Applying Surface Charge Attraction to Synthesizing TiO 2 / Ag Composition for VOCs Photodegradation

This study utilized a unique method to prepare TiO2/Ag composites by operating the pH value of synthesis solution, which causes the opposite charges on the surface between TiO2 and Ag to enhance the combination affinity of TiO2 and Ag due to electron attraction. The results showed that the TiO2/Ag prepared in solution at pH 5.8 was characterized as high TiO2 dispersibility and the optimal combination between TiO2/Ag, due to the opposite surface charges and the greatest difference in zeta potentials between TiO2 and Ag. The prepared TiO2/Ag was utilized to process acetone photodegradation, and the results illustrated that TiO2/Ag prepared in solution at pH 5.8 showed the best performance in photodegrading acetone. With increasing TiO2 coated on Ag in TiO2/Ag, the photodegradation efficiency of acetone was enhanced. In terms of the average efficiency via TiO2 coating amount, 10 wt% was the most optimal efficient proportion of TiO2/Ag for the photodegradation of acetone.


INTRODUCTION
Titanium dioxide (TiO 2 ) is widely applied and was proven efficient for decontamination, sterilization, and deodorization, as well as removing NO x and volatile organic compounds (VOCs) (Kim et al., 2006;Zuo et al., 2006;Yu et al., 2007).However, agglomeration of TiO 2 is common due to van der Waals forces of nanoparticles, which reduces the contact surface of TiO 2 with contaminants, and thus lowers the photocatalysis activity.This problem can be ameliorated by coating TiO 2 onto a carrier or adding a surfactant and dispersant.
Common carriers used with TiO 2 include SiO 2 (Herrmann et al., 1997;Kamegawa et al., 2006), ceramics (Sen et al., 2005), zeolite (Hashimoto et al., 2001), and activated carbon (Cordero et al., 2007), among which SiO 2 is the most widely applied, but it has no obvious porous characteristic or function of electron trapping, thus it is not significantly conducive to photocatalysis efficiency.Porous materials, such as zeolite and activated carbon with large specific surface areas and high absorption quantities, are used as catalysis carriers with TiO 2 , thereby enhancing the photocatalysis efficiency by absorbing contaminants, but they are not applied under high temperatures.Related works modified platinum (Pt) (Cho et al., 2004) and sliver (Ag) (Ohtani et al., 1993) on the surface of TiO 2 to increase the photocatalysis efficiency.Several studies reported that the photocatalytic efficiency of Pt/TiO 2 was better than that of Ag/TiO 2 , but the latter is more economical than other precious metals for the surface modification of TiO 2 (Vamathevan et al., 2002).Sclafani and Herrmann (1998) indicated that in addition to postponing the recombination between electrons and holes, the application of Ag is much more favorable at adsorbing oxygen to enhance photocatalysis.Zhang and Yu (2005) found that silver could reduce toxic TiO 2 after exposure, and Ag/TiO 2 had a better photocatalytic efficiency than other metals.Lin et al. (2007) used Ag as the catalysis carrier to prepare a TiO 2 /Ag photocatalyst, and indicated the excellent performance on dye decoloration.In their result indicated that Ag carrier can also employ electron scavenging function to alleviate recombination of electron-hole pairs, which were excited from TiO 2 under irradiation, to increase photocatalysis effect.Common methods of coating TiO 2 onto carriers include spraying, spin coating, dip coating (Herrmann et al., 1997), liquid-phase deposition (Yu et al., 2005), and a sol-gel method (Moonsiri et al., 2004;Sen et al., 2005), among which the sol-gel method is most frequently used because it can be easily mixed with other materials by a simple process.However, the sol-gel method requires a subsequent calcination process, and the high temperature can cause the volume of TiO 2 to shrink and then agglomerate, or even alter the crystal structure of TiO 2 .In addition, to avoid agglomeration of TiO 2 particles during application, a surfactant must be added and the particles should be evenly dispersed, in order to increase the contact area between the reactive target and catalyst (Ding et al., 2000;Zainal and Lee, 2006).But the surfactant is usually an organic solvent, which may be degraded under UV irradiation thus inhibiting the photocatalysis process.Key and Maass (2001) indicated that the electron orbital of Ag is 36 [Kr]4d 10 5s 1 , and the 5s 1 electron in the Ag orbital will attract a hydrogen proton (H + ).In an alkaline solution, the positively charged H + of hydroxyl ions (OH -) is attracted to the 5s 1 of the electron orbital of Ag, and the negatively charged O -of OH -will be on the outside of the Ag surface; thus, Ag particles are negatively charged in solution.In contrast, in an acidic solution, Ag particles are positively charged.For metal oxides, the surface hydroxyl group in the solution has the Brönst amphiphilic characteristic.When the pH varies, the particle surface will be positively or negatively charged.In an acidic solution, the particle is positively charged due to protonation, but in an alkaline solution, the particle is negatively charged because of deprotonation.Whether the particle surface is positively or negatively charged is demarcated by the pH value of the solution at the isoelectric point of the particle itself.If the pH value of the solution makes TiO 2 have the same positive charge, and Ag has the other same negative charge, this could resolve TiO 2 agglomeration due to repulsion of similar charges without utilizing a dispersant; this would enable the binding of TiO 2 to Ag due to the attraction between their oppositely charged surfaces, regardless of the presence of an adhesive.
In this study, TiO 2 was coated onto an Ag carrier in solution by altering the pH to synthesize a TiO 2 /Ag photocatalyst.In the process, the pH of the prepared solution was controlled to change the surface electrical properties of the particles.This process was operated at room temperature without calcination, which saved energy and avoided particle agglomeration due to high temperatures.Besides being a carrier, Ag can also be used as an electron captor to postpone the recombination of electron-hole pairs, excited from TiO 2 under UV irradiation, and thus improve the photocatalysis performance.In this study, we explored the photodegradation efficiency of acetone by TiO 2 /Ag composites prepared with various pH values, compared the photodegradation performance with various TiO 2 coated amount on Ag in TiO 2 /Ag, and discussed the factors influencing the photodegradation efficiency of acetone by TiO 2 /Ag according the materials analysis.

