Photodesorption Of Oxygen Research Articles

Sub-ppm Formaldehyde Detection by n-n TiO2@SnO2 Nanocomposites.

Formaldehyde (HCHO) is an important indicator of indoor air quality and one of the markers for detecting lung cancer. Both medical and air quality applications require the detection of formaldehyde in the sub-ppm range. Nanocomposites SnO2/TiO2 are promising candidates for HCHO detection, both in dark conditions and under UV illumination. Nanocomposites TiO2@SnO2 were synthesized by ALD method using nanocrystalline SnO2 powder as a substrate for TiO2 layer growth. The microstructure and composition of the samples were characterized by ICP-MS, TEM, XRD and Raman spectroscopy methods. The active surface sites were investigated using FTIR and TPR-H2 methods. The mechanism of formaldehyde oxidation on the surface of semiconductor oxides was studied by in situ DRIFTS method. The sensor properties of nanocrystalline SnO2 and TiO2@SnO2 nanocomposites toward formaldehyde (0.06–0.6 ppm) were studied by in situ electrical conductivity measurements in dark conditions and under periodic UV illumination at 50–300 °C. Nanocomposites TiO2@SnO2 exhibit a higher sensor signal than SnO2 and a decrease in the optimal measurement temperature by 50 °C. This result is explained based on the model considering the formation of n-n heterocontact at the SnO2/TiO2 interface. UV illumination leads to a decrease in sensor response compared with that obtained in dark conditions because of the photodesorption of oxygen involved in the oxidation of formaldehyde.

Non-aligned ZnO nanowires composited with reduced graphene oxide and single-walled carbon nanotubes for highly responsive UV–visible photodetectors

A comprehensive comparison study of the UV–visible photoresponse properties of non-aligned carbothermal ZnO nanowires (ZNWs) composited with reduced graphene oxide (RGO) and single-walled carbon nanotubes (SWCNTs) is presented. The RGO/ZNWs composites (GZWs) and SWCNTs/ZNWs composites (CZWs) have been successfully synthetized through UV-assisted photochemical reduction of GO in ZNWs suspension and ultrasonic stirring technology, respectively. The optimal contents of RGO and SWCNTs incorporated into ZNWs were determined for achieving maximum photoresponse, respectively. Experimentally, the optimal GZWs and CZWs exhibit significant enhancement in photoelectric properties than pure ZNWs. It is demonstrated that the GZWs-based photodetector has higher photoresponsivity (∼16 AW-1) than CZWs-based photodetector under low UV irradiation (<10 μWcm−2) and at 1.0 V bias, while the CZWs-based photodetector has higher on/off current ratio (8.41 × 104) and faster response time (1.99 s of rise-time and 0.39 s of decay-time) than GZWs-based one. The detail physics mechanism on the effects of RGO and SWCNTs on photoelectric characteristics of ZNWs is presented with the interface charge transfer due to the chemisorption and photo-desorption of oxygen.

ВЛИЯНИЕ СВЕТА НА ПРОВОДИМОСТЬ И ПРОЦЕССЫ АДСОРБЦИИ–ДЕСОРБЦИИ АКЦЕПТОРОВ НА ПОВЕРХНОСТИ ТОНКИХ ПЛЕНОК СdTe:Sb

Изучено влияние внешних факторов (атмосфера, освещение) на проводимость тонких пленок CdTe:Sb. Обнаружено, что на зависимости действительной части комплексной электрической проводимости σ*=σ'+jσ''от времени под действием света с энергией кванта больше Eg наблюдается две области: резкий рост на начальном этапе и последующий спад. Показано, что наблюдаемый эффект снижения проводимости под действием света обусловлен фотодесорбцией молекул кислорода и/или воды с поверхности образца. Предложена модель «двуслойной» проводимости, представляющей собой цепь с «параллельно» соединёнными поверхностной и объемной проводимостью.&#x0D; Авторы выражают признательность к. х. н. Гапановичу М. В. за помощь в приготовлении образцов.&#x0D; Работа выполнена при финансовой поддержке проекта РФФИ № 16-08-01234 и госзадания № 01201361850.

