Understanding the Chemical Bond in Semiconductor/MXene Composites: TiO2 Clusters Anchored on the Ti2C MXene Surface

First-principles calculations on titania clusters (TiO2)n (n=5 and 10) supported on the pristine Ti2C (0001) surface were carried out to understand the properties of semiconductor/MXene composites with implications in (photo)-catalysis. The reported results reveal a high exothermic interaction accompanied by a substantial charge transfer with a concomitant, notorious, deformation of the titania nanoclusters. The analysis of the density of states analysis of the composite systems evidences a metallic character with titania related states crossing the Fermi level. The picture of the chemical bonds is completed by the analysis of X-Ray Photoelectron Spectra (XPS) features, evidencing clear shifts of the C(1s) and O(1s) related peaks relative to the isolated systems that have a quite complex origin. This detailed analysis provides insights to experimentalists interested in the design and synthesis of these systems with possible applications in catalysis.

Toward a Rigorous Theoretical Description of Photocatalysis Using Realistic Models

This Perspective aims at providing a road map to computational heterogeneous photocatalysis highlighting the knowledge needed to boost the design of efficient photocatalysts. A plausible computational framework is suggested focusing on static and dynamic properties of the relevant excited states as well of the involved chemistry for the reactions of interest. This road map calls for explicitly exploring the nature of the charge carriers, the excited-state potential energy surface, and its time evolution. Excited-state descriptors are introduced to locate and characterize the electrons and holes generated upon excitation. Nonadiabatic molecular dynamics simulations are proposed as a convenient tool to describe the time evolution of the photogenerated species and their propagation through the crystalline structure of photoactive material, ultimately providing information about the charge carrier lifetime. Finally, it is claimed that a detailed understanding of the mechanisms of heterogeneously photocatalyzed reactions demands the analysis of the excited-state potential energy surface.

Challenges of modeling nanostructured materials for photocatalytic water splitting

Understanding the water splitting mechanism in photocatalysis is a rewarding goal as it will allow producing clean fuel for a sustainable life in the future. However, identifying the photocatalytic mechanisms by modeling photoactive nanoparticles requires sophisticated computational techniques based on multiscale modeling. In this review, we will survey the strengths and drawbacks of currently available theoretical methods at different length and accuracy scales. Understanding the surface-active site through Density Functional Theory (DFT) using new, more accurate exchange-correlation functionals plays a key role for surface engineering. Larger scale dynamics of the catalyst/electrolyte interface can be treated with Molecular Dynamics albeit there is a need for more generalizations of force fields. Monte Carlo and Continuum Modeling techniques are so far not the prominent path for modeling water splitting but interest is growing due to the lower computational cost and the feasibility to compare the modeling outcome directly to experimental data. The future challenges in modeling complex nano-photocatalysts involve combining different methods in a hierarchical way so that resources are spent wisely at each length scale, as well as accounting for excited states chemistry that is important for photocatalysis, a path that will bring devices closer to the theoretical limit of photocatalytic efficiency.

Oscillation in Excited State Lifetimes with Size of Sub-nanometer Neutral (TiO2)n Clusters Observed with Ultrafast Pump-Probe Spectroscopy

Neutral titanium oxide clusters of up to 1 nm in diameter (TiO₂)_n, with n < 10, are produced in a laser vaporization source and subsequently ionized by a sequence of femtosecond laser pulses. Using a 400 nm pump and 800 nm probe lasers, the excited state lifetimes of neutral (TiO₂)_n clusters are measured. All clusters exhibit a rapid relaxation lifetime of ∼35 fs, followed by a sub-picosecond lifetime that we attribute to carrier recombination. The excited state lifetimes oscillate with size, with even-numbered clusters possessing longer lifetimes. Density functional theory calculations show the excited state lifetimes are correlated with charge carrier localization or polaron-like formation in the excited states of neutral clusters. Thus, structural rigidity is suggested as a feature for extending excited state lifetimes in titania materials.

