Synthesis and Visible Photocatalytic Activities of Poly(aminobenzoic acid)/TiO2 Nanocomposites

Three kinds of polymers, poly(p-aminobenzoic acid) (PPA), poly(m-aminobenzoic acid) (PMA), and poly(o-aminobenzoic acid) (POA), were prepared by oxidizing p-, m-, and o-aminobenzoic acid with (NH 4 ) 2 S 2 O 8 in acidic solution, respectively. Poly(aminobenzoic acid)/TiO 2 nanocomposites PPA/TiO 2 , PMA/TiO 2 , and POA/TiO 2 were prepared by adsorption of PPA, PMA, and POA polymers on surface of TiO 2 nanocrystals ST01 and P25, respectively. The polymers and the poly(aminobenzoic acid)/TiO 2 nanocomposites were studied by FT-IR andUV-visible spectra, TG-DTA analysis, SEMobservation, andmeasurements of isotherms and adsorption model of the polymers on TiO 2 nanocrystals. Furthermore, the visible photoelectrochemical and photocatalytic activities of these nanocomposites were investigated by the photocatalytic decomposition of water and methylene blue under visible irradiation. These nanocomposites have exhibited higher and different the abovementioned activities. This difference can be attributed to the influence of site of amino group in aminobenzoic acid.


Introduction
Among the various semiconductor photocatalysts, titanium dioxide is one of the most popular and promising materials, because it is stable in various solvents under photoirradiation, it is also available commercially, and it can induce various types of redox reactions [1]. The enhancement of photocatalytic activity is needed for applications of photocatalytic reactions. It is thus important to increase efficiencies absorbing and utilizing light energy. But TiO 2 photocatalyst can absorb only the UV light with a wavelength region of below 400 nm that is only about 4% energy in the solar spectrum. Recently, there have been several attempts to enhance visible photocatalytic activity of TiO 2 , such as modification of TiO 2 band gap by inducing oxygen-vacancy [2], replacing oxygen site with nitrogen [3], substituting Ti site with Cr, Fe, or Yb [4,5], and modification of TiO 2 by adsorbing organic dye molecules as sensitizer on the surface [6,7]. To develop suitable sensitizer, some studies have been carried out on the synthesis of organic dyes [8][9][10][11][12]. Polyaniline (PANI), as a well-known conducting polymer, has attracted a considerable interest in recent years because of its good stability, electrical, electrochemical, and optical properties [13][14][15]. The polyaniline and its derivatives can be expected to be utilized as a new type of photocatalytic sensitizer, because they are p-type semiconducting material. Very recently, we reported synthesis of poly(o-aminobenzoic acid) (POA), adsorption of the polymer on TiO 2 nanocrystals, and the behavior as photocatalytic sensitizer in the photocatalytic reaction [16]. In this study, three chemical reagents p-aminobenzoic acid (PAB), m-aminobenzoic acid (MAB), and o-aminobenzoic acid (OAB) were selected as the starting monomer of the polymerization for comparing the activities of these polymers as photocatalytic sensitizer, because they have a carboxyl group located on the para-, meta-, or ortho-position of amino-group in the benzene ring, it will be polymerized into a poly (aminobenzoic acid) with straight-chain structure. In these polymers, the -NH 2 groups can be utilized in the polymerization reaction, and carboxyl groups can be used to combine to Ti(IV) on TiO 2 surface by a multi-bridging chelating coordination [16]. The synthesized poly(aminobenzoic acids) and the poly(aminobenzoic acid)/TiO 2 nanocomposites obtained by adsorption of the polymer on TiO 2 nanocrystals were studied by FT-IR and UV-visible spectra, TG-DTA analysis, SEM observation, and measurements of isotherms and adsorption model of the polymers on TiO 2 nanocrystals. Furthermore, the visible photoelectrochemical and photocatalytic activities of these nanocomposites were investigated by the photocatalytic decomposition of water and methylene blue under visible irradiation. These nanocomposites have exhibited higher and different the above-mentioned activities. This difference can be attributed to the influence of site of amino group in aminobenzoic acid.

