Synthesis of Indium Borate and Its Application in Photodegradation of

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Synthesis of Indium Borate and Its Application in Photodegradation of 4-Chlorophenol Jixiang Yuan,† Qiang Wu,† Peng Zhang,† Jianghong Yao,† Tao He,*,‡ and Yaan Cao*,† †

College of Physics and Teda Applied Physics School, Nankai University, Tianjin 300071, China National Center for Nanoscience and Technology, Beijing 100190, China

‡

ABSTRACT: Indium borate has been prepared by a sol−gel method. The structure, morphology, and photophysics of the resultant photocatalysts have been studied via the techniques of X-ray diffraction (XRD), transmission electron microscopy (TEM), and diffuse reflectance UV−visible light spectroscopy. These photocatalysts have been used to photodegrade 4chlorophenol. The photocatalytic activity depends on the annealing temperature during preparation. It is found that borates can exhibit a high photodegradation activity under UVlight irradiation, for which the efficiency can be higher than that of as-prepared TiO2. This is explained according to the results of fluorescence spectra and valence band X-ray photoelectron spectroscopy (XPS). It is confirmed by the results of time-resolved photoluminescence decay spectra; i.e., the lifetime of electrons and holes involved in the radiative process can be longer for the borates than that for TiO2. This implies that indium borate can be a promising photocatalyst for future applications in treatment of environment contaminants.

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INTRODUCTION The 4-chlorophenol (4-CP), a hazardous waste and toxic pollutant, has been widely used in many fields such as pharmacy and dyes over the past few decades. It shows high toxicity and strong irritant on the organisms. Since it is difficult to purify the wastewater contaminated by 4-CP, it is critical to develop an efficient method to degrade it. Compared to conventional treatment processes, a promising approach is to employ a photocatalyst to oxidize it at room temperature using clean solar energy.1,2 Many photocatalysts have been designed and prepared to achieve this. Among them, TiO2 is the most widely studied one due to its nontoxicity and high photostability.3 However, so far, the photocatalytic efficiency for all of these catalysts is still relatively low, which keeps them away from practical applications. Hence, it is important to develop new photocatalysts to degrade 4-CP with a high efficiency. Indium borate (InBO3) is an optoelectronic material with a good chemical stability,4−6 which is often used as the host of luminescence materials. It is a wide-gap semiconductor, which makes it with a lower recombination rate of photoproduced charge carriers. Moreover, the holes photogenerated in it can be involved in decomposing organic molecules since the potential of its valence band is positive enough (vs SHE). However, so far, only few reports about InBO3 as a photocatalyst have been reported. One example is that InBO3 with a calcite structure can be used for water splitting under UV-light irradiation.7 There are no reports about InBO3 as a photocatalyst for degradation of organic contaminants. Here, we report that InBO3 can have a high photocatalytic activity on degradation of 4-CP under UV-light irradiation, for which the efficiency can be higher than that of as-prepared TiO2. © 2012 American Chemical Society

EXPERIMENTAL SECTION Photocatalyst Preparation. At room temperature, 5.5 mL of indium(III) chloride solution (1 mol·L−1) was mixed with 50 mL of deionized water under vigorous stirring for 15 min, followed by addition of 720 mg of boric acid. A 16 mL of sodium hydroxide solution (1 mol·L−1) was then added dropwise. After aging for 24 h at room temperature, the resultant white precipitate was washed with deionized water to remove the unreacted reagent and dried at 373 K for 12 h. The obtained white powder was then divided equally into three parts and annealed in air for 2.5 h, respectively, at 723, 923, and 1073 K, which was, respectively, designated as IBO-723, IBO923, and IBO-1073. The reference sample, TiO2, was prepared by a sol−gel method via hydrolysis of tetrabutyl titanate (Ti(OC4H9)4). At room temperature, 1 mL of concentrated HCl solution (12 mol·L−1) and 12 mL of Ti(OC4H9)4 were added dropwise to 40 mL of anhydrous ethanol under vigorous stirring. The mixture was continuously stirred until the TiO2 gel was formed. After aging at room temperature for 24 h, the asprepared TiO2 gel was dried at 373 K for 12 h and then annealed in air at 723 K for 2.5 h.8,9 The deionized water (18.2 MΩ·cm) was used in all of the experiments. All the chemicals were purchased from Tianjin Letai Chemical Industry Co., Ltd. Photocatalyst Characterization. X-ray diffraction (XRD) pattern was collected on a Rigaku D/max 2500 X-ray diffraction spectrometer (Cu Kα, λ = 1.54056 Å). The crystal size was Received: Revised: Accepted: Published: 2330

