PANI

Article pubs.acs.org/JPCC

Microscale Hierarchical Three-Dimensional Flowerlike TiO2/PANI Composite: Synthesis, Characterization, and Its Remarkable Photocatalytic Activity on Organic Dyes under UV-Light and Sunlight Irradiation Na Guo, Yimai Liang, Shi Lan, Lu Liu, Junjun Zhang, Guijuan Ji,* and Shucai Gan* College of Chemistry, Jilin University, Changchun 130026, PR China ABSTRACT: A microscale hierarchical 3D flowerlike TiO2/PANI composite with enhanced photocatalytic activity was synthesized via a sol−gel method, and the as-synthesized samples were characterized by XRD, SEM, TEM, FT-IR, UV− vis adsorption, BJH, and TGA/DSC. The outcome of the photocatalytic experimental demonstrating the TiO2/PANI hybrid with a Ti/ANI (aniline) molar ratio of 1:1 (denoted as T/P) showed high photocatalytic activity upon the degradation of Congo red (CR) and methyl orange (MO) under both UV-light and sunlight irradiation. The unique hierarchical 3D flowerlike structure endows the T/P hybrid with a large surface area of 38.81 m2/g. The intermeshed PANI nanoflakes could make full use of the light resource by multiple reflections between the nanoflakes. Moreover, the intrinsic cavity of the hollow TiO2 nanoparticles can also increase the light-capturing efficiency. The synergistic effect between PANI and TiO2 hollow nanoparticles results in a reduction of the photoinduced electron−hole recombination rate as well as enhanced photocatalytic activity under UV-light and sunlight. Given the unique spatial structure and high photocatalytic characteristics of the T/P composite, there is great potential for applications in water treatment.

1. INTRODUCTION Global researchers have been engaged in developing and improving a more effective method to deal with increasingly severe organic dye pollution. Photocatalysis, which uses a semiconductor as the photocatalyst and can degrade a wide scope of organic contaminants effectively, has gained the upper hand as the subject of much research for environmental contaminant treatment1 due to its optimal cost, high activity, and no secondary pollution in the environment.2 TiO2 as a remarkable photocatalyst has been broadly applied in photocatalysis on account of its high photocatalytic activity, nonpoisonous nature, and ease of preparation.3 However, TiO2 also has its own shortcomings, including a high charge recombination rate of photoinduced electron−hole pairs, a critical drawback of the ineffective utilization of solar energy, a small surface area, and so forth. These fatal weaknesses impair its applications to a great extent. To conquer these problems, tremendous endeavors have been made such as combining titania with adsorbents,4−6 doping metals7 or nonmetals,8,9 and other semiconductors.10 More recently, conducting polymers with spatially extended π conjugation, such as polythiophene, polypyrrole, polyaniline (PANI), and their derivatives, have been consider as prospective photosensitizer in TiO2 photocatalysts.11−13 Modifying TiO2 with conducting conjugated polymers has been proven to expand its spectral scope to visible light effectively as well as enlarge the surface area.3,14 Simultaneously, conjugated polymers possess distinct electrical © 2014 American Chemical Society

and optical properties, such as high migration rates of electrons, environmental friendliness, and high absorption coefficients in the visible-light spectrum.1,15 A wise incorporation of TiO2 with conducting polymers with appropriate structures is significant due to their potential synergistic effect or extra added functionalities.16 Polyaniline (forbidden band gap is 2.8 eV),17 which possesses all of the merits of the above-mentioned conjugated polymer,2,15,18 is a good candidate for TiO2 sensitization. At the present time, several approaches to preparing TiO2/PANI composites with various morphologies and enhanced photocatalytic activities have been reported. Kim et al.19 prepared TiO2/PANI composites by in situ oxidative template polymerization on the surface of negatively charged TiO2 particles with the morphology of the core−shell structure. Zhang10 et al. fabricated well-designed hierarchical nanostructures with a 2D layered structure homogeneously grown on the electrospun (1D) TiO2 nanofibers. Liao et al.3 fabricated a core−shell PANI/M-TiO2 photocatalyst by a hydrothermal method and a chemisorption approach. The as-prepared catalyst exhibited high catalytic activity on methyl orange under UV-light and sunlight irradiation. However, the structures of the photocatalyst reported previously were onedimensional (1D), two-dimensional (2D), or three-dimensional Received: May 7, 2014 Revised: July 16, 2014 Published: July 24, 2014 18343

