Wire-like Pt on mesoporous Ti0.7W0.3O2 Nanomaterial with Compelling Electro-Activity for Effective Alcohol Electro-Oxidation

Finding out robust active and sustainable catalyst towards alcohol electro-oxidation reaction is major challenges for large-scale commercialization of direct alcohol fuel cells. Herein, a robust Pt nanowires (NWs)/Ti0.7W0.3O2 electrocatalyst, as the coherency of using non-carbon catalyst support and controlling the morphology and structure of the Pt nanocatalyst, was fabricated via an effortless chemical reduction reaction approach at room temperature without using surfactant/stabilizers or template to assemble an anodic electrocatalyst towards methanol electro-oxidation reaction (MOR) and ethanol electro-oxidation reaction (EOR). These observational results demonstrated that the Pt NWs/Ti0.7W0.3O2 electrocatalyst is an intriguing anodic electrocatalyst, which can alter the state-of-the-art Pt NPs/C catalyst. Compared with the conventional Pt NPs/C electrocatalyst, the Pt NWs/Ti0.7W0.3O2 electrocatalyst exhibited the lower onset potential (~0.1 V for MOR and ~0.2 for EOR), higher mass activity (~355.29 mA/mgPt for MOR and ~325.01 mA/mgPt for EOR) and much greater durability. The outperformance of the Pt NWs/Ti0.7W0.3O2 electrocatalyst is ascribable to the merits of the anisotropic one-dimensional Pt nanostructure and the mesoporous Ti0.7W0.3O2 support along with the synergistic effects between the Ti0.7W0.3O2 support and the Pt nanocatalyst. Furthermore, this approach may provide a promising catalytic platform for fuel cell technology and a variety of applications.

particle agglomeration and coalescence of the Pt nanocatalysts 11,13,14 , resulting in good sensitivity and activity of the Pt-based electrocatalyst.
To be the best of our knowledge, there are a limited number of researches on the coherency of the non-carbon catalyst support and one-dimensional Pt nanowires to develop a robust electrocatalyst towards alcohol electro-oxidation reaction. In this work, we demonstrated the Pt NWs/Ti 0.7 W 0.3 O 2 catalyst toward methanol electro-oxidation and ethanol electro-oxidation which was successfully fabricated via the simple chemical reduction route at room temperature, only using formic acid (HCOOH) as reducing agents. These observational results indicated that the Pt NWs/Ti 0.7 W 0.3 O 2 is promising anodic catalysts for methanol electro-oxidation reaction (MOR) and ethanol electro-oxidation reaction (EOR), which can alter the conventional Pt NPs/C electrocatalysts. For instance, the robust Pt NWs/Ti 0.7 W 0.3 O 2 electrocatalyst exhibited the lower onset potential (~0.1 V vs. NHE for MOR and ~0.2 V vs. NHE for EOR), higher mass activity (~355.29 mA/mg Pt for MOR and ~325.01 mA/mg Pt for EOR) and higher I f /I b ratio (~2.70 for MOR and ~1. 35 for EOR) as well as much higher electrochemical stability relative to the Pt NPs/C catalyst. The high mass activity and superior stability of the robust Pt NWs/Ti 0.7 W 0.3 O 2 electrocatalyst could be derived from combining the merits of the one-dimensional Pt nanostructures and the mesoporous Ti 0.7 W 0.3 O 2 catalyst support, as well as the synergistic effect between the mesoporous Ti 0.7 W 0.3 O 2 catalyst support to the Pt nanoforms. Finally, this research can provide robust catalysts platforms for fuel cell technologies and other applications such as solar cells, water splitting. was prepared via the one-pot solvothermal route without employing surfactant/stabilizer or further heat treatment 8 (see Figs S1-S6, Supplementary Information). In this work, the one-dimensional (1D) Pt nanocatalysts were directly grown on the mesoporous Ti 0.7 W 0.3 O 2 support via a facile and simple chemical reduction approach at room temperature, only utilizing formic acid (HCOOH) as reducing agents (Fig. 1a). The formed structure lattice of platinum nanowires (NWs) over the Ti 0.7 W 0.3 O 2 support was investigated by means of X-ray diffraction (XRD) measurement. As can be seen in Fig. 1b, three typical diffraction peaks of face-centered cubic (fcc) structure (JDCPS 04-0802) of the platinum metal were clearly observed at 39.76°; 46.24° and 67.45° with respect to the crystal (111), (200) and (220) facets. Importantly, the strongest peak of platinum metal was located at 39.76° correspond to crystal (111) facets, which implied that the platinum