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In situ Composition-Transforming Fabrication of BiOI/BiOIO3 Heterostructure: Semiconductor p-n Junction and Dominantly Exposed Reactive Facets Hongwei Huang, Ke Xiao, Kun Liu, Shixin Yu, and Yihe Zhang Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.5b01101 • Publication Date (Web): 20 Nov 2015 Downloaded from http://pubs.acs.org on November 21, 2015

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Crystal Growth & Design

In situ Composition-Transforming Fabrication of BiOI/BiOIO3 Heterostructure: Semiconductor p-n Junction and Dominantly Exposed Reactive Facets Hongwei Huang,* Ke Xiao, Kun Liu, Shixin Yu, Yihe Zhang* Beijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid Wastes, National Laboratory of Mineral Materials, School of Materials Science and Technology, China University of Geosciences, Beijing 100083, China

Corresponding Author *E-mail: Hongwei Huang [email protected]; Tel: 86-10-82332247; Yihe Zhang [email protected] ; Tel: 86-10-82323433; Address: School of Materials Science and Technology, China University of Geosciences, Beijing 100083, China

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ABSTRACT: We for the first time disclose the integrated effects of a semiconductor p-n heterojunction and dominantly exposed reactive facets that are enabled in a facile way. Unlike most of reported semiconductor heterojunctions that are constructed by compositing the individual components, in this work, we report the compositiontransformation fabricating BiOI/BiOIO3 heterostructure via an in situ reduction route by using thiourea as reducing agent. This reducing process enables BiOIO3 dominant exposure of {010} reactive facet, and the exposed percentage can be effectively tuned by mono-controlling the thiourea concentration. The photocatalysis and photoelectrochemical properties of samples are assessed by surveying the decomposition of methyl blue (MB) and photocurrent generation under simulated solar light or visible light illumination. The heterostructured BiOI/BiOIO3 nanocomposites unfold drastically strengthened photoreactivity, in which the MB degradation rate is over 85% for 1 h irradiation and the photocurrent density rises more than 3 times higher than the pristine sample. This enhancement should be ascribed to the formation of a steady p-n junction between the ntype BiOIO3 and p-type BiOI as well as dominantly exposed reactive facets. Separation and transfer of photoinduced charges are thereby greatly boosted as verified by the electrochemical and photoelectrochemical results. This work paves a novel way for fabrication of semiconductor p-n junction via composition transformation, and furnishes new perspective into the designing of crystal reactive facet.

KEYWORDS: Photocatalysis, BiOIO3, BiOI, p-n junction, Reactive facet


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INTRODUCTION Semiconductor photocatalysis has been broadly recognized as a promising and clean technique for relieving the crises on energy shortage and environmental deterioration.1−3 Different semiconductors play various roles in the photocatalytic process. Semiconductors with wide-band-gap (WBG) usually possess strong reduction and oxidation power due to their further energy band potentials away from Fermi level, whereas they suffer from the low efficiency in utilizing the full solar spectrum. On the other side, a narrowband-gap (NBG) semiconductor can effectively responds to visible light, it is nevertheless subjected to weak photoinduced redox ability. To overcome the above drawbacks inherent to the single photocatalyst, fabrication of heterojunctions by coupling a WBG semiconductor with a NBG semiconductor shows considerable potential with integrated superiority.4-8 Bismuth-based oxides have recently aroused great interest owing to their high photocatalytic performance.9-15 As a newly discovered Bi-based nonlinear optical (NLO) material, bismuth oxide iodate (BiOIO3) was found to be an efficient photocatalyst for removal of liquid and gaseous pollutants.16-18 The layered structure inherent in BiOIO3 crystal enables it internal self-built electric field, which could enhances the charge separation efficiency. In particular, the non-centrosymmetrical crystal structure of BiOIO3 strengthens the internal polarization electric field, further facilitating the separation of photoexcited electron-hole pairs.17 However, the relatively large band gap (～ 3 eV) of BiOIO3 confines severely its photoabsorption to a narrow region with shorter wavelength. Therefore, BiOIO3 heterojunctions have been constructed to increase the response to visible light as well as the photocatalytic activity.19-23 Brick-red colored BiOI is a NBG photo3 ACS Paragon Plus Environment


