Ti3+-Promoted High Oxygen-Reduction Activity of Pd Nanodots

One-dimensional nanocrystals favoring efficient charge transfer have attracted enormous attentions, and conductive nanobelts of black titania with a u...
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Ti3+-Promoted High Oxygen-Reduction Activity of Pd Nanodots Supported by Black Titania Nanobelts Xiaotao Yuan,†,‡ Xin Wang,†,‡ Xiangye Liu,‡ Hongxin Ge,‡ Guoheng Yin,§ Chenlong Dong,‡ and Fuqiang Huang*,‡,§ ‡

State Key Laboratory of Rare Earth Materials Chemistry and Applications and Beijing National Laboratory for Molecular Sciences, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, People’s Republic of China § State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, People’s Republic of China S Supporting Information *

ABSTRACT: One-dimensional nanocrystals favoring efficient charge transfer have attracted enormous attentions, and conductive nanobelts of black titania with a unique band structure and high electrical conductivity would be interestingly used in electrocatalysis. Here, Pd nanodots supported by two kinds of black titania, the oxygen-deficient titania (TiO2−x) and nitrogen-doped titania (TiO2−x:N), were synthesized as efficient composite catalysts for oxygenreduction reaction (ORR). These composite catalysts show improved catalytic activity with lower overpotential and higher limited current, compared to the Pd nanodots supported on the white titania (Pd/TiO2). The improved activity is attributed to the relatively high conductivity of black titania nanobelts for efficient charge transfer (CT) between Ti3+ species and Pd nanodots. The CT process enhances the strong metal−support interaction (SMSI) between Pd and TiO2, which lowers the absorption energy of O2 on Pd and makes it more suitable for oxygen reduction. Because of the stronger interaction between Pd and support, the Pd/TiO2−x:N also shows excellent durability and immunity to methanol poisoning. KEYWORDS: black titania nanobelts, charge transfer, oxygen reduction reaction, Pd nanodots, strong metal−support interaction (SMSI)

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INTRODUCTION One-dimensional nanomaterials, such as nanobelts, nanorods, nanotubes and nanowires, have attracted a lot of attention because of their dimensional confinement, high specific surface area and fast charge transfer efficiency.1−4 Among these materials, black titania nanobelts has shown promising application in photovoltaics,5,6 photocatalysis,7−10 and microwave absorption11,12 because of its special band structure and relatively high conductivity. Compared to the white titania, the black titania contains a vacancy band under the conduction band, which makes TiO2 have light absorption in the visible light and the near-infrared regions.13−15 The vacancy band is mainly contributed by Ti3+ which is accompanied by the formation of oxygen vacancy.16 The self-doping of Ti3+ endows black TiO2 with relatively high carrier density compared to the white TiO2, as presented in our previous works.10,17−19 In this respect, the black TiO2 can be a promising catalyst support for electrocatalysis. Besides good electrical conductivity, the Ti3+ can also enhance the strong metal−support interaction (SMSI) between the metal catalyst and TiO2.20 The SMSI is thought to originate from the electron transfer between Ti3+ and metal nanodots,21 which can be detected by the spectroscopy measurements.22 There are more Ti3+ sites exiting in black © 2016 American Chemical Society

TiO2 than white TiO2. So the black TiO2 is supposed to be a more suitable catalyst support than the white TiO2. To verify the special interaction between noble metals and black TiO2, we synthesized the Pd nanodots supported on black titania nanobelts as an oxygen reduction reaction (ORR) catalyst. Titania supported Pd nanodots has been used as an ORR catalyst before, however, the intrinsic titania shows poor conductivity. Because of this, the TiO2−C or TiO2−Ti composites were used as supports.23,24 However, it is hard to tell the pivotal role of TiO2 in these composites, because there are carbon and metallic titanium in them. And these composites did not change the interaction between Pd and TiO2. As mentioned above, we think the black titania will be a more suitable support for Pd nanodots in ORR. The defective black titania (TiO2−x) was prepared by a special Al-reduction approach as reported in our works published before. The Pd nanodots supported on white titania nanobelts was also prepared for comparison. As a result, the Pd/TiO2−x shows better ORR performance than the Pd/TiO2 with lower Received: June 12, 2016 Accepted: September 26, 2016 Published: September 26, 2016 27654

