Surface Doping

Surface doping of oxide colloids and nanostructured electrodes with transition metal ions and complexes is of great interest for improving efficiency and selectivity of photocatalysts and photoelectrodes. Such surface ions as electron donors or acceptors play an important role as catalytic active centers, in charge transfer and in adsorption. There were many publications on this subject and we will try to bring forward the most 

At the adsorption onto titanium dioxide surface, vanadium ions form, at the beginning, randomly distributed isolated V4+ centers with typical D4h symmetry (gy gx, Ay A for the unpaired electron). At higher vanadium concentrations, monolayers and 

The spectroscopic study of the nature of vanadium oxide [VOx] supported on a high surface area Ti02 (anatase) indicated the formation of three different VOx structures [48] a) isolated V4+ ions, part of which was coordinatively unsaturated, strongly bonded to the surface hydroxy groups of the support b) bidimensional clusters of VOx with mainly V5+ after calcinations, reducible under mild conditions to V4+ and also to V3+ to some extent (these species weakly interact with the support surface) c) V205 appeared when cove-rage was about the monolayer and was presented as bulk multiplayer structures. The authors observed the existence of at least two different isolated surface V4+ species, which caused splitting of the low-field hfs lines in parallel orientation. 

As an example, Fig. 8.14 presents typical EPR spectra at 77 K of the Degussa P25 particles after different time of incubation in 0.65 M ascorbic acid solution [215], EPR analysis indicates the presence of at least two distinct V4+ species the sharp signals, overlapping with a broad single line of the aggregated V4+ centers. Similar spectra were observed in [48, 160, 163], etc. 

It follows from Fig. 8.14 that a relative part of such aggregated centers decreases in time, transforming into isolated centers. Analogous changes have also been observed for the Hombicat-100 samples [217]. In contrast, Zr02 particles doped with vanadium did not change their EPR spectra with time. 

---

In eadi specific case the choice of an adsorbent, electrophysical parameters and the method of registration of its change as well as the choice of various pre-adsorption treatment techniques of the surface of adsorbent is dictated by the type and nature of analytical problem to be solved. For instance, if particles active from the standpoint of the change in electrophysical parameters of semiconductor adsorbent occur on the surface of the latter due to development of a chemical reaction involving active particles, it is natural to use either semiconductor material catalyzing the reaction in question or if this is not possible specific surface dopes accelerating the reaction. Above substances are used as operational element of the sensor. If such particles occur as a result of adsorption from adjacent volume, one can use semiconductor materials with maximum adsorption sensitivity to the chosen electrophysical parameter with respect to a specific gas as operational element. 

Sometimes there is a significant difference in bulk and surface doping, as reported in the case of B and V co-doping, where improvement in the degradation of methylene blue was observed only when V was present at the surface of the photocatalyst [69]. 

For the electrochemical capacitors of Carbon-Ni Oxide system with aqueous KOH solution it is expedient to use carbonaceous graphite materials with expanded structure and modified surface. The best results were achieved with carbon surface doped with Boron, which makes this carbon superior than other conductive additives used in this study, due to its... 

The models of the ZnS (110) surface doped with Cu " and Fe ions are presented in Fig. 9.14. These models are relaxed using GGA with CASTER The rule of... 

The PDOS of the relaxed surface doped with Cu and Fe is presented in Fig. 9.15 and Fig. 9.16 respectively. It can be seen that the valence band of the surface doped with Cu is predominantly composed of Cu (3d) orbital which extends from 0 to -5 eV. This is in excellent agreement with Andrea s XPS studies (1999). There is no obvious gap between the occupied and unoccupied levels. The S (3p) and Zn (4p) orbits are located at the bottom of the conduction band. 

Similar PDOS distribution can be seen on the ZnS surface doped with Fe ions. The dominant state in valence band is Fe (3d) orbital, and the conduction band is composed of S (3p) and Zn (4p) orbital. This result indicates that doping Cu or Fe ions on the ZnS surface reduces the band gap of the ZnS. This kind of reduction will produce lot of surface state in bulk ZnS forbidden band. 

Nakato Y, Tsumura A, Tsubomura H (1982) Efficient photoelectrochemical conversion of solar energy with n-type silicon semiconductor electrodes surface doped with IIIA elements. Chem Lett 1071-1074... 

Results of a first principles calculation shows that the antiferromagnetic state is more stable than the ferromagnetic state, and that the energy gap decreases with the Mn composition (Zhao, Y.-J. et al. 2001b). The reason for the discrepancies between theoretical expectations and experimental results is not clear it may stem from the substitution of Ge for Mn in surface-doped samples. More recent plane-wave pseudopotential and KKR-CPA calculations show that the intrinsic defects are responsible for the stabilization of the ferromagnetic state (Mahadevan and Zunger 2002 Kamatani and Akai 2001b). 

