TiO2 Nanotubes with Different Ag Loading to Enhance Visible-Light Photocatalytic Activity

An improved photocatalytic activity of semiconductor materials using incorporation of the noble metals such as Ag, Au, and Pt is a promising technology. In this study, Ag nanoparticle-TiO2 nanotube structures (Ag-TNTs) have been investigated as a photocatalyst in different irradiation conditions using different characterization techniques. The results indicate that Ag nanoparticles dispersed uniformly on the TNTs’ surface without any change in TNTs’ morphology. In addition, Ag-TNTs exhibited lower photoactivity than the TNTs under UV irradiation. In contrast, Ag-TNTs increased the photoactivity in comparison with TNTs and the photocatalytic performance under sunlight irradiation. These phenomena could be contributed to the appearance of Ag nanoparticles on the nanotube surface.


Introduction
Various oxide semiconductors have been studied such as ZnO, TiO 2 , CdS, WO 3 , and SnO 2 .TiO 2 has attracted an excellent attention in various applications such as water treatment, air purification, self-cleaning surfaces, antibacterial, and water-splitting catalysts for hydrogen generation [1][2][3] due to high catalytic efficiency, nontoxicity, being environmentally friendly, wide band gap, and low cost.In recently years, one-dimensional (1D) TiO 2 nanostructures, such as nanorods, nanotubes (TNTs), and nanofiber, have been successfully fabricated and initially effective in environmental treatment [4,5].In addition, TNTs remarkably possess superior properties compared with other forms of 1D TiO 2 since TNTs have good mechanical and chemical stability, large surface area, and charge transport property and are highly reproducible [6].
TNTs have been fabricated using different methods such as anodic oxidation, template method, and hydrothermal method [7][8][9].Among these technologies, the hydrothermal method may be the best choice due to cost saving, simple processing, and high quality [10].Unfortunately, a drawback of TiO 2 is its wide band gap (  = 3.0−3.2eV).Therefore, electron-hole pairs generation can only be attained by UVlight irradiation.Moreover, its high recombination rate of photogenerated electron-hole pairs on the surface results in low photocatalytic efficiency.Various strategies have been adopted for improving the photocatalytic efficiency of TiO 2 and extending the absorption spectrum into the visible light region.Making a composite semiconductor by modifying TiO 2 with metal ions is an effective method to extend the light absorption of TiO 2 to the visible light.The transition metal ions act as electron reservoirs to suppress the electronhole pairs recombination and promote interfacial electron transfer process that increases the surface reactivity.Previous reports have demonstrated that the photocatalytic property of TiO 2 can be improved in combination with transition metal ions, such as Fe, Ni, and Cu, as well as noble metal (Ag, Pt, and Au) [11].Among these noble metals, Ag is the most suitable for incorporating with TiO 2 to improve the photocatalytic activity and open possibility for further applications in the antibacterial field due to its strong antimicrobial effect individually.The previous studies have been focused on the photocatalytic properties and the antibacterial ability of Ag nanoparticles loaded TiO 2 under UV light and visible light [12][13][14][15].Nainani et al. [12] reported that Ag-TiO 2 shows higher photocatalytic activity compared to TiO 2 and its absorption and shifting to longer wavelength (visible range) due to lowering of its band gap.Li [13] also reported that the Ag-TiO 2 thin film exhibits higher photocatalytic activity than Degussa P-25 TiO 2 under visible illumination.It is attributed to the deposition of Ag species on the surface layer of TiO 2 ; an appropriately deposited Ag species on the surface of TiO 2 can effectively capture the photoinduced electrons and holes; besides the photoinduced electrons can quickly be transported to the oxygen adsorbed on the surface of TiO 2 .Furthermore, Yi et al. [14] studied Ag/TNT heterojunctions and compared its photocatalytic activity with TNT, P25, and Ag/TNT under visible light due to their extraordinary localized surface plasmon resonance (LSPR) property of Ag nanocrystals and the high adsorption capability of TNTs with the large specific surface area.Gupta et al. [15] compared the antibacterial activity of TiO 2 with Agdoped TiO 2 nanoparticles on various strains (Staphylococcus aureus, Pseudomonas aeruginosa, and Escherichia coli) under visible light irradiation.Their studies indicated that the enhanced bactericidal activity in the dark and under UV illumination is due to the synergistic antibacterial effect of the photocatalytic reaction of the TiO 2 coating and silver nanoparticles in the matrix.However, these previous works focused on the photocatalytic property of Ag-TiO 2 under UV or visible light irradiation; there are a few reports about the photocatalytic property of Ag-TiO 2 under sunlight condition [16].Besides that, the average sunshine at 150 kcal/m 2 in Vietnam is between 2.000 and 5.000 hours, which leads to a potential to synthesis application materials in the photocatalytic and antibacterial field.Therefore, in this work, Ag-TiO 2 nanotubes (Ag-TNTs) will be fabricated using the hydrothermal method and photoreduction.The morphology, chemical composition, and crystalline phase of the Ag-TNTs are investigated using various characterization techniques.In addition, the photocatalytic activity of the Ag-TNTs is also evaluated using the absorption of Methylene Blue (MB) aqueous solution at  = 664.6 nm under UV light and sunlight irradiation conditions.The photocatalyst mechanism is also discussed in detail.

