Rutile RuxTi1-xO2 nanobelts to enhance visible light photocatalytic activity

We herein report on the synthesis by a facile sol-gel method without templates for preparing rutile RuxTi1-xO2 (x = 0.16; 0.07; 0.01) nanobelts with exposed (001) facets. The rutile nanobelts with exposure (001) facets, favor the separation photogenerated electron-hole pairs and inhibit the recombination of the electron-hole pairs resulting in the increase of the number of main superoxide and hydroxyl radicals. The photocatalytic properties of the rutile RuxTi1-xO2 nanobelts were evaluated by discoloring of MB (methylene blue) dye under sunlight irradiation at an intensity of 40000 lx. It was also done a thorough interface analysis to determine the band energy.

It should be noted that the larger lattice constants and d-spacing values are due to upon Ru doping. This is due to the larger ionic radius of Ru than that of Ti in the rutile, the structural relaxation follows the Vegard's law 16 . The d-spacing value of the rutile sample for (110) plane is in agreement to the interatomic dimension observed at HRTEM microscopy. Figure 2 shows the HRTEM images of Ru x Ti 1-x O 2 (x = 0.16) nanobelts sample. The TEM images confirm the formation of one-dimensional nanobelts nanostructures. Rutile TiO 2 nanobelts have very small thicknesses, widths of 6-20 nm and can be 100-200 nm in length.
The HRTEM image shows the (101) and (002) atomic planes with lattice spacings of 3.499 Å (1/r) and 4.255 Å (1/r), respectively 17 . The interfacial angle between these two crystalline facets was found to be 67.27° and it was determined by Fast-Fourier Transform (FFT) image (Fig. 2). Deviation with one degree may be due to ruthenium from the crystalline lattice. These results even if less than the theoretical value, reveal that the interfacial angle between rutile is in good agreement with the theoretical value of the angle between the (101) and (001) planes. HRTEM images suggest that the prepared sample behaved like a well-crystallized heterostructure nanobelts.
Rutile Ru x Ti 1-x O 2 nanobelts have an octahedral arrangement, where either Ti or Ru atoms prefer a coordination number of 6. The layered arrangements of octahedrons facilitate their growth in the (001) direction as nanobelt like geometry 14 .
The UV-Vis diffuse reflectance spectroscopy (DRS) spectra of all samples are shown in Fig. 3A. The DRS method is employed to determine the band gap energy for photocatalysts. The intense absorption feature in the range of 200 nm-408 nm is characteristic of the TiO 2 and corresponds to the band gap energy of 3.2 eV as for rutile phase. The visible absorption band in the range 408-627 nm corresponds to the band gap energy from 2 to 3.2 eV and can be assigned to charge transfer transition of the donor (Ru 4+ → Ru 5+ + e − , Ru 3+ → Ru 4+ + e − ) or acceptor (Ru 4+ → Ru 3+ + h + ) type 18,19 .
This suggests that rutile Ru x Ti 1-x O 2 photocatalysts could be active in the visible light region. The specters UV-vis from rutile Ru x Ti 1-x O 2 photocatalysts were transformed to the absorption specters according to the Kubelka Munk theory:   www.nature.com/scientificreports www.nature.com/scientificreports/ XPS measurements have been performed to analyze the surface composition and oxidation states of Ti and Ru in the samples. The survey spectrum for the Ru x Ti 1-x O 2 (x = 0.16), with the highest ruthenium content (Fig. 3B), confirms the complete removal of chlorine in the samples TiO 2 doped. The absence of the peaks in area 197.9 eV and 199.5 eV (Cl 2p3/2 and Cl 2p1/2) were observed. Figure 3C shows the XPS spectra of Ti 2p on the surface of each sample. The binding energies at 454.0 and 459.7 eV (Ti 2p3/2 and Ti 2p1/2) correspond to octahedral coordinated Ti 4+ state 19,20 . The energy difference between the two peaks is 5,7 eV which is consistent with the energy  www.nature.com/scientificreports www.nature.com/scientificreports/ difference between the level of spin-orbit splitting coupling effect. c (Fig. 3C). The substitution of Ru ions into the TiO 2 lattice can induce electronic structure.
