AVS 69 Session SS+HC-TuA: Photochemistry

Seawater electrolysis provides a viable method to produce clean hydrogen fuel. To date, however, the realization of high-performance photocathodes for seawater hydrogen evolution reaction has remained challenging. Here, we introduce n+-p Si photocathodes with dramatically improved activity and stability for hydrogen evolution reaction in seawater, modified by Pt nanoclusters anchored on GaN nanowires (Fig 1). We find that Pt-Ga sites at the Pt/ GaN interface promote the dissociation of water molecules and spilling H* over to neighboring Pt atoms for efficient H2 production. Pt/GaN/Si photocathodes achieve a current density of −10 mA/cm2 at 0.15 and 0.39 V vs. RHE and high applied bias photon-to-current efficiency of 1.7% and 7.9% in seawater (pH = 8.2) and phosphate-buffered seawater (pH = 7.4), respectively. We further demonstrate a record-high photocurrent density of ~169 mA/cm2 under concentrated solar light (9 suns). Moreover, Pt/GaN/Si can continuously produce H2 even under dark conditions by simply switching the electrical contact. This work provides valuable guidelines to design an efficient, stable, and energy- saving electrode for H2 generation by seawater splitting.

Understanding the reactivity of TiO₂ particles with different polymorphs and morphologies is important for many photocatalytic applications. Here, the reactivity of individual anatase TiO₂ microcrystals with a large percentage of (001) facet was monitored and studied using operando photoluminescence microscopy. The photoreduction of resazurin on anatase microcrystals shows that the photoreduction rate on each microcrystal was different although the microcrystals had comparable sizes and exposed the same facets. The reaction rate changes from no reactivity to higher than that of the anatase (001) bulk single crystal. The reaction rate of the anatase microcrystals depends on the morphology and the structure of each particle. The reactivities of the microcrystals with mixed anatase-rutile phases and after the anatase-to-rutile phase transformation have also been monitored.

Photo-electrocatalysis involving complex oxide-water interfaces is a highly promising technology for the sustainable production of fuels. However, probing these complex interfaces and gaining atomistic insights is still very challenging for current experimental methods, and is often only possible through accurate computational simulations. In this talk I will discuss some of our recent work on the application of ab-initio based molecular simulations to understand the structure and dynamics of interfacial water on photoelectrochemically relevant oxide surfaces. Specific topics will include proton transfer at the aqueous TiO₂ and IrO₂ interfaces and the influence of surface atomic structure on the water dissociation fraction and hydroxyl lifetimes at the interface.

Aluminum oxide films are usually grown on aluminum metal by anodization in a liquid electrolyte (thick films) or heating in the presence of oxygen gas (thin films). A third way is to expose the aluminum to ultraviolet radiation (UV) in the presence of water vapor. We devised a model of such oxidation that combined descriptions of photoemission from the Al metal, electron-phonon scattering in the oxide, Al³⁺ ion transport in the oxide, and the adsorption and ionization of H₂O on the oxide surface. It also accounted for UV-induced desorption of H₂O and the effect of the Al³⁺ ion flux on the surface reactions.

The model’s five free parameters were fit to our measurements of UV-induced oxidation of aluminum. The UV, which was produced by filtering synchrotron radiation, comprised wavelengths from 150 nm to 480 nm, and the H₂O pressure was varied between 3 × 10^-8 mbar and 1 × 10^-4 mbar. Exposures lasted from 3 hours to 20 days. An exposure with oxygen instead of water caused oxidation consistent with the background H₂O pressure; the oxygen caused no additional oxidation.

The introduction of organic molecules onto the surface of metal oxides through surface grafting provides the ability to tailor the surface properties towards an increased specificity and control of interactions. In the field of hybrid organic-inorganic materials, organophosphonic acid (PA) grafted metal oxides are becoming increasingly more prominent given their versatility in surface tuning and their specific merits in applications ranging from supported metal catalysis⁽¹⁾, hybrid (photo)-electric devices⁽²⁾, biosensing⁽³⁾ and sorption and separation processes.⁽⁴⁾ While synthesis-properties-performance correlations are being studied for organophosphonic acid grafted TiO₂, their stability and the impact of exposure conditions on possible changes in the interfacial surface chemistry remain unexplored. In addition, a differentiation in the stability of the organic group (carbon chain) and the M-O-P bonds is missing. In this study⁽⁵⁾, the impact of different ageing conditions on the evolution of the surface properties of propyl- and 3-aminopropylphosphonic acid grafted mesoporous TiO₂ over a period of 2 years is reported, using solid-state ³¹P and ¹³C NMR, ToF-SIMS, EPR and XPS as main techniques. In humid conditions under ambient light exposure, PA grafted TiO₂ surfaces initiate and facilitate photo-induced oxidative reactions, resulting in the formation of phosphate species and degradation of the grafted organic group with a loss of carbon content ranging from 40 to 60 wt%. Since exposure under dry air does not result in ageing phenomena, humidity and more specifically, the interactions of adsorbed water with the grafted surface, play a fundamental role in the ageing process. By revealing the underlying ageing mechanism, solutions were provided to prevent degradation. This work creates critical awareness in the research community working on hybrid titania materials and other possible photo-active materials to evaluate changes in photo-activity and stability after surface grafting.

1. F. Forato et al., Chem. - A Eur. J. 24, 2457–2465 (2018).

2. H. Chen, W. Zhang, M. Li, G. He, X. Guo, Chem. Rev. 120, 2879–2949 (2020).

3. N. Riboni et al., RSC Adv. 11, 11256–11265 (2021).

4. G. A. Seisenbaeva et al., RSC Adv. 5, 24575–24585 (2015).

5. N. Gys et al., Chempluschem. 88 (2023), doi:10.1002/cplu.202200441.

In 1989, a novel technique, Laser-XPS, was developed. Laser-XPS uses XPS to probe the core level surface chemical physics of various solid state materials while the materials are held in an ultra-high vacuum (UHV) chamber and irradiated with CW tunable organic dye or argon ion lasers. These two tunable CW lasers provide energy in the 1.9-3.5 eV range (655-351 nm, 44-81 Kcal/mol) with power levels ranging from 100-1,000 mW. A dynamicmode of operation uses XPS to directly measure the electronic nature of the photo-excited states, the photo-thermal effects, and the lifetimes of the associated initial and final states produced by the tunable CW laser irradiation. Several reversible phenomena, which are wavelength or power level dependent, were observed via the dynamic mode. These phenomena include: energy shifts in XPS signals, changes in XPS peak widths, charge control quality, and phosphorescence during XPS.A sequential mode of operation makes it possible to study the non-reversible effects of irradiating materials under UHV conditions with CW lasers. Non-reversible phenomena observed via the sequential mode include: surface chemical reactions, elimination of adventitious carbon contamination, elimination of oxygen species associated with the presence of water, hydroxides, or carbonates, color changes, outgassing, and melting. The initially expected energy shifting of a specific XPS signal within a complex spectrum of XPS signals from very similar chemical species due to narrow,selectivephoto-excitation of specific valence bands, was, however, not realized in this preliminary study.