Page 142 - Read Online

Chen et al. Microstructures 2023;3:2023025 https://dx.doi.org/10.20517/microstructures.2023.12 Page 11 of 31

Coupling TiO NPs with superior conductive carbon-based support materials has been reported as one of
2
the most effective solutions to enhance their low electrical conductivity. Wang et al. presented a
photochemical deposition method for loading Pt NPs onto a composite TiO -C support (Pt/TiO -C) and
2
2
then heated it at 300 °C under H /N atmosphere for 2 h to induce the SMSI effect . Such a structure
[72]
2
2
enhances not only the overall conductivity of the catalyst system but also the TiO -induced SMSI effect that
2
strongly encapsulates the Pt NPs to facilitate the electron transfer and prevent the migration and
aggregation. As a result, the resulting catalysts exhibit excellent ORR activity and durability in high-
temperature environment. Additionally, embedding TiO films onto carbon matrix is another fabrication
2
strategy. Shi et al. reported a sonochemical reaction method to synthesize a continuous ultrathin (~1.5 nm)
TiO -coated carbon nanotube (CNT) as the support of Pt NPs . Their investigations revealed that the
[73]
2
improved ORR activity of Pt/TiO /C catalysts is attributed to the SMSI effect between Pt NPs and TiO -
2
2
coating and the bifunctional mechanism of TiO . Notably, either TiO NPs or a layer in these composites
2
2
serves as a channel that allows electron transfer between Pt NPs and carbon-based support. Therefore, the
TiO NPs or coating should not be too big or thick (< 5 nm) to facilitate electrical conductivity. In addition,
2
the TiO NPs or coating should also be conformally continuous over the entire carbon surface to prohibit
2
direct contact between the carbon support and the Pt NPs. Therefore, effective tuning of TiO particle size
2
or film layers thickness in these composites and selective deposition of Pt NPs in composites are key to
improving the SMSI effect and thus increasing the ORR activity and durability of the catalyst.
Recently, an attractive method to enhance the electrical conductivity of titanium dioxide is the introduction
of structural defects such as oxygen vacancies (V ) on the surface of TiO , which can construct the
[74]
2
o
Pt-V -Ti interaction and induce the SMSI effect to optimize the ORR catalytic activity by reducing the
o
desorption free energy of Pt-OH, Pt-O, or O . In addition, the SMSI effect reduces the binding energy
2
between the active sites and the adsorbed oxygen species, which is beneficial for the ORR process . One
[75]
common method to introduce V in TiO is by doping the metal oxides with other metals, such as W [76-78] ,
2
o
Nb [78-84] , Cr [85,86] , Mo [87,88] , Ta [89,90] , V , and more. In general, similar electron donations from the substrate to
[91]
metals and induced SMSI effect between them can be observed in various doping systems. Synoptically, a
screening strategy was proposed by Tsai et al. to quickly find out potential supports and dopants (Ti M O
x
1-x
y
supports, where M = Nb, W, Mo, Ru, etc.) in which based on a method of high-throughput DFT
calculations . To predict properties by M dopants, a systematic guide map has been produced for TiMO
[92]
2
substrate that contains data of metal-induced electronic states (MIES, as shown in Figure 6A), the formation
energy of oxygen vacancies (E ), the adsorption energy of single Pt atoms (E ), and charge states of
1Pt
Ovac
deposited Pt (Δδ ).
Pt
Another effective strategy to enhance ORR performance is to construct nanostructured TiO support, such
2
as microspheres, nanofibers, nanotubes, nanosheet assembly, and nanorods. For example,
Murphin Kumar et al. presented a Pt NPs decorated one-dimensional (1D) TiO nanorod (Pt/TiO NRs)
2
2
with remarkably enhanced electronic conductivity and excellent strong coupling of Pt NPs with the TiO
2
NRs support compared to the Pt/TiO (Comm) catalyst (as shown in Figure 6B) . The results verified that
[93]
2
the as-prepared Pt/TiO NRs composite nanostructures exhibited excellent ORR performance and stability,
2
which are mainly attributed to the unique 1D morphology of the TiO NRs providing a greater surface area
2
and the SMSI effect enhancing electron transfer rate at their functional interface.
Cerium oxides (CeO )
2
Recently, CeO has attracted plenty of attention as ORR catalytic support because of its lower price and
2
corrosion resistance in acidic media. What is more, CeO support could switch between Ce and Ce
4+
3+
2
oxidation states due to its abundant oxygen vacancies, which is beneficial to the storage and release of lattice