The Raman spectra for rutile Ru x Ti 1-x O 2 photocatalysts are displayed in Fig. 3D, where two bands features of tetragonal rutile TiO 2 (space group D 4h ) at 445 and 610 cm −1 were assigned to E g (planar O-O vibration) and A 1g (Ti-O stretch) modes 20. The broad band at 235 cm −1 was attributed to the multiple photon scattering process. The absence of the peaks to features attributable to RuO 2 from 528 cm −1 and 646 cm −1 corresponds to E g and A 1g modes 21 , was evident in all of the samples, confirming the lattice substitution of the RuO 2 in the rutile TiO 2 . Ruthenium doping induces a low red-shifts with increasing Ru concentration in all of the samples, probably due to the formation of oxygen defects or Ru-O-Ti linkages (Fig. 3D).
The photocatalytic properties of the rutile Ru x Ti 1-x O 2 nanobelts have been evaluated by photocatalytic degradation of methylene blue (MB) under sunlight at the 40000 lx intensity. The photocatalytic results are displayed in Fig. 5A. It was observed that the photodegradation process of methylene blue took place faster in the presence of rutile doped with ruthenium compared with undoped rutile. The increase of ruthenium content gave a decrease of the band gap that can be correlated with the increase absorption to the visible light range, and respectively enhance of the photocatalytic activity. It was observed that the photodegradation process of methylene blue took place faster in the presence of rutile Ru x Ti 1-x O 2 (x = 0.16) nanobelts compared to the other synthesized photocatalysts. It can be observed that after irradiation for 90 min, about 90% of the methylene blue has been degraded for the Ru 0.16 Ti 0.84 O 2 nanobelts which are above the results of the rutile TiO 2 (74 wt.%).
The kinetic curves of sunlight MB degradation over each catalyst are depicted in Fig. 5B and show that the degradation of MB dye follows pseudo first-order kinetics law, lnC/C 0 = k app t, where k app is the pseudo-first order constant rate.
The constant rate k app , was determined by plotting the lnC/C 0 versus irradiation time. It can be observed that with increasing of the ruthenium content the constant rate k app rises from 0.0125 min −1 to 0.0227 min −1 ( Table 2).
On the basis of the experimental results, a possible mechanism of the enhanced photocatalytic activity over the rutile Ru x Ti 1-x O 2 is proposed in Fig. 6.
The enhanced photocatalytic activity caused by ruthenium doping and to the ability of Ru to capture the photogenerated holes on the TiO 2 (valence band). The conduction band can be calculated by using the empirical equation 22 : Ecb = Evb − Eg and Evb of semiconductor were estimated by the following equation: www.nature.com/scientificreports www.nature.com/scientificreports/ Evb = X − Ee + 1/2Eg, where Ecb is the conduction band, Evb is the valence band, X the geometric mean of the Mulliken, Ee is the energy of free electrons on the hydrogen scale (~4.5 eV) and Eg is the bandgap value of semiconductor, respectively. The values of Ecb, Evb are displayed in Table 3.
It can be observed that hydrogen peroxide and peroxide radicals can form. The photocatalytic process is shown in Fig. 6. Excitaded electrons under sunlight jumped from VB (valence band) to CB (conduction band) and generate charge carriers (electron-hole pairs). Due to the nanobelts structure, the electron-hole pairs are moved to the photocatalyst surface, where electrons and holes are involved in redox reactions on the surface. The photocatalytic degradation efficiency can be due to the reactive (001) facets, these having a strong ability to dissociate water molecules to form hydrogen peroxide and peroxide radicals, contributors in the photo-oxidation process 16 .    www.nature.com/scientificreports www.nature.com/scientificreports/ Discussions Ru x Ti 1-x O 2 nanobelts were synthesized by the sol-gel method by doping TiO 2 with RuO 2 . The reaction was carried out in HCl medium (c > 10 mol/l), which facilitated the formation of nanobelts nanostructures. RuO 2 has been chosen for doping since both RuO 2 and TiO 2 have the same crystalline crystal structure, the crystalline parameters have almost equal values, coordinate number 6 for both Ti and Ru. From the diffraction spectrum, the rutile phase is observed for all nanobelts with high crystallinity. Also, doping TiO 2 with ruthenium has a double role: decrease the bandgap and facilitating light from the visible field in the photocatalysis process, as well as to inhibit the recombination of the electron-hole pairs. UV-vis absorption spectra were transformed using the Kubelka