September 22, 2011 January 18, 2012 January 19, 2012 January 19, 2012 dx.doi.org/10.1021/es203333k | Environ. Sci. Technol. 2012, 46, 2330−2336

Environmental Science & Technology

Article

oxidizing agent after oxidation at 150 °C for 120 min in the COD reactor.11−13

calculated using Scherrer equation. The BET specific surface area was determined by the nitrogen adsorption−desorption isotherm measurement at 77 K (Micromeritics Tristar 3000, Shimadzu). The transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) analysis were conducted using a Philips Tecnai G2 F20 instrument, for which the samples were prepared by applying a drop of ethanol suspension onto an amorphous carbon-coated copper grid and dried naturally. X-ray photoelectron spectroscopy (XPS) measurements were carried out with a Kratos Axis Ultra DLD spectrometer using a monochromated Al Kα X-ray source (1486.6 eV). The binding energy was calibrated using the adventitious C1s peak of 284.8 eV. Diffuse reflectance UV−visible (UV−vis) absorption spectra were recorded on a UV−vis spectrometer (U-4100, Hitachi). Photoluminescence (PL) spectra and the timeresolved PL decay curve were acquired using a time-resolved spectrofluorometer with a femtosecond (fs) laser system and an intensified CCD camera (ICCD) spectrograph (LAVISION, PicoStar HR12 Camera System). The fs laser pulse (800 nm, 120 fs, 1 kHz) was converted to another one (400 nm, 120 fs, 1 kHz) by a regenerative amplifier. The wavelength accuracy is 0.1 nm, and the time resolution for the time-resolved PL spectrum is 50 ps. All the measurements were carried out at room temperature (25 ± 2 °C) unless stated otherwise. Calculation. Generalized gradient approximation (GGA) based density-functional theory (DFT) was used to calculate the electronic band structure and density of states (DOS) for InBO3.10 The plane wave energy cutoffs were taken to be 340 eV, and the k-point set was 6 × 6 × 2. The core electrons were replaced with ultrasoft core potentials. The valence electronic configurations for O, In, and B atoms were 2s22p4, 4d105s25p1, and 2s22p1, respectively. Photocatalysis. The as-prepared photocatalysts were used to photodegrade 4-CP in a 100 mL photochemical reactor with 10 mg of photocatalyst suspended in 4-CP solution (5 × 10−5 mol·L−1, 40 mL, pH = 5.74) under UV-light irradiation. A sunlamp (Philips HPA 400/30S, Belgium, λ ≥ 300 nm) was used as light source. The UV-light intensity on the sample surface was 0.2 W·cm−2. The reactor was perpendicular to the light beam and located 15 cm away from the light source. All the suspensions were stirred at 25 ± 2 °C in the dark for 30 min with continuously bubbled oxygen at a flux of 5 mL·min−1 to reach adsorption equilibrium before photoirradiation. The solution concentration was monitored with a UV−vis spectrometer (UV-1601PC, Shimadzu) using 4-aminoantipyrine as the chromogenic reagent. The photodegradation was repeated at least three times with different batches of photocatalysts prepared using the same recipe. The control experiment was performed under identical conditions but without catalysts. The concentration of organic species can be calculated by the determination of chemical oxygen demand (COD), which is performed using a COD reactor (DRB2000, HACH) via a semimicro modified dichromate method suitable for the determination of low COD concentrations (1−35 mg l−1).11 Typically, the COD measurements were carried out before and after UV-light photodegradation of a 4-CP suspension containing 10 mg of photocatalyst.12 After a small fraction of suspension was taken out from the reactor at different time intervals, the particles inside were removed immediately by centrifugation and filtration. The COD values of the resultant solution were then determined by measuring the residual