Experimental
2.1.Materials Preparation.TNTs were fabricated by the hydrothermal method using 4.2 g of TiO 2 powder dispersed in 120 ml of 10 M NaOH aqueous solution by the magnetic stirrer for 4 hours at 50 ∘ C; the TiO 2 + NaOH suspension was then heated at 130 ∘ C for 22 hours in an autoclave.After filtration and centrifugation process, the white product was washed with distilled water until pH ∼ 9; then 2 M HNO 3 acid was then slowly added to the distilled water until pH 7 and, finally, washed again with not distilled water at 80 ∘ C to remove the residue of sodium.Next, this product was dried at 60 ∘ C for 4 hours; after that the white powder was annealed in air at 400 ∘ C for 2 hours with an increment of 5 ∘ C/min.Ag-TNTs were prepared using the photoreduction method of AgNO 3 and TNTs under UV light.Firstly, 0.2 g TNTs were added to 50 ml distilled water and then stirred for 15 min to disperse the TNTs suspension.Different amounts of AgNO 3 powder were dissolved in 50 ml distilled water and mixed by the magnetic stirrer at room temperature.Secondly, AgNO 3 solution dispersed slowly into the TNTs suspension and was stirred for 15 min to mix in the substances.Thirdly, the solution was stirred for 2 hours under UV irradiation.The Ag-TNTs samples were labeled as Ag-TNTs-2.5,Ag-TNTs-5, and Ag-TNTS-10 based on the concentration of Ag: 2.5 wt.%, 5.0 wt.%, and 10 wt.%, respectively.

Characterization Techniques.
The morphology, crystalline phase, and chemical components of Ag-TNTs were characterized by transmission electron microscopy (TEM, JEM-1400), X-ray diffraction (XRD, Bruker D8-Advance), and AAS (AA-6650, Shimadzu, Japan), respectively.The cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) results were obtained using an electrochemical workstation (Biologic SAS, model 1).A threeelectrode system was used for cyclic voltammetry (CV) constituting a counter electrode (platinum mesh), a reference electrode (Ag/AgCl), and a working electrode.The working electrode was prepared by a drop casting method.5 ml of graphene oxide 0.1 M and 0.05 g TNTs (or Ag-TNTs) solution were dropped on the platinum electrode and allowed to dry out at room temperature.This was repeated until a thin film was formed on top of the platinum electrode, which served as a working electrode.These electrodes were dipped in an electrochemical cell containing 100 ml KCl 0.1 M. Scans were performed between potential ranges of −1 V to 1 V at a scan rate of 10 mVs −1 for 100 cycles.

Photocatalytic Property Tests.
The photocatalytic activity of TNTs and Ag-TNTs with a different AgNO 3 /TiO 2 ratio was investigated by the absorption in an aqueous solution containing MB under different irradiation conditions.25 mg of each sample was dispersed in 10 ml MB aqueous solution (50 mg/l) and irradiated by UV light (25 W, Reptile UVB100-PT 2187) and sunlight.Every 5 min interval time, TNTs were separated from MB aqueous solution using a centrifuge and filter paper; UV-Vis spectrophotometer (UV-2450; Shimadzu, Tokyo, Japan) was then used for monitoring the absorption of MB solutions at a wavelength of 664.6 nm.