dx.doi.org/10.1021/jp5044927 | J. Phys. Chem. C 2014, 118, 18343−18355

The Journal of Physical Chemistry C

Article

above powders at 500 °C for 4 h. The products were termed HT-500. Our previous work has demonstrated that the TiO2 nanoparticles which were calcined at 500 °C show only a pure anatase phase and possess a better photocatalytic activity than those calcined at 400, 600, and 700 °C. 2.3. Synthesis of a Microscale Hierarchical ThreeDimensional Flowerlike TiO2/PANI Composite. The TiO2/PANI hybrid was synthesized by the chemical oxidation polymerization of aniline in the presence of colloidal TiO2 nanoparticles at 0 °C in an ice−water bath, and APS was used as the oxidant in this process.28 In a typical procedure, the asprepared HT-500 powders were suspended in 40 mL of ethanol under ultrasonication for 30 min to reduce the aggregation of the TiO2 nanoparticles, termed solution A. The desired amount (Ti/ANI = 1:1) of aniline was dissolved in 40 mL of a HCl (1 mol/L) solution and termed B. We then mixed the above two solutions A and B and stirred them for another hour. After that, 20 mL of a HCl (1 mol/L) solution containing APS ((NH4)2S2O8) with an equimolar ratio of aniline was then slowly added dropwise to a well-dispersed suspension with continuous stirring at 0 °C in an ice− water bath. After 4 h, an ideal degree of polymerization was gained, and the dark-green precipitate was obtained after centrifugation. Then the precipitate was washed repeatedly with ethanol and deionized water. Finally, fine dark-green powders were achieved by drying the as-obtained precipitate in vacuum at 60 °C for 12 h. The samples were termed T/P. Samples with different Ti/ANI molar ratios (2:1, 1:2,and 1:5) were prepared. They were termed 2T/P, T/2P, and T/5P, respectively. For comparison, the 1D structure TiO2/PANI photocatalyst was prepared under the guidance of the method developed by Tai et al.29 2.4. Characterization. The crystallite phases were identified by X-ray diffraction (XRD) analysis for powders via a D/Max-IIIC (Rigaku, Japan) with Cu Kα radiation. Morphological analysis was carried out with an S-4800 field emission scanning electronmicroscope (FE-SEM, Hitachi, Japan) with an acceleration voltage of 3 kV. The particle size and inner structure of the prepared products were observed with a Hitachi 8100 transmission electron microscope (TEM, Hitachi, Tokyo, Japan). The infrared spectra of the samples were obtained from KBr pressed pellets on a Nexus 670 infrared Fourier-transform spectrometer (Nicolet Thermo, Waltham, MA). An X-ray photoelectron spectra (XPS) measurement was carried out on a PHI-5000CESCA system with Mg K radiation (hν =1253.6 eV). Containment carbon (C 1s = 284.6 eV) was used to calibrate the binding energies. The X-ray anode was run at 250 W, and the high voltage was kept at 14.0 kV with a detection angle of 540°. UV−vis spectrophotometry (UV-2000, Shmadzu, Japan) was used to measure the optical absorption edge of the samplesover the wavelength range of 200 to 700 nm. The Brunauer−Emmett−Teller (BET) surface area of the samples was analyzed by nitrogen adsorption on a Micromeritics ASAP2020 nitrogen adsorption apparatus (USA). Before nitrogen adsorption measurements, the samples were degassed in vacuum at 120 °C for 5 h. The BET surface area was calculated with a multipoint BET method using the adsorption data over relative pressure (P/P0) ranging from 0.01 to 0.99. The Barrett−Joyner−Halenda (BJH) method was used to determine the pore-size distributions of the samples. The pore volume of the sample was calculated by the volume of nitrogen adsorbed at the relative pressure (P/P0) of 0.98.

(3D) with core@shell morphology. There have few reports on the 3D structure of TiO2/PANI with a special morphology composite.20,21 To the best of our knowledge, there has been no report on the hierarchical 3D flowerlike TiO2/PANI composite photocatalyst. In the present work, a unique hierarchical 3D flowerlike TiO2/PANI composite photocatalyst was synthesized via a simple sol−gel method. The as-prepared photocatalyst possesses a unique 3D flowerlike superstructure and excellent photocatalytic activity upon the degradation of MO and CR under both UV light and natural sunlight. Flowerlike microscale particles were composed of 2D thin PANI nanoflakes, where the hollow TiO2 nanoparticles were adhere to, having a thickness of 40−50 nm and widths and lengths in the range of 1−2 μm. The photocatalysis experimental results demonstrated the priority of modification with PANI, and the photocatalytic performance was indeed improved when hollow TiO2 was modified with PANI. Recycled stability tests indicated that the high photocatalytic stability of T/P composites and the presence of PANI in the photocatalyst contributed to photocorrosion inhibition. The remarkable photocatalytic activity and photocatalytic capability of T/P are mainly ascribed to the following three factors: (1) Light-harvesting efficiency. The intermeshed PANI nanoflakes on the exterior can surely enhance light harvesting by multiple reflections of UV light/ sunlight.3,5,22,23 (2) The interfacial reaction process. The hierarchical flowerlike superstructure possesses a large specific surface area, which remarkably increased the local concentration of dye in the vicinity of the catalyst photoactive layer compared to the bulk solution, resulting in rapid and efficient contact between dye molecules and transitory living •O2− or •OH on the photocatalyst surface. (3) Separation efficiency of photogenerated charges. PANI in itself is an excellent electron donor and a good hole acceptor under UV-light and visiblelight excitation.15 Moreover, the heterojunction established between PANI and TiO2 could suppress the recombination of the photoinduced electron−hole pairs, with the synergistic effect between PANI and TiO2 causing it to possess dramatic photoactivity.

2. EXPERIMENTAL SECTION 2.1. Materials. All reagents were of analytical grade and purchased from Beijing Chemical Reagent Research Company and used as received without further purification. Deionized water was used for all experimental processes. 2.2. Fabrication of TiO2 Hollow Nanoparticles. Hollow TiO2 nanopheres were synthesized using carbon spheres as a template. Carbon spheres were first synthesized via a hydrothermal reaction as previously reported.24,25 The TiO2 shell was fabricated under the guidance of the theory developed by Ming et al.,26 and the hollow TiO2 nanoparticles have been prepared successfully in our previous work.27 In a typical experiment, 0.1 g of carbon spheres was dispersed into 50 mL of absolute ethanol under the condition of ultrasonic vibrication, and then 2 mL of tetrabutyl titanate (TBOT) was added. After stirring for 1 h, 60 mL of deionized water was dropped into the suspension under continuous stirring. The mixture was stirred for another 12 h to complete the reaction. Brown powders were obtained after the centrifugation of the mixtures, washing the precipitates several times with deionized water and ethanol, and drying the powders at 80 °C for 12 h. The hollow TiO2 nanoparticles were obtained by calcining the 18344



