Citation

Mei Wang, Yuanxu Liu, Dan Li, Junwang Tang, Weixin Huang. Isoelectric point-controlled preferential photodeposition of platinum on Cu₂O-TiO₂ composite surfaces[J]. Chin. Chem. Lett., 2019, 30(5): 985-988  

Mei Wang^a, Yuanxu Liu^b, Dan Li^a, Junwang Tang^c, Weixin Huang^a,*     

Photocatalytic water splitting has been extensively studied as an ideal method to produce hydrogen since the report of the Honda-Fujishima effect of water splitting using a TiO₂ electrode [1]. It consists of photoexcitation of semiconductor photocatalyst to generate electrons and holes in the bulk, transfer of photoexcited carriers from the bulk to the surface, and photoexcited electrons-participated water reduction to H₂ and photoexcited holes-participated water oxidation to O₂ on the surface [2-4]. Among various semiconductor photocatalysts for photocatalytic water splitting, TiO₂ is considered as a potential commercial photocatalyst due to the favorable electronic energy band structure and high photo-chemical stability [5-7]. However, TiO₂ photocatalyst suffers from the wide band gap of around 3.2 eV [8-10] and rapid recombination of photoexcited electronhole pairs [5, 6]. Great effort has been thus devoted to suppressing the recombination of photoexcited electron-hole pairs to increase the efficiency of TiO₂ in photocatalytic water splitting, and the cocatalyst strategy is very effective. Noble metals (Pt, Pd, Au and Ag) and transitional metal oxides have been demonstrated as efficient co-catalysts for TiO₂ to suppress the charge recombination via the transfer of photoexcited electrons and holes from TiO₂ to the cocatalyst, respectively [11-20]. Recently space-separated noble metal and transitional metal oxide co-catalysts have been successfully loaded on TiO₂ to allow simultaneous transfer of photoexcited electrons and holes from TiO₂ respectively to the noble metal and transitional metal oxide co-catalysts [11-13].

Utilizing photoexcited electrons to reduce metal cations, photodeposition emerges as a convenient method to load metal particles on semiconductors [21-28]. Photodeposition was also successfully used for a simultaneous loading of Au and CoO_x respectively on electron-enriched {010} face and hole-enriched {110} face of BiVO₄ nanocrystals [11]. However, there are few works reported on photodeposition of metal particles on oxide composite surfaces. Cu₂O acts as a nice hole-scavenger co-catalyst for TiO₂ [29-31]. In this letter, we report the structures and photocatalytic performances in water reduction of Pt/Cu₂O-TiO₂ composite photocatalysts prepared by photodeposition of Pt particles on Cu₂O-TiO₂ composite surfaces. The results reveal an interesting isoelectric point-controlled preferential adsorption and photodeposition of Pt species on Cu₂O-TiO₂ composite surfaces.

Experimental details are described in the Supporting Information. Pt with loadings of about 0.5% was photodeposited on TiO₂ and Cu₂O-TiO₂ composites employing H₂PtCl₆ aqueous solutions as the Pt precursor. Under the photodeposition condition, the pH values of the solutions were measured to be 3.6. Table 1 summarizes compositions and BET specific surface areas of various photocatalysts. Both Pt loadings and BET specific surface areas are similar for all photocatalysts. Fig. 1 shows photocatalytic H₂ productions as a function of reaction time of various photocatalysts in photocatalytic water reduction illuminated with simulated solar light. It can be seen that the photocatalytic H₂ production increases linearly with the reaction time for various photocatalysts, indicating the stability of these photocatalysts. The calculated mass-specific photocatalytic H₂ production rates of all photocatalysts are summarized in Table 1. 0.5%Pt/TiO₂ exhibits a H₂ production rate of 4.8 μmol g^-1 h^-1, but the H₂ production rate of Pt-Cu₂O-TiO₂ composite photocatalysts keeps decreasing with the loading of Cu₂O. These results suggest that the Pt and Cu₂O species in Pt/Cu₂O-TiO₂ composite photocatalysts exert a negative effect, instead of a synergetic effect, on promoting the photocatalytic performance of TiO₂.

