The Photocatalytic Activity of SiO2-TiO2/Graphite and Its Composite with Silver and Silver oxide

This research study the mixed semiconductor of SiO2-TiO2 which was immobilized on graphite substrate and also studies the effect of surface modification on its mixed semiconductor. The surface modification was carried out by electrode position of Ag from 0.4 M of AgNO3 solution at various applied current. The electrode position was conducted for 30 minutes at 0.004; 0.008; 0.010; 0.012 and 0.014 A. In the electrode position cell, SiO2-TiO2/Graphite was used as cathode and a graphite rod was used as anode. The weight of deposited Ag was analyzed gravimetrically. The current efficiency of electrode position was calculated by comparing its experimental weight to its theoretical weight founded from calculation using Faraday’s equation for electrolytic cell. Meanwhile, the photo electrochemical testing was carried out to investigate the efficiency of induced photon to current conversion; it was measured as %IPCE (% Induced Photon to Current Efficiency). The crystallinity and crystal structure of the prepared materials were analyzed by X-ray diffraction and their surface morphology was analyzed by Scanning Electron Microscope (SEM). This research found that silver was deposited as silver metal, Ag and as silver oxide, AgO. The electrode position efficiency at applied current of 0.014 A is 92.30 % with diameter of cluster is 7 11.9 mm. It is founded that Ag and AgO deposition enhanced the efficiency of photon conversion into current up to 89.92 %. The optimum %IPCE value is at 28.6 % of Ag content. Ag-SiO2-TiO2/G has higher photo conversion than Ag-TiO2/G, shown by the %IPCE value at 300 nm is 83.25 % higher than Ag-TiO2/G. It indicates the significant role of silica network in photo excitation mechanisms in the composite material. ©2014 BCREC UNDIP. All rights reserved


Introduction
Semiconductor plays significant roles in human life due to it becomes a promising technology for environmental remediation such as for air and water purification, wastewater treatment and also as material for generating energy such as for solar cell and for battery [1].Those important roles inspire some researchers to conduct researches on semiconductor materials in order to increase their performance.Mixed photo catalyst has been studied by some researchers [2][3][4] due to those mixed photo catalyst perform better charge separation than single semiconductor and therefore suppress electron-hole recombination resulting better photo catalytic performance.That semiconductor also might be engineered to be expanded their photoresponse especially into visible light area by mixing it with other material that has good photorespon in visible light, such as CdS or inorganic complex that can serve as sensitizer.Linsebigler et al. [5] synthesized a CdS-TiO2 composite and found that the coupling semiconductor shows enhancement of photo catalytic efficiency through charge separation, i.e. hole-excited electron separation then each charge can move freely to the semiconductor surface and serve as oxidator or reductor.Xe and Yuan [6] doped Ce 4+ into Titania sol and nanocrystalline Titania and found that Ce 4+ doped-titania sol has higher photo catalytic efficiency than nanocrystallite Titania.Meanwhile, Nugraheni [7] prepared composite of TiO2 with SiO2 with Na2SiO3 was used as precursor for silica and TiCl4 was used as precursor for TiO2.A 16 mM cetyl trimethyl ammonium (CTAB) solution was used as mesopore template.
There are some methods for preparation of photo catalytic semiconductor that have been investigated by some researchers, such as spin coating method [8][9], magnetron sputtering [10], chemical bath deposition [11,12], chemical vapor deposition [13] and also dip coating method [14,15].Immobilization of semiconductor on a substrate has been considered as an effort to reduce cost of post treatment, such as for separation of the photo catalyst powder from the degraded solution.Moreover, recovery and recycling process can be conducted easier for the immobilized form.Therefore, some researchers interest to conduct researches on the immobilized form of semiconductor, such as Zhou et al. [15] who have prepared SiO2-TiO2 composite on glassy ceramic substrate by dip coating method from tetrabutyl orthotitanate (Ti(Obu)4) and tetraethyl orthosilicate (TEOS) as precursors.Meanwhile, Whang et al. [14] prepared a thin film of SiO2-TiO2-PDMS composite by dip coating method from TEOS and polydimethylsiloxane (PDMS) which were mixed with titanium isopropoxide (TiOPr) as precursors and used acid solution as catalyst.The sol-gel and hydrothermal method of synthesis have been considered as good alternative for SiO2-TiO2 preparation due to no requirement of high temperature.Sol-gel method for mesoporous silica-titania composite is also reported by Shao et al. [2] with TiOCl solution and sodium silicate as precursor in the absence of CTAB surfactant.Meanwhile, Nichi et al. [16] used sol-gel preparation of nanoscale TiO2/SiO2 composite and Huang et al. [17] also used sol-gel method on the synthesis of polyaniline-modified Fe2O3/SiO2/TiO2 composite.A sol-gel route also applied by Wang et al. [18] to prepare a polyimide/silica/titania nanohybrids.It was found by Wang and coworkers that introduction of SiO2 detain the aggregation behavior of TiO2 on polyimide matrix.
Metal deposition on semiconductor surface is a modification method of semiconductor in accordance to enhance photo catalytic activity of the semiconductor.The enhancement of photo catalytic activity as the result of metal surface modification has been recorded since 1980 by Sato and White [19] in photo catalytic conversion of H2O into O2 and H2.Loganathan et al. [20] doped silver, gold and platinum into TiO2 nanoparticle, and found that Au doped TiO2 (0.5 wt%) has the highest photo catalytic activity due to cathodic influence of gold in suppressing the electron-hole recombination during the reaction.In our previous study, metal surface modification on TiO2/Graphite by electrode position of Cu and Ag and found the enhancement of photo catalytic activity in disinfection on Escherichia coli-contaminated water [12].Ag metal has high standard reduction potential, i.e. 0.799 Volt, it indicates low possibility to be oxidized and high possibility to receive electrons and become the electron trapper for the excited electrons, e -, in conduction band, then it can increase separation between the excited electrons to the holes, h + and allows e -and h + to move freely to surface and become oxidiser and reductor for the targeted molecules.
This paper discusses the investigation of the photo catalytic activity of TiO2-SiO2 on graphite substrate and its modification with Ag and AgO deposition.Investigates the effect of Ag and AgO on the photo catalytic activity.Then compare the results to the photocatalytic activity of TiO2/Graphite and Ag-TiO2/Graphite which were published previously, in order to determine the contribution of the presence of SiO2.The photo catalytic activity has been determine through the measurement of % Induce Photon to Current Efficiency (%IPCE).

