ISSN 00360244, Russian Journal of Physical Chemistry A, 2014, Vol. 88, No. 13, pp. 2476–2485. © Pleiades Publishing, Ltd., 2014.

PHOTOCHEMISTRY AND MAGNETOCHEMISTRY

Heterostructural BiOI/TiO2 Composite with Highly Enhanced Visible Light Photocatalytic Performance1 Dongfang Zhang College of Science, Huazhong Agricultural University, Wuhan 430070, P.R. China email: [email protected] Received March 26, 2014

Abstract—Binary BiOI/TiO2 hybrid material was synthesized via a solgel method combined with chemical etching. The asprepared powders were characterized by Xray diffraction (XRD), transmission electron microscopy (TEM), UV–Vis diffuse reflectance spectroscopy (DRS), photoluminescence spectra (PLS) and photocurrent response tests. Under visible light (λ > 420 nm), BiOI/TiO2 degraded methyl orange (MO) effi ciently and displayed much higher photocatalytic activity than that of pure BiOI. Moreover, BiOI/TiO2 can effectively promote photooxidation of other organic dyes like rhodamine B (RhB), crystal violet (CV) and –

methylene blue (MB). In addition, the quenching effects of different scavengers proved that reactive O 2 and h+ played the major role in the MO degradation. The photocatalytic activity enhancement of BiOI/TiO2 is closely related to the strong absorption in the visible region, and the efficient charge separation derived from the matching band potentials between BiOI and TiO2, as well as the low recombination rate of the electron hole pairs due to the heterojunction formed between BiOI and TiO2. Keywords: BiOI, electron scavengers, reaction mechanism. DOI: 10.1134/S0036024414130044 1

1. INTRODUCTION The elimination of toxic chemicals from wastewa ter is presently one of the most important subjects in pollutant control [1]. These pollutants may originate both from industrial or household areas. The search for effective means of removing these compounds is of interest to regulating authorities everywhere. It has been demonstrated in the last two decades that semi conducting materials mediated photocatalytic oxida tion of organic compounds can be an alternative to conventional methods for the removal of organic pol lutants from water and air [2]. A variety of semicon ducting oxides acting as photocatalysts have been used. Most attention was given to TiO2 because of its high photocatalytic activity, resistance to photocorro sion, low toxicity and low cost. Despite these achieve ments and remarkable advantages, heterogeneous photocatalysis based on TiO2 has to cope with signifi cant limitations [3]. In general, photocatalytic reac tions rates are moderate, and consequently this tech nology is not appropriate for high throughput pro cesses, as for example in the decontamination of heavily polluted industrial effluents [4]. However, the most important drawback of photocatalysis is derived from the mismatch between the TiO2 band gap energy and the sunlight spectra, which overlap only in the 1 The article is published in the original.

UVA (400–320 nm) and UVB (320–290 nm) ranges. As a consequence, this technology can use only less than 6% of the solar energy, and its potential as a sus tainable technology cannot be entirely fulfilled [5, 6]. This fact has profoundly influenced research in photo catalysis, so that modification of TiO2 to achieve effi cient photoactivation in the visible spectrum is an active field of research. During the last few years an increasingly great number of new photocatalysts have been synthesized and tested as possible alternatives to TiO2, while TiO2 as an extensively investigated photocatalyst suffers severe constraints in practical applications due to lim ited visible light absorbance and poor quantum yield caused by the fast recombination of photogenerated electronhole pairs. Therefore, it is necessary to cou ple TiO2 with narrow band gap semiconductors capa ble of harvesting visible light to form heterostructures or nanocomposites. This approach has been reported to be an effective strategy, which can improve photo catalytic efficiency because of the efficient separation of photogenerated electronhole pairs. In this respect, bismuth oxyiodide (BiOI) is an attractive material due to its good optical and electrical properties and semi conductor characteristics. Possessing a narrow band gap (1.7–1.9 eV) and a layered structure of alternate [Bi2O2]2+ sheets and the I slabs, BiOI demonstrates