Munk function and the values of the bandgap energy for all samples with which the values of conduction bands and valence bands were calculated. In both the UV-vis and the transformed Kubelka Munk spectra, there are two types of transitions: interband between the 2p orbitals of the oxygen and the orbitals of the ruthenium and an interband type d-d between the orbitals of Ru (1.65-1.81 eV) [23][24][25] . TiO 2 is considered a n-type semiconductor. There is a decrease in the bandgap from 3.17 to 2.55 eV as the content of Ru concentration increases in the samples, there is also an increase in the conductive band value, respectively, a decrease of valence value of nanobelts as the increase in doping with Ru. This favors the splitting of water and the formation of oxidoreduction species. Corroborated with the shape of nanostructures with surface exposure (001) facets showed the performance of efficient photocatalysts in the degradation of organic compounds. Doping with Ru induces a stress in the crystal of Ru x Ti 1-x O 2 , which is revealed by an angle deviation of 68.3° to 67.3° for the (001) facets. Crystal stress is also noticeable in Raman spectra by easily moving to redshifts the bands assigned to Eg and A1g corresponding to the rutile phase of TiO 2 . The XPS spectrum shows a slight (low) displacement of the corresponding Ti 2p 3/2 and Ti 2p 1/2 blue-shifts spectra due to ruthenium doping. The high performance of the photocatalysts is due both to the nanobelts form and to the ruthenium doping and the visible light access to the photodegradation process. The superoxide and hydroxyl radicals obtained on the photocatalyst surface favor the mineralization reaction of the organic dye by in CO 2 and H 2 O.
In conclusion, the rutile Ru x Ti 1-x O 2 nanobelts with exposure of (001) facets were successfully synthesized by a simple sol-gel method chemical route using hydrochloric acid and ethanol as capping and stabilizing agents. The superior photocatalytic activity of rutile nanobelts can be assigned to the band gap energy reduction from 3.2 to 2.55 eV and due to its small size, high surface area and of exposed highly reactive (001) facets. Also, the architecture of the heterostructure with exposure (001) facets favors the separation of photogenerated electron-hole pairs and inhibit the recombination of the electron-hole pairs resulting in the increase of the number of main superoxide and hydroxyl radicals.
A useful application of green technology can be the utilization of the synthesized photocatalysts in the remediation of the environment by decomposition of organic compounds under the sunlight.

Methods
Materials. The chemicals used in this work were of analytical reagent. Titanium n-butoxide Ti(OBu) 4 , ruthenium chloride (RuCl 3 xH 2 O), hydrochloric acid (HCl), methylene blue (MB) and NH 4 OH were purchased from Sigma-Aldrich. All solutions were prepared with distilled water. characterization. X-Ray diffraction (XRD) of the samples was analyzed at ambient temperature on a Bruker D8 Advance diffractometer using the characteristic Kα radiation of copper at a voltage of 40 kV and a current of 40 mA. XRD patterns were collected in the 2θ range between 5° and 80°.
The UV-vis diffuse reflectance spectra of rutile Ru x Ti 1-x O 2 were obtained by using a Jasco UV-Vis V-550 spectrophotometer in the wavelength range from 200 to 900 nm with an integrating sphere assembly. The sample was diluted with MgO (ratio 1:6) and then mechanically mixed. The UV-vis absorption was transformed according to the Kubelka Munk function, ∞ F R ( ), for infinite thick samples. The sample surface elements and their oxidation states were analyzed by Thermo Scientific K-Alpha X-ray Photoelectron Spectrometer (XPS) system with Al K-alfa radiation. The Raman spectra of samples were registered using a DXR Raman Microscope from Thermo Scientific.
The morphology of the samples was characterized by transmission electron microscope Tecnai ™ G2 F20 TWIN  ites were evaluated via the degradation of MB under sunlight with a light intensity I = 40000 lx. For photocatalysis analysis, 0,1 g of the photocatalyst was suspended in a beaker with 50 ml aqueous of MB C 0 = 10 −4 mol/l. Before exposure to sunlight, the reaction systems were kept in the dark for 30 min to reach absorption-desorption equilibrium between Ru x Ti 1-x O 2 photocatalysts and MB solution. The absorption intensity of MB was monitoring by using a UV-vis spectrophotometer Jasco UV-vis V-540. At 30 min intervals, 2 ml solution was collected and it was measured absorption at 665 nm (sample is recovered).