Materials and Methods
The chemicals were used in this research have pro analysis grade.Titanium (IV) chloride (TiCl4) (Merck) was used as TiO2 precursor, Sodium Silicate (Na2SiO3) (Merck) as precursor of silica, CTAB surfactant (Merck) for linking agent between silica-titania network to graphite substrate and also as pore template of composite, graphite rods (product of ABC battery industry, Indonesia) were sliced into thin tablets and being used as substrate after being cleaned with deionized -water and ethanol.An-
In preparation of SiO2-TiO2 mixture, 6.2 ml (which is similar to 0.070 mol) of Na2SiO3 was poured into 20 ml ethanol, then 2.7 ml of HCl 37 %, 3 ml of H2O and a definite amount of CTAB 16 mM ( the definite amount was based to calculation to get ration of mol of H2O to Na2SiO3 is 2.385).In other vessel, 2.9 ml of TiCl4 was dissolved in 10 ml of methanol and 10 ml of isobutanol was added to reduce the hydrolysis rate of TiCl4.
The Na2SiO3 solution was being refluxed and TiCl4 solution was added gradually under stirred condition at 70 °C.After all TiCl4 was added into Na2SiO3 solution and the mixture has became homogeneous, then the reflux process was stopped and the mixture was sealed and stirred for 3 days until it became gel.A 4.12 ml of NH4OH solution then being added to the gel followed by heating at 70 °C and stirred for 1 hour.Then it was added by 5.9 mL aquadest and it was heated at 120 °C for 1 day.The produced SiO2-TiO2 powder was then being calcined at 1100 °C [7].
The immobilized SiO2-TiO2 mixture was prepared by dip coating method, by dissolved 0.2 g of SiO2-TiO2 mixture in 25 ml methanol and then 2 graphite tablets with diameter of each 0.8 cm were dipped into these solution for 5 minutes and then dipped repeatedly with 5 minutes for each dipping during 18 hours.The immobilized SiO2-TiO2 mixture in graphite substrate then being cleaned by deionized-water and being heated at 110 °C for 4 hours [21].
Silver was deposited electrically from 0.4 M of AgNO3 solution at various currents of 0.004; 0.006; 0.008; 0,010; 0.012 and 0.014 A. Electrode position process was carried out for 30 minutes in Analytic Analyzer Electrolysis Yanaco AES-2D.
Gravimetric analysis was carried out to measure the weight of silver deposited.Crystallinity and crystal structure of the prepared materials were analyzed by XRD (X-ray diffraction Shimadzu 6000) at 2θ of 20-80 °.Morphological analysis was conducted on the prepared materials by Scanning Electron Microscope (Analytical Scanning Electron Microscope Jeol JSM-6360LA).Meanwhile, the %IPCE was measured by using Deuterium lamp and Wolfram lamp (installed in UV Vis Spectrophotometer Seiki-Ogawa, the specification of the lamps are 100 mA and 10 mV).A solution of 0.1 M of Kalium Iodide and 0.1 M of Iodide solution in acetonitryle was used as electrolyte.A graphite rod was used as counter electrode.The current produce during light induction was recorded by a multimeter (Sanwa Multimeter Digital CD 751, scaling in mA, mV and W).