Preparation of TiO 2 /Ag Composites
Formation of the Ag carrier is based on the principle of oxidation reduction.By putting a flat piece of copper in a silver nitrate solution, which was alkaline, the silver particles were separated on the copper, according to the chemical equation: Since the electrostatic potential energy of the equation was 0.5 V, the reaction was spontaneous without the addition of external energy.The process to obtain the synthesized nano-TiO 2 /Ag was as follows.The pH value of preparation solution was controlled using ammonia or nitric acid, TiO 2 (P25, Degussa) was put in the solution and stirred to be evenly suspended After adding the Ag carrier, the solution was mixed and finally washed, filtered, and dried.When the theoretical weight ratio of TiO 2 on the surface to the weight of Ag was 10: 100, it was defined as 10 wt% TiO 2 /Ag.

Test Procedure for the Photodegradation of Acetone by TiO 2 /Ag
The photocatalysis reactor was a quartz tube with an inner diameter of 2.2 cm, embedded in an 8-W ultraviolet lamp (UVA, with a wavelength of 365 nm and a light intensity of 1.78 mW/cm 2 ).The outside of the tube was covered with black acryl to reduce the impacts of external light sources on the reaction, and a fan was used for ventilation and to eliminate impacts of the heat emitted by the lamp.During the experiment, the coating length of the photocatalyst in the glass tube was 12 cm.The inner wall of reactor was treated as the frosted glass, the TiO 2 /Ag were weighed as 0.95 ± 0.05 g, dispersed in water and then coated onto the inner wall of the reactor.The reactor was then ready for the subsequent catalysis test after being dried at 110°C for 8 h and cooled to ambient temperature.The TiO 2 /Ag could be fixed on the inner wall well, and not be blown away under the experimental flowrate.The TiO 2 /Ag photocatalysis test system was operated in a continuous-flow manner, and the entire system was placed in a constant-temperature refrigerator (at 25°C).A compressor was used to supply high-pressure air, and the air stream was passed through silica gel and a high-efficiency particulate air (HEPA) filter to respectively remove humidity and particles.The air flow rate was controlled by a mass flow controller (MKS 1179A, USA); the gaseous acetone flow was produced by air flow passing through an impinger filled with an acetone solution.The diluted air flow and gaseous acetone flow were well mixed in the mixing chamber, the acetone concentration in the test air flow was stabilized as 200 ppm, and the air flow entering the reactor was exposed to the photocatalysis process.After 30 min of air inflow, the acetone was saturated, thus avoiding the photodegradation efficiency being influenced by adsorption of the photocatalyst.UVA light exposure was initiated to photodegrade the acetone via TiO 2 /Ag.The concentration of acetone was analyzed by gas chromatography coupled with flame ionization detection (GC-FID, SRI-8610C, CA, USA), and the photodegradation efficiency of acetone was defined as η = (C inlet -C outlet )/C inlet × 100%, where C inlet and C outlet were the respective inlet and outlet acetone concentrations.
