In this paper, ZnO nanoparticles were modified by depositing different amount of noble metal Ag or Pd on their surfaces with a photoreduction method. The as-prepared ZnO samples were principally characterized by X-ray photoelectron spectroscopy (XPS) and surface photovoltage spectroscopy (SPS), and their activity was evaluated by a photocatalytic degradation reaction of phenol solution. The effects of noble metal modification on surface composition, photoinduced charge transfer behavior and photocatalytic activity of ZnO samples were mainly investigated. The results show that, after an appropriate amount of Ag or Pd is deposited on the ZnO surfaces, the O 1s XPS spectrum shifts to higher binding energy, indicating that the content of surface hydroxyl oxygen (OH) increases. And the SPS intensity greatly decreases, indicating that the photoinduced electrons are easily captured by adsorbed oxygen via noble metal clusters so that the recombination of photoinduced electron and hole pairs is effectively inhibited. These are responsible for the enhancement in the photocatalytic activity. Moreover, the effects of Pd modification on photocatalytic activity is greater than that of Ag, which can be explained by means of surface hydroxyl content as well as charge separation situation.

Nanoparticle TiO2/Ti films were prepared by a sol-gel process using Ti(OBu)4 as raw material, the as-prepared film samples were also characterized by TG-DTA, XRD, TEM, SEM, XPS, DRS, PL, SPS and EFISPS testing techniques. TiO2 nanoparticles experienced two processes of phase transition, i.e. amorphous to anatase and anatase to rutile at the calcining temperature range from 450 to 700℃. TiO2 nanoparticles calcined at 600℃ had similar composition, structure, morphology and particle size with the internationally commercial P-25 TiO2 particles. Thus, the conclusion that 600℃ might be the most appropriate calcining temperature during the preparation process of nanoparticle TiO2/Ti film photocatalysts could be made by considering the main factors such as the properties of TiO2 nanoparticles, the adhesion of nanoparticle TiO2 film to Ti substrate, the effects of calcining temperature on Ti substrate and the surface characteristics and morphology of nanoparticle TiO2/Ti film for the practice view. The Ti element mainly existed on the nanoparticle TiO2/Ti(3) film calcined at 600℃ as the chemical state of Ti4þ, while O element mainly existed as three kinds of chemical states, i.e. crystal lattice oxygen, hydroxyl oxygen and adsorbed oxygen with increasing band energy. Its photoluminescence (PL) spectra with a peak at about 380 nm could be observed using 260nm excitation, possibly resulting from the electron transition from the bottom of conduction band to the top of valence band. The PL peak position was nearly the same as the onset of its diffuse reflection spectra (DRS) and surface photovoltage spectroscopy (SPS), demonstrating that the effects of the quantum size on optical property were greater than that of the Coulomb and surface polarization. The PL spectra with two peaks related to the anatase and rutile, respectively, could be observed using the excited wavelength of 310 nm. Weak PL spectra could be observed using the excited wavelength of 450 nm, resulting from surface states. In addition, during the experimental process of the photocatalytic degradation phenol, the photocatalytic activity of nanoparticle TiO2/Ti film with three layers calcined at 600℃ was the highest.

Heterogeneous photocatalysis is a promising technique valuable for environmental purification. Nano-sized semiconductors such as ZnO and TiO2, which is one of the most basic functional materials, have emerged as effective photocatalyst materials. The surface photovoltage spectra (SPS) can be an effective method for quickly evaluating the photocatalytic activity of semiconductor materials since it can provide a rapid, non-destructive monitor of the semiconductor surface properties such as surface band bending, surface and bulk carrier recombination and surface states, mainly showing the carrier separation and transfer behavior with the aid of light, especially the electric-field-induced surface photovoltage spectra (EFISPS), in which SPS is combined with the electric-field-modified technique. In this review, the basic principles, measurement and applications of the SPS and EFISPS are mainly discussed together with some fundamental aspects like the electric properties of semiconductor surface and the principle of electric field effect. In particular, the applications of SPS to nano-sized semiconductors such as ZnO and TiO2 in heterogeneous photocatalysis are emphasized, which involve mainly evaluating the photocatalytic activity by analyzing semiconductor surface properties such as the separation efficiency of photoinduced carriers under illumination by the SPS measurement, highlighting our own contributions. The results show that the weaker the surface photovoltage signal is, the higher the photocatalytic activity is in the case of nano-sized semiconductor photocatalysts.

In this paper, ZnO nanoparticle photocatalysts were modified by depositing Pd on their surfaces with a photoreduction method. We mainly investigated the modification mechanisms as well as the effects on the photocatalytic activity of ZnO nanoparticles of deposited Pd by means of XPS and SPS (Surface Photovoltage Spectroscopy), and the effects of Pd content on SPS responses were also discussed from the point of the electronic energy level. The results showed that the content of crystal lattice oxygen on the surface of ZnO nanoparticle decreased after an appropriate amount of Pd was deposited, while that of adsorbed oxygen increased, indicating that Pd was mainly deposited on the crystal lattice oxygen. At the same time, the intensity of SPS responses of ZnO nanoparticles remarkably decreased. In addition, the activity of ZnO nanoparticles could be greatly improved by depositing an appropriate amount of Pd in the gas phase photocatalytic oxidation of n-C7H16. Thus, it could be concluded that the increase in surface content of adsorbed oxygen could facilitate the photocatalytic reaction, and there were close relationships between the SPS response and photocatalytic activity, i.e. the weaker the SPS response, the higher the photocatalytic activity, of Pd-deposited ZnO nanoparticles.

TiO2 nanoparticles capped with sodium dodecylbenzenesulfonate (DBS) are synthesized by a sol-hydrothermal process using tetrabutyl titanate and DBS as raw materials. The effects of surface-capping DBS on the surface photovoltage spectroscopy (SPS), photoluminescence (PL) and photocatalytic performance of TiO2 nanoparticles are principally investigated together with their relationships. The results show that the surface of TiO2 nanoparticles can be well capped by DBS groups while the pH value and added DBS amount are controlled at 5.0 and 2% of TiO2 mass weight, respectively, and the linkage between DBS groups and TiO2 surfaces is mainly by means of quasi-sulphonate bond. The intensities of SPS and PL spectra of TiO2 obviously decrease after DBS-capping, while the activity can greatly increase during the photocatalytic degradation of Rhodamine B (RhB) solution, which are mainly attributed to the electron-withdrawing character of the DBS groups. Moreover, the enhancement of photocatalytic activity of DBS-capped TiO2 is also related to the increase in the capability for adsorbing RhB.