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catalyst with a band gap of ～ 1.7 eV and always acts as an efficient photosensitizer for enhancing the light absorption of other semiconductors, for instance TiO2, ZnO, BiOCl, etc.24-28 Given that the I-/(IO3)- is a redox couple, BiOI/BiOIO3 composites may be fabricated via phase-transformation in an in-situ oxidation or reduction process. This approach might enable an intimate interfacial interaction for efficiently promoting the charge transfer. On the other hand, the design and manipulation of reactive facets of crystals gain worldwide attention for that they provide physicochemical property tailoring in microscopic scale.29,30 In addition to the broad researches on the active {001} facets of anatase TiO2, it is also found recently that the exposure of active facets can largely enhances the photocatalytic activity of the bismuth compounds, such as BiOCl, BiOBr, Bi2O2CO3, etc.31-34 Taking into account the layered structure of BiOIO3 analogous to BiOCl, engineering the active facets of BiOIO3 may be achieved by certain means. Herein, we report the integration of p-n heterojunction fabrication and active exposing facet manipulation in BiOI/BiOIO3 hetrostructures. The BiOI/BiOIO3 p-n heterojunctions are constructed via In situ partial reduction of BiOIO3 to BiOI. In order to avoid being reduced of Bi3+, a modest reducing agent, thiourea has been selected compared to other agents with strong reducing ability, like NaBH4. It is very fascinating to find that the reduction process from BiOIO3 to BiOI results in the exposure of {010} reactive facet of BiOIO3, and the exposed percentage can be effectively tuned by controlling the thiourea amount. Via the in-situ growth of BiOI on BiOIO3, the photoresponsive range of the composites is orderly extended from UV to visible light. In comparison with the pure BiOIO3 and BiOI, the BiOI/BiOIO3 nanocomposites exhibit significantly promoted photo4 ACS Paragon Plus Environment

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reactivity pertaining to methyl blue (MB) decomposition and photocurrent generation under simulated solar light. This enhancement is attributed to the cooperative effect of formation of BiOI/BiOIO3 p-n junction and dominantly exposed reactive facets, which can greatly facilitate the separation and transfer of charge carriers. Our work demonstrates a novel route for fabrication of semiconductor heterojunction without relying on external compositing.

EXPERIMENTAL SECTION Synthesis. All the raw materials used here are of AR purity and without further purification. The BiOI/BiOIO3 composites were synthesized via a facile in situ reduction reaction using thiourea as reducing agent. First, BiOIO3 precursors were prepared based on the hydrothermal method.18 Then, different amounts of thiourea (0.01, 0.02, 0.05, 0.1, 0.2, 0.5, and 1 mmol) were separately added to the 20 mL of BiOIO3 (1 mmol) suspension. The resulting suspension was stirring magnetically for 5 h at room temperature. Then the precipitates were collected by centrifugation and then dried at 60°C overnight to get the final samples. The samples prepared with thiourea concentrations of 0%, 1%, 2%, 5%, 10%, 20%, 50% and 100% are denoted as BiOIO3, BB-1, BB-2, BB-3, BB-4, BB-5, BB6 and BiOI.

