Surface science and chemical studies of NiO/single crystal TiO2 heterostructure photocatalysts

Photocatalysis on semiconductor metal oxide surfaces has attracted considerable attention as a sustainable environmentally friendly method for water/air purification and hydrogen production by water splitting. Among semiconducting metal oxides TiO2 has been intensively investigated as a promising photocatalyst candidate. However, despite many efforts, its photocatalytic activity is far from a practical level mainly due to inefficient charge carrier separation and resulting charge carrier recombination. An advantageous strategy to address this issue is the development of heterostructures by coupling to a metal to form a Schottky junction or to metal oxides to create a p-n junction at their interface in order to prevent the recombination by vectorial charge carrier separation at these energy junctions. On the other hand it was revealed over the past decade that crystal facets play a decisive role in trapping of charge carriers and thus photocatalytic redox reactions. Thus, selective deposition of metal or metal oxides onto specific facets would enhance the photocatalytic activity by improving charge separation. To achieve higher activities, two methods, the supercritical fluid chemical deposition route and the photodeposition method, were investigated to deposit selectively p-type NiO onto specific facets of ntype TiO2 single crystalline nanoparticles to establish a p-n junction. The resulting NiO/TiO2 nanocrystals were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX), N2 sorption measurements, UV-visible diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). The heterojunction photocatalysts showed higher photocatalytic efficiency than pure TiO2 for the decomposition of organic dyes. Particularly, 0.1-0.25 wt % of NiO was the optimal loading amount, showing the highest activity. To elucidate the role of crystal facets of TiO2 and the effect of selective deposition of NiO, rutile (001), rutile (110), anatase (001), and anatase (101) surfaces with different surface states were prepared and their electronic properties were systematically compared by XPS and UPS measurements. Furthermore, water adsorption onto the different surfaces were also investigated. Regardless of surface stoichiometry, the Fermi level position of the anatase (001) surface is situated higher than that of the anatase (101) surface in energy while that of the rutile (001) surface is located lower than that of the rutile (110) surface. This can explain why photo-generated electrons and holes preferentially migrate to the (101) and (001) facets on TiO2 anatase crystals, respectively. Work function values of these oriented surfaces vary depending upon the surface states related to distribution and amount of oxygen vacancies as well as adsorbed oxygen peroxo species on the surface. In order to experimentally determine energy band alignments, interface experiments were performed by stepwisely depositing NiO onto above well-defined oriented TiO2 surfaces. The enhanced photocatalytic activity of NiO/TiO2 heterostructure nanoparticles were rationalized on the basis of the obtained band alignments. The information of electronic properties of different oriented TiO2 under various surface states would provide a new insight to construct the optimal energy band alignment of the heterostructure system with TiO2. In addition, the concept of heterojuction nanocrystals where co-catalysts are selectively deposited should find practical application to purify the environment and to sustainably produce renewable hydrogen

Surface science and chemical studies of NiO/single crystal TiO2 heterostructure photocatalysts

Les photocatalyseurs à base de TiO2 ont été l’objet d’une grande attention comme une méthode durable de purification de l’air ou de l’eau, et de production d’hydrogène par décomposition de l’eau. Une stratégie avantageuse consiste à développer des héterostructures par couplage avec un autre oxyde métallique former une jonction de type Schottky ou avec un autre oxyde métallique pour créer une jonction p-n à l’interface de manière à prévenir les recombinaisons via une séparation de charge « vectorielle » à ces jonctions. De plus, les facettes cristallines jouent un rôle crucial dans le piégeage des porteurs de charge et, donc,dans les réactions rédox photoactivées. Ainsi, le dépôt sélectif de métal ou d’oxyde métallique sur des facettes spécifiques de nanocristaux de TiO2 devrait augmenter l’activité photocatalytique par l’amélioration de la séparation des charges. Dans ce travail, nous avons combiné l’emploi du cocatalyseur de type p NiO pour former des jonctions p-n avec son dépôt sélectif sur des nanocristaux de TiO2 anatase exposant des facettes bien définies. Par ailleurs, des expériences modèles de physique de surface ont été menées pour étudier les propriétés électroniques de ces hétérojonctions.TiO2 photocatalysts have attracted attention as a sustainable method for water/air purification and hydrogen production by water splitting. An advantageous strategy is the development of heterostructures by coupling metal oxides to create a p-n junction at their interface in order to prevent there combination by vectorial charge carrier separation at these energy junctions. In addition, crystal facets play a decisive role in the trapping of charge carriers and thus photocatalytic redox reactions. Thus, selective deposition of metal or metal oxides onto specific facets would enhance the photocatalytic activity by improving charge separation. In this work, we have combined the usage of p-type NiO co-catalyst to form p-n junction with its selective deposition onto the specific facet of oriented TiO2nanocrystal photocatalysts. Furthermore, the physical model experiments have been performed to investigate the electronic properties of these heterojunctions

Science de surface et propriétés chimiques d'hétérostructures NiO/TiO2 monocristallin

