18 research outputs found
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
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
Recommended from our members
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 ”mol hâ1 (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 lightâmatter interactions
Iridium-doping as a strategy to realize visible light absorption and p-type behavior in BaTiO3
BaTiO3 is typically a strong n-type material with tuneable optoelectronic
properties via doping and controlling the synthesis conditions. It has a wide
band gap that can only harness the ultraviolet region of the solar spectrum.
Despite significant progress, achieving visible-light absorbing BTO with
tuneable carrier concentration has been challenging, a crucial requirement for
many applications. In this work, a p-type BTO with visible-light absorption is
realized via iridium doping. Detailed analysis using advanced spectroscopy
tools and computational electronic structure analysis is used to rationalize
the n- to p-type transition after Ir doping. Results offered mechanistic
insight into the interplay between the dopant site occupancy, the dopant
position within the band gap, and the defect chemistry affecting the carrier
concentration. A decrease in the Ti3+ donor levels concentration and the
mutually correlated oxygen vacancies upon Ir doping is attributed to the p-type
behavior. Due to the formation of Ir3+ or Ir4+ in-gap energy levels within the
forbidden region, the optical transition can be elicited from or to such levels
resulting in visible-light absorption. This newly developed Ir-doped BTO can be
a promising p-type perovskite-oxide with imminent applications in solar fuel
generation, spintronics and optoelectronics.Comment: 21 pages, 8 figure
Dual Excitation Transient Photocurrent Measurement for Charge Transfer Studies in Nanocarbon Hybrids and Composites
The final publication is available via https://doi.org/10.1002/admi.201600244.A new technique, dual excitation transient photocurrent measurement (DETPM), which enables to deconvolute intrinsic photoresponse of nanocarbons from interfacial charge transfer processes in nanocarbon-inorganic hybrids and composites, is reported. The technique can be extended to other nanocarbon-based systems with a wide range of semiconductors and allows quantitative analysis of the charge transfer process.EuropÀische Kommissio
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 activityâin comparison to non-porous counterpartsâwe 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 â often used as catalyst for the CNT growth â 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
Beware of Doping: Ta<sub>2</sub>O<sub>5</sub> Nanotube Photocatalyst Using CNTs as Hard Templates
Nanostructuring constitutes
a promising strategy to increase efficiency
and stability of contemporary photocatalysts. Here, we report on the
synthesis of highly crystalline Ta<sub>2</sub>O<sub>5</sub> nanotubes
(NTs) by using carbon nanotubes (CNTs) as sacrificial hard templates
and elucidate the role of residual Fe nanoparticlesîžoften used
as catalyst for the CNT growthîžon their photocatalytic performance
toward H<sub>2</sub> evolution. We show that, when using as grown
CNTs, the resulting Ta<sub>2</sub>O<sub>5</sub> 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 Ta<sub>2</sub>O<sub>5</sub> 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 originates from atomic doping and formation of a segregated
FeO<sub><i>x</i></sub> phase within the Ta<sub>2</sub>O<sub>5</sub> 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