Results and Discussion
The synthesis of SiO2-TiO2 mixture through sol-gel method proceed in two steps, i.e. the hydrolysis, when the precursor reacts with water and followed by condensation process that changes the solution to become sol.After solvent vaporization, gel will be formed and after drying process the silica-titania mixture is formed.The diffraction pattern of silica-titania powder is described in Figure 1.The diffraction pattern in Figure 1 shows the characteristic peaks of the SiO2-TiO2, which are signed as ST.The ST peaks are dominated by some broaden peaks that indicates the amorphous phase.The mixture also contain rutile TiO2 phase and there is no characteristic peaks of anatase.As high calcinations temperature, 1100 °C, might transform the anatase phase to rutile phase, as it was studied by Shao et al.Electrodeposition of Ag from AgNO3 solution at applied current of 0.004 A and 0.014 A produced composite material with diffraction patterns described in Figure 2. The diffraction patterns in Figure 2 shows the appearance of new peaks which are identified as Ag at 2θ 38.09 ° that matches to Ag standard diffraction ICSD#44387 with space group of FM3M, a face centered cubic structure and a peak at 2θ 45.37 ° is identified as Ag that matches to Ag standard diffraction ICSD#64707 with space group of P63/MMC, a primitive cubic structure.Some new peaks at 2θ 34.35, 37.35 and 56.85 ° which