In the material analysis, a field emission scanning electron microscope (FE-SEM, LEO 1530, Germany) and X-ray powder diffractometer (XRD, Panalytical X' Pert Pro MRD, Holland) were utilized to observe the surface of the prepared TiO 2 /Ag and determine the crystal structure.Xray photoelectron spectroscopy/electron spectroscopy for chemical analysis (XPS/ESCA, Thermo VG ESCAlab 250, USA) was used to identify the chemical state of surface elements of TiO 2 /Ag.Operating various pH values, the zeta potential was measured with a zetasizer (Malvern zetasizer nano ZS, UK), and calculated via smoluchowski equation, respectively.The transmittance of the supernatant in the TiO 2 /Ag solution under various pH values, after centrifugation, was analyzed by a UV-Vis spectrophotometer (Hitachi U3012, Japan).The practical weight percentage of TiO 2 coated on Ag was examined by inductively coupled plasma mass spectrometry (ICP-MS, AGILENT, 7500A).

Effect of Operational pH in Synthesis Process on Zeta Potential of TiO 2 and Ag Carrier
pH values of the preparation solutions were adjusted by adding HNO 3 and NH 4 OH.In acidic solutions, water separates into H + or further reacted with H + to form hydronium ions (H 3 O + ).H + or H 3 O + was attracted to the particle surface to cause positively charged.On the contrary, the particle surface would attract OH -and be charged negatively in the alkaline solution.Fig. 1 was illustrated the effect of operational pH in synthesis process on zeta potential of TiO 2 and Ag carrier, respectively.The isoelectric point (pI) of a compound is defined as the pH at which the compound has a net charge of zero.Operated at pH values below the pI, the surface of compound would carry a net positive charge; on the contrary, operating above the pI, the compound would carry a net negative charge.In order to measure the pI, Ag and TiO 2 were dispersed in various pH environments and detected by zeta potential.The results obtained that the zeta potential presented the zero net charge when pH value at pH 4 and 6.8 in Ag and TiO 2 , respectively.It demonstrated that the pI of Ag and TiO 2 were 4 and 6.8, respectively.
In acidic conditions (pH 3-6) or neural and alkaline conditions (pH 7-11), the absolute value of the TiO 2 zeta potential was larger than that of Ag.This is because TiO 2 is a hydrophilic colloid.The surface hydrophilicity of the metal oxide, TiO 2 , was greater than that of the metal, Ag, thus H + , H 3 O + , and OH-in solutions with different pH values were more easily charged by binding of the hydrogen bond and particle surface, so the potential of the TiO 2 surface charge was very high.Comparatively, the absolute value of the zeta potential of Ag was low, and the mechanism by which silver attracted ions to the surface was to attract H + ions by the 5s 1 in the [Kr]4d 10 5s 1 silver orbital.This differs from the direct absorption of charged ions onto the surface of metal oxides; so it was concluded that the zeta potential of Ag was low because the space charge that could be contained by the orbital electron was limited.In an analysis of the surface potential, when the pH value was 4-6.8, the TiO 2 surface was positively charged, Ag was negatively charged, and they bound to each other due to electrostatic attraction TiO 2 being inclined to the surface of silver particles might have been influenced by the thermal diffusion of the solution, evenly distributing among the silver particles and producing chemical bonds.This study used the attraction of positive and negative charges to spontaneously create TiO 2 and evenly coat the Ag carrier.

