Boosting photocatalytic performance of inactive rutile TiO2 nanorods

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Boosting photocatalytic performance of inactive rutile TiO nanorods under solar light irradiation: Synergistic effect of acid treatment and metal oxide co-catalysts 2

Love Kumar Dhandole, Mahadeo A. Mahadik, Su-Gyeong Kim, HeeSuk Chung, Young-Seok Seo, Min Cho, Jungho Ryu, and Jum Suk Jang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b02104 • Publication Date (Web): 30 Jun 2017 Downloaded from http://pubs.acs.org on June 30, 2017

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ACS Applied Materials & Interfaces

Boosting photocatalytic performance of inactive rutile TiO2 nanorods under solar light irradiation: Synergistic effect of acid treatment and metal oxide co-catalysts Love Kumar Dhandolea, Mahadeo A. Mahadika, Su-Gyeong Kima, Hee-Suk Chungb, Young-Seok Seoa, Min Choa, Jung Ho Ryu*c, and Jum Suk Jang*a a

Division of Biotechnology, Brain Korea 21 Plus Program, Advanced Institute of Environment and Bioscience, College of Environmental and Bioresource Sciences, Chonbuk National University, Iksan, 54596, Korea

b

Analytical Research Division, Korea Basic Science Institute, Jeonju, Jeollabuk-do, 54907, South Korea

c

Mineral Resources Research Division, Korea Institute of Geoscience and Mineral Resources, Daejeon 34132, Korea

Supporting Information Placeholder ABSTRACT: In the present work we accomplish the

boosting photocatalytic performance by synergistic effect of acid treatment and transition metal oxide co-catalysts on molten salt rutile TiO2 nanorods. FTIR and XPS (oxygen deconvolution) results confirmed that the amount of hydroxyl groups increased on the surface of rutile TiO2 nanorods (TO-NRs) after acid treatment. HR-TEM analysis revealed fine dispersion of metal oxide on the surface of acid treated TiO2 nanorods (ATO-NRs). The photocatalytic activities of as-prepared (TO-NRs), acid treated (ATO-NRs), metal oxide loaded (MTO-NRs), and both acid treated and metal oxide loaded (MATONRs) nanorods were compared based on the rate kinetics and dye degradation efficiencies. Cobalt oxide (1 wt%) loaded and 1.0 M acid treated TiO2 nanorods (Co/ATO-NR) exhibited the higher photocatalytic degradation efficiency for Orange-II dye degradation and inactivation of S. typhimurium pathogen compared to other photo-catalysts under solar irradiation. Photoelectrochemical analysis demonstrated that the charge transfer process in Co/ATO-NR is significantly higher than the untreated samples. The improved photocatalytic activity of inactive TO-NRs might be due to enhanced charge transfer of finely dispersed metal oxides on the rich OH surface of acid treated TiO2 nanorods. KEYWORDS: Synergistic effect; co-catalyst; Acid treatment; Transition metal oxides; Dye degradation

1. INTRODUCTION In the past years, physical treatment methods and photocatalytic degradation are used as the main ways to remove organic and synthetic chemical

dyes from wastewater.1-3 However, physical adsorption simply transfers pollutants from one phase to another instead of completely degrading them.4 Photocatalytic degradation is considered as a green technique for complete removal of organic dyes.5 In photocatalytic degradation process, different kinds of binary metal oxides6 are used as photocatalysts to effectively treat water pollutants. TiO2 is the most efficient metal oxide photocatalyst7, 8 because of its distinguished advantages such as low cost, high stability, chemically inertness, and eco-friendliness.9 However, TiO2 has disadvantages such as low charge separation efficiency which limits its photocatalytic activity.10 Many attempts have been made to enhance the photocatalytic activity and charge separation efficiency of TiO2-based photocatalyst materials, including surface treatment of TiO2 nanomaterial,11 codoping or doping of metals,12, 13 noble metal oxides co-doping,14 and transition metal oxide loading.15 Recent studies16 have shown that surface treatment with inorganic acids could improve the hydroxylsurface attachment, resulting in better photocatalytic activity. Park et al. have demonstrated the effect of acid treatment on TiO2 surface.17 The hydroxyl- and water- rich surface of HCl treated TiO2 can act as a Brønsted acid (proton donor) site and degrade rhodamine-B dye solution successfully compared to H2SO4 treated TiO2 where SO42- of acid treated TiO2 behaves as OH• scavenger.17 However, to extend the absorption threshold of TiO2 to visible light and improve the charge transfer mechanism, metal or metal oxide nanoparticles (noble or transition metals) have been loaded onto the surface of TiO2.15 Thus in metals oxide deposited on TiO2 photocatalyst, the inter-

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face at metals oxide and TiO2 will facilitate the trapping of conduction band exited electrons,18 whereas noble or transition metals oxide deposited on TiO2 surface can act as co-catalyst to induce trapping of photogenerated positive charged carries (holes).14 However, due to cost ineffectiveness and supply shortage (not abundant on earth), the noble metal loading is unsuitable for commercial photocatalysis application. Recently, due to easy availability and cost effectiveness, transition metal oxides have attracted research interest over the noble metal oxides. Liu et al. have performed a study on transition metal oxide clusters incorporated TiO2 nanosheet synthesized by one pot reaction method and found that ultra-thin transition metal oxide nanoparticles (co-catalysts) have better photocatalytic water oxidation compare to noble (Ru and Ir) metals.15 Jang et al. have studied the mechanism of metal oxide/BiVO4 p-n junction composite material synthesized by ureaprecipitation and wet impregnation method.18 They found that copper and cobalt oxides photocatalyst had higher photocatalytic degradation activity over orange-7 dye under visible light.18 Zhang et al. have synthesized cobalt modified porous single crystalline LaTiO2N for water oxidation and found that the cobalt species of 5 nm deposited on the surface of LaTiO2N has better charge separation followed by highly efficient water oxidation performance under visible light when compared to IrO2 (Ir, noble metal) metal oxide.19 Thus, the presence of metal deposited on TiO2 can increase the longer electron−hole pair by forming Schottky-barrier between the deposited metal and TiO2, consequently increasing the rate of photocatalytic reaction. Therefore, transition metal oxides loaded TiO2 might be an important and promising technique to degrade organic dye pollutants from water. Photocatalytic generated reactive oxygen species as hydroxyl radicals was also found as a primary agent for the bacteria cell inactivation. 20 In this study, we prepared metal oxide (M = MnO, NiO, Co2O3, and CuO) loading over the surface of HCl treated TO-NRs (sample denoted as MATONRs) and studied its synergistic effect on photocatalytic degradation of Orange (II) dye and inactivation of S. typhimurium pathogen under solar irradiation. First, uniform rutile TO-NRs were synthesized via molten salt flux method. To functionalize the surface of TO-NR by forming more OH groups for effective catalytic properties, this TO-NR was then acidified up to 9 h with HCl. Secondly, in order to improve the charge separation of photogenerated electrons

