Oxygen vacancies and interfaces enhancing photocatalytic hydrogen production in mesoporous CNT/TiO2 hybrids

We demonstrate a unique route towards hierarchical assemblies of mesoporous TiO2 and CNT/TiO2 photocatalysts by a combination of electrospinning and sol\u2013gel methods. The resulting materials exhibit a mesoporous network of highly crystalline, well-connected inorganic nanocrystals with an order of magnitude higher photocatalytic activity than individualised TiO2 nanoparticles. The in-situ combination of small amounts of MWCNTs with TiO2 to form an electrospun CNT/TiO2 hybrid further enhanced the oxide photoactivity considerably, reaching hydrogen evolution rates of 1218 \u3bcmol/h in water splitting in the presence of sacrificial reagents under UV irradiation. We also discuss the effect of oxygen vacancies on the oxide crystallisation and phase transformation. These vacancies lead to inter-bandgap states and a lower flat band potential that facilitates the photocatalytic process

Ordered Mesoporous TiO2 Gyroids: Effects of Pore Architecture and Nb-Doping on Photocatalytic Hydrogen Evolution under UV and Visible Irradiation

Pure and Nb-doped TiO2 photocatalysts with highly ordered alternating gyroid architecture and well-controllable mesopore size of 15 nm via co-assembly of a poly(isoprene)-block-poly(styrene)-block-poly(ethylene oxide) block copolymer are synthesized. A combined effort by electron microscopy, X-ray scattering, photoluminescence, X-ray photoelectron spectroscopy, Raman spectroscopy, and density functional theory simulations reveals that the addition of small amounts of Nb results in the substitution of Ti4+ with isolated Nb5+ species that introduces inter-bandgap states, while at high concentrations, Nb prefers to cluster forming shallow trap states within the conduction band minimum of TiO2. The gyroidal photocatalysts are remarkably active toward hydrogen evolution under UV and visible light due to the open 3D network, where large mesopores ensure efficient pore diffusion and high photon harvesting. The gyroids yield unprecedented high evolution rates beyond 1000 µmol h−1 (per 10 mg catalyst), outperforming even the benchmark P25-TiO2 more than fivefold. Under UV light, the Nb-doping reduces the activity due to the introduction of charge recombination centers, while the activity in the visible triple upon incorporation is owed to a more efficient absorption due to inter-bandgap states. This unique pore architecture may further offer hitherto undiscovered optical benefits to photocatalysis, related to chiral and metamaterial-like behavior, which will stimulate further studies focusing on novel light–matter interactions

Mesoporous Semiconductors: A New Model To Assess Accessible Surface Area and Increased Photocatalytic Activity?

The final publication is available via https://doi.org/10.1021/acsaem.8b01123.Mesoporous photocatalysts have gained tremendous attention in the past decade by demonstrating that increased surface area and porosity can strongly improve their performance. In fact, all reports on mesoporous semiconductors corroborate this scenario. But is it possible to quantify and compare the reported advantages of the mesopores and the increased surface area between different works? In this contribution, we present a model that can evaluate the improvements in photocatalytic activity achieved by the introduction of mesoporosity independent of synthetic or test conditions. We exemplify this methodology focusing on photocatalytic hydrogen/oxygen evolution with sacrificial reagents, but also include examples of CO2 reduction and electrocatalysis. By correlating the relative increase in surface area to the relative increase in activity—in comparison to non-porous counterparts—we show that the origin of mesoporosity can have a pronounced influence on the activity enhancement and that different semiconductor materials behave quite differently. Our model can serve as a starting point for the community to extract and compare key information on mesoporous photocatalysts, to put results into context of existing data, and to compare the performances of various catalytic systems much better.German Research Foundation (DFG

Beware of doping: Ta2O5 nanotube photocatalyst using CNTs as hard templates

The final publication is available via https://doi.org/10.1021/acsaem.8b00006.Nanostructuring constitutes a promising strategy to increase efficiency and stability of contemporary photocatalysts. Here we report on the synthesis of highly crystalline Ta2O5 nanotubes (NTs) by using carbon nanotubes (CNTs) as sacrificial hard templates and elucidate the role of residual Fe nanoparticles – often used as catalyst for the CNT growth – on their photocatalytic performance towards H2 evolution. We show that when using as grown CNTs, the resulting Ta2O5 NTs contained detectable amounts of Fe and possessed negligible photocatalytic activity. When CNTs were, however, purified from Fe by thermally annealing the CNTs at 2100°C, the same synthetic procedure yielded pure Ta2O5 NTs that showed a 40-fold increase in activity compared to the Fe-containing counterpart. A complementary set of analytical techniques in a combination with additional model experiments indicate that the detrimental effect of the residual Fe on the photocatalytic activity originate from atomic doping and formation of a segregated FeOx phase within the Ta2O5 matrix that can both act as efficient electron traps. Our result highlights that the presence of residual catalyst needs to be taken into account when using CNTs as hard templates and generally illustrates a possible effect of unintentional dopants that are often not considered in preparing functional nanostructures.Deutsche Forschungsgesellschaf

Polyoxometalates on Functional Substrates: Concepts, Synergies, and Future Perspectives

Polyoxometalates (POMs) are molecular metal oxide clusters that feature a broad range of structures and functionalities, making them one of the most versatile classes of inorganic molecular materials. They have attracted widespread attention in homogeneous catalysis. Due to the challenges associated with their aggregation, precipitation, and degradation under operational conditions and to extend their scope of applications, various strategies of depositing POMs on heterogeneous substrates have been developed. Recent ground-breaking developments in the materials chemistry of supported POM composites are summarized and links between molecular-level understanding of POM-support interactions and macroscopic effects including new or optimized reactivities, improved stability, and novel function are established. Current limitations and future challenges in studying these complex composite materials are highlighted, and cutting-edge experimental and theoretical methods that will lead to an improved understanding of synergisms between POM and support material from the molecular through to the nano- and micrometer level are discussed. Future development in this fast-moving field is explored and emerging fields of research in POM heterogenization are identified

Effects of Periodic Pore Ordering on Photocatalytic Hydrogen Generation with Mesoporous Semiconductor Oxides

Crystalline and 3D continuous mesoporous quaternary CsTaWO6 semiconductors are prepared with different degrees of long‐range periodic order and local order, respectively, to investigate the influence of periodic pore order on the photocatalytic performance in hydrogen evolution of mesoporous photocatalysts. The degree of long‐range order of the mesopores is changed by modifying the ratio between metal precursors and soft polymer template poly(isoprene‐b‐styrene‐b‐ethylene oxide) (PI‐b‐PS‐b‐PEO; ISO) in the sol–gel synthesis. Long‐range periodic order is found to have no appreciable advantage compared with an only locally ordered continuous pore system. On the contrary, nonperiodically ordered mesopores result in higher activity toward photocatalytic hydrogen evolution, even with slightly smaller pore diameter and lower cumulative pore volume. Most importantly, it is shown that pore connectivity and heterogeneous pore systems in mesoporous photocatalysts play a major role for hydrogen evolution when other parameters are confirmed to be not rate limiting

Correction: Elucidating the formation and active state of Cu co-catalysts for photocatalytic hydrogen evolution (vol 9, pg 21958, 2021)

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