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The two broad peaks focused at 2θ = 20.70° and 25.52° in curve a are ascribed to the periodicity parallel and perpendicular to the polymer chain, which indicated that the HCl-doped PANI XRD pattern was in its semicrystalline phase.14 The main peaks at 2θ values of 25.3, 37.9, 48.1, 54.0, 55.2, and 62.8° in curves c and d can be indexed to the (101), (004), (200), (105), (211), and (204) faces of anatase TiO2,2,32 respectively, which match well with JCPDS powder diffraction pattern 21− 1272 (shown in curve b). The broad peak presented in curve d can be attributed to the diffraction peak of PANI, confirming the coexistence of PANI with TiO2 hollow spheres. Compared to HT-500, the XRD patterns of T/P shown in curve d are hardly altered in peak positions, indicating that PANI as a modifier did not influence the lattice structure of TiO2, which would be very favorable to the photodegradation process of the synthesized photocatalyst. The morphologies (Figure 2) of both HT-500 and the TiO2/ PANI hybrid were examined by field emission scanning

Differential scanning calorimetry−thermogravimetric analysis (DSC−TGA 1600 LF, Mettler Toledo, Switzerland) was carried out at a O2 gas flow rate of 60 mL·min−1 and a heating rate of 10 °C·min−1. The IPCE was acquired in dc mode by using monochromatic incident light of 1016 photons/cm2 from a 100 mW cm−2 white bias light source (CEP-2000, BunkoKeiki). The PL spectra were obtained by using a Hitachi F7000 spectrophotometer fitted with a 150 W xenon lamp as the excitation source. 2.5. Photocatalytic Experiments. CR and MO were chosen as representative target contaminants for the photodegradation process to estimate the photocatalytic capability of the as-prepared TiO2/PANI hybrid photocatalyst under UVlight (λ < 400 nm) and natural sunlight irradiation. TiO2 (P25) was used as the reference under identical experimental conditions. In a typical procedure, 0.05 g of photocatalyst was dispersed into 20 mL of the above-mentioned organic dyes aqueous solution. A high-pressure Hg UV lamp (GGZ175, 175 W) with a maximum emission at 365 nm served as the light source. To obtain a good dispersion and establish an adsorption−desorption equilibrium between organic molecules and the photocatalyst, the suspensions were shaken for 20 min continuously in the dark prior to irradiation. During the photodegradation process, the samples were collected every 20 min, and the catalyst was removed by centrifugation. A UV−vis spectrometer (UV-2550, Shimadzu, Japan) was used to measure the organic dye concentration before and after degradation. The adsorption capability (qe) and degradation efficiency (D) were calculated using the following equations30,31 qe =

(C0 − Ce)V W

D=

C0 − Ce × 100% C0

where C0 is the initial concentration of target organic dye solution (mg/L), Ce is the equilibrium concentration after degradation/adsorption (mg/L), V is the volume of target organic dye solution (mL), and W is the mass of the prepared catalyst (g).

3. RESULTS AND DISCUSSION 3.1. Physicochemical Properties of T/P Composites. The X-ray diffraction patterns (XRD) of HT-500 and T/P are presented in Figure 1.

Figure 2. (a) SEM images of HT-500. (b) TEM images of HT-500. (c) Low-magnification SEM images of T/P. (d) SEM images of the inner structure of T/P. (e) High-magnification SEM image of T/P. (f) TEM image of T/P.

electron microscopy (FE-SEM) and transmission electronmicroscopy (TEM). Figure 2a depicts the SEM image of hollow TiO2 nanoparticles, and the shell of the hollow TiO2 spheres with a rough and porous surface with a diameter of 200 nm was seen. The typical TEM image of a monodisperse hollow TiO2 sphere (Figure 2b) shows that its shell thickness is about 60 nm, the cavity diameter is approximately 80 nm, and pores are seen clearly on it. A unique uniform hierarchical flowerlike superstructure emerged after modification by PANI (Figure 2c). The inner hollow TiO2 nanospheres located on the surface of the PANI nanoflakes are connected to each other (Figure 2d). The high-magnification SEM image of a single microflower is given in Figure 2e, clearly showing the

Figure 1. XRD patterns of (a) pure PNI, (b) JCPDS 21-1272, (c) HT500, and (d) T/P. 18345



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T/P composite are summarized in Table 1. According to Katoch,38 the peak shift indicates that there exists chemical

intermeshed PANI nanoflakes at the external microscale particles forming the microscale flower, with the diameter ranging from 2 to 3 μm. Further observation can be obtained from the TEM image of the hybrids in Figure 2f, where it can be seen that the microscale flowers were composed of PANI nanoflakes and the TiO2 hollow spheres were attached to the sheet randomly. The large surface area of PNAI nanoflakes furnished more spaces for TiO2 to be located, avoiding TiO2 nanoparticles from aggregating. Meanwhile, the large surface area increased the contact area between the semiconductor and the sensitization, which provide convenience for the charge− hole transfer. The interaction established between the TiO2 nanoparticles and PANI is shown by the FT-IR spectra. The FT-IR spectra of pure PANI, HT-500, and T/P are shown in Figure 3a−c,

Table 1. Peak Locations in the FT-IR Spectra of Pure PANI and the Hybrids

PANI 2T/P T/P T/2P T/5P

plan bending mode of the C−H bond (cm−1)

quinonoid unit of doped PAN

C−C stretching vibration of the benzenoid ring (cm−1)