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excellent visiblelightdriven catalytic activity, which can be used as a possible sensitizer in view of its strong visible light absorptivity. These properties make BiOI an ideal option as a new visible light photocatalyst for degrading organic pollutants [7]. Recently, promising results have been achieved using BiOI in the photocat alytic degradation of organic compounds. Nonethe less, the rapid recombination of photoinduced elec trons and holes greatly lowers its quantum efficiency [8–10]. Therefore, it is of great interest to improve the generation and separation of photoinduced electron hole pairs in BiOI for further applications. The present work is based on the idea that hetero structures of BiOI coupling with TiO2, where BiOI was employed as a novel sensitizer to produce an excellent visiblelight induced photocatalytic properties. Methyl orange (MO) was used as model pollutant to evaluate the photocatalytic activity of the BiOI/TiO2 hybrid under visible light (λ > 420 nm). Additionally, various scavengers were introduced to the photocata lytic reaction system to explore the effects of different reactive oxygen species in the MO degradation pro cess. The underlying mechanism of photocatalytic efficiency improvement of the BiOI and TiO2 coupling was proposed on the basis of the above results. 2. EXPERIMENTAL 2.1. Catalyst Preparation All the reagents were analytical grade and were used as such without any further purification. Deionized water prepared on an ultra pure water system type Smart2pure made by TKA company of (Germany) was used throughout. BiOI/TiO2 composite material was synthesized by a sol–gel combined with chemical etching method, in which chemicals Bi(NO3)3·5H2O, tetrabutyl titanate (Ti(OBu)4, TBOT), NaOH, C2H5OH, and HNO3 (65%), all of analytical grade, were used as the starting materials. Firstly, 20 mL TBOT was added into 34 mL of C2H5OH, and 0.012 mol Bi(NO3)3·5H2O was dissolved into 1.0 mL HNO3 aqueous solution to get homogeneous solution. Then, solution of 5 mL deionized water and 5 mL C2H5OH was slowly dropped into the above solution with continuous stirring. The mixture was stirred for 30 min until sol was formed. After aging for 24 h, the sols transformed into wet gels. Next, the amorphous precipitate was obtained by centrifugation, followed by washing with distilled water and anhydrous ethanol for several times to remove all remaining chemicals. After that, the wetgel precursor was heated in an oven at 60°C for 8–10 h to evaporate excess solvent. The dried precipitate or xerogel precursor was calcined at 450°C for 3 h to get the yellow powders. BiOI/TiO2 heterostructure was then obtained through low tem perature chemical etching method using hydroiodic acid (HI) as an etching agent. The powders were dis persed into 30 mL of deionized water and 5 mL of HI RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A

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(0.0012 mol) solutions. For comparison, pure BiOI and TiO2 powders were also prepared by using the same procedure without adding TBOT or Bi(NO3)3·5H2O. 2.2. Catalyst Characterization Xray powder diffraction (XRD) data were recorded at room temperature on Xray diffractometer (XRD6000, Shimadzu Corporation) using CuKα radiation (λ = 0.15418 nm), operated at 40 kV and 100 mA. The size and morphology of the samples were characterized by transmission electron microscopy using HITACHI H_7650 instrument at 80 kV equipped with Gatan 832 CCD camera. The percent age of UV–Vis reflectance of the catalysts was mea sured by diffuse reflectance spectroscopy (DRS) for the powder form of the catalysts using a scan UV– Vis–NIR spectrophotometer (Varian Cary 500) in the region of 200–800 nm. The spectrophotometer equipped with an integrating sphere assembly and polytetrafluoroethylene was used as a reflectance material in the UV–Vis absorbance experiment. The photoluminescence (PL) emission spectra of the sam ples were measured with a RF5301 PC spectrofluoro photometer (Shimadzu Corporation) by using the 320 nm line of a Xe lamp as excitation source at room temperature. For the fabrication of the photoanode in photoelectrochemical measurements, asprepared samples were obtained by mixing 2 mL of ethanol and 30 mg of asprepared powder homogeneously. The as prepared samples were spread on an indium tin oxide (ITO) conducting glass and allowed to dry under ambient conditions. The photocurrents were mea sured with an electrochemical analyzer (CHI660B, CHI Shanghai, Inc.) in a standard threeelectrode system with the asprepared sample as the working electrode, a Pt foil as the counter electrode, and Ag/AgCl (saturated KCl) as the reference electrode. A 500W Xe arc lamp equipped with a 420 nm ultravi olet cutoff filter was utilized as the light source. A 0.6 M Na2SO4 aqueous solution was used as the elec trolyte. 2.3. Evaluation of Photocatalytic Activity Methyl orange (MO) was chosen as a model organic compound to evaluate the photoactivity of the prepared samples and the effects of photocatalytic conditions on their catalytic performance. The artifi cial light photocatalytic activity test is conducted in a quartz tube photoreactor with a cylindrical configura tion. Photoirradiation employed a 500 W xenon arc lamp equipped with an UV cutoff filter (emission wavelength λ > 420 nm) as visible light source. The distance between the surface of reaction solution and light source is adjusted to about 15 cm and the temper ature of the photoreactor was maintained at 25 ± 1°C by water circulation. A certain amount of photocata Vol. 88

