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Revealing the Synergy of Mono/Bimetallic PdPt/TiO Heterostructure for Enhanced Photoresponse Performance Jun Li, Chang-Hai Liu, Mohammad Norouzi Banis, Daniel Vaccarello, Zhifeng Ding, Sui-Dong Wang, and Tsun-Kong Sham J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b08144 • Publication Date (Web): 15 Oct 2017 Downloaded from http://pubs.acs.org on October 21, 2017

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The Journal of Physical Chemistry

Revealing the Synergy of Mono/Bimetallic PdPt/TiO2 Heterostructure for Enhanced Photoresponse Performance Jun Li†,∥, Chang-Hai Liu‡,§,∥, Mohammad Norouzi Banis¶, Daniel Vaccarello†, Zhi-Feng Ding†, Sui-Dong Wang*,§ and Tsun-Kong Sham*,†,¶ †

Deparment of Chemistry and ¶Soochow University-Western University Joint Centre for

Synchrotron Radiation Research, University of Western Ontario, 1151 Richmond Street, London, Ontario N6A 5B7, Canada ‡

School of Material Science & Engineering, Jiangsu Collaborative Innovation Center of

Photovoltaic Science and Engineering, Changzhou University, Changzhou, Jiangsu 213164, PR China §

Soochow Univesity-Western University Joint Centre for Synchrotron Radiation Research,

Insitute of Functional Nano & Soft Materials (FUNSOM), Soochow University, Suzhou, Jiangsu 215123, PR China ∥

J. Li and C. H. Liu contributed equally to this work

*Corresponding email: [email protected]; [email protected] 1 ACS Paragon Plus Environment


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Abstract

This work reports a synergistic study of a noble metal (NM)/semiconductor hybrid structure featuring mono/bimetallic PdPt/TiO2 heterostructure prepared by a facile anodizationhydrothermal process. The electronic interaction between TiO2 nanotubes (NTs) and mono/bimetallic PdPt nanoparticles (NPs), as well as the bimetallic PdPt interplay, has been thoroughly investigated by various X-ray techniques. Particularly, X-ray absorption near edge structure (XANES) at the Ti L3,2-edge and O K-edge was employed to understand the NM sensitization effect on the local structure of TiO2 host. Meantime, d-charge redistribution within Pd and Pt upon alloying and loading on TiO2 NTs were disclosed at the Pt L3-edge and Pd L3edge XANES, respectively. Consistent results were given from their corresponding X-ray photoemission spectroscopy (XPS) analysis. In addition, extended X-ray absorption fine structure (EXAFS) at the Pt L3-edge and Pd K-edge was also performed to unravel the atomic distribution and intermetallic interaction of Pd and Pt upon alloying. Finally, the synergy within the mono/bimetallic PdPt/TiO2 heterostructure was examined by size, compostion and structure of the as-attached NM NPs assisted with photoresponse performance analysis.

Introduction As one of the mostly explored photocatalysts to harness solar energy, TiO2 has been investigated for decades due to its low-cost, nontoxicity, abundance, and superior chemical stability. Nevertheless, two major issues pertaing to TiO2 still remain as challenges for practical functionalization: (i) large band gap with the UV-only photoabsorption and (ii) the fast recombination of photoexcited electron/hole (e-/h+) pair.1 Many efforts have been made to resolve the current conundrums; to date, major interests have focused on the modifications of

2 ACS Paragon Plus Environment

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TiO2 with metallic/nonmetallic doping, and semiconductor and noble metal (NM) couplings.1 Among these, the sensitization of TiO2 with NM (i.e., Pt, Pd, Au and Ag) nanoparticles (NPs) recently has attracted enormous attention. Indeed, research on NM/TiO2 hybrid structure has made a prominent process towards solar energy utilization.2-5 The design of this approach is not only because NM can enhance the photoabosorption due to its localized surface plasma resonance,5-8 but also because the equalized Fermi level energy of NM/TiO2 heterojunction is lower than the conduction band energy of TiO2,9 which allows the efficient electron transfer from the conduction band of TiO2 to the attached NM NPs. Meantime, the formation of the Schottky barrier between TiO2 and the attached NM NPs (Figure S1) would block the electron back-transfer, resulting in efficient e-/h+ separation. Regarding the efficiency of the NM/TiO2 hetreostructure, the general consensus is that the work function of NM predominantly determines the overall photoresponse performance as a NM with a larger work function would induce a larger potential difference between the conduction band of TiO2 and the Fermi level of NM, which facilitates a more efficient electron transfer from the former to the latter.4, 10 Indeed, Pt (-5.65 eV) with its comparatively larger work function than Pd (-5.2 eV), Au (-5.1 eV) and Ag (-4.7 eV), provides a better synergy with TiO2, resulting in the oustanding photoresponse performance among others.11, 12 Moreover, it is known that the use of bimetallic system is a strategy to gain new properties towards better catalytic performance (e.g., activity and stability) compared to monometallic counterparts. In order to further boost the photoactivity of Pt/TiO2, alloying of Pt with the other NMs, PtM(M: Pd, Au and Ag)/TiO2, outperformed Pt/TiO2 from many perspectives.5, 13, 14 Among these, the construction and test of PdPt/TiO2 ternary structure have shown its popularity due to the following reasons: (1) Pt and Pd have a very good miscibility with only 0.77% lattice mismatch, which results in the easy