TiO2 photocatalysts have attracted attention as a sustainable method for water/air purification and hydrogen production by water splitting. An advantageous strategy is the development of heterostructures by coupling metal oxides to create a p-n junction at their interface in order to prevent there combination by vectorial charge carrier separation at these energy junctions. In addition, crystal facets play a decisive role in the trapping of charge carriers and thus photocatalytic redox reactions. Thus, selective deposition of metal or metal oxides onto specific facets would enhance the photocatalytic activity by improving charge separation. In this work, we have combined the usage of p-type NiO co-catalyst to form p-n junction with its selective deposition onto the specific facet of oriented TiO2nanocrystal photocatalysts. Furthermore, the physical model experiments have been performed to investigate the electronic properties of these heterojunctions.Les photocatalyseurs à base de TiO2 ont été l’objet d’une grande attention comme une méthode durable de purification de l’air ou de l’eau, et de production d’hydrogène par décomposition de l’eau. Une stratégie avantageuse consiste à développer des héterostructures par couplage avec un autre oxyde métallique former une jonction de type Schottky ou avec un autre oxyde métallique pour créer une jonction p-n à l’interface de manière à prévenir les recombinaisons via une séparation de charge « vectorielle » à ces jonctions. De plus, les facettes cristallines jouent un rôle crucial dans le piégeage des porteurs de charge et, donc,dans les réactions rédox photoactivées. Ainsi, le dépôt sélectif de métal ou d’oxyde métallique sur des facettes spécifiques de nanocristaux de TiO2 devrait augmenter l’activité photocatalytique par l’amélioration de la séparation des charges. Dans ce travail, nous avons combiné l’emploi du cocatalyseur de type p NiO pour former des jonctions p-n avec son dépôt sélectif sur des nanocristaux de TiO2 anatase exposant des facettes bien définies. Par ailleurs, des expériences modèles de physique de surface ont été menées pour étudier les propriétés électroniques de ces hétérojonctions

The Work Function of TiO₂

Polycrystalline anatase thin films, (001)- and (101)-oriented anatase TiO₂ single crystals and (001)- and (110)-oriented rutile TiO₂ single crystals with various surface treatments were studied by photoelectron spectroscopy to obtain their surface potentials. Regardless of orientations and polymorph, a huge variation of the Fermi level and work function was achieved by varying the surface condition. The most strongly oxidized surfaces are obtained after oxygen plasma treatment with a Fermi level ∼2.6 eV above the valence band maximum and ionization potentials of up to 9.5 eV (work function 7.9 eV). All other treated anatase surfaces exhibit an ionization potential independent of surface condition of 7.96±0.15 eV. The Fermi level positions and the work functions vary by up to 1 eV. The ionization potential of rutile is ∼0.56 eV lower than that of anatase in good agreement with recent band alignment studies

The Work Function of TiO2

Polycrystalline anatase thin films, (001)- and (101)-oriented anatase TiO 2 single crystals and (001)- and (110)-oriented rutile TiO 2 single crystals with various surface treatments were studied by photoelectron spectroscopy to obtain their surface potentials. Regardless of orientations and polymorph, a huge variation of the Fermi level and work function was achieved by varying the surface condition. The most strongly oxidized surfaces are obtained after oxygen plasma treatment with a Fermi level ∼2.6 eV above the valence band maximum and ionization potentials of up to 9.5 eV (work function 7.9 eV). All other treated anatase surfaces exhibit an ionization potential independent of surface condition of 7.96 ± 0.15 eV. The Fermi level positions and the work functions vary by up to 1 eV. The ionization potential of rutile is ∼0.56 eV lower than that of anatase in good agreement with recent band alignment studies

Nickel oxide selectively deposited on the {101} facet of anatase TiO2 nanocrystal bipyramids for enhanced photocatalysis

International audienceFacet-engineered anatase TiO2 with NiO nanoparticles heterocontacts were successfully prepared by selective photodeposition of NiO nanoparticles onto the {101} facet of the top-truncated bipyramidal TiO2 anatase nanocrystals coexposed with {001} and {101} facets. The morphology and electronic properties of the resulting 0.1–10 wt % NiO-decorated TiO2 were investigated by X-ray diffraction, high-resolution electron microscopy, N2 sorption analysis, and UV–vis spectroscopy. Furthermore, a careful determination of the energy band alignment diagram was conducted by a model experiment using XPS and UPS to verify charge separation at the interface of the NiO−TiO2 heterostructure. The model experiment was performed by stepwise deposition of NiO onto oriented TiO2 substrates and in-situ photoelectron spectroscopy measurements without breaking vacuum. Core levels showed shifts of 0.58 eV toward lower binding energies, meaning an upward band bending in TiO2 at the NiO–TiO2 interface. Furthermore, 0.1 wt % NiO–TiO2 exhibited 50% higher activities than the pure TiO2 for methylene blue (MB) photodecomposition under UV irradiation. This enhanced photocatalytic activity of NiO–TiO2 nanocomposites was related to the internal electric field developed at the p–n NiO−TiO2 heterojunction, leading to vectorial charge separation. Finally, mechanistic studies conducted in the presence of carrier or radical scavengers revealed that holes dominantly contributed to the photocatalytic reactions in the case of NiO–TiO2 photocatalysts while electrons played the main role in photocatalysis for the pure TiO2 materials

