Shape and Composition Effects on Photocatalytic Hydrogen

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Shape and Composition Effects on Photocatalytic Hydrogen Production for Pt-Pd Alloy Cocatalysts Muhua Luo, Pan Lu, Weifeng Yao, Cunping Huang, Qunjie Xu, Qiang Wu, Yasutaka Kuwahara, and Hiromi Yamashita ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b04388 • Publication Date (Web): 21 Jul 2016 Downloaded from http://pubs.acs.org on July 24, 2016

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ACS Applied Materials & Interfaces

Shape and Composition Effects on Photocatalytic Hydrogen Production for Pt-Pd Alloy Cocatalysts Muhua Luo1#, Pan Lu1#, Weifeng Yao1*, Cunping Huang2, Qunjie Xu1*, Qiang Wu1, Yasutaka Kuwahara3 and Hiromi Yamashita3

1

Shanghai Key Laboratory of Materials Protection and Advanced Materials in Electric Power,

College of Environmental & Chemical Engineering, Shanghai University of Electric Power, Shanghai 200090, P. R. China. 2

Aviation Fuels Research Lab, FAA William J. Hughes Technical Center, Atlantic City

International Airport, NJ 08405, USA. 3

Division of Materials and Manufacturing Science, Graduate School of Engineering, Osaka

University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan.

*

Corresponding author: [email protected] and [email protected]

#

These authors contributed equally to this study and share first authorship

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ACS Paragon Plus Environment


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Abstract The shape and composition effects of platinum-palladium Pt-Pd alloy nanoparticle cocatalysts on visible-light photocatalytic hydrogen evolution from an aqueous ammonium sulphite solution have been reported and discussed. The activity of Pt-Pd nanoparticles loaded Pt-Pd/CdS photocatalysts are affected based on both the Pt-Pd alloy nanoparticles’ shape and their compositions. In this research, two shapes of Pt-Pd nanoparticles have been studied. One is Pt-Pd nanocubes enclosed by {100} crystal planes and the other is nanooctahedra covered with {111} crystal facets. Results show that the photocatalytic turnover frequency (TOF), defined as moles of hydrogen produced per surface mole of Pt-Pd metal atom per second, for Pt-Pd nanocubes/CdS (Pt-Pd NCs/CdS) photocatalyst can be 3.4 times more effective than Pt-Pd nanooctahedra/CdS (Pt-Pd NOTa/CdS) nanocomposite photocatalyst. Along with the shape effect, the atomic ratio of Pt to Pd can also impact the efficiency of Pt-Pd/CdS photocatalysts. When the Pt to Pd atomic ratio changes from 1:0 to about 2:1, the rate of hydrogen production increases from 900 μmol/h for Pt NCs/CdS catalyst to 1837 μmol/h for Pt-Pd (2:1) NCs/CdS photocatalyst—a 104% rate increase. This result suggests that the 33 mol% of more expensive Pt can be replaced with less costly Pd, resulting in a more than 100% hydrogen production rate increase. The finding of this research will lead to the research and development of highly effective catalysts for photocatalytic hydrogen production using solar photonic energy. Key Words: Shape effect, Pt-Pd alloy cocatalysts, CdS, Hydrogen production, Visible light photocatalysis

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1. Introduction Photocatalytic hydrogen production from water is an important process for new energy development. To reduce charge recombination and over potential of proton reduction, platinum and other precious metal cocatalysts are required to be loaded on the surface of main photocatalysts to construct metal/semiconductor interfaces.1-6 Pt nanoparticles (NPs) are the most studied cocatalyst, and are also the best hydrogen evolution reaction (HER) catalyst in electrocatalytical hydrogen production systems.1, 6-7 Due to high cost and limited reserves, the performance improvement of platinum NPs to reduce its loading requires long term research efforts. In photocatalytic hydrogen production only limited research results on the activity enhancement of Pt NPs can be found in literature. In contrast, however, to increase the electrocatalytic activity of Pt catalysts so as to reduce the amount of high cost Pt loading have long been a research front.8-12 Pt alloy based catalysts are found to significantly reduce the amount of high cost Pt loading since the replacement of platinium by a less costly alloy can exhibit superior performance in comparison to monometallic Pt catalysts. Pt-Pd alloy nanostructures are especially favourable for enhancing platinum activities