Effect of Operational pH in Synthesis Process on Surface Characterization of TiO 2 /Ag
Fig. 2 illustrated the surface morphology of TiO 2 /Ag prepared in various pH conditions, observed via SEM.
Operating the solution at pH 3, TiO 2 and Ag were repulsing each other because of the same surface electrical charges, hence nano-TiO 2 particles could not easily attach to the surface of Ag but aggregation with themselves, as shown in Fig. 2(a).In Fig. 2(b), TiO 2 /Ag prepared in the solution at pH 5.8, TiO 2 and Ag were existed opposite surface charges allowing TiO 2 /Ag combined each other; moreover, TiO 2 particles were well-distribution and less aggregation appeared on the surface on the surface of Ag.When TiO 2 /Ag prepared in the solution at pH 8, TiO 2 and Ag were existed both negative surface charge which make a strong repulsive force between TiO 2 and Ag, therefore produced the few TiO 2 particle deposited on Ag (illustrated in Fig. 2(c)).
The practical weight percentage of TiO 2 coated on Ag carrier examined by ICP-MS was listed in Table 1.The theoretical 10 wt%TiO 2 /Ag prepared in the solution at various pH showed the different result in practical weight percentage TiO 2 coated on Ag, which were contributed to surface electrical properties between TiO 2 and Ag.Due to the electric attraction between of TiO 2 and Ag operating at pH 5.8, the practical weight percentage of TiO 2 in theoretical 10 wt%TiO 2 /Ag was 9.3%; however, the repulsion of same charges existing on the surface of TiO 2 and Ag, the practical weight percentage of TiO 2 in theoretical 10 wt%TiO 2 /Ag operating at pH 3 and 8 were 6.5 and 5.1, respectively.

Effect of Operational pH in Synthesis Process on Combination Strengths between TiO 2 and Ag of TiO 2 /Ag
To understand the effect of operational pH in synthesis process on combination strengths between TiO 2 and Ag, the transmittance of the supernatant in synthesis solutions for preparing 10 wt%TiO 2 /Ag at various pH values was measured.After centrifuging, the UV-Vis spectrometer was used to analyze the transmittance of the supernatant, according to adsorbed wavelength of TiO 2 on 365 nm.After centrifugation at 4000 rpm, the transmittance of the supernatant in solutions at pH 3 and 8 for prepare 10 wt% TiO 2 /Ag solution were only 1-12%, and the liquid looked turbid.The results indicated that there was still considerable TiO 2 suspended in solutions at pH 3 and 8, due to the same electrical properties between TiO 2 and Ag lacking of combination affinity.The supernatant in a solution operated at pH 5.8 for synthesizing TiO 2 /Ag was pellucid, and the transmittance was 73-84%.This indicates that when Ag and TiO 2 were prepared in a pH 5.8 solution, and their surface electrical properties were opposite, the both were attracted and combined together due to electrostatic forces.TiO 2 indeed attached to the Ag carrier, and was not easily peeled off by external forces.The isoelectric point of Ag and TiO 2 were 4 and 6.8, respectively; thus the optimal pH value operating on 5.8, which causing the differences in the zeta potential and surface charges, enhances to synthesize TiO 2 /Ag characterized as stable structure.

Effect of Operational pH in Synthesis Process on Crystal Structure and Electronic State of TiO 2 /Ag
It was observed from the XRD spectra (Fig. 4) that regardless of the pH values, the signal intensity of Ag in 10 wt% TiO 2 /Ag was strong, especially for pure silver.The signal intensity of TiO 2 coated on 10 wt%TiO 2 /Ag was weaker due to the less coating quantity, and was still a composed of anatase and rutile crystallites.For samples prepared in solutions with various pH values, peaks of the main characteristics were similar to each other, without a significant difference; thus the initial structure of the TiO 2 /Ag didn't change at different pH values.
Fig. 5 shows the XPS spectrum of TiO 2 /Ag prepared at various pH values, the characterizing elemental composition and chemical states could be determined via the results of XPS.The 2p 3/2 peaks of Ti (IV) in TiO 2 /Ag prepared at various pH values were on 458.6 eV without conspicuous shift, thus chemical states of TiO 2 didn't alter during synthesis process.Both 3d 5/2 peaks of the Ag in TiO 2 /Ag prepared at pH 3 and 8 was around 368.1 eV, which indicated that Ag existed in the metallic form.However, the 3d 5/2 peak of the Ag in TiO 2 /Ag prepared at pH 5.8 shifts to 367.8 eV, indicating Ag existed as Ag 2 O.