Results and Discussions
Figure 1 shows the TNTs and Ag-TNTs with the different AgNO 3 /TiO 2 ratio.TNTs show the fairly uniform length with a diameter of 11.78 nm with the diameter of Ag-TNTs samples ranging 8-12 nm with the loading of Ag nanoparticles on the tube.It indicated that the appearance of Ag nanoparticles does not affect the morphology of TNTs.At the lowest concentration (2.5 wt.%), there are only a few Ag nanoparticles on the surface of TNTs, when the concentration increases (5.0 wt.%) the density of Ag nanoparticles on  TNTs is denser, with the increase nearly covering the TNTs.At the highest concentration (10 wt.%) (Ag-TNTs-10) the size of Ag (Figure 1(d)) is larger than Ag-TNTs-5 sample (Figure 1(c)) and its density is insignificant.These results are fully consistent with the results of AAS analysis in Table 1 and can be explained as follows: the increasing Ag content in samples leads to increasing aggregation phenomena on the surface; in modification process, the filtering and washing possible wash away Ag from TNT-Ag.Therefore, ratio of Ag content after modification process does not agree with ratio of Ag addition.TNTs surface is explained by the mechanism following the reaction equations [17].First, the TNTs aqueous solution is dispersed in AgNO 3 solution, followed by strong magnetic stirring for 2 hours.Silver ions are then adsorbed onto the walls of TNTs due to magnetic stirring under UV irradiation condition.The total deposit of the adsorbed Ag + onto the outer surface of TNTs is stronger than onto the inner surface due to the inconvenience of silver ions to travel into smaller areas of inner surface nanotubes: The UV irradiation is an extremely important step; TNTs absorb UV radiation and produce electron-hole pairs at the surface of the semiconductor.The photoinduced conduction band electrons are responsible for reducing silver cations into the metallic silver: The UV-vis absorption spectra of TNTs and Ag-TNTs with different AgNO 3 /TiO 2 ratio are displayed in Figure 3.The absorption spectrum of Ag-TNTs samples shows single broadening at the range of 385.7-504.9nm, attributing to the charge-transfer from the valence band to the conduction band.The optical band gap energy (  ) of TNTs and Ag-TNTs was estimated by a plot of [ℎ]] 1/2 versus photon energy (ℎ]), using a tangential interception to the -axis;   of TNTs and Ag-TNTs would be calculated by Kubelka-Munk functions and was presented in Figure 3(b).Table 2 shows the wavelength of absorption () and the corresponding band gap (  ) for TNTs, Ag-TNTs-2.5,Ag-TNTs-5, and Ag-TNTs-10.  of Ag-TNTs samples were narrower than that of TNTs, and the reflectance spectra of Ag-TNTs samples significantly shifted toward a longer wavelength (red shift) compared with TNTs.It proves that the surface modification significantly expanded absorption wavelength region of TNTs due to the surface plasmon resonance of Ag nanoparticles on the surface of TNTs.The photocatalytic ability of TNTs and Ag-TNTs samples is evaluated by the absorption of MB in the presence of samples under the UV and sunlight irradiation.The absorption peak of MB appeared at 664.6 nm wavelength, and MB absorption efficiency varied with the structural properties of the catalysts.Figure 4(a) shows the absorption spectra of MB solution TNTs and Ag-TNTs with a various AgNO 3 /TiO 2 ratio under UV irradiation for 30 min.The efficiency of MB absorption is changed corresponding to the structural properties of the catalysts.The absorption spectra for TNTs show the biggest change under UV irradiation condition as shown in Figure 4(a).The degradation efficiency of MB solution of Ag-TNTs-2.5,Ag-TNTs-5, Ag-TNTs-10, and TNTs is about in turn 94.32, 91.46, 90.00, and 99.56, within 45 min (Figure 4(c)), respectively.This result indicated that TNTs show better photocatalytic activity than Ag-TNTs samples under UV irradiation.In contrast, under sunlight condition, Ag-TNTs samples exhibit better photocatalytic activity than TNTs (Figure 4   of MB solutions in Ag-TNTs-5 is about 96.30 while Ag-TNTs-2.5,Ag-TNTs-10, and TNTs turn in about 95.41, 88.69, and 85.10 within 20 min (Figure 4(d)).It is found that 5.0 wt.% Ag is optimum to achieve the highest degradation efficiency of MB solution under sunlight irradiation.According to the result from Figures 4(a), 4(c), and 4(d), the Ag deposition has the benefits of the photocatalytic activity of all Ag-TNTs samples than TNTs under sunlight irradiation condition.The deposition of Ag nanoparticles on the TNTs surface improved the absorption of MB for the following reasons, and these phenomena were the same as in the previous studies [18,19]: (i) the formation of Schottky barrier at the Ag nanoparticles-TNTs junction results in Ag nanoparticles deposited on TNTs which have often been applied to act as electron reservoirs to suppress the electron-hole recombination; thus more holes are available for