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External View |
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Internal View |
The High Temperature Kelvin Probe (HTKP-2000) may be applied for the determination of work function (WF) and its changes as a function of time.
The HTKP-2000 is a unique surface sensitive equipment that may be used for investigation of materials at elevated temperatures and under controlled gas phase environment of atmospheric pressure (it does not require vacuum).
The HTKP-2000 may also be applied for in situ surface monitoring of materials during their processing. The probe is sensitive in an atomic scale with respect of the outermost surface layer.
Contents
The probe may be applied in several areas for the following determinations:
AREA |
PROPERTIES |
Solid-State Chemistry |
Defect chemistry, non-stoichiometry, chemical diffusion, gas/solid reactions |
Solid-State Physics |
Charge transport, semiconducting properties, bi-dimensional structural transitions |
Materials Science |
Equilibration kinetics, phase transitions in the boundary layer, electronic structure |
Metallurgy |
Mechanism and kinetics of oxidation and reduction processes |
Catalysis |
Mechanism and kinetics of chemisorption and catalytic processes |
Sensorics |
Determination of the sensing signal at the gas/solid interface |
Temperature |
RT - 1200 K |
Gas Environment |
Static/Dynamic |
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SIZE |
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Height |
1000 mm |
Base |
400x400 mm |
Weight |
21 kG |
Specimen's Size |
Approx 10x10x2mm |
Vibration Frequency |
30 Hz - 100 KHz |
Accuracy |
± 0.5 mV |
The HTKP-2000 and the ancillary equipment are shown, schematically, in Figure 1. The probe incorporates the following integral components: (1) vibrating system, (2) vibrating capacitor, and (3) tuning system (Figures 2 and Figures 3). The vibrating capacitor is formed of a specimen located on support and upper reference electrode. The electrode is put into vibrations using piezoceramic element. The distance between the capacitor plates (about 0.1 - 0.2 mm) is tuned using a micrometer attached to the sample support.
A tube-type furnace is located in the middle part (at the sample level). Upper and lower water coolers aim at preventing the upper and lower parts against heating. Gas inlet and outlet are connected to a gas flow system. The probe is gas tightened by two fringes. The specimen is located on a support. A rack allows to remove the lower part of the probe and to replace the specimen. Performance of the probe and its applications were reported in refs. [1-3].
Additional equipment items required for HTKP-2000 include: high voltage amplifier, voltmeter, frequency generator, integrator, oscilloscope, chart recorder, personal computer, temperature controllers, lock-in amplifier, scanner, gas-flow system, and oxygen sensor.
Figure 1. The HTKP-2000 and ancillary equipment
Figure 2. The HTKP-2000. Internal view (schematic)
WHY SURFACE PROPERTIES AT ELEVATED TEMPERATURES ARE IMPORTANT.
Properties of materials interfaces, such as surfaces and grain boundaries, are entirely different from those of the bulk phase [4,5]. It appears that most of materials properties are strongly influenced or even determined by interfaces. Concordantly, interfaces hold the key to tailoring materials properties for specific industrial applications.
There is an increasing need to understand interface properties and phenomena in situ during processing of materials at elevated temperatures and controlled gas phase composition. Application of the most of surface sensitive tools based on electron and ion spectroscopy, such as XPS, SIMS, ISS and LEED, is limited to room temperature (when materials are quenched) and high vacuum (when surface composition of compounds are different than those under atmospheric pressure).
The HTKP-2000 is a very powerful tool in hands of solid state chemists and materials scientists that allows to have an insight into the unknown world of surface properties at elevated temperatures under atmospheric pressure.
PRINCIPLE
Performance principle of HTKP is based on the determination of contact potential difference (CPD) between studied surface and a reference surface of known WF:
Eq(1)
where 
is WF of the studied specimen and reference surface, respectively.
While absolute WF of compounds has a complex meaning, the WF changes can be used for monitoring of several properties, such as chemisorption, description, catalytic processes, gas-solid equilibration, diffusion in the surface layer, structural transitions within the surface layer, redox processes and related charge transfer. According to Eq (1) we have:
Eq(2)
where 
denotes changes. Therefore, knowledge of the 
component is required for the determination of 
. The electronic circuitry for the determination of CPD using the HTKP-2000 is shown in Figure 4.