Strain- and surface-induced modification of photoluminescence from self-assembled GaN/Al0.5Ga0.5N quantum dots: strong effect of capping layer and atmospheric condition

We report on the influence of a capping layer on the photoluminescence properties of self-assembled GaN quantum dots grown on an Al0.5Ga0.5N template. Self-assembled GaN quantum dots show a large quantum confined Stark shift and long carrier recombination time due to strong built-in spontaneous and piezoelectric polarization fields. Nevertheless, owing to strong carrier localization and suppressed nonradiative processes, these quantum dots have a high-quantum efficiency even at room temperature. Here, we show that the capping thickness has an important role on the optical properties of the GaN quantum dots. The radiative and nonradiative recombination processes of quantum dots are strongly affected by adjusting the capping thickness, and the GaN quantum dots with 12 monolayers-thick Al0.5Ga0.5N capping layer show a remarkably high internal quantum efficiency of more than 80% at room temperature. We also studied photoluminescence quenching and enhancement for surface (uncapped) quantum dots caused by photoadsorption and photodesorption of oxygen.

Oxygen photo-adsorption related quenching of photoluminescence in group-III nitride nanocolumns

GaN and InGaN nanocolumns of various compositions are studied by room-temperature photoluminescence (PL) under different ambient conditions. GaN nanocolumns exhibit a reversible quenching upon exposure to air under constant UV excitation, following a t−1/2 time dependence and resulting in a total reduction of intensity by 85–90%, as compared to PL measured in vacuum, with no spectral change. This effect is not observed when exposing the samples to pure nitrogen. We attribute this effect to photoabsorption and photodesorption of oxygen that modifies the surface potential bending. InGaN nanocolumns, under the same experimental conditions do not show the same quenching features: The high-energy part of the broad PL line is not modified by exposure to air, whereas a lower-energy part, which does quench by 80–90%, can now be distinguished. We discuss the different behaviors in terms of carrier localization and possible composition or strain gradients in the InGaN nanocolumns.

The size dependence of tin oxide atomic cluster nanowire field effect transistors

Tin oxide (SnO2) cluster nanowires have been fabricated using atomic clusters as building blocks.Nanowires with widths of less than 100 nm were defined using electron beam lithographyfollowed by deposition of Sn clusters which were subsequently thermally oxidized. Thecluster nanowires were used to fabricate field effect transistors. The transistors weren-type and demonstrated a clear size-dependent behaviour. With zero gate bias thenarrowest wires were depleted, both the carrier concentration and the conductionquickly increasing with wire width. This behaviour is attributed to the formationof a surface depletion region caused by Fermi level pinning. The width of thedepletion region was estimated using the carrier concentration calculated fromthe transistor threshold voltages. The change in the wire conductance with UVillumination was also investigated. Illumination with 365 nm light increases theconduction by up to 40 times. This is attributed to a combination of an increase incarriers due to photo-desorption of oxygen from the surface of the wires and anincrease in the mobility due to a reduction of inter-grain potential barrier height.

Photon-driven electron and atomic processes on solid-state surface in photoactivated spectroscopy and photocatalysis

Several results of “spectro-manometric” investigations of photoinduced processes in gas–solid systems pioneered by A.N. Terenin in the Leningrad State University which gave rise to the “photonics of heterogeneous systems” continued by his disciples are discussed. The step-by-step experimental investigations of mechanisms of interrelated processes in electronic and atomic subsystems on UV–vis illuminated surface of wide-bandgap oxides are presented. A variety of complementary experimental methods such as the mass-spectrometry, the optical, luminescence and ESR spectroscopies, and the UV-photoelectron spectroscopy have been adapted to carry out in situ investigations in three phases: gas–adsorbate–surface. The primary acts of electron subsystem excitation in the sub-bandgap range are the spectral-selective and site-sensitive excitations of F- and V-type centers. The role of photoadsorption and photodesorption of oxygen as relaxation channels competing with radiative and non-radiative decay is treated. Spectral and kinetic parameters of photoexcited centers and structures of adsorbate complexes are determined. Highly active oxygen species arise at photoactivated surface, allowing oxygen vacancies to form or to heal and also to oxidize a number of adsorbed molecules. Using the time-of-flight (TOF) spectroscopy, the composition and the kinetic energy distributions of photodesorbed particles are analyzed for to obtain the dynamics of bond rearrangements and bond breaking. The results are discussed in the framework of a charge transfer approach in excited clusters. The potentialities of the presented results for probing and characterization of the electronic and atomic structure of active sites on the surface, as well as for a regulation of a number and a composition of surface defects are discussed. We have demonstrated on an example of NO x /Al 2O 3 and (NO + CO)/TiO 2 systems that proper defects of F, F + and V-type can be used for sensitization of photocatalysts to a long-wave spectral range.