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102

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212 30

40

302

310

220

60

+

+

110

204

+

211

105

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+ + 204

114

+

+ 004

101

+

1 20

+

+

200

+

2 +

200

112 004

+

115

+ +

3 101

Intensity. a.u.

+

70

80 2θ, deg

Fig. 1. The XRD patterns of (1) TiO2, (2) BiOI, and (3) BiOI/TiO2 powders.

lyst powder (30 mg) is added to 120 mL aqueous methyl orange (30 mg/L) solution. Before turning on the lamp, the dispersion was stirred in dark for 120 min within which MO adsorption equilibrium had been achieved. The photocatalytic decomposition of MO aqueous solution was characterized by a UVvisible spectrometer based on the Beer–Lambert law. During the irradiation experiments, aliquots (6 mL) are with drawn from the suspension at regular time intervals and are immediately centrifuged at 8000 rpm for 20 min to remove solids. The concentration of MO after illumination is monitored at λ = 464 nm using UV–Vis spectrophotometer (Spectrumlab 2450, Shi madzu). The degradation rate of the MO can be deter mined by the formula: η, % = (B0 – Bt)/B0, where B0, Bt represents the initial absorbance and the absorbance after at time t of the aqueous MO, respectively. Besides, other organic dyes like rhodamine B (RhB), crystal violet (CV) and methylene blue (MB) were also employed as model pollutants to evaluate the photo oxidation ability of asprepared BiOI/TiO2 catalyst. 3. RESULTS AND DISCUSSION Xray diffraction (XRD) was used to identify prod uct phases and evaluate the corresponding crystallite size. Figure 1 displays the XRD patterns of the aspre pared powders. The XRD pattern of the sample pre pared by the direct hydrolysis of TBOT shows diffrac tion peaks around 2θ of 25.3°, 37.9°, 47.8°, 54.1°, 55.2°, and 63.1°, which could be indexed to the char acteristic peaks (101), (004), (200), (105), (211), and

(204) of anatase TiO2 (JCPDS card file no. 21272). The asprepared BiOI is well crystallized in a single phase, all of the diffraction peaks can be assigned to the tetragonal phase BiOI (space group P4/nmm (129), PDF card no. 100445). No other impurities can be detected. The intense and narrow diffraction peaks reveal the good crystallinity of the pure BiOI sample. The standard intensity of the (110) peak is much weaker than that of the (102) peak, which could be expressed as I(110)/I(102) = 0.55. In our case, the value of the I(110)/I(102) is close to that of the refer ence indicating that BiOI does not anisotropically grew along the (110) plane. By comparison, for TiO2 and TiO2 in the asprepared BiOI/TiO2 heterostruc tures synthesized via solgel route the diffraction peaks are not sharp and have relatively weak intensities. Meanwhile, the characteristic peaks of BiOI were decreased to some extent with the incorporation of TiO2. No diffraction peaks of other bismuth com pounds were observed, indicating complete conver sion to BiOI. Generally, Scherrer equation is used to calculate the average crystallite size of TiO2 or BiOI based on the full width at half maximum (FWHM) of the main (101) or (102) diffraction peaks. In the present case, the FWHM of anatase peaks for BiOI/TiO2 nanocomposites is slightly broadened with the incorporation of BiOI, suggesting a size decrease for anatase crystallite. The results indicate that pure TiO2 has an average crystallite size of 10.8 nm, while the mean crystallite size of TiO2 is about 6.2 nm in the BiOI/TiO2 composite, hinting the presence of BiOI