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formation of bimetallic alloy with convenient doping/deposition of one to the other;15-17 (2) alloying of Pt with Pd, other than with Au or Ag, on TiO2 would arouse stronger Coulombic interaction at the interface of bimetallic PdPt and TiO2 due to the comparatively larger work function of Pd than that of Au and Ag,18 leading to a more efficient synergy within the heterostructure towards a better photoresponse performance;4, 5, 11, 12 (3) upon PdPt alloying, the charge redistribution within Pd and Pt would positively contribute to catalytic activity, since both Pd and Pt have unoccupied d states just above the Fermi level. 19-21 However, while extensive efforts have been undertaken to optimize the catalytic performance of PdPt/TiO2 ternary structure, to our knowledge, little analytic study regarding the synergy within this ternary structure from a fundamental point of view has been reported.6, 22 Of which the bimetallic PdPt NPs, in particular, are less characterized and understood. Henceforth, a comprehensive investigation of this bimetallic system pertaining to its electronic, structural and chemical interactions will greatly facilitate the optimization of the photoresponse performance of the system. To this end, we here report a facile synthesis and comprehensive characterization of bimetallic PdPt/TiO2 hybrid structure. One-dimensional (1D) TiO2 nanotube (NT) prepared from electrochemical anodization was selected as a host structure for the secondary NM deposition due to its unique geometry and alignment over other TiO2 forms such as nanoparticle, nanowire and nanobelt.1,

9

Then a facile hydrothermal process was utilized over sol-gel,8,

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photodeposition,5, 6 and electrochemical2, 24 synthesis, to prepare the NM/semiconductor hybrid structure, which demonstrates a better control over the size, composition and quantity of asdeposited NM NPs.3 Afterwards, X-ray techniques were applied on the mono/bimetallic PdPt/TiO2 heterostructures to reveal the electronic and structural interplay associated with their


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photoresponse performance. Specifically, X-ray absorption near edge structure (XANES) analysis on the Ti and O sites were performed to disclose the interaction between NM and TiO2. XANES together with the extended X-ray absorption fine structure (EXAFS) measurements on Pd and Pt sites were carried out not only to corroborate the NM/TiO2 interaction, but also to unveil the bimetallic interaction upon PdPt alloying. Of which XANES elucidated the local chemical environment (e.g., local symmetry and oxidation state) of element of interest (i.e., Ti, O, Pd and Pt), whereas EXAFS shed light on the modification of Pd/Pt local structure (e.g., bond distance, degree of disorder and coordination number) upon alloying and deposition on TiO2 host. Experimental Methods Sample synthesis. The preparation of highly ordered TiO2 NTs (TNTs) using an electrochemical anodization procedure has been previously reported.9, 25 Briefly, a home-made two-electrode cell was utilized for Ti anodization where the Ti foil (0.1 mm thick, Goodfellow) with a size of 2 cm × 0.5 cm was employed as the anode and a Pt wire was used as the cathode. An ethylene glycol-based electrolyte with the inclusion of 0.3 wt % NH4F (ACS, 98 % min, Alfa Aesar) and 2 vol % H2O was prepared for the anodization experiment. In order to fabricate TNTs with a good geometry and alignment for the following NM NP deposition, a two-step anodization procedure was carried out. At first, a 50 V constant potential was applied on the initial Ti foil for 4 h to initiate the first anodization, then the as-anodized first layer was ultrasonically removed in 1 M HCl from the Ti substrate. Later on, a second anodization was conducted at 50 V for 30 mins with the same but fresh electolyte. After rinsing with ethanol for several times to remove the electrolyte surplus, as-grown TNTs (attached on Ti substrate) were



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dried with N2 gas. The crystallization of as-grown amorphous TNTs to anatase ones was achieved by performing a post-annealing at 450 ℃ for 2 h. Mono/bimetallic NM NPs were deposited onto anatase TNTs using a facile hydrothermal method.9 Anatase TNTs together with a 15 mL of bath solution including 150 mg of polyvinylpyrrolidone (PVP, Mw ≈ 55000, Sigma-Aldrich), 56 mg of NaI (ACS, 99.5 %, SigmaAldrich) and a certain amount of NM precursor(s), were loaded into a 50 mL Teflon-lined stainless-steel autoclave and heated under sealing for 2 h at 180 ℃. Of which the total mole of NM precursor(s), i.e., PdCl2 (99%, Sigma-Aldrich) and PtCl2 (99.9%, Alfa Aesar), was kept at a constant value of 0.01 mmol while the molar ratio of PdCl2/PtCl2 varied from 1/0, 2/1, 1/1, 1/2 and 0/1 to prepare the nominal Pd, Pd2Pt1, Pd1Pt1, Pd1Pt2 and Pt NPs decorated on TNT, hence denoted as Pd-TNT, Pd2Pt1-TNT, Pd1Pt1-TNT, Pd1Pt2-TNT and Pt-TNT, respectively. Subsequently, the as-formed NM/TNT speciemens were rinsed with ethanol for several times and dried with N2 gas. Characterization. The morphologies of TNTs before and after NM decoration were characterized using a LEO (Zeiss) 1540 XB SEM. Elemental analysis was obtained by energy dispersive X-ray (EDX) spectroscopy where an EDX detector operating at 10 kV was attached on the SEM facility. Transmission electron microscopy (TEM, FEI Quanta FRG 200F, 200 kV) and the high-angle annular dark-field scanning TEM (HAADF-STEM) were applied to characterize the morphology and elemental mapping of NM NPs, respectively. X-ray photoemission spectroscopy (XPS, Kratos Axis Ultra DLD, monochromatic Al Kα) was employed to examine the electronic structures of NM NPs. X-ray absorption fine structure (XAFS) measurements were collected at the Canadian Light Source (CLS, Saskatoon, SK, Canada). Ti L3,2-edge and O K-edge XANES spectra were


