Structural Evolution of Chemically-Driven RuO2 Nanowires and 3-Dimensional Design for Photo-Catalytic Applications

Growth mechanism of chemically-driven RuO2 nanowires is explored and used to fabricate three-dimensional RuO2 branched Au-TiO2 nanowire electrodes for the photostable solar water oxidation. For the real time structural evolution during the nanowire growth, the amorphous RuO2 precursors (Ru(OH)3·H2O) are heated at 180 °C, producing the RuO2 nanoparticles with the tetragonal crystallographic structure and Ru enriched amorphous phases, observed through the in-situ synchrotron x-ray diffraction and the high-resolution transmission electron microscope images. Growth then proceeds by Ru diffusion to the nanoparticles, followed by the diffusion to the growing surface of the nanowire in oxygen ambient, supported by the nucleation theory. The RuO2 branched Au-TiO2 nanowire arrays shows a remarkable enhancement in the photocurrent density by approximately 60% and 200%, in the UV-visible and Visible region, respectively, compared with pristine TiO2 nanowires. Furthermore, there is no significant decrease in the device’s photoconductance with UV-visible illumination during 1 day, making it possible to produce oxygen gas without the loss of the photoactvity.

photocurrent with ~100% increase compared to the pristine TiO 2 nanowires 9 . The photocatatytic activities of TiSi 2 /graphene nanoparticles and Nb/N-doped anatase TiO 2 nanoparticles were reported to be improved by the loading of RuO 2 nanoparticles 10,11 . Also, the RuO 2 -Au composites enhanced the catalytic property with ~30% than pure Au 12 . Thus, most of the catalysts are formed as a form of fine nanoparticles. The photocatalytic activity may be enhanced by the advantages imparted by the 3D branched nanostructure, as mentioned above. Furthermore, since 2012, it is also interesting that the price of Ru has decreased by approximately 40% on sales in Asia and U.S. 13 .
In spite of its great potential toward future real applications, on the other hand, the growth of highly single crystalline RuO 2 nanowires has been recently reported by thermal oxidation or chemical vapor deposition (CVD) of appropriate Ru-based precursors and reactive sputtering using pure Ru metal targets 7,14,15 . However, it is still challenging to make RuO 2 nanowires with well-defined crystal structures with desirable density in real applications. For the RuO 2 nanowires grown by a CVD, a process referred to as vapor-solid (VS) growth model was suggested via the formation of gaseous RuO 4 species which is a highly volatile and very low melting point intermediate 14 . A thermal conversion from amorphous oxides nanoparticle precursor was also suggested to synthesize single crystalline nanowires of oxide materials 15 . However, it is necessary to explore this mechanism in greater detail in order to fully demonstrate the preferential and unidirectional crystal growth of RuO 2 .
Here, we focus on the real-time structural evolution of chemically-driven RuO 2 nanowires for the growth mechanism and report a facile method to fabricate RuO 2 branched Au-TiO 2 nanowire arrays for the photostable solar water oxidation. The in-situ synchrotron x-ray diffraction (in-situ SXRD) shows the direct transition of the amorphous RuO 2 precursors (Ru(OH) 3 ·H 2 O) to crystalline RuO 2 nanoparticles with the tetragonal crystallographic structure at 180 °C , without the intermediate change formation. In the high-resolution transmission electron microscope (HRTEM), we find that most of the crystallized nanoparticles are in the vicinity of the nanowires and the amorphous region is Ru enriched by about 10% in comparison to the RuO 2 nanowire. This means that the growth proceeds by Ru diffusion to the nanoparticles, increasing the size of the nanoparticles, eventually, forming the nanowires. At low temperature less than 250 °C , the diffusion is so slow, thereby, the nanowires are very short (less than 1 μ m at the growth time of several hours). We believe that this process is supported by the nucleation theory. We also carefully explore the photocatalytic performance of the water splitting of the branched nanowires and the performance is compared with pristine TiO 2 nanowires. We found that the photoactivity was effectively enhanced in the entire UV-visible region, especially more effective in visible light, due to the high catalytic properties of the RuO 2 for the water oxidation and the efficient plasmonic absorption with efficient charge separation to TiO 2 and Au interface, catalyzing the oxygen evolution of the 3D RuO 2 branched structure on the Au surface. The electrodes were quite stable during long-term use (we believe that it may ascribe to the high-crystalline properties of the RuO 2 nanowires).