Synthesis of Poly(aminobenzoic acid).
Poly(amin-obenzoic acid) was synthesized by oxidizing aminobenzoic acid with an oxidizing agent of peroxodisulfate ammonium (NH 4 ) 2 S 2 O 8 in an acidic solution. A mixed solution containing 0.1 mol/L p-aminobenzoic acid, 0.15 mol/L (NH 4 ) 2 S 2 O 8 , and 0.2 mol/L HNO 3 was reacted at 40 ∘ C for 24 h with stirring. After the reaction, the precipitated solid was separated from the solution by centrifugation, then washed with distilled water, and finally dried at 40 ∘ C. The polymer poly(p-aminobenzoic acid) (PPA) is obtained. The synthesis methods of polymers poly(m-aminobenzoic acid) (PMA) and POA were similar to that of PPA. In the synthesis, the molar ratio of aminobenzoic acid/peroxodisulfate ammonium/nitric acid is selected as 1/3/1 for PMA and 1/1/3 for POA, respectively, for obtaining products with larger absorbance on visible light and pledging higher reacting speed.

Adsorption of Polymer on TiO 2
Nanocrystals. Ishihara ST01 and Degussa P25 were used as TiO 2 nanocrystal samples in adsorption experiments. The adsorption experiment of the polymer on TiO 2 nanocrystals was carried by a batch method. TiO 2 nanocrystal powder sample (5 mg) was added into an ethanol solution (5 mL) of the polymer in a concentration range of 0.06 to 0.5 g⋅L −1 , and then stirred at room temperature for 72 h. After the adsorption, the liquid phase was separated from the solid phase by centrifugation, and then the concentration of the polymer in the solution was analyzed using SHIMADZU UV-2450 spectrophotometer. The solid phase was washed with ethanol for 3 times and dried at 40 ∘ C. The TiO 2 nanocrystal sample was calcined at 450 ∘ C for 30 min before using in the adsorption experiment [17]. A TiO 2 nanocrystalline film on conducting glass (FTO coated glass) was prepared by coating the conducting glass surface with ST01 or P25 TiO 2 nanocrystal paste and then calcined at 450 ∘ C for 60 min. A polymer/TiO 2 nanocomposite film was obtained by soaking the TiO 2 film in 0.3 g⋅L −1 polymer ethanol solution for 24 h.

Physical Analysis.
FT-IR spectra of the samples were measured on a PERKIN ELMER SPECTRUM ONE spectrophotometer at a resolution of better than 2 cm −1 using the KBr technique. UV-visible spectra were recorded on a SHIMADZU UV-2450 spectrophotometer. Scanning electron microscopy (SEM) observation was performed on JEOL JSM-5500S. Nitrogen gas adsorption was carried out on a QUANTACHROME AUTOSORB-1-MP apparatus. The specific surface area was calculated from the adsorption data using the Brunauer-Emmett-Teller (BET) method. Simultaneous thermogravimetry and differential thermal analysis (TG-DTA) data were obtained on a SHIMADZU DTG-60H thermal analyzer at a heating rate of 10 ∘ C/min in air. In the photoelectrochemical and electrochemical measurements, a Hokuto-Denko BAS100B electrochemical analyzer was used.

Measurement of Photocatalytic
Activity. The visible photocatalytic activity of polymer/TiO 2 nanocomposite was characterized by methylene blue (MB) degradation method and photoelectrochemical method. In the MB degradation method, the nanocomposite powder sample (30 mg) was added in the MB aqueous solution (150 mL, 5 ppm), and then irradiated with a 300 W, 110 V TAI incandescent lamp made by Toshiba Corp. in Japan located at 1 m from the MB solution. The spectral range of the lamp is mainly in visible range and containing about 5% UV-light. The concentration of MB in the solution was measured using SHIMADZU UV-2450 spectrophotometer. A blank experiment was carried simultaneously using 5 ppm MB solution without TiO 2 to deduct MB degradation by direct photoreaction. The amount of MB adsorbed on sample surface was evaluated by desorbing MB from the sample with a 0.1 mol/L HCl solution after photocatalytic degradation experiment. The decrease amount of MB by the adsorption was removed from the degradation amount by the photocatalytic reaction. A sample of polymer-free TiO 2 was used also as the reference in the visible photocatalytic active study.