REFERENCE ELECTRODE
The HTKP-2000 is equipped with a Pt reference electrode. Figure 5 shows the WF changes of Pt, 
Pt, at elevated temperatures vs oxygen partial pressure, p(O2), [6]. These data may be used for the determination of using Eq(2). The determination of WF of the reference electrode in reducive atmospheres requires its calibration.
BASIC TERMS
WF is the work required to remove an electron from the EF level to the level just outside the surface where the electron is beyond electrostatic interactions [3]. Concordantly, the WF changes are equal to the EF changes
Eq(3)
Figure 6 shows WF of n-type semiconducting materials (in terms of a flat band model) where 
are WF values of non-stoichiometric oxides exhibiting low and high oxygen content.
Figure 6. Effect of p(O2) on Fermi energy at gas/solid equilibrium
The WF changes of oxide materials involve mainly the following two components [2,3]:
Eq(4)
where 
denote the WF changes related to (1) lattice nonstoichiometry and related concentration of defects, and (2) potential drop across space charge caused by chemisorption.
At high temperatures, when gas/solid equilibrium can be established rapidly, the WF changes are determined by the 
component. At moderate temperatures, when changes gas phase composition, such as p(O2), result in changes in the concentration of chemisorbed species (while the bulk remains quenched) the WF changes are determined by the 
component (Figure 7).
SENSITIVITY
The HTKP-2000 is extremely sensitive to the outermost surface layer. Figure 8 shows the changes of both WF, 
, and electrical conductivity, 
, of TiO2 during adsorption (A) and desorption (D) of oxygen at 300 K [7]. As seen, A results in WF increase by 0.7 eV and D leads to WF decrease by 0.4 eV (
denote reversible and irreversible WF change, respectively). As also seen, D results in a WF change by 
while 
remains constant.
These data indicate that WF is substantially more sensitive to the changes in the adsorbed layer while is not.
Figure 8. Effect of oxygen adsorption (A) and desorption (D) on WF of TiO2 at roomtemperature
Defect Chemistry and Nonstoichiometry of Zirconia
Figure 9 shows the WF changes of yttria-doped (10%) ZrO2 at 1073 and 1173 K during oxidation [6]. As seen, oxidation at 1173 K results in a rapid increase of WF by 0.186 eV which is reached within 1 min and then remain constant. This is in agreement with the theoretical determined from the Nernst relation:
E
q(5)
where R is gas constant, T - absolute temperature, F - Faraday constant and p(O2)' and p(2)'' denote the initial and final p(O2).
Figure 9. Isothermal WF changes of Y-doped ZrO2 at 1073 - 1173 K during oxidation
As seen, oxidation at 1073 K results in substantially larger WF change (0.3 eV) while the theoretical is 0.17 eV.The difference between the two (0.13 eV) is the WF component corresponding to O2 chemisorption. It is interesting to note that after the initial rapid WF increase within 1 min the subsequent WF change exhibits a slow drift.
As seen, the dynamics of the WF changes at both 1073 and 1173 K, corresponding to the charge transfer at the oxygen/zirconia interface, may be monitored within seconds. These WF data are essential for evaluation of solid electrolytes and electrodes in respect of the impact of interface processes on their performance in electrochemical devices [6].
The mechanism of incorporation of foreign ions into the surface layer of ionic solids differs essentially from that in the bulk phase. These two mechanism may be determined by simultaneous measurements of surface sensitive properties, such as WF, and a bulk sensitive property, such as Seebeck effect.
Figure 10 shows the effect of Cr on EF of NiO determined according to defect chemistry (curve 1), Seebeck effect (curve 2) and WF (curve 3) [8]. As seen, there is a good agreement between the theory (curve 1) and Seebeck effect data (relevant to bulk properties) indicating that incorporation of Cr results in the formation of donors in the bulk phase of NiO.
Figure 10. Effect of Cr on Fermi energy of NiO according to theory (1), in the bulk (2) and at the surface (3)
At the same time the WF data (relevant to the surface) indicate that Cr leads to the formation of acceptors at the surface [4,8]. This phenomenon is essential for correct understanding the effect of doping the solids with aliovalent ions on properties.