Scheme 1. Schematic diagram for the formation of BiOI/BiOIO3 composites



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Characterization. The crystallinity phase of the prepared samples was studied by Xray diffraction (XRD) on a CuKα (λ=0.15418 nm) Bruker D8 X-ray diffractometer with. Field emission scanning electron microscopy (FE-SEM) images were obtained on a Hitachi S-4800 field emission scanning electron microscope operated at 10.0 kV, and a H-800 Hitachi transmission electron microscopy (TEM) was allowed to characterize the microstructure. Surface chemical properties of samples were investigated by X-ray photoelectron spectroscopy (XPS) with 150 W Al Kα X-ray irradiation. The PerkinElmer Lambda 35 UV–vis spectrometer was used to obtain the UV-Visible diffuse reflectance absorbance (DRS). Photoluminescence (PL) spectra were recorded on a fluorescence spectrophotometer (Hitachi F-4600). Photodegradation Experiment. The photocatalysis activity was studied by decomposition of MB (1×10-5 mol/L) under simulated solar light (500 W xenon lamp) at ambient temperature.35 The energy intensity is measured by a light- meter (FZ-A) to be about 21.2 mW/cm2. 50 mg of photocatalyst powder was added into 50 mL of MB aqueous solution in a quartz tube. Before photoreaction, the mixture was magnetically stirred in the darkness for 20 min to ensure the adsorption-desorption equilibrium of MB and the catalyst. When exposing to light, about 4 mL of suspension was taken for every 20 min, and 6 ACS Paragon Plus Environment

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centrifuged to remove the solids. Then, the absorption spectra of solution were analyzed on a Cary 5000 UV−vis spectrophotometer. Active Species Trapping Experiment. In the photodegradation process, different agent scavengers were introduced to determine the types of active species. 1,4benzoquinone (BQ), isopropyl alcohol (IPA) and disodium ethylenediaminetetraacetate (EDTA) were added as the scavengers of superoxide radical (•O2−), hydroxyl radical (•OH) and the hole (h+), respectively.36,37 Photoelectrochemical Tests. A CHI 660E electrochemical analyzer with a threeelectrode quartz cell was applied to perform the photoelectrochemical measurements. The counter electrode and reference electrode were the platinum wire and saturated calomel electrode (SCE), respectively. The photocatalyst films coated on ITO were used as the working electrode and the electrolyte was 0.1 M Na2SO4 solution. A 500 W Xe lamp was used as the simulated solar light source, and the photocurrent response and electrochemical impedance spectra was measured at 0.0 V under illumination. The frequency range of electrochemical impedance spectra is 1-82520 Hz. The working electrodes are sampled by a dip-coating method: 25 mg of photocatalyst was added into1 mL ethanol to produce a suspension, then it was dropped on a 20 mm × 40 mm indium–tin oxide (ITO) glass electrode. Afterwards, the electrodes were placed in air for 1 day to remove the ethanol. RESULTS AND DISCUSSION XRD analysis. The crystalline phase change in the reduction process of BiOIO3 was detected by XRD. As depicted in Figure 1a, the pristine BiOIO3 shows a series of sharp diffraction peaks, which can be indexed into the orthorhombic BiOIO3 (ICSD # 262019).18 When the BiOIO3 was reduced by thiourea, the tetragonal phase of BiOI 7 ACS Paragon Plus Environment


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(JCPDS file 10-0445) emerges. With elevating the thiourea concentration, the BiOI diffraction peaks were gradually enhanced from BB-1 to BB-6 in the BiOI/BiOIO3 composites. It is important to note from the enlarged XRD pattern (Figure 1b) that the intensity of (040) peak of BiOIO3 first gradually increases, and then diminishes with raising the reduction agent (thiourea) amount. Figure 1c presents the peak intensity ratios of (040)/(002) of different BiOI/BiOIO3 composites. It revealed the same trend that the BB4 sample exhibits the largest intensity ratio value of (040)/(002), which implies that the BB-4 undergoes the highest exposure of {010} facet. It is concluded that the reduction process from BiOIO3 to BiOI can effectively tune the exposed percentage of {010} facet of BiOIO3, and excessive BiOI products may cover on the {010} facet of BiOIO3 conversely reducing the exposure ratio.


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Figure 1. (a) XRD patterns and (b) Enlarged XRD patterns of BiOIO3, BiOI and BiOI/BiOIO3 composites; (c) Diffraction peak intensity ratio of (040)/(002).