C−N stretching mode of the quinonoid ring (cm−1)

1075 1078 1078 1080 1082

1139 1144 1142 1143 1145

805 806 808 807 809

1309 1312 1315 1316 1314

bonding between PANI and TiO2, which could be further evidence of the existence of an interaction between PANI nanoflakes and hollow TiO2 nanospheres.38−40 Further proof is given in the XPS study. As shown in Figure 4, the XPS survey spectrum (Figure 4a) demonstrated that there exist four elements in the hybrid

Figure 3. FT-IR spectra of (a) pure PANI, (b) HT-500, and (c) T/P.

respectively. Characteristic peaks of PANI can be observed in curve a. The peak at 3219 cm−1 results from the N−H stretching vibration, and the peak around 2955 cm −1 corresponds to the aromatic C−H vibration. The peaks at 1572 and 1491 cm−1 are assigned to N=Q=N and N−B−N stretching vibrations (Q and B represent the quinonoid structure and benzenoid structure), respectively. Additional, the band at 1139 cm−1 is ascribed to the quinonoid unit of doped PANI. All of the above peaks are distinct characteristics of the PANI backbone. Furthermore, the band at 1309 cm−1 is caused by the C−N stretching vibration of the benzenoid unit. The band at 1075 cm−1 is attributed to the plane bending vibration of C−H. The peak at 805 cm−1 is linked to C−C and C−H of the benzenoid unit.33,34 The strong absorption within 550−800 cm−1 in curve b is associated with the Ti−O−Ti stretching vibrations,35,36 which is the typical adsorption in anatase TiO2. The peaks at 3458 cm−1 in all of the spectra correspond to the −OH group stretching vibrations, which come from absorbed water molecules and the surface hydroxyls on the TiO2 particles.37 Characteristic absorption peaks of both PANI and TiO2 can be found from the T/P composite in curve c, which proved the existence of TiO2 in the PANI nanoflakes. Furthermore, a relatively strong IR absorption peak at 3458 cm−1 is associated with the hydrogen bonding of the O atom in TiO2 and the H atom in curve c, which can provide evidence that there exists an interaction between PANI nanoflakes and the hollow TiO2 nanosphere in the interface of nanoheterostructures.19 Beyond that, positions of the peaks linked to PANI are shifted toward higher wavenumbers in the FT-IR spectra. The peak positions associated with the pure PANI and

Figure 4. XPS study of the T/P (a) survey spectrum, (b) C 1s, (c) O 1s, and (d) N 1s.

photocatalyst, they are C, O, N, and Ti. The C 1s spectrum is shown in Figure 4b at binding energies of 284.7, 286.1 and 288.4 eV; these three binding energies are related to different forms of carbon. The peak at 284.7 eV is ascribed to C−C and CC, while the peaks at 286.1 and 288.4 eV are related to C− O−Ti and CO, respectively.1 The XPS spectrum of O 1s is shown in Figure 4c, and the peaks at 530.2 and 531.5 eV are assigned to Ti−O−Ti and H bonding of TiO2 and PANI.14 The N 1s XPS spectrum is shown in Figure 4d. It can be seen that there exist four forms: the peak at 400.3 eV linked to pyrrolic N, the peak at 398.8 eV related to pyridinic N, the peak at 401.8 eV resulting from the interaction between N+ and protons from the acid doping, and the peak at 399.6 eV caused by the interaction between the metal ions and nitrogen (M− N).41 According to the XPS study, in C−O−Ti, M−N bonds emerged in the T/P hybrid, which is further evidence of the 18346



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3 times as large as that of HT-500 (13.38 m2/g). Figure 6c describes that the PSD of the HT-500 sample is unimodal, and the peak pore diameters occur at 6.4 nm, while after incorporation into PANI nanoflakes the peak pore diameter shifts to 1.1 nm (Figure 6d). The surface area, pore size, and pore volume values of the prepared samples are given in Table 2. Obviously, the T/P photocalyst displayed a relative high

existence of an interaction between PANI nanoflakes and hollow TiO2 nanospheres. The UV−vis diffuse reflectance spectra of T/P, HT-500, and P25 are shown in Figure 5. It can be seen clearly that, due to

Table 2. Textural Properties of As-Synthesized Photocatalyst Samples samples

surface area, m2/g

pore size, nm

pore volume, cm3/g

HT-500 2T/P T/P T/2P T/5P

13.38 33.32 38.81 18.50 12.30

21.86 23.80 20.32 25.86 21.22

0.07305 0.1764 0.1932 0.1176 0.06691

specific surface area in comparison to others. It can be seen that the specific surface area of the prepared hybrid samples became smaller as the amount of PANI increased. Owing to the large BET surface area, the T/P hybrid possesses a strong adsorption capability and would adsorb more organic dye onto its surface than would HT-500 and P25. TGA/DSC results of pure PANI and T/P samples are shown in Figure 7 and its inset. Figure 7a,b shows TGA curves of

Figure 5. UV−vis diffuse reflectance spectra of (a) P25, (b) HT-500, (c) T/P, and (d) pure PANI.

transitions in the PANI molecules, pure PANI can absorb both UV and visible light.18 Additionally, because of the combination of PANI and TiO2, the T/P hybrid can absorb not only UV light but also visible light. The result shows that PANI is capable of sensitizing TiO2 efficiently and extending the spectrum of TiO2 to the visible-light range.39 The T/P hybrid can be excited by absorbing UV and visible light simultaneously to generate more electron−hole pairs, which is beneficial to improving the photocatalytic activity. The N2 adsorption−desorption isotherm and pore size distribution (PSD) of HT-500 and T/P samples are shown in Figure 6 (main panel and inset). The two N2 adsorption−

Figure 7. TGA curves of (a) pure PANI and (b) T/P with inset DSC curves of (c) pure PANI and (d) T/P.