Formation of Ti2AuN from Au-Covered Ti2AlN Thin Films: A General Strategy to Thermally Induce Intercalation of Noble Metals into MAX Phases

Thermally induced intercalation of noble metals into non-van der Waals ceramic compounds presents a method to produce a new class of layered materials. We recently demonstrated an exchange reaction of Au with A layers of MAX phase carbides with plentiful combinations of A and M elements. Here, we report the first substitution of Al with Au in a Ti2AlN MAX phase nitride at an elevated temperature without destroying the original layered structure. These results bolster the generalization of the Au intercalation for the A elements in MAX phases with diverse combinations of M, A, and X elements. Furthermore, we propose crucial factors to achieve the exchange reaction: there should be a chemical potential for the A element to dissolve in or react with noble metals to intercalate; the noble metals should be inert to the initial metal carbides/nitrides; and it is necessary to choose the reaction temperature that allows balanced interdiffusion of the noble metals and A elements.Funding Agencies|Swedish Research CouncilSwedish Research Council [2017-03909, 642-2013-8020]; Knut and Alice Wallenberg (KAW) FoundationKnut &amp; Alice Wallenberg Foundation; Swedish Foundation for Strategic Research (SSF)Swedish Foundation for Strategic Research [EM16-0004]</p

Supercritical CO2-assisted deposition of NiO on (101)-anatase-TiO2 for efficient facet engineered photocatalysts

NiO/(101)-anatase-TiO2 heterostructure nanoparticles were synthesized by depositing NiO onto the (101) facet of anatase crystals via the supercritical fluid chemical deposition (SFCD) route. Thorough characterization experiments performed by various techniques (XRD, UV-Vis DRS, N2 sorption, HR-TEM, EDX, and XPS) indicate that the SFCD method allowed a good dispersion of NiO onto the TiO2 nanoparticles for NiO amounts below 2 wt%. Compared to pure TiO2, the 0.1–1 wt% NiO–TiO2 nanocomposites showed enhanced photocatalytic properties for methylene blue (MB) and methyl orange (MO) decomposition under UV light irradiation, the 0.25 wt% NiO–TiO2 system leading to the highest efficiencies. The photocatalytic properties were then rationalized in terms of the acidic properties and electronic structures of the NiO–TiO2 nanocomposites. This higher photocatalytic activity was mainly related to the heterocontact at the interface of the NiO–TiO2 crystallites and to the enhanced reaction rates at the NiOx surface

Discovering the determining parameters for the photocatalytic activity of TiO2 colloids based on an anomalous dependence on the specific surface area

The photocatalytic (PC) performance of titanium dioxide (TiO2) nanoparticles strongly depends on their specific surface, the presence of crystal defects, their crystal phase, and the exposed crystal facets. In order to understand which of these factors contributes most significantly to the PC activity of TiO2 colloids, all of them have to be individually analyzed. This study entails the synthesis of five anatase nanocrystal samples. By maintaining the same reactant ratios as well as hydrothermal sol–gel synthesis route and only varying the autoclaving time or temperature, different crystallite sizes are obtained under comparable experimental conditions. A decrease in PC performance with increase in specific surface area is found. Such an unexpected counterintuitive result establishes the basis for a better understanding of the crucial factors that ultimately determine the PC activity. These are investigated by studying nanocrystals bulk and surface structure and morphology using a selection of complementary analysis methods (X‐ray photoelectron spectroscopy (XPS), X‐ray absorption fine structure (XAFS), X‐ray diffraction (XRD)…). It is found that a change in the nanocrystal morphology from an equilibrium state truncated tetragonal bipyramid to a more elongated rod‐like structure accompanied by an increase in oxygen vacancies is responsible for an augmented PC activity of the TiO2 nanocrystals

Elucidating the formation and active state of Cu co-catalysts for photocatalytic hydrogen evolution

The design of active and selective co-catalysts constitutes one of the major challenges in developing heterogeneous photocatalysts for energy conversion applications. This work provides a comprehensive insight into thermally induced bottom-up generation and transformation of a series of promising Cu-based co-catalysts. We demonstrate that the volcano-type HER profile as a function of calcination temperature is independent of the type of the Cu precursor but is affected by changes in oxidation state and location of the copper species. Supported by DFT modeling, our data suggest that low temperature (&amp;lt;200 degrees C) treatments facilitate electronic communication between the Cu species and TiO2, which allows for a more efficient charge utilization and maximum HER rates. In contrast, higher temperatures (&amp;gt;200 degrees C) do not affect the Cu oxidation state, but induce a gradual, temperature-dependent surface-to-bulk diffusion of Cu, which results in interstitial, tetra-coordinated Cu+ species. The disappearance of Cu from the surface and the introduction of new defect states is associated with a drop in HER performance. This work examines electronic and structural effects that are in control of the photocatalytic activity and can be transferred to other systems for further advancing photocatalysis.Funding Agencies|SSF Foundation [EM16-0004]; Austrian Science Fund (FWF)Austrian Science Fund (FWF) [P32801]; TU-D doctoral college (TU Wien); TU Wien Bibliothek</p