Numerous advances of societal significance, such as CO2 conversion to fuels, hydrogen production by water splitting, and new materials with self-cleaning and antifogging properties, are based on heterogeneous photocatalysis. *
Strategies to increase charge separation in the photocatalyst include decorating the surface with metal nanoparticles (NPs). *
The Schottky barrier at the metal–semiconductor interface and the associated electric field in the space charge region increases the efficiency of separation of electrons and holes and their diffusion. Under UV illumination, metal NPs in direct contact with the semiconductor can efficiently attract photogenerated electrons from the TiO2 conduction band, while holes are accumulated in the valence band of the oxide. This spatial separation of charge carriers results in an increased lifetime and, therefore, the catalytic efficiency. *
This strategy significantly increases the photocatalytic reduction efficiency of CO2, the selectivity toward methane synthesis, the photoproduction of hydrogen, and the photo-oxidative degradation of NO by increasing the efficiency by 700% relative to unmodified TiO2. In addition, studies reveal that interfacial sites at the TiO2–Au NPs activate adsorbed molecular species. *
However, the charge-transfer process at the TiO2–metal NP junction scale is poorly understood, and yet, it is key to enable the design of more efficient photoactive materials. *
On the experimental side, ultrahigh vacuum studies using scanning tunneling microscopy and atomic force microscopy (AFM) have been used for atomic-scale characterization of TiO2 sensitized with metal clusters and with organic dyes used in solar cells, which contributed to our current knowledge of charge transfer in model metal/TiO2(110) systems, including Pt and small Pt clusters. *
In the article “Photoinduced Charge Transfer and Trapping on Single Gold Metal Nanoparticles on TiO2” Monica Luna, Mariam Barawi, Sacha Gómez-Moñivas, Jaime Colchero, Micaela Rodríguez-Peña, Shanshan Yang, Xiao Zhao, Yi-Hsien Lu, Ravi Chintala, Patricia Reñones, Virginia Altoe, Lidia Martínez, Yves Huttel, Seiji Kawasaki, Alexander Weber-Bargioni, Victor A. de la Peña ÓShea, Peidong Yang, Paul D. Ashby and Miquel Salmeron present a study of the effect of gold nanoparticles (Au NPs) on TiO2 on charge generation and trapping during illumination with photons of energy larger than the substrate band gap. *
They elucidate the effect that the Au NP decorated on TiO2 has on the photogeneration, charge transfer, and trapping. *
In order to study the process at the single Au NP level the authors used a novel characterization technique, photoassisted Kelvin probe force microscopy (PA-KPFM).*
PA-KPFM was implemented by operating in noncontact amplitude modulation mode (AM-AFM) with phase-locked loop (PLL) detection and force gradient feedback. In this technique, two different types of modulations were applied to the cantilever: (1) a mechanical excitation at its resonance frequency and (2) an electrical excitation at lower frequency. As the AFM tip scanned the sample, three types of simultaneous feedback were performed: (1) topography feedback to keep the mechanical oscillation amplitude constant by regulating the tip–sample distance (AM-AFM); (2) a phase-locked loop (PLL) nullifies the phase difference between the driving signal and the cantilever mechanical oscillation, keeping the oscillation at resonance by controlling the excitation frequency; (3) a bias voltage is applied to the tip in order to nullify the electrostatic force or force gradient. Hence, the CPD between the tip and sample is obtained at each pixel, as described in published literature. *
The authors used a commercially available Atomic Force Microscope together with NANOSENSORS AdvancedTEC ATEC-EFM tip view AFM probes. (Platinum iridium coating on AFM tip and AFM cantilever, typical resonance frequency 85 kH, typical force constant 2.8 N/m). For the applied electrical excitation, the chosen amplitude was 2 V (rms) at a frequency of 7 kHz. *
A key point of the authors’ technique is to achieve stable noncontact operation. Topographic noncontact feedback relies on the attractive force between the AFM tip and sample that decreases the oscillating amplitude of the AFM cantilever as the AFM tip approaches the surface, making possible noncontact imaging of the surface and simultaneous acquisition of topography, frequency, and CPD data in a single pass. *
By means of photoassisted Kelvin probe force microscopy (PA-KPFM), Monica Luna et al. have been able to achieve high-resolution SPV imaging of single Au NPs of 3 nm diameter simultaneously with the topographic image. This allowed them to compare the differential charge transfer to the Au NP and to the TiO2 surface when the system was exposed to light of band gap energy and to measure its dependence on light irradiance.
By examining the effect of illumination on single Au NPs they were able to relate nanoscale observations with macroscopic results from TiO2/AuNP–electrolyte interfaces under different conditions. *
The authors found that the photoinduced electron transfer from TiO2 to the Au NP increases logarithmically with light intensity due to the combined contribution of electron–hole pair generation in the space charge region in the TiO2–air interface and in the metal–semiconductor junction. Their measurements on single particles provide direct evidence for electron trapping that hinders electron–hole recombination, a key factor in the enhancement of photo(electro)catalytic activity. *
They could also show that the observed electrochemical behavior of TiO2 photoelectrodes containing large amounts of gold nanoparticles (Au NPs) is in line with the conclusions obtained by photoassisted Kelvin probe force microscopy (PA-KPFM). *
This parallel study by PA-KPFM and electrochemical experiments provides a deeper understanding of the charge transfer to the metal NP and to the TiO2 and allows to correlate the photoinduced charge generation and transfer in Au/TiO2 interfaces with the enhancement in photocatalytic H2 production. *
Figure 4 from “Photoinduced Charge Transfer and Trapping on Single Gold Metal Nanoparticles on TiO2” by Monica Luna et al :
Topography (top) and VCPD (bottom) images of a 3 nm diameter Au NP on bare TiO2(110) (a) in the dark and (b) under UV illumination. VCPD profiles are shown at the bottom. Color scale in topography: (a) 3.6 nm and (b) 4 nm. A CPD image of a larger sample area can be found in the Supporting Information (Figure S10).
NANOSENSORS AdvancedTEC tip view AFM tip
*Monica Luna, Mariam Barawi, Sacha Gómez-Moñivas, Jaime Colchero, Micaela Rodríguez-Peña, Shanshan Yang, Xiao Zhao, Yi-Hsien Lu, Ravi Chintala, Patricia Reñones, Virginia Altoe, Lidia Martínez, Yves Huttel, Seiji Kawasaki, Alexander Weber-Bargioni, Victor A. de la Peña ÓShea, Peidong Yang, Paul D. Ashby and Miquel Salmeron
Photoinduced Charge Transfer and Trapping on Single Gold Metal Nanoparticles on TiO2
ACS Appl. Mater. Interfaces 2021, 13, 42, 50531–50538
DOI: https://doi.org/10.1021/acsami.1c13662
Open Access: The article “Photoinduced Charge Transfer and Trapping on Single Gold Metal Nanoparticles on TiO2” by Monica Luna, Mariam Barawi, Sacha Gómez-Moñivas, Jaime Colchero, Micaela Rodríguez-Peña, Shanshan Yang, Xiao Zhao, Yi-Hsien Lu, Ravi Chintala, Patricia Reñones, Virginia Altoe, Lidia Martínez, Yves Huttel, Seiji Kawasaki, Alexander Weber-Bargioni, Victor A. de la Peña ÓShea, Peidong Yang, Paul D. Ashby and Miquel Salmeron is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/.