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BiOI

200 nm

TiO2

Fig. 2. A representative pattern of transmission electron microscopy (TEM) image of composite photocatalyst BiOI/TiO2 heterostructure.

could suppress the crystal growth of TiO2 to certain degree, probably due to the insulation effect of BiOI in BiOI/TiO2 heterostructures. The morphology and microstructure of asprepared BiOI/TiO2 coupling particles was investigated and a typical TEM image was given in Fig. 2. As illustrated in Fig. 2, the particles display inorganized slicelike morphology with size of about 100 nm and thickness of about 8 nm. Moreover, it can be observed that the TiO2 nanoparticles (arrow shown in Fig. 2) with size of about 30 nm exhibited rel atively smooth surface, after coupled with BiOI, small hetero nanoparticles with size of about 8 nm over the surface of TiO2 can be clearly observed. The TEM results confirm the presence of BiOI deposits on the surface of TiO2 particles although these particles dis play chippinglike morphology with slight agglomera tion. The sizes of these agglomerates are not uniform and their diameters are in the range of several nanom eters to more than 100 nm. The drastic aggregation of particles might be caused by incomplete removal of solvents present in the interstices of crystalline BiOI/TiO2 coupling particles, which would be dis persed well in ethanol (ultrasonication) and it reveals that the particles have rough surface structure with woolly cloudlike morphology, which conveys high adsorption abilities. Moreover, Fig. 2 shows that the BiOI/TiO2 sample consisted of large numbers of irreg ular plates with rough surfaces. These plates were 100– 200 μm in width and 10–20 nm in thickness. The TEM image in Fig. 2 suggest that the morphology of BiOI/TiO2 heterostructure depends highly on the content of BiOI in the heterostructures, which were composed of some irregularly shaped aggregates excepts for plates, indicating that TiO2 aggregates were well wrapped on the surface of the BiOI. The pure TiO2 sample had a flowershaped superstructure con structed of plenty of aggregated small particles (not shown here). The TEM observation reveals that the coexistence of BiOI and TiO2 significantly affects the RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A

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morphologies and crystal growth habits of both com ponents. The UV–Vis diffuse reflectance spectra of the as prepared samples were recorded to obtain insight into their light absorption characteristics and the interac tions of the photocatalytic materials with photon energies directly and typical results were shown in Fig. 3. It shows that pure TiO2 primarily absorb the ultraviolet light with a wavelength below 400 nm and the absorbance of TiO2 in the visible light region is negligible. This absorption property was ascribed to the intrinsic band gap absorption of anatase TiO2. In contrast, pure BiOI and BiOI/TiO2 exhibit significant optical response in the visible region. For the assyn thesized BiOI sample, its absorption edge extends nearly to the whole spectra of visible light, ranging from 240 to 720 nm. The absorption edge of BiOI/TiO2 shift blue compared with that of pure BiOI sample, implying the sizes of BiOI in the BiOI/TiO2 sample is smaller than the corresponding value of pure BiOI. Approximately, the absorption edge of the as prepared TiO2, BiOI, and BiOI/TiO2 are located at 396, 713, and 605 nm in the nearUV and visible light regions, respectively, and these results are consistent with the white, red and carmine red colors of TiO2, BiOI, and BiOI/TiO2 powders, respectively. The reduction in the band gap energy of the asprepared sample was determined by the following equation: Eb = 1240/λ, where Eb is the band gap energy (eV) and λ is the wavelength (nm) of the absorption edge in the spectrum. Accordingly, the band gap of BiOI and het erostructured BiOI/TiO2 are evaluated to be 1.74 and 2.05 eV, while the band gap of pure TiO2 is estimated to be 3.13 eV. The broadening in band width of hetero structured BiOI/TiO2 compared with that of pure BiOI is thought to be formation of intermediate level between BiOI and TiO2. In a photocatalytic reaction, the activity is largely affected by the recombination of the photoinduced electrons and holes, which will decrease the quantum yield [11]. It is known that the recombination of elec tron–hole pairs can release energy in the form of fluo rescence emission. Lower fluorescence emission intensity implies lower electronhole recombination rate and corresponds to higher photocatalytic activity [12]. Using an ultraviolet light with a 320 nm wave length as excitation source, the fluorescence emission spectra of BiOI, TiO2, and BiOI/TiO2 hybrids are pre sented in Fig. 4. As can be seen, TiO2 exhibited two obvious emission peaks at about 400 (3.1 eV) and 450 nm. The former can be attributed to bandband PL phenomenon, and the later is ascribed to excitonic PL which mainly results from surface oxygen vacan cies and defects of TiO2 nanoparticles. Compared with that of TiO2, the two emission peak intensities of BiOI decreased considerably, indicating that the recombi nation of photogenerated charge carrier is inhibited Vol. 88