DOI: 10.1021/acsami.6b07062 ACS Appl. Mater. Interfaces 2016, 8, 27654−27660

Research Article

ACS Applied Materials & Interfaces

solution by 50 min sonication and 2 h of magnetic stirring to get a catalyst ink. The electrochemical tests were performed on a glassy carbon electrode, which has a diameter of 5 mm. For each test, the electrode was modified by 10 μL of catalyst ink. Cyclic Voltammetry (CV). The cyclic voltammetry test was conducted in a three-electrode system by a Chenhua CHI 660D electrochemical workstation.The 0.1 M KOH was used as electrolyte. Hg/HgO filled with 1 M KOH was used as reference electrode. Pt wire was used as the counter electrode. The glassy carbon electrode modified with catalyst was used as working electrode. The electrolyte was saturated by O2 before the tests for 30 min. The O2 was bubbled in to the electrolyte during the whole process. The CV data was collected from 0.2 V to −0.8 V at 50 mV s−1 For comparison, CV tests were also conducted in the electrolyte, which was saturated by Ar before the tests for 30 min, and the Ar was bubbled during the whole test. Rotating Disk Electrode (RDE) Measurement. The preparation of electrode was the same to that of the CV measurement. The data of linear sweep voltammetry (LSV) was collected from 0.2 to −0.8 V at a scan rate of 10 mV s−1. The rotating speed of the electrode was varied from 250 to 2000 rpm. Koutecky−Levich plots (J−1 vs ω−1/2) were acquired according to the limiting diffusion current of each rotating speed. The electrons transfer number can be calculated from the Koutecky−Levich equation27

overpotential and larger limited current. And when nitrogen was introduced to the titania (TiO2−x:N), the amount of Ti3+ sites were further increased due to reduction of energy cost for oxygen vacancy formation by N-doping.25 The Pd/TiO2−x:N shows even higher ORR activity than Pd/TiO2−x. The increasing ORR activity from Pd/TiO2 to Pd/TiO2−x:N are well in accordance with the increasing amount of Ti3+ sites. The results indicate that the SMSI between Pd and TiO2 is indeed enhanced by Ti3+. To be specific, the charge transfer from Ti3+ to Pd nanodots leads to an electron-rich Pd surface. The electron enrichment lowers the O2 absorption energy on Pd nanodots, which makes Pd a more suitable catalyst for ORR.26 Because of the strong interaction between support and nanoparticles, the prepared Pd/TiO2−x:N also shows excellent stability and immunity to methanol poisoning.

