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The large-scale deployment of proton-exchange membrane water electrolyzers for high-throughput sustainable hydrogen production requires transition from precious noble metal anode electrocatalysts to low-cost earth-abundant materials. However, such materials are commonly insufficiently stable and/or catalytically inactive at low pH, and positive potentials required to maintain high rates of the anodic oxygen evolution reaction (OER). To address this, we explore the effects of a dielectric nanoscale-thin layer, constituted of amorphous TiO2, on the stability and electrocatalytic activity of nanostructured OER anodes based on low-cost Co3O4. We demonstrate a direct correlation between the OER performance and the thickness of the atomic layer deposited TiO2layers. An optimal TiO2layer thickness of 4.4 nm enhances the anode lifetime by a factor of ca. 3, achieving 80 h of continuous electrolysis at pH near zero, while preserving high OER catalytic activity of the bare Co3O4surface. Thinner and thicker TiO2layers decrease the stability and activity, respectively. This is attributed to the pitting of the TiO2layer at the optimal thickness, which allows for access to the catalytically active Co3O4surface while stabilizing it against corrosion. These insights provide directions for the engineering of active and stable composite earth-abundant materials for acidic water splitting for high-throughput hydrogen production.

Controllable concomitant production of CO and H2(syngas) during electrochemical CO2reduction reactions (CO2RR) is expected to improve the commercial feasibility of the technology to mitigate CO2emissions as the generated syngas can be converted into useful chemicals using the commercial Fischer–Tropsch (FT) process. Herein, we demonstrate the ability of a Co single-atom-decorated N-doped graphitic carbon shell-encapsulated cobalt nanoparticle electrocatalyst (referred as Co@CoNC-900) to controllably produce syngas at low overpotentials during CO2RR. Through the engineering and modulation of dual active sites for CO2RR (modified carbon shell with encapsulated Co) and hydrogen evolution reaction (Co–N4moieties) within Co@CoNC by varying the annealing temperature, we are able to tune the H2: CO ratio from 1: 2 to 1: 1 to 3: 2 over a wide range of applied potentials (−0.5 V to −0.8 V versus reversible hydrogen electrode, RHE). This versatile control of H2: CO ratio in CO2RR reaction brings up significant opportunity of using CO2and H2O and renewable energy for producing a range of chemicals.

Silica has the potential to enhance the performance of ceria–zirconia as a support for the dry reforming of methane; however, controlling the integration of silica with the ceria–zirconia using flame spray pyrolysis (FSP) is a significant challenge. To address this challenge, an asymmetrically variable double-FSP (DFSP) system was established to control the SiO2interaction with Ce0.7Zr0.3O2. The engineered materials were then utilized as supports for Ni for the dry reforming of methane. Initially, silica formation during FSP synthesis was examined where it was revealed that, at a low precursor concentration (<1.5 M tetraethyl orthosilicate in xylenes), the physical characteristics of the silica varied differently in relation to what is typically encountered during FSP synthesis. Explicitly, on using a 0.5 M tetraethyl orthosilicate precursor, increasing the FSP feed rate provided an increase in the specific surface area from 217 m2/g at 3 mL/min to 363 m2/g at 7 mL/min. Adopting this knowledge on silica formation under these conditions, the asymmetrical DFSP system was then exploited to regulate the integration of ceria–zirconia with the silica. To restrict the silica from coating the particles during DFSP, the intersection distance along the silica flame was tuned from 18.5 to 28.5 cm, whereas the distance along the ceria–zirconia flame was fixed at 5 cm. It was found that at short intersection distances the ceria–zirconia provided sites for silica nucleation and growth, resulting in high surface-area silica encapsulating the ceria–zirconia. At large intersection distances, encapsulation of the ceria–zirconia by silica was suppressed. An enhanced oxygen storage capacity and basicity along with the small Ni sizes facilitated by the longer intersection distances produced the most selective catalyst for the dry reforming of methane.

The interface at the metal oxide–carbon hybrid heterojunction is the source to the well-known “synergistic effect” in catalysis. Understanding the structure–function properties is key for designing more advanced catalyst–support systems. Using a model MnIII–Oxsingle-layer catalyst on carbon, we herein report a full elucidation to the catalytic synergism at the hybrid heterojunction in the oxygen reduction reaction (ORR). The successful fabrication of the single-layer catalyst from bottom-up is fully characterized by the X-ray absorption fine structure and high-resolution transmission electron microscopy. For oxygen electrocatalysis over this model hybrid heterostructure, our results, from both theory and experiment, show that the synergistic ORR truly undergoes a cooperated two-step electrocatalysis with catalytic promotion (ΔEonset= 60 mV) near the heterojunction and over the single-layer catalyst through an interfacial electronic interplay, rather than an abstruse transition towards a one-step dissociative pathway. Finally, we report a superior peroxide-reducing activity of 432.5 mA cm–2mg(M)–1over the MnIII–Oxsingle-layer.

