Single-Molecule Vibrational Spectroscopy of H2O on Anatase TiO2(101)

Atomic level understanding of water on titanium dioxide (TiO2), in particular the mineral form anatase, is exceptionally important, considering its practical applications including photocatalytic water-splitting, photoinduced hydrophilicity , and water purification.(1-3) Although much effort has been devoted to the study of water adsorption and dissociation on anatase, investigating the fundamental steps of single water splitting remains challenging. These challenges arise from the difficulty of chemical identification of single molecular water and its dissociation products. Here, we have unequivocally identified single water molecules and hydroxyls on the TiO2 anatase (101) surface using scanning tunneling microscopy (STM) in combination with inelastic tunneling spectroscopy (IETS). Labeling of single molecules on the semiconductor surface was confirmed by demonstrating the isotope shift of the vibrational signatures and by first-principles density functional theory (DFT). The chemical identification of individual water molecules on anatase opens a direct path to follow the water splitting process on the single molecule level.

Atomic Scale Observation of Chemical and Electronic Properties of Metal-Oxide Surfaces

Christian Alexander Stephan Dette

To meet the future global energy demand - an estimated additional 10 terawatts (TWs) per year in 2050 - a diversification of energy sources and fuels is needed. Solar energy represents a prominent alternative energy source. An important material for these solar-based applications is titanium dioxide (TiO2), thanks to its stability, band alignment and abundance. TiO2 is either used as a scaffold or to create photogenerated electrons and holes to perform catalytic reactions. However, the large bandgap of TiO2 yields device efficiencies that are too low to be economically sound. To make TiO2-based devices competitive, the electronic and chemical properties of TiO2 need to be clearly understood. In this work, using scanning tunneling microscopy (STM) together with spectroscopy techniques (tunneling spectroscopy (STS) and inelastic tunneling spectroscopy (IETS)), we study the electronic and structural properties of pristine TiO2 anatase (101), the most technologically relevant polymorph of TiO2. In particular, STM-IETS was applied for the first time to obtain chemical identification of adsorbed species on the semiconducting TiO2 surface. For each step, density functional theory (DFT)-based calculations were performed to support our findings. Using STM, we showed that the high reactivity of step edges along the [-111] direction stems from oxygen vacancies (VOs). As studied with STS, this non-stoichiometric step edge exhibits a bandgap reduced by 2 eV. Furthermore, a higher amount of adsorbates are present on this step edge, suggesting a higher chemical reactivity. Going one step further, we created a novel surface phase consisting of undercoordinated Ti atoms, increasing the amount of VOs over the whole surface phase. This new surface phase exerts the same behavior as the step edges, reducing bandgap and enhancing reactivity. On the other hand, by exposing the anatase surface to excess oxygen at elevated temperatures, we reduced the overall surface reactivity. This reduction was achieved by formation of an oxygen network, acting as a passivating layer on top of the TiO2 anatase (101) surface. Additionally, the excess oxygen fills vacant positions at the highly-reactive step edges along the [-111] direction, reducing the overall surface reactivity even further. It is important to note that the preparation procedures of both surface phases with enhanced and reduced surface reactivity are cheap and reversible, only modifying standard ultra-high vacuum (UHV) cleaning methods without any additional materials. Improved characterisation of the interaction between water and the TiO2 anatase (101) surface represents another important aspect of this work. Although photocatalytic water-splitting on TiO2 has been used for over 40 years, fundamental insights of the reaction mechanisms are still missing. In this thesis work, we labeled individual H2O and OH molecules on the semiconducting surface by detecting their vibrational modes with STM-IETS. Through clear identification of adsorbed species, we demonstrated that water can thermally dissociate on the TiO2 anatase (101) substrate without an additional light source. Furthermore, for the first time, we could structurally identify formation of a well-ordered water monolayer on TiO2 anatase (101). The work presented here opens new paths towards fundamental understanding of surface reactions on metal oxides, especially water on TiO2 anatase (101), to improve future solar energy conversion devices.

EPFL2016

Alignment of energy levels in amorphous oxides and at semiconductor-water interfaces