PANI and T/P, respectively. Clearly, both curves show remarkably sharp weight losses between 100 and 600 °C. The relative weight loss is 39.8% in the hybrid due to the thermal degradation of the polymer chains,19 which is in contrast to the weight loss of nearly 100% observed for pure PANI in curve a. After 600 °C, the curve becomes a straight line. Figure 7c,d shows the DSC curves of pure PANI and hybrid T/P, and a strong, sharp exothermic peak appears at around 400 °C, which is in the temperature range of significant weight loss and is assigned to the combustion of polymer chains. Likewise, after 600 °C, no endothermic/exothermic peak appears. 3.2. Formation Process of the T/P Composite. The formation process of the T/P hybrid can be described as in Figure 8. First, a homogeneous ANI/HCl solution was prepared and stirred into the ice−water bath, and then the HT-500 colloid was added to the above system because of the

Figure 6. N2 adsorption−desorption isotherms of (a) HT-500, (b) T/ P, with the inset containing pore size distribution curves for (c) HT500 and (d) T/P.

desorption isotherms obtained for the samples (Figure 6a,b) are of type IV according to IUPAC,3 with curves a and b showing significant hysteresis at a relative pressure of P/P0 between 0.1 and 1.0 and 0.6−1.0, respectively, this demonstrates that both the TiO2 nanoparticles and the T/P composite photocatalyst belong to the mesoporous material, which indicates that there exists mesoporosity in the two samples. The BET surface area of T/P-500 is 38.81 m2/g, which is nearly 18347



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However, excess PANI content leads to a decrease in the MO/ CR degradation rate because the higher contents of PANI nanoflakes would cause overlapping agglomerates and block the hierarchical flowerlike structure according to Figure 9c−f, and excess PANI nanoflakes cover up the hierarchical structure and hinder the direct contact of TiO2 with the dyes, with all of these primary factors resulting in low photocatalytic activity. As stated above, T/P displayed a higher photodegradation efficiency than other samples; therefore, T/P was selected in the next experimental process. 3.4. Photodegradation of MO and CR. To decide the optimal initial concentration and the pH value of organic dye solution on the photocatalytic performance, a series of comparative photodegradations of MO and CR were performed in the presence of HT-500, T/P, and Degussa P25 under UV-light irradiation at room temperature. The studies were carried out at an MO and CR initial concentration ranging from 40 to 120 mg/L (in this set of experiment, the pH of the suspension was not adjusted). The pH value ranges from 5 to 10 (for this section, the concentration of the solution is 50 mg/L). The results are shown in Figures 10 and 11.

Figure 8. Schematic diagram for the formation mechanism of the hierarchical 3D flowerlike TiO2/PANI composite.

adsorption of HT-500. Precursor (ANI) was enriched around the surface of HT-500 and then polymerized into PANI nanoflakes as the APS was dropped in. The interaction created between TiO2 nanoparticles and PANI has been proven by the FT-IR spectra and XPS study. 3.3. Effect of Different Molar Ratios of Ti/ANI on the Photodegradation Capability. A prominent difference in degradation capability was observed in samples with different molar ratios of Ti/ANI. To analyze that, a series of comparative studies for the liquid-phase degradation of MO and CR were carried out in the presence of different catalysts. The studies in this section were carried out at a dye concentration of 50 mg/L. The degradation efficiency toward MO and CR in the presence of above obtained photocatalysts with different molar ratios of Ti/ANI is presented in Figure 9a,b. According to it, the

Figure 10. (a) Effect of initial concentration on the photodegradation of MO. (b) Effect of pH values on the photodegradation of MO. (c) Comparison of the maximum degradation rate of different catalysts. (d) Kinetic linear simulation curves of MO degradation over the different catalysts. (All of the above experiments were carried out under UV-light irradiation, T = 25 °C, catalyst dose = 0.05 g.)

Figure 9. Effect of different Ti/ANI molar ratios on the photocatalytic degradation of (a) MO and (b) CR and the SEM images of (c) 2T/P, (d) T/P, (e) T/2P, and (f) T/5P.

optimum photocatalytic performance was obtained when the molar ratio of Ti/ANI was 1:1, with the photodegradation efficiency toward MO and CR being 83 and 95%, respectively. Appropriate PANI content causes a uniform dispersion on the TiO2 surface, which is beneficial to the transfer and separation of the electrons and holes. Below or exceeding that proportion, the photodegradation efficiency is lower than that. When the molar ratio of Ti/ANI is 2:1, the percentage of ANI is too low to react with TiO2 nanoparticles completely. Therefore, adsorption and photocatalytic active sites become limited. On the other hand, as an electron donor, PANI in the 2T/P hybrid will inevitably provide fewer electrons than will other hybrids. As we all know, hVB+ and eCB− occupy important positions in degradation of organic molecules because they are responsible for the production of hydroxyl radicals and superoxide anions.42

Figures 10a and 11a demonstrate that the degradation efficiency of the two dyes reached a maximum value at 80 mg/L and decreased thereafter. The reason can be interpreted as follows: when the concentration of organic dyes was below 80 mg/L, the activated molecules, which would be involved in the photocatalytic reaction, are too few to achieve efficient collision with the photocatalyst, so the photocatalytic reaction would be delayed. However, with an excess concentration of dyes, dyes molecules have a relative surplus after the adsorption capacity of the catalyst reached equilibration. Meanwhile, the photons get intercepted by the excess organic dye molecules before they can reach the catalyst surface; consequently, the degradation efficiency decreased.43 18348