In the photoelectrochemical measurement, the polymer/ TiO 2 nanocomposite film was used as working electrode, and irradiated in a quartz cell containing sodium sulfate supporting electrolyte solution using a Xe lamp (Asahi Spectra USA LAX-Cute, VIS 400-700 nm) with a light intensity of 3000 W/m 2 . The masked-off irradiated area was 1.13 cm 2 . A Pt plate and an Hg/Hg 2 Cl 2 /KCl sat electrode were used as counter and reference electrodes respectively. An external bias was applied, and photocurrent was measured using the electrochemical analyzer.

Synthesis and Characterization of Poly(aminobenzoic acid)
. The polymers were synthesized by mixed the solutions Journal of Nanomaterials 3 of aminobenzoic acid and peroxodisulfate ammonium. The aminobenzoic acid has an aromatic structure similar to aniline that can be polymerized to polyaniline by an oxidation reaction [18,19]. The long conjugated system of polyaniline leads to absorption of visible light in a wide wavelength region and electric conduction. It is expected that aminobenzoic acids can be polymerized also to a poly(aminobenzoic acid). The mechanism of the polymerizing reaction is similar to that of aniline [14]. In the reaction, (NH 4 ) 2 S 2 O 8 is used as an oxidizing agent and the strongly acidic environment requiring for the reaction is provided by nitric acid. When (NH 4 ) 2 S 2 O 8 solution is added into the aminobenzoic acid solution, free radicals as initiating agent produced by breaking peroxodisulfate can attract the activated hydrogen of amino group of aminobenzoic acid, resulted new free radical of aminobenzoic acid molecular. The new free radical can attract aminobenzoic acid molecular, and then acted with the initiating agent free radical for completing a substitution reaction on the aromatic ring by free radicals. This substitution product can be attracted and lengthened further by free radical of aminobenzoic acid molecular. The product with enough great molecular weight can be precipitated from reacting solution. Therefore, polymerizations of lengthened chain can be ended, and formed PPA, PMA and POA respectively. The carboxyl groups on the poly(aminobenzoic acid) can be utilized for the adsorption of this polymer on TiO 2 surface similar to the case of ruthenium bipyridryl derivative dyes [20].
The optical absorption properties of synthesized polymers are dependent also on their synthesis conditions, such as the concentration of (NH 4 ) 2 S 2 O 8 oxidizing agent and HNO 3 . The influences produced by amount of oxidizing agent and the acid are different in the synthesis of PPA, PMA, and POA because of the difference of site of amino group in aminobenzoic acid. The increase of amount of oxidizing agent is not advantage to enhance the visible light absorption of polymers PPA and POA, but is advantage for polymer PMA. The increase of HNO 3 concentration is advantage to enhance the visible light absorption of polymer POA, but lead to decrease greatly the visible light absorption of polymers PPA and PMA and color of the sample change to light.