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EXPERIMENTAL SECTION

Preparation of H2Ti3O7 Nanobelts. The H2Ti3O7 nanobelts were prepared by a hydrothermal procedure.4 At first, 0.4 g of P25 powders was dispersed in 70 mL of 10 M NaOH solution. Then the mixture was transferred to a 100 mL Teflon-lined autoclave. The autoclave was treated at 180 °C for 48 h. After cool down, the white powder was filtrated and washed with deionized water and ethanol for 3−4 times. Then, the obtained white powders were dispersed in an aqueous solution of HCl (0.1 mol L−1) under constant magnetic stirring for 48 h. The produced H2Ti3O7 nanobelts were collected by filtration and washed with deionized water until the residual of HCl was removed. The H2Ti3O7 nanobelts was then transferred into a 100 mL Teflon-lined autoclave with 80% of the volume filled by the aqueous solution of H2SO4 (0.02 mol L−1). The autoclave was treated at 100 °C for 12 h. The product was collected and washed with deionized water and ethanol 3−4 times. Then, it was dried overnight at 60 °C. Preparation of TiO2, TiO2−x and TiO2−x:N Nanobelts. TiO2 nanobelts were prepared by annealing H2Ti3O7 nanobelts under 550 °C for 4.5 h in air. The oxygen deficient TiO2−x nanobelts were synthesized by an Al reduction reaction which has been reported in our previous works.10 Typically, the as-prepared H2Ti3O7 nanobelts and aluminum powder were placed separately in a quartz tube in a two zone tube furnace and then the quartz tube was evacuated to a pressure of ∼2 Pa. The temperature of aluminum was kept at 850 °C while the sample was kept at 550 °C for 4.5 h. The N-doped titania (TiO2−x:N) was prepared by annealing H2Ti3O7 at 620 °C under NH3 flow for 4h. The gas flow was controlled at 300−400 sccm (standardstate cubic centimeter per minute). The color of TiO2 nanobelts is white while the color of TiO2−x and TiO2−x:N is black. Preparation of Pd/TiO2, Pd/TiO2−x, Pd/TiO2−x:N. First, 16.7 mg of PdCl2 was dissolved in a solution of 9 mL of deionized water and 1 mL of 37% HCl, then 30 mg of TiO2 was dispersed in the solution. The mixture was treated by an ultrasonic process for 10 min. Eighty milligrams of NaBH4 was dissolved by 10 mL of deionized water. The NaBH4 solution was added dropwise to the mixture under magnetic stirring. The mixture was filtrated and the obtained product was washed with deionized water 2−3 times. The Pd/TiO2−x and Pd/ TiO2−x:N were prepared with the same procedure. The mass loading of Pd is 25 wt % of the total catalyst. Characterization. The microstructures of the samples were studied by transmission electron microscopy (TEM) using a Tecnai F20 S-Twin electron microscope working at 200 kV. Scanning electron microscope (SEM) images were acquired by Hitachi S-4800 electron microscope working at 5 kV. The crystallinity was studied by Powder X-ray diffraction (XRD) performed on a Bruker D2 diffractometer with monochromatized Cu Kα radiation (λ = 1.5418 Å). The surface state of the samples were studied by the X-ray photoemission spectroscopy (XPS) performed on an Axis Ultra Photoelectron Spectrometer Electrochemical Measurements. Eight milligrams of catalyst and 2 mg of acetylene black were dispersed in 2 mL of 0.5% Nafion

1 1 1 1 1 = + = + J JL Jk Jk Bω1/2 B = 0.62n(D0)2/3 FC0ν−1/6 Jk = nC0 where J is the current density acquired from experiment, Jk is the kinetic current density while JL is the diffusion limiting current density, ω is the angular velocity, n is electron transfer number, D0 is the diffusion coefficient (O2, 1.9 × 10−5 cm2 s−1), F is the Faraday constant (96485 C mol−1), v is the kinematic viscosity of the electrolyte (0.01 cm2 s−1), C0 is the bulk concentration of O2 that is taken as 1.2 × 10−6 mole for the case of 0.1 M KOH, and k is the electron-transfer rate constant. Rotating Ring-Disk Electrode (RRDE) Measurement. For the RRDE tests, the electrode was prepared just the same as CV test. The disk of the ring-disk electrode has a diameter of 6 mm, which is different from the rotating disk electrode used in the RDE test. The disk potential was varied from 0.2 V to −0.8 V at a scan rate of 10 mV s−1,whereas the ring potential was fixed at 1.5 V vs RHE (reversible hydrogen electrode). The electron transfer number and % HO2− were calculated from the equations below

n=4

Id Id + Ir /N

HO−2 = 200

Ir / N Id + Ir /N

The Id and Ir represent disk current and ring current, respectively, N represents the current collection efficiency of the Pt ring, which was calculated to be 0.39 from the measurement of K3Fe[CN]6 reduction reaction.28 RHE Calibration. Because the Hg/HgO (1 M KOH 0.098 V vs NHE) electrode was used as reference electrode in the measurements. The data was calibrated relative to RHE as follows

In 0.1 M KOH, E(RHE)

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= E(Hg/HgO) + 0.098 + 0.0592pH (pH 13)

RESULTS AND DISCUSSION Pd nanodots supported on defective black titania (Pd/TiO2−x) was first synthesized to verify the positive effect of Ti3+ on the 27655