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measured at the Spherical Grating Monochromator (SGM, E/∆E > 5000) beamline,26 of which surface-sensitive total electron yield (TEY) detection mode by monitoring the specimen current was applied to characterize the local structure modification of TiO2 host upon NM decoration. Pd L3-edge XANES spectra was recorded at the Soft X-ray Microcharacterization Beamline (SXRMB, E/∆E > 5000),27 whereas the Pt L3-edge and Pd K-edge XAFS experiments were performed at the Hard X-ray MicroAnalysis (HXMA, E/∆E > 5000) beamline.28 Fluorescence yield collected by a solid state detector was utilized for the XAFS measurements of NM NPs where the energy calibration was implemented by comparing to the XAFS spectra of high purity metal Pt and Pd foils. EXAFS analysis. Standard procedures were used to analyze the EXAFS data via the IFEFFIT pacakage.29 Briefly, energy calibration and spectral normalization of the raw absorption spectrum were carried out using Athena software, of which a cubic spline function was used to fit the background above the absorption edge. Then the normalized EXAFS function recorded in energy space, χ(E), was transformed to χ(k) where k is the photoelectron wave vector. To emphasize on the interatomic interaction of NM, χ(k) was multipllied by k2 to amplify the EXAFS oscillations in the mid k region. Subsequently, k2-weighted χ(k) with a k range of 3.2~12.5 Å-1 for both Pt L3-edge and Pd K-edge was Fourier transformed from k space to R (radial distribution) space to differentiate the EXAFS oscillation from different coordination shells. Afterwards, EXAFS data recorded in R space was transferred to Artemis software for EXAFS fitting analysis. The fittings of the first shells between 1.6 and 3.3 Å for Pt, and between 1.6 and 3.2 Å for Pd were conducted. The phase and amplitude functions of Pt-Pt, Pt-Pd, Pd-Pt and Pd-Pd were calculated with FEFF. S0/∆σ2 values of 0.83/0.0045 for Pt and 0.76/0.0052 for Pd were determined from Pt and Pd foils, which were individually applied to the Pt and Pd



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EXAFS fittings of NM NPs. A bond distance of 2.77 Å, 2.75 Å and 2.76 Å was used for the EXAFS fitting of Pt-TNT, Pd-TNT and bimetallic PdPt-TNT specimens, respectively. From these standard EXAFS fitting practices, structural parameters of NM NPs were calculated, including coordination number (CN), bond distance (R), inner potential shift (∆E0) and DebyeWaller factor (∆σ2). Photoelectrochemical (PEC) Measurements. PEC experiments were performed in a custom three-electrode PEC cell attached on a CHI 832a potentiostat (CH Instruments, Austin TX), of which NM/TiO2 NT film adhered to Ti substrate (with a constant area exposed by masking with a resistive polyimide tape) was straightly used as the working electrode, whereas a Pt coil and a standard calomel electrode (SCE) were employed as the counter and reference electrodes, respectively. The electrolyte was 1 M KOH (ACS, 95%, Caledon Laboratory Chemicals) solution. During the photocurrent collection, a 150 W Newport lamp with an AM 1.5D filter and a ThorLabs SC10 shutter were combined to produce the chopped light source. Linear sweep voltammetry (LSV) measurements were carried out with a linear sweep from 0.4 V to -0.4 V, whereas the chronoamperometry tests were recorded under 0 V for 70 s. Results and Discussion The morphology of various TNT specimens before and after NM NP decoration is shown in Figure 1. Prinstine TNT (Figure 1a and 1b) displays an inner tube size of ~80 nm, a tube wall thickness of ~30 nm and a tube length of ~3.2 μm (Figure S2). After hydrothermal process, PdTNT (Figure 1c and 1d), Pd2Pt1-TNT (Figure 1e and 1f), Pd1Pt1-TNT (Figure 1g and 1h), Pd1Pt2TNT (Figure 1i and 1j) and Pt-TNT (Figure 1k and 1l) specimens all demonstrate uniform deposition of NM NPs on the surface of nanotubular TiO2 host (inner and outer tube walls). The weight percentage of loading NM NPs in all specimens is approximately 3 wt. % from EDX


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analysis (not shown), and the bulk Pd/Pt molar ratios of bimetallic NPs are determined to be 2.15, 0.75 and 0.22 for the nominal Pd2Pt1, Pd1Pt1 and Pd1Pt2, respectively. Several interesting features are noted. First, the bulk Pd/Pt molar ratio of bimetallic NP deviates from its nominal molar ratio (determined from precursor concentrations) once the initial concentration of PtCl 2 increases, and it appears that Pd1Pt1-TNT and Pd1Pt2-TNT are Pt-rich, particularly for the latter specimen. Second, the overall particle size of the as-deposited NM becomes larger while the NP adhesion rate (i.e., number of NPs per unit area) drops with the increase of PtCl 2 initial concentration, hence, the as-formed Pt NPs of Pt-TNT specimen (Figure 1k and 1l) shows the largest particle size of ~20 nm. Similar size trend of PdPt alloy with a Pt-rich nature has been reported elsewhere.30-32 A closer TEM examination of the large Pt NPs is indicated in the inset of Figure 1k, which clearly reveals the multiparticulate morphology (i.e., aggregation) of these NPs attached on the TNT substrate.



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Figure 1. Top/side SEM views of (a/b) TNT, (c/d) Pd-TNT, (e/f) Pd2Pt1-TNT, (g/h) Pd1Pt1-TNT, (i/j) Pd1Pt2-TNT and (k/l) Pt-TNT. A closer TEM exmination on the morphology of the asformed Pt NPs in Pt-TNT specimen is indicated as the inset of (k). All the scale bars of SEM images are 100 nm. HAADF-STEM was employed to confirm the bimetallic character of the as-attached NM NPs as illustrated in Figure 2. Representative NPs selected from Pd2Pt1-TNT (Figure 2a), Pd1Pt1-TNT (Figure 2b) and Pd1Pt2-TNT (Figure 2c) specimens all demonstrate their bimetallic nature with homogeneous Pd and Pt distributions as shown in their respective STEM mappings.