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and holes, transition metal oxide co-catalysts were loaded onto as-prepared (TO-NRs) and acid treated (ATO-NR) NRs by wet impregnation method. These transition metal oxides were served as hole acceptor co-catalysts. Lastly, the synergistic effect between acid treatment and co-catalysts loading on nanorods was determined for Orange (II) degradation by forming the series of (i) HCl treated TO-NRs; (ii) metal oxide coated TO-NRs; and (iii) HCl treated and metal oxide coated TO-NRs. The optimum condition to achieve the maximum degradation was determined. Moreover, cobalt oxide loaded ATO-NR sample was successfully used for inactivation of S. typhimurium pathogen. Furthermore, effective photogenerated electron transfer in acid treated TO-NR was also studied by photocatalytic solar hydrogen evolution from reduced Pt loaded ATO-NRs. 2. EXPERIMENTAL SECTION Na2HPO4 (Kanto chemicals, 99%), NaCl (JUNSEI chemicals, 99.5%), Cu(NO3)2.3H2O (JUNSEI, 99%), Ni(NO3)2.6H2O (JUNSEI, 97%), Co(NO3)2.6H2O (Aldrich, 98%), Mn(NO3)2.4H2O (Aldrich, 97%), H2PtCl6. 6H2O (Kojima), Orange (II) sodium salt (Aldrich, 85%) were used without any further purification. HCl (Assay 35%, JUNSEI) acid was diluted with deionized water (DI) (CBNU, pH 7) in 25% volume ratio. 2.1 Synthesis of TiO2 (TO-NRs) nanorods. Synthesis of molten salt TO-NRs was performed as described in our previous work.21 Briefly, commercially available TiO2 nanopowder (P25, Degussa), NaCl, and Na2HPO4 in a ratio of 1:4:1 (by wt %) were ground together with a mortar and pestle for one hour to prepare homogeneous mixture. The ground powder was then calcined in a box furnace at 825˚C for 8 hours. After cooling down to ambient temperature, the mixture was filtered with a vacuum filtration system. Excess amount of boiled DI water was used to wash and remove all soluble salts. The washed powder was collected on a filter paper (< 5 µm) and dried over night at 80˚C inside a hot air oven. 2.2 Preparation of acid-treated TiO2 (ATO-NRs) nanorods. For acid treatment, 300 mg of asprepared nanopowder was added into 300 mL of diluted HCl solution and stirred for 9 hours at ambient temperature. The resulting mixture was then centrifuged with excessive amount of DI (22˚C) water. The

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washed powder was then collected onto a filter paper (< 5 µm) by using the vacuum filtration system. The filtration process was repeated until the pH of the filtrate reached 7.0. The powder was then dried in a hot air oven at 80˚C overnight. Finally, dried samples were ground with a mortar and pestle for further experiments. Similar acid treatment process was followed to prepare samples with different concentrations (i.e., 0.1 M, 0.5 M, 1.0 M, 1.5 M and 2.0 M) of HCl-treated TO-NRs. 2.3 Preparation of metal oxide loaded TiO2 (MTO-NRs) nanorods. Metal oxides (M = MnO, NiO, Co2O3, and CuO) loaded photocatalysts were prepared by wet impregnation method. Briefly, metal nitrate precursors were dissolved (by 1 wt %) in 15 g DI water to prepare metal hydroxide solutions. The as-prepared 250 mg TO-NRs and quantified amount of metal hydroxide solution were mixed together by adding 3 mL ethanol in agate mortar. Solvent was then removed by evaporation at room temperature. Homogeneous mixing was followed by adding ethanol solvent repeatedly three times. The dried samples were calcined at 300˚C for 2 h to obtain metal oxide diffused site. These samples were named MTO-NRs catalysts. Similar procedure was used to obtain metal oxide loaded and acid treated TiO2 nanorod samples named MATO-NRs. Platinized ATO-NRs was prepared by 1 wt % of Pt metal impregnated to the acid treated TO-NR mixing together by adding 3 mL ethanol in agate mortar. Sample was calcined at 300oC for 1h in box furnace and same sample was kept for 400oC for 2h inside tubular furnace to get reduced Pto form under continuous H2 gas flow. 2.4 Characterization. X-Ray diffraction (XRD) structural analysis was performed using a PANalytical X’pert Pro MPD diffractometer equipped with Cu Kα radiation source (wavelength Kα1 = 1.540598 Å, Kα2 = 1.544426 Å) operated at 40 kV,30 mA at a scan rate of 0.03˚ 2θ s-1 and 2θ angle of 5° to 80°. Scanning electron microscopy (SEM) observation was carried out using a field emission scanning electron microscope (FESEM) (SUPRA 40VP, Carl Zeiss, Germany) equipped with X-ray energy dispersive spectrometer (EDS). Photocatalytic experiments were performed under one sun (1000 W/m2) irradiation using solar light source (Abet Technologies Inc., USA). UV-vis diffuse reflectance spectroscopy (UV-vis DRS) was performed using a Shimadzu UV-2600 UV-visspectrophotometer. Fourier transform infrared spec-

tra (FTIR) of the TO NR based samples were recorded on a Paragon 1000 Spectrometer (Perkin Elmer) with a signal resolution of 1 cm−1. Thermogravimetric analysis (TGA) measurements were carried out on an SDT Q600 (V20.9 Build 20) instrument (Artisan Technology Group, Champaign, IL) under N2 atmosphere with a heating rate of 10 °C/min. X-ray photoelectron spectroscopy (XPS, Thermo Scientific XPS spectrometer) equipped with a monochromatic Al Kα X-ray source (hν = 1486.6 eV) was used for valence state determination and elemental quantification of samples. Transmission electron microscopy (TEM) was performed using a JEOL JEM-3100F transmission electron microscope operated at 200 kV. The sample for TEM was prepared by placing a drop of the sample suspension in ethanol on a standard carbon-coated copper grid. The surface area of nanorods was measured with a Micromeritics ASAP 2010, Autopore III 9420 instrument. The evolved H2 was analyzed by a gas chromatography (GC) equipped with a TCD detector and a molecular sieve 5 Å packed column. 2.5 Photocatalytic degradation experiment. Organic dye degradation experiments were performed in a Pyrex glass vessel at atmospheric pressure and ambient temperature. Briefly, 50 mg of as-prepared catalyst was dispersed in 45 mL of 25 µM aqueous Orange (II) sodium salt dye (pH 7.0) under continuous magnetic stirring. Initially, the solution was stirred for 30 min in the dark to ensure good adsorption equilibrium between the catalyst and the solution. Reaction was then initiated under one sun irradiation (AM 1.5) using 150 W Xenon arc lamp (Abet, Japan) for 5 hours. UV–vis spectrophotometer (Shimadzu UV-2600 UV-vis-spectrophotometer) was used to monitor the dye degradation process. During the course of reaction, dark sampling was performed for five hours in order to examine the dark activity of the sample as background test. Dye sample of 1.4 mL was withdrawn by using a syringe filter (pore size 0.2 nm) at an interval of 60 min. Maximum dye absorbance wavelength (λmax) at 484 nm was used for absorbance measurement. Dye degradation efficiency was calculated using the following equation: 22

