Facet-Dependent Photocatalytic Deposition of Metal Nanoparticles Onto Rutile TiO2
Authors: Akiyama Tomoki , Nagakawa Haruki , Tatsuma Tetsu 
Published: December 2024

Deposition of noble metal nanoparticles onto semiconductor such as TiO2 is important for improving photocatalytic activities with metallic co-catalysts and for achieving plasmon-induced charge separation. In that context, facet-selective deposition of metal nanoparticles is attracting attention. There are many reports on the facet-selective deposition, and the selectivity has been explained in terms of different work functions depending on facets.1,2 However, it is likely that factors other than work function also contribute to the selectivity. Therefore, in this work, we focus also on the number of electrons involved in the reactions and the surface and interfacial energies of the semiconductor and metal, so as to understand the mechanisms of the selective deposition and to control the deposition sites and morphologies of the metal nanoparticles. Here, we employed rutile TiO2 particles with exposed crystal planes as a semiconductor material and deposited Au, Pt, and Ag nanoparticles onto the TiO2 particles by photocatalytic means.
Rutile TiO2 particles with exposed crystal planes were synthesized by a flux method. Anatase TiO2 as a precursor was heated at 1000 °C in a NaCl molten salt in air. The TiO2 particles thus obtained (50 mg) was added to 50 vol% aqueous ethanol (5 mL). The solution was purged with nitrogen and a metal precursor, HAuCl4, H2PtCl6, or AgNO3, was added to the solution, followed by irradiating with a UV LED lamp (365 nm) for 10 min, for deposition of Au, Pt, or Ag, respectively, onto the TiO2 particles.
From XRD measurements and SEM observation, it was revealed that the obtained TiO2 particles have rutile structure and (110) and (111) planes. The average exposed area ratio between (110) and (111) planes was estimated to be around 7:3. After photocatalytic deposition of Au, we found 87% of the Au nanoparticles were deposited on (110) planes. Even higher selectivity was observed for Pt nanoparticles; 94% of them were deposited on (110) planes. In contrast, interestingly, 75% of Ag nanoparticles were found at (111) planes. Other Ag nanoparticles were found on the edges in between (110) planes. Namely, facet selectivity was different for Au, Pt, and Ag, and the observed difference was difficult to explain solely in terms of work function.
We therefore focused on difference in the number of electrons required for the metal deposition reactions. In the case of Au and Pt, these metals are deposited by multi-electron reduction reactions; [AuCl4]- and [PtCl6]2- require 3 and 4 electrons, respectively, to be deposited as metal. It is known that the conduction band minimum of the (110) plane is lower (i.e., more positive in potential) than that of the (111) plane. Therefore, photoexcited electrons should tend to accumulate at (110) planes, and multi-electron reductive deposition of Au and Pt should be promoted there. In contrast, Ag nanoparticles are deposited through one-electron reduction of Ag+, which should not require accumulation of excited electrons, and the deposition occurs even at (111) planes. Because the (111) plane has higher surface energy than the (110) plane, the former could be readily stabilized by coating with Ag, which has much lower surface energy than the rutile TiO2(111) plane. When such an anisotropic growth of metal into nanoplates occurs on a semiconductor, the interfacial energy between them should be low enough. It is expected that a metal crystal lattice is easily distorted to match that of a semiconductor, if the Young's modulus for the metal is low. The Young's modulus of Ag is actually lower than those of Au and Pt,3 indicating that Ag is advantageous to cover the TiO2 surface, even if there is some lattice mismatch between them, resulting in low interfacial energy.
In summary, it was shown that facet-selective deposition and morphology control of metal nanoparticles are possible to some extent for rutile TiO2. Also, it was shown that, not only work function, but also the number of electrons involved in the deposition reactions and the surface and interfacial energies for the metal and semiconductor are also important in the facet-selective deposition. These findings would allow us to improve activity of metal co-catalysts and to control plasmon resonance properties of metal nanoparticles.
[1] T. Takata, et al., Nature, 581, 411 (2020).
[2] R. Li, et al., Nat. Commun., 4, 1432 (2013).
[3] C. J. Price and S. P. Hepplestone, J. Mater. Chem. C, 11, 14278 (2023).

Figure 1