It has been reported that photogenerated electrons and holes can be directed toward specific crystal facets of a semiconductor particle, which is believed to arise from the differences in their surface electronic structures, suggesting that different facets can act as either photoreduction or photo-oxidation sites. This study examines the propensity for this effect to occur in faceted, plate-like bismuth molybdate (Bi2MoO6), which is a useful photocatalyst for water oxidation. Photoexcited electrons and holes are shown to be spatially separated toward the {100} and {001}/{010} facets of Bi2MoO6, respectively, by facet-dependent photodeposition of noble metals (Pt, Au, and Ag) and metal oxides (PbO2, MnOx, and CoOx). Theoretical calculations revealed that differences in energy levels between the conduction bands and valence bands of the {100} and {001}/{010} facets can contribute to electrons and holes being drawn to different surfaces of the plate-like Bi2MoO6. Utilizing this knowledge, the photo-oxidative capability of Bi2MoO6was improved by adding an efficient water oxidation co-catalyst, CoOx, to the system, whereby the extent of enhancement was shown to be governed by the co-catalyst location. A greater oxygen evolution occurred when CoOxwas selectively deposited on the hole-rich {001}/{010} facets of Bi2MoO6compared to when CoOxwas randomly located across all of the facets. The elevated performance exhibited for the selectively loaded CoOx/Bi2MoO6was ascribed to the greater opportunity for hole trapping by the co-catalyst being accentuated over other potentially detrimental effects, such as the co-catalyst acting as a recombination medium and/or covering reactive sites. The results indicate that harnessing the synergy between the spatial charge separation and the co-catalyst location on the appropriate facets of plate-like Bi2MoO6can promote its photocatalytic activity.

Au nanoparticles with different sizes (10, 20, 30, and 50 nm) were synthesized using a seed-assisted approach and anchored onto Pt/TiO2employing 3-mercaptopropionic acid as the organic linker. The sizes of the Au nanoparticles were controlled within a narrow range so that the size-dependent surface plasmonic resonance effect on sensitizing Pt/TiO2can be thoroughly studied. We found that 20 nm Au nanoparticles (Au20) gave the best performance in sensitizing Pt/TiO2to generate H2under visible-light illumination. Photoelectrochemical measurements indicated that Au20-Pt/TiO2exhibited the most efficient “hot” electrons separation among the studied catalysts, correlating well with the photocatalytic activity. The superior performance of Au-supported Pt/TiO2(Au20-Pt/TiO2) compared with Au anchored to TiO2(Au20/TiO2) revealed the important role of Pt as a cocatalyst for proton reduction. To elucidate how the visible-light excited hot electrons in Au nanoparticles involved in the proton-reduction reaction process, Au20/TiO2was irradiated by visible light (λ > 420 nm) with the presence of Pt precursor (H2PtCl6) in a methanol aqueous solution under deaerated condition. Energy-dispersive X-ray spectroscopy mapping analysis on the recovered sample showed that Pt ions could be reduced on the surfaces of both Au nanoparticles and TiO2support. This observation indicated that the generated hot electrons on Au nanoparticles were injected into the TiO2conduction band, which were then subsequently transferred to Pt nanoparticles where proton reduction proceeded. Besides, the excited hot electrons could also participate in the proton reduction on Au nanoparticles surface.

Bioinspired nanotopography artificially fabricated on titanium surfaces offers a solution for the rising issue of postoperative infections within orthopedics. On a small scale, hydrothermal etching has proven to deliver an effective antimicrobial nanospike surface. However, translation to an industrial setting is limited by the elevated synthesis temperature (150 °C) and associated equipment requirements. Here, for the first time, we fabricate surface nanostructures using comparatively milder synthesis temperatures (75 °C), which deliver physicochemical properties and antimicrobial capability comparable to the high-temperature surface. Using a KOH etchant, the simultaneous formation of titania and titanate crystals at both temperatures produces a one-dimensional nanostructure array. Analysis indicated that the formation mechanism comprises dissolution and reprecipitation processes, identifying the deposited titanates as hydrated layered tetra-titanates (K2Ti4O9·nH2O). A proposed nanospike formation mechanism was confirmed through the identification of a core and outer shell for individual nanostructures, primarily comprised of titanates and titania, respectively. Etching conditions dictated crystalline formation, favoring a thicker titanate core for nanorods under higher synthesis temperatures and etchant concentrations. A bactericidal investigation showed the efficacy against Gram-negative bacteria for a representative low-temperature nanosurface (34.4 ± 14.4%) was comparable to the higher temperature nanosurface (34.0 ± 17.0%), illustrating the potential of low-temperature hydrothermal synthesis. Our results provide valuable insight into the applicability of low-temperature etching protocols that are more favorable in large-scale manufacturing settings.

