Accepted Manuscript Title: Enhanced Electronic and Electrochemical Properties of Core-Shelled V2 O5 -Pt Nanowires Authors: Ko-Ying Pan, Da-Hua Wei PII: DOI: Reference: S0169-4332(17)32648-X http://dx.doi.org/10.1016/j.apsusc.2017.09.033 APSUSC 37112 To appear in: APSUSC Received date: Revised date: Accepted date: 6-6-2017 1-9-2017 5-9-2017 Please cite this article as: Ko-Ying Pan, Da-Hua Wei, Enhanced Electronic and Electrochemical Properties of Core-Shelled V2O5-Pt Nanowires, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.09.033 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. Enhanced Electronic and Electrochemical Properties of Core-Shelled V2O5-Pt Nanowires Ko-Ying Pan and Da-Hua Wei* Institute of Manufacturing Technology and Department of Mechanical Engineering, National Taipei University of Technology (TAIPEI TECH), Taipei 10608, Taiwan, R.O.C. *Corresponding author: email@example.com Highlights 1. We adopted thermal evaporation without using precursors for V2O5 NWs and Pt@ V2O5 NWs. 2. The electrical resistivity of V2O5 NWs and three kinds of Pt@V2O5 NWs (50, 100 and 150 cycles) are 1.716x107, 9.091x105, 4.456x104 and 1.961x103 Ω, respectively. 3. The value of IF/IR of Pt@V2O5-150 cycles NWs is the largest of these four samples, so the Pt@ V2O5-150 cycles is the best material for fuel cell. ABSTRACT Platinum nanoparticles (Pt NPs) were decorated on vanadium pentoxide nanowires (V2O5 NWs) to form the core-shelled vanadium-platinum nanowires (Pt@V2O5 NWs) and their electrochemical activities for methanol oxidation were investigated. The synthetic procedure involved the synthesis of abundant vanadium pentoxide nanowires (V2O5 NWs) by a direct vapor-solid growth process (VS method), followed by atomic layer depositions (ALD) of platinum nanoparticles (Pt NPs) onto the V2O5 NWs. After the physical examinations, three designed deposition parameters (50, 100 and 150 cycles) of Pt NPs onto the V2O5 NWs by ALD process were successful. From the measurements of current-voltage (I-V) and cyclic voltammetry (CV) curves respectively, both the conductivity and the ratio of the forward anodic peak current (IF) to the reverse anodic peak current (IR) are enhancing proportionately to the deposition cycles of ALD process, which denotes that coating Pt atomic layers onto V2O5 nanowires indeed improves the catalytic performances than that of pure V2O5 nanowires. Keywords: Pt@V2O5 nanowires, VS method, Atomic layer deposition, I-V curve, CV curve, Catalyst 1. Introduction For four decades, in the field of fuel cell technology and electro-catalysis, various oxidation states of vanadium have received considerable interest, including VO, V2O3, VO2 and V2O5 [1-6]. Because of the compositions of single and mixed valence states are used to obtain highly desirable manufactures and plans to reduce pollution, and vanadium is one of the abundant metallic elements within our planet’s crust. Most of all, vanadium pentoxide (V2O5) is widely adopted in actuators, sensors, field-emitters, lithium ion batteries, catalysis, electrochromic devices and supercapacitors, owing to its intriguing microstructures and electronic properties [3, 7-13]. To our best knowledge, V2O5 is an n-type semiconductor with a bandgap of 2.4 eV, and it owns a special layered structure, which is composed of layers of VO5 square pyramids; also, these VO5 square pyramids regularly link at edges or corners. Various approaches have been developed to fabricate V2O5 nanostructure, such as thermal chemical vapor evaporation, hydrothermal treatment, sol-gel process, pulsed laser deposition, and so on [1-15]. Mostly, the solution-based methods such as in-situ hydrothermal growth  and procedure of binder-free V2O5 as a cathode for rechargeable aluminum battery  have been widely used to prepare V2O5 nanocomposites. However, the previously published papers demonstrate that two big issues diminish the advantages of V2O5; one condition is the low conductivity; the other is poor microstructure stability. Above two severe disadvantages cause poor retention capacity while the long term cycling is working in the cell. In order to improve the poor retention capacity resulted from features of V2O5, many manufactures for fabricating core-shelled metal@V2O5 , oxide semiconductor@V2O5 nanocomposites  have been still developing rapidly.  