% = 1 −

× 100 [1] where A0 is the initial absorbance of the dye solution before reaction and At is the dye absorbance during reaction at time t. A0

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2.6 Photo-electrochemical measurements. All photochemical measurements were carried out in a 0.1 M Na2S + 0.02 M Na2SO3 electrolyte by using potentiostat instrument (COMPACTSTAT.e, Ivium, Netherland). TiO2 working electrodes prepared by electrophoretic deposition were illuminated using a simulated sunlight source under irradiation of 100 mW cm−2. Photocurrent measurements (J–V) were carried out using a conventional three-electrode electrochemical cell with Pt wire and Ag/AgCl (saturated KCl) as counter electrode and reference electrode, respectively. Electrochemical impedance spectroscopy (EIS) measurements were implemented on a portable potentiostat equipped with electrochemical interface and impedance analyzer. Experimental EIS data were fitted to suitable equivalent circuit model using Z View program (Scribner Associates Inc.). 2.7 Culture and analysis of bacteria. The microorganisms used in the experiments were gram negative and used Salmonella typhimurium (SL1344), a pathogenic microorganism, and cultivation was performed with Luria-Bertani (LB) Broth and LB agar plate. Briefly, 300 mL of LB broth was inoculated with S. typhimurium and cultured for 18 h at 180 rpm in a 37°C shaking incubator. The incubated S. Typhimurium was placed in a 50 mL conical tube, centrifuged (4,000 rpm, 10 min) using a centrifuge, and washed two times with PBS (phosphate buffer solution). The washed S. typhimurium was resuspended in 30 mL of PBS. Analysis of S. typhimurium was conducted in a spread plate method on LB agar and cultured in a 37°C incubator for 24 h. The formation of colony was counted and expressed as cfu/mL (Colony forming unit). The stock solution concentration of S. typhimurium was 1x1010 cfu/ mL. The initial concentration in the inactivation experiment ranged 1x105 cfu/ mL and was obtained by diluting stock solution. In the inactivation experiment, samples were diluted to 1/1, 1/10, and 1/100. 0.1 mL of each sample was dispensed on LB agar plate and S. typhimurium was counted by spread plate method. All the experiments proceeded with three replicate, and the inactivation efficiency was measured by the averages and the deviation of inactivation level according to the contact time (Log N/N0, N0: initial S. typhimurium population (cfu/ mL), N: remaining S. typhimurium population at contact time t (cfu/ mL)) 2.8 Inactivation experiments. Inactivation experiments were performed in a 50 mL quartz reactor

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with a 30 mL suspension. A suspension containing 0.5 g/ L Photocatalytic material (ATO-NRs or Co/ATO-NRs), 1 mM PIPES (1, 4piperazinediethanesulfonic acid) buffer and S. typhimurium (1 x 105 cfu/ mL) were placed in a quartz reactor and was carried out with a stirrer. In order to illuminate the UVA region (315~400 nm), black light blue (BLB) lamps were attached to the light source. Details of the device were described by Cho et al. 23 Light intensity was measured by ferrioxalate actinometry method24 using four lamps (1.2 x 10-6 Einstein/ Ls).20 The temperature was maintained at 21±1 °C by the air-cooling system installed in the reactor and the pH was adjusted to 7.0 using PIPES buffer. In addition, in order to confirm the presence or absence of ROS formation, inactivation experiment was also performed under the condition of addition of 30 mM methanol (MeOH). 3. RESULTS AND DISCUSSION 3.1 Characterization and photocatalytic activity of TO and ATO-NRs. XRD patterns of as-prepared (TO-NRs) and acid treated TO-NRs (ATO-NRs) are shown in Figure S1. Their peaks are analogous to each other, exhibiting similar rutile phases (JCPDS 894202).Characterized XRD peaks were observed at 2θ of 27.5°, 36.1°, and 54.4° corresponding to (110), (101), and (211) crystal planes of the rutile phase, respectively. When the position and intensity of diffraction peaks were compared, a continuous decrement in major diffraction peak intensity at 2θ = 27.5˚ was observed for ATO-NRs treated with 0.5 and 1.0 M of HCl, whereas a strong intensity appeared for ATONR treated with 2.0 M, confirming that high HCl concentration could affect grain size and crystallinity.25 Field emission scanning electron microscopy (FE-SEM) results of as-prepared TO-NRs and ATONRs are shown in Figure 1. The diameter of these asprepared NRs was observed in the range of 100 to 300 nm with a length of c.a. 1.5 to 5 µm. After acid treatment, the surface of NRs became clean. When the HCl concentration was increased, the thickness of NRs was slightly reduced as shown in Figure 1D. Such

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Figure 1. FESEM images of (A) TO-NRs, (B) ATONRs [0.5 M], (C) ATO-NRs [1.0 M], (D) ATO-NRs [2.0 M], and TEM high-resolution images of (E) asprepared TO-NRs and (F) ATO-NRs [1.0 M].

slight reduction in thickness of NRs was due to etching of outer surface layer of the as- prepared sample in the HCl solution and allowed attachments of more OH ion to the surface of NRs. When the concentration of HCl was further increased, NRs was dissolved more quickly. Transmission electron microscopy (TEM) analysis (Figure 1E and 1F) further demonstrated that the one dimension ATO-NR (1.0 M) had a tetragonal morphology with clean surface site after acid treatment with slightly reduced thickness. The clean surface site of ATO-NRs indicates reduction of sodium ions during the acid treatment. Results of FESEM/ EDX analysis of acid treated samples confirmed that sodium ions were reduced to negligible amount. This is due to the replacement of Na+ with H+ during the acid treatment through ion-exchange.26 EDX data of Na/Ti ratio are summarized in Table S1a. FT-IR spectra of the as-prepared TO-NR and ATO-NR are shown in Figure 2A. The broad peaks at 3100- 3600 cm-1 and a peak at 1634 cm-1 corresponded to stretching vibration of O-H and bending vibration of surface H2O, respectively.27,28 As shown in Figure