nanocatalysts were formed along the (111) direction. Interestingly, no signal of the segregation of tungsten and titanium dioxide (TiO 2 ) was detected in the XRD pattern (Fig. 1b), suggesting that the mesoporous Ti 0.7 W 0.3 O 2 catalyst support possessed the highly stable structure in reduction media with the long reaction time. Furthermore, the transmission electron microscopy (TEM) was implemented to investigate the morphology of the Pt nanoforms on the mesoporous Ti 0.7 W 0.3 O 2 support. Figure 1c,d shows the morphology of Pt nanocatalyst to be the wire-like shape with the length ~40 nm and ~5 nm in diameter. The disuniform size of particles could be interpreted due to the agglomeration phenomena when growing the PtNWs on the surface of the Ti 0.7 W 0.3 O 2 supports. Besides, TEM images (Fig. S7, Supplementary Information) exhibited the catalyst morphology to be the rhombus and sphere that could be explained due to the agglomeration of support materials and one-dimensional (1D) Pt nanocatalyst which maybe covers overall the surface of Ti 0.7 W 0.3 O 2 support due to the high Pt loading (50 wt%) on the support. Moreover, HR-TEM image (see Fig. 1e) exhibited the fringe with a lattice spacing of ~2.3 Å corresponding to the (111) crystal plane of fcc Pt, confirming the oriented formation of Pt toward (111) facets on the surface of the supports. The mechanism of the growth of the Pt nanowires (NWs) on the Ti 0.7 W 0.3 O 2 support could occur in a similar manner to that of Pt NWs on carbon spheres, CNT or other supports that reported in previous works [14][15][16][17][18][19] . Typically, Pt nuclei are deposited on the surface of support during the reduction of H 2 PtCl 6 by HCOOH. Next, the freshly formed Pt nuclei act as sites for further nucleation via the continual absorption and reduction of Pt (IV) ions resulting in the formation of clustered particles. The very low reduction rate of the formic acid (HCOOH) at room temperature, which environmental favors for the anisotropic development of platinum nuclei along the (111) direction 15,16 . From this mechanism, Pt nuclei are deposited on the surface of Ti 0.7 W 0.3 O 2 support during the reduction of H 2 PtCl 6 by HCOOH to form Pt nuclei that act as sites for further nucleation via the continual absorption and reduction of Pt (IV) ions resulting in the formation of clustered particles and form the Pt NWs on the surface of Ti 0.7 W 0.3 O 2 supports under the very low reduction rate and long-time reaction (72 hours) at room temperature. These outcomes suggest that the simple chemical reduction route using formic acid is a suitable approach to design the Pt NWs/ Ti 0.7 W 0.3 O 2 electrocatalyst.

Results
In order to further investigate the surface characterization of the as-prepared Pt NWs/Ti 0.7 W 0.3 O 2 , Pt NWs/C and Pt NPs/C electrocatalysts, the X-ray photoelectron spectroscopy (XPS) measurement was performed. These XPS results (see Fig. 2) indicated that the Pt 4f 5/2 and Pt 4f 7/2 peaks of the Pt NWs/C and Pt NPs/C electrocatalyst were located at 74.08 eV and 70.80 eV, respectively, which could be which assigned to zero-valent of Pt 20,21 . Interestingly, the Pt NWs/Ti 0.7 W 0.3 O 2 exhibited the Pt 4f 5/2 and Pt 4f 7/2 at 73.75 eV and 70.47 eV, respectively. It means that the negative shift to the low binding energy of Pt 4 f 5/2 and Pt 4f 7/2 in the Pt NWs/Ti 0.7 W 0.3 O 2 catalyst are ascribable to the electronic transfer from Ti 0.7 W 0.3 O 2 support to the Pt catalysts that carbon support can not exhibit the mechanism 2,6,10,22 . This results in the downshift d-band center of Pt nanocatalyst 6,22 , which normally found out in the conventional Pt-M alloy implying that the mesoporous Ti 0.7 W 0.3 O 2 catalyst support could play a key role as co-catalyst for the Pt metal that a simple carbon support cannot 6    www.nature.com/scientificreports www.nature.com/scientificreports/ compared to the Pt NWs/C and the traditional Pt NPs/C in N 2 -purged 0.5 M H 2 SO 4 aqueous solution via the cyclic voltammetry measurements. As can be seen in Fig. 3a, these catalysts show the multiple peaks in the hydrogen adsorption/desorption regions, implying that the high crystallinity of these electrocatalysts 11,14,18 . The electrochemical surface area (ECSA) of the Pt NWs/Ti 0.7 W 0.3 O 2 , Pt NWs/C, and conventional Pt NPs/C electrocatalysts, calculated from