and polymer@V2O5 On the other hand, atomic layer deposition (ALD) is a cutting-edge technology, which has been considered a powerful coating skill to form three-dimensional uniformity. Also, it depends on serial self-terminating gas-solid reactions, which has been applied for coating sequential atomic layers of organic metal on the basic materials. Moreover, ALD is generally adopted in thin-film process of semiconductors manufacturing technology due to its good performance of controllable thickness and complicated shape of atomic monolayer . Although V2O5 has been widely discovered experimentally, there have been rare literatures on the function and application of Pt@V2O5 NWs for the catalysts. In this task, in advance, thermal evaporation was demonstrated to synthesize pristine V2O5 nanowires via VS method. These nanostructures, V2O5 nanowires, have been grown on the silicon chips without a catalyst. Then, Pt nanoparticles - known to be a high conductivity and good catalytic material- have been grown uniformly onto the surface of V2O5 NWs by ALD process. The samples were characterized by scanning electron microscopy (SEM), energy-dispersive X-ray spectrometer (EDS) and high-resolution transmission electron microscope (HRTEM). As to electronic and electrochemical properties, the synthesized V2O5 and Pt@V2O5 nanowires have been investigated and compared via using current-voltage (I-V) curves and cyclic voltammetry (CV) curves, respectively. 2. Experimental Method The vanadium pentoxide nanowires (V2O5 NWs) used here were fabricated on silicon (100) substrates, by thermal evaporation, similar to that reported for GaN nanorods synthesis [22, 23]. The whole preparation tactics for constructing pristine V2O5 nanowires and Pt@V2O5 nanowires are sketched in the Figure 1. High purity V2O5 powders (Alfa, 99.5%, 325 mesh) in an alumina boat and several silicon substrates were situated in the other alumina oxide (Al2O3) boat. Then, authors placed these two Al2O3 boats and a space length is 5 cm between them in the heating area of a quartz tube, as illustrated in Fig. 1(a). The commercial Si (001) substrates were prepared without coating metallic precursors, chemical etchings or physical damages but purged ultrasonically in acetone solution. During the growth of V2O5 nanowires, the pressure inside the quartz tube was kept up at 1 Torr by a rotary pump. The working conditions of this vapor-solid process (VS) were as follows: (1) The working temperature had gone up to 850°C with 20°C/min at a constant flow rate of 20 sccm Ar gas. (2) Maintaining the working temperature at 850°C for 1 hour was at a continual flow rate of mixed gas of 20 sccm Ar and 10 sccm O2. (3) Cooling the working temperature to the room temperature was under a fixed flow rate of 20 sccm of Ar gas. After the formations of pristine V2O5 nanowires, Pt nanoparticles were deposited on the nanowires surface to form Pt@ V2O5 nanowires by ALD of Pt after 50, 100 and 150 cycles using alternative exposures to MeCpPtMe3 (Aldrich, 98%) and oxygen (99.99%) at 250°C, as sketched in Fig. 1(b). In the ALD process, the receptacle of MeCpPtMe3 was kept at 45°C, suppling a stable flux of MeCpPtMe3 at a fixed pressure of 3 mTorr to the reactor. In every single cycle, 1 second of MeCpPtMe 3 pulse and 5 seconds of O2 pulse were separated by 20 seconds of N2 (99.99%) purge . The morphology, crystal structure and chemical composition of the V 2O5 and Pt@V2O5 nanowires were analyzed by a field emission scanning electron microscope (FESEM, JEOL, JSM-6500F), a high-resolution transmission electron microscope (HRTEM, JEOL, JEM-2010), and energy-dispersive X-ray spectroscopy (EDS, Oxford), respectively. Regarding the electronic property of V2O5 and Pt@V2O5 nanowires, the current-voltage (I-V) curves were measured between -10V and 10V by Keithley 4200. As for the electrochemical property of V2O5 and Pt@V2O5 nanowires, the cyclic voltammetric (CV) curve was plotted by an Electrochemical Workstation (CHI 627E) to investigate the potential of oxidation peak and reduction peak. The test cell includes three kinds of electrodes and one electrolyte in a beaker. A saturated calomel electrode (SCE) and a platinum foil were adopted as the reference electrode and counter electrode, separately. The working electrodes were made by mixing 80 wt% V2O5 or Pt@V2O5 nanowires, 10 wt% polyvinylidene difluoride (PVdF) and 10 wt% carbon black in N-methyl pyrrolidinone (NMP) solvent to become a slurry. The slurry was spread onto an Al2O3 foil and evenly distributed on the Al2O3 foil, then the sample was placed in the oven to transform slurry into a solid. 1 M H2SO4+ 1M CH3OH solution was prepared as the electrolyte in the beaker-style three-electrode test cell. The cyclic voltammetry (CV) was conducted in a voltage range between -0.1 and 1 V at 50 mVS-1 scan rate at room temperature. 3. Results and Discussion Figure 2(a) exhibits the SEM image of abundant V2O5 nanowires; it is very clear that pure V2O5 nanowires are with diameter about 100 nm and length more than 10 μm, which have been directly obtained by VS process. Figures 2(b), 2(c) and 2(d) show the SEM images of three conditions (50, 100 and 150 cycles) of Pt@V2O5 nanowires, respectively. Judging from these three SEM images of Pt@V2O5 nanowires, three kinds of ALD process in this work are highly successful because every final product still keeps the same shape as the initial V2O5 nanowires. The phenomena definitely prove that Pt monolayers were extremely deposited uniformly and well-ordered onto pure V2O5 nanowires via the surface-control process of ALD. Furthermore, the micromorphology and microstructure of one single Pt@V2O5 nanowire were inspected by a classical observation of TEM. In Fig. 2(e), the SEM-EDS result of spot 1 on the surface indicates the co-existence of V and O, and the percentages of V and O atoms are 27.89 and 72.11, which is approaching to the composition ratio of V2O5. To observe the surfaces of V2O5 nanowires exactly, the low-magnification and high-resolution TEM images of V2O5 samples were taken in Fig. 3. Figures 3(a) and 3(b) are typical low-magnification TEM images of an individual V2O5 and Pt@V2O5 (50 cycles) NWs, which demonstrate the diameters are 103 nm and 108.54 nm, respectively. The high resolution TEM images of Pt@V2O5 (50 cycles) NWs are shown in Figs. 3(c) and 3(d), and Fig. 3(c) reveals that the thickness of Pt layer is 2.77 nm. From Fig. 3(d), the HRTEM image shows the combination of Pt NPs and V2O5 NW very well with highly ordered and continuous states. The lattice spacing between two adjacent lattice planes is 0.22 nm, corresponding to the (111) plane of face-centered cubic (fcc) Pt. Figures 3(e) and 3(f) are the TEM-EDS results on the spots 2 and 3 marked in Figure 3(b), respectively. The TEM-EDS acquires from an individual Pt@V2O5 NW (50 cycles) indicates that the elements of the product consists entirely V, O and Pt. In the next analysis, authors would certify the Pt is certain whether metallic or oxidative states via doing XPS. The TEM images of a single Pt@V2O5 NW (100 cycles and 150 cycles) are presented in Figs. 3(g) and 3(h), respectively. From the two photos, the thicknesses of Pt layer are 6.24 nm and 8.89 nm, respectively. The XPS patterns of V2O5 and Pt@V2O5 nanowires are shown in Fig. 4, and Fig. 4(a) is the survey scan for V2O5 and Pt@V2O5 samples. From Fig. 4(b), three peaks at 517.7, 525.2 and 530.5 eV are associated with orbits of V2p3/2, V2p1/2 and O1s, respectively. It should be noted that the binding energy difference (ΔE) between orbits of V2p3/2 and O1s is 12.8 eV, which is consisted with the previous literature , so the