the oxidation reaction, leading to higher photocatalytic activity of TNTs; (ii) more MB molecules are absorbed on the surface of Ag-TNTs than on the TNTs surface, enhancing efficient transfer of the photoexcited electron from MB to the conduction band of TNTs in a self-photosensitization pathway under irradiation; (iii) Ag-TNTs absorb visible light via surface plasmon resonance owing to Ag nanoparticles on the surface TNTs; the plasmon photocatalysis is taking place by injection of electron across the conduction band of TNTs enhancing the surface electron excitation and electron-hole separation.Therefore, the photocatalytic performance of the Ag-TNTs could be better under sunlight condition, whereas Ag nanoparticles deposited on the surface TNTs show the lower photocatalytic activity under UV irradiation condition.It may be explained that the efficiency absorption of MB of Ag-TNTs samples does not significantly contribute to enhancing the TNTs photocatalytic activity under UV; MB molecules are not excited by UV irradiation.Furthermore, the Ag surface plasmon resonance is not excited under UV irradiation.The Ag content has an important effect on the photocatalytic activity of Ag-TNTs.These results indicated that Ag content of 5.0 wt.% shows the best degradation performance of MB solution under different irradiation.It implies that the photocatalytic activity or the effective electron-hole separation depends on an amount of Ag in TNTs specimens.It could be attributed to the Ag-TNTs surface that acts as electronhole separation centers, as mentioned in [20].The electron transfers from the TiO 2 conduction band to metallic Ag nanoparticles at the interface because the Fermi level of TiO 2 is higher than that of Ag metal.It results in the formation of Schottky barrier at metal-semiconductor contact region and improves the photocatalytic activity performance of TiO 2 [21].However, at an amount of 5.0 wt.% of the silver, that is, a decrease of photocatalytic activity, it can be explained by the following effects [17,22,23]: (i) the increasing amount of Ag on the surface of TNTs forms a barrier hindering the contact of MB dye molecules with TNTs [22]; (ii) the Ag nanoparticles prevent the light absorption of TNTs because all of the active sites on the TNTs will be covered by Ag deposition, reducing the efficiency of charge separation increased by the large number of negatively charged Ag particles on the surface TNTs [17,23]; and (iii) it can also act as the recombination center of electrons and holes, leading to the decrease of photocatalytic activity of TiO 2 .
In addition, EIS spectra are used to compare the electronic transportation properties of the TNTs electrode and the Ag-TNTs samples electrode (Figure 5), which is an effective tool for probing the photogenerated charge separation and transport properties [24][25][26].The equivalent circuit contains  1 ,  PE , and  2 , where  1 is the bulk resistance of the electrolyte and electrodes and  PE and  2 are the capacitance and the resistance formed at the interfaces of the electrodes, respectively.The arc radius of Ag-TNTs samples is smaller than TNTs, which shows that the capacitance ( PE ) and the resistance ( 2 ) of Ag-TNTs are larger and smaller than TNTs.A smaller arc radius indicates that the transfer rate and the separation rate of photogenerated electron-hole pairs on the Ag-TNTs arrays' surface become higher due to the appearance of Ag on the surface of TNTs [19].These results suggest that the photocatalytic Ag-TNTs are higher than TNTs.EIS results are fully consistent with the results of photocatalytic property tests in Figure 4.

Conclusions
TNTs were successfully modified with Ag nanoparticles using the photoreduction method.The TNTs have a fairly uniform length of several hundred nanometers and the diameter of about 11.78 nm with Ag nanoparticles decorated on the nanotube surface.Photocatalytic decomposition of MB by Ag-TNTs samples depended on Ag content and irradiation conditions.The TNTs exhibited better photocatalytic activity than Ag-TNTs under the UV irradiation due to the adsorption of dye molecule on the surface of Ag-TNTs and self-degradation of MB.Under sunlight, Ag-TNTs samples showed better photocatalytic activity than TNTs due to the effective electron-hole separation at the interface and the surface plasmon resonance of Ag species.The best photocatalytic performance was obtained when Ag content is 5.0 wt.%.Based on the efficient photocatalytic activity of Ag-TNTs under sunlight, Ag-TNTs have promise for application in the antibacterial field under different light irradiation.

Figure 4 :
Figure 4: Absorption spectra of MB under (a) UV irradiation and (b) sunlight in the presence of different catalysts.And photocatalytic degradation of MB under (c) UV irradiation and (d) sunlight using various photocatalysts.

Table 1 :
AAS result of the Ag concentrations in the Ag-TNTs samples.