Results and Discussion
The schematic diagrams outlining the fabrication process of 1D RuO 2 nanowires and 3D RuO 2 branched Au-TiO 2 nanowires are shown in Fig. 1a and detailed information described in Experimental section. Scanning electron microscopy (SEM) images in Fig. 1b shows the product (of a reaction carried out at 250 °C ) to consist of long, randomly oriented RuO 2 nanowires without any catalyst particles at the end of their tips. The nanowires (0.5 ~ 1 μ m long and 130 ~ 170 nm in diameter) grow out of the plane of the substrate in the rectangular shape. Figure 1c shows RuO 2 nanowires directly grown on hydrothermally-grown TiO 2 nanowires with Au nanoparticles (AuNPs), producing 3D branched nanowires. The lengths of most of the nanowires fall in the range 60 ~ 80 nm. The mean nanowire width was determined to be 25 nm. The smaller RuO 2 nanowires in the 3D branched structures ascribes to the decrease in the amount of the precursor, which is an important factor in determining the length and width of the nanowires. Figure 2a shows SEM images of the as-grown RuO 2 nanowires on a Si substrate synthesized by heating the RuO 2 precursors in air atmosphere in the range of 150 ~ 250 °C during 4 hrs. At low temperature less than 150 °C , only nanoparticles were observed and there were no nanowires on the substrates. In ex-situ x-ray diffraction (XRD), there are no peaks in the spectra, meaning that the nanoparticles are still amorphous. As the temperature increases to 180 °C , three peaks corresponding to the (110), (101), and (211) planes were clearly observed, confirming that the amorphous nanoparticles start to be crystallized at the temperature. In the SEM image, many straight nanowires with apparent tapering at the end and without any cluster at the tip were also seen. Further increase in the temperature enhances the growth of the nanowires, increasing the intensity of the three peaks, and produces additional peak corresponding to the (200) plane. At 250 °C , the length and diameter of the nanowires are in the range of 150 nm and 18 nm, respectively. The length and diameter of the nanowires increase as the temperature increases, while the aspect ratio decreases with the temperature (Supplementary Fig. S1).  211)) of RuO 2 nanowires as a function of temperature (scale bar, 100 nm). The peak at 33.2 ° is due to the substrate. (c) The in-situ synchrotron radiation diffraction from the growth of RuO 2 nanowires as the temperature increases to 150 ~ 600 °C . Each vertical line is one XRD scan, with the intensity represented by the color. The first scan is at the left, and time progresses right. (d) The change in the intensity of (101) and (110) plane with time and inset image is full time.
In order to further investigate the synthetic process, such as intermediate phase formation over annealing time and temperature, in-situ SXRD experiments were carefully performed in the course of the growth process. In-situ SXRD data provides direct identification of the solids and a sequence of events in the formation of different objects and their growth. The ruthenium hydroxide precursors are put on the stages and then heated by external power sources up to 600 °C with the ramping speed of 7 °C /min. 42 data points for 2 minutes at each temperature interval were obtained. However, experience tells that the measured temperature is not equal to that of the nanoparticles, which will be lower. In the initial stage of the heating, there is no peak. As the temperature increases to about 300 °C , two peaks corresponding to the (110) and (101) planes are observed. However, we could observe any other peaks, indicating the spontaneous growth of the RuO 2 phases from the amorphous phases without any intermediate phase formed.