Effect of Operational pH in Synthesis Process on TiO 2 /Ag for Photodegradation Efficiency of Acetone
Fig. 6 shows the photodegradation efficiency of acetone by 10 wt% TiO 2 /Ag prepared at various pH values.The inlet concentration of acetone was 200 ppmv, and the retention time was 41 seconds in the photodegradation experiment.Prior to photocatalysis process, the acetone was adsorped on TiO 2 /Ag for saturation in order to reducing impact of adsorption on photodegradation efficiency.In addition,   only UVA illumination and Ag illuminated via UVA didn't show any ability for degrading acetone in the pre-experiment.TiO 2 /Ag prepared in a solution at pH 5.8 showed the best photodegradation efficiency for acetone, and the efficiency was as high as 91%, followed by TiO 2 /Ag prepared in a solution at pH 8, the efficiency of which was 81% for the sample preparations.TiO 2 /Ag prepared in a solution at pH 3 appeared the worst efficiency.Effect of retention time on photodegradation efficiency of acetone via TiO 2 /Ag prepared at various pH values was illustrated in Fig. 7.The photodegradation efficiency of acetone would improve with the increasing retention time.The straight-line slope in the figure indicates a highly linear relation, demonstrating that photocatalysis in this experiment  followed a first-order reaction kinetic equation, and the reaction rate constant could be simulated by Langmuir-Hinshelwood kinetics (Matthews, 1987;Sang and Sung, 2002), given as r = -dC/dt = kC.Integrating the L-H kinetic equation, -ln(C/Co) = kt could be given, where r is rate of photodegradation, C is acetone concentration after the photocatalysis process, while Co is the inlet acetone concentration, k is rate constant, and t is retention time.
Reaction rate constants (k) and correlation coefficients (r square, R 2 ) of the photodegradation of acetone by TiO 2 /Ag prepared in solutions at various pH values were listed in Table 2.
As TiO 2 and Ag were mixed in solution at pH 5.8, the electrical properties of between TiO 2 and Ag were opposite due to the differences of zeta potentials, and then TiO 2 would be anchored stably and dispersed uniformly on Ag.Thus the best photodegradation efficiency of acetone could be obtained on TiO 2 /Ag prepared in solution at pH 5.8.Contrarily, the photodegradation efficiency of acetone via TiO 2 /Ag synthesized in solution at pH 3 or 8 was decreased, which was contributed to TiO 2 aggregated or dispersed partially on Ag surface.

Effect of TiO 2 Coated Amount on TiO 2 /Ag for Photodegradation Efficiency of Acetone
The above results showed that operating at pH 5.8 was optimal condition solution for synthesizing well-structured TiO 2 /Ag.The following experiments discussed the effect of TiO 2 coated amount on TiO 2 /Ag for photodegradation efficiency of acetone, according to the above optimal pH.The initial concentration of acetone was 200 ppm, and the retention time was 21 s in the experiment.As illustrated in Fig. 8, with an increase of the proportion of TiO 2 enhanced the photodegradation efficiency of acetone, and thus the main factor influencing the efficiency was the TiO 2 content.
In addition, the photodegrdation efficiency of acetone via P25 TiO 2 was added to compare, and the weight of P25 coated on the reactor was same with the TiO 2 consisting of 10 wt% TiO 2 /Ag.The result indicated the photodegrdation efficiency of acetone via 10wt% TiO 2 /Ag was superior to that of P25, which was contributed to Ag carrier acting the electron-capturer.As TiO 2 /Ag illuminated with UVA, the electron (e -) and hole were excited from surface of TiO 2 ; in the meanwhile, the Ag carrier formed as Ag 2 O also excited e --hole + pair under UVA illuminate (Zhang et al., 2003), Ag 2 O would reacted with e − reducing to metallic Ag.These e -reacted with oxygen molecular to form reactive species O 2 •-, and also react with surface Ti 4+ to produce reactive center surface Ti 3+ .Meanwhile, the metallic Ag would capture electron as Ag -, and then react with oxygen molecular to form O 2 •-; Ti 4+ on TiO 2 surface would react with Ag -form Ti 3+ .The photogenerated holes (h + ) from TiO 2 would also react with Ag − as metallic Ag.The more reactive species of O 2 •-and reactive center of Ti 3+ (Liu et al., 2004) producing in the TiO 2 /Ag improved the photocatalysis efficiency of acetone superior to photocatalysis ability of TiO 2 .The acetone would react with the increasing O 2 •-and degrade to CO 2 and H 2 O.The relevant formula reactions about above-mentioned mechanism were shown as below: Ti 4+ + e -→ Ti 3+ (6) Ag + e -→ Ag - ( 7) Ag -+ Ti 4+ → Ag+ Ti 3+ (9) Considering the average photodegradation efficiency of acetone determined by the coating amount of TiO 2 , the efficiency divided by the TiO 2 content was defined as E (eff/ratio), as shown in Fig. 9. 10 wt%TiO 2 /Ag could be obtained the optimal efficiency.Although there was considerable TiO 2 in 20 or 40wt% TiO 2 /Ag, the excessive amount of TiO 2 could not benefit the photocatalysis ability to degrade acetone due to TiO 2 aggregation.In additional, there was not the other organic byproduct, which could be detected via GC-FID in the exhaust after photocatalysis process.There was not any particle found in the end of reactor, thus the TiO 2 /Ag coated on the reactor showed the excellent stability.1.