Experiments have been done where the polymer film is grown on an electrode surface, doped, and then removed from the electrochemical cell and placed... 

In principle, the utility of TiOi can be improved enormously by optical sensitization towards visible light. This can be achieved by surface doping or dye sensitization and several reports have described dye sensitization of TiOi powders.In each case, the mechanism of sensitization appears to involve charge injection from the excited state of the dye into the conduction band of the semiconductor. Such methods permit Hi evolution to be observed from TiOi coated with 8-hydroxyquinoline using visible light. 

While the effect of cation impurities on the surface chemistry of MgO has been investigated in detail, very little is known about anion substitution. Defect formation and excitation energies for S and Se -doped bulk MgO have been calculated [182,183] but there are no data for the surface. In the bulk it has been estimated that the presence of S or Se impurities result in a outward relaxation of the Mg neighbors of 6% and 8%, respectively [182]. A recent report of the 0 "-S exchange reaction on MgO has been reported [184]. The reaction involves adsorption of CS2 on MgO powders and the subsequent exchange reaction with formation of COS and of S ions probably located at the low coordinated sites. It has been found that the basicity of the MgO surface doped with sulfur ions is drastically modified with respect to that of pure MgO [184]. 

In addition, one needs not to deal with impure surfaces so much because the reaction takes place in solution and surface layers (oxides for example) may dissolve under appropriate potential conditions. Several procedures for achieving activation are also known for electrocatalysts. Ion implantation may be a convenient method of activating and modifying an electrocatalyst. Activation takes place by the introduction of defects and modification by the production of surface-doped layers of varying composition. 

The quantity of alkali metal retained on the MgO surface and the concentration of the newly created ionic superbasic centres depends on the position of metal in the Periodic Table. The greater the electropositivity in the sequence sodium, potassium, caesium, the greater is the reactivity with surface acceptor centres of MgO surfaces. It is possible that metals having lower ionization energy, such as potassium or caesium (Table 1), react with these surface centres of MgO, which are not affected by sodium atoms. In consequence an oxide surface that has been heated to a particular temperature is able to bind more caesium than sodium atoms. The increase of the quantity of metal retained on MgO surfaces is not followed by a simultaneous increase in the number of newly created ionic superbasic centres. The largest quantity of such centres is formed on MgO surfaces doped with potassium. It is interesting to note that in the case of MgO-K and MgO-Cs systems two types of superbasic centres occur, one with a basic strength of 33 < H < 35, the second one with H > 35 (Table 1). ... 

The concentrations of donor centres that cause the creation of an anion radical from adsorbed nitrobenzene molecules are given in Table 3. They have been established on an MgO surface doped with different alkali metals and on silica and alumina doped with metallic solution. 

Keeping abrupt profiles of dopants is also required to apply the FLA technique to solar cell fabrication process. Figure 11.8 shows secondary ion mass spectroscopy (SIMS) profiles of Cr and P atoms in the bottom layers and of B in the surface doping layer of p-i-n Si stacked films before and after FLA, the structure of which is also schematically shown [34]. The abrupt profiles of the surface B atoms as well as of the bottom Cr and P atoms are maintained after FLA, which results from millisecond-order rapid annealing, and shows the possibility of immediate formation of p-i—n poly-Si structure with only one irradiation of flash lamp for p-i-n stacked a-Si layers. 

Sanders, D. Simon, U., High-throughput gas sensing screening of surface-doped ln203, J. Comb. Chem. 2007, 9, 53-61... 

CoTi03 is a p-type semiconductor and was introduced by Chu et al. for the detection of ethanol.36 Lanthanum volume doped CoTi03 showed higher conductivity and better ethanol sensitivity of the material as shown in Siemons.29 The use of HT-IS enabled the analysis of surface doping with different amounts of Au, Ce, Pd, Pt, Rh, and Ru in order to find out the highest selectivity and sensitivity of the respective material composition. Measurements of CoTi03/La samples at different temperatures and under different atmospheres will be shown in order to illustrate the high reproducibility of the impedance measurements and sensitivities of identical samples on different positions. 

The second approach is based on addition of a precious metal or oxide catalyst (Fig. 12.1b). The design of a catalyst usually requires a large number of experiments because various catalyst materials are available and their concentrations should be optimized. In this respect, the combinatorial method is advantageous. Indeed, there are reports on the design of catalysts or surface doping elements that promote the selective gas sensing reaction.48"50... 

The surface doping of the 30/70 Cu/ZnO catalyst was performed after the catalyst was reduced in 2% H2/N2 at 250 C by an addition and total evaporation to dryness under flowing nitrogen of an alkali or an alkaline earth hydroxide solution. In this manner, specimens of the 30/70 Cu/ZnO catalyst were doped with 0.4 atomic Z of LiOH, NaOH,... 

---