A precipitate was formed by aminobenzoic acid reacted with (NH 4 ) 2 S 2 O 8 in acidic solution. Colors of precipitates are black for POA, brown for PPA, and brown-black for PMA, respectively. Their SEM images were shown in Figure 1. It is observed that their morphology show hollow ball-like for POA (Figure 1(a)), grain-like for PPA (Figure 1(b)) and branch-like for PMA (Figure 1(c)), respectively. The solubility of the polymer samples in water solvent is low, while it is high in ethanol solvent. The UV-visible absorption spectra of synthesized polymers and their monomers in ethanol solvent are shown in Figure 2. The absorption spectrum of N719 dye in ethanol solvent is also shown in Figure 2 for the comparison. It is observed that all monomer have not absorption in visible light range, while the polymer PPA, PMA, and POA reveal a broad absorption band between 400 and 700 nm. The correlating data are summarized in Table 1. Their absorption peaks red shift to 300 and 391 nm for PPA, 331 and 346 nm for PMA, and 350 and 550 nm for POA compared with their monomers. The absorbance coefficients of the polymers in visible light region are much larger than that of N719 dye used in dye-sensitized solar cell. The average value of absorbance coefficient in 400-600 nm increases in the order of N719 < PMA < PPA < POA. The results indicate that the conjugation extent and activity absorbing on visible light of the molecular chain increase in the order of PMA < PPA < POA because their monomers have a same molecular weight. The polyaniline also shows two absorption peaks around 360 and 600 nm in water solvent, which can be attributed to the - * transition of the benzenoid rings and the exciton transition of the quinoid rings, respectively [21]. This fact indicated that the carboxylic acid groups substituted on the benzene rings cause a blue shift due to their electronwithdrawing nature [18]. Figure 3 showed the TG and DTA curves of the polymers. As shown in Figure 3, the decomposition of main part approaching 95% occurred at 200 to 650 ∘ C and formed broad exothermic peak corresponding to the burnout of the polymer. The broad peaks indicate they are a mixture with different polymeric degree. The results illustrate they are thermal stable polymers.
The FT-IR spectra of synthesized polymers and their monomers samples are shown in Figure 4. The correlating data are summarized in Table 1. It is found that the main differences between the IR spectra of the monomer and the polymer are vibration bands of the amino groups.   The bands at 823 and 871 cm −1 in FTIR spectra of POA are assigned to bend vibration of C−H of aromatic ring occurring 1,2,4-disubstituted. In the chemical structure of POA, whose scheme has been shown in literature 16, the ortho-position (occupied by a nitrogen atom) to one carboxylate group at a given monomeric unit would be located in meta-position relative to the carboxylate group of the neighboring unit. One nitrogen atom is located the para-position to other nitrogen atom on an aromatic ring. In fact, ortho-, meta-, and paraaminobenzoic acids should give polymers with quite the same chemical structure. But there are some differences in their chemical structure. The bands at 837, 867, and 683 cm −1 in FTIR spectra of PMA are assigned to bend vibration of C−H of aromatic ring occurring 1,3,5-disubstituted. The result is close to the theoretical analysis since there are the steric effect and the substituent effect. Therefore, in the chemical structure of PMA, one nitrogen atom is located the metaposition to other nitrogen atom on an aromatic ring. The bands at 845 and 871 cm −1 in FTIR spectra of PPA are assigned to bend vibration of C−H of aromatic ring occurring 1,2,4-disubstituted. The result agreed with the theoretical analysis by the steric effect and the substituent effect in this synthesis. Therefore, in the structure of polymer PPA, one nitrogen atom is located the ortho-position to other nitrogen atom on an aromatic ring. the adsorption isotherms of synthesized polymers on TiO 2 nanocrystal samples ST01 and P25 at room temperature. The experimental data fit the Langmuir isotherm for all these samples. Thus the adsorptions of PPA, PMA, and POA on the TiO 2 nanocrystals can be explained using the Langmuir monolayer adsorption model [23]. The Langmuir equation can be represented in the linear formula (1):

Formation of Polymer/TiO
where is PPA, or PMA or POA uptake (mg⋅mg −1 ), is the equilibrium concentration of the polymer in the solution (mg⋅mL −1 ), is the saturation (maximum) adsorption capacity (mg⋅mg −1 ), and ad is the adsorption constant (mL⋅mg −1 ). From the fitting of experimental data by plotting / against , the saturation capacity ( ) and the adsorption constant ( ad ) for PPA, PMA, and POA adsorptions were evaluated, respectively, and listed in Table 2. The adsorption parameters of dye N719 on TiO 2 nanocrystals sample are also shown in Table 2 for the comparison.  It is observed that the majority adsorption constant ad value for polymers is larger than that for N719, except PMA adsorption on P25. This result suggests the adsorptions of polymers on TiO 2 nanocrystals are stronger than that of N719. The saturation capacity value (mg/mg (TiO 2 )) for polymers is approach or larger than that for N719. The molecular weight of chemical composition corresponding per -COOH group for the polymers is much smaller than that of N719. If the value is in M(-COOH)/mg (TiO 2 ) unit, where M(-COOH) is amount (mole) of -COOH group adsorbed on TiO 2 , the value (mmole/g (TiO 2 )) of polymers are much larger than that of N719.