8-9, 12-14

. Platinum and palladium bimetallic

nano-dendrites have demonstrated higher efficiencies for oxygen reduction reaction than widely used platinum catalysts due to their crystalline structure and high surface area.9 It should be pointed out that the efficiency of a precious metal/semiconductor photocatalyst during a hydrogen production process can be determined by the activity of the metal cocatalyst. Our previous research 15-17 has indicated that with higher electrocatalytic activity of Pt NPs, Pt loaded CdS (Pt/CdS) photocatalyst for photocatalytic hydrogen evolution is also higher. Since the performance of Pt-Pd alloy nanostructures depends on shape and composition of Pt-Pd NPs, research into finding high efficiency Pt-Pd alloy cocatalysts has accelerated to find an approach to increase photocataltic activity of Pt-Pd/CdS photocatalyst while also reduce platinum consumption for hydrogen production. This paper reports the preparation and characterizations of two shapes of Pt-Pd alloy nanoparticle cocatalysts and their applications for photocatalytic H2 evolution. Pt-Pd alloy nanocubes (NCs) are covered by {100} crystal facets and nanooctahedra (NOTa) are covered by {111} planes that are deposited onto a commercial CdS photocatalyst, forming Pt-Pd (NCs)/CdS 3



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and Pt-Pd (NOTa)/CdS photocatalysts. Along with hydrogen production rate, photocatalytic turnover frequency (TOF), defined as hydrogen production per surface Pt-Pd metal atom per second, is used to evaluate the efficiencies of Pt-Pd loaded CdS for H2 production via photocatalytic oxidation of ammonium sulphite solution.

2. Experimental 2.1 Synthesis of Pt-Pd nanocubes (NCs) and nanooctahedra (NOTa). Pt-Pd alloy NCs and NOTa were synthesized using a method reported by Huang et al. 12 For the preparation of 1:1 Pt to Pd atomic ratio Pt-Pd (1:1) NCs, stoichiometric of Na2PdCl4 and K2PtCl4 were dissolved in 10 mL DMF with a certain amount of KI and PVP as the capping agents. After sonication for 10 minutes the solution was heated at 130 ºC for 5 hours. The samples were then centrifuged to separate the solution from the solid samples. The solid Pt-Pd nanoparticles were washed several times with ethanol and cyclohexane. Complete removal of PVP was achieved using the NaBH4 and TBA treatment method, which our group had developed previously.17 Pt-Pd NCs with Pt to Pd atomic ratios of 2:1, 3:1 and 1:2 were synthesized separately by adjusting the molar ratio of Na2PdCl4 and K2PtCl4 precursors. The synthesis of Pt-Pd NOTa is similar to those of Pt-Pd NCs, except that 0.5 mmol KI solution was replaced with 0.5 mmol NaCl solution. 2.3 Preparation of Pt-Pd/CdS composite photocatalysts. 0.25 mg prepared Pt-Pd NCs or Pt-Pd NOTa was added to 100 mL deionized water containing 0.05 g commercial CdS photocatalyst. The resulting solution was stirred for 2 hours at room temperature. After centrifugation, the obtained powders were washed and dried at 60 °C. UV-Vis absorption spectra indicate that the Pt-Pd nanoparticles were completely deposited onto cadmium sulfide particles (Fig. S1, Supporting Information). The (Pt-Pd) NPs’ loading was 0.5 wt.%, Pt-Pd mass divided by the total mass of CdS and Pt-Pd NPs. 2.3 Visible light photocatalytic hydrogen production. The visible light photocatalytic H2 production was achieved using a photocatalytic system (Perfectlight Co., Labsolar-III). In detail, 0.05 g prepared Pt-Pd/CdS photocatalyst was added into a 100 mL aqueous (NH4)2SO3 solution (1.0 M) and then transferred to the photocatalytic H2 production system. The photolyte was then vacuum-degassed for 30 min. A 300 W Xe lamp was

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used as a visible light source using an optical UV cut-off filter (> 420 nm). H2 evolution was determined using an on-line gas chromatograph. 2.4. Photoelectrochemical characterization. The photocurrent measurements were taken using the procedure reported by W. Choi et al.18-19 A three-electrode assembly was selected to detect the photocurrent generated by the prepared Pt-Pd/CdS photocatalysts. A glassy carbon electrode (GCE) with 0.07 cm2 surface area and a saturated calomel electrode (SCE) were used as the working electrode and the reference electrode, respectively. A large size Pt foil was used as the counter electrode. Aqueous FeCl3 (1.0 mM) was used as the electron shuttle. 10.0 vol.% methanol was used as the electron donor. The electrolyte was an aqueous solution consisting of 0.1 M NaNO3 and 0.1 M HClO4. The electrolyte was purged with N2 gas before and during the experiment. (Pt-Pd)/CdS photocatalyst powder was suspended in the electrolyte and magnetically stirred during the photocurrent measurements. The light source was a 300 W Xe light with a UV light filter (λ > 420 nm). The light source was the same as that used in photocatalytic hydrogen production. The electrical bias potential for photocurrent measurements was + 0.6 V vs. SCE. 2.5. Materials Characterization. The materials and catalyst characterizations can be found in detail in the supporting information.