DOI: 10.1021/acsami.6b07062 ACS Appl. Mater. Interfaces 2016, 8, 27654−27660

Research Article

ACS Applied Materials & Interfaces

TiO2 and Pd/TiO2−x show obvious O2 reduction current in the O2 saturated electrolyte (Figure 3a). However, the Pd/TiO2−x

catalytic process. The Pd supported on white titania (Pd/TiO2) was also prepared for comparison. The synthesis of Pd/TiO2−x is illustrated in Figure 1. The H2Ti3O7 was used as raw material

Figure 1. Schematic diagram of the synthesis of Pd/TiO2−x. Figure 3. (a) CV curves of Pd/TiO2 and Pd/TiO2−x on a glassy carbon electrode in O2-saturated (solid line) and Ar-saturated 0.1 M KOH (dash line); (b) RDE measurement of Pd/TiO2 and Pd/TiO2−x under 2000 rpm.

and a special Al-reduction process was applied. XRD was performed to confirm the structure and phase purity of the prepared samples. As shown in Figure 2a, there are six peaks at

shows more positive onset potential (∼0.94 V vs RHE) and larger limited current density (∼5.3 mA cm−2) than Pd/TiO2 (∼0.90 V vs RHE; ∼ 4.8 mA cm−2). The improved ORR activity is attributed to the enhanced strong metal−support interactions (SMSI) between TiO2 and Pd nanodots. The SMSI often occurs between noble metals and some transition metal oxides (TiO2, V2O3, Nb2O3, Ta2O5, etc.), which can be partially reduced. Upon reduction, some low-valence cations, which may have special interaction with metal particles are acquired. For metals supported on TiO2, a localized electronic charge transfer can be found between Ti3+ states and the metal surface atoms.22 In our cases, as illustrated in Figure 4b, the

Figure 2. (a) XRD patterns of Pd/TiO2 and Pd/TiO2−x; (b) SEM image of Pd/TiO2−x; (c) TEM and (d) HRTEM images of Pd/TiO2−x.

25.2, 37.7, 47.9, 53.8, 55, and 62.5° which can be attributed to anatase TiO2 (JCPDS No. 21−1272), whereas the peaks at 40.1 and 46.7°are attributed to Pd (JCPDS No. 46−1043). The XRD patterns indicate that there is no impurities exiting in the composites. The morphology of the samples was studied by SEM and TEM. The Figure 2b shows the SEM image of Pd/ TiO2−x, the thicknesses of the nanobelts are 20−40 nm, the widths are 50−200 nm, and the lengths range up to several micrometers. The Pd nanodots, which can be seen from the high magnification inset, are uniformly located around the nanobelts. The highly dispersed Pd nanodots can also be seen in the TEM images (Figure 2c). The corresponding highresolution TEM (HRTEM) image (Figure 2d) reveals that the Pd nanodots have average size of about 4−6 nm, with an interplanar spacing of 0.23 nm, which corresponds to the (111) planes of Pd. The TiO2 nanobelts show an interplanar spacing of 0.35 nm, which corresponds to the (101) planes of anatase. The TEM images of Pd/TiO2 were shown in the Figure S1a, b. The ORR activity of Pd/TiO2 and Pd/TiO2−x was measured in a three-electrode configuration in 0.1 M KOH. Both of Pd/

Figure 4. (a) ORR catalytic process of Pd/TiO2; (b) ORR catalytic process of Pd/TiO2−x with charge transfer between Ti3+ and Pd.

charge transfer between Ti3+ and Pd nanodots leads to an electron-rich Pd surface. The charge transfer influences the adsorption energy of O2 on Pd nanodots. The adsorption of O2 on catalyst is crucial for the ORR process and it depends on the coupling between the oxygen 2p states and the Pd 3d states. The coupling leads to the bonding and antibonding states. The filling of antibonding states can weaken the coupling, which lowers the adsorption energy of O2 on Pd. The case is similar to the Pt nanodots supported by two-dimensional Ti3C2X2 (X = OH, F), the charge transfer from Ti3C2X2 to Pt enhance the ORR activity of Pt.29 The reduced absorption energy makes Pd a more proper ORR catalyst which can also be achieved by alloying with other metals as has been reported in literature.26 It is also a common view in the literature that the d states everage energy of the surface atoms strongerly influence the bonding energies of the adsorbate.30−34 As is shown in Figure 27656