XPS analysis. The oxidation state and surface chemical composition of BiOIO3, BiOI and BiOI/BiOIO3 composite (BB-4) are investigated by XPS. As shown in the XPS survey spectra (Figure 2a), the three samples display signal peaks of Bi 4p, Bi 4d, Bi 4f, O 1s, I 3p and I 3d, and C 1s is resulted from the adventitious carbon. As shown in Figure 2b, the BB-4 exhibits the characteristic Bi3+ peaks at 164.56 and 159.25 eV, which are assigned to Bi 4f2/5 and Bi 4f2/7, respectively.15 The peak of O 1s at 530.39 eV observed for the three samples corresponds to the lattice O (Figure 2c). Compared with the pristine BiOI and BiOIO3, the BB-4 displays two sets of I 3d peaks (Figure 2d). The two strong peaks around 635.45 and 623.98 eV are attributed to the I 3d3/2 and I 3d5/2 states of I5+,22 and the other two peaks at 630.63 and 619.10 eV are the characteristic of I 3d3/2 and I 3d5/2 states of I-.26 Additionally, obvious binding energy shift is observed for both the I5+ 3d and I- 3d peaks of BB-4 in comparison with the two pristine samples. Unlike simply mechanical mixing with no chemical bonding, the shift in binding energy demonstrate that the I5+ ions in BiOIO3 and I- ions in BiOI may undertake chemical interaction with the O2- or Bi3+ ions in the opponent counterparts. Namely, intimate interfacial interaction occurs between the BiOI and BiOIO3 phases. The above XPS results evidence the successful fabrication of BiOI/BiOIO3 composite with close interfacial interaction.



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Figure 2. XPS of BiOIO3, BiOI and BB-4: (a) Typical survey spectra; Highresolution XPS spectra of the (b) Bi 4f, (c) O 1s, and (d) I 3d.

SEM and TEM Investigation. The morphology of pristine BiOIO3, BiOI and BiOI/BiOIO3 composites are studied by SEM. The pristine BiOIO3 is composed of thick slabs with very smooth surface and neat cut edges, and the crystal size is about several micrometers (Figure S1a). When thiourea is introduced, the morphology of BiOIO3 is significantly changed. As seen from Figure S1b, the smooth surface of BiOIO3 large crystal is destroyed, and some nanosheets are grown on the surface of BiOIO3. This is because of that partial BiOIO3 is reduced to BiOI by thiourea as displayed in Scheme 1. With increasing the thiourea concentration, we can observe that the morphology of Bi10 ACS Paragon Plus Environment

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OIO3 crystals is damaged more severely and the amount of as-born BiOI nanosheets on BiOIO3 rises accordingly (Figure S1c-g). When an excess thiourea concentration is used (like 100%), the original thick-slab crystals all transform to tiny nanosheets (Figure S1h), corresponding to the achieving of pure phase of BiOI as confirmed by the above XRD results. TEM is used to confirm the morphological change and investigate the microstructure of the BiOI/BiOIO3 composites. Figure 3a verifies the neat and smooth surface of the bare BiOIO3 crystals and the size is about several hundreds of nanometers, whereas the BiOI samples are composed of uniform nanosheets with diameter of approximately 50 nm as shown in Figure 3f. With respect to the BiOI/BiOIO3 composites (Figure 3b-e), more and more BiOI nanosheets are grown from the surface of BiOIO3 with the increase of thiourea amount, consistent with the SEM results. Figure 4a and b show the HRTEM images of BB-4, which can be used to determine the exposed facets.38 The clear lattice fringes with interplanar spacing of 0.302 nm can be indexed in the (102) plane of BiOI, and the fringes with interplanar spacing of 0.280 and 0.281 nm corresponds to the (110) plane of BiOI, indicating that the main exposed facet of BiOI is {001} facet, as illustrated in Figure 4d. Besides, the lattice spacing of the crystals that are closely connected to the BiOI nanosheets is determined to be 0.289 nm and 0.290 nm, which can be assigned to the (002) plane of BiOIO3. Combined with the XRD results, it can be concluded that the dominantly exposing facet of BiOIO3 may be {010} facet (Figure 4c). For further confirmation, the fast Fourier transform (FFT) image of BiOIO3 is shown in Figure S2. It indicated that the facets corresponding to the FFT spots are determined to be (002) and (200) planes of BiOIO3, respectively. Thus, the exposed facet of BiOIO3 is {010} facet. 11 ACS Paragon Plus Environment


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As displayed in the Figure 4e and f, the {001} facet and {010} facet are the reactive crystal facets of BiOI and BiOIO3, respectively. The coupling of the two kinds of reactive crystal facets is considered to be favorable for the separation and transfer of charge carriers.

