that also exhibited ordered nanotextures, demonstrating that inorganic phototropic growth can effect tunable, template-free generation of ordered CdSe nanostructures over macroscopic length scales