3. Results and discussion 3.1 Morphologies and structures of (Pt-Pd) nanocrystals. Fig. 1 and Fig. S2 (Supporting Information of this paper) show typical TEM images for Pt-Pd alloy nanocubes. The Pt-Pd nanocubes with 1:1 platinum to palladium atomic ratio (1:1 Pt-Pd NCs) are comprised of uniform cubic nanocrystals (Fig 1A). The shape selectivity of the nanocubes is about 93%. A single Pt-Pd cubic nanoparticle HRTEM reveals that the particle is a single crystal with defined fringes (Inset of Fig. 1A). The edge lattice spacing of the crystal is 0.193 nm, which agrees with the interplanar spacing of {200} of the face-centered cubic (f.c.c) Pt or Pd crystals. This observation suggests that Pt-Pd alloy NCs are covered by {100} planes. By measuring over 200 nanoparticles in different zones of TEM images, the average edge length of the Pt-Pd nanocubes was determined to be 7.3 nm. The histogram of nanocubes for the prepared 1:1 Pt-Pd

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NCs is shown in Fig 1B. Energy dispersive spectroscopic (EDS) analysis indicates that the Pt-Pd nanocubes contain both platinum and palladium atoms. The concentration profiles of platinum and palladium crossing a nanocube suggests that platinum and palladium atoms exist homogeneously in the particles (Fig. 1D). This observation suggests that by varying the molar ratio of K2PtCl4 to Na2PdCl4 we are able to prepare (Pt-Pd) alloyed NCs with Pt to Pd molar ratios of 2:1, 3:1 and 1:2. The measurements show that despite the difference in the Pt to Pd atomic ratios the average edge lengths of Pt-Pd NCs are very close to each other. For example, the average edge lengths for the 1:2 Pt-Pd NCs and 2:1 Pt-Pd NCs are 7.9 nm and 7.7 nm, respectively (Fig. S2, Supporting Information). The synthesis process for {111} crystal plane bounded Pt-Pd NOTa is similar to the preparation of Pt-Pd NCs except that NaCl is used as a capping agent. TEM images (Fig. 2A) show the dispersive (Pt-Pd) nanooctahedral shaped particles prepared under this research condition. The shape selectivity of octahedral morphology is 78%. The size distribution histogram of the prepared (Pt-Pd) NOTa is illustrated in Fig 2B. The average edge length of Pt-Pd NOTa is 4.7 nm. The HRTEM image (Inset in Fig. 2A) reveals that the prepared Pt-Pd NOTa are single crystals. The interplanar edge spacing of an octahedron is 0.228 nm, which is close to {111} plane of Pt (0.226 nm) or Pd crystals (0.22 nm). The formation of Pt-Pd alloy NOTa is also confirmed by the EDS analysis (Fig. 2C). The Pt and Pd concentration line-scanning profiles crossing an octahedral Pt-Pd nanocrystal are shown in Fig. 2D. Both Fig. 2C and Fig. 2D indicate that the particles are comprised of both platinum and palladium atoms. The mechanism of Pt-Pd nanocrystal formation with different enclosed facets is attributed to the different chemisorption energy of halides on a single-crystalline metal surface. The absorption order is I‾ > Br‾ > Cl‾.20-21 Different addition of halide ions leads to selective adhesion or activation of certain facets, resulting in the formation of different shaped Pt-Pd nanocrystals.12 XRD patterns agree well with the standard face centered cubic (f.c.c.) diffraction patterns of pure platinum and palladium crystals. As shown in Fig. 3, the Pt-Pd nanocrystals have peaks at diffraction angles (2θ) of 39.9°, 46.4°, 68.0° and 81.7°, which are the characteristic diffraction peaks (111), (200), (220) and (311) for the f.c.c. Pt-Pd alloys crystalline, respectively.

22-23

The

formation of Pt-Pd nanostructures with different Pt to Pd molecular ratios is facilitated by the minimal lattice mismatch (0.77%) between Pd and Pt.13 The XRD peak diffraction angles of Pt-Pd 6


