Facile synthesis of Rh/Ti3+-TiO2 nanocomposites and its photodisinfection properties on Staphylococcus aureus under visible-NIR excitation

Jingtao Zhang School of Food and Bioengineering, Zhengzhou University of Light Industry Shurui Liu Zhengzhou University of Light Industry Qinwen Wang Zhengzhou University of light industry Jing Yao Zhengzhou University of Light Industry Yin Liu Zhengzhou University of Light Industry Bingkun Liu Zhengzhou University of Light Industry jun xiao (  jxiao14b@imr.ac.cn ) Institute of Metal Research, Chinese Academy of Sciences Hengzhen Shi Zhengzhou University of Light Industry


Introduction
Photodisinfection technology based semiconductors has attracted attention of many researchers as a novel potential bacterial inaction technology in food safety area [1]. Among various studied, TiO 2 was one of the most attractive photocatalyst owing to its properties of high photocatalytic e ciency, high chemical and physical stability, and low cost [2][3][4][5]. Previous reports have demonstrated that the TiO 2 photocatalyst can be used for food microorganism disinfection under UV or visible light irradiation [6][7][8][9].
Jing and Hung reported that the Escherichia coli O157: H7 cell can be inactivated with TiO 2 nanoparticle embedded cellulose acetate lms under UV-A light illumination [1]. Muranyi et al. found that the Kocuria rhizophil could be reduced 3.3 orders of magnitude on titanium dioxide coated glass slide after 4 h of UV-A light exposure [8]. Xu et al. studied the antibacterial effect of the graphene oxide and chitosan biopolymer loaded TiO 2 , and found that the synthesized nanocomposites exhibited high antibacterial activity against Aspergillus niger and Bacillus subtilis [9]. The photocatalytic antimicrobial performance of a TiO 2 nanocomposite with low-density polyethylene (LDPE) lm and its fresh-keeping test for fresh pear were studied by Li et al [10]. However, the low quantum yields and poor e ciency of visible-light use are the two primary challenges for practical applications of pristine TiO 2 [11,12]. A number of methods have been used to overcome these challenges, such as doping with metal or non-metal elements, grouping with other semiconductors, and grouping with plasmonic metal [2,13,14]. Recently, studies on Ti 3+ self-doped TiO 2 have been carried out due to the ability of this process to overcome the above disadvantages [15][16][17]. The addition of oxygen vacancies (Ov) or Ti 3+ to the altered TiO 2 grid greatly improves the photocatalytic ability within the band of visible light [18,19]. Ti 3+ , when self-doped, also increases the e ciency of separation of the photogenerated charge carriers [20]. The Rh 3+ -modi ed TiO 2 exhibited much higher photoactivity than TiO 2 modi ed by either Cu 2+ or Fe 3+ [21].
In this study, we synthesized rhodium-modi ed and Ti 3+ self-doped TiO 2 (Rh/Ti 3+ -TiO 2 ) nanocomposites using a facile, solvothermal method. In the Vis-NIR area, we found a signi cantly rate of absorption in the synthesized Rh/Ti 3+ -TiO 2 nanocomposite samples after analyzing the structure of their crystals using TEM, XRD, and ESR methods. We also con rmed the Ti 3+ by XPS and ESR analysis. Antibacterial activity was tested using foodborne pathogenic bacteria of Staphylococcus aureus under both visible light and near-infrared light irradiation. Our disinfection results demonstrated that S. aureus could be disinfected via illumination with Vis-NIR light in the Rh/Ti 3+ -TiO 2 nanocomposite.