The above spontaneous growth of the nanowires during the oxidation of precursors without the use of conventional templates or catalysts has received significant attention in the nanotechnology community. The VS model was usually proposed to understand the growth of ZnO and SnO 2 nanowires based on the evaporation of the precursors 16,17 . However, the RuO 2 synthesis is carried out at low temperature of ~180 °C, at which the equilibrium vapor pressures of the pure elements are negligibly small, therefore cannot be ascribed to the model. The growth of the nanowires may ascribe to the diffusion of the cations during the oxidation, forming the tip-like geometry of a single nanowire, in which the driving force for the diffusion is related to the internal stress associated with the phase formation at the CuO/Cu 2 O interface. Also, metal-oxides nanowires such as VO 2 , V 2 O 5 , MoO 2 , MoO 3 , and Fe 3 O 4 , can be grown via the diffusion and solidification of nanodroplets near the bulk melting point [18][19][20][21][22][23] .
The detailed growth mechanism was characterized by bright-field TEM (BFTEM) and HRTEM images, as shown in Fig. 3. Figure 3a is a BFTEM image of a single nanowire grown at 190 °C with a diameter of approximately 30 nm. The HRTEM image and the corresponding fast-Fourier transformed (FFT) of the nanowire reveal highly ordered lattice fringes, demonstrating that the nanowire is a defect-free single crystal (Fig. 3b). The RuO 2 nanowire has identified as a tetragonal crystalline (110) phase. In the vicinity of the region where the nanowire starts to be grown, there are some crystallized RuO 2 nanoparticles that are ~4 nm in size are clearly seen in the vicinity of the nanowires, while most of the nanoparticles exist as a form of amorphous elsewhere (Fig. 3c,d). Any additional Ru-based crystalline phases except the RuO 2 phase, tetragonal crystallographic structure, are not observed. This implies that the nanowires are grown from the amorphous nanoparticles by the direct recrystallization process.
Previously, it was reported that the amorphous RuO 2 precursors started to lose the water of the hydration to create the anhydrous RuO 2 nanoparticles and Ru metals at 90 °C, as follows 24 ; The rate of Ru/RuO 2 content was approximately 0.3 in N 2 ambient, meaning that the nanoparticles are amorphous, and increased with the annealing temperature. Actually, it was observed that the amorphous region in Fig. 3c, about 20 nm away from the nanowire, was Ru enriched by about 10% in comparison to the RuO 2 nanowire in Fig. 3b, determined by energy dispersive spectroscopy (EDS). However, most of nanoparticles are seen to be crystalline in the vicinity of the nanowire. The size of the nanoparticles is in the range of 2 ~ 4 nm. According to the nucleation theory, the critical radius (r*) of the nucleation critically depends on the surface energy (γ), the latent heat of fusion (ΔH f ), the melting temperature (T m ), and the growth temperature (T g ), which is given by, Here, the melting temperature and latent heat of fusion are 1200 °C and 1.66 × 107 J/m 3 , respectively. The surface free energy of (110) and (001) planes are 1.136 J/m 2 and 1.398 J/m 2 , respectively 24,25 . In this case, the critical thickness of RuO 2 is calculated to be about 2 nm at 190 °C , similar with those observed in the TEM images (Fig. 3d). The observation of the RuO 2 crystalline nanoparticles between the amorphous phase and the nanowire may suggests that the amorphous phase is crystallized before the growth of the nanowires.
The growth rate of the nanowires is mainly limited by the rate of transport of mass, the diffusion rate of Ru and RuO 2 materials produced by the heating process of RuO 2 precursor. The activation energy (Δ E) for the migration of Ru and RuO 2 of Δ E is supposed to be 384 kJ/mol and 3140 kJ/mol, respectively through latent heat of fusion (Ru: 24 kJ/mol, RuO 2 : 314 kJ/mol) 26 . This indicates that Δ E of RuO 2 is about 8 times of magnitude of larger than that of Ru, meaning that the diffusion of Ru metals is the dominating process. The concentration gradient of Ru from the amorphous phase to the nanowires promotes the diffusion, enhancing the growth of the nanowires. We also believe that the Ru metals migrate from the bottom to top region of the nanowires through the surface because the bulk diffusion becomes significant at only high temperature. In O 2 ambient, the Ru reacts with oxygen, producing the RuO 2 on the surface 27 . Actually, at free oxygenless atmosphere such as ambient gases such as nitrogen or under vacuum conditions, there are no nanowires on the substrates. The RuO 2 nanowires were successfully grown on the Au-decorated TiO 2 nanowires, by precisely controlling the concentration of precursors and the growth temperature. HRTEM image in Fig. 4a shows that single-crystalline RuO 2 nanowire is directly grown on the Au nanoparticle. EDS elemental mapping of Fig. 4b clearly also confirms each elemental distribution.