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convenient for organic molecules to contact TiO2 hollow nanoparticles. Moreover, large PANI nanoflakes can provide convenience for the organic dyes and end products to move in or out of the catalyst. This demonstrated that the as-prepared T/P composite can remove the contaminant effectively even in the dark or under weak light irradiation conditions. Figures 10d and 11d depict that the photodegradation of MO/CR over the different catalyst is roughly in agreement with the Langmuir−Hinshelwood pseudo-first-order reaction: ln(C0/Ct) = kt,45 where C0 is the adsorption equilibrium of organic dyes, Ct is the concentration of organic dyes at time t, and k is a kinetic constant. The corresponding k (kinetic constants) and R2 (regression coefficients) are calculated and provided in Table 3. Obviously, the kinetic constants for MO/ Table 3. k and R2 of Different Catalysts Photodegraded on MO/CR under UV-Light and Sunlight Irradiation degradation of MO

Figure 11. (a) Effect of initial concentration on the photodegradation of CR. (b) Effect of pH values on the photodegradation of CR. (c) Comparison of maximum degradation rate of different catalysts. (d) Kinetic linear simulation curves of CR degradation over the different catalysts. (All the above experiments were carried under UV-light irradiation, T = 25 °C, catalyst dose = 0.05 g.)

T/P (UV light) HT-500 (UV light) P25 (UV light) T/P (sunlight) HT-500 (sunlight) P25 (sun light)

The pH of the solution is a vital parameter in the photodegradation of organic dyes because it determines the surface charge of the photocatalyst, the electrostatic interaction between the photocatalyst surface and organic dye molecule, and the number of charged radicals generated during the photocatalytic oxidation process.44 The pH of the solutions was adjusted with HCl or NaOH solution by using a pH meter. Figures 10b and 11b show the degradation efficiency at different pH values. The optimum photocatalytic performance was acquired at pH 6, with the photodegradation capability being 92% (MO) and 96% (CR). The maximum degradation capability of T/P under UV-light and natural sunlight irradiation was measured under the optimum experimental conditions. For this section, the concentrations of the organic dyes are 80 (MO) and 100 mg/L (CR) at pH 6. In the meantime, blank reactions were performed in the presence of PANI and no catalyst under the same experiment conditions. The results are presented in Figures 10c and 11c. It can be seen that after 2 h of UV-light irradiation both of these dyes showed negligible photodegradation (less than 1%) without catalyst or in the presence of PANI. It is notable that the degradation rate of MO and CR over T/P was much quicker compared to the degradation over HT-500 and P25. Additionally, the maximum photocatalytic capability was achieved using T/P to remove about 95% of the MO and about 98% of the CR from the solution. Thus, the results exactly demonstrated the superiority of the modification of the PANI. In fact, it is noteworthy that the concentration of the organic dyes in the presence of T/P also decreased after shaking in the dark, and the decreasing in the concentration is mainly due to its powerful adsorption capability, in which the maximum adsorption capability of T/P toward MO and CR is 180 and 232 mg/g, accordingly. According to the SEM/TEM and BET surface area studies, the unique hierarchical 3D flowerlike structure really provides more adsorption sites and makes it

kinetic constant (k, h−1)

R

0.036

0.9996

0.0086

0.9981

0.0065

0.9936

0.031

0.9982

0.012

0.9986

0.011

0.9971

2

degradation of CR T/P (UV light) HT-500 (UV light) P25 (UV light) T/P (sunlight) HT-500 (sunlight) P25 (sunlight)

kinetic constant (k, h−1)

R2

0.044

0.9997

0.019

0.9960

0.011

0.9940

0.037

0.9997

0.015

0.9960

0.012

0.9945

CR degradation with T/P are 0.036 and 0.044 h−1, respectively. The kinetic constants of MO and CR photodegradation with the T/P hybrid were 5 and 4 times as large as that of P25, which confirmed the superiority of the modification of PANI and the unique hierarchical flowerlike structure in the further improvement of photocatalytic activity. Here, the efficient photocatalytic capability of T/P under UV-light irradiation resulted from the following factors. Primarily, the adsorption capability can play a positive role in photocatalytic performance.2 After modification by PANI, the molecule structure changed a lot, which resulted in a significant contribution to the photodegradation process. The unique hierarchical flowerlike structure indeed enlarges the specific surface area. The large specific surface area on the one hand offers more adsorption sites, remarkably increasing the local concentration of organic dyes in the vicinity of the TiO2 photoactive layer compared to the other part of the solution46 and enhancing the interfacial reaction process.3 In the second place, the unique hierarchical flowerlike structure could enhance the light-capturing efficiency by multiple reflections of light between the intermeshed PANI nanoflakes.3,5 Third, the modification of PANI decreases the rapid recombination rate of the photogenerated electron−hole and the radiative recombination rate of self-trapped excitons1 The photoluminescence (PL) spectrum is often used to study the separation efficiency of the photogenerated electron−hole pairs in semiconductor particles.1,47 Figure 12 illustrated the PL spectra of T/P, HT-500, and P25 with the excitation wavelength being 470 nm, which can be assigned to the band gap recombination of electron−hole pairs.18 It is observed that the PL intensity of T/P is much lower than that of P25 and 18349