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DONGFANG ZHANG

1

Intensity. a.u.

3

2

300

400

500

600

700 Wavelength, nm

Relative intensity. a.u.

Fig. 3. UV–Vis diffuse reflectance spectra of (1) TiO2, (2) BiOI, and (3) BiOI/TiO2 powders.

1 2

3 390

420

450

480

510 540 Wavelength, nm

Fig. 4. Fluorescence emission spectra of (1) TiO2, (2) BiOI, and (3) BiOI/TiO2 powders.

greatly in the BiOI sample. Moreover, it can be found that both BiOI and TiO2 show higher intensity of emis sion spectra than that of the asprepared BiOI/TiO2 hybrid. In case of pure BiOI and TiO2, the photoin duced electrons and holes might recombine rapidly because of the narrow band gaps of BiOI or thermody namic factors taking place in single TiO2 semiconduc

tor. However, in the case of BiOI/TiO2 hybrid, the photoinduced carriers can migrate easily between BiOI and TiO2 due to their matching band potentials and therefore the electronhole recombination rate is greatly decreased. The lowest emission intensity of BiOI/TiO2 hybrid indicates it has the lowest electron hole recombination rate, suggesting that the electrons

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and holes have longer lifetime and may form more amount of reactive species. This result shows that BiOI/TiO2 heterojunction is helpful to inhibit the recombination of photoinduced carriers and improve the corresponding photocatalytic activity. Due to chemical stability and lightfast character, azobenzene dye MO was chosen as a representative model pollutant to investigate the photocatalytic per formance of the asprepared pure BiOI and BiOI/TiO2 powders under visible light (λ > 420 nm) illumination. Figures 5a and 5b illustrate temporal evolution of the spectral changes during photodegra dation of MO over asprepared BiOI and BiOI/TiO2 samples under visible light illumination. The band at 464 nm, which is attributed to the azo bond (–N=N–), was used to monitor the photocatalytic degradation process. It can be seen that the absorbance at 464nm decreased gradually with irradiation time. However, photocatalytic performances of different photocata lysts on degradation of MO were also distinct. The photodegradation of MO is much slower than the counterpart and only 50% of MO is degraded after 120 min under visiblelight irradiation. By compari son, about 91% of MO is decolorized after 120 min over asprepared BiOI/TiO2 hybrid photocatalyst, exhibiting efficient photocatalytic activity under visi blelight irradiation (λ > 420 nm). Although there might be photolysis for MO, the preliminary test con firms only 1.6% of MO is degraded after 120 min under visiblelight irradiation. These results of above comparison signify that MO degradation in the present study is through a photocatalytic process. The photocatalytic property of BiOI/TiO2 as tested sug gests that the degradation efficiency of MO is signifi cantly greater than that of asprepared BiOI samples under the same conditions, and the maximum absor bance at 464 nm almost completely disappeared after 120 min of irradiation, which indicates destruction of the conjugated structure occurred. In addition, RhB, CV and MB dyes were also employed as model pollut ants, as displayed in Fig. 6, asprepared BiOI/TiO2 was also exhibited excellent photooxidation ability towards these organic dyes. Normally, semiconductor photocatalysis involves the generation of electrons in the conduction band and holes in the valence band within a semiconductor upon light irradiation at energies equal to or greater than the band gap of the semiconductor. Subsequently, the utilization of photoexcited charge carriers to ini tiate redox reactions with suitable substrates on the semiconductor surface. To further understand the het erojunction effect on the photocatalytic activity enhancement of BiOI/TiO2 heterostructures, we care fully studied the photoinduced charge transfer proper ties of the BiOI/TiO2 heterostructures. The photoelec trochemical cell (PEC) responses of BiOI and the as prepared BiOI/TiO2 heterostructures were recorded under visiblelight irradiation (λ > 420 nm). Figure 7 RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A