The PEC performance of the TiO 2 -based photoelectrodes was examined in a three-electrode configuration with Ag/AgCl as the reference electrode and a Pt wire as the counter electrode. Figure 5a shows the photocurrent-potential characteristics of the TiO 2 nanowires, AuNPs decorated TiO 2 nanowires, and RuO 2 branched AuNPs-TiO 2 nanowires recorded in 0.5 M Na 2 SO 4 electrolyte (pH = 7.2) under AM 1.5G illumination of 100 mW/cm 2 . The dark scans revealed a small background current of ~2 × 10 −3 mA/cm 2 , negligible compared to the photocurrent densities of all photoelectrodes. Upon illumination with white light, the pristine TiO 2 nanowires electrodes show a photocurrent density of 0.673 mA/cm 2 at 1.23 V vs. RHE (0.61 V vs. Ag/AgCl). As AuNPs are decorated onto the nanowires, the photocurrent density increases to 0.909 mA/cm 2 at the same potential, associated with the surface plasmonic resonance effect of the AuNPs 28 . It was also observed that under visible-light illumination, the photocurrent density of AuNPs-TiO 2 nanowires was increased by 1.93 times, in comparison with pristine TiO 2 nanowires, obtained by adding a 420 nm long-pass filter to the white source, as shown in Fig. 5b. The formation of RuO 2 nanowires on AuNPs-TiO 2 nanowires increases the photocurrent density to 1.052 mA/cm 2 , around 56% enhancement, compared with pristine TiO 2 nanowires. It is believed that the RuO 2 promotes the electron transfer to the AuNPs due to the high catalytic properties for the water oxidation and the efficient hole injection to RuO 2 to AuNPs, catalyzing the oxygen evolution ( Supplementary Fig. S2). This process is prominent seen under visible light. We also observed that the photocurrent of the branched nanowires significantly increased over approximately 0.7 V vs. RHE, compared to that of the AuNPs-TiO 2 nanowires, with a slight shift in the onset voltage. This may be related to the flat band voltage as 0.41 eV of the work function difference between Au and RuO 2 . The slight shift in the onset voltage may also be ascribed to the low overpotential on the Au-TiO 2 nanowires associated with the oxygen reduction 29,30 .
For the long photostability of the electrodes, the photocurrent was measured under illumination conditions of illuminated with white light at 1.23 V vs. RHE as a function of time up to 24 hrs. Figure 5c shows that the photocurrent decreases slowly up to 24 hrs after a little increase in photocurrent within 3 hrs, which may ascribe to the build-up of hydrogen concentration on Pt electrodes. The rapid decrease in the photocurrent of the efficient photoelectrodes as major limitation to commercialization was not observed here 31 , which may ascribe to the highly crystalline properties of the RuO 2 nanowires and the good chemical stability of the TiO 2 . Gas chromatographic measurements under the illumination conditions described above also shows that there is no significant difference between the calculated oxygen amount from the photocurrent density and the oxygen evolution amount. This may imply that the RuO 2 branched Au-TiO 2 electrodes has stoichiometric amount reaction for H 2 and O 2 evolution and avoid the back reaction, showing quite stable performance during the gas evolution.

