(Invited) Surface Photovoltage Analysis of Photocatalytic Materials
Authors: Dittrich Thomas 
Published: May 2020

A brief review of surface photovoltage (SPV) techniques and their application to photocatalytic materials is given. SPV is defined by the light-induced change of the contact potential difference (∆CPD) between two electrodes, i.e. SPV signals can be obtained whenever photogenerated charge carriers are separated in space [1]. SPV signals depend on processes of light absorption, photogeneration, directed charge transfer, charge transport and recombination. Therefore, SPV techniques can be well applied to the analysis of electronic, transport and recombination properties of photoactive materials [2].
Basics of SPV are introduced including the relaxation of a disturbed space charge, the center of charge approach and mechanisms of charge separation. Principles of SPV measurements will be given and illustrated with applications. Depending whether fixed capacitors, Kelvin probes, electron beams or photoelectrons are applied (see also figure 1), SPV signals can be measured at crystals, layers or powders of photocatalysts with high sensitivity over wide ranges in time or high resolution in space and in different ambient (gases, vacuum, electrolytes). In-operando SPV measurements are realized.
For measurements with fixed capacitors, SPV signals are coupled out directly with a high-impedance buffer. Due to the discharge of the measurement capacitor via a very large measurement resistance, the application of fixed capacitors is limited to transient [3] and modulated [4] SPV measurements. With fixed capacitors, the widest range in time ((sub)ns...ms...h, transient, example: nano-composite of TiO2/In2S3 [5]) and the highest sensitivity (10...100 nV, modulated, see [6] for defect analysis in (FA0.85MA0.1Cs0.05)PbBr3) are reached whereas no additional interactions are involved. Attention will be paid to the application of transient SPV spectroscopy [7] at constant photon flux for the investigation of defect states and charge transfer across interfaces [8] (c-Si(n++)/TiO2). Furthermore, principles of random walk simulations for the modeling of transient [9] and modulated [10] SPV signals in small systems will be discussed [2].
With Kelvin probes, the ac current between a sample and a vibrating electrode (vibrating electrode [11], most applied technique, see, for example [12]) or the electrostatic force between the sample surface and a vibrating tip electrode (Kelvin probe force microscopy [13] [14], KPFM) are nulled by applying an external bias potential. The highest resolution in space is reached for SPV measurements with KPFM (tens of nm). Examples will be shown for SPV measurements with vibrating electrodes for TiO2 [15] and KPFM for Cu2O [16] and BiVO4 with co-catalysts [17].
In SPV measurements with electron beams [18, 19], a low electron current is kept constant with a compensating bias potential. In photoelectron spectroscopy, core levels shift with regard to ∆CPD. Additional interactions of the sample with electrons and/or x-rays are possible. The application of synchrotron radiation allows for SPV measurements at extremely short times [20, 21]. Further opportunities opened for SPV measurements by ambient pressure photoelectron emission spectroscopy [22].
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Figure 1