Fabrication and characterizations of Rh/Ti 3+ -TiO 2 Nanocomposite
We synthesized the Rh/Ti 3+ -TiO 2 samples using a one-pot solvothermal reaction, which is brie y described below. First, we dissolved 6 mmol titanium tetrachloride and 0.06 mmol rhodium chloride hydrate (1% molar ratio of titanium tetrachloride) in 30 mL of ethanol, which resulted in a mixture. Then we added 30 mL of NaOH ethanol solution in order to get a xed molar ratio of NaOH to TiCl 4 and RhCl 3 of 4:1 and 3:1, at room temperature. During this process, we observed a precipitate while the reaction occurred, for 30 min under vigorous agitation. Lastly, we placed the mix into a 100 mL Te on-lined autoclave bottle where it was stored for 4 h at 180 ℃. The mix was then allowed to cool to room temperature. Following the solvothermal reaction, we washed the precipitate three times with distilled water and ethanol, after which they were dried at 80 ℃ for 12 h. After the precipitate dried, it was crushed into a ne powder and marked as 1% Rh/Ti 3+ -TiO 2 . We prepared samples with different Rh molar ratios of 3%, 5%, and 7% Rh according to this method, while the TiO 2 without Rh was also prepared for use as a reference.
We analyzed the structure of the crystal using powder X-ray diffraction (XRD) with Cu Kα radiation on a D8 Advance X-ray Diffractometer (Bruker, Germany). The scan rate was 0.5°/min, while the scan range of 2θ was 20° to 80°. Using a JEM-2100 transmission electron microscope (JEOL, Japan), we examined images from the transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). We analyzed Ti 3+ using Electron Spin Resonance (ESR) spectroscopy on a JES-FA200 (JEOL, Japan) electron spin resonance instrument at 110 K. We studied traces of Ti, Rh, and O in the Rh/Ti 3+ -TiO 2 sample via X-ray photoelectron spectroscopy (XPS), and performed XPS measurements with an ESCALAB250 X-ray photoelectron spectrometer (Thermo Fisher Scienti c Inc., MA) with an Al K anode (1,486.6 eV photon energy, 300 W). The optical absorbance was calculated from measurements of diffuse re ectance, which we got from a U-3600 UV-VIS-NIR spectrophotometer (Shimadzu, Japan). A xed concentration of 1 mg photocatalyst/mL S. aureus suspension was used in the experiments. 10 mg photocatalyst with 9.9 mL buffer solution was rst injected into a sterile 60 mm × 15 mm Petri dish and was dispersed ultrasonically for 10 min. Then, 0.1 mL S. aureus suspensions (ca. 10 9 CFU/mL) was added into the Petri dish, so that the initial S. aureus concentration used in the photoctalytic disinfection experiments was ca. 10 7 CFU/mL.
A 300 W xenon lamp (HSX-F300, Beijing NBET Technology Co. Ltd., Beijing, China) was used for photocatalytic inactivation experiments, and the light with wavelengths below 400 nm and above 700 nm was blocked by glass lters for the disinfection under visible light. In addition, the light with wavelengths below 800 nm and above 1100 nm was blocked by other glass lters for the disinfection under NIR light.
A cooling water circulating device was used for keep the temperature under NIR light irradiation (see schematics S1). The light intensity striking the cells was at ca. 30 mW/cm 2 , as measured by a FZ-A optical Radiometer (Photoelectric Instrument Factory of Beijing Normal University, Beijing, China).