DOI: 10.1021/acsami.6b07062 ACS Appl. Mater. Interfaces 2016, 8, 27654−27660

Research Article

ACS Applied Materials & Interfaces

Figure 5. XPS results of Pd/TiO2−x:N. (a) Survey spectrum; high-resolution spectrum of (b) Ti 2p; (c) Pd 3d spectrum of Pd/TiO2, and Pd/ TiO2−x:N; and (d) N 1s.

Figure 6. (a) RDE measurement of Pd/TiO2−x, Pd/TiO2−x:N, Pd/C, and Pt/C; (b) RDE measurement (2000 rpm) with different rotating speed and K-L plot of Pd/TiO2−x:N; (c) RRDE measurement (2000 rpm) and (d) calculated electron transfer number (n) and peroxide percentage of Pd/ TiO2−x:N.

reduce the formation energy of oxygen vacancy, so more oxygen vacancy and the derived Ti3+ sites were achieved. X-ray photoelectron spectroscopy (XPS) was performed to analyze the surface status of Pd/TiO2−x:N. Figure 5b shows the Ti 2p spectrum of Pd/TiO2−x:N, the peak located at ca. 458.6 and 464.3 eV are assigned to the Ti4+ 2p3/2 and 2p1/2, respectively. The spectrum also shows obvious peaks of Ti3+,

3, Al-reduction is a feasible way to enhance the SMSI between Pd and TiO2 nanobelts and thus improve the ORR catalytic activities. Ti3+ has played an important role in the ORR process. Increasing Ti3+ sites may further improve the ORR activity. In order to get more Ti3+ sites, nitrogen-doping was introduced to the TiO2 nanobelts (TiO2−x:N). N-doping can significantly 27657

DOI: 10.1021/acsami.6b07062 ACS Appl. Mater. Interfaces 2016, 8, 27654−27660

Research Article

ACS Applied Materials & Interfaces

Figure 7. (a) Rotating-disk voltammograms of Pd/TiO2−x:N before and after 2000 CV segments (2000 rpm); (b) chronoamperometric responses of Pd/TiO2−x:N with addition of CH3OH after 50 s.

did not show much decay (Figure 7a). It indicates that the material is very stable during the electrochemistry process in basic electrolyte. The Pd/TiO2−x:N also shows improved resistance to methanol crossover effects compared to the acetylene black supported Pd nanodots. Both cycling stability and methanol immunity are important for the practical use of catalyst in fuel cells. The excellent durability and immunity to methanol poisoning can also be attributed to the strong interaction between Pd and TiO2−x:N nanobelts. The stronger interaction between Pd and the support avoids the Pd nanodots from aggregating during the long-term test. The methanol poisoning may originate from the adsorption of CH3OH on the catalytic active site of the Pd nanodots. As mentioned above, the charge transfer between TiO2−x:N and Pd can weaken the adsorption of CH3OH, so the immunity to methanol poisoning of the catalyst was enhanced.