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nanocrystals are almost overlapped when the Pt to Pd molar ratio changes from 2:1 to 1:2. The broadened XRD peaks widths of the samples indicate that the prepared Pt-Pd alloys are ultra-fine nanoparticles. 3.2 EDS, XPS and EXAFS analyses for Pt-Pd nanocrystals after photocatalytic hydrogen production The obtained Pt-Pd alloy nanocrystals were dissolved in 100 mL pure water containing a certain amount of commercial CdS powder. The CdS average particle size was 45.6 nm with 15.9 m2/g BET surface area. The suspended water was stirred at room temperature for 2 h for Pt-Pd nanocrystals to deposit on the CdS surface. The complete attachment of Pt-Pd nanocrsytals was confirmed by a reported UV-Vis absorption method previously reported (Figure S1, Supporting Information).24 Fig. 4 shows a typical TEM image of the CdS photocatalysts loaded with 0.5 wt.% Pt-Pd (1:1) NCs. The Pt-Pd alloy particle was confirmed using EDS analysis for a darker particle in the TEM image. The TEM analysis further confirms that the shapes and sizes of Pt-Pd nanoparticles remain unchanged after 20 hours of photocatalytic hydrogen production. The same results were observed for Pt-Pd NOTs on CdS (Fig. S3, Supporting Information).These results indicate that the Pt-Pd loaded CdS is photochemically stable under the photocatalytic condition. In this research, both X-ray photoelectron spectroscopy (XPS) and extended X-ray absorption fine structure (EXAFS) spectra were applied in the analyses of Pt-Pd/CdS photocatalysts. Fig. 5 depicts the XPS spectra of Pt 4f and Pd 3d derived from Pt-Pd (1:1) NCs/CdS photocatalyst after 20 hours of photocatalytic reaction. Results indicate that the Pt 4f7/2 and Pt 4f5/2 bands peaked at 71.4 and 74.7 eV, respectively, which are in good agreement with 70.5 to 72.0 eV for Pt-Pd binary alloy systems.25 The maximum Pd 3d5/2 and 3d3/2 binding energies of Pt-Pd alloy particles are 335.8 and 341.0 eV corresponding to metal state Pd.26 The XPS analysis results confirm that Pt or Pd in Pt-Pd NCs/CdS photocatalysts are not in an oxidized state after photocatalytic hydrogen production. The same observation was also confirmed by EXAFS spectral analysis. (Figure S4, Supporting Information) 3.3 Activities of Pt-Pd/CdS photocatalysts The visible light (λ > 420 nm) photocatalytic hydrogen production for Pt-Pd/CdS photocatalysts was carried out using an aqueous ammonium sulphite solution as a sacrificial reagent. Previous results have confirmed that CdS photocorrosion can be avoided in the photocatalytic 7



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oxidation of (NH4)2SO3 solution.24, 27-28 Experimental results show that no photocatalytic activity of Pt-Pd/CdS under the dark condition was observed. Fig. 6 shows visible light (λ > 420 nm) photocatalytic H2 evolution over Pt-Pd NCs/CdS and Pt-Pd NOTa/CdS photocatalysts. The rate of H2 production over bare CdS is almost zero compared to those for Pt-Pd/CdS photocatalysts. As shown in Fig. 6, the rate of H2 production over Pt-Pd (1:1) NCs/CdS and Pt-Pd (1:1) NOTa/CdS are 1583 µmol/h and 1073 µmol/h, respectively. The quantum efficiency of Pt-Pd (1:1) NCs/CdS (54.0% at 420 nm) is 1.48 times higher than that of Pt-Pd (1:1) NOTa/CdS catalyst (36.6% at 420 nm) at the same 0.5 wt.% Pt-Pd metal loading. The catalytic activities of Pt-Pd/CdS photocatalysts can be normalized to the turnover frequency (TOF) defined as specific hydrogen production rate per surface Pt or Pd metal atom per second. As pointed out by Huang and coworkers, the shape and composition effects of Pt-Pd electrocatalysts can be directly compared using TOF results.12 Detailed TOF calculations for Pt-Pd NCs/CdS and Pt-Pd NOTa/CdS catalysts are listed in the supporting information of this paper. As shown in Fig. 6B and Table 1, the TOF of Pt-Pd (1:1) NCs/CdS (3.28 S-1) is about 3.4 times greater than that of Pt-Pd (1:1) NOTa/CdS photocatalyst (0.97 S-1). This result suggests Pt-Pd NCs/CdS catalysts are more effective for photocatalytic hydrogen production than Pt-Pd NOTa/CdS catalyst. Fig. 7 compares the life span of Pt-Pd (1:1) NCs/CdS and Pt-Pd (1:1) NOTa/CdS photocatalysts. In four photocatalytic hydrogen production cycles, the hydrogen evolution rates are almost identical. During 20 hours of reaction a total of 30.1 mmol H2 was produced over Pt-Pd (1:1) NCs/CdS photocatalyst. The turnover numbers, defined as the molar ratio of the total hydrogen evolved (30.1 mmol) per mole of CdS photocatalyst (0.346 mmol) and per mole of Pt-Pd cocatalysts (1.66 µmol), are 87 and 18133, respectively. The high turnover numbers indicate that the hydrogen produced resulted from the photocatalytic reduction of water during the oxidation of the aqueous ammonium sulfite solution rather than from the photooxidation of CdS photocatalyst or Pt-Pd cocatalyst. Table 1 summarizes the effects of shapes and compositions of Pt-Pd alloys for Pt-Pd/CdS catalysts. To illustrate both effects, Fig. 8 depicts hydrogen evolution rates from both Pt-Pd NCs/CdS and Pt-Pd NOTa/CdS. The effects of shapes and compositions of Pt-Pd alloy cocatalysts on TOFs are shown in Fig. 9. Results show much higher hydrogen production rates and TOFs of Pt-Pd NCs/CdS photocatalysts than those of Pt-Pd NOTa/CdS photocatalyst. The TOFs are 4.09, 8