Table 1

Table 1 Compositions, BET specific surface areas and mass-specific photocatalytic H2 productions of various photocatalysts.

Table 1 Compositions, BET specific surface areas and mass-specific photocatalytic H2 productions of various photocatalysts.

Figure 1

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Fig. 1. Photocatalytic H2 production as a function of reaction time of various Pt/Cu2O-TiO2 under simulated solar light irradiation.

Fig. 2A shows XRD patterns of various photocatalysts, in which all observed diffraction patterns could be indexed to anatase TiO₂ (JCPDS card: No. 21-1272). No peaks associated with Cu or Pt species could be identified. This may be due to the fine dispersions of copper and platinum species on TiO₂ and/or the low loadings of copper and platinum in all photocatalysts. Surface structures of all photocatalysts were characterized with XPS. As shown in Fig. S1 (Supporting information), all Pt/Cu₂O-TiO₂ composite photocatalysts exhibit the same Ti 2p_3/2 binding energy at 458.8 eV and Cu 2p_3/2 binding energy at 932.4 eV that respectively correspond to TiO₂ and Cu₂O [30]. However, they exhibit Cu₂O loadingdependent Pt 4f XPS features (Fig. 2B). A peak at around 75.6 eV corresponds to an energy loss peak of TiO₂, and other peaks at 70.9/74.3, 72.8/76.1, and 74.2/77.5 eV can be assigned to the 4f_7/2/4f_5/2 components of metallic Pt, Pt²⁺, and Pt⁴⁺ species, respectively [32-34]. Fig. 2C shows the Pt speciation in various photocatalysts. 0.5% Pt/TiO₂ exhibits dominant metallic Pt species with minor Pt²⁺ species. With the Cu₂O loading of Pt/Cu₂O-TiO₂ increasing, the metallic Pt species decreases and could not be observed in Pt/1% Cu₂O-TiO₂; the Pt²⁺ species increases, reaches the maximum in Pt/0.5%Cu₂O-TiO₂ and then slightly decreases in Pt/1%Cu₂O-TiO₂; and the Pt⁴⁺ species emerges in Pt/1%Cu₂O-TiO₂. These XPS results demonstrate that the Pt species changes from the metallic Pt to the Pt cations in our Pt/Cu₂O-TiO₂ photocatalysts prepared by photodeposition as the Cu₂O loading increases.

Figure 2

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Fig. 2. (A) XRD patterns, (B) Pt 4f XPS spectra with peak-fitting results, and (C) calculated Pt speciation of (a) 0.5%Pt/TiO2, (b) Pt/0.005%Cu2O-TiO2, (c) Pt/0.05%Cu2O-TiO2, (d) Pt/0.5%Cu2O-TiO2 and (e) Pt/1%Cu2O-TiO2 photocatalysts. The red line in Fig. 2A represents the standard XRD pattern of anatase TiO2 (JCPDS Card No. 21-1272). The scatter points and solid lines in Fig. 2B respectively represent the original XPS spectra and peak-fitted XPS spectra.