Dynamics of O 2 photodesorption from metal clusters: A significant difference from bulk behaviour

Photodesorption of O 2 from size-selected Ag n O 2 - cluster anions with n = 2, 3, 4 and 8 was studied using time-resolved photoelectron spectroscopy (TR-PES). The spectra indicate that relaxations of photo-excited Ag n O 2 - clusters with n = even numbers accompany ultrafast direct O 2 photodesorption. For the odd-numbered cluster Ag 3 O 2 - , in contrast, a long-lived excited state is observed, since O 2 might be dissociatively chemisorbed, suppressing direct photodesorption of oxygen. Both, direct desorption and long-lived excited states, have not been observed from adsorbate covered metal surfaces, suggesting unique photochemical properties of such small clusters.

ZnO−Latex Hybrids Obtained by Polymer-Controlled Crystallization: A Spectroscopic Investigation

Micro- and submicrosized ZnO-polymer hybrid materials were synthesized by precipitating zinc oxide from an aqueous medium in the presence of poly(styrene-acrylic acid) latex nanoparticles, prepared by miniemulsion polymerization. Up to 10 wt % of the latex becomes incorporated into the crystals. Although the long-range order of the inorganic material is essentially not altered by the polymer, studies by photoluminescence (PL) spectroscopy and electron paramagnetic resonance (EPR) show that the latex particles influence the optical and paramagnetic properties of the hybrids, which can be correlated with changes in the defect structure. Typical PL emission spectra showed a narrow UV peak and a defect-related broad band in the green-yellow spectral region. The former emission is attributed to exciton recombination, whereas the latter seems to be related with deep-level donors. Latex acts as a quencher of the visible emission, and compared to pure ZnO, ZnO-latex hybrids show a significantly lower PL intensity in the visible range. A noticeable dynamic behavior of the PL, clearly more remarkable in the presence of latex, was observed, and it is explained in terms of photodesorption of oxygen adsorbed at surface positions. EPR provided additional information about crystal defects and species with unpaired electrons. All EPR spectra showed a single signal at g approximately 1.96, whose intensity and temperature dependence did not correlate with those of the PL visible band. These findings indicate that the green-yellow emission and the EPR signal of our samples have a different physical origins.

Photodesorption of oxygen from carbon nanotubes

We propose to use photodesorption as a noninvasive tool to clean carbon nanotubes from oxygen, offering a clear advantage over thermal and chemical treatments. The detachment of chemisorbed oxygen from atomic vacancies is triggered by a resonant Auger process, initiated by an $\mathrm{O}\phantom{\rule{0.3em}{0ex}}1s\ensuremath{\rightarrow}\mathrm{O}\phantom{\rule{0.3em}{0ex}}2p$ transition, which leaves two holes in the $\mathrm{O}\phantom{\rule{0.3em}{0ex}}2s$ level. In the electronically excited state, oxygen desorbs spontaneously, with no damage to the carbon network or the cylindrical nanotube shape. Subsequent reaction of oxygen atoms with ${\mathrm{H}}_{2}$ molecules is shown to prevent reoxidation of the nanotube.

Photo- and Thermal Annealing-Induced Processes in Carbon Nanotube Transistors

ABSTRACTPhotoinduced conductivity changes and effects of thermal annealing in carbon nanotube transistors have been examined. Low-intensity ultraviolet light significantly reduces the p-channel conductance while simultaneously increasing the n-channel conductance. A combination of optical absorption and electron transport measurements reveals that these changes occur without variations in dopant concentrations. Measurements with different metals reveal that UV induces oxygen desorption from the electrodes rather than from nanotubes. In Ti-nanotube contact where the Schottky barrier plays an important role, photodesorption of oxygen mainly occurs from the native oxide of Ti electrodes. Decrease in the p-channel conductance arises from the metal work function change which causes larger hole Schottky barrier. Non-Schottky Pd-contacted nanotube transistors do not show photodesorption effects with low intensity UV. Thermal annealing of nanotube transistors with Ti/Au electrodes also leads to the disappearance of the photodesorption effects. However, a noticeable p-doping is observed to upon air exposure after thermal annealing.

Hot-atom mechanism in photodesorption of molecular oxygen from a stepped platinum (113) surface

The photodesorption of oxygen admolecules was studied on a stepped Pt(113)=(s)2(111)×(001) surface with 193 nm irradiation at 110 K. Multidirectional desorptions were found to collimate at ±12–20° and ±45–49° off the surface normal and also along the surface normal in a plane along the trough. The first component is always dominant, and the weak second component only appears at higher oxygen coverages. The normally directed desorption is not significant. The translational energy of desorbing O2 peaks around 15–20° and 50°, confirming the inclined desorptions. It is proposed that these inclined components are due to the desorption induced by the impact of oxygen admolecules with hot oxygen atoms from the photodissociation of adsorbed molecular oxygen, emitted along the trough. A simple cosine distribution was found to fit the thermal desorption from oxygen admolecules and also the recombinative desorption of oxygen adatoms. The 193 nm irradiation also produces additional, less tightly bound oxygen adatoms, which yield a desorption component collimated at 15° from the surface normal in the step-down direction.