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4.52, 3.28, and 2.56 S-1 for the Pt to Pd molar ratios of 3:1, 2:1, 1:1 and 1:2 for Pt-Pd NC/CdS, respectively. In contrast, the highest TOFs value of the Pt-Pd NOTa loaded CdS is only 1.5 S-1 (Fig. 9 and Table 1). At a same Pt to Pd molar ratio, the TOFs for Pt-Pd NC/CdS are greater than those for Pt-Pd NOT loaded CdS. As shown in Fig. 8 and Fig. 9, both hydrogen rate and TOF curves peak at about 2:1 Pt to Pd atomic ratio. This result indicates that when 33 mol% of Pt is replaced with Pd, the rate of H2 evolution for Pt-Pd NCs/CdS increases from 900 μmol/h for Pt NCs/CdS catalyst to 1837 μmol/h for Pt-Pd (2:1) NCs/CdS photocatalyst, more than a 104% rate increase (Fig. 8). Similarly, the TOF increases from 2.72 S-1 for Pt NC/CdS to 4.52 S-1 for Pt-Pd (2:1)/CdS, more than a 66% increase (Fig. 9). All these results are achieved at a 33 mol% Pt loading reduction. The composition effect of Pt-Pd NCs can be explained with the volcano curve of hydrogen adsorption energy for Pt-Pd alloys

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The main function of a Pt based cocatalyst during

photocatalytic hydrogen production is the adsorption of protons (H+(aq)) from a photolyte. The adsorbed protons receive electrons from Pt particles and are reduced to hydrogen atoms (H(ad)). Two hydrogen atoms form a hydrogen molecule (H(ad) + H(ad) = H2(g)). If a cocatalyst does not adsorb protons the hydrogen production rate will be close to zero. But when a cocatalyst has very strong hydrogen adsorption energy, the hydrogen atoms (H(ad)) will have difficulty in combining to form gaseous hydrogen. Therefore a lower hydrogen production rate is the result. The reason that Pt is the most desirable catalyst for almost all hydrogen related applications is that among metal based catalysts hydrogen adsorption energy for Pt is an optimal value. This optimized hydrogen adsorption energy determines its higher Pt electrode exchange current density, resulting in a higher hydrogen evolution rate for a photocatalytic process. Pd is also a very desirable hydrogen evolution catalyst but the rate of hydrogen evolution is lower than that of Pt. The addition Pd to Pt to form Pt-Pd alloy may further fine-tune alloy’s hydrogen adsorption free energy and lead to an improved hydrogen evolution rate. As indicated in Fig. 8 and Fig.9, when Pd mol.% is at about 30% (Pt to Pd molar ratio of 2:1), both hydrogen evolution rate and turn over frequency reach their maximum values. Based on the discussion above, the involvement of Pd optimizes Pt hydrogen adsorption energy and finally promotes the hydrogen rate. Detailed discussion must rely on density function calculation and is beyond the scope of this paper. Fig. 8 and 9 also shows the shape effect of Pt-Pd nanoparticles at the same Pt to Pd atomic ratio. Hydrogen evolution rates for Pt-Pd (1:1) NCs/CdS and Pt-Pd (1:1) NOTa/CdS are 1583 μmol/h and 9



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1073 μmol/h, respectively. This result indicates that when Pt-Pd (1:1) alloy nanoparticles change from octahedron to cubic shape the activity of Pt-Pd/CdS increase by 47.5%. Interestingly, the change of TOF from Pt-Pd (1:1) NOTa/CdS to Pt-Pd (1:1) NCs/CdS is much more significant than the increase of the hydrogen production rate. As shown in Table 1 and Fig. 9, the TOF for Pt-Pd (1:1) NOTa/CdS and Pt-Pd (1:1) NCs/CdS increase from 0.97 S-1 to 3.28-1, a 3.4 times increase. The TOF of Pt-Pd (1:1) NOTa/CdS (0.97 S-1) is even lower than that of Pt NCs/CdS (2.72 S-1) and Pd NCs/CdS (1.75 S-1) photocatalysts under the same reaction condition (Fig.9). This is due to the higher surface atom density of {111} than that for {100}. These experimental results further indicated that {100} crystal plane surrounded Pt-Pd NCs/CdS exhibits much higher activity than the {111} facet terminated Pt-Pd NOTa/CdS for photocatalytic hydrogen production. The higher photocatalytic efficiency of Pt-Pd NCs/CdS is also ascribed to the higher activity of {100} planes, which was observed in the catalytic hydrogenation reactions with Pt-based catalysts.10, 12 The different TOFs of Pt-Pd NCs as functions of the Pt to Pd atomic ratio may, however, be the result of electronic coupling between platinum and palladium atoms.11 A higher TOF of Pt-Pd NCs also may indicate higher charge separation efficiency. To understand the influence of Pt-Pt loading on the rate of electron transfer at the interface of CdS and photolyte, photocurrents generated by Pt-Pd/CdS photocatalysts were determined using Fe3+(aq) ion, acting as an electron shuttle and methanol as a hole scavenger. As shown in Fig. 10, the photocurrent for Pt-Pd (1:1) NCs/CdS is significantly greater than that of Pt-Pd (1:1) NOTa/CdS. The much higher photocurrents of Pt-Pd/CdS than those of bare CdS photcatalysts indicate that the Pt-Pd nanocrystals on CdS decrease the electron-hole recombination rate by trapping electrons in the Pt-Pd alloy particles and subsequently accelerating the interfacial electron transfer from Pd-Pt alloy to Fe3+ in the solution. As indicated in Fig. 10, the sequence of photocurrent increase follows the order of Pt-Pd (1:1) NCs/CdS > Pt-Pd (1:1) NOTa/CdS >> bare CdS. This suggests that the interfacial electron transfer from CdS to Pt-Pd nanoparticles is more efficient on Pt-Pd (1:1) NCs/CdS than on Pt-Pd (1:1) NOTa/CdS. The higher rate of efficient interfacial electron transfer indicates the recombination rate of electrons and holes has been reduced. Comparing with Pt-Pd NOTa/CdS, this can explain the enhanced photocatalytic efficiency of Pt-Pd NCs/CdS for hydrogen production. The higher rate of electron transfer from CdS to Pt-Pd also shows that the impedance between the interfaces of Pt-Pd NCs and CdS is lower than that of Pt-Pd NOTa and CdS. The lower 10