Figure 2. HAADF-STEM images of (a) Pd2Pt1-TNT, (b) Pd1Pt1-TNT and (c) Pd1Pt2-TNT hybrid specimens, where their corresponding elemental mappings of Pd and Pt are shown separately on their right sides. All the scale bars are 10 nm. To evaluate the photoresponse performance, photoelectrochemical (PEC) measurements were performed using a three-electrode assembly in 1 M KOH electrolyte. The associated linear sweep voltammetry (LSV) results are illustrated in Figure 3a, in which all species show the steady photocurrent densities at a potential range of -0.4 ~ 0.4 V relative to SCE standard electrode with 10 ACS Paragon Plus Environment

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a variation of the current density among different samples. In general, TNT specimens decorated with NM NPs deliver the much enhanced photocurrent than the pristine TNTs, underscoring the synergy between NM/TiO2 for effective charge separation. As suggested in previous reports, the photo-excited electron would preferentially transfer from the conduction band of TiO2 to NM sites, thus hindering its recombination with the valence band hole and greatly enhancing the photocurrent.3, 9 It is because the Fermi energy levels of NMs as well as their alloys are lower than the conduction band energy of TiO2, and the formation of Schottky barrier between TiO2 and NM would prevent the reverse electron transfer from the latter to the former (Figure S1).5

Figure 3. (a) Linear sweep voltammetry (LSV) measurements and (b) chronoamperometry tests under 0 V of TNT (red), Pd-TNT(orange), Pd2Pt1-TNT(green), Pd1Pt1-TNT(cyan), Pd1Pt2TNT(blue), and Pt-TNT (purple) under chopped illuminations.



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Figure 3b shows the corresponding chronoamperometry (at 0 V versus SCE) results. It is apparent that monometallic decoration of TNT (Pd-TNT or Pt-TNT) would promote its photocurrent density twice as high as TNT. More interestingly, the bimetallic NM/TiO2 specimens like Pd2Pt1-TNT and Pd1Pt1-TNT exhibit the more pronounced photoresponse performance where both show a photocurrent density three times higher than TNT. The superior outperformance of bimetallic PdPt-TNT system suggests the heterogeneity of NMs upon alloying would establish a synergy for the trap of photoinduced electrons, which can positively contribute to the enhanced photoactivity. 20, 21, 33 Nevertheless, bimetallic Pd1Pt2-TNT produces a comparable photocurrent density value with Pt-TNT, which validates the fact that the photoresponse performance of NM/TiO2 hybrid structure is highly associated with the size, composition and structure of as-attached NM NPs.34 The larger size of NM NP in Pd1Pt2-TNT specimen associated with its lower NP adhesion rate (Figure 1), compared to those in Pd-TNT, Pd2Pt1-TNT and Pd1Pt1-TNT (Figure 1), might directly contribute to its weaker photoresponse performance, i.e., less effective e-/h+ separation.35 It also explains why we observe the lower photocurrent of Pt-TNT than that of Pd-TNT although the reverse case would be expected as the Fermi level energy of Pt is lower than that of Pd (i.e., Pt has a higher electron affinity than Pd), albeit the Fermi level pinning does not always follow the Schottky rule.11, 18 Moreover, the bulk molar ratio of Pd/Pt in nominal Pd1Pt2-TNT specimen is 0.22, thus its predominant Pt composition associated with its poorer photoresponse performance correlates with literature that positive synergetic effect within PdzPt1-z alloys is mainly achieved from Pd dominant species.34, 36, 37

More importantly, the structure of NM NPs and their interaction with TiO2 substrate would

play a dominant role in the photoresponse performance of NM/TiO2 composite, which we will further delineate below.


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We first performed XANES measurements at the Ti and O sites recorded in surface-sensitive TEY mode to examine the NM loading effect on the TiO2 substrate.25 Figure 4a displays the Ti L3,2-edge TEY XANES of various samples of interest as well as anatase and rutile TiO2 standards for comparison. The interpretation of Ti L3,2-edge characteristics of TiO2 has been well documented in literature.9, 38-40 In short, peaks a and b arise from the electronic transitions of Ti 2p3/2 to t2g and eg bands of Ti 3d states, whereas features c and d can be attributed to the Ti 2p 1/2 to Ti 3d-t2g and eg electronic transitions, respectively. Note that the major difference between anatase and rutile TiO2 at their Ti L3,2-edge XANES is their inverse peak intensity ratio of b1/b2 where ratio values larger and lower than 1 suggest the D2d and D2h local symmetries of Ti atoms in anatase and rutile TiO2 structures, respectively.9 By comparing to the standards, all samples of interest (except the as-grown which shows the typical amorphous structure without peak b splitting)25 exhibit the distinct Ti L3,2-edge features attributable to anatase TiO2 before and after the NM decoration, meaning that the surface anatase host structure is well maintained during the NM NP growth. Further evidence is provided by considering the O K-edge TEY XANES. As demonstrated in Figure 4b, where peaks A and B originate from the O 1s electronic transitions to O 2p states hybridized with Ti 3d-t2g and eg states, features C and D are assigned to the electronic transitions from O 1s to O 2p-Ti 4sp hybridized states, and the presence of peak E indicates the long range order of as-characterized TiO2 structure.25, 39 It is worth mentioning that the peak D splitting as well as the blue energy shifts of peaks C and E in rutile TiO2 compared to anatase phase, insinuates their different O local symmetries (multiple scattering pathways). Again, apart from the amorphous character of the as-grown specimen, all other samples of interest exhibit anatase XANES spectral patterns before and after NM decoration, in line with the Ti L3,2-edge results discussed above. It is clear that the electronic structure of anatase TiO2 host remains intact