Figure 11. Phase diagram of the Fe-O2 system
Figure 12. CPD vs log p(O2) for the FeO phase at 1073 K
HTKP-2000 is the unique tool that allows the determination of structural and phase transitions within the surface layer. Figure 11 shows the CPD vs log p(O2) for the Fe-O2 system at 1073 K across the the stability ranges of 
, FeO and Fe3O4 (Figure 12) [9].
Figure 13. Phase diagram of the W-O2 system
Figure 14. CPD vs log p(O2) for tungsten oxides at 1023 K
The rapid CPD changes during oxidation/reduction allow to determine the p(O2) related to phase transitions while the slope CPD vs log p(O2) may be used for evaluation of defect chemistry. Figure 13 shows the spectrum of the CPD changes vs log p(O2) for the W-O2 system at 1023 K (Figure 14) that allows to identify the conditions of transformations of the Magneli structures [10].
These phase and structural transitions may be detected at temperatures substantially lower than those required to bring the system into equilibrium. Consequently, the HTKP-2000 is the only tool that may be used for the determination of phase diagrams at lower temperatures (below equilibrium).
Figure 15. CPD vs time for the Co-O2 system during oxidation and reduction
Figure 15 shows both the CPD changes and log p(O2) vs time for the Co-O2 system within the CoO/Co3O4 boundary [11]. These results indicate that:
Figure 16. Bulk and surface defect diagram for CoO
Figure 16 shows a diagram illustrating the effect of p(O2) on defect chemistry of the surface layer of CoO. These data indicate that (i) defect chemistry of the surface layer of ionic solids is entirely different than that of the bulk phase, (ii) HTKP-2000 is the only tool able to determine its defect structure and the low dimensional structural transitions that are limited to the outermost surface layer. Similar effect of p(O2) on WF was reported for Fe2O3 (Figure 17) [12].
Figure 17. CPD vs time during oxidation and reduction of Fe2O3
The HTKP-2000 may be used for monitoring the transport within the surface layer and related charge transfer. Figure 18 shows the WF changes vs time during sorption of small oxygen doses (equivalent to the monolayer coverage) for NiO [13].
Figure 18. WF vs time for NiO during sorption of small oxygen doses
The initial WF increase and its subsequent decrease are related to oxygen chemisorption and its incorporation into the NiO lattice, respectively. Therefore, the initial charging may be used for monitoring the rate of oxygen chemisorption with respect to surface coverage and the valency of the chemisorbed species.
Consequently, the subsequent discharging the surface, that is rate controlled by chemical diffusion, may be used for the determination of the diffusion coefficient within the surface layer [14]. Thus determined diffusion data may also serve for evaluation of transport properties of the surface layer.
Knowledge of the mechanism and kinetics of metals oxidation in its very initial stage is crucial for undertaking the right measures aiming at corrosion inhibition. Unfortunately, the classical methods applied in studies of metals oxidation, such as thermogravimetry, cannot be applied for monitoring of the oxidation process within its initial stage because of their sensitivity limit. The HTKP-2000 is the only tool able to monitor the mechanism and kinetics of the oxidation process.
Figure 19. WF changes during isothermal oxidation of Ti
Figure 19 shows the WF changes vs time during the very initial stage of Ti oxidation in the range 373-773 K [15]. The initial WF decrease corresponds to oxygen penetration into the surface metal layer while the subsequent increase is caused by adsorption of oxygen and oxide layer formation.
Fig. 19 illustrates the effect of temperature on the mechanism and kinetics of the oxidation process. Similar studies were reported by Delchar and Tompkins [16] for nickel oxidation.
The formation of SiO2 layer on Si is important for performance of silicon-based solar cells and, specifically, the effect of passivation of Si for photo-voltaic cells. Pre-oxidation conditions, such as surface preparation, as well as oxidation conditions (temperature, gas phase environment and time) are critical for the properties of thus formed SiO2 layer and its impact on passivation.
WF is the only surface sensitive property that can be applied for in situ monitoring of Si oxidation at elevated temperatures and under controlled gas phase environment of well defined oxygen activity.