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impedance in the interface of Pt-Pd (1:1) NCs and CdS results from the cubic shaped nano Pt-Pd alloy particles.

Conclusions Highly uniform Pt-Pd nanocubes (Pt-Pd NCs) exposed with {100} crystal planes and Pt-Pd nanooctahedra (Pt-Pd NOTa) covered by {111} crystal facets were synthesized via a facile hydrothermal method. The photochemical activity and the stabilities of Pt-Pd alloy nanocubes/CdS and Pt-Pd nanoocthedra/CdS for solar photocatalytic hydrogen production were investigated based on the photocatalytic oxidation of (NH4)2SO3 in water. The results showed that all the Pt-Pd alloy nanocrystals loaded Pt-Pd/CdS photocatalysts are active for visible light photocatalytic H2 production. The catalytic activities Pt-Pd/CdS photocatalysts depend highly on both the shape and composition of platinum and palladium alloy nanoparticles. Pt-Pd nanocubes (covered by {100} crystal facets) loaded Pt-Pd NCs/CdS photocatalysts demonstrate much higher activity than the {111}-facet-bounded Pt-Pd nanooctahedra loaded Pt-Pd NOTa/CdS photocatalyst. This fundamental research points to a new approach in the research and development of less expensive, but high performing, cocatalysts for photocatalytic solar H2 production.

Acknowledgments The National Natural Science Foundation of China (21103106, 21107069), Shanghai Key Project for Fundamental Research (13JC1402800), the “Dawn” Program of Shanghai Education Commission (11SG52) and Science and Technology Commission of Shanghai Municipality (14DZ2261000) support this research and are appreciated.

Supporting Information Available: Additional figures (PDF) and TOF calculations.

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Yang, J.; Wang, D.; Han, H.; Li, C., Roles of Cocatalysts in Photocatalysis and

Photoelectrocatalysis. Acc. Chem. Res. 2013, 46 (8), 1900-1909. 7.

Zou, X.; Zhang, Y., Noble Metal-free Hydrogen Evolution Catalysts for Water Splitting. Chem.

Soc. Rev. 2015, 44 (15), 5148-5180. 8.

Lee, H.; Habas, S. E.; Somorjai, G. A.; Yang, P., Localized Pd Overgrowth on Cubic Pt

Nanocrystals for Enhanced Electrocatalytic Oxidation of Formic Acid. J. Am. Chem. Soc. 2008, 130 (16), 5406-5407. 9.

Lim, B.; Jiang, M.; Camargo, P. H. C.; Cho, E. C.; Tao, J.; Lu, X.; Zhu, Y.; Xia, Y., Pd-Pt