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during the secondary NM NP growth, indicating that the NP decoration takes place on the surface (i.e., no doping). Nevertheless, a noticeable broadening between features A and B upon NM coating is observed as indicated by orange arrows in Figure 4b, which most likely can be traced to the interfacial interactions between the d orbital of NM NP and the O 2p-Ti 3d hybridized states of TiO2 substrate.18, 41 A closer examination of the peak valley between peak A and B among NM/TiO2 specimens show the peaks become narrower again as the Pt content increases, corroborating that the size increase is accompanied by a reduced NM-TiO2 interface which would reduce the interfacial interaction between the TiO2 host and adsorbed NM NPs.

Figure 4. (a) Ti L3,2-edge and (b) O K-edge XANES spectra of various samples of interest in comparison with that of anatase and rutile TiO2 standards as recorded in TEY mode.


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XPS analysis on the Pt and Pd sites has been carried out to investigate the electronic structure of NM NPs, Figure 5a shows the Pt 4f XPS spectra, which are calibrated to the C 1s at 284.5 eV. A cursory look on these spectra reveals two dominant peaks at ~70.3 eV and ~73.6 eV assigned to Pt 4f7/2 and 4f5/2, indicating that they are of metallic character.22, 35 It is noteworthy that the binding energies for Pt 4f7/2 in Figure 5a are lower than that reported for bulk Pt, which has a typical binding energy range of 70.9~71.1 eV for Pt 4f7/2.22, 42 It indicates that Pt atoms locate in an electron-rich environment at the interface.18 Another possible contribution is electron redistribution via TiO2 host and supported NM NP interaction. It underscores the strong interaction within the NM/TiO2 hybrid structure.9, 35 Upon alloying with Pd, Pt 4f of Pd1Pt2-TNT exhibits a positive energy shift compared to that of Pt-TNT, and further gradual shifts of Pt 4f towards the higher binding energy regions are observed for Pd1Pt1-TNT and Pd2Pt1-TNT. It suggests a charge redistribution takes place within the PdPt bimetallic system where a charge transfer from Pt to Pd is indicated. To verify this, Pd 3d XPS spectra of Pd as well as bimetallic PdPt species supported on TNT are shown in Figure 5b. Several points can be noted. At first, a broad peak centered at ~330 eV can be attributed to the 4d3/2 character of Pt, of which its intensity (not observed in Pd-TNT), as expected, becomes more pronounced as the Pd/Pt molar ratio of bimetallic NPs decreases. Second, Pd 4d5/2 of all specimens exhibits a binding energy at ~334.2 eV, which is evidently lower than that reported in literature for bulk Pd at ~335 eV,22, 43 further corroborating the strong NM/TiO2 interaction as the work function of TiO2 is lower than that of Pd (5.2 eV).9, 11 It also validates the fact that NM deposition most likely occurs on the electron-rich surface defects of TiO2 host, and the removal of these surface defects, which usually act as e-/h+ trapping sites, upon NM deposition contributes positively to the photoresponse performance.5, 9, 44 Moreover, the alloying of Pd with Pt induces a negative shift



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of Pd 3d binding energy, and a further red shift of Pd 3d is observed as the Pt content increases in the bimetallic NPs. It insinuates the alloying effect that Pd gains electron from Pt, which echoes the Pt 4f XPS results as discussed above. To further explore the alloying effect, the Pd 3d XPS peak splitting analysis has been conducted. As shown in Figure 5c~5f, both Pd0 and an oxidized Pdδ+ species are resolved,22, 34 of which the Pdδ+ percentage (relative to the total Pd 3d contribution) decreases from 45.1 % in Pd-TNT to 19.4% in Pd1Pt2-TNT albeit the absolute intensity is overemphasized as noted above due to many body effect. It results from the Pd charge redistribution upon alloying (i.e., an electron transfer from Pt to Pd), and/or the lesser portion of Pd surface atoms exposed to TiO2 lattice once alloying with Pt as well as the formation of larger NPs (i.e., lower surface-to-volume ratio) as indicated in Figure 1.

Figure 5. (a) Pt 4f XPS spectra of Pt-TNT (1), Pd1Pt2-TNT (2), Pd1Pt1-TNT (3), and Pd2Pt1-TNT (4). (b) Pd 3d XPS spectra of Pd-TNT (i), Pd2Pt1-TNT (ii), Pd1Pt1-TNT (iii), and Pd1Pt2-TNT 16 ACS Paragon Plus Environment

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(iv). (c~f) Further Pd 3d XPS peak splitting analysis of Pd-TNT, Pd2Pt1-TNT, Pd1Pt1-TNT, and Pd1Pt2-TNT, respectively; Black circles and correlated red curves show the individually experimental and fitted spectra, where the latter is decomposed into components of Pt 4d3/2 (blue), Pd0 (green), and Pdδ+ (magenta). The XPS spectra in (a) and (b) are vertically shifted for clarity. It must be noted however, the shoulder peak is exaggerated somewhat due to the fact that the Pd 3d peaks are not single particle peaks, there are manybody contributions, thus the peak shape is not a Voight function. Therefore, the Pd 3d shoulder is overestimated. To better understand the charge redistribution within PdPt alloys, we further explored the unoccupied d band states above the Fermi level at both Pt and Pd sites as the d band of NM is directly associated with the catalytic activity.9, 17, 32, 45 At first, the Pt L3-edge XAFS spectra of various samples of interest are illustrated in Figure 6a, of which the similarity of the Pt L3-edge XAFS pattern of samples of interest as that of Pt metal is indicative of the metallic Pt fcc structure in all Pt species, in good agreement with the Pt 4f XPS results (Figure 5a). Notably, the whiteline (WL) feature indicated by orange rectangle arises from the Pt 2p3/2 to 5d5/2,