Figure 2. (A) FT-IR patterns and (B) High resolution O 1s XPS spectra of (a) TO-NRs, (b) ATO-NRs [0.5 M], (c) ATO-NRs [1.0 M], and (d) ATO-NRs [2.0 M] respectively. 2A, the intensity of the peaks of surface hydroxyl groups and surface adsorbed H2O were increased with acid concentration (up to 1.0M) and then decreased at higher concentration of HCl (2.0 M) as shown in Figure 2A (d).29. This decrement is due to the most of the surface sodium ions were removed and concentration of alkaline chlorine (Cl-) was increased over the surface of NR. A low peak at 2345 cm-1 indicated that the carbon dioxide molecule was adsorbed onto the surface of TO-NRs.30, 31 To further confirm the existence of surface hydroxyl groups, XPS spectra of the as-prepared TO-NRs and ATONRs were recorded and results are shown in Figure 2B. As indicated in Figure S2A, the XPS peaks for Ti 2p, O 1s, and C 1s were appeared clearly at a binding energy of c.a. 457.90 eV, 530.01 eV, and 286 eV, respectively. High resolution XPS spectra of Ti 2p and O 1s for both TO-NR and ATO-Ns are shown in Figure S2B and Figure 2B, respectively. The doublet peaks of Ti 2p3/2 and Ti 2p1/2 spin-orbit confirmed that titanium ascribed Ti4+ oxidation state.32, 33 The O 1s peak was deconvoluted into three peaks (as

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Figure 3. Zeta potential of (a) TO-NRs, (b) ATO-NRs [0.5 M], (c) ATO-NRs [1.0 M], and (d) ATO-NRs [2.0 M]. summarized in Table S1a), the first peak at binding energy around ~529.6 eV belonging to Ti-O bond and other two at the binding energies around c.a. 530.5 and 531.9 eV corresponds to oxygen in the hydroxyl groups and surface absorbed water, respectively.34, 35, 36 The fitted values corresponding to deconvoluted O 1s XPS peaks of hydroxyl group is summarized in Table S1b. As shown in Figure 2B, physically absorbed surface-water contents of the TO-NRs were higher than those of ATO-NRs, indicating that the TO-NRs contained sodium ions which could easily adsorb the water molecules from atmospheric moisture.37 This adsorbed surface water content in 1.0 M ATO-NRs was decreased and the intensity of hydroxyl groups (at 530.5 eV) was increased, which ascribed the fact that water adsorbed sodium ions removed by HCl treatment and hydroxyl groups can easily react with clean surface site of NR to form Ti-OH bonds in the ATO (as shown in Figure 2B (c)).38 Further increment in acid concentration 2.0 M lead to remove surface hydroxyl groups and appeared low intensity peak as compared to 1.0 M ATO. This XPS analysis of hydroxyl groups in ATONRs was completely correlated with the FT-IR results. Yu et al. 38 have also suggested that the presence of hydroxyl groups in TiO2 can cause of chemical adsorption of water molecules to form Ti-OH bonds and physically adsorbed H2O (sodium adsorbed OH) to be easily desorbed under ultra-high vacuum condition of the XPS system.39 To further support the amount of hydroxyl groups in TO-NR based samples; the thermogravimetric analysis (TGA curves) was performed as shown in Figure S3. The thermal behaviors and corresponding weight losses were determined from the TGA curves for TO-NRs, ATO-NRs, MTO-NRs and

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MATO-NRs (MTO and MATO–NRs are discussed in further sections). All TGA curves measured from the TO-NR based samples exhibit three distinct weight losses. The first and second step in the weight loss curve represents the dehydration of water molecules adsorbed by sodium ions and surface of the TiO2, respectively.40, 41 The high weight losses in the first step of TO (0.20%) and MTO (0.17%) can be attributed to the more affinity of sodium ions toward the water molecules in untreated samples.37 The acid treated samples showed less weight loss in the first two steps. The careful analysis of TGA curves indicates that the low weight loss (0.21 to 0.14 %) was observed for TO and MTO samples in the third step as compared to treated (ATO and MATO) samples. However, the ATO and MATO samples with the clean surface by acid treatment showed a high weight loss during third step. Last, third step represents complete dehydration of hydroxyl groups on the surface of TiO2, which exists as the form of Ti-OH bonding in ATO sample. In ATO-NR sample the weight loss in the third step is found more as compared to the TO-NR sample. However, cobalt oxide loaded MATO-NR sample showed highest weight loss during third step among all other samples. This can be attributed to the combined dehydration of bond water (Ti-OH) and dehydroxilation of metallic hydroxides.42-44 The zeta potential is a function of surface charges of particles and is depends upon pH, solvent, and surrounding medium. Surface charges of the asprepared TO-NRs and ATO-NRs were determined at different pH values and the obtained results are shown in Figure 3. At pH of 3, the Zeta potential values of TO-NRs and 2.0 M ATO-NRs were 21.6 mV and 26.4 mV, respectively. At pH of 5, the 1.0 M ATONRs showed a high zeta value of 27.5 mV. These results indicated that surface OH- groups of ATO-NRs could adsorb protons (H+ ions) under acidic condition. Therefore, suspended particles would exhibit more positive potential.45, 46 These results supported the results of FT-IR and TEM analyses that the asprepared TO-NRs consisted of sodium ions and surface water molecules. Under acidic condition, surface Na+ ions were replaced by H+ protons due to protonexchange. Therefore, TO-NRs and ATO-NRs showed similar positive potentials. Low values of zeta potential (below -30 mV) indicate that particles in dispersion tend to repel each other without agglomeration.47, 48 The isoelectric point (point of zero charge) of the as-prepared TO-NRs was at 4.042 (isoelectric pH), which was in good agreement with the values