Bimetallic Nanodendrites with High Activity for Oxygen Reduction. Science 2009, 324 (5932), 1302-1305. 10. Tsung, C. K.; Kuhn, J. N.; Huang, W.; Aliaga, C.; Hung, L. I.; Somorjai, G. A.; Yang, P., Sub-10 nm Platinum Nanocrystals with Size and Shape Control: Catalytic Study for Ethylene and Pyrrole Hydrogenation. J. Am. Chem. Soc. 2009, 131 (16), 5816-5822. 11. Zhou, W. P.; Yang, X.; Vukmirovic, M. B.; Koel, B. E.; Jiao, J.; Peng, G.; Mavrikakis, M.; Adzic, R. R., Improving Electrocatalysts for O2 Reduction by Fine-tuning the Pt-support Interaction: Pt Monolayer on The Surfaces of A Pd3Fe (111) Single-crystal Alloy. J. Am. Chem. Soc. 2009, 131 (35), 12755-12762. 12. Huang, X.; Li, Y.; Li, Y.; Zhou, H.; Duan, X.; Huang, Y., Synthesis of PtPd Bimetal Nanocrystals with Controllable Shape, Composition, and Their Tunable Catalytic Properties. Nano Lett. 2012, 12 (8), 4265-4270. 13. Habas, S. E.; Lee, H.; Radmilovic, V.; Somorjai, G. A.; Yang, P., Shaping Binary Metal Nanocrystals Through Epitaxial Seeded Growth. Nature Mater. 2007, 6 (9), 692-697. 14. Li, X.; Liu, Y.; Hemminger, J. C.; Penner, R. M., Catalytically Activated Palladium@platinum Nanowires for Accelerated Hydrogen Gas Detection. ACS Nano 2015, 9 (3), 3215-3225. 15. Luo, M.; Yao, W.; Huang, C.; Wu, Q.; Xu, Q., Shape Effects of Pt Nanoparticles on Hydrogen Production via Pt/CdS Photocatalysts Under Visible Light. J. Mater. Chem. A 2015, 3 (26), 13884-13891. 16. Luo, M.; Yao, W.; Huang, C.; Wu, Q.; Xu, Q., Shape-controlled Synthesis of Pd Nanoparticles for Effective Photocatalytic Hydrogen Production. RSC Adv. 2015, 5 (51), 40892-40898. 17. Luo, M.; Hong, Y.; Yao, W.; Huang, C.; Xu, Q.; Wu, Q., Facile Removal of Polyvinylpyrrolidone (PVP) Adsorbates From Pt Alloy Nanoparticles. J. Mater. Chem. A 2015, 3 (6), 2770-2775. 18. Kim, H.-i.; Kim, J.; Kim, W.; Choi, W., Enhanced Photocatalytic and Photoelectrochemical

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13



35

A

B

Frequency (%)

30 25 20 15 10 5 0 5

6

7

8

9

10

Particle size (nm) Cu

D

C C

Pt

Cu

Cu Pt Pt

Pd

0 0

Pd Pt

D Intensity/a.u

C

Counts / a. u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 23

5

10

Pd

Pt

15

Energy / KeV

20

5nm

25

2

4

6

8

10

12

14

Position/nm

Fig. 1. Morphology and structural analyses for Pt-Pd (1:1) alloy nanocubes (Pt-Pd (1:1) NCs). (A) TEM and HRTEM images, (B) nanoparticle size distribution, (C) selected area energy dispersive spectroscopy and (D) line-scanning Pt and Pd concentration profiles across a cubic Pt-Pd particle.

14


16

Page 15 of 23

35

A

B

Frequency(%)

30 25 20 15 10 5 0

3.2

4.0

4.8

5.6

6.4

Particle size (nm)

C

Pd Pt

D

Cu

C

Intensity/a.u

Counts / a. u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Pt Cu

Cu Pt

Pd

0 0

5

10

Pt Pt

Pd

15

Energy / KeV

20

5nm

25

2

4

6

8

10

12

Position/nm

Fig. 2. Morphology and structural analyses for Pt-Pd (1:1) alloy nanooctahedra (Pt-Pd (1:1) NOTa). (A) TEM and HRTEM images, (B) nanoparticle size distribution, (C) selected area energy dispersive spectroscopy and (D) line-scanning Pt and Pd concentration profiles across a cubic Pt-Pd particle.

15



(111)

Pt-Pd(2:1) NCs Pt-Pd(1:2) NCs Pt-Pd(1:1) NCs Pt-Pd(1:1) NOTa (200)

Intensity / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 23

(220)

(a)

(311) (222)

Pt-Pd(2:1) NCs

(b)

Pt-Pd(1:2) NCs

(c)

Pt-Pd(1:1) NCs

(d)

Pt-Pd(1:1) NOTs

0 30

40

50

60

70

80

90

2 θ / degree

Fig. 3. XRD patterns of prepared Pt-Pd (2:1) nanocubes (NCs) (a), Pt-Pd (1:2) NCs (b), Pt-Pd (1:1) NCs (c) and Pt-Pd (2:1) nanooctahedra (NOTa) (d).

16


Page 17 of 23

A

B

C C

Counts / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Cd S Cu

Pd Cu 0 0

5

Cu Pt Pt 10

Pd 15

20

Cd

Cd 25

30

Energy / KeV

Fig. 4. TEM images of (1:1) Pt-Pd NCs/CdS before (A) and after (B) photocatalytic hydrogen production; (C) Selected area energy dispersive spectroscopic (EDS) of a single metallic particle.

17



Pt 4f7/2

A

B

Pd 3d5/2

Intensity (a.u.)