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RESULTS AND DISCUSSION Structure and Morphology of Photocatalyst. Crystalline structure of different borates and TiO2 reference sample was studied using XRD patterns (Figure 1). The TiO2 annealed

Figure 1. XRD patterns of samples for different borates and TiO2 reference.

at 723 K exhibits an anatase structure.8,9 All the diffractive peaks for IBO-923 and IBO-1073 match well with the characteristic feature of InBO3 crystal (JCPDS, No. 170933).5,6,14 The peaks at 24.3°, 31.6°, 37.4°, 45.0°, and 51.9° can be indexed to (102), (104), (110), (202), and (116) crystal planes of InBO3, respectively. Hence, IBO-923 and IBO-1073 samples are in crystalline state with a hexagonal lattice and calcite structure.4−6,14 Only two weak peaks were observed at 24.3° and 31.6° for IBO-723 (Figure 1), indicating that it is largely in amorphous state.14,15 The values of specific surface area (SBET), cell volume and cell parameters of borate samples decrease with the increase of annealing temperature (Table 1). Table 1. Cell Parameters and BET Surface Area for Different Samples sample

a = b (nm)

c (nm)

cell volume (10−3 nm3)

SBET (m2g−1)

IBO-723 IBO-923 IBO-1073 TiO2

0.4823 0.4823 0.4820 0.3792

1.5457 1.5450 1.5441 0.9499

311.42 311.24 310.71 136.59

55.3 3.9 1.0 63.8

As the crystal size increases with the increase of annealing temperature due to crystal growth, the BET surface area decreases with the increase of annealing temperature and, thereby, the amount of crystalline phase of borate.14,15 Figure 2 shows TEM and HRTEM images of IBO-1073 sample. Accurate structural information about crystal structure and planar spacing can be extracted via 2D fast Fourier transform (FFT) analysis of HRTEM image. Fringe spacing of (104) and (110) crystallographic plane for hexagonal lattice structure are determined to be 2.86 Å (Figure 2B) and 2.42 Å (Figure 2C), respectively. FFT analysis (Insets of Figure 2) clearly reveals the bright spots for both (104) and (110) planes, indicating that a single crystal of IBO-1073 has been successfully prepared.16,17 2331

dx.doi.org/10.1021/es203333k | Environ. Sci. Technol. 2012, 46, 2330−2336


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Figure 2. Images of (A) TEM and (B, C) HRTEM for IBO-1073. Insets are FFT analysis of square area selected in the corresponding image. Scale bar is 200 nm for (A) and 5 nm for (B, C), respectively.

Figure 3. The electronic band structure (along high symmetry lines in Brillouin zone) of InBO3 calculated using DFT.

Photophysics of Photocatalyst. Figure 3 shows the electronic band structure (along high symmetry lines in Brillouin zone) of InBO3 obtained by DFT calculation. The zero energy level lies in the top of valence band (VB), corresponding to the highest state occupied by electrons. Hence, all electrons are located in the VB. The band structure indicates that the photoabsorption can arise from both indirect and direct bandgap transition. The top of VB was H point (−0.333, 0.667, 0.500), and the bottom of conduction band (CB) was G point (0.0, 0.0, 0.0). The direct bandgap at H point is calculated to be ∼4.60 eV, and the indirect bandgap is ∼2.31 eV. Figure 4 shows the calculated density of states (DOS) of InBO3 . The projected contributions to total DOS from O (2s22p4), In (4d105s25p1), and B (2s22p1) are also included in the figure. It is found that, as expected, the top of VB is dominated by O 2p states, as well as a small fraction of contribution from B 2p, In 5p, In 4d, and In 5s. The bottom of CB is dominated by In 5s states, with a small fraction of contribution from O 2s, In 5p, B 2s, and B 2p. The O 2p states also partially contribute to the CB. Except In 5s, all the states