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Figure 4. (A) Effect of as-prepared and acid treated NR (ATO-NRs) photocatalyst samples over photocatalytic Orange (II) dye degradation under solar light. (a) as-prepared NRs, (b) 0.5 M, (c) 1.0 M, and (d) 2.0 M, (a’) and (c’) represents (a) and (c) samples dark condition respectively. (B) Orange (II) dye degradation by 1.0 M TO-NRs photocatalyst. reported by other literatures of TiO2 colloidal.49, 50 Due to protonation of surface, the isoelectric pH values of acid treated samples shifted to neutral pH along with increasing concentrations of acid used for treatment. Their isoelectric pH values were decreased back to low pH after attaining Ti-OH bond (in 2.0M).51, 52 To study the effect of acid treatment on photocatalytic activity of TO-NRs, degradation of Orange (II) dye experiment was performed under solar irradiation. Results are shown in Fig. 4A. Orange (II) dye is a water soluble anionic dye. The molecular structure of dye contains a benzene ring and an azo linkage with a naphthalene ring. The azo linkage has a maximum absorption in the visible region of 484 nm. It provides N=N bonding to both chromophores benzene and naphthalene. The photocatalytic decomposition of Orange (II) involves active radical species (OH•, HOO• and O•2-) which can easily attack the linkage (N=N) bond compared to aromatic rings, leading to decomposition in the visible region.53 The degradation rate of Orange (II) by 1.0 M ATO-NRs photocatalyst was found to be higher than that by

TO-NRs or ATO-NRs treated with other concentrations of HCl. Blank experiments (dark condition, black dotted line in Figure 4A) showed negligible or imperceptible degradation. The absorption peak at 484 nm was chosen as a measure for the concentration of Orange (II) dye as shown in Figure 4B for further degradation studies. The apparent kinetics of disappearance of Orange (II) was determined by monitoring the concentration of this substance at various time intervals using a UV–vis spectrophotometer. It was found that the absorbance value of the Orange (II) dye was decreased with the experimental time. The change in concentration value of the dye solution during its degradation was used to find out the reaction kinetics.22 When the dye concentration is low, the degradation mechanism of the dye is of Langmuir–Hinshelwood (L–H) type. It can be described by a pseudo-first order reaction. The variations in (A/A0) as a function of illumination time are shown in Figure 4A, where A0 is the initial concentration and A is the concentration of Orange (II) at time t.54 After 300 min of solar light irradiation, about 74.17% of Orange (II) was degraded by 1.0 M ATO-NRs (Figure 4B). Generally, the photocatalytic degradation activity completely depends on the surface properties of the catalyst. In the present study, higher degradation of Orange (II) was achieved by 1.0 M ATO-NRs due to their sodium free cleaned surface which allowed direct interaction between Ti atoms and water molecules to establish TiOH bonds by chemisorption (Figure 2A). Acid treatment can promote sodium free site over the adsorbed water molecules which may re-associate with Ti atoms to form Ti-OH bonds by transferring protons to the adjacent oxygen atoms.55-57 Similarly, at high acid concentration (2.0 M), Cl- removed most of the surface OH- groups, causing less Ti-OH bonds on the surface of ATO-NRs and reducing the photocatalytic activity of the material (Figure 2B).58 The effects of transition metal oxide loaded TO-NRs on photocatalytic degradation of Orange (II) dye were compared to those of acid treated and nontreated TO-NRs. Results are shown in Figure S4. Background test appeared as dotted line parallel to X-axis in photocatalytic diagram, suggesting that there was no adsorption in darkness for samples. The photocatalytic activities of all transition metal oxides (1 wt%) loaded TO-NRs (MTO-NRs) catalysts are summarized in Table S2. Among different transition metal oxides loaded MTO-NRs, 1 w % cobalt oxide showed the maximum degradation efficiency (at

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94.22%) within 300 min and was used for further studies. No new XRD peak was observed for the 300˚C calcined Co, Cu, Mn, or Ni transition metal oxide (1 w %) loaded TO-NR samples (Figure S 5A). This result might be due to the less amount of loading of metal oxide over TO-NRs.59 The absorption spectra of the resulting MTO-NRs showed an increased absorption and shift in the visible light in comparison with pure TO-NRs. Such change in light absorption was attributed to the loading of transition metal ions into TO-NRs (Figure S5B). 60, 61 Amongst all transition metal (Co, Cu, Mn and Ni) oxide loaded catalysts, cobalt oxide incorporated sample showed absorbance in the visible region (c.a. 688 nm). The outer surface of MTO-NRs appeared rough after metal loading as shown in Figure S5C-F. Whether acid treatment and transition metal loading for cobalt oxide incorporated sample might have synergistic effect of on dye degradation was unclear. Therefore, to examine this, the samples were prepared using both acid treatment and transition metal loading on TO-NRs and were used further for dye degradation. 3.2 Synergistic effect of acid treatment and metal oxide loading on TO-NRs. XRD and FESEM characterizations confirmed that MATO-NRs maintained their parental crystallinity and morphology of TONRs (Figure S1) after calcination at 300˚C (Figure S6 and S7A-D). High resolution XPS spectra of metal oxide loaded untreated and acid treated samples are shown in Figure 5. Doublet peaks of Ti 2p and three oxygen peaks of O1s were observed in the high resolution XPS spectra (Figure 5A and B) as discussed earlier. Positively charged protonated surface of ATO-NRs can strongly interact with metal hydroxide loading solution to form Ti-OH bonds. These results indicated that the hydroxyl group in each metal oxide loading could withstand calcination at 300˚C. It was enhanced in MATO-NRs compared to that in ATO-NR sample. High resolution XPS spectra of cobalt oxide loaded TO-NRs and ATO-NRs revealed doublet peaks at binding energy of 780.49 eV and 797.33 eV corresponding to Co 2p oxidation state of Co 2p3/2 and Co 2p1/2 positon, respectively (Figure 5C).

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In Figure 5D, sodium Na 1s peak in untreated Co/TO-NRs (for further study, in place of writing ‘M = metal oxide’ we abbreviated by element identity; e.g. M = Co/ or Cu/ or Mn/ or Ni/) appeared at binding energy of 1071.4 eV. It disappeared after acid treatment (1.0 M), which was in good agreement with the previous photocatalytic results (Figure 4A) of acid treated samples where 1.0 M HCl treatment showed higher activity. To determine the structural and chemical properties, high-resolution TEM (HRTEM) and annular dark field-scanning TEM (ADFSTEM) were performed for acid treated (ATO-NR) and untreated (TO-NRs) samples after cobalt loading. Low magnification TEM image of acid treated and cobalt loaded TO-NRs (Co/ATO-NR) are shown in Figure 6A. It possessed a flat nanorod surface with intermittent cobalt nanoparticles. To gain detailed structural information of Co/ATO-NR, HR-TEM analysis was performed for the surface area of NR. As shown in Figure 6B, Co/ATO-NR sample well maintained the single crystalline form of TiO2. TiO2 growth direction of nanorod was observed in the (002) direction (d-spacing measured 1.48 Å).