3/2

transitions,46 thus the WL intensity/peak area underneath is related to the unoccupied density of states of Pt 5d character, and a higher WL intensity/larger peak area represents a larger 5d vacancy of Pt species. A direct comparison of Pt L3-edge WLs is shown at the right side of Figure 6a, while the pristine Pt NP in Pt-TNT exhibits a lower WL intensity than Pt foil, the Pt L3-edge WL intensity of bimetallic NPs becomes stronger as the Pd content increases. On one hand, the decrease of WL intensity of pristine Pt NP compared to the bulk Pt foil implies the gain of d charge of as formed Pt NPs, further corroborating the interfacial interaction between TiO2 host and Pt NP with a charge transfer from the former to the latter. It is because an enhancement of Pt L3-edge WL intensity would be expected once the Pt size decreases to a nanoscale as the 17 ACS Paragon Plus Environment


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reduction of CN and then the more effective hybridization within Pt NP would positively contribute to the d-charge depletion of Pt.47 On the other hand, the increment of Pt L3-edge WL intensity of bimetallic PdPt NPs suggests the d-charge redistribution of Pt upon alloying with Pd, where a d-charge depletion of Pt is concluded.

Figure 6. (a) Pt L3-edge and (b) Pd L3-edge XANES spectra of various samples of interest together with Pt and Pd foils for comparison. Second, d-charge redistribution study at the Pd site has also been carried out by focusing on the Pd L3-edge XANES (Figure 6b) which probes the Pd 2p3/2 to 4d5/2, 3/2 transitions at the WL region indicated by an orange rectangle. Similarly, the Pd L3-edge XANES spectra of samples of interest resemble that of Pd foil, signifying the metallic Pd fcc structure in all Pd species. More importantly, the overlay of WLs shows the much stronger WL intensity of Pd species in NM NPs than that of Pd foil, and the Pd L3-edge WL intensity drops with the introduction of Pt. On one hand, the strong WL intensity of Pd NP compared to that of bulk Pd foil exemplifies the NM size 18 ACS Paragon Plus Environment

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effect where a d-charge depletion upon nanosizing is induced due to the reduction of coordination and stronger s-d hybridization within Pd states.9 On the other hand, the drop of Pd L3-edge WL intensity upon alloying with Pt unequivocally indicates the d-charge gain of Pd with the inclusion of Pt, in accord with the above Pd 3d XPS analysis (Figure 5b~5f). Therefore, both Pt and Pd species essentially exist in their metallic forms in NM NPs, and indeed, the d-charge redistributions occur at both Pt and Pd sites where a d-charge depletion of Pt and a d-charge gain of Pd are double confirmed from XPS and XAFS analysis. This results points to a homogeneous Pd-Pt alloy formation. Additionally, EXAFS analysis at the Pt L3-edge and Pd K-edge were performed to scrutinize the PdPt bimetallic interaction. As shown in Figure 7, the Fourier transform of the EXAFS oscillations from both Pt and Pd perspectives share the similar evolution trend. It should be noted that because of the nolinearity in the the k depedence of the phase and amplitude, the radial distribution will not appear as a simple symmetric peak. Two observations can be highlighted. First, the radial distribution amplitudes of the prinstine Pt/Pd NPs decrease comparing to that of bulk Pt/Pd foil, implying that a considerable disorder takes place upon nanosizing which mainly can be assigned to a decrease in CN (will be confirmed later). Second, with the increasing content of Pd and Pt guests in their respective Pt and Pd hosts, further amplitude reductions together with their shifts from both Pt and Pd perspectives are observed, which unambiguously arises from the Pt-Pd interaction due to the incoherent Pt-Pt/Pd-Pd and Pt-Pd EXAFS oscillations in k space.46 It evidently provides the formation of bimetallic PdPt alloy with their strongly structural interaction from a spectroscopic view. It must be noted that the splitting into several resonances between 1.5 and 3.3 Å from both Pt and Pd cases results from the phase interference and nonlinear k dependence (amplitude and phase of Pt and Pd are not linear in k) instead of a



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reflection of different shells. In fact, for heavy metal like Pt and Pd, its bond distance can not be directly read from the radial distribution scale with a simple phase correction owing to the nonlinearity of the phase and amplitude in k space.9, 46 Alternatively, an EXAFS fitting practice is necessary in order to extract the CN and bond distance.