Figure 4. The DOS of InBO3 calculated using DFT, as well as the projected state densities of O, In, and B. To clearly observe the VB and CB, a zoom-in of total DOS is shown in the inset of (a).

contribute to the CB at high energy levels. The contribution of O 2p states to CB may make its level rise, resulting in widening the bandgap. 2332



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that the photoabsorption is very weak since its energy is very high. Hence, the contribution to photocatalysis from the direct bandgap is no longer considered in this work. Photocatalytic Degradation of 4-CP. The photocatalysts have been used to photodegrade 4-CP under UV-light irradiation (Figure 6A). Both TiO2 and borates have a very low adsorption capability of 4-CP after 1 h (about 2−4%, inset of Figure 6A). The 4-CP can hardly be degraded in the control experiment (photolysis). The TiO2 shows a degradation rate (ln(C0/C)) of ∼66.0% after 1 h irradiation (Table 2). The ln(C0/C) after 1 h irradiation for IBO-723, IBO-923, and IBO1073 is about 40.0%, 67.3%, and 96.0%, respectively. Hence, IBO-923 and IBO-1073 exhibit a higher photocatalytic activity

Figure 5 is the diffuse reflectance UV−vis absorption spectra of TiO2, IBO-723, IBO-923, and IBO-1073 samples. The strong absorption peak at 340 nm for TiO2 is attributed to the band-to-band transition, for which the absorption onset edge is ∼400 nm, corresponding to a bandgap of ∼3.10 eV.8,9 For

Table 2. Photodegradation of 4-CP under UV-Light Irradiation sample controlc IBO-723 IBO-923 IBO1073 TiO2

Figure 5. Diffuse reflectance UV−vis spectra for TiO2, IBO-723, IBO923, and IBO-1073 samples.

IBO-1073 sample, two absorption peaks appear in the spectra, which are suggested to correspond, respectively, to indirect bandgap transition and direct bandgap transition. The bandgap for the absorption peak centered on ∼256 nm is determined to be ∼3.31 eV since its absorption onset edge is 375 nm. The bandgap for the second one is ∼5.60 eV as its onset edge is at 220 nm. It is suggested that the former (3.31 eV) is due to the indirect transition, while the latter (5.60 eV) is the direct transition. In addition, redshift occurs in the absorption edge for the borates when the annealing temperature increases since the crystallinity of borate increases with the increase of annealing temperature, as evidenced by XRD. It is notable that the experimentally determined bandgaps are larger than the calculated values for both indirect and direct bandgaps. This difference points at a common failure when the local density approximation is applied to the semiconducting systems.18,19 Hence, a “scissors operator” is usually introduced, allowing a shift of the CB. Here, the energy of 1.0 eV can be chosen to make the calculated data match the experimental ones. In addition, the bandgap for borates has been reported to be above 5 eV in the literature,7,20 which agrees with the one corresponding to the direct transition reported here. It is suggested that it contributes very little to the photocatalysis in

a

degradation rate (Δc/c0) (%)a

−1 b

t1/2 (min)

specific photocatalytic activityc (mol g−1 h−1)

10−4 10−2 10−2 10−2

956.1 81.5 37.3 12.9

0.80 × 10−4 1.35 × 10−4 1.92 × 10−4

1.80 × 10−2

38.5

1.32 × 10−4

k (min )

0.043 0.400 0.673 0.960

7.25 0.85 1.86 5.36

0.660

× × × ×

b

After reaction for 1 h. Apparent rate constant deduced from the linear fitting of ln(c0/c) versus reaction time. cControl refers to the photolysis of 4-CP.

than TiO2, while IBO-723 is lower. Both specific photocatalytic activity and photodegradation rate of IBO-1073 are ∼1.5 times of that of TiO2 (Table 2). Similar phenomena were observed by the change in COD before and after the photodegradation of 4-CP (Figure 6B), which can reveal the degree of mineralization and/or degradation of an organic upon irradiation.13 Considering photocatalysis may lead to stoichiometric photomineralization of organic compounds,13 the COD of a given photocatalyst can be assessed, by tracing the change of dissolved oxygen concentration under the photocatalytic conditions. The reduction of COD value after 1 h irradiation for the control, TiO2, IBO-723, IBO-923, and IBO-1073 is about 2.5%, 58.3%, 37.5%, 60.0%, and 92.5%, respectively, so 4-CP can not only be degraded but also be mineralized efficiently by the borates. Therefore, the borate can be an effective catalyst for 4-CP photodegradation.