the charge of hydrogen adsorption, are around 63.48 m 2 /g; 56.73 m 2 /g, respectively, which is approximate half that of the conventional Pt/C electrocatalyst (~130.32 m 2 /g) (Fig. 3b). The low ECSA values of the Pt NWs/Ti 0.7 W 0.3 O 2 and the Pt NWs/C catalysts versus that of the Pt NPs/C could be accounted for the reducing boundaries of the 1D morphology of the nanowires relative to the 0D morphology of the nanoparticles 23 . Besides, the accelerated durability test (ADT) in N 2 -purged 0.5 M H 2 SO 4 at a scan rate of 50 mV/s was also employed to investigate the electrochemical stability of as-prepared electrocatalysts. After the 5000 cycling test, the ECSA loss of the Pt NWs/Ti 0.7 W 0.3 O 2 catalysts was estimated to be 11.89% of initial ECSA value, meanwhile, the ECSA value of Pt NWs/C and conventional Pt NPs/C was significantly degraded to be ~19.33% and ~27.45% of initial ECSA value, respectively ( Fig. 3c-e). The enhanced stability of the Pt NWs/Ti 0.7 W 0.3 O 2 catalyst could be ascribed to the inherent structural and chemical durability and the superior corrosion resistance of the TiO 2 -based oxide in acidic and oxidative environments 6 .
The cyclic voltammetry measurement was carried out in N 2 -purged 10 v/v % CH 3 OH/0.5 M H 2 SO 4 at a scan rate of 50 mV/s to evaluate the electrocatalytic activity towards methanol electro-oxidation reaction (MOR) of the Pt NWs/Ti 0.7 W 0.3 O 2 catalyst. Figure 4a compares the CV curves of the Pt NWs/Ti 0.7 W 0.3 O 2 , the Pt NWs/C and the conventional Pt NPs/C electrocatalysts. Compared with other as-obtained electrocatalysts, the Pt NWs/ Ti 0.7 W 0.3 O 2 catalyst exhibited the highest mass activity (355.29 mA/mg Pt ), which is ~1.23-fold and ~1.57-times higher than those of the Pt NWs/C (288.79 mA/mg Pt ) and conventional Pt NPs/C (226.40 mA/mg Pt ) catalysts, respectively, albeit it's the low electrochemical surface area (ECSA) value. The great electrocatalytic activity of the Pt NWs/Ti 0.7 W 0.3 O 2 catalysts is ascribable to the formed Pt nanoforms along the (111) orientation, which possessed the most activity towards methanol electro-oxidation owing to the low poisoning rate 24 . Furthermore, the potential at which the methanol oxidation starts (i.e., the onset potential, E onset ) of the Pt NWs/Ti 0.7 W 0.3 O 2 catalyst was found to be the lowest (~0.1 V vs. NHE), which is negatively shifted about 200 mV and 350 mV with respect to the Pt NWs/C (~0.3 V vs. NHE) and the conventional Pt NPs/C (~0.45 V vs. NHE) (see Fig. 4b), implying that the methanol electro-oxidation reaction (MOR) on the Pt NWs/Ti 0.7 W 0.3 O 2 catalyst was performed easier and faster than the Pt NWs/C and Pt NPs/C electrocatalyst. Consequently, the ratio of the forward peak current density (I f ) and a negative-going current density (I b ) is generally represented the resistance to the poisoning of the accumulation of carbonaceous species 2,25 . Interestingly, the Pt NWs/Ti 0.7 W 0.   www.nature.com/scientificreports www.nature.com/scientificreports/ support as well as the advantages of one-dimensional (1D) structure of Pt nanocatalyst such as (i) long segments of smooth crystal planes, (ii) a low number of surface defects, leading to good sensitivity and and activity for methanol electro-oxidation reaction 11,18 and (iii) unique one-dimensional (1D) Pt morphology, resulting in improving mass transport and electron transfers during electrocatalytic reactions 11,17,18,27,28 .
In order to examine the durability of the Pt NWs/Ti 0.7 W 0.3 O 2 catalyst towards MOR, the 5000 potential cycling measurement was conducted in N 2 -purged 10 v/v % CH 3 OH/0.5 M H 2 SO 4 solution at a scan rate of 50 mV/s. Figure 4c-e show the CVs of three different electrocatalysts before and after the test. As results indicated that the Pt NWs/Ti 0.7 W 0.3 O 2 electrocatalysts exhibited superior stability in comparison with the Pt NWs/C, and the conventional Pt NPs/C catalysts. Particularly, the Pt NWs/Ti 0.7 W 0.3 O 2 catalyst demonstrated the deterioration of the mass activity to be around 12.05% of the initial mass activity, which was ~2.18-times and ~3.06-fold lower than those of the Pt NWs/C (~26.36%), and the conventional Pt NPs/C electrocatalysts (~36.83%), respectively (see Fig. 4f). It can be concluded that the stability of the Pt NWs/Ti 0.7 W 0.3 O 2 was greatly enhanced towards the MOR.