Figure 5. High resolution XPS spectra of MATO-NRs samples. (A) Ti 2p, (B) O 1s, (C) Co 2p, and (D) Na 1s of (a) Co/TO; (b) Co/ATO; (c) Cu/ATO; (d) Mn/ATO, and (e) Ni/ATO NRs. (where ‘M = metal oxide’ further abbreviated by element identity; e.g. M = Co/ or Cu/ or Mn/ or Ni/= 1wt %)

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Figure 6. High resolution TEM images (A, B, E and F), STEM image (C and G) and EDX elemental mapping images (D and H) of cobalt oxide loaded TO-NR (A, B, C and D) and ATO-NRs (E, F, G and H) photocatalysts. Arrangement of the atomic structure of a single crystal was confirmed by Fast Fourier Transformation (FFT) (Figure 6B, inset). ADF-STEM analysis and corresponding EDS mapping results of Co/ATO-NR sample are shown in Figure 6C and D. In Figure 6C, we found evenly distributed contrast along the ATONR and partially enhanced contrast on the surface due to Z (atomic number)-contrast in ADF-STEM. Regarding chemical information, uniform distribution of Ti, O, and Co was found in Figure 6D. In addition, Co nanoparticles partially observed on the surface were found to be more intensive. The surface of untreated Co/TO-NR sample (Figure 6E and F) appeared to be rather rough compared with Co/ATONR sample (Figure 6A). There was more coverage of Co particle on rough surface due to the higher surface density of these nanoparticles as shown in HRTEM image. To determine the chemical components, ADF-STEM analysis and EDS mapping of Co/TO-NR were performed and the results are shown in Figure 6G and H. As shown in Figure 6G, a relatively different contrast was observed for treated sample (Figure 6C) compared to untreated (Figure 6G) due to Zcontrast in ADF-STEM. Also, we found uniform Ti and O components and partially distributed Co signal on the entire untreated sample. Ti, O, and Co EDS spectra are shown in the bottom of the mapping image. The copper element detected in the spectrum was TEM copper grid. Removal of surface nanoparti-

cles after HCl etching reduced the concentration of sodium ions (Figure S7E and F), allowing an evendispersion of cobalt oxide nanoparticle on the surface of ATO-NR. Results of UV-Vis-DRS absorbance of MATO-NRs revealed no significant visible absorbance or peak shifting compared to MTO-NRs (Figure. S8A). In order to investigate the efficient transition metal co-catalyst, the photocatalytic dye degradation activities of 1 w % of MATO-NR samples was studied and are shown in Figure S8B. Ni/ATO-NR sample showed higher activities compared to Ni/TO-NR (Figure S4). However, manganese oxide loaded photocatalytic material failed to show any improvement under untreated (Mn/TO-NR) condition or acid treated (Mn/ATO-NR) conditions. The activity of cobalt oxide loaded ATO-NR sample was significantly enhanced compared to copper oxide loaded onto ATO-NR sample. It degraded the Orange (II) dye very quickly (98.57% within two hours) and complete degradation of Orange (II) dye was achieved within three hours. The main factors that contributed to the enhanced photocatalytic degradation of Orange (II) dye could be due to efficient electron–hole separation in MATO-NRs. Photogenerated electrons and holes can react with H+ and OH− ions dissociated from the surface adsorbed H2O molecules to form adsorbed hydrogen and OH radicals (•OH).62 In a photocatalytic

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oxidation reaction, hydroxyl radicals (•OH) are active species that play significant roles in photocatalytic reaction. FT-IR spectra of 1.0 M acid treated and 1wt % transition metal oxide loaded TO-NR samples are shown in Figure S8C. In the FT-IR spectra, 1.0 M HCl treated samples had broad peak at around 3648-2971 cm-1 corresponding to hydroxyl groups (with small decrease in intensity after loading transition metal oxides) and a peak at 1628 cm-1 corresponding to surface adsorbed water molecules, thus rigorously retaining the same position after calcination at 300˚ C.63 The residues of carbon content (CO2) were decreased after metal interaction (peak at 2345 cm-1) possibly due to oxygen released from CO2 molecules. After transition metal oxide loading, a new peak at 1098 cm-1 corresponding to the interaction between Ti-O bond and transition metal oxide was observed in the site of TiO2. Both 1wt % of 1.0 M acid treated Cu/ATO-NRs and Co/ATO-NRs samples showed deep and sharp peak at 1098 cm-1. These results are in good agreement with the synergistic activity data, indicating that catalyst surface layer rich in Ti-OH and metal oxides could bind to Ti-O bond and contribute to charge transfer in the metalsemiconductor interface. 3.3 Optimization of cobalt oxide loading. The optimization of cobalt oxide loading was verified by using different concentrations (0.5 1.0, 1.5, and 2.0 wt %) of cobalt nitrate aqueous solutions that impregnated over ATO-NRs (as method was described in the experimental section) (as shown in Figure 7A). The photocatalytic activity was found to be increased when the amount of cobalt oxide loading was increased. The maximum photocatalytic activity for Orange (II) dye degradation was achieved after 1.0 wt % cobalt oxide loading over ATO-NR (Co/ ATONR). The dye degradation efficiency of Co/ ATO-NR over Orange (II) dye for first two hours 98.57% was observed (inset image of Figure 7B shows the Orange (II) fresh dye before and after two hour degraded, colorless solution). However, further increasing the amount of cobalt oxide loading (wt %) result in slight decrease in photocatalytic activity. The improved photocatalytic activity for Co/ ATO-NR was observed due to the synergistic effect of acid treatment and cobalt oxide loading. Metal oxide particles of 1.0 wt % cobalt oxide were distributed as an ultrathin layer over the surface of

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Figure 7. (A) Synergistic effect of acid treatment and cobalt oxide loading over photocatalytic dye degradation for optimization of cobalt oxide at different concentrations. (a) 0.5 wt%, (b) 1.0 wt%, (c) 2.0 wt% and (d) TO-NRs under solar light. (B) Synergistic effect of acid treatment and 1 wt% cobalt oxide loading over photocatalytic degradation of Orange (II) dye. (Inset show Orange (II) dye color before and after photocatalysis reaction for 1.0 wt% of cobalt oxide photocatalyst.) ATO-NRs as described earlier (Figure 6.) Thus, hetero junction will form near the interface of metal oxide and ATO-NR semiconductor. The interface of metal oxides and semiconductors closely related to the thickness of the cobalt oxide layer. Thick cobalt oxide layer can interrupt the charge transfer between cobalt oxide and ATO-NR semiconductor, leading to decreased photocatalytic activity.64 Therefore, 1 wt % cobalt oxide loading set as optimum concentration for this study. The photodegradation efficiencies of photocatalysts loaded with different concentrations of cobalt oxide are summarized in Table S3.