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between ln(C0/Ct) of MO and CR and the photodegradation time in the presence of T/P, HT-500, and P25. Analogously, it is obviously demonstrated that the degradation capability of T/ P toward MO and CR over is 90 and 96% after 2 h of sunlight illumination, respectively. It is also found that the photodegradation MO and CR agrees with pseudo-first-order kinetics under natural sunlight (Figure 14b,d), and the corresponding kinetic constants (k) and regression coefficients (R2) were calculated and are shown in Table 3. The kinetic constants of both MO and CR photodegradation with T/P composite photocatalysts were 3-fold higher than that of P25. The results revealed that T/P hybrid can absorb visible light to generate electron−hole pairs and then decompose the organic dye. For this section, according to Figure 5, the as-made T/P hybrid can adsorb not only UV light but also visible light, which is significant for catalysts to generate more electron−hole pairs. The synergistic effect between TiO2 and PANI is another factor in the high photocatalytic activity of the T/P hybrid. The asprepared T/P composites take full advantage of the outstanding properties of pure PANI, for PANI is both an electron donor and hole acceptor in its undoped or partially doped state.15 The unique high electron mobility of the spatially extended πbonding conjugated system suppresses the recombination process of photogenerated electron−hole pairs.1 All of the above factors lead to notable photocatalytic activity. The degradation of MO and CR was carried out in the presence of the as-synthesized 1D photocatalyst to make a comparison of the photocatalytic activity between synthesized 3D structure T/P and the 1D structure. Figure 15a shows the morphology of the 1D structure photocatalyst; it shows a meshlike structure in which the particle dimensions fall into the nanometer range.29 Figure 15b presents the optimal initial concentration (upside) and pH value (downside) for the 1D catalyst. Figure 15c,d show comparisons of the photodegradation of MO/CR between the above-synthesized 1D and 3D photocatalysts. Clearly, the 3D photocatalyst displays a comparative advantage in the photodegradation of MO/CR both under UV-light irradiation and natural sunlight irradiation. The 3D structure photocatalyst T/P possesses a higher photodegradation efficiency than its 1D counterpart, which means that when irradiated under the same light-intensity conditions the 3D structure photocatalyst (T/P) shows a higher photoconversion efficiency. To put it into another way, T/P can utilize its light resources more effectively. The monochromatic incident photon-to-electron conversion efficiency (IPCE) was measured to evaluate the photoconversion efficiency of the T/P hybrid (Figure 16). It can be seen that T/ P displayed a higher IPCE from 400 to 750 nm wavelength than did the 1D sample. At about 500 nm, the measurement of the peak IPCE of the 3D sample is 76%, and that of the 1D sample is 50%, and a dramatic increase of 26% is obtained. The results indicate that the as-synthesized microscale hierarchical 3D flowerlike TiO2/PANI (T/P) greatly improved the utilization of light resources.48,49 According to the above experimental research, a probable mechanism for charge transfer and photocatalytic organic dyes over the T/P catalyst is put forward and illustrated in Figure 17a,b. The band gaps of TiO2 and PANI are 3.2 eV3 and 2.8 eV,8 respectively, which means that TiO2 can absorb only UV light and PANI can absorb both UV and visible light. Therefore, the as-synthesized T/P composite should display the double advantages of TiO2 and PANI.

Figure 12. PL spectra of as-prepared samples P25, HT-500, and T/P.

HT-500. The remarkable reduction of PL intensity implies a high charge separation rate. This shows that PANI as a modifier can surely suppress the recombination process of the photogenerate electron−hole pairs. Figure 13a,c shows the variation of UV−vis spectra concerning MO and CR degradation over the T/P hybrid.

Figure 13. Absorption change spectra of (a) MO and (c) CR aqueous solutions in the presence of T/P under different UV-light irradiation times. Color changes of (b) MO and (d) CR with the irradiation times increasing.

The characteristic peaks of MO and CR are 460 and 497 nm, which are used to determine their concentration. The lower absorbance intensity of catalyst means a higher catalysis efficiency,17 and a rapid decrease in absorbance intensity was observed in Figure 13a,c. And after 120 min under UV-light irradiation, the absorbance intensity nearly disappeared, which indicates that there was almost no MO/CR existing in the system. A more intuitive visualization of the corresponding color changes can be seen in Figure 13b,d. The orange/red color of the initial solution gradually fades as the irradiation time is increased. In order to evaluate the photocatalytic activity of T/P under natural sunlight, we carried out the degradation of MO and CR under sunlight irradiation. Figure 14a,c shows the relationship 18350



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Figure 14. Comparison of maximum degradation rate on (a) MO and (c) CR in the presence of different catalysts. Kinetics of (b) MO decomposition and (d) CR decomposition over the different catalysts. (All of the above experiments were carried out under natural sunlight irradiation, T = 25 °C, catalyst dose = 0.05 g.)

Figure 15. (a) SEM images of the 1D sample. (b) Optimum initial concentrations (upside) and pH values (downside) of organic dyes for the 1D sample. (c, d) Comparisons of the degradation efficiency of MO and CR in the presence of 1D and 3D photocatalysts.

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position of water (eq 2) and in the presence of O2 (eq 3). •O2− may from organic peroxides in the presence of organic scavengers (eq 4). At the same time, the synergistic effect, which is caused by the energy-level matches15,47 (CB of TiO2 matches well with the LUMO of PANI and the VB of TiO2 matches well with the HOMO of PANI), impels the excitedstate electrons on the LUMO of PANI to be injected into the CB of the TiO2, and at the same time, holes on the VB of TiO2 migrate to the HOMO of PANI. Holes in the photocatalyst allow the direct oxidation of MO/CR to reactive intermediates (eq 5)50 or indirectly by the •OH radicals to form final decomposition products (eq 6).15 The recombination process of the electron−hole pairs is suppressed, and charge separation and stabilization are reached. Consequently, the efficient electron−hole separation leads to a remarkable enhancement of photocatalytic MO/CR degradation in the T/P composite system. The synergistic effect between PANI and TiO2 hollow nanoparticles is found to lead to an improved photogenerated carrier separation.

