Electron Work Function–An Effective Parameter for In-situ Reflection of Electron Activities in Various Processes

Electron work function (EWF) is the minimum energy required to move electrons at the Fermi level from inside a conducting material to its surface with zero kinetic energy [1,2]. This fundamental parameter largely reflects electron activities and is directly related to chemical, physical, and mechanical properties of materials [2-8]. Recent studies have shown that EWF can be used as a sensitive parameter to characterize many surface properties of biomaterials. As an example [9], the affinity of medical implant materials for bacteria can be characterized by EWF. Figure 1A illustrates the adhesive forces of nanocrystalline stainless steel samples with different gran sizes. As shown, as the grain size decreases, the adhesion decreases due to the fact that the passive film becomes more protective, blocking the interaction between the steel and surrounding media. Such a trend is consistent with the variations in electron work function, as shown in Figure 1B. A higher surface EWF corresponds to a higher degree of reluctance for interacting with the environment. The decrease in the number of bacteria bound to the surfaces with an increase in EWF provides direct evidence.

Electron work function (EWF) is the minimum energy required to move electrons at the Fermi level from inside a conducting material to its surface with zero kinetic energy [1,2]. This fundamental parameter largely reflects electron activities and is directly related to chemical, physical, and mechanical properties of materials [2][3][4][5][6][7][8]. Recent studies have shown that EWF can be used as a sensitive parameter to characterize many surface properties of biomaterials. As an example [9], the affinity of medical implant materials for bacteria can be characterized by EWF. Figure 1A illustrates the adhesive forces of nanocrystalline stainless steel samples with different gran sizes. As shown, as the grain size decreases, the adhesion decreases due to the fact that the passive film becomes more protective, blocking the interaction between the steel and surrounding media. Such a trend is consistent with the variations in electron work function, as shown in Figure 1B. A higher surface EWF corresponds to a higher degree of reluctance for interacting with the environment. The decrease in the number of bacteria bound to the surfaces with an increase in EWF provides direct evidence.
Another example is the use of EWF in analyzing the photocatalyical activity of TiO2 nanotubes (TNTs). TNTs have been used for different applications such as DNA biosensor [10], detection of bacteria [11] and toxic compounds [12]. TNTs have also been explored for immobilizing proteins and enzymes in biomaterial and biosensor applications [13,14]. Many of the application are related to the photocatalytic activity of TNTs, which is usually evaluated by variations in photocurrent and absorbance under illumination. However, the photocurrent response or absorbance does not provide sufficient information for full understanding of the photocatalytic behaviour of TNAs.
Recent studies have demonstrated that the electron work function is a very sensitive parameter for in-situ analysis of photocatalytic activity of TNTs [15]. Figure 2 illustrates variations in the work function of TiO 2 nanotubes with different tube lengths (corresponding to the anodization duration; the larger the anodization duration, the longer the nanotubes). Figures 2A, C, E and G illustrate variations in EWF of the TNAs illuminated successively by lights having wavelengths of 670 nm, 650 nm, 550 nm, 450 nm, 420 nm, 400 nm and 390 nm. As shown, EWF of the TNAs was relatively stable when they were illuminated by lights of 670 nm and 620 nm, indicating that the photonic energies of the lights are not sufficient to excite electrons in the TNAs. While when the wavelength of incident light is in the absorption range of TNAs lower than 550 nm, electrons can be excited accompanied with an apparent decrease in EWF. The observed decrease in EWF with a decrease in the wavelength of incident light is an indication of photoninduced electron-hole separation, which requires photons with their energy above a certain level.
The TNAs were also illuminated successively by the lights with different wavelengths in a reversed order. Corresponding variations in EWF are illustrated in Figures 2B, D, F  pairs could be re-separated under the illumination with the lower-energy photons, but the electrons could only be excited to relatively lower energy states that correspond to higher EWF. This range of longer wavelengths, which could not suppress e h − + − recombination, extended to 420-670 nm for TNAs-12 h and TNAs-16 h. It appears that longer TNAs do not only provide more recombination centers as suggested [16,17], but also have larger driving forces for e h − + − recombination.
The above observations demonstrate that EWF is related to the photocatalytic process involving electron-hole separation and recombination. The study clearly shows that EWF is an effective parameter reflecting the photocatalytic behavior of TiO 2 nanotubes, and helps to get an insight into the semiconductors' photo-activities with different views that may not be achieved using traditional techniques, such as diffuse reflection spectrum and photo-electrochemical measurement.
In conclusion, EWF has been demonstrated to be a sensitive parameter with great potential for a variety of sensor applications. Anodization potential=20 V; Annealing temperature=45°C [15].