Adsorption and photodesorption of oxygen on the surface and crystallite interfaces of sputtered ZnO films

The oxygen adsorption and photodesorption of r.f. sputtered ZnO films having the preferred growth orientation with the c-axis parallel to the substrate have been investigated. Electric-field-induced oxygen adsorption was observed on these polycrystalline films. Strong photoresponses were obtained when samples were irradiated with UV light, and were assumed to arise from oxygen photodesorption on both the surface and crystallite interfaces. Surface photodesorption produced a large, slow photoresponse, while the photoresponse produced by oxygen photodesorption of the crystallite interfaces was relatively weak and fast. Doping the surface layer of ZnO films with nitrogen could stabilize the surface structure, subsequently eliminating surface adsorption. The fast photoresponse originating from photodesorption of crystallite interfaces could appear as a result. The above results indicate that the conductivity of sputtered polycrystalline ZnO films is controlled by surface and intercrystallite depletion barriers, ascribed to the presence of ionized adsorbates on the surface and crystallite interfaces.

The adsorption and photodesorption of oxygen on the TiO2(110) surface

We have investigated the adsorption and thermal conversion of molecular oxygen (O2) states on the TiO2(110) surface by making use of the distinct photodesorption behavior of each adsorption state. Oxygen chemisorbs at the oxygen vacancy defect sites on the annealed TiO2(110) surface at 105 K to a saturation coverage of less than 0.12 monolayers (ML), producing mostly the α-O2 species which is observed to undergo slow photodesorption. Upon heating this surface to above 250 K, the α-O2 is converted to the β-O2 state which can photodesorb at a significantly higher rate. The β-O2 species dissociates above 400 K to produce atomic oxygen, eliminating the oxygen anionic vacancies. Both the α- and β-photodesorption processes have a threshold energy at the TiO2 band gap (3.1 eV), indicating a substrate excitation mediated process. The photodesorption time-profile is fitted with an exponential decay function with a cross section of ∼8×10−17 cm2 for the α-O2 and ∼1.5×10−15 cm2 for the β-O2 species at a photon energy of 3.94 eV.

Synchrotron-Radiation-Induced Modification of Silicon Dioxide Film at Room Temperature

Thermally grown SiO2 films were irradiated by synchrotron radiation at room temperature and changes in film properties were analyzed to clarify the irradiation effects and explain the mechanism of the photo-stimulated desorption (PSD) of SiO2. The irradiated SiO2 was chemically weakened throughout the film of 89-nm thickness by Si-O bond breaking following core electron excitation. The broken bonds were recombined by postannealing at 850°C. The irradiated surface was deoxidized by oxygen desorption and the remaining Si-rich layer was evaporated by postannealing over 700°C. Results indicate that irradiation effects are bond breaking and the desorption of oxygen and that thermal effects are the recombination of broken bonds and the evaporation of the deoxidized layer. It is supposed that the PSD of SiO2 at elevated temperature consists of photo-desorption of oxygen and thermal evaporation of SiO.

Drastic changes in electronic properties of a ZnO-SiO2-Au device by ultraviolet illumination