Figure 3. TEM images of (a) BiOIO3; (b) BB-1; (c) BB-3; (d) BB-4; (e) BB-6 and (f) BiOI.


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Figure 4. (a,b) HRTEM images of BB-4; Schematic orientation illustration of (c) BiOIO3 and (d) BiOI; Crystal structures of (e) BiOIO3 and (e) BiOI

Optical Properties. DRS spectra of BiOIO3, BiOI and BiOI/BiOIO3 composites are shown in Figure 5a. The absorption edge of pristine BiOIO3 crystals is situated at approximately 410 nm, whereas BiOI displays broad photoabsorption in visible region that extends to about 675 nm. For the BiOI/BiOIO3 composites, their absorption edges are all located between that of BiOIO3 and BiOI, and the photoabsorption orderly increases with the rise of the thiourea amount. This evidences that not only the BiOI component gradually increases in the BiOI/BiOIO3 composites by raising the thiourea concentration, but also



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the photoresponse of BiOIO3 can be significantly enhanced by BiOI, which is beneficial for the photoexcitation process. Band gap energies of BiOIO3 and BiOI can be determined by the following equation:39 αhv=A(hv-Eg)n

(1)

where α, Eg, v and h represent absorbance, band gap, photon frequency and Planck’s constant. Because both BiOIO3 and BiOI are indirect-transition semiconductors,18,28 their band gap energies can be obtained from the plot of (Abs)1/2 Vs Energy. As shown in Figure 5b, the band gap of BiOIO3 and BiOI was calculated to be 2.99 and 1.75 eV, respectively.

Figure 5. (a) UV−vis diffuse reflectance spectra and (b) Band gap of BiOIO3, BiOI and BiOI/BiOIO3 composites.

Photocatalysis Activity of BiOI/BiOIO3 p-n Junctions. Figure 6 shows the photocatalytic degradation efficiency of MB over BiOIO3, BiOI and BiOI/BiOIO3 composites under simulated solar light (500 W xenon lamp) for 1 h. The BiOIO3 and BiOI can only separately decompose 56.1% and 44.2% of MB within 1 h irradiation. In contrast to the 14 ACS Paragon Plus Environment

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two pristine compounds, enhanced photocataltic activities were observed for all the BiOI/BiOIO3 composites. By increasing the thiourea concentration, the photodegradation efficiency of the BiOI/BiOIO3 composites display a trend of first increase and then decrease, achieving a maximum at BB-4 with a MB degradation efficiency of 85%. The asprepared BiOI/BiOIO3 junctions also excel the BiOI/g-C3N4 p-n junction from the reference, which can degrade about 60% of MB under 1 h irradiation.40 The activity trend may be explained as that: Certain amount of BiOI can boost the visible-light absorption and charge separation of BiOIO3 to promote the photoreactivity, whereas excessive BiOI would reduce the activity due to its low efficiency. Figure S3 shows the absorption spectra of BB-4, where the photoabsorption curves centering at 664 nm gradually descend with prolonging the illumination time, indicating the destruction of MB molecules.

Figure 6. Photocatalytic degradation efficiency of MB over BiOIO3, BiOI and BiOI/BiOIO3 composites under simulated solar light for 1 h.