Results And Discussion
anatase phase. Figure 2 displays the TEM image of the Rh/Ti 3+ -TiO 2 nanocomposite. The XRD diffraction peak of Rh is clearly visible in the XRD patterns, while the signal strengthened as the atomic ratio of Rh/Ti increased [22]. Figure 2 displays the TEM image of the Rh/Ti 3+ -TiO 2 nanocomposite. Figure 2(a) displays nanosized particles with non-uniform shapes, with the average particle size being ~ 5 to 10 nm. Figure 2(b) displays the high-resolution TEM (HRTEM) image. The d-spacing was set at ~ 0.35 nm. This aligns with the (101) plane at TiO 2 . A group of lattice planes is easily identi ed on one nanocrystallite, with d-spacing at ~ 0.22 nm, corresponding to the (111) plane of Rh [23]. These lattice planes are clearly visible, which concurs with the results of our XRD analysis.
Using XPS, we examined both the surface components as well as the chemical valence state of the 5% Rh/Ti 3+ -TiO 2 nanocomposite (Fig. 3). After analyzing the XPS spectrum we found traces of Ti, Rh, and O (Fig. 3a). The Ti 2p XPS spectra of the Rh/Ti 3+ -TiO 2 nanocomposite was used to determine binding energies at 464.7, 463.9, 458.7, and 458.1 eV, which align, respectively, with Ti 4+ 2p 1/2 , Ti 3+ 2p 1/2 , Ti 4+ 2p 3/2 , and Ti 3+ 2p 3/2 [15], (Fig. 3b). ESR is particularly useful for detecting the existence of Ti 3+ , due to its high sensitivity to species containing unpaired electrons. The presence of Ti 3+ in the 5% Rh/Ti 3+ -TiO 2 nanocomposite was also identi ed using low-temperature ESR. A sharp and steep signal at g = 1.999 indicates the existence of Ti 3+ [24] in the Rh/Ti 3+ -TiO 2 nanocomposite (Fig. 4). In Fig. 3c, the binding energies of Rh 3d 3/2 at 312.2 eV and Rh 3d 5/2 at 307.5 eV can be attributed to the Rh 0 [23] valence states (Fig. 2c). The weak peak binding energies of Rh 3d 3/2 at 314.0 eV and Rh3d 5/2 at 309.0 are attributed to the Rh 3+ valence states. There are a number of factors that in uence the incidence of Rh 3+ : partial oxidization during treatment, high surface activity, and a small particle size. The binding energy of 530.0 eV can be attributed to the O lattice of TiO 2 , while the peak at 532.0 eV is due to the O lattice of Ti 3+, or chemisorbed hydroxyl groups (Fig. 3d). Figure 5 shows the light absorbance of Rh/Ti 3+ -TiO 2 nanocomposite. Pure TiO 2 powder (without adding Rh) was also tested as a control. The Rh/Ti 3+ -TiO 2 nanocomposite showed a clear shift of absorbance in the visible and NIR light range (1100 nm > λ > 400 nm). We also observed that as the atomic ratio of Rh increased, the visible-NIR light absorbance capacity of the Rh/Ti 3+ -TiO 2 nanocomposite was also increased. The photographs in Fig. 5 are digital photos of the synthesized TiO 2 and Rh/Ti 3+ -TiO 2 nanocomposite materials. We observed the darkest color of the 7% Rh/Ti 3+ -TiO 2 nanocomposite, which coincides with the absorbance spectrum of visible light.  nanocomposites, the survival ratio of S. aureus was approximately 4.7%, 0.56%, 0.0049%, and 0.056%, respectively. An Rh content of 5% displayed the strongest disinfection rate, while the lower disinfection rate of the 7% Rh/Ti 3+ -TiO 2 nanocomposite could be due to the increased amount of Rh. Without visible light illumination, the survival ratio of S. aureus remained constant at 50% after 4 h of treatment with 5% Rh/Ti 3+ -TiO 2 nanocomposite, which could be explained by mental rhodium nanoparticles accepting the electron of the cell membrane. In Fig. 6b and Figure S1, the survival ratio of S. aureus was approximately 6.3% under treatment with the 5% Rh/Ti 3+ -TiO 2 nanocomposite and NIR light illumination.

Conclusions
In conclusion, we successfully synthesized a metallic, rhodium-modi ed, and Ti 3+ self-doped TiO 2 nanocomposite photocatalyst using a facile solvothermal method. Our results con rmed that the Ti 3+ ion and metallic rhodium were both present in the synthesized sample. This indicates that, when illuminated with both visible and NIR light, the synthesized nanocomposite demonstrates enhanced antibacterial activity as compared to pure TiO 2 nanoparticles. Figure 1 X-ray diffraction patterns of TiO2 nanoparticles and Rh/Ti3+-TiO2 nanocomposite.     Optical absorbance of Rh/Ti3+-TiO2 nanocomposite, compared with that of TiO2 nanoparticle. The inset pictures were the color of the synthesized Rh/Ti3+-TiO2 nanocomposite.