Representative TEM and HRTEM images of Pt/TiO₂ and Pt/Cu₂OT-iO₂ photocatalysts are shown in Fig. 3, Figs. S2 and S3 (Supporting information). The identified lattice spacings of 0.35, 0.21, 0.23, 0.26 and 0.22 nm arise from TiO₂(101), Cu₂O(200), Pt(111), PtO(101) and PtO₂(011), respectively [29, 30, 35, 36]. Fig. S3 gives FT-transformed patterns of TRTEM images of individual particles to distinguish nanoparticles with similar observed lattice spacings. As reported in our previous paper [29], TiO₂ exhibits a rod shape, and Cu₂O forms a thin film on TiO₂ in Cu₂O-TiO₂ composites up to 1%Cu₂O-TiO₂, forming TiO₂ (core)/Cu₂O (thin film shell) rod structures in which the Cu₂O shell thickness increases with the Cu₂O loading. Pt/TiO₂ and Pt/Cu₂O-TiO₂ photocatalysts remain rod shapes, and Cu₂O exists as thin layers on TiO₂ in all Pt/Cu₂O-TiO₂ photocatalysts, but exposed TiO₂ surfaces are always present. This is also confirmed by the DRIFTS results of CO adsorption on Pt/Cu₂O-TiO₂ photocatalysts (Fig. S4 in Supporting information) in which the vibrational band of CO adsorption at Ti(IV) sites is observed for Pt/Cu₂O-TiO₂ photocatalysts [37]. Thus the original TiO₂ (core)/Cu₂O (thin film shell) rod structures of Cu₂O-TiO₂ composites get destroyed after the photodeposition processes in H₂PtCl₆ aqueous solutions. Metallic Pt nanoparticles of 2–3 nm can be easily identified in Pt/TiO₂, Pt/0.005%Cu₂O-TiO₂ and Pt/0.05%Cu₂O-TiO₂ but few PtO nanoparticles is found although the XPS results suggest the presence of minor PtO species. This could be attributed to the generally much higher dispersion of Pt oxides than Pt metal supported on oxide surfaces. In Pt/0.5%Cu₂O-TiO₂ and Pt/1%Cu₂OT-iO₂ photocatalysts with Pt oxides as the dominant Pt species, PtO and PtO₂ nanoparticles of 3–4 nm are easily observed but Pt nanoparticles can only be occasionally found. It is found that the Pt species supported on TiO₂ are always metallic Pt nanoparticles while those supported on Cu₂O include metallic Pt nanoparticles in Pt/0.005%Cu₂O-TiO₂ and Pt/0.05%Cu₂O-TiO₂ but then are exclusively PtO and PtO₂ nanoparticles in Pt/0.5%Cu₂O-TiO₂ and Pt/1% Cu₂O-TiO₂. Meanwhile, much more Pt species are photodeposited on the Cu₂O surface of Cu₂O-TiO₂ composites than on the TiO₂ surface. This agrees with the XPS results of dominant Pt oxides species since the photodeposited Pt species on TiO₂ is mainly metallic Pt nanoparticles.

Figure 3

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Fig. 3. Representative TEM and HRTEM images of (A1 and A2) 0.5%Pt/TiO2, (B1, B2, B3) Pt/0.005%Cu2O-TiO2, (C1, C2, C3) Pt/0.05%Cu2O-TiO2, (D1, D2, D3) Pt/0.5%Cu2O-TiO2, and (E1, E2, E3) Pt/1%Cu2O-TiO2. Lattice spacings of 0.35, 0.21, 0.23, 0.26 and 0.22 nm correspond to TiO2(101), Cu2O(200), Pt(111), PtO(101) and PtO2(011), respectively.

The above spectroscopic and microscopic characterization results demonstrate that the presence of Cu₂O layers on TiO₂ strongly affects the photodeposition processes of Pt. The photodeposited Pt species is preferentially formed on the Cu₂O surface, and changes from metallic Pt nanoparticles to PtO and PtO₂ nanoparticles as the Cu₂O layers thicken. It is reasonable that the synthesized Pt/Cu₂O-TiO₂ photocatalysts with dominant PtO and PtO₂ nanoparticles on Cu₂O surface and few Pt nanoparticles on TiO₂ surface are less active than the synthesized Pt/TiO₂ catalyst with Pt nanoparticles in photocatalytic water reduction, as experimentally observed.