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depicts the currentvoltage curves for BiOI and BiOI/TiO2 under several on/off visiblelight irradia tion cycles. It can be seen that the photocurrent rap idly decreases to zero as long as the light was turned off, and the photocurrent remained a constant value when the light was on. The reducibility of these results is very good. For each sample, an anodic photocurrent peak, which decayed rapidly followed by a steady cur rent, appears at the initial time of irradiation. When the light is switched off, a cathodic peak can also be observed. The initial current is due to the separation of electron–hole pairs at the semiconductor/electrolyte interface: holes are trapped or captured by reduced species in the electrolyte, while the electrons are trans ported to the back contact substrate. The decay of the photocurrent indicates that a fraction of the holes reaching the semiconductor surface, instead of cap turing electrons from the electrolyte, either recom bines with electrons from the conduction band and/or accumulates at the surface. After recombination of the excessive holes with electrons, the generation and transfer of electron–hole pairs reach an equilibration and form a constant current. When the light is inter rupted, the holes accumulated in the surface state still continue to recombine, and a cathodic peak is observed. It is clear that each sample was prompt in generating photocurrent with a reproducible response to on/off cycles, demonstrating the effective charge transfer and successful electron collection for the sam ples within the PEC. In comparison with pure BiOI, BiOI/TiO2 exhibited an increased photocurrent den sity, demonstrating that the photoinduced electrons and holes in BiOI prefer to separate and further trans fer to the TiO2 because of the p–n junction between them. As a result, the recombination of the photoge nerated electron–hole pairs is greatly reduced by the internal electrostatic field in the junction region. It is generally accepted that the dyes and organic pollutants can be photodegraded via photocatalytic oxidation (PCO) process. A large number of main reactive oxygen species (ROSs) including h+, ·OH, – and · O 2 involved in PCO process. Therefore, the effects of some scavengers on the degradation of MO were examined in attempt to elucidate the reaction mechanism. In this study, tertbutanol (tBuOH) was added to the reaction system as a ·OH scavenger, and KI was introduced as a scavenger of h+, benzoquinone – (BQ) was adopted to quench · O 2 . The results imply that in the presence of tBuOH or KI, the photodegra dation of MO was inhibited negligible compared with no scavenger at the same conditions, indicating the minor roles of ·OH and h+ for MO degradation. In – order to examine the role of · O 2 BQ (2.0 mM) was – used as a · O 2 quencher, and the addition of BQ could greatly decrease the photocatalytic efficiency of MO at airequilibrated condition, which shows that the MO Vol. 88

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DONGFANG ZHANG 1.8 (a)

Pure BiOI

1.6 1.4

0 min 20 min 40 min 60 min 80 min 100 min 120 min

1.2 1.0 0.8 0.6 0.4

Absorbance, a.u.

0.2 0 300

400

500

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700 800 Wavelength, nm

1.8 (b)

BiOI/TiO2

1.6 1.4

0 min 20 min 40 min 60 min 80 min 100 min 120 min

1.2 1.0 0.8 0.6 0.4 0.2 0 300

400

500

600

700 800 Wavelength, nm

Fig. 5. The temporal UV–Vis absorption spectral changes of MO dye degradation during the photocatalytic process over (a) pure BiOI and (b) BiOI/TiO2 samples. –

photodegradation is caused by · O 2 to a large degree. These observations confirm the proposed assumption that dissolved oxygen is a precursor of main oxidant, that is, the role of dissolved oxygen is to trap the pho – togenerated electrons to produce · O 2 radicals, which participate in photocatalytic process to accelerate the MO photodegradation. The addition of BQ showed stronger effects in PCO process of MO, suggesting that · O –2 played comparatively important role for MO deg

radation, whereas dissolved oxygen can act as a photo – generated electron scavenger to give · O 2 and ·OH active species, the latter has been proven not to be the major oxidation species in this process. In brief, MO molecules were decolorized on the surface of photo catalyst by photocatalytic process. According to the effects of scavengers, the decomposition of MO mole cules was attributed to the action of h+ via direct hole oxidation process and the oxidation action of the gen – erated · O 2 radicals. Dissolved oxygen could trap the

