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.

Pt and PbO 2 on the specific orientations of rutile and anatase via photodeposition, indicating that the facets help in the separation of photoinduced electrons and holes. It was found that electrons tend to be transferred to the (101) facets, whereas the holes are driven to the (001) surfaces. This result suggests that anatase (101) surface provides the effective reduction site, whereas anatase (001) works as the oxidation site. Tachikawa et al. [5] investigated facet dependant photocatalysis on anatase with single-molecule fluorescence imaging and kinetic analysis by using redox-responsive fluorogenic dyes. On the single crystal of anatase coexposed with the (101) and (001) facets, the fluorogenic dyes are preferentially reduced on the (101) facet rather than the (001) facet. This finding confirms that photogenerated electrons preferentially migrate to and are trapped at the (101) facet. Such a charge carrier separation was observed for different metal oxides such as Cu 2 O, WO 3 , and BiVO 4 . [3,6] Furthermore, based on the charge separation between different facets in the crystal, Li et al. [7] demonstrated a drastic enhancement of photocatalytic activities by selectively depositing reduction and oxidation cocatalysts onto the reductive and oxidative facets of BiVO 4 crystals. In summary, there is a strong need of deeper understanding of the mechanism of charge separation between different crystal facets.Surface properties of the TiO 2 anatase have been studied by a number of experimental and theoretical investigations without providing a clear reason for different photocatalytic efficiencies. [8] However, the charge separation and trapping are conventionally explained by the different energy levels of different facets due to the surface atomic arrangement and coordination. [9] Recently, a first-principles calculation predicted that the Fermi level of the (001) facet is located at a lower energy level than that of the (101) facet. [9a] Thus, a so-called surface heterojunction would be formed between the (101) and (001) facets due to the original difference of their surface Fermi levels in a crystal exposed with both facets. As a result, photogenerated electrons and holes could preferentially migrate to the (101) and (001) facets, thereby exhibiting different photocatalytic activities on these facets. However, the Fermi level shown in the density of states is located near valence band maximum for the (101) and enters even into valence band for the (001) surface meaning that the (101) surface is a p-type semiconductor and Single crystalline anatase is used to prepare well defined (001) and (101) surfaces in ultrahigh vacuum (UHV) in different states: sputtered, annealed, stoichiometric, and oxidized. The electronic properties of the well-defined surfaces are investigated by X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy after UHV transfer. The Fermi level of (001) facets for all applied surface conditions is lower than that of the (101) facets by 150-450 meV. The energy differen...

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...

Facet-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.

Deposition of NiO on the (101) facet of anatase nanocrystals by the SFCD route yields nanocomposites more efficient than pure anatase TiO2 for the photodecomposition of both anionic and cationic dyes.

The photocatalytic (PC) performance of titanium dioxide (TiO 2 ) 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 TiO 2 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 TiO 2 nanocrystals. PhotocatalysisThe ORCID identification number(s) for the author(s) of this article can be found under https://doi.

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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