which located at ca. 458.0 and 462.2 eV. The two peaks can be assigned to the Ti3+ 2p3/2 and 2p1/2. As expected, the intensity of Ti3+ peaks in Pd/TiO2−x:N is much stronger than that of the Pd/TiO2 and Pd/TiO2−x (Figure S2 a,c). Figure 5c shows the Pd 3d spectrum. The peaks located at 334.7 and 340.0 eV are attributed to the Pd 3d5/2 and 3d3/2 states, and the peaks located at 335.7 and 340.8 eV corresponds to the Pd2+ 3d5/2 and 3d3/2 states. The appearance of Pd2+ peaks may originate from the partial oxidation of the Pd surface, since the Pd particles are very small. The charge transfer between Pd and the black titania can also be indirectly proved by the XPS spectrum of the Pd particles (Figure 5c). The binding energy of Pd supported by black TiO2−x:N shows obvious down shift (∼0.6 eV) compared to the Pd supported by white TiO2.The N 1s spectrum can be deconvoluted into two peaks, the peak located at 396.0 eV is assigned to the substitutional nitrogen (Ti−N) and the peak at 399.3 eV is attributed to interstitial nitrogen (Ti−O−N) in the TiO2−x:N. The RDE test was performed to evaluate the ORR activity of Pd/TiO2−x:N. As is shown in Figure 6a, the Pd/TiO2−x:N shows more positive half-wave potential (∼0.81 V vs RHE) and larger limited current density (∼5.7 mA cm−2) compared to the Pd/TiO2−x (∼0.80 V vs RHE ; ∼5.3 mA cm−2). The results confirm that the Pd/TiO2−x:N has a higher catalytic activity toward ORR. The Pd/TiO2−x:N even shows comparable activity with the Pd nanodots on Carbon black. The polarization curves for the ORR on Pd/TiO2−x:N electrode performed at different rotating speeds are presented in Figure 6b. The inset is the corresponding K-L plot. The electron transfer numbers (n) was calculated from the slope of K-L plot. The n is 4.02 (0.5 V vs RHE) for Pd/TiO2−x:N. The rotating ring−disk electrode (RRDE) measurements were also carried out to determine the ORR catalytic pathways of the composite. The n and yield of peroxide species during the whole ORR process are displayed in Figure 6d, which indicates that the catalytic process on Pd/TiO2−x:N is dominated by a four electron (4e−) pathway, which shows comparable activity with the Pd/C (Figure S5). The RRDE performance is consistent with the K-L plots. The improved ORR catalytic activity from Pd/TiO2 to Pd/TiO2−x:N is well in accordance with the increasing amount of Ti3+. The results confirm that Ti3+ has enhanced the SMSI effect between Pd and TiO2 nanobelts. In addition to good ORR activity, the Pd/TiO2−x:N also shows excellent stability and immunity to methanol poisoning. After 2000 segments CV scans between −0.8 and 0.2 V (vs Hg/ HgO) in 0.1 M KOH, the RDE performance of Pd/TiO2−x:N

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CONCLUSION In summary, Al-reduced and nitrogen-doped black TiO2 are explored as reliable support for Pd nanodots. After reduction, the Pd/TiO2−x shows lower overpotential and higher limited current for ORR than the untreated one (Pd/TiO2). The improved ORR activity is attributed to the charge transfer between Pd and the reduction derived Ti3+ center. The charge transfer enhances the strong metal−support interactions (SMSI) of Pd and TiO2 and leads to an electron-rich Pd surface. The electron-rich surface lowers the absorption energy of O2 on Pd, which contributes to the higher ORR activity. More Ti3+ center is acquired by nitrogen-doping which further reduce the O2 absorption energy. As expected, the Pd/ TiO2−x:N shows even higher ORR activity than Pd/TiO2−x. The results are well in accordance with the increasing amount of Ti3+. Because of the enhanced SMSI effect between Pd and TiO2−x:N, the composites also show excellent stability and immunity to methanol poisoning.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b07062. TEM images of Pd/TiO2 and Pd/TiO2−x:N; XPS spectrum of Pd/TiO2 and Pd/TiO2−x; RDE measurements of Pd/TiO2 and Pd/TiO2−x under different rotation speeds; RRDE measurements of Pd/C; stability test of Pd/C (PDF) 27658

DOI: 10.1021/acsami.6b07062 ACS Appl. Mater. Interfaces 2016, 8, 27654−27660

Research Article

ACS Applied Materials & Interfaces

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions †

X.Y. and X.W. contribute equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported from National Key Research and Development Program of China (Grant 2016YFB0901600), NSF of China (Grants 61376056, 51402335, and 11404358), National Program of China (Grant 2016YFB0901600) and Science and Technology Commission of Shanghai (Grants 13JC1405700 and 14520722000).

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DOI: 10.1021/acsami.6b07062 ACS Appl. Mater. Interfaces 2016, 8, 27654−27660

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DOI: 10.1021/acsami.6b07062 ACS Appl. Mater. Interfaces 2016, 8, 27654−27660