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The valence band and conduction band potentials of BiOI/TiO2 heterostructure were estimated in this study according to the following empirical equation:

(a)

0.8

E VB = X − E e + 0.5 × E 0 , 0 min 15 min 30 min 45 min 60 min 75 min 90 min

(1)

(2) ECB = E VB − E 0 , where EVB and ECB are the valence band and conduc tion band edge potentials, respectively, X is the elec 0.4 tronegativity of the semiconductor, which is the geo metric mean of the electronegativity of the constituent atoms, and the values of X for BiOI and TiO2 are 0.2 almost 5.99 and 5.85 eV, Ee is the energy of free elec trons on the hydrogen scale (about 4.5 eV), E0 is the 0 band gap energy of the semiconductor. Following the 400 450 500 550 600 above empirical equation, the EVB of BiOI was calcu (b) lated to be 2.36 eV. ECB (conduction band edge poten 1.2 tial) can be determined by ECB = EVB – E0. The band gap of the asprepared BiOI sample is about 1.74 eV, 1.0 thus, the ECB of BiOI was estimated to be 0.62 eV. In addition, the EVB of TiO2 was calculated to be 2.92 eV. 0.8 The band gap of the asprepared TiO2 sample is about 0 min 20 min 3.13 eV, thus, the ECB of TiO2 was estimated to be 0.6 40 min ⎯0.21 eV. It is known that photocatalytic processes are 60 min based on electronhole pairs generated by means of 80 min 0.4 band gap excitation. The photoinduced electron and hole could migrate to the surface to react with the 0.2 adsorbed reactants in the desired process, or undergo an undesired recombination. Therefore, the genera tion and separation of the photoinduced electronhole 0 400 450 500 550 600 650 700 750 800 pairs are the key factors to influence a photocatalytic reaction [13]. According to the above analysis, it is evi (c) 1.0 dent that the interaction between ptype BiOI and ntype TiO2 is responsible for the efficient generation and separation process under visible light excitation. 0.8 BiOI has so narrow band gap energy (1.74 eV) that it could be irritated easily by visible light (λ > 420 nm, 0 min energy less than 2.95 eV) and consequently induce the 15 min 0.6 30 min generation of photoelectrons and holes. In fact, pure 45 min BiOI with narrow band gap energy results in rapidly 60 min recombined photoelectrons and holes, this is why 0.4 75 min BiOI usually show poor photocatalytic activity. Mean 90 min while, TiO2 possesses both strong electron affinity and 0.2 high electron conductivity but with a wide energy gap of 3.13 eV, thus it could not be excited by the visible light irradiation in our experiments. Accordingly, MO 0 550 600 650 700 750 800 could not be effectively degraded on TiO2 as well. 500 Wavelength, nm However, it is deduced that the unusually high cata lytic efficiency of the BiOI/TiO2 composite originates Fig. 6. The change of spectra with time during the RhB, from the unique bandmatching between these two CV, and MB dyes photocatalytic degradation process over semiconductors. Under visible light irradiation, the asprepared BiOI/TiO2 samples. electrons in the VB of BiOI are excited to its CB, inducing the formation of holes in its VB. As a result, photogenerated electrons to reduce the recombination the holes generated in the VB of BiOI can be used for various oxidation reactions. In the case of BiOI/TiO – of h+ and e– in concert with the formation of · O 2 rad heterostructures, we can observe that the photoin2 icals oxidant. Hydroxyl radical was verified to be insig duced electrons in the reformed conduction band (CB) potential edge on the surface of BiOI could eas nificant for the decomposition of MO.

Absorbance, a.u.

