Quasi-one dimensional metal oxide semiconductors: Preparation, characterization and application as chemical sensors

The effects of UV illumination on the electronic properties and gas sensing performance of ZnO are reported. It is found that UV light improves the sensitivity and the sensor response and recovery rate. By investigating the photoresponse behavior of ZnO, we observe that the electrons generated by UV light promote the adsorption of oxygen and form the photoinduced oxygen ions [O2−(hv)]. These ions [O2−(hv)] are responsible for the room-temperature gas sensing phenomena and promise enhanced sensor performance through further optimization.

UV-activated room-temperature gas sensing mechanism of polycrystalline ZnO

Slow photoconductivity transients were comprehensively studied in ZnO films prepared by spray pyrolysis of the zinc-nitrate solution. Surface charge controlled the film conductivity, and it was possible to reversibly change the conductivity by many orders of magnitude using short-term annealing in hydrogen and oxygen. Under illumination, the conductivity of as-grown films may increase by several orders of magnitude, depending on the dark conductivity. Photoconductivity was due to the capture of nonequilibrium holes at surface oxygen states to produce an equivalent number of excess electrons in the conduction band. Reverse process of the photoconductivity relaxation is determined by an electron tunneling mechanism to the surface oxygen states.

Carrier mobility and density contributions to photoconductivity transients in polycrystalline ZnO films

The effect of post-processing treatments on inflection points in current–voltage curves of roll-to-roll processed polymer photovoltaics

In this chapter models of adsorption processes are reviewed. Both adsorption from the gas phase and from the liquid phase onto a solid are discussed, but adsorption from the gas phase is emphasized. Much of the material in earlier chapters is obviously pertinent background information for the topic of adsorption processes. In Section 5.2.4 the various degrees of interaction of gases at a solid surface, physical adsorption (physisorption), chemisorption, and finally the development of a new phase were described. In the remainder of Chapter 5, the forms of bonding of an adsorbate to the surface were reviewed. This chapter will build on these concepts, moving on to discussions of amount adsorbed, the kinetics of adsorption/desorption, and electron transfer during adsorption (ionosorption).

Adsorption and Desorption

Photoconductive rise and decay curves of porous sintered zinc oxide are studied as a function of time (0.3 to 107 sec), ambient oxygen pressure (5×10—5 to 760 mm), temperature (110° to 373°K), incident light intensity (factor of 103), and various cycling procedures in regard to illumination. The photoconductivity is shown to be entirely due to the desorption of chemically adsorbed oxygen atoms from the surfaces of the porous sample. Light of wavelength near the fundamental absorption edge forms electron‐hole pairs of which the hole is captured by the oxygen, converting the oxygen to physically adsorbed. The physically adsorbed oxygen easily desorbs and the electron remains to increase the conductivity. The kinetics of the adsorption process when the light is removed are found to obey the Elovich equation, dq/dt=a exp(—bq), where q is the density of adsorbate, and a and b are constants. It is believed that the limiting process is electron transfer through the Schottky barrier, since it results in an equati...

Donald A. Melnick

Papers

Zinc Oxide Photoconduction, an Oxygen Adsorption Process