Conclusion
We report the chemically-driven RuO 2 nanowires and a facile method to fabricate three-dimensional RuO 2 branched Au-TiO 2 nanowire arrays for the photostable electrodes in PEC water oxidation. The amorphous RuO 2 precursors (Ru(OH) 3 ·H 2 O) are heated at 180 °C , producing the RuO 2 nanoparticles with the tetragonal crystallographic structure and Ru enriched amorphous phases, observed by the real time structural evolution during the growth process through the in-situ SXRD and the HRTEM images. Growth proceeds by Ru diffusion to the nanoparticles, followed by diffusion to the growing surface of the nanowire in oxygen ambient, supported by the nucleation theory. The RuO 2 branched Au-TiO 2 nanowire arrays shows a remarkable enhancement in the photocurrent density by approximately 60% and 200%, in the UV-visible and Visible region, respectively, compared with pristine TiO 2 nanowires at the same potential. Furthermore, there is no significant change in the device's photoconductance with UV-visible illumination during 1 day, making it possible to produce oxygen gas without the loss of the photoactivity. This may ascribe to the highly crystalline properties of the RuO 2 nanowires and means the good photostability of the designed electrode.  Methods Synthesis of RuO 2 nanowires and Growth Mechanism. Highly-crystalline RuO 2 nanowires were grown by a simple annealing process of the RuO 2 precursor via a simple reaction between RuCl 3 ·xH 2 O and NaOH by carefully controlling pH (~9.00) of the aqueous solution at room temperature, as described previously 14 . This was found to be so simple and facile method for the growth of the nanowires. During this process, the amorphous RuO 2 precursor (Ru(OH) 3 ·H 2 O) were washed several times with deionized (DI) water to remove the remaining chlorides and sodium hydroxide, followed by the redispersion in DI water. For the detailed growth mechanism, it was spread on a Si (001) wafer and after drying at room temperature for 2 hrs, the sample are heated at the temperature ranging from 150 to 250 °C for 4 hrs.
Growth of 3D Branch-Shaped Nanostructure with Au nanoparticles. For the efficient oxygen production, TiO 2 nanowire arrays were first grown on fluorine-doped tin oxide (FTO) substrate by the hydrothermal method, reported elsewhere 32 . After washing with DI water, the sample was annealed in air at 450 °C for 4 hrs to improve the crystallinity of the nanowires. The AuNPs are formed by metal aggregation. The ~1 nm thick (mass thickness) Au film was then deposited on the TiO 2 nanowires using e-beam evaporation at a very slow deposition rate of less 0.1 Å/s and annealed at the temperature 250 °C for 1 min in nitrogen gas. The base pressure was maintained at 2.0 × 10 −6 Torr. It produced well-separated AuNPs covering the nanowire uniformly. Finally, the RuO 2 precursor mixed in DI water (RuO 2 precursor are well-controlled from 3 × 10 −6 to 6 × 10 −4 wt%) are dropped into the sample and dried at 2 hrs, followed by the annealing process in atmosphere.

Microstructural analysis.
The real time structural evolution during the growth process was done by the in-situ synchrotron radiation diffraction as the temperature increases to 150 ~ 600 °C , which was carried out at the 3D beamline at Pohang Accelerator Laboratory (PAL). For high-resolution XRD measurements, the wavelength of the incident X-ray was set at 1.488 Å by a double-bounce Si (111) monochromator. The SEM was done using a PHILIPS XL30S with an accelerating voltage of 5 kV. The HRTEM images were collected using a Cs-corrected JEM-2100 operated at 200 kV.
Photoelectrochemical Characterization. The PEC properties of the electrodes were studied in a three electrode cell with Ag/AgCl as the reference electrode and a Pt wire as the counter electrode. The working electrode area is in the range of ~0.2 cm 2 . 0.5 M Na 2 SO 4 aqueous solution (pH = 7.2) was used as an electrolyte for PEC measurements. Linear sweeps and time-profiled current generation were obtained by an IviumStat electrochemical analyzer, with Ag/AgCl as reference and Pt wire as counter electrode under simulated sunlight with a 150 W xenon lamp coupled with an AM 1.5 global filters (100 mW/ cm 2 ). For visible light measurement, a long-wave pass filter (λ > 430 nm) was placed in front of the light source.