Bulletin of Chemical Reaction Engineering & Catalysis, 9 (1), 2014, 48
Copyright © 2014, BCREC, ISSN 1978-2993 are identified as AgO based on the standard diffraction of AgO ICSD#27667.Morphological analysis by SEM also shows the present of Ag and AgO deposits as described in Figure 3. Ag and AgO clusters which were produced at 0.004 A of current have smaller size than that were produced by electrode position at 0.014 A, therefore the morphology of composite surface which were produced at 0.004 A is smoother, as it is described in Figure 3(b) and 3(c).The gravimetrical analysis to the prepared composites shows the different of composite weight as function of current, as listed in Table 1.
(1) Meanwhile, the theoretical weight (Wtheo) was calculated based on Faradic equation for electrode position (Equation ( 2)). is number of electrons contribute to the reaction (equation ( 3) and ( 4)) and F is the Faradic number (96500 C).H2O(aq)  ½ O2(g) + 2 H + (aq) + 2e - EºA = 1.23 Volt (3) Ag + (aq) + e - Ag(s) EºC = 0.799 Volt ( Gravimetrical analysis data in Table 1 shows that the higher the current was applied in electrodeposition process, the current efficiency decreases, except at 0.014 A. These probably due to some deposited silver fall out and the experimental weight then become non linear function to the applied current as it is stated in Faradic equation (Equation ( 2)).At applied current of 0.014 A, the current efficiency is high, i.e. 92.30 %.This probably due to different morphological characteristic of deposited Ag at 0.014 A. SEM images (Figure ( 3)) shows that the deposited Ag at 0.014 A has more regularly form, which is similar to cubic form with short side 7 mm and long side of 11.9 mm.Meanwhile, Ag and AgO which were deposited at current less than 0.014 A has irregularly form and smaller size.Those irregularly deposition may cause the connection between metal deposited and SiO2-TiO2 surface is weak and unstable.
Photo electrochemical testing on the prepared materials measured the %IPCE value that represents the conversion efficiency of induced photon to current at various wavelength of light (200 -700 nm) (Figure 4).Those %IPCE represents the photo catalytic activity of each material due to the electrons that passed the external circuit and were detected by amperemeter were electrons that were excited by photon from valence band to conduction band then flowed to the current collector and meas-ured by amperemeter.
Figure 4 shows that deposition of Ag and AgO on SiO2-TiO2/Graphite enhances the %IP-CE value up to 89.92 % at ultraviolet wavelength, 300 nm and enhances the %IPCE value up to 84.06 % at visible light (500 nm).These prove that Ag and AgO could serve as electron trapper then reduce the possibility of electronhole recombination.Schemes of the proposed mechanisms in SiO2-TiO2/G and Ag-SiO2-TiO2/G are depicted in Figure 5.Meanwhile, the relation between weights of metal deposited with the conversion efficiency of induced photon to current (%IPCE) at 300 nm is described in Figure 6.Polynomial trend line application to those %IPCE data found that the optimum silver content will be 28.6 %.Meanwhile, the comparison between %IPCE of Ag-SiO2-TiO2/Graphite, the material that is discussed in this paper with Ag-TiO2/Graphite, the material which was published in our previous paper [12] Table 1.Weight of deposited silver as function of applied current and its calculated current efficiency (%) based on calculation using Equation ( 1  Figure 7 shows that Ag-SiO2-TiO2/G has higher %IPCE than Ag-TiO2/G.It indicates that SiO2 plays significant role in photo excitation mechanism.At induce photon of wavelength lower than 350 nm (comparable to more than 3.47 eV as the gap energy of SiO2) the electrons in valence band of SiO2 also can be excited to conduction band and contribute to the photo activity of this Ag-SiO2-TiO2/Graphite.The existence of Ag and AgO at SiO2 surface can serve as electron trapper then flows those electrons to the external circuit.
Moreover, SiO2 itself also can serve as electron trapper for the excited electron from valence band of TiO2 and this mechanism can reduce the possibility of electron-hole recombination.Hu et al. [22] who prepared the mixture of SiO2/TiO2 and TiO2 core in SiO2 shell (SiO2-TiO2) found that the recombination rate is decreased by the existence of SiO2.This explanation was based on photo luminescence, PL, spectrum.PL emissions is the result of the recombination of the excited electrons with holes, therefore the lower the PL intensity indicates the decrease in recombination rate [22].Mean- while, in Ag-TiO2/Graphite, only electrons of TiO2 that contributes to photo excitation and also only Ag and AgO that can serve as electron trapper.Therefore, the photo catalytic activity, as it is refered by %IPCE value is lower than Ag-SiO2-TiO2/G.The proposed schemes for photo excitation mechanism in Ag-SiO2-TiO2/G and Ag-TiO2/G are described in Figure 8.

Conclusions
Surface metal electrodeposition on SiO2-TiO2/Graphite produced deposited of Ag and AgO.The highest current efficiency of electrodeposition is at 0.004 A of applied current, i.e. 99.404 %.The results show that Ag and AgO deposition on TiO2-SiO2/Graphite composite significantly enhance the photocatalytic activity up to 89.92 % under UV irradiatio and up to 84.06 % under visible light irradiation.The optimum %IPCE value can be reached when the Ag and AgO content is 28.6 %.The conversion efficiency of induced photon to current of Ag-SiO2-TiO2/G is higher than Ag-TiO2/G.It indicates the existence of SiO2 contribute significantly to the photo activity of material.

Figure 1 .
Figure 1.Diffraction patterns of TiO2-SiO2 mixture after being heated at 1100 °C.R refers to Rutile and ST refers to SiO2-TiO2 phase

( 2 )Figure 2 .Figure 3 .
Figure 2. The diffraction patterns of Ag-TiO2-SiO2/Graphite, Ag was deposited at 0.004 A and 0.014 A of current.Ag* refers to characteristic peak of Ag corresponding to ICSD#64707