Biochar nanoparticles (BC NPs), produced by low temperature pyrolysis (350 °C) of microalgae (Nannochloropsis sp.) and nutshells, are proposed as low-cost and sustainable co-catalysts to promote the photocatalytic activity of TiO2 nanotube (NT) arrays towards the degradation of methylene blue (MB) used as an organic pollutant model molecule. BC NPs (size < 25 nm) were obtained by treating bulk BC (i.e., biomass after pyrolysis) by sonication–centrifugation cycles in a water solution. The filtered BC NPs dispersion was deposited by simple drop-casting on the TiO2 NT support. The BC loading was varied by performing multiple depositions. Photocatalytic experiments under UV light (365 nm) revealed that the decoration with BC NPs significantly improves the TiO2 photoactivity. Such enhancement is mainly influenced by the amount of BC deposited; upon optimizing the BC deposition conditions, the rate of photocatalytic degradation of MB increases approximately three times with respect to bare TiO2, almost irrespective of the nature of the raw material. The greater photocatalytic activity of BC-TiO2 can be attributed to the synergistic combination of reactant/product adsorption and catalytic degradation of the adsorbed organic pollutant, as well as an improved charge carrier separation and electron transfer.
This study explores an ultrarapid electrochemical self-doping procedure applied to anodic titanium dioxide (TiO2) nanotube arrays in an alkaline solution to boost their performance for electroanalytical and photocatalytic applications. The electrochemical self-doping process (i.e., the creation of surface Ti3+ states by applying a negative potential) is recently emerging as a simpler and cleaner way to improve the electronic properties of TiO2 compared to traditional chemical and high-temperature doping strategies. Here, self-doping was carried out through varying voltages and treatment times to identify the most performing materials without compromising their structural stability. Interestingly, cyclic voltammetry characterization revealed that undoped TiO2 shows negligible activity, whereas all self-doped materials demonstrate their suitability as electrode materials: an outstandingly short 10 s self-doping treatment leads to the highest electrochemical activity. The electrochemical detection of hydrogen peroxide was assessed as well, demonstrating a good sensitivity and a linear detection range of 3–200 µM. Additionally, the self-doped TiO2 nanotubes exhibited an enhanced photocatalytic activity compared to the untreated substrate: the degradation potential of methylene blue under UV light exposure increased by 25% in comparison to undoped materials. Overall, this study highlights the potential of ultrafast electrochemical self-doping to unleash and improve TiO2 nanotubes performances for electroanalytical and photocatalytic applications.
Abstract In this work, NiCu co‐catalysts on TiO 2 are studied for photocatalytic hydrogen evolution. NiCu co‐catalyst films are deposited at room temperature by argon plasma sputtering on high aspect‐ratio anodic TiO 2 nanotubes. To tune the Ni : Cu atomic ratio, alloys of various compositions were used as sputtering targets. Such co‐catalyst films are found to be amorphous with small nanocrystalline domains. A series of parameters is investigated, i. e., i) Ni : Cu relative ratio in the sputtered films, ii) NiCu film thickness, and iii) thickness of the TiO 2 nanotube layers. The highest photocatalytic activity is obtained with 8 μm long TiO 2 nanotubes, sputter‐coated with a 10 nm‐thick NiCu films with a 1 : 1 Ni : Cu atomic ratio. This photocatalyst reaches a stable hydrogen evolution rate of 186 μL h −1 cm −2 , 4.6 and 3 times higher than that of Ni‐ and Cu‐TiO 2 , respectively, demonstrating a synergistic co‐catalytic effect of Ni and Cu in the alloy co‐catalyst film.
The complete dissolution of silicate-containing materials, often necessary for elemental determination, is generally performed by microwave-assisted digestion involving the forced use of hydrofluoric acid (HF). Although highly efficient in dissolving silicates, this acid exhibits many detrimental effects (e.g., formation of precipitates, corrosiveness to glassware) that make its removal after digestion essential. The displacement of HF is normally achieved by evaporation in open-vessel systems: atmospheric contamination or loss of analytes can occur when fuming-off HF owing to the non-ultraclean conditions necessarily adopted for safety reasons. This aspect strongly hinders determination at the ultra-trace level. To overcome this issue, we propose a clean and safe microwave-assisted procedure to induce the evaporative migration of HF inside a sealed "vessel-inside-vessel" system: up to 99.9% of HF can be removed by performing two additional microwave cycles after sample dissolution. HF migrates from the digestion solution to a scavenger (ultrapure H2O) via a simple physical mechanism, and then, it can be safely dismissed/recycled. The procedure was validated by a soil reference material (NIST 2710), and no external or cross-contamination was observed for the 27 trace elements studied. The results demonstrate the suitability of this protocol for ultra-trace analysis when the utilization of HF is mandatory.
We used in situ X-ray absorption spectroscopy (XAS) to investigate the composition-performance correlation of Ni-SrTiO 3 photocatalysts active for water splitting.After preparation and exposure to ambient conditions, the Ni particles on SrTiO 3 consist of Ni(0) and Ni(II) phases, with a 4:1 at % ratio, in a metal/oxide core/shell configuration, as confirmed by XPS and TEM-EDX.In situ XAS experiments using an aqueous slurry of the Ni-SrTiO 3 photocatalyst and simultaneous continuous exposure to 365 nm light with a power density of 100 mW cm -2 and the Xrays do not reveal significant changes in oxidation state of the Ni particles.Contrarily, when the X-rays are discontinuously applied, UV excitation leads to oxidation of a significant fraction of Ni(0) to Ni(II), specifically to NiO and Ni(OH) 2 phases, along with cocatalyst restructuring.Ni dissolution or oxidation to higher valence states (e.g., Ni(III)) was not observed.The UV lightinduced oxidation of Ni(0) causes the hydrogen evolution rate to drop to similar rates as observed for pristine SrTiO 3 , suggesting that Ni(0) is the active phase for H 2 generation.Our results underscore the importance of assessing the effects of (continuous) X-ray exposure to (photo)catalyst-containing aqueous slurries during in situ XAS experiments, which can significantly influence the observation of compositional and structural changes in the (photo)catalysts.We ascribe this to X-ray induced water photolysis and formation of free electrons, which in this study quench SrTiO 3 photoholes and prevent Ni oxidation.
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