Application of the pt nWs/ti 0.7 W 0.3 o 2 towards ethanol electro-oxidation reaction (EOR). Until now, ethanol has emerged as green fuel sources, which can alter for methanol because of lower toxicity and market cost, however, one of the most major challenges is the development of the electrocatalyst with the great electrocatalytic stability and activity towards ethanol electro-oxidation reaction (EOR) 11 . With the unique electrocatalytic properties towards MOR, we also further evaluated the catalytic activity and stability of the Pt NWs/Ti 0.7 W 0.3 O 2 electrocatalyst for the EOR. Figure 5a shows the CV curves of three different electrocatalysts in N 2 -purged 10 v/v % C 2 H 5 OH/0.5 M H 2 SO 4 solution at a scan rate of 50 mV/s. By comparing the positive-going and negative-going EOR waves in terms of the peak potential and peak current, the Pt NWs anchored over the mesoporous Ti 0.7 W 0.3 O 2 catalyst support  www.nature.com/scientificreports www.nature.com/scientificreports/ exhibited the higher electrocatalytic activity than the Pt NWs/C and the conventional Pt NPs/C electrocatalyst. On the positive-going sweep, both the onset potential and ethanol electro-oxidation potential of the Pt NWs/Ti 0.7 W 0.3 O 2 catalyst were negatively shifted compared to those of the conventional Pt NPs/C to be ~300 mV and ~40 mV, respectively (see Fig. 5a), implying the better CO-tolerance of the as-prepared Pt NWs/Ti 0.7 W 0.3 O 2 catalyst due to the facile removal of the adsorbed carbonaceous intermediate species 14,29 . Moreover, the mass activity of the Pt NWs/Ti 0.7 W 0.3 O 2 catalyst was found to be around 325.01 mA/mg Pt , which was ~1.91-fold and ~2.35-times higher than those of the Pt NWs/C (~169.73 mA/mg Pt ), and the traditional Pt NPs/C (~137.98 mA/mg Pt ), respectively (Fig. 5b), suggesting that the catalytic activity towards ethanol electro-oxidation reaction (EOR) of the Pt NWs/Ti 0.7 W 0.3 O 2 catalyst was drastically improved. The great mass activity of the as-obtained Pt NWs/Ti 0.7 W 0.3 O 2 catalyst could result from the strong interaction between the Pt nanocatalyst and Ti 0.7 W 0.3 O 2 support which provides active species for catalytic reaction resulting in enhancing the dehydrogenation of ethanol [30][31][32][33]    www.nature.com/scientificreports www.nature.com/scientificreports/ The electrocatalytic stability of the Pt NWs/Ti 0.7 W 0.3 O 2 towards EOR was further investigated via the accelerated durability test (ADT) in N 2 -purged 10 v/v % C 2 H 5 OH/0.5 M H 2 SO 4 aqueous solution. These outcomes indicated that the Pt NWs/Ti 0.7 W 0.3 O 2 catalyst possessed the superior stability towards ethanol electro-oxidation in comparison with the Pt NWs/C and the conventional Pt NPs/C electrocatalysts. For instance, after 5000 cycling test, the mass activity of the Pt NWs/Ti 0.7 W 0.3 O 2 electrocatalyst was found to be around 283.89 mA/mg Pt with respect to the deterioration to be ~12.65% of the initial mass activity (~325.01 mA/mg Pt ), meanwhile, the Pt NWs/C and conventional Pt NPs/C electrocatalysts showed the decay to be around ~30.09% (from 169.73 mA/ mg Pt dropped to 118.66 mA/mg Pt ) and ~45.36% (from 137.98 mA/mg Pt to 75.41 mA/mg Pt ), respectively (see Fig. 5c-f). The significant degradation of the Pt NWs/C and the conventional Pt NPs/C electrocatalyst is attributable to the poor durability of the carbon-based support resulting in the detachment/dissolution, Ostwald ripening of the Pt nanocatalyst 2 .