Oxidative gases are chemisorbed on the semiconductor surface by capturing electrons of a semiconductor and then are desorbed by releasing electrons of chemisorption states [1]. The releasing of electrons can be attained by thermionic emission, field emission, and hole injection. The thermionic process is predominant as the temperature is raised. In general, chemisorptive states of oxidative gases are located at a few electronvolts below the bot tom of the conduction band of an n-type semiconductor, e.g. 0.8 to 1.0 eV for O2 on the ZnO surface [2], so that a high temperature is necessary to excite electrons from the chemisorptive states into the conduction band. On the other hand, the field-assisted desorption of oxygen was recently confirmed in an c~-F%O3-Au diode, as reported in a previous paper [3]. Desorption by hole injection was found in the photodesorption of oxygen from ZnO powder [4]. However, the desorption by hole injection has not been qualitatively studied, therefore the purpose of this work was to elucidate this process, and it is applied to develop a gas sensor with high sensitivity even at room temperature. For this purpose, a Z n O SiO2-Au device is fabricated to generate effectively holes at the semiconductor surface by band gap illumination, and the adsorption and desorption of oxygen is intensively investigated by considering the change in the leakage current of the device. ZnO film was deposited on a glass substrate (Corning 7059) using an r.f. magnetron sputtering apparatus. The sputtering was performed at 50W r.f. power under 1 Pa of an 95% Ar + 5% 02 atmosphere for 135 min. SiO2 was subsequently deposited on the sputtered ZnO surface. Sputtering conditions were the same as those for the ZnO sputtering. Gold was deposited on the SiO2 surface by vacuum evaporation. A high-pressure 500 W mercury lamp was used as a light source for ultraviolet illumination. Photocurrent spectra were measured using a 500 W xenon lamp, a monochromator , a light chopper (80 Hz) and a lock-in amplifier. Impedance measurements of the Z n O SiO2-Au device were carried out for the assignment of the resistance change before and after ultraviolet illumination. Two peaks of (0 0 2) and (0 0 4) reflections were seen in X-ray diffraction patterns of the sputtered ZnO film, except for a negligible weak peak of(1 0 3) reflection. Thus, the sputtered ZnO film had a c-axis

An ESR Study on Oxygen Adsorption and Illumination of Vacuum Treated V2O5/TiO2

Abstract Oxygen adsorption on vacuum treated TiO2 doped with vanadium results in the formation of O2- species. It is argued that substitutional V3+ ions act as electron donors. The oxygen radicals are stabilized in the neighbourhood of titanium ions. Depending on the oxygen pressure photo-adsorption or photodesorption of oxygen is observed.

Photo-adsorption and photo-desorption of oxygen on highly hydroxylated TiO2 surfaces. Part 3.—Role of H2O2 in photo-desorption of O2

Download options Please wait... Article information DOI https://doi.org/10.1039/F19807601535 Article type Paper Download Citation J. Chem. Soc., Faraday Trans. 1, 1980,76, 1535-1546 BibTex EndNote MEDLINE ProCite ReferenceManager RefWorks RIS Permissions Request permissions Photo-adsorption and photo-desorption of oxygen on highly hydroxylated TiO2 surfaces. Part 3.—Role of H2O2 in photo-desorption of O2 G. Munuera, A. R. González-Elipe, J. Soria and J. Sanz, J. Chem. Soc., Faraday Trans. 1, 1980, 76, 1535 DOI: 10.1039/F19807601535 To request permission to reproduce material from this article, please go to the Copyright Clearance Center request page. If you are an author contributing to an RSC publication, you do not need to request permission provided correct acknowledgement is given. If you are the author of this article, you do not need to request permission to reproduce figures and diagrams provided correct acknowledgement is given. If you want to reproduce the whole article in a third-party publication (excluding your thesis/dissertation for which permission is not required) please go to the Copyright Clearance Center request page. Read more about how to correctly acknowledge RSC content. Search articles by author Guillermo Munuera Augustin R. González-Elipe Javier Soria Jesús Sanz

Photoconducting properties of cadmium sulphide-lithium thin films formed by the chemical bath deposition method

The cadmium sulphide-lithium thin films used in this work were prepared by the chemical bath deposition technique. In this simple process uniform large-area films of semiconducting and photoconducting cadmium sulphide can be formed on rotating substrates. The photoconducting and optical properties of these films were studied. The photoconducting properties studied included the rise and decay of the photoconductivity, the spectral response of the photoconductivity and the variation of the time constants with the wavelength of excitation and with the intensity of excitation. The photoconductivity rise curve for the first excitation was analysed considering the role of photodesorption of oxygen to determine the oxygen trap level, which is 0.78 eV below the conduction band. Although it was analysed using an old method developed by R.H. Bube, the oxygen trap level obtained is in good agreement with that found from the decay curve. The variation of the time constants with the intensity of illumination is explained on the basis of quasi-Fermi levels and the pinning of the quasi-Fermi levels somewhere in the band gap. This pinning arises from the high density of the filled traps at quasi-Fermi levels.

Photo-adsorption and photo-desorption of oxygen on highly hydroxylated TiO2 surfaces. Part 2.—Study of radical intermediates by electron paramagnetic resonance

The effect of pretreatment temperature on the u.v. photo-adsorption of O2 on TiO2 has been studied by e.p.r. Different oxygen species are found if the sample is completely hydroxylated or strongly dehydroxylated, this being related to two different photo-adsorption mechanisms. O2H· and V-centres are found for the hydrated samples and O–2, O–3 O3–3 for the dehydroxylated samples. The origin of all these species is discussed.