Figure 20. Effect of SiO2 thickness (formed on Si) on WF
Figure 20 shows the results of the studies of Lassabatere et al [17] who applied WF measurements for monitoring the formation of SiO2 layers formed on Si in the temperature range 1023 - 1273 K under p(O2) ranging between 0.1 and 10 Pa.
As seen, oxidation in its initial stage results in a WF decrease as a result of oxygen penetration into the surface layer of Si. Subsequent oxidation results in WF increase assuming its maximum when the thickness of the SiO2 surface layer is about 2.1 nm. Further oxidation results in a decrease of WF which above 3 nm assumes a constant WF value.
Understanding the electronic mechanism of catalytic processes is of key importance for processing catalysts with enhanced activity and selectivity and selecting optimal conditions of the catalytic process. The HTKP-2000 is the only surface sensitive tool that can be applied for in situ monitoring of catalytic processes and, therefore, able to determine the temperature of the formation of active complexes and related charge transfer.
Figure 21. CPD changes vs temperature for Ge catalyst in (1) CO, (2) O2 and (3) CO+O2 mixture
Figure 21 shows CPD vs temperature characteristics of Ge during heating (1) in vacuo, (2) in oxygen, and (3) in the CO+O2 gas mixture [18]. As seen, the curves 1 and 2 are monotonous while the CPD vs T for the reaction mixture (curve 3) exhibit a sudden drop at the temperature at which the active complex is formed (95°C). This drop indicates that the active centre exhibits acceptor properties.
Figure 22. Activation energy of CO oxidation on NiO vs WF
Figure 22 shows that the activation energy of CO oxidation on NiO and Cr-doped NiO exhibit a linear change vs WF [19]. This dependence indicates that:
The studies using WF measurements, and the experimental material which so far has been accumulated, indicates that the gas/solid system is a complex (sandwich - type) system involving (Fig. 23) [3,4]:
Figure 23. Schematic illustration of the gas/solid interface
From the practical point of view the most abundant gas/solid system is the oxygen/metal oxide system. The oxygen/metal system may only be considered at very low temperatures at which the reactivity between oxygen and the metal is very limited. At elevated temperatures, when metal is covered with a surface layer, the gas/solid interface should be considered as an oxygen/metal oxide interface.
The adsorption layer on metals and metal oxides may be established at lower temperatures at which the equilibrium between the gas phase and the solid phase is not reached. Then a quasi-equilbrium between the gas phase and the adsorption layer may be reached.
At elevated temperature, at which the mobility of ions within the solid becomes effective, a quasi-equilibrium between the gas phase, and adsorbed layer and the outermost surface layer (overlayer) may be reached. Then the properties of the overlayer are determined by the gas phase composition. It was shown that properties of this bi-dimensional surface layer are entirely different than those of the bulk phase of the solid [4,5].
At higher temperatures, at which the ionic transport within the solid becomes more rapid, an equilibrium may be established between the gas phase, the adsorption layer, the overlayer and a sub-layer may be established. Although the sub-layer exhibits similar structural properties than those of the bulk phase its chemical composition is different. Specifically, the sub-layer exhibits segregation-induced concentration gradients [4].
Finally, at very high temperatures, when the gas/solid system reaches equilibrium, all the items forming the gas/solid interface are determined by the equilibrium and, specifically, by the temperature and p(O2) corresponding to equilibrium.
Awareness is growing that properties of materials, and specifically of functional materials, are determined by the properties of the overlayer and the sub-layer rather than by the bulk phase. Consequently, processing of the materials of outstanding properties for specific industrial applications requires that the overlayer and the sub-layer exhibit specific structure and composition. This may be achieved through interface processing [4,5].
It appears that the HTKP-2000 is a very powerful tool in hands of materials scientists in monitoring interface phenomena during their processing and, consequently, in understanding the role of these phenomena in the formation of the materials that are required for specific applications.
Mr. Ross K. Druitt
Managing Director of Sialon Ceramics Pty. Ltd. and Wallarah Minerals Pty. Ltd.
130 Tall Timbers Rd, Doyalson North, NSW 2262, Australia
Tel. +61 2 4358 4994; Fax. +61 2 4358 1348Sialon Ceramics ABN: 57 002 988 543; Wallarah Minerals ABN: 77 002 503 399
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