During the photodeposition process, the [PtCl₄]^2- precursor needs to adsorb on the oxide surface prior its photoreduction. Depending on its isoelectric point and the pH value of aqueous solution, an oxide surface in the aqueous solution is negatively, neutrally, or positively charged. We measured Zeta potentials of TiO₂, Cu₂O, and various Cu₂O/TiO₂ composites as a function of pH values (Fig. 4), from which the isoelectric point of TiO₂, 0.005% Cu₂O/TiO₂, 0.05%Cu₂O/TiO₂, 0.5%Cu₂O/TiO₂, 1%Cu₂O/TiO₂ and commercial Cu₂O is determined as 3.1, 4.8, 5.3, 5.8, 7.1 and 10.2, respectively. The pH value of employed H₂[PtCl₄] aqueous solution is measured to be 3.6, thus the TiO₂ surface of Cu₂O-TiO₂ composites is locally negatively charged while the Cu₂O surface is locally strongly positively charged. Based on these observations, we proposed a photodeposition mechanism of Pt on Cu₂O-TiO₂ composites (Fig. 5). Under the employed photodeposition conditions, the negatively-charged [PtCl₄]^2- precursor adsorb majorly on the strongly positively-charged Cu₂O surface of Cu₂O-TiO₂ composites but minorly on the negatively-charged TiO₂ surface, resulting in the preferential adsorption of [PtCl₄]^2-precursor on Cu₂O surface of Cu₂O-TiO₂ composites. Illuminated with light, both Cu₂O and anatase TiO₂ are photoexcited to produce the electron-hole carriers, and the excited electrons in the conduction band of Cu₂O tend to transfer to the conduction band of TiO₂ at the Cu₂O-TiO₂ junction interface while the excited holes in the valence band of TiO₂ tend to transfer to the valence band of Cu₂O. This leads to electron-rich TiO₂ and hole-rich Cu₂O in Cu₂OT-iO₂ composites. Thus, the [PtCl₄]^2- species on electron-rich TiO₂ surface can be facilely reduced to form metallic Pt nanoparticles while those on hole-rich Cu₂O surface can not be adequately reduced and tend to form PtO nanoparticles and even PtO₂ nanoparticles. The reduction extent of [PtCl₄]^2- species on Cu₂O surface decreases as its thickness on TiO₂ increases. Thus, oxide isoelectric point plays an important role in the photodeposition of Pt on Cu₂O-TiO₂ composite surfaces via preferential adsorption of [PtCl₄]^2- precursor. These reveal the important role of surface chemistry in photo-induced chemical processes on solid surfaces [38].

Figure 4

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Fig. 4. Zeta potentials of TiO2, Cu2O, and various Cu2O/TiO2 composites as a function of pH values of aqueous solution.

Figure 5

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Fig. 5. Schematic illustration of the photodeposition process of H2PtCl6 on Cu2O/TiO2 composite. The yellow, red, blue spheres represent TiO2, Cu2O, Pt species, respectively.

In summary, we have successfully investigated the photodeposition processes of Pt on Cu₂O-TiO₂ composite surfaces employing H₂PtCl₆ aqueous solution as the precursor and revealed a key role of oxide isoelectric point-controlled adsorption of [PtCl₄]^2- precursor in the photodeposition of Pt on Cu₂O-TiO₂ composites. Under the photodeposition conditions, the PtCl₆2-precursor adsorb dominantly on positively-charged and holeenriched Cu₂O surface of Cu₂O-TiO₂ composites, but minorly on negatively-charged and electron-enriched TiO₂ surface. This leads to the dominant formation of Pt oxide particles on Cu₂O surface but few Pt metal particles on TiO₂ surface. Consequently, the activity of resulting Pt/Cu₂O-TiO₂ composite photocatalysts in photocatalytic water reduction decreases as the Cu₂O content increases. These results deepen the understanding of photodeposition processes on oxide composite surfaces.

This work was financially supported by the National Key R & D Program of Ministry of Science and Technology of China (No. 2017YFB0602205), the National Natural Science Foundation of China (Nos. 21525313, 91745202, 21703001) and the Changjiang Scholars Program of Ministry of Education of China.

Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.cclet.2019.01.017.