The chronoamperometry measurement in N 2 -purged 10 v/v % C 2 H 5 OH/0.5 M H 2 SO 4 aqueous solution at the immobilized potential of 0.7 V for 7200 s was carried out to investigate the electrocatalytic stability of the Pt NWs/Ti 0.7 W 0.3 O 2 electrocatalyst towards EOR. As can be seen in Fig. 6, the Pt NWs/Ti 0.7 W 0.3 O 2 catalyst exhibited the initial mass activity to be 187.04 mA/mg Pt , which is higher than those of the Pt NWs/C (~178.99 mA/mg Pt ) and the conventional Pt NPs/C (~175.21 mA/mg Pt ). After 7200 s test, the mass activity of the Pt NWs/Ti 0.7 W 0.3 O 2 electrocatalyst was remained to be around 116.80 mA/mg Pt , which is ~1.54-fold and ~8.44-fold higher than those of the Pt NWs/C (75.90 mA/mg Pt ) and the Pt NPs/C (13.84 mA/mg Pt ) at the same time, respectively. The decay rate of the as-prepared catalysts is showed in order: Pt NWs/Ti 0.7 W 0.3 O 2 (~0.59 mA/mg Pt .min) < Pt NWs/C (~0.86 mA/cm 2 .min) < Pt NPs/C (~1.34 mA/mg Pt .min). The superior durability of the Pt NWs/Ti 0.7 W 0.3 O 2 electrocatalyst relative to the conventional Pt NPs/C could be interpreted due to the lower vulnerability to dissolution, Ostwald ripening and aggregation of the 1D Pt structure (nanowires) than 0D Pt structure (nanoparticles) 11,13,14 . In addition, the high corrosion resistance of TiO 2 -based oxide in acidic and oxidative environments 14   www.nature.com/scientificreports www.nature.com/scientificreports/ ultrasonically dissolved into the above solution for 15 min to create a homogeneous suspension. Afterward, the as-prepared suspension was stored at room temperature for 72 hours to fabricate the 50 wt % Pt NWs/Ti 0.7 W 0.3 O 2 catalyst. Finally, the obtained product was rinsed copiously with purified water and then dried at 80 °C overnight for further analysis. For comparison, the 50 wt % Pt NWs were grown on Vulcan XC-72 support at the same condition.
Material characterization. The structure information of the formed Pt nanowires over the mesoporous Ti 0.7 W 0.3 O 2 support was measured via the X-ray diffraction (XRD) measurement operated on a D2 PHASER-Brucker using Cu K α radiation at 30 kV. The transmission electron microscopy (TEM) measurement was conducted on the JEOL-LEM 1400 microscope at an accelerating voltage of 3800 V to examine the morphology of the as-prepared Pt NWs/Ti 0.7 W 0.3 O 2 catalyst. Furthermore, the X-ray photoelectron spectroscopy (XPS) was implemented to investigate the surface properties of the as-obtained Pt NWs/Ti 0.7 W 0.3 O 2 electrocatalyst.
Electrochemical properties. An EC-LAB Electrochemistry instrument (Bio-Logic SAS) with an Ag/ AgCl/Sat. KCl electrode, and a Pt gauze, as well as glassy carbon electrode with 5 mm in diameter with respect to a reference electrode and the counter electrode as well as a working electrode, were used for investigating all electrochemical tests. The electrocatalytic activity towards methanol electro-oxidation reaction (MOR) and ethanol electro-oxidation reaction (EOR) of the as-obtained catalysts was recorded at a scan rate of 50 mV/s in N 2 -statured 10 v/v % CH 3 OH/0.5 M H 2 SO 4 solution and N 2 -statured 10 v/v % C 2 H 5 OH/0.5 M H 2 SO 4 solution, respectively. Furthermore, the ADT test was performed in the range of 0 V to 1.0 V (vs. NHE) for 5000 cycles at a scan rate of 50 mV/s at room temperature in N 2 -purged 10 v/v% CH 3 OH/0.5 M H 2 SO 4 and in N 2 -purged 10 v/v% C 2 H 5 OH/0.5 M H 2 SO 4 for methanol electro-oxidation reaction (MOR) and ethanol electro-oxidation reaction (EOR), respectively. All potential ranges in this work were reported with the normal hydrogen electrode (NHE) scale. The catalyst ink preparation: the catalyst powder was ultrasonicated in a solution comprising ethanol absolute and 0.5% Nafion within 30 min. Before the catalyst ink placement, the surface of the glassy carbon disk was polished with 0.5 µm BAS and then washed with ethanol as well as purified water. To start with, the catalyst electrode was activated by 100 cycles at a scan rate of 50 mV/s. In this work, the Pt loading onto the glassy carbon electrode was maintained at 0.13 mg/cm 2 in all electrochemical tests.