Photoelectrochemical Properties and Investigations on Photocatalytic Mechanism. The transient photocurrent response can indicate the generation and separation ef15 ACS Paragon Plus Environment


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ficiency of photoexcited charge carriers.41 Figure 7a presents the transient photocurrent densities of BiOIO3 and BB-4 electrodes. Compared with the pure BiOIO3, the BB-4 electrode shows a highly promoted current generation, which is approximately 3.5 times higher than that of BiOIO3. The obviously enhanced photocurrent intensity of BB-4 revealed the positive effect of BiOI on facilitating the generation and separation efficiency of photoexcited charge carriers of BiOIO3. To further investigate the photoelectrochemical properties, we have measured the I-V curves of BiOI, BiOIO3 and BiOI/BiOIO3 (BB4) electrodes under visible light (λ>420 nm) illumination. As BiOIO3 cannot be excited by visible light (λ>420 nm), the contribution of BiOI within the composites can be indicated by comparing the photocurrent curves of BiOI/BiOIO3 and pristine BiOI. As shown in Figure 7b, all the three electrodes exhibit enhanced photocurrent densities with increasing the bias voltage. Though BiOIO3 almost cannot contribute to the photocurrent, the BB-4 electrode still shows a slightly higher current density in contrast to the pristine BiOI. This observation further confirms the positive effect of BiOI/BiOIO3 p-n junction on charge separation and transfer.


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Figure 7. (a) Transient photocurrent-time curves of pristine BiOIO3 and BB-4 electrodes under UV-vis light irradiation ([Na2SO4] = 0.1 M); (b) Current densities of BiOIO3, BiOI and BB-4 electrodes under visible light (λ>420 nm) illumination ([Na2SO4] = 0.1 M).

The interfacial charge transfer of BiOIO3 and BB-4 electrodes is investigated by electrochemical impedance spectra (EIS).28 In comparison with BiOIO3, both the typical EIS Nyquist plots (Figure 8a) and high-resolution plots in high frequency zone (Figure S4) indicate a smaller arc radius of BB-4, indicative of a lower interfacial charge-transfer resistance of BB-4. In order to gain a deep insight into charge transfer, the equivalent circuit (inset of Figure 8) as well as a quantitative analysis was proposed. In the equivalent circuit, Cdl indicates the electric double layer capacitor and Q is the constant-phase element. R, RCT and RΩ represent the reaction resistance, charge transfer resistance, and ohmic resistance, respectively. Generally, the arc radius corresponds to the RCT. In the current work, RCT indicates the difference of impedance spectra at the intersection of low frequency and high frequency. Accordingly, the RCT values of BiOIO3 and BiOI/BiOIO3 (BB-4) are separately estimated to be 41370 and 37090 Ω. It further demonstrates that the BiOI/BiOIO3 junction favors more efficient charge transfer.The results from above photoelectrochemical experiments demonstrated that the separation and migration processes of photoexcited electron-hole pairs are greatly forwarded in the BiOI/BiOIO3 composite. Photoluminescence (PL) emission spectra can be applied to further clarify the separation efficiency of charge carriers.42 As seen from Figure 8b, the BiOIO3, BiOI and BiOI/BiOIO3 composites all show emission peaks around 420 nm, consistent with the emission characteristic of bismuth oxides. In contrast to the pristine BiOIO3 and BiOI, all the 17 ACS Paragon Plus Environment


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BiOI/BiOIO3 composites display greatly decreased emission intensities, indicating that the recombination rate of photogenerated electrons and holes is largely hindered.

Figure 8. (a) EIS Nynquist plots and the proposed equivalent circuit (inset) of pristine BiOIO3 and BB-4 under UV-vis light irradiation ([Na2SO4] = 0.1 M); (b) PL spectra of BiOIO3, BiOI and BiOI/BiOIO3 composites.

To speculate the photocatalytic mechanism, radical trapping experiments are performed to capture the reactive species generated in the degradation process. From the Figure 9, we can see that the degradation rate was almost not influenced by the addition of isopropyl alcohol (IPA, a scavenger of •OH), excluding the effect of •OH. However, both the benzoquinone (BQ, a scavenger of •O2−) and ethylenediaminetetraacetate (EDTA, a scavenger of h+) have significant impediment in MB degradation, revealing that •O2− and h+ are the critical active species.36,37


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Figure 9. Photodegradation curve of MB over BB-4 alone and with the addition of EDTA, IPA, or BQ.