Pt 4f5/2

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 23

64

66

68

70

72

74

76

Binding Energy (eV)

78

80

82

Pd 3d3/2

332

334

336

338

340

342

344

346

Binding Energy (eV)

Fig. 5. XPS spectra of Pt 4f (A) and Pd 3d (B) in Pt-Pd (1:1) NCs/CdS photocatalyst collected after visible-light-induced (λ > 420 nm) photocatalytic hydrogen production.

18


4

8000

Pt-Pd(1:1) NCs/CdS Pt-Pd(1:1) NOTs/CdS CdS

A

Pt-Pd (1:1 ) NCs/CdS

B

3 -1

TOF (S )

6000

4000

2000

2

Pt-Pd (1:1 )NOTa/CdS

1

0

0

0

1

2

3

4

5

Irradiation Time (hour)

Fig. 6 (A) Irradiation time course for H2 evolution over Pt-Pd (1:1) NCs/CdS, Pt-Pd (1:1) NOTa/CdS and bare CdS photocatalysts. (B) The photocatalytic turnover frequency (TOF) of Pt-Pd NCs/CdS and Pt-Pd NOTa/CdS photocatalysts.

Pt-Pd (1:1) NCs/CdS Pt-Pd (1:1) NOTa/CdS

8000

Hydrogen Production (µ mol)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Hydrogen Production (µmol)

Page 19 of 23

First run

Second run

Third run

Fourth run

6000

4000

2000

0 0

2

4

6

8

10 12 14 16 18 20

Irradiation Time (h) Fig. 7. Life spans of Pt-Pd (1:1) NCs/CdS and Pt-Pd (1:1) NOTs/CdS photocatalysts.

19



Hydrogen Production Rate (µ µ mol/h)

1900 1700

Pt-Pd NCs/CdS

1500 1300 1100 900

Pt-Pd NOTa/CdS 700 500 300 0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

90.0

100.0

Pd mol.%

Fig. 8. Shape and Pd concentration effects on the hydrogen production rate

5.00

Turnover Frequency (TOF) (1/sec)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 23

4.50 4.00 3.50

Pt-Pd NCs/CdS 3.00 2.50 2.00 1.50

Pt-Pd NOTa/CdS

1.00 0.50 0.00 0

10

20

30

40

50

60

70

80

90

100

Pd mol.%

Fig.9. Turnover frequency of Pt-Pd NCs/CdS and Pt-Pd NOTa/CdS photocatalysts

20


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Photocurrent Density (µ A/cm2)

Page 21 of 23

300

(0.5 wt%)Pt-Pd(1:1) NCs/CdS

200

(0.5 wt%)Pt-Pd(1:1) NOTa/CdS

100 Light on

Pure CdS

0 0

500

1000

1500

Time (sec) Fig. 10. Electron shuttle-mediated photocurrent measurements in the suspensions of CdS, Pt-Pd (1:1) NCs/CdS and Pt-Pd (1:1) NOTa/CdS. (Conditions: Catalysts concentration: 0.5 g/L photolyte. Aqueous photolyte composition: 0.1M NaNO3, 1.0 mM FeCl3 and 10 vol.% CH3OH. Light Source: 300W Xe light with a cutoff filter (λ > 420 nm); Bias potential: 0.6V (vs. SCE)).

21



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 23

Table 1. Properties of Pt-Pd nanoparticles and photocatalytic activities of Pt-Pd/CdS photocatalysts Mole of Surface atoms (mol)

H2 Evolution Rate of Pt-Pd/CdS (µmol/h)

TOF （S-1）

Catalyst

Shape

Average Edge Length (nm)

Pt NCs*

Cube

8.2

1.84*10-7

900

2.72

Pt-Pd (3:1) NCs

Cube

7.3

2.21*10-7

1629

4.09

Pt-Pd (2:1) NCs

Cube

7.9

2.26*10-7

1837

4.52

Pt-Pd (1:1) NCs

Cube

7.3

2.68*10-7

1583

3.28

Pt-Pd (1:2) NCs

Cube

7.7

2.82*10-7

1301

2.56

Pd NCs#

Cube

8.9

3.11*10-7

814

1.45

Pt-Pd (3:1) NOTa

Octahedron

4.9

5.16*10-7

1131

1.22

Pt-Pd (2:1) NOTa

Octahedron

5.2

5.08*10-7

1369

1.50

Pt-Pd (1:1) NOTa

Octahedron

4.7

6.17*10-7

1073

0.97

Pt-Pd (1:2) NOTa

Octahedron

5.3

6.06*10-7

951

0.87

*

Reported data J. Mater. Chem. A 2015, 3, 13884. #Reported data RSC Adv. 2015, 5, 40892.

22


Page 23 of 23

TOC graphic

-1

Turnover Frequency (TOF) (S )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4

Pt-Pd (1:1 ) NanoCubes loaded CdS

3

2

1

Pt-Pd (1:1) NanoOctahedra loaded CdS

0

23


Shape and Composition Effects on Photocatalytic Hydrogen

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