Figure 6. Photocatalysis of different catalysts. (A) Concentration of 4-CP as a function of UV-light irradiation time during photodegradation with different catalysts. (B) The corresponding change in COD before and after the photodegradation. Inset of (A) is the adsorption ratio of 4-CP with different catalysts in dark after 1 h. 2333



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Mechanism of 4-CP Photodegradation. As an effective photocatalyst, its potential of VB and CB should be located in an appropriate position so that photogenerated charge carriers can take part in decomposing organic molecules. Here, IBO1073 is used as an example for borate catalysts. Figure 7 is the XPS VB spectra for TiO2 and IBO-1073 samples, which can be used to determine the VB position in a semiconductor.21,22 The energy levels are aligned using the work function of the XPS

Similarly, the energy levels of defects in TiO2 are determined to be −0.58 V (480 nm) and −0.36 V (525 nm) (Figure 8B). Under UV-light irradiation, the photogenerated electrons in the defects and CB can be directly captured by the adsorbed O2 as their potential is negative enough, resulting in formation of O2− (eq 1).2,24 The O2− can react with electrons to form H2O2 (eq 2), which can further react with the electrons to produce OH− and •OH (eq 3). The •OH can also be obtained via the reaction of holes with OH− or surface hydroxyl (eq 4). The 4CP molecules absorbed on the catalyst surface can be oxidized by both O2− and •OH (eqs 5 and 6),1 as well as directly by the holes (eq 7) since their potential is positive enough. Eventually, the 4-CP molecules can be photodegraded into CO2 and H2O.

Figure 7. Valence band XPS spectra for TiO2 and IBO-1073 samples. The Fermi level (Ef) is shown too.

instrument (4.10 eV), and the binding energy of the onset edge of O2p peak reveals the energy gap between the VB maximum and Fermi level.21 As shown in Figure 7, this energy level is determined, respectively, to be ∼ +2.40 and +2.85 eV for TiO2 and InBO3 by linearly extrapolating the leading edge at its maximum slope point to the baseline.22 Hence, the potential of VB top is determined to be 2.45 V (vs SHE) for IBO-1073 and 2.00 V for TiO2. As mentioned above, the bandgap for TiO2 is 3.10 eV in light of the absorption spectrum, and the indirect and direct bandgap for IBO-1073 is 3.31 and 5.60 eV, respectively. The potential of CB bottom for IBO-1073 and TiO2 is thus determined to be −0.86 (indirect bandgap), −3.15 (direct bandgap), and −1.10 V, respectively. Since PL for both borates and TiO2 is closely related to the defects in semiconductor, energy levels of these defects can be determined using PL spectra.8,9,23 For all the borate catalysts, a strong peak at ∼470 nm is observed (Figure 8A). Therefore, the energy level of defects in IBO-1073 is determined to be −0.19 V. Here, the hump at ∼515−540 nm is not discussed.

O2 + e → O2−

(1)

O2− + 2H+ + e → H2O2

(2)

H2O2 + e → OH− + •OH

(3)

OH− + h+ → •OH

(4)

4‐CP + O2− → CO2 + H2O

(5)

4‐CP + •OH → CO2 + H2O

(6)

4‐CP + h+ → CO2 + H2O

(7)