Figure 7. Fourier transform of the EXAFS spectra of various samples of interest together with that of Pt and Pd foils at the (a) Pt L3-edge ∆k = 3.2 – 12.5 Å-1 and (b) Pd K-edge ∆k = 3.2 – 12.5 Å-1. EXAFS simulations were performed with FEFF.48 The quality of EXAFS fitting of each sample of interest is shown in Figure S3 (Pt L3-edge) and Figure S4 (Pd K-edge), the associated fitting parameters are summarized in Table 1 with an uncertainty of ~10%. In general, all Pt and Pd species exhibit a CN value lower than 12 (a value for bulk Pt and Pd foils), which corroborates the formation of NM NPs and amplitude reduction of EXAFS oscillations as shown 20 ACS Paragon Plus Environment

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in the FT display in Figure 7. Moreover, by comparing to the bulk bond distance of Pt-Pt at 2.77 Å and Pd-Pd at 2.75 Å, a shorter metallic bond distance is achieved, suggesting the contracted NM structure upon nanosizing. Consistent results are reported elsewhere.9,

17

A particular

emphasis is given on the CN values of bimetallic species, which shows that whereas Pd1Pt2-TNT specimen has a total Pt CN[Pt-Pt]+[Pt-Pd] of 11.2 and a total Pd CN[Pd-Pd]+[Pd-Pt] of 9.8, the total Pt CN[Pt-Pt]+[Pt-Pd]/Pd CN[Pd-Pd]+[Pd-Pt] of Pd1Pt1-TNT and Pd2Pt1-TNT specimens are 10.5/11.8 and 10.9/11.6, respectively. It suggests that as-synthesized bimetallic NPs possess an approximately uniform composition of Pt and Pd. Table 1. First shell EXAFS fitting results for various samples of interest. Sample

Edge

Scatter

CN

R

∆E0

(Å)

(eV)

∆σ2 (×10-3 Å2)

Pt-TNT

Pt L3-edge

Pt-Pt

10.5

2.76

2.2

4.1

Pd1Pt2-

Pt L3-edge

Pt-Pt

10.1

2.76

-1.1

4.0

Pt-Pd

1.1

2.75

-0.6

4.0

Pd-Pd

3.6

2.76

0.3

3.0

Pd-Pt

6.2

2.76

1.6

3.0

Pt-Pt

7.3

2.75

-2.3

3.2

Pt-Pd

3.2

2.74

0.6

3.2

Pd-Pd

6.1

2.73

-1.1

4.4

Pd-Pt

5.7

2.73

-1.8

4.4

Pt-Pt

6.8

2.75

0.1

3.7

Pt-Pd

4.1

2.74

-1.3

3.7

Pd-Pd

9.5

2.74

5.7

5.3

Pd-Pt

2.1

2.74

5.5

5.3

Pd-Pd

11.4

2.74

-1.3

5.7

TNT Pd K-edge

Pd1Pt1-

Pt L3-edge

TNT Pd K-edge

Pd2Pt1-

Pt L3-edge

TNT Pd K-edge

Pd-TNT

Pd K-edge



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To further gain insights on the atomic distribution and the extent of alloying of Pt and Pd in bimetallic PdPt forms which strongly correlate with catalytic activity,32 a structural model based on CN values from EXAFS fitting was applied.49 In short, the atomic distribution of Pt in PdPt alloy is defined as: Pobsd = CNPt-Pd/(CNPt-Pd + CNPt-Pt). Likewise, the atomic distribution of Pd in PdPt alloy is described as: Robsd = CNPd-Pt/(CNPd-Pt + CNPd-Pd). Then the extent of alloying (J) of Pt and Pd can be calculated by JPt = Pobsd/Prandom × 100% and JPd = Robsd/Rrandom × 100%, respectively. Of which Prandom and Rrandom individually stand for the extent of alloying of Pt and Pd in a perfectly uniform PdzPt1-z (z value is obtained from EDX analysis) alloy, and they can be determined from the molar ratios of Pt and Pd in the ideal situation. The calculated results are shown in Table 2. Table 2. Alloying extent of Pt and Pd in bimetallic specimens based on EXAFS fitting parameters. Nominal/EDX

Pobsd

Robsd

Prandom

Rrandom

JPt

JPd

Pd1Pt2/Pd0.22Pt1

0.098

0.633

0.180

0.820

54.4

77.2

Pd1Pt1/Pd0.75Pt1

0.305

0.483

0.429

0.571

71.1

84.6

Pd2Pt1/Pd2.15Pt1

0.376

0.181

0.683

0.317

55.1

57.1

Generally, a higher J value denotes a higher heterometallic coordination of the metal species (Pt or Pd); i.e., stronger bimetallic interaction. Hence, a comparison within Table 2 shows that both Pt and Pd atoms in Pd1Pt1-TNT hold the highest heterometallic coordination whereas Pd2Pt1-TNT demonstrates the lowest. Recall the photoresponse performance comparison from Figure 3, we conclude that the extent of alloying is quite relevant to the catalytic activity where Pd1Pt1-TNT delivers the best photocurrent with its highest heterometallic coordination for both Pt and Pd species. Nevertheless, NP size plays an important role in photocatalysis as well, which 22 ACS Paragon Plus Environment

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accounts for the much better photoresponse performance of Pd2Pt1-TNT (comparable to that of Pd1Pt1-TNT) with its smaller PdPt NPs than that of Pd1Pt2-TNT with its larger PdPt NPs although the overall extent of alloying of the former is lower than the latter. Conclusions Mono/bimetallic PdPt/TiO2 hybrid structures were fabricated for a synergistic investigation with the application of advanced synchrotron X-ray spectroscopies. Of which highly ordered TiO2 NTs were prepared using a two-step anodization procedure and subsequently used as a host structure, then secondary NM NPs were uniformly deposited on the surface of TiO2 NTs with a facile hydrothermal process. SEM and HAADF-STEM results confirmed that the nanotubular structure remained intact after NM NP coating and the uniformity of bimetallic PdPt NPs as attached, respectively. Meanwhile, an aggregation of NP with its overall size increase was observed with the increase of Pt content. Photoelectrochemical measurements were carried out to evaluate the synergy within NM/TiO2 composite. The results exhibit the trend of Pd2Pt1/TiO2 ≈ Pd1Pt1/TiO2 > Pd/TiO2 > Pt/TiO2 ≈ Pd1Pt2/TiO2 > TiO2. As the efficient charge transfer from photoexcited TiO2 to NM NPs accounts for the enhanced photoresponse performance of NM/TiO2 compared to that of pristine TiO2 NTs, a size effect results in the higher photocurrent density of Pd/TiO2 than that of Pt/TiO2. In order to interpret the outperformance of bimetallic Pd2Pt1/TiO2 and Pd1Pt1/TiO2 species among others, an electronic structure examination correlated with bimetallic composition was conducted. XANES analysis at the Ti L3,2-edge and O K-edge revealed that the attached NM NPs were essentially on surface with the TiO2 NTs remaining intact, whereas Pt L3-edge and Pd L3-edge XANES results individually show d-charge depletion of Pt and d-charge gain of Pd upon alloying, in line with the associated XPS findings. Additionally, EXAFS study at the Pt L3-edge and Pd K-edge were further performed to unveil