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In addition, we followed the similar experiment with the dye degradation experiments to confirm the presence of free radicals. The generation of free radicals has been detected by a decrease in photocatalytic activity after adding a reactive oxygen species scavenger solvent. The studies of Di Valentin et al.67 and Y. Liao et al.68 were shown the effect of organic alcohol solvents in photocatalysis reaction. These studies were suggested that in the presence of organic solvent, e.g. methanol (the order of strong

Figure 8. Kinetics of S. typhimurium inactivation by (a) ATO-NRs, (b) Co/ATO-NRs and (c) Co/ATO-NRs with MeOH (21±1°C; pH: 7.0; [C]0: 0.5 g/L; [N]0: 1x105 cfu/mL; light intensity (six lamps): 1.8 x 10-6 Einstein/L s; [MeOH]0: 30 mM) (Inset images show inactivation of S. typhimurium by ATO-NRs and Co/ATO-NRs for control, 90 min and 180 min time periods), (b’) Co/ATO-NRs under dark condition. 3.4 Inactivation of Salmonella typhimurium by ATO-NRs and Co/ATO-NRs. Photochemically generated reactive oxygen species (ROS; OH radical, superoxide radical, etc.) from the aqueous suspension the of ATO-NRs and Co/ATO-NRs were also applied for the S. typhimurium inactivation (Figure 8). Despite the degradation of orange (II) dye (Figure 7), no remarkable degradation of S. typhimurium was observed in the presence of ATO-NRs (Figure 8a), since the generated amount of ROS is not enough to inactivate Salmonella. Interestingly, rapid inactivation efficiency (about 2 log, 99%) was observed in the presence of Co/ATO-NRs within 180 min (Figure 8b). But, Co/ATO-NRs didn’t show any activity under dark condition (Figure 8b’). Co/ATO-NRs demonstrate that not only orange (II) dye degrade, but also disinfect bacteria. These results indicate that the synergy effect of cobalt oxide accelerates the redox reaction and generates a large amount of ROS. As a result of an experiment in which Co/ATO-NRs was excessively added to MeOH used in the ROS scavenger (Figure 8c), it was confirmed that the inactivation level was greatly reduced as in ATO-NRs. This is because the produced ROS did not inactivate S. typhimurium and reacted with excess MeOH. Consequently, it is clear evidence that S. typhimurium is inactivated as a result of ROS generation. 65, 66

Figure 9. (A) J–t curves measured at 0 V versus Ag/AgCl under chopped simulated sunlight illumination. (B) Nyquist plots of (a) TO-NR, (b) ATO-NRs, (c) Co/TO-NRs, and (d) Co/ATO-NRs photoelectrodes in 0.1 M Na2S/0.02 M Na2SO3 electrolyte. The inset shows the equivalent circuit model used for fitting the data (where ‘M = metal oxide’ further abbreviated by element identity; e.g. M = Co/ or Cu/ or Mn/ or Ni/= 1wt %).

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Figure 10. Photogenerated charge carrier transfer mechanisms in cobalt oxide loaded ATO-NR. to the low scavenge activity: glycerol > tert-butanol > iso-propanol > methanol > formic acid) could slow down the photocatalytic reaction by scavenge the photo-generated reactive oxygen species (ROS).69 So, the photocatalytic experiment conditions for ROS scavenger test were fixed similar to that of photocatalytic dye degradation experiments. In our tests, we fixed the concentration of Orange dye (25 µM) and vary the concentrations of methanol as reactive oxygen species scavenger solvent. 1w% Co/ ATO-NR photocatalyst was added to the mixed solutions. The photocatalytic activity was observed for three hours under simulated solar irradiation (AM 1.5). The high photocatalytic degradation activity was observed without methanol as shown in Figure S9. A decreased in photocatalytic activity was observed with the increment of the concentration of methanol solvent in the mixture solution. These results indicated that ROS generation is affected due to methanol solvent and significantly observed the existence of ROS generation in the photocatalytic reaction. 3.5 Synergistic improvement in photocurrent generation and charge transfer mechanism. The synergy of photocatalytic degradation by TO-NR electrode prepared by acid treated and cobalt oxide loaded hybrid is further explored by photocurrent measurements. The photocurrent density-voltage (J– V) curves of the TO-NRs (as-prepared), ATO-NRs (1.0 M), MTO-NRs (cobalt oxide loaded) and MATONRs (cobalt oxide loaded on acid treated NRs) in the dark and under solar light illumination are shown in Figure S10A. The as-prepared sample (a) showed negligible photocurrent density (1.5 μA cm-2). However, the 1.0 M (optimal concentration) of HCl treat-

ed TiO2 films had outstanding photoelectrochemical performance (current-density J; 8 μA cm-2) compared to TO-NRs as seen in the inset of Figure S10A and Figure 9A. The improvement in photocurrent with negative shift in onset potential of TO-NRs upon HCl treatment is due to the negatively shifted Fermi level70 and decreased electron–hole recombination near the ﬂat band potential.71 Again, the photoactivity of cobalt oxide loaded TO-NRs (Co/TO-NR) was higher than the acid treated or the as-prepared TO-NRs. It reached a maximum value of 26 μA cm-2 (current density J) (Figure S10B). The increased value of Jph was due to the higher surface barrier potential and narrower space charge region after cobalt oxide loading onto TO-NRs compared to acid treated or the as-prepared TO-NRs. This increases the electron–hole separation efficiency, thereby increasing the photoactivity.72, 73 However, after cobalt oxide loading, the transient photocurrent was decreases with time (Figure 9A). This is due to small ions as Co2+ may replace Ti4+ ions in the surface layer and create oxygen vacancies which can behave as electron traps, resulting in the decrease of electrons in the conduction band. Thus, the photoactivity of cobalt oxide loaded TiO2 sample were slightly lower with time compared to those of acid treated or as-prepared TO-NRs. This finding is in agreement with the results of Whang et al.74 When cobalt oxide was loaded onto acid treated TO-NRs, a noticeable synergistic effect on photoactivity was observed. The significant improvement of photocurrent (current density J; 128 μA cm-2) in cobalt oxide loaded upon treating TiO2 film with HCl solution indicates that surface treatment is very important parameter for effective charge transfer at the surface of TO-NR. The

