Pt Single Atoms on TiO₂ Polymorphs—Minimum Loading with a Maximized Photocatalytic Efficiency

Abstract

For more than 20 years, Pt/TiO2 represents the benchmark photocatalyst/co‐catalyst platform for photocatalytic hydrogen (H2) generation. Here, single atom (SA) Pt is decorated on different polymorphs of TiO2 (anatase, rutile, and the mixed phase of P25) using a simple immersion anchoring approach. On P25 and anatase, Pt SAs act as highly effective co‐catalyst for pure water splitting with a photocatalytic H2 evolution activity (4600 µmol h−1 g−1)—on both polymorphs, SA deposition yields a significantly more active photocatalyst than those decorated with classic Pt nanoparticles or conventional SA deposition approaches. On rutile, Pt SAs provide hardly any co‐catalytic effect. Most remarkable, for P25, the loading of Pt SAs from precursor solution with a very low concentration (<1 ppm Pt) leads already to a maximized co‐catalytic effect. This optimized efficiency is obtained at 5.3 × 105 atoms µm−2 (at macroscopic loading of 0.06 at%)—for a higher concentration of Pt (a higher density of SAs), the co‐catalytic efficiency is significantly reduced due to H2/O2 recombination. The interactions of the SA Pt with the different polymorphs that lead to this high co‐catalytic activity of SA Pt at such low concentrations are further discussed.

Single Atoms in Photocatalysis: Low Loading Is Good Enough!

“…On both types of sputtered layers, SA Pt deposition was carried out from a H 2 PtCl 6 solution at three different concentrations (0.005 mM, 0.05 mM, and 2 mM) using a deposition process described previously. 15,17,28 Figure 1a−c shows HAADF-STEM images for deposition from differently concentrated Pt precursor solutions. For all precursor concentrations, Pt SA decoration is apparent on the anatase TiO 2 surface (the lattice spacing of 0.354 nm corresponds to the (101) plane of anatase).…”

Single-Atom-Based Catalysts for Photocatalytic Water Splitting on TiO2 Nanostructures

“…The Pt SAs on the Co 3 O 4 have shown a higher valence state of ~4+ and a larger population of unoccupied Pt 5d states [90]. Similar investigations were also performed to investigate the Pt 5d occupation state of Pt SA compared to small clusters or nanoparticles (on graphene) [91], for Pt SAs on FeOx [92] or on different polymorphs of TiO 2 nanoparticles [93] and for bi-metallic SAs of PtAu [94].…”

“…Recently Wei et al [94] reported on the synthesis of bi-metallic single-atoms on different facetted TiO2 ({001},{101}), where the metal-support interactions that originated from the coordinatively unsaturated sites contributed to the anchoring of the atomic cocatalysts-namely, onto {001}-TiO2 through Pt-O and Au-O bonds, while on {101}-TiO2 Similarly, the coordination of Pt SAs on different supports (TiO 2 [93]) or of other SAs (Au on TiO 2 [82]) can also be evaluated. For example Wan et al [82] reported on the isolated single atomic dispersion of Au on defective TiO 2 , with an Au-O shell (R = 1.6 Å) and an Au-Ti shell (R = 2.4 Å), without Au-Au and Au-Cl coordination peaks (compared with Au foil and HAuCl 4 as reference).…”

“…Similarly, Li et al [90] have further evaluated the EXAFS analysis of the Pt SAs on the various supports, see Figure 4b, and attributed to the coordination number of Pt-O (peak at 1.60 Å) the subsequent trend Co 3 O 4 > CeO 2 > ZrO 2 > graphene, for the Pt SAs decorated different substrates, and the absence of the Pt-Pt coordination peak suggested the absence of Pt NPs or clusters. Additionally, the Pt/Co 3 O 4 curve showed a second shell peak at 2.56 Å (attributed to Pt-Co coordination), distinct from those of Pt foil (2.37 Å) and PtO 2 (2.68 Å) [90].Similarly, the coordination of Pt SAs on different supports (TiO 2[93]) or of other SAs (Au on TiO 2[82]) can also be evaluated. For example Wan et al[82] reported on the isolated single atomic dispersion of Au on defective TiO 2 , with an Au-O shell (R = 1.6 Å) and an Au-Ti shell (R = 2.4 Å), without Au-Au and Au-Cl coordination peaks (compared with Au foil and HAuCl 4 as reference).XANES and EXAFS measurements are also advantageous for investigating single-atoms on MOF, zeolites, or graphitic layers, see as examples Ye et al[95] for Co SAs on MOF (Co-N bonding detected in EXAFS), Yin et al[96] for Co on N-doped carbon, or Choi et al[97] for Pt on zeolites.…”

Light-Induced Defect Formation and Pt Single Atoms Synergistically Boost Photocatalytic H2 Production in 2D TiO2-Bronze Nanosheets

“…One of the major challenges to address is that Pt SAs without strong binding sites can be thermodynamically unstable due to their high surface energies and hence tendencies to agglomerate during multiple reaction cycles. For these reasons, a high surface area support with sufficient strong coordination sites is necessary to firmly anchor Pt SAs onto the TiO 2 matrix. − …”