0.6

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DONGFANG ZHANG Photocurrent (× 10 ⎯5 A) 4.5 4.0

Light off Light on 2

3.5 3.0 2.5 2.0

1

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0

100

200

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700 800 Time, s

Fig. 7. Photocurrent responses of pure (1) BiOI and (2) BiOI/TiO2 heterostructure in 0.6 M Na2SO4 aqueous solutions under visiblelight irradiation (λ > 420 nm) at 0.5 V vs. Ag/AgCl.

ily flow to a lower CB potential edge of TiO2. In this case, BiOI acts as a sensitizer to absorb visible light. When BiOI is illuminated by photon energy less than 2.95 eV (λ > 420 nm), electrons in the valence band of BiOI could be excited up to a higher potential edge (⎯0.59 eV). The reformed CB edge potential of BiOI is more active than that of TiO2 (–0.21 eV). Hence, photoinduced electrons on the BiOI surface would easily transfer to TiO2, leaving the holes on the BiOI valence band. Then, what is the fate of the accumu lated electrons in the CB of BiOI? Considering that the reformed CB level of BiOI is –0.59 V (vs. NHE), direct electron transfer to oxygen molecules requiring • –0.284 V (vs. NHE) or formation of H O 2 requiring ⎯0.046 V (vs. NHE) will not be difficult, as shown in Eqs. (3) and (4), respectively,

O2 + e − → • O2− , +

−

E0 = –0.284 V (vs. NHE),

HO•2 ,

(3)

E0 = –0.046 V (vs. NHE). (4) O2 + H + e → Thus we guess that the accumulated electrons in the CB of BiOI would be transported to oxygen species through the singleelectron processes. In such a way, the recombination of photogenerated electronhole pairs could be effectively inhibited and the corre sponding photocatalytic properties would be greatly improved [14]. Moreover, the formed junction between BiOI and TiO2 in the heterostructural photo catalyst could further prevent the recombination between charge carriers. Before contact, the conduc tion band edge of BiOI is lower than that of TiO2, and the Fermi level of BiOI is also lower than that of TiO2.

After contact, the Fermi level of BiOI is moved up, while the Fermi level of TiO2 is moved down until an equilibrium state is formed. Meanwhile, consistent with the rising up and/or descending of the Fermi level, the whole energy band of BiOI is raised up while that of TiO2 is descended, and as a result, the conduc tion band edge of BiOI is higher than that of TiO2. At the equilibrium, the inner electric field is formed, thus the ptype BiOI region has the negative charge while ntype TiO2 has the positive charge. Under visible light illumination, BiOI could be easily excited and induced the generation of photoelectrons and holes. With the formation of a p–n junction and its energy band structure at equilibrium, the excited electrons on the conduction band of the ptype BiOI transfer to that of ntype TiO2, and simultaneous holes remain in the pBiOI valence band. The migration of photoge nerated carriers can be promoted by the internal field. Thus, the photogenerated electronhole pairs will be separated effectively by the p–n junction formed in the pBiOI/nTiO2 interface, and the recombination of electronhole pairs can be reduced. The separated electrons and holes are then free to initiate reactions with the reactants adsorbed on the photocatalyst sur faces with enhanced photocatalytic activity. As a result, the asprepared BiOI/TiO2 hybrid material could show better photocatalytic properties than that of BiOI or TiO2 in the degradation of MO under visible light irradiation.

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4. CONCLUSION In general, we synthesized BiOI/TiO2 heterostruc tures through a solgel method combined with chemi cal etching. The presence of BiOI could extend the spectral response of TiO2 from the UV to visible region, enabling the heterostructures to effectively degrade the organic dye pollutants under visible light irradiation. The asprepared BiOI/TiO2 exhibited excellent performance in the degradation of MO and displayed much higher photocatalytic activity than single BiOI under visible light (λ > 420 nm). It is attributed to the lower recombination rate of the elec tronhole pairs. Photocatalysis mechanism investiga tions demonstrate that the degradation of MO over the asprepared BiOI/TiO2 under visible light is mainly via direct h+ oxidation mechanism and superoxide oxida tion pathway. Heterostructural BiOI/TiO2 may be a promising efficient composite photocatalyst for envi ronmental purification. ACKNOWLEDGMENTS This work was supported by Natural Science Foundation of Hubei Province of China (project no. 2011CDB148) and the Fundamental Research Funds for the Central Universities (program no. 2013QC026) REFERENCES 1. Y. Zhang, D. L. Li, Y. Chen, X. H. Wang, and S. T. Wang, Appl. Catal. B 86, 182 (2009). 2. S. W. Zhang, J. X. Li, H. H. Niu, W. Q. Xu, J. Z. Xu, W. P. Hu, and X. K. Wang, Chem. Phys. Chem. 78, 192 (2013).

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