It has reported that the carboxylic acid or carboxylate groups of organic molecules can coordinate to the TiO 2 surface by various bonding modes, such as (a) unidentate mode, (b) chelating mode (4-membered bidentate), and (c) bidentate bridging mode [24][25][26]. Since the asymmetric and symmetric IR absorptive bands of carboxylate groups are different each other in these binding modes, these modes can be identified by the IR spectrum data. It has been reported that if the wavenumber difference between the asymmetric and symmetric bands in the adsorbed state is lower than that in the free-state, then the anchoring mode is bidentate chelating or bidentate bridging, while if the wavenumber difference is greater or equal to that in the free-state, then anchoring mode is unidentate [27]. Figure 6 showed the FT-IR spectra of the nanocomposite samples obtained by adsorbing PPA, PMA, and POA on  and ](C-OH) bands of carboxylic acid groups (-COOH) decreased largely after being adsorbed on TiO 2 for PPA, PMA, and POA, and the decreased intensity is follow as PPA > PMA > POA. The result reveals -COOH groups were deprotonated to -COO − groups after the adsorptions, and the order of deprotonated degree in accord with that of the decreased intensity. As shown in Figures 4 and 6, the wavenumber differences between the carboxylate group asymmetric and symmetric bands of POA, PPA and PMA in the free state are 221, 194 and 225 cm −1 , and that in adsorbed state are 232, 201 and 227 cm −1 , respectively, for the polymer/ST01 nanocomposite sample. The wavenumber differences are larger than that in the free-state, and the results are identical for the polymer/P25 nanocomposite sample. The results suggest that the polymers anchor as unidentate coordination to the TiO 2 . It has been reported that the carboxylate groups in N719 anchor to the TiO 2 surface with bidentate chelating or bridging binding mode [16,20]. Although the bidentate chelating and bridging binding modes are more stable than the unidentate mode, meaning that the adsorption of N719 must be more stable than that of the polymers. But the adsorption isotherm results revealed that adsorption constant ( ad ) of the polymers is larger than that of N719 (Table 2). This fact can be explained by formation of a multibridging chelating mode in the polymer/TiO 2 nanocomposite [16]. Each polymer molecule has many carboxylate groups, and each carboxylate group can coordinate to TiO 2 surface with the unidentate mode, which forms the multi-bridging chelating mode, and enhances the stability of the adsorption.