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the intermetallic interaction, hence the atomic distributions together with the extent of alloyings of Pt and Pd. Based on the observations and the rationalization of size, composition and structure contributions to the synergy of NM/TiO2 system, a physical picture, in which homogeneous mixing of Pd and Pt and small NP size facilitate the photoresponse performace, emerges. This work gains new insights into the rational design of NM/TiO2 co-catalyst for achieving high photoresponse performance, and provides a protocol for understanding the synergy within mono/bimetallic NM/semiconductor hybrid structure with in-depth synchrotron X-ray spectroscopy analysis, which can be widely applied to other synergistic nanostructures. Associated Content Supporting Information. This includes the schematic illustration of noble metal (NM)/TiO2 heterojunction, SEM and fitted first-shell EXAFS results at the Pt L3-edge and Pd K-edge, respectively. This material is available free of charge via the Internet at http://pubs.acs.org. Author Information Corresponding Authors *[email protected]; [email protected] Notes The authors declare no competing financial interests. Acknowledgement Research at the University of Western Ontario is supported by the Discovery grant of the Natural Science and Engineering Research Council of Canada (NSERC), the Canada Research Chair (CRC) Program, the Canada Foundation for Innovation (CFI), and the Interdisciplinary 24 ACS Paragon Plus Environment

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Initiative (IDI) grant of the University of Western Ontario (UWO). Research at Soochow University is supported by the National Natural Science Foundation of China (Nos. 61675143, 11661131002). The work at the Canadian Light Source (CLS) is supported by CFI, NSERC, the University of Saskatchewan, the Government of Saskatchewan, Western Economic Diversification Canada, the National Research Council Canada, and the Canadian Institute for Health Research (CIHR). We thank Dr. T. Regier, Dr. Q. F. Xiao, Dr. Y. F. Hu and Dr. N. Chen for their technical support at the SGM, SXRMB and HXMA beamlines at CLS. Dr. J. Li acknowledges the receipt of support from the CLS Graduate and Post-Doctoral Student Travel Support Program. References 1. Ge, M. Z.; Cao, C. Y.; Huang, J. Y.; Li, S. H.; Chen, Z.; Zhang, K. Q.; Al-Deyabd, S. S.; Lai, Y. K. A Review of One-Dimensional TiO2 Nanostructured Materials for Environmental and Energy Applications. J. Mater. Chem. A 2016, 4, 6772-6801. 2. Mushtaq, F.; Asani, A.; Hoop, M.; Chen, X. Z.; Ahmed, D.; Nelson, B. J.; Pane, S. Highly Efficient Coaxial TiO2-PtPd Tubular Nanomachines for Photocatalytic Water Purification with Multiple Locomotion Strategies. Adv. Funct. Mater. 2016, 26, 6995-7002. 3. Ye, M. D.; Gong, J. J.; Lai, Y. K.; Lin, C. J.; Lin, Z. Q. High-Efficiency Photoelectrocatalytic Hydrogen Generation Enabled by Palladium Quantum Dots-Sensitized TiO2 Nanotube Arrays. J. Am. Chem. Soc. 2012, 134, 15720-15723. 4. Li, M. J.; Yu, Z. B.; Liu, Q.; Sun, L.; Huang, W. Y. Photocatalytic Decomposition of Perfluorooctanoic Acid by Noble Metallic Nanoparticles Modified TiO2. Chem. Eng. J. 2016, 286, 232-238. 5. Melvin, A. A.; Illath, K.; Das, T.; Raja, T.; Bhattacharyya, S.; Gopinath, C. S. MAu/TiO2 (M = Ag, Pd, and Pt) Nanophotocatalyst for Overall Solar Water Splitting: Role of Interfaces. Nanoscale 2015, 7, 13477-13488. 6. Jiang, X. L.; Fu, X. L.; Zhang, L.; Meng, S. G.; Chen, S. F. Photocatalytic Reforming of Glycerol for H2 Evolution on Pt/TiO2: Fundamental Understanding the Effect of Co-Catalyst Pt and the Pt Deposition Route. J. Mater. Chem. A 2015, 3, 2271-2282. 7. Zhao, G. L.; Kozuka, H.; Sakka, S. Preparation of TiO2 Coating Films Containing Pd Fine Particles by Sol-Gel Method. J. Sol-Gel Sci. Technol. 1995, 4, 37-47. 8. Zielinska-Jurek, A.; Hupka, J. Preparation and Characterization of Pt/Pd-Modified Titanium Dioxide Nanoparticles for Visible Light Irradiation. Catal. Today 2014, 230, 181-187. 9. Li, J.; Sham, T. K.; Ye, Y.; Zhu, J.; Guo, J. Structural and Optical Interplay of PalladiumModified TiO2 Nanoheterostructure. J. Phys. Chem. C 2015, 119, 2222-2230.



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