as-synthesized nanowires prepared by thermal evaporation could be completely verified as vanadium pentoxide nanowires (V2O5 NWs). In Fig. 4(a), three peaks from Pt 4p, 4d and 4f orbitals mean that Pt coatings indeed were deposited onto the surface of V2O5 nanowires. The above results reveal that the Pt@V2O5 nanowires are able to fabricate via using a procedure consists of the thermal evaporation and the subsequent ALD processes of Pt. Figure 4(c) is Pt 4f spectrum and Pt 4f7/2 located at 71.2 eV implies that the atomic-layer-deposited Pt on the V2O5 nanowires is the form of metallic Pt. The electric properties of the V2O5 NWs and three kinds of Pt@V2O5 NWs (50, 100 and 150 cycles) are also evaluated and compared. A schematic drawing of a fabricated V2O5 NWs or Pt@V2O5 NWs- based device for electric test is shown in Fig. 5(a). Figure 5(b) displays the current-voltage (I-V) curves of all V2O5 and Pt@V2O5 samples, and all curves appear to be linear with the applied voltage, also indicating that Ohmic contact forms are between these nanostructures and the Ag electrodes, confirming that the V2O5 NWs and Pt@V2O5 NWs- based devices are well connected and have low contact resistance. The electrical resistivity can be expressed as R=ρ×L/A, and charge transfer between the nanostructures can be ignored. The electrical resistivity of V2O5 NWs and three kinds of Pt@V2O5 NWs (50, 100 and 150 cycle) are 1.716x107 Ω, 9.091x105 Ω, 4.456x104 Ω and 1.961x103 Ω, respectively. That ALD of numerous Pt particles on V2O5 NWs causes the formation of Pt films that cover the V2O5 NWs surface and upgrade the charge transfer would be the root cause of this phenomenon . For making a comparison of the electrochemical properties among V2O5 NWs and Pt@V2O5 NWs, the CV technique was used to measure the electrocatalytic activities. The typical CV curves of V2O5 nanowires and Pt@V2O5 NWs (50, 100 and 150 cycles) in 1 M H2SO4+ 1M CH3OH solution at the scan rate of 50 mVs-1 at room temperature is plotted in Fig. 6, performed in a potential range of -0.1 V to 1 V at the 100th cycle [26, 27]. As shown in Fig. 6, the oxidation peak and the reduction peak of pristine V2O5 nanowires along the wine dashed line are at 0.608 and 0.372 V, respectively. The onset potentials for methanol oxidation are 0.477, 0.302, 0.133 and 0.126 V, corresponding with pristine V2O5 nanowires and Pt@V2O5 nanowires (50, 100 and 150 cycles), respectively. There is no doubt that the better dispersion of Pt NPs deposited by ALD process on the pristine V2O5 NWs surface do the trick for the methanol oxidation owing to their high surface area and good conductivity. As given in Fig. 6 and Table 1, the IF/IR values of the Pt@V2O5 nanowires (50,100 and 150 cycles) are 1.574, 1.859 and 1.886, respectively. The ratios of the forward anodic peak current (IF) to the reverse anodic peak current (IR) are used to evaluate the catalyst tolerance to the poisoning species on the surfaces of the electrodes. The higher IF/IR value signifies that the removal of poisoning species on the catalysts surfaces is more energy efficient, so Pt@V2O5 NWs get better catalytic activity toward methanol oxidation than that of pristine V2O5 nanowires [28-32]. The demonstration of catalytic activity for Pt@V2O5 NWs is similar to that of some bifunctional compositions derived from published literatures [30-32]. At the first step of the improvement of the catalytic activity between Pt NPs and V2O5 NWs, the methanol shows favor to binding with Pt atoms on surface and then is produced CO absorbed species by dehydrogenation. The products, COad intermediates, are deemed to be the major poisoning species while the electro-oxidation of methanol is in progress [28, 30]. During the oxidation reaction, the related schematic mechanism of methanol adsorption turns out in four steps as shown in Fig. 7, making various products owing to dissociation of the molecule that could be formulated as following reactions and can be expressed as (Eqs. (1)–(4)) Pt-(CH3OH)ad→ Pt-(CH2OH)ad + H+ + e- (1) Pt-(CH2OH)ad→ Pt-(CHOH)ad + H+ + e- (2) Pt-(CHOH)ad→ Pt-(COH)ad + H+ + e- (3) Pt-(COH)ad→ Pt-(CO)ad + H+ + e- (4) Second, thanks to the better affinity of V2O5 NWs being oxygen-containing species, a large amount of OHad has been made to provide rational CO oxidation rates on V2O5 NWs than Pt NPs. The OHad species are essential for the oxidative removal of COad intermediates, which causes the better catalytic activity and longer lifespan for the entire methanol oxidation on Pt@V2O5 NWs. The above chemical reaction in methanol electro-oxidation can be illustrated as following (Eqs. (5)–(8)) [30, 32], V2O5 +2H| → 2VO2 + H2O (5) 4VO2 + 4H| → 4VO2+ + O2 + 2H2O (6) VO2+ + H2O → VOOH- + H+ (7) COad + VOOH → CO2 + VO2 | + H- + e (8) For instance, in our designed core-shelled nanostructures, the CV curve shows that Pt@V2O5 NWs (150 cycles) in the solution containing methanol at the oxidation peak, take place Pt-CO bonds on themselves surfaces. After the negative sweep, the CV curve reveals that CO molecules are reduced to CO2 at the oxidation peak of negative sweep, as drawn in Fig. 7. The above results show that Pt@V2O5 nanowires exhibit higher catalytic activity to methanol oxidation with the increasing Pt deposition cycles of ALD process, on account of the following two reasons. First, metallic Pt nanoparticles are deposited on as-prepared V2O5 NWs uniformly, which can additionally lessen the effect of aggregation derived from V2O5 NWs and make plenty of accessible immobilization sites inV2O5 NWs during the interaction of methanol oxidation. 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Kondratenko, Similarity and differences in the oxidative dehydrogenation of C2–C4 alkanes over nano-sized VOx species using N2O and O, Catalysis Today, 142 (2009) 138. Figure Captions Figure 1 (a) Schematic diagram illustrating the thermal evaporation set-up with three main processes including evaporation, reaction and condensation. (b) Sketched picture of ALD process and its working conditions. Figure 2 SEM image of (a) V2O5 nanowires, (b) Pt@V2O5 nanowires-50 cycles, (c) Pt@V2O5 nanowires-100cycles, (d) SEM image of Pt@V2O5 nanowires-150 cycles; (e) SEM-EDS analysis at spot 1 of V2O5 nanowires. Inset: SEM image of spot 1. Figure 3 Low-magnification TEM image of (a) a V2O5 nanowire, (b) a Pt@V2O5 nanowire (50 cycles). HRTEM images of (c) and (d) a Pt@V2O5 nanowire (50 cycles). TEM-EDS analysis of (e) and (f) on the spot 2 and 3 in Figure 3(b). Low-magnification TEM images of (g) a Pt@V2O5 nanowire (100 cycles) and (h) a Pt@V2O5 nanowire (150 cycles). Figure 4 XPS spectra of the core-shelled nanowires: (a) Survey scan, (b) V 2p and O 1s, and (c) Pt 4f. Figure 5 (a) Schematic illustration of the fabricated device for electric measurements. (b) The measured I-V curves of V2O5 NWs and Pt@V2O5 NWs (50,100 and 150 cycles). Figure 6 The measured CV curves of V2O5 NWs and Pt@V2O5 NWs (50,100 and 150 cycles). Figure 7 Schematic diagram illustrating the oxidation and reduction reactions in CV characterization. (a) V2O5 NWs Pt 4f Pt@V2O5 NWs-50 cycle Pt@V2O5 NWs-100 cycle Intensity (a.u.) Pt@V2O5 NWs-150 cycle Pt 4d5/2 Pt 4d3/2 V(LMM) O(KVV) O1s V2p V2s N1s C1s V(LML) O2s 0 100 200 300 400 500 600 700 800 900 1000 1100 Binding energy (eV) (b) V2p3/2 ΔE=12.8 eV O1s 530.5 eV (c) 517.7 eV Pt 4f7/2 Intensity (a.u.) 71.2 eV V2p1/2 525.2 eV Intensity (a.u.) Pt 4f5/2 515 520 525 530 535 Binding energy (eV) 70 75 80 Binding energy (eV) 85 6.0x10-4 4.0x10-4 Current (A) 2.0x10-4 7 V O NWs (R=1.176x10 Ω) 2 5 5 Pt@V O NWs- 50 cycles (R=9.091x 10 Ω) 2 5 4 Pt@V O NWs- 100 cycles (R=4.546x 10 Ω) 2 5 3 Pt@V O NWs- 150 cycles (R=1.961x 10 Ω) 2 5 0.0 -2.0x10-4 -4.0x10-4 -6.0x10-4 -10 -5 0 Voltage (V) 5 10 12 V2O5 NWs 10 Pt@V2O5 NWs-50 cycles Pt@V2O5 NWs-100 cycles Pt@V2O5 NWs-150 cycles 2 Current (mA/cm ) 8 6 4 2 0 -2 -0.2 0.0 0.2 0.4 0.6 Potential(V/SCE) 0.8 1.0 Table Caption Table 1 Comparison of electrocatalytic activity of Pt@V2O5 NWs (50,100 and 150 cycles).