https://escholarship.org/content/qt2h61k4tr/qt2h61k4tr.pdf?t=p2atsn

Low energy electron diffraction and electron spectroscopy studies of the clean (110) and (100) titanium dioxide (rutile) crystal surfaces

Photoconductivity is studied in individual ZnO nanowires. Under ultraviolet (UV) illumination, the induced photocurrents are observed to persist both in air and in vacuum. Their dependence on UV intensity in air is explained by means of photoinduced surface depletion depth decrease caused by oxygen desorption induced by photogenerated holes. The observed photoresponse is much greater in vacuum and proceeds beyond the air photoresponse at a much slower rate of increase. After reaching a maximum, it typically persists indefinitely, as long as good vacuum is maintained. Once vacuum is broken and air is let in, the photocurrent quickly decays down to the typical air-photoresponse values. The extra photoconductivity in vacuum is explained by desorption of adsorbed surface oxygen which is readily pumped out, followed by a further slower desorption of lattice oxygen, resulting in a Zn-rich surface of increased conductivity. The adsorption-desorption balance is fully recovered after the ZnO surface is exposed to air, which suggests that under UV illumination, the ZnO surface is actively "breathing" oxygen, a process that is further enhanced in nanowires by their high surface to volume ratio.

/pdf/photoinduced-oxygen-release-and-persistent-photoconductivity-4x8riley6g.pdf

Photoinduced oxygen release and persistent photoconductivity in ZnO nanowires

ZnO surfaces adsorb oxygen in the dark and emit CO2 when exposed to white light, reminiscent of the lungs of living creatures. We find that this exchange of oxygen with the ambient affects the integrity of the ZnO surface. Thus, it forms a basis for several interesting surface phenomena in ZnO, such as photoconductivity, photovoltage, and gas sensing, and has a role in ZnO electrical conduction. Using x-ray photoelectron spectroscopy on ZnO nanowires, we observed a decomposition of ZnO under white light and formation of oxygen-depleted surface, which explains photoconductivity by the electron donation of oxygen vacancies. Our findings suggest that the observed decomposition of the ZnO lattice may only take place due to photon-induced reduction of ZnO by carbon containing molecules (or carbo-photonic reduction), possibly from the ambient gas, accounting in a consistent way for both the reduced demands on the energy required for decomposition and for the observed emission of lattice oxygen in the form of CO2. The formation of oxygen-vacancy rich surface is suggested to induce surface delta doping, causing accumulation of electrons at the surface, which accounts for both the increase in conductivity and the flattening of the energy bands. Using surface photovoltage spectroscopy in ultra high vacuum, we monitored changes in the deep level spectrum. We observe a wide optical transition from a deep acceptor to the conduction band, which energy position coincides with the position of the so called “green luminescence” in ZnO. This green transition disappears with the formation of surface oxygen vacancies. Since the oxygen vacancies are donors, while the green transition involves surface acceptors, the results suggest that the initial emission of oxygen originates at the defect sites of the latter, thereby eliminating each other. This suggests that the green transition originates at surface Zn vacancy acceptors. Removing an oxygen atom from a Zn vacancy completes the vacancy to become a full ZnO molecule vacancy, which does not produce deep levels. Our results explain why ZnO finds use as an electrical detector for oxygen and for carbon containing gas molecules. They may also shed new light on photocatalytic uses of ZnO. It is suggested that similar surface phenomena may affect other semiconducting oxides.

Interaction of light with the ZnO surface: Photon induced oxygen "breathing," oxygen vacancies, persistent photoconductivity, and persistent photovoltage