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Table 1. EIS data of fitting of equivalent circuit model. Sample/ EIS paRs R1 C[PE] Jph at oV rameters, JPh Ω Ω μF μA TO_NR (a) 33 124740 10.4 1.5 Acid TO_NR (b) 32 42684 10 8 Co loaded TO_NR 32 5669 15.6 26 (c ) Co loaded Acid 30 709 12.9 185 treated TO_NR (d)

increase in photocurrent depends on electron transport and charge recombination in photoelectrode. To gain further insights into the effect of acid treatment and cobalt oxide loading on charge transfer and recombination processes at the interfaces of electrode/electrolyte, electrochemical impedance spectroscopy (EIS) analysis was conducted. Nyquist plots for (a) TO-NR, (b) ATO-NRs, (c) Co/ TO-NRs, and (d) Co/ ATO-NRs under illumination at 0 V vs. Ag/ AgCl are shown in Figure 9B. The EIS spectrum contained only a single semicircle. Experimental data were fitted to an equivalent circuit model consisting of RS and a RC unit containing resistance in parallel with a constant phase element (CPE) (Figure 9B inset). The series of resistance RS accounts for the resistance of the FTO and/or the electrolyte. R1 represents the charge transfer resistance from the surface states to the solution. Chemical capacitance (CPE) is a constant phase element of the capacitance corresponding to R1. The fitted data are summarized in Table 1. The larger the R1 value, the more strongly the charge recombination rate, which will result in lower photocurrent densities. Notably, the resistance of TO-NRs (R1) was affected after acid treatment and cobalt oxide loading. The R1 value was reduced by a one and two order less than the as-prepared electrodes (acid treated or cobalt oxide loaded TO-NRs). Such decrease in R1 value is beneficial for charge transport. However, the cobalt oxide loaded and acid treated TiO2 electrode showed much smaller radius of the semicircle with drastically decreases of the R1 value, thereby enhancing the charge transport at the electrode/electrolyte interface. On the basis of the studied photocatalytic activities and photoelectrochemical measurements, the photocatalytic mechanism of cobalt oxide loaded and acid treated TO- NRs is schematically shown in Figure 10. The co-sensitization effect of acid treatment and cobalt oxide loading provides an efficient

pathway for the separation and transfer of photogenerated holes. The enhanced photoactivity of HCl treated TO-NR leads to higher photocurrent with a lower shift of the onset potential of TO-NR. This might be due to the H+ ion at the surface of TiO2.56 In other words, cobalt oxide loaded and acid treated TO-NRs might have contributed to a fast interfacial charge transfer by trapping of positively charged holes in semiconductors.75 In order to examine the effectivity of acid treated rutile TiO2 nanorods towards electron transfer we performed the solar hydrogen production. Figure S11 shows the time course of H2 evolution from the aqueous electrolyte containing methanol solvent under solar light irradiation. In this, hydrogen is produced from reduction of protons in water by the photoelectrons. The result indicated that reduced Pt on the surface of ATO-NRs contributes to electron deficiency inside ATO-NRs and consumed the photogenerated electron for solar hydrogen evolution from water.76 The charge transfer mechanism for cocatalyst hole trapping can be explained as follows. Under light irradiation, the photogenerated charge carriers in acid treated TiO2 are excited from valence band (VB) to conduction band (CB). These excited electrons are then captured by dissolved oxygen in aqueous solution to produce reactive oxygen species (O2•). In contrast, the holes remaining behind in the VB of ATO-NR are trapped by cobalt oxide and transferred to H2O to form hydroxyl radicals (OH•). These photogenerated reactive hydroxyl radicals (OH•) and oxygen species (O2•) can interact with dye molecules rapidly and degrade them efficiently. Therefore, efficient photogenerated charges (electron−hole pairs) are separated at the contact surfaces of cobalt oxide loaded and acid treated TiO2 photocatalysts, thus increasing the lifetime of charge carriers. Effective separation of electron and hole pairs is the main contributor to the improvement of photocatalytic abilities. 4. CONCLUSIONS In this work, transition metal oxide (MnO, NiO, Co2O3 and CuO) loaded ATO-NRs were successfully prepared by wet impregnation method. Acid treatment removes surface sodium ions and clean site of TO-NR induced more -OH groups to form Ti-OH bonds by chemisorption. FT-IR, XPS and TGA characterization results confirmed the presence of hydroxyl groups after acid treatment. This acid treated surface of TO-NR support the highly dispersive and

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thin loading of cobalt oxide nanoparticles which was confirmed by TEM/ EDS mapping. Furthermore, the fine dispersion of metal oxide nanoparticles favored the charge transfer across the interface of metal oxide co-catalyst nanoparticle and TO-NR. Cobalt oxide co-catalyst could facilitate the acceptance of photogenerated holes from the TiO2 surface, leading to the separate electrons and holes. Due to higher charge separation efficiency and -OH rich surface, 1 wt% cobalt oxide loaded acid treated TiO2 nanorods showed synergistic effect on photocatalytic degradation of Orange (II). Co/ ATO-NR co-catalysts achieved a photocatalytic degradation efficiency of 98.57 % within 120 min under solar irradiation, which was much higher and faster than the other photocatalyst samples. Moreover, Co/ ATO-NR cocatalysts were showed high photocatalytic inactivation activity towards S. typhimurium pathogen. Rapid inactivation efficiency (about 2 log, 99%) was observed within 180 min. Photocatalytic hydrogen evolution results of platinized ATO-NRs have suggested that acid treated nanorod photocatalyst also more effective for solar hydrogen production from water. This study has shown the synergistic effect of acid treatment and transition metal oxide impregnation over the inactive molten salt rutile type TiO2 to improved photocatalytic activity under solar light irradiation and its application in different fields. ASSOCIATED CONTENT Supporting Information: Tables of band gap energies, binding energies, EDAX(Na/Ti wt%) and dye degradation efficiencies of ATO-NRs; Table of dye degradation efficiency of Co/ATO-NRs; XRD, XPS and UV-DRS of ATO-NRs; TGA analysis, dye degradation, XRD, UV-DRS and FE-SEM of MTO-NRs; XRD, FE-SEM, EDS, UV-DRS, dye degradation and FT-IR of MATO-NRs; Methanol effect on ROS generation, J-V curve under solar light illumination for Co/TOo NRs; photocatalytic hydrogen production over Pt loaded ATO-NRs.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (Jum Suk Jang), [email protected] (Jung Ho Ryu)

Notes

The authors declare no competing financial interest

ACKNOWLEDGMENT This research was supported by the BK21 Plus Program, Basic Science Research Program (2012R1A6A3A04038530) funded by the Korean National Research Foundation (NRF) and C1 Gas Refin-

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Boosting photocatalytic performance of inactive rutile TiO2 nanorods

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