Figure 16. IPCE spectra of (a) 3D and (b) 1D.

Figure 17. Mechanism of electron−hole migration under (a) UV light and (b) sunlight (e−, electron; h+, hole).

When irradiated with UV light, as shown in Figure 17a, both the TiO2 hollow nanoparticles and the PANI nanoflakes adsorbed photons and then generated photoelectron−hole pairs (eq 1). Subsequently, large numbers of •OH and superoxide radicals (•O2−) were generated by the decom-

T/P + hv → hVB+ + eCB−

(1)

hVB+ + H 2O → H+ + •OH

(2)

eCB− + O2 → •O2−

(3)

•O2− + MO/CR → MO/CR − OO•

(4)

hVB+ + MO/CR → oxidation of MO/CR

(5)

•OH + MO/CR → degradation of MO/CR

(6)

Figure 18. Cycling runs of (upside) MO and (downside) CR. 18352



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4. CONCLUSIONS A microscale hierarchical 3D flowerlike TiO2/PANI composite with enhanced photocatalytic activity was fabricated by a sol− gel method, and the T/P nanocomposites can be excited by UV light and sunlight simultaneously, which broadens the potential of their application as photocatalysts. The photocatalytic degradation experiments demonstrate that the as-prepared catalyst (T/P) displayed a high photocatalytic activity and adsorption capability toward azo dyes (MO and CR) under both UV light and natural sunlight under their own optimum experimental conditions. Additionally, the circulation photocatalytic experimental test demonstrated that T/P still shows a high photocatalytic activity after three measurements. The study revealed that the prepared composite exhibits a high potential for applications in the treatment of contaminated water.

When irradiated under natural sunlight, as shown in Figure 17b, it can be found that TiO2 can be excited under visible light irradiation, but only a few electrons and holes were generated because UV light makes up merely 5% of sunlight.12 Fortunately, PANI can absorb visible light to induce the π− π* transition, delivering the excited-state electrons of the HOMO orbital to the LUMO orbital.40 On the basis of the synergistic effect, the excited-state electrons from the PANI molecules can be injected into the CB of TiO2 and finally react with oxygen on the surface. And then a series of reactions mentioned in the above equations (1−6) occur. In this situation, as a photosensitizer, PANI makes wonderful contributions to the still highly photocatalytic activity under sunlight.51,52 It is beyond question that, hVB+ and eCB− occupy an important position in the degradation of organic molecules because they are responsible for the production of hydroxyl radicals and superoxide anions, and the most important is that •OH would oxidize the organic dyes. The repeatability of MO/CR degradation on the T/P photocatalyst was test repeatedly three times under the same conditions. After centrifugal separation the catalyst was used immediately for further runs without any treatment. As shown in Figure 18 (upside and downside), after three runs of the degradation reaction, the photodegradation capability of T/P toward MO/CR exceeds 90% under UV-light irradiation. The photodegradation efficiency shows a small decrease of about 1%. The decreased performance of photocatalyst in the degradation of organic dyes may be caused by the blockage of photocatalytic active sites by the strong adsorption of the dye intermediate2 and the inevitable slight photocorrosion of the catalyst. Moreover, this material can rapidly settle from the slurry system and is easier to separate (Figure 15) than the conventional photocatalytic materials. As stated above, the enhanced photocatalytic activity of the as-prepared T/P composite under UV-light and sunlight irradiation may be owing to the mutual influence of several factors. Primarily, in terms of the molecule structure, the unique hierarchical 3D flowerlike structure could make full use of the light resources by multiple reflections between the crossed/ intermeshed PANI nanoflakes. Additional, the intrinsic cavity of the hollow TiO2 nanoparticles can also increase the lightcapturing efficiency. Second, the outside crossed/intermeshed PANI nanoflakes have a large specific surface area, which provides more adsorption sites and photodegradation activity sites, remarkably increasing the local concentration of dye in the vicinity of the catalyst photoactive layer compared to that of the whole solution, thus promoting the interfacial reaction process. Besides, the outside PANI nanoflakes will stabilize TiO2 particles on the nanoscale through interfacial chemical bonds to protect them from photocorrosion. Third, as a photosensitizer as well as a modifier, conjugated PANI plays a vital role in the separation efficiency and stabilization of the charge carrier due to its delocalized electron in the π-bonding conjugated system. Last, high absorption coefficients in the visible-light spectrum endow the T/P hybrid system with enhanced photocatalytic activity under sunlight compared to that of pure TiO2 hollow nanoparticles and P25. In summary, all these factors contributed to the higher photoactivity of T/P powders.

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AUTHOR INFORMATION

Corresponding Authors

*(G.J.) Phone: 86 431 88502259. E-mail: [email protected]. *(S.G.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by Mineral and Ore Resources Comprehensive Utilization of Advanced Technology Popularization and Practical Research (MORCUATPPR) founded by the China Geological Survey (grant no. 12120113088300). It was also supported by Key Technology and Equipment of Efficient Utilization of Oil Shale Resources (no. OSR-05) and the National Science and Technology Major Projects (no. 2008ZX05018).

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