The concept Photocatalysis and, of greater import here, Heterogeneous Photocatalysis were first introduced in the second decade (1910–1920) of the 20th century according to the CAPLUS and MEDLINE databases (SciFinder). This review reports a brief historical perspective on the origins of the two concepts, whether implied or explicitly stated, in some detail up to about the mid-1980s when heterogeneous photocatalysis witnessed the beginning of an exponential growth, with particular emphasis on the use of nanosized TiO2 particles in powdered form as the (so-called) photocatalyst of choice in environmental applications because of its inherent properties of abundance and chemical stability in acidic and alkaline aqueous media (in the dark), in contrast to ZnO that had been the metal oxide of choice in the early days. The early workers in this area often used the term photosensitization rather than the current popular term photocatalysis, used since the early 1980s. The term Photocatalysis appeared in the literature as early as 1910 in a book by Plotnikow (Russia) and a few years later it was introduced in France by Landau. The review also reports on contributions during the early years by Terenin at the University of St. Petersburg (previously Leningrad, Soviet Union), and in the decade spanning 1975–1985 contributions by Bard's group at the University of Texas at Austin (USA) as well as those of other groups. Some activities into the conversion of light energy to chemical fuels (e.g. H2) during the 1975–1985 decade are also considered.

https://link.springer.com/content/pdf/10.1039/c2pp25026h.pdf

On the genesis of heterogeneous photocatalysis: a brief historical perspective in the period 1910 to the mid-1980s.

In situ solid-state NMR methodologies have been employed to investigate the photocatalytic oxidation of ethanol (C2H5OH) over a TiO2-coated optical microfiber catalyst and two other TiO2-based catalysts. Adsorption of ethanol on the surface of the TiO2/optical microfiber catalyst formed a strongly hydrogen-bonded species and a Ti ethoxide species. In situ UV irradiation experiments under 13C magic angle spinning (MAS) conditions reveal the formation of two main reaction intermediates, 1,1-diethoxyethane (CH3CH(OC2H5)2) and acetic acid, under dry conditions. The catalyst was shown to be highly effective for the degradation of ethanol as complete photooxidation of ethanol was observed to form acetic acid and CO2. These results were compared to those using a monolayer catalyst supported on porous Vycor glass and powdered TiO2. Solid-state NMR investigations on TiO2 powder modeled after temperature-programmed desorption experiments confirm the identities of the hydrogen-bonded and Ti ethoxide species and show...

Ethanol Photocatalysis on TiO2-Coated Optical Microfiber, Supported Monolayer, and Powdered Catalysts: An in Situ NMR Study

A dynamic mass spectrometer system is described which utilises fast detection techniques to follow pressure surges of gaseous product initiated by incidence of 30 µs flashes of u.v. photons onto ZnO or TiO2 surfaces pretreated in vacuo at 10–6 Nm–2. A difficulty inherent to the system is appearance of “system” transients corresponding to flash-initiated growth in ion currents at m/e= 44, 28, 16 and 12, corresponding to photo-desorption of carbon oxides from walls of the system. Addition of ZnO samples to the system caused appearance of a new transient at m/e= 32, consistent with photoassisted release of O2 from the ZnO surface. Rutile samples similarly treated did not give rise to a comparable O2 transient. Presence of electron-attaching methyl halides at pressures 10–5-10–3 N m–2 over the ZnO or TiO2 surfaces during flashes resulted in only a small transient at m/e= 19, corresponding to growth of CD3H from CD3I/ZnO with ϕ(CD3H)≈ 10–6. Presence of adsorbed, hole-trapping methanol and ethanol resulted in photo-assisted formation of formaldehyde or acetaldehyde and observed yields correspond approximately to quantum efficiencies of 10–4 for these photoassisted dehydrogenation processes. Use of the deuterated ethanol showed that photodesorption occurred accompanied by H/D exchange involving one deuterium atom in the originally adsorbed alcohol. For flash-illuminated ethanol/ZnO systems the mass distribution of transients could best be accounted for as a combination of flash desorption of alcohol, photoassisted formation of acetaldehyde and photoassisted formation of ethylene. The importance of photodegradation of strongly adsorbed ethanol in these photoassisted surface processes was demonstrated by observations that a ratio of 1 : 4 : 9 observed between them when ethanol was present over the ZnO surface during the flash changed to 1 : 10 : 80 when flashes were deliverd to the surface after all gaseous alcohol was removed. For ethanol/TiO2 systems, flash desorption of alcohol, plus photoassisted formation of acetaldehyde and ethylene were the main flash-initiated processes. Results are briefly compared with various models of the flash illuminated gas/oxide interface

Eoin Finn

Papers

Photo-assisted surface reactions studied by dynamic mass spectrometry