Zhendong Guo

We conduct a detailed investigation of defects in two representative amorphous oxides: amorphous Al2O3 (am-Al2O3) and TiO2 (am-TiO2), by combining ab initio molecular dynamics (MD) simulations and hybrid functional calculations. Our results indicate that oxygen vacancies and interstitials occur neither in am-Al2O3 nor in am-TiO2 due to structural rearrangements which assimilate the defect structure, and that the injection of extra holes can lead to the formation of O-O peroxy linkages. In am-Al2O3, hydrogen is found to be amphoteric. Based on localized Wannier functions, we identify the nominal charge state and the composition of the defect core units related to C and N impurities in am-Al2O3, which are found to depend on the total charge set in the simulation cell. Through the adopted electron counting rule, we assess that carbon and nitrogen impurities are only found in neutral and in singly positive charge states, respectively, indicating that none of them gives charge transition levels in am-Al2O3. In addition, those defect core units are shown to incorporate a varying number of oxygen atoms, by which their formation energy is dependent on the oxygen chemical potential. In addition, we propose an exchange mechanism for hole transport in am-TiO2 that relies on the simultaneous breaking and forming of O-O peroxy linkages that share one O atom. Through the use of nudged-elastic-band calculations, a hopping path as long as 1.2 nm with barriers of 0.3-0.5 eV is demonstrated, suggesting that hole diffusion through O-O peroxy linkages is viable in am-TiO2. In this work, we also determine the band alignment between various semiconductors and liquid water by combining MD simulations of atomistic interface models, electronic-structure calculations at the hybrid-functional and GW levels, and a computational standard hydrogen electrode (SHE). Our study comprises GaAs, GaP, GaN, CdS, ZnO, SnO2, rutile and anatase TiO2. For each semiconductor, we generate atomistic interface models with liquid water at the pH corresponding to the point of zero charge. The MD simulations are started from initial configurations, in which the water molecules are either molecularly (m) or dissociatively (d) adsorbed on the semiconductor surface. The calculated band offsets are found to be strongly influenced by the adsorption mode at the semiconductor-water interface, leading to differences larger than 1 eV between m and d models of the same semiconductor. We then assess the accuracy of various ab initio electronic-structure schemes in determining the band alignment. In the last part, we try to evaluate photocatalysts for water-splitting by considering the surface coverage and the energy alignment. We determine surface concentrations of water molecules, protons, and hydroxyl ions adsorbed at the semiconductor-water interfaces mentioned above as a function of pH. This is achieved through the calculation of the acidity constants at the surface sites, which are derived from ab initio MD simulations and a grand-canonical formulation of adsorbates. It is finally shown how the potential of a semiconductor material as photocatalysts for water splitting can be inferred by combining the nature of the surface coverage and the alignment of the band edges to the relevant redox levels.

EPFL2019

Single molecule fluorescence and phosphorescence studies using a scanning tunneling microscope

Abhishek Grewal

This thesis investigates novel single-molecule luminescence phenomena at their inherent, sub-molecular length scale. The microscopic understanding of luminescence processes will be crucial for the continued improvement of organic optoelectronic and semiconductor devices.In this work, a scanning tunneling microscope (STM) is combined with optical spectroscopy and Hanbury Brown and Twiss interferometry to image topography, electronically characterize, and analyze the vibrational mode excitations and emission statistics, all at the atomic scale. Moreover, the precise STM tip positioning allows tuning the coupling between the molecular exciton and the tip-induced plasmon with sub-molecular spatial control, which is key to the decay rate enhancement of molecular phosphors.A prototypical platinum phthalocyanine (PtPc) molecule, decoupled from a metal substrate by a thin film of NaCl, provides a model platform for examining STM-induced bipolar fluorescence, phosphorescence, and energy upconversion electroluminescence (UCEL). First, an STM investigation of only the ionic insulator NaCl demonstrates that both valence and conduction bands are of anionic Cl- character. This is relevant for the coupling between the molecule and the metal substrate. Theoretical considerations reveal a dominant contribution from low-lying molecular orbitals under certain conditions.Voltage and polarity of fluorescence onsets can be controlled using different metal substrates because their different work functions tune the energy alignment of the molecular orbitals with respect to the substrate Fermi level. For PtPc atop an Au(111) substrate, fluorescence is observed at both voltage polarities, even under tunneling conditions where UCEL occurs. This remarkable behavior allows the testing of different emission mechanisms and reveals new general guidelines for designing future optoelectronic devices. An analysis of photon emission statistics from a single molecule under UCEL conditions refutes previously proposed excitation mechanisms. It guides theory towards a novel mechanism, briefly introduced in the thesis.PtPc atop NaCl on Ag(111) exhibits fluorescence, and in this thesis, its phosphorescence is also observed by local excitation in the STM. Electronic characterization reveals a simultaneous voltage onset of both types of emission. Phosphorescence is also detected using tip-enhanced resonant photoexcitation of the singlet state. Since direct optical excitation of the triplet state is forbidden, this observation is the first demonstration of intersystem crossing in a tip-controlled single-molecule experiment. Exciton-plasmon coupling can be varied by fine positioning the tip in proximity to the molecule. This allows tuning the triplet emission intensity through control of the singlet lifetime. These findings are directly transferrable to highly topical phosphorescent OLEDs that employ plasmonic enhancement.The phenomenon of plasmonic antibunching is discovered for electron tunneling from a few monolayer thick C60 films to the STM tip. The layer thickness and the applied voltage control the sequential tunneling of charges by a Coulomb blockade mechanism. This mechanism eventually causes the emission of one photon at a time. Such a plasmonic source allows for a higher single-photon emission rate than an individual molecule. The design principles for other semiconducting films are discussed, suggesting a quest for similar nanostructured single-photon emitters.

EPFL2022