Nickel work function

The significance and impact of surface science were now becoming very apparent with studies of single crystals (Ehrlich and Gomer), field emission microscopy (Sachtler and Duell), calorimetric studies (Brennan and Wedler) and work function and photoemission studies (M.W.R.). Distinct adsorption states of nitrogen at tungsten surfaces (Ehrlich), the facile nature of surface reconstruction (Muller) and the defective nature of the chemisorbed oxygen overlayer at nickel surfaces (M.W.R.) were topics discussed. 

Figure 2.1 Real-time photoemission study (hv = 6.2 eV) of the interaction of oxygen (Po2 = 10- Torr) with a nickel surface at 300 K. The photocurrent decreases initially (A B), then recovers (B-C), before finally decreasing (CD). Surface reconstruction occurs (B-C) with further support from studies of the work function. The work function measured by the capacitor method15 increases by 1.5 eV with oxygen exposure at 80 K followed by a rapid decrease on warming to 295 K and an increase on further oxygen exposure at 295 K. These observations suggest that three different oxygen states are involved in the formation of the chemisorbed overlayer. (Reproduced from Refs. 15, 42).

However, when the reductions were carried out with lithium and a catalytic amount of naphthalene as an electron carrier, far different results were obtained(36-39, 43-48). Using this approach a highly reactive form of finely divided nickel resulted. It should be pointed out that with the electron carrier approach the reductions can be conveniently monitored, for when the reductions are complete the solutions turn green from the buildup of lithium naphthalide. It was determined that 2.2 to 2.3 equivalents of lithium were required to reach complete reduction of Ni(+2) salts. It is also significant to point out that ESCA studies on the nickel powders produced from reductions using 2.0 equivalents of potassium showed considerable amounts of Ni(+2) on the metal surface. In contrast, little Ni(+2) was observed on the surface of the nickel powders generated by reductions using 2.3 equivalents of lithium. While it is only speculation, our interpretation of these results is that the absorption of the Ni(+2) ions on the nickel surface in effect raised the work function of the nickel and rendered it ineffective towards oxidative addition reactions. An alternative explanation is that the Ni(+2) ions were simply adsorbed on the active sites of the nickel surface. 

It is particularly helpful that we can take the Cu-Ni system as an example of the use of successive deposition for preparing alloy films where a miscibility gap exists, and one component can diffuse readily, because this alloy system is also historically important in discussing catalysis by metals. The rate of migration of the copper atoms is much higher than that of the nickel atoms (there is a pronounced Kirkendall effect) and, with polycrystalline specimens, surface diffusion of copper over the nickel crystallites requires a lower activation energy than diffusion into the bulk of the crystallites. Hence, the following model was proposed for the location of the phases in Cu-Ni films (S3), prepared by annealing successively deposited layers at 200°C in vacuum, which was consistent with the experimental data on the work function. 

The LEC structure that involves the addition of ionic dopants and surfactants to the printable inks enables the ability to print a top electrode without restriction by the work function of the metal. Silver, nickel, or carbon particle-based pastes are generally the preferred printable electron injecting electrodes however, the shape and size of the particles combined with the softening properties of the solvent can create electrical shorts throughout the device when printed over a thin polymer layer that is only several hundred nanometers thick. For optimal performance, the commercially available pastes must be optimized for printing onto soluble thin films to make a fully screen-printed polymer EL display. 

Worked Example 3.5. A new means of extracting nickel from its ore is being investigated. The first step is to crush the rock to powder, roast it, and then extract soluble nickel species (as Ni " ") into an aqueous solution. The activity, a(Ni +), is monitored by a potentiometric method, where a wire of pure nickel metal functions as an electrode and is immersed in aliquot samples taken from the plant. This wire monitors the electrode potential NP+,Ni.If 2+,Ni =-0-230 V, what is if afNi " ) = 10... 

N2O decomposition on nickel increases suddenly at the Curie point, owing to a smaller electronic interaction in accordance with the foregoing explanation. On the other hand, the electronic work function of nickel, according to Cardwell (71), above the Curie point is 0.2 volt higher than below it. The transfer of metal electrons to the N2O molecules, therefore, occurs less readily above the Curie point. Hence the bond between an 0 atom and N2 in the adsorbed N2O molecule is not weakened so much at temperatures above the Curie point. Thermal decomposition of N2O, therefore, requires a higher energy of activation. ... 

The lone electrons of the 0 atom in the H2O molecule can also become part of the electron gas in the metal surface and reduce its work function. So Schaaff (75) observed an increase of the photoelectric emission of platinum in the presence of water vapor. On the other hand an adsorbed layer of H2O molecules on the surface of a thin nickel film decreases the electric resistance of the film (18). 

Indeed Mignolet (14) has not observed any change of the contact potential upon contacting ethane with nickel at room temperature. If the adsorption takes place at 90°K., however, the contact potential unexpectedly changes by the relatively large amount of A t/i, 2 = -(-0.77 volt i.e., the work function decreases. In other words the electrons of the C2H6... 

A similar behavior of the resistance of transparent nickel films was observed with the adsorption of triphenylmethane and naphthalene. In these cases the resistance decreased by 0.033 and 0.035% (18). As does the adsorption of benzene, the adsorption of triphenylmethane and of copper phthalocyanine lowers the electronic work function of a platinum surface (76). 

Fig. I. (left). Work functions of films prepared by evaporation of nickel on top of a copper film, followed by sintering and admission of carbon monoxide. O, Fresh , after sintering (200 C) A, after admission of CO (3 x 10 8 Torr). From Sachtler and Dorgelo (4b).

X-ray diffraction work showed the existence of two phases. The work function data suggest that the copper-rich alloy in the two-phase system is located at the surface and the nickel-rich phase below the surface. To check this, CO was admitted at a pressure of 10 8 Torr. The gas is strongly adsorbed on nickel, but not on copper at such low pressures. The work function of copper was not altered. The binary alloys showed a constant increase in work function between 0.04 and 0.11 eV. Therefore, the adsorbing surface belonged to the copper-rich phase. Chemisorption of H2 on Ni-Cu films (40) leads to essentially the same conclusions. At temperatures below the miscibility gap, several classes of alloy systems characterized by their concentration ranges can be distinguished (4c), as illustrated in Fig. 3 ... 

The variations of the work function with adsorption. For the combination hydrogen-nickel... 

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