Facile Synthesis of Potassium-Doped Titanium Oxide Nanostructure (KTiOxs)/AlO(OH) Composites for Enhanced Photocatalytic Performance

Generally, nanoparticles (NPs) are used as photocatalysts, which sometimes results in difficulties in the separation and recycling of photocatalysts from suspensions after their application in water and wastewater treatment, which hinders industrial applications of NPs that are too fine to be removed by gravitational settling. This can be solved by using support NPs to overcome these problems. -OH enrich AlO(OH), which is produced by a steam coating process, has been could be used as a possible support, because the -OH groups on the surface can interact with foreign molecules; thus, various composite functional materials can be prepared. Potassium doped titanium oxide NPs, which are produced by a wet corrosion process, namely KTiOxs, have been selected as photocatalysts, because KTiOxs have sufficient K+ ions, thereby expecting the chemical bonding with -OH group from AlO(OH). This study fabricated a novel photocataysis system made by combining KTiOxs as catalysts and AlO(OH) as the catalysts’ support, namely KTiOxs/AlO(OH) composites. The KTiOxs nanowires, obtained from 10 mol/L of a KOH solution treated with Ti and AlO(OH) at 280 °C for 24 h through a steam coating process, yielded the highest surface area and the highest photocatalytic performance.

Role of surface termination in forming type-II photocatalyst heterojunctions: the case of TiO2/BiVO4

In this work we investigate TiO₂ and BiVO₄ nanostructures by means of density functional theory (DFT) calculations, to provide an estimate of the band alignment in TiO₂/BiVO₄ interfaces, highly active in photo-electrochemistry and photocatalytic water splitting. Calculations were carried out with both DFT range separated and self-consistent dielectric dependent hybrid functionals (HSE06 and PBE0_DD). The impact of systems' size has been investigated. The converged electronic levels of TiO₂ and BiVO₄ surfaces have been used to predict the band alignment in TiO₂/BiVO₄ heterostructures. Results indicated that when TiO₂ (101) surface is matched with BiVO₄ (110), a type-II alignment is obtained, where the band edges of BiVO₄ are higher in energy that those of TiO₂. This picture is favorable for charge-carriers separation upon photoexcitation, where electrons move toward TiO₂ and holes toward BiVO₄. On the contrary, if TiO₂ (101) is interfaced to BiVO₄ (010) the offset between the band edges is negligible, thus reducing the driving force toward separation of charge carriers. These results rationalize the dependence on the facet's exposure of the observed photocatalytic performances of TiO₂/BiVO₄ composites, where the TiO₂ (101)/BiVO₄ (110) interface outperforms the TiO₂ (101)/BiVO₄ (010) one.

Photocatalytic activity and the electron transport mechanism of titanium dioxide microsphere/porphyrin implanted with small size copper

To obtain efficient photocatalysts by coupling architectures, developing novel materials and elucidating the charge transport mechanism at the semiconductor interface are vital. Herein, a special nanocomposite (TiO2 microsphere/CuNPs/THPP) for photocatalytic hydrogen production was facilely fabricated with copper nanoparticles (CuNPs) as the interfacial linker of the TiO2 microspheres and meso-tetra(p-hydroxypheny)porphyrin (THPP). The assembly mode of the nanocomposite was studied in detail. It was found that the CuNPs implanted at the interface of the TiO2 microspheres and THPP can dramatically strengthen the interaction between the TiO2 microspheres and THPP, and improve the separation and transfer of photo-produced charges. Therefore, the nanocomposite displayed excellent performance for photocatalytic hydrogen production. Moreover, by recycling hydrogen production, it is demonstrated that the nanocomposite was a highly efficient and long-term stable photocatalyst. By investigating the energy band location and the charge transfer, the photocatalytic mechanism over the special nanocomposite was explored and proposed to explain the better activity of the TiO2 microsphere/CuNPs/THPP photocatalytic system. It will be helpful to provide deep insights into the construction of efficient photocatalytic systems.

Toward an understanding of electronic excitation energies beyond the molecular orbital picture

Can we gain an intuitive understanding of excitation energies beyond the molecular picture?