CONCLUSIONS
Effects of various pH values in the synthesis solution on the structure stability of TiO 2 /Ag and their application to the photodegradation of acetone were discussed in this study.Altering the pH value of the solution would influence difference of the zeta potentials between the surface of TiO 2 and Ag particles, respectively.When the pH value of the synthesis solution was controlled on 5.8, the surface charges of TiO 2 and Ag particles were opposite and the difference of charges was the maximum; then TiO 2 and Ag were attracted and combined to each other due to electrostatic forces.The charges on the surface of each TiO 2 particle were the same and repulsive to each other, thus forming uniform TiO 2 dispersion on Ag.The characteristics analysis of 10wt% TiO 2 /Ag prepared in solution at pH 5.8 has indicated the stable combination between TiO 2 and Ag, the chemical states of TiO 2 didn't alter during synthesis process; however, Ag carrier existed as Ag 2 O from metallic Ag.
Photodegradation efficiency of acetone reached upto 91% via 10wt% TiO 2 /Ag prepared in solution at pH 5.8, obviously better than TiO 2 /Ag prepared in other conditions, and could be fitted with the Langmuir-Hinshelwood kinetic model.The excellent photocatalysis performance of 10wt% TiO 2 /Ag prepared in solution at pH 5.8 solution was attributed to the opposite surface charges of between TiO 2 and Ag, due to the differences of zeta potentials; hence, TiO 2 would be anchored stably and dispersed uniformly on Ag in 10wt% TiO 2 /Ag.
Besides acting as an electron capturer, Ag carrier characterized as Ag 2 O state also generated electron-hole pairs under UVA illumination, photodegradation efficiency of acetone of 10wt% TiO 2 /Ag could be enhanced superior to that of P25.Coating more TiO 2 to the surface of TiO 2 /Ag would improve the photodegradation efficiency; however, the excessive TiO 2 amount on Ag could not benefit the photocatalysis ability to degrade acetone due to TiO 2 aggregation.Furthermore, 10 wt% TiO 2 /Ag presented the optimal TiO 2 dosage for the photodegradation efficiency of acetone.

Fig. 1 .
Fig. 1.Zeta potentials of TiO 2 and Ag in solutions with different pH values.

Fig. 2 .
Fig. 2. Observation of the surface of TiO 2 /Ag prepared in solutions with different pH values.

Fig. 3 .
Fig. 3. Transmittance of the supernatant of TiO 2 /Ag prepared in solutions with different pH values after centrifugation.

Fig. 7 .
Fig. 7. Analysis and comparison of the photodegradation kinetics of acetone by TiO 2 /Ag prepared in solutions with different pH values.

Fig. 10 .
Fig. 10.Analysis of UV-Vis spectra on the surface of TiO 2 /Ag with different theoretical weight percents of TiO 2 .

Table 1 .
Practical weight percentage of TiO 2 coated on Ag examined by inductively coupled plasma mass spectrometry.