Activities of Polymer/
2 Nanocomposites in Visible Photocatalytic Reaction. The nanocomposites prepared by adsorbing polymer on surface of P25 or ST01 nanoparticles were employed in visible photocatalytic degradation of methylene blue (MB) to measure their visible photocatalytic activities. Figure 7 showed the changes of the degradation ratio of MB with photocatalytic reaction time for POA/P25, PMA/P25 and PPA/P25 nanocomposite samples and P25 under the incandescent lamp irradiating conditions. The results showed the degradation ratio of MB at 330 min as 44.1%, 32.2%, 29.3%, and 21.1% for PMA/P25, POA/P25, PPA/P25, and no sensitized P25, respectively. The PMA/P25 nanocomposite showed the highest degradation rate in all testing samples. And that for all polymer/P25 nanocomposite samples is higher than the polymer-free P25 sample under visible light irradiating conditions. Furthermore, the photocatalytic degradation process was studied on the dynamics. The related data are summarized in Table 3. It can be found that these reactions are first order reaction and the reaction rate is proportional to the concentration of MB. The rate constant increased according to the sequence of P25 < POA/P25 < PMA/P25 < PPA/P25, indicating the reaction activation energy of the order of decreasing, therefore, the catalytic activity of the catalysts is enhanced in this order. These results indicate the visible light sensitizing effect of PPA, PMA, and POA on TiO 2 nanocrystal surface for the photocatalytic reaction. Figure 8 showed UV-visible spectra of MB solution degraded for the polymer/ST01 and N719/ST01 nanocomposites samples under indoors-light irradiating conditions for 3d. It is found that the visible photocatalytic activity of POA/ST01 sample is higher than that of others. The result showed the degradation ratio of MB at 3d as 77.5%, 66.3%, 47.2%, 22.3%, and 16.1% for POA/ST01, PMA/ST01, PPA/ST01, N719/ST01,  and no sensitized ST01, respectively. This result suggests that samples from POA and PMA have strong adsorbing character and higher visible photocatalytic activity than that of others. And the syntactic polymer/ST01 samples used as a photocatalyst have more advantages than N719/ST01 sample in visible photocatalytic activity, synthesis condition and producing cost.
The visible photocatalytic activity of the polymer/TiO 2 nanocomposite is further demonstrated by the results of photoelectrochemical study. Figure 9(a) is a typical currentvoltage curve obtained by irradiating intermittently the PPA/ST01 nanocomposite film electrode with a 100 W Xe lamp with 400-700 nm wavelengths. When the potential was scanned from −0.3 to 1.2 V (versus SCE), the photocurrent decreased slightly with increasing the applied potential, and so for the dark current, which is due to electrochemical oxidation reaction of PPA on the electrode [22]. The hydrogen gas evolution was observed at the Pt counter electrode accompanying the photocurrent, suggesting the reduction of H + to H 2 . Figure 9(b) is a typical current-voltage curve obtained at the same conditions for PMA/ST01 nanocomposite. It is observed that the almost constant photocurrent is much lower than that of PPA/ST01 nanocomposite, and the dark current closed to zero. The polymer-free ST01 film was also employed for the comparison, but without photocurrent was observed in the current-voltage curve under the same conditions (Figure 9(c)). The results indicate PPA/ST01 and PMA/ST01 nanocomposites are the material with visible photoelectrochemical activity, and the activity of PPA/ST01 nanocomposites is more than three times PMA/ST01 in the photocatalytic reaction. Figure 10 showed current-time curves of PPA/ST01 film at zero applied pressure (Figure 10(a)) and POA/ST01 film when applied pressure is 200 mV (Figure 10(b)) in aqueous solution containing 0.1 mol/L of sodium sulfate in the dark and under irradiation with light (400-700 nm). It is observed that the photocurrent for PPA/ST01 film is still much higher than that at applied pressure as 200 mV for POA/ST01 film although the applied pressure closes zero. The above results suggest that visible photoelectrochemical activity of PPA/ST01 nanocomposites is the highest among the three.

Conclusions
Three kinds of poly(aminobenzoic acid) were synthesized by oxidizing p-, m-, or o-aminobenzoic acid with an oxidizing agent of peroxodisulfate ammonium in an acidic solution. The influences produced by the amount of oxidizing agent and the acid are different in the synthesis of PPA, PMA, and POA because of the difference of site of amino group in aminobenzoic acid. The polymer/TiO 2 nanocomposite can be obtained by adsorbing the polymer on TiO 2 surface and shows high stability, due to that the carboxylate groups anchor to the TiO 2 surface with the multi-bridging chelating mode. PMA/P25 and PPA/ST01 nanocomposites exhibit much higher visible photocatalytic activity than others in the photocatalytic reaction, and PPA/ST01 nanocomposite reveals higher visible photoelectrochemical activity than others. Therefore, they have a potential application as visible light photocatalyst.