Photocatalytic Activity. The alignment of energy levels (Figure 9) can be used to explain why IBO-1073 exhibits a higher activity than as-prepared TiO2, which is drawn according to the aforementioned results of UV−vis absorption spectra, valence band XPS, and PL spectra. Here, it is noted that the energy levels for TiO2 are different from the literature, which is ascribed to that TiO2 can exhibit different energy levels if they are prepared by different methods. The energy level of CB bottom (indirect) for IBO-1073 locates closer to the energy level of O2/O2− (∼−0.78 V vs SHE)25 than that for TiO2, and the energy level of defects in IBO-1073 locates closer to the energy level of O2/H2O2 (∼0.38 V) than that for TiO2. This may facilitate the formation of O2− and •OH (eqs 1−3). Moreover, the potential of VB top for IBO-1073 is more positive than that for TiO2, leading to a higher oxidization capacity and, thereby, also in favor of the formation of •OH (eq 4). It is suggested that •OH may play a major role in the photodegradation.26 Moreover, the holes with a higher oxidization power can also oxidize 4-CP more

Figure 8. The PL spectra of (A) different borate catalysts and (B) TiO2. 2334



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slow PL decay indicates a prolonged recombination process in IBO-1073 and, thus, results in a higher photocatalytic activity than TiO2. Similarly, the τ2 value for IBO-923 (2.612 ns) is longer than that for TiO2 and, accordingly, exhibits a higher activity. The τ2 value for IBO-723 (1.312 ns) is shorter than that for TiO2 and shows a lower activity. It is noted that photocatalytic activity of borate photocatalyst increases with the increase of annealing temperature (Table 2), which correlates to the PL results. The IBO-1073 has the longest τ2 value, followed by IBO-923, and IBO-723 has the shortest τ2 (Table 3). Since the PL intensity is sensitive to the lifetime, accordingly, IBO-1073 exhibits the weakest PL intensity, followed by IBO-923, and IBO-723 has the strongest PL intensity (Figure 8). This may be related to the increase of crystallinity with the increasing temperature as well as the change in surface states. Further study is undergoing to elucidate this.

Figure 9. Schematic diagram for the alignment of energy levels for IBO-1073 and TiO2 (not drawn to scale).

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effectively via direct oxidization (eq 7). Thus, IBO-1073 exhibits a higher photocatalytic activity than as-prepared TiO2, albeit its bandgap is slightly wider than that of TiO2. The photocatalytic activity is closely related to the behavior of photogenerated electrons and holes. The photogenerated electrons first fall into the defects from the CB via a nonradiative process and then recombine with the holes in VB to give rise to fluorescence emission. Hence, the lifetime of electrons in the defects can be used to evaluate the photocatalytic activity, which is measured by PL technique (Figure 10). For metal oxides, PL decay curve comes from a combination of nonradiative and radiative processes.27,28 The fast decay component of the PL (τ1) is normally attributed to

AUTHOR INFORMATION

Corresponding Author

*Fax: +86-22-66229310 (Y.C.). Tel: +86-22-66229598 (Y.C). E-mail: [email protected] (Y.C); [email protected] (T.H.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by National Natural Science Foundation of China (50872056, 51072082, and 51043010). T.H. also thanks National Research Fund for Fundamental Key Projects No. 973 (2011CB933200), Ministry of Science and Technology of China (2010DFA64680), and the HundredTalent Program of Chinese Academy of Sciences. We thank Prof. Ying Ma from Institute of Chemistry, Chinese Academy of Sciences, for helping us to carry out the theoretical calculation.

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Figure 10. The time-resolved PL decay curve for different catalysts, excited at 400 nm and monitored at 470 nm for borates and 480 nm for TiO2.

the nonradiative relaxation process related to defects of materials, and the longer PL lifetime range (τ2) arises from the radiative process related to the recombination of photogenerated electrons and holes. The τ1 and τ2 values of different samples have been calculated via double exponential decay fitting (Table 3). Results show that the τ2 value for IBO-1073 is 4.538 ns, which is longer than that for TiO2 (1.528 ns). This

Table 3. Values of the Calculated Decay Time Constant τ1 and τ2 through Double Exponential Decay Fitting for the Corresponding Samples τ1 (ns) τ2 (ns)

TiO2

IBO-723

IBO-923

IBO-1073

0.442 ± 0.036 1.528 ± 0.182

0.468 ± 0.097 1.312 ± 0.007

0.626 ± 0.122 2.612 ± 0.233

1.289 ± 0.084 4.538 ± 0.577

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