The anatase–rutile mixed-phase photocatalysts have attracted extensive research interest because of the superior activity compared to their single phase counterparts. In this study, doping of Sn4+ions into the lattice of TiO2facilitates the phase transformation from anatase to rutile at a lower temperature while maintaining the same crystal sizes compared to the conventional annealling approach. The mass ratios between anatase and rutile phases can be easily manipulated by varying the Sn-dopant content. Characterization results reveal that the Sn4+ions entered into the lattice of TiO2by substituting some of the Ti4+ions and distributed evenly in the matrix of TiO2. The substitution induced the distortion of the lattice structure, which realized the phase transformation from anatase to rutile at a lower temperature and the close-contact phase junctions were consequently formed between anatase and rutile, accounting for the efficient charge separations. The mixed-phase catalysts prepared by doping Sn4+ions into the TiO2exhibit superior activity for photocatalytic hydrogen generation in the presence of Au nanoparticles, relatively to their counterparts prepared by the conventional annealling at higher temperatures. The band allignment between anatase and rutile phases is established based on the valence band X-ray photoelectron spectra and diffuse reflectance spectra to understand the spatial charge separation process at the heterojunction between the two phases. The study provides a new route for the synthesis of mixed-phase TiO2catalysts for photocatalytic applications and advances the understanding on the enhanced photocatalytic properties of anatase–rutile mixtures.

The current work demonstrates the importance of WO3crystallinity in governing both photoenergy conversion efficiency and storage capacity of the flower structured WO3electrode. The degree of crystallinity of the WO3electrodes was varied by altering the calcination temperature from 200 to 600 °C. For the self-photochargeability phenomenon, the prevailing flexibility of the short-range order structure at low calcination temperature of 200 °C favors the intercalation of the positive cations, enabling more photoexcited electrons to be stored within WO3framework. This leads to a larger amount of stored charges that can be discharged in an on-demand manner under the absence of irradiation for H2generation. The stability of the electrodes calcined at 200 °C, however, is compromised because of the structural instability caused by the abundance insertion of cations. On the other hand, films that were calcined at 400 °C displayed the highest stability toward both intercalation of the cations and photoelectrochemical water splitting performance. Although crystallinty of WO3was furthered improved at 600 °C heat treatment, the worsened contact between the WO3platelets and the conducting substrate as induced by the significant sintering has been more detrimental toward the charge transport.

Visible-light-active BiVO4photocatalyst prepared by a one-step flame spray pyrolysis demonstrates the structural evolution from amorphous to crystalline scheelite-tetragonal and further to scheelite-monoclinic (the photocatalytic active phase). Up to 95% scheelite-monoclinic content, the rest being scheelite-tetragonal, can be achieved in situ by exposing the collection filter to higher flame temperature. Reasonable activity in terms of photocatalytic O2evolution was obtained with the increase in crystallinity and scheelite-monoclinic content. Although analogous postcalcination of BiVO4improves crystallization and phase transformation, it inevitably induces vacancy defects that are detrimental to the photocatalytic activity. Hence a facile aqueous acid treatment on the flame-made BiVO4is introduced, which in the presence of small addition of Bi and V promotes full transformation to scheelite-monoclinic and reduces charge trapping defects. As a result, the photocatalytic O2evolution activity was increased by a remarkable 5 folds compared to the best performing untreated flame-made BiVO4.

This work reports the use of sodium fluoride (in ethylene glycol electrolyte) as the replacement of hydrofluoric acid and ammonium fluoride to fabricate long and perpendicularly well-aligned TiO2nanotube (TNT) (up to 21 μm) using anodization. Anodizing duration, applied voltage and electrolyte composition influenced the geometry and surface morphologies of TNT. The growth mechanism of TNT is interpreted by analyzing the current transient profile and the total charge density generated during anodization. The system with low water content (2 wt %) yielded a membrane-like mesoporous TiO2film, whereas high anodizing voltage (70 V) resulted in the unstable film of TNT arrays. An optimized condition using 5 wt % water content and 60 V of anodizing voltage gave a stable array of nanotube with controllable length and pore diameter. Upon photoexcitation, TNTs synthesized under this condition exhibited a slower charge recombination rate as nanotube length increased. When made into cis-diisothiocyanato-bis(2,2̀-bipyridyl-4,4̀-dicarboxylato) ruthenium(II) bis (tetrabutyl-ammonium)(N719) dye-sensitized solar cells, good device efficiency at 3.33 % based on the optimized TNT arrays was achieved with longer electron time compared with most mesoporous TiO2films.