Photocatalysis mechanism is closely related to the type and band structures of semiconductors, especially the heterojunctional photocatalyst. Mott-Schottky (M-S) methods were employed herein to determine the semiconductor types of BiOIO3 and BiOI as well as their flat-band potentials.15,43 It is well known that the slope of linear 1/C2 vs. potential curves is positive for n-type semiconductor and negative for p-type semiconductor.44,45 As shown in Figure 10, the slope of linear 1/C2-potential curves of BiOIO3 (Figure 10a) and BiOI (Figure 10b) is separately positive and negative, indicating that BiOIO3 and BiOI are n-type semiconductor and p-type semiconductor, respectively. By extrapolation to 1/C2 = 0, the flat potentials of BiOIO3 and BiOI are separately determined to be -0.8 and 1.92 V, versus the saturated calomel electrode (SCE).



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Figure 10. Mott−Schottky curves of (a) BiOIO3 and (b) BiOI. According to the above experimental results on charge carrier behavior and band energy levels, the mechanism for enhanced photocatalytic activity is speculated as follows: First, BiOIO3 and BiOI can form a p-n heterojunction for efficient charge separation. BiOIO3 is an n-type semiconductor whose Fermi level (EF) is close to conduction band (CB), and BiOI is a p-type semiconductor with the EF adjacent to the valence band (VB). When BiOI in-situ grows on the surface of BiOIO3, the band levels of BiOIO3 shifts downward, while the band levels of BiOI shifts upward until an EF equilibrium between BiOIO3 and BiOI was achieved. Eventually, the BiOIO3/BiOI p-n heterojunction was formed as shown in the right top of scheme 2. Based on this band structure, the photogenerated electrons (e-) would be transferred from the CB of BiOI to that of BiOIO3 and meanwhile the holes (h+) could be clustered on the VB of BiOI, due to the inducement of the built-in electric field in the space charge region.4 This band configuration can effectively promote the separation of photogenerated charge carriers. On the other hand, the BiOIO3/BiOI p-n heterojunction are constructed by coupling their reactive crystal facets, namely BiOI {001} facets and BiOIO3 {010} facets in particular. In this case, the internal 20 ACS Paragon Plus Environment

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self-built electric field of the two layered crystals can favorably transport the e- and h+ along the opposite direction, as illustrated in the right bottom of scheme 2. In this way, the charge separation was further greatly facilitated and the recombination is also largely reduced. Therefore, the photocatalytic activity of BiOIO3/BiOI composite is highly promoted.

Scheme 2. Schematic diagram for the possible charge separation process of BiOI/BiOIO3 composite.

CONCLUSIONS A thiourea-assisted room-temperature reduction route is utilized to in-situ reduce partial BiOIO3 to BiOI for fabricating the heterostructured BiOI/BiOIO3 p-n junctions. This 21 ACS Paragon Plus Environment


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synthetic approach leads to the dominant exposure of {010} active facet of BiOIO3, and the tunable facet-exposing percentage is also achieved. Due to the cooperative effects of p-n junction between the n-type BiOIO3 and p-type BiOI and the active exposing facets, the BiOI/BiOIO3 p-n junction exhibits photocatalysis and photoelectrochemical enhancement in MB degradation and photocurrent generation in comparison with the pristine BiOIO3 and BiOI. The highly promoted separation and transfer of the photogenerated e--h+ pairs are confirmed by electrochemical impedance spectroscopy as well as PL spectra. The present study shed new light on design and synthesis of semiconductor p-n junctions with active exposing facet by a facile route.

ASSOCIATED CONTENT Supporting Information SEM image, FFT image, temporal absorption spectral patterns and high-resolution electrochemical impedance spectra in high frequency zone. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * Hongwei Huang, E-mail: [email protected]. Tel: 86-10-82332247; [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT 22 ACS Paragon Plus Environment

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This work was supported by the National Natural Science Foundations of China (Grant No. 51302251), the Fundamental Research Funds for the Central Universities (No. 2652013052 and No. 2652015296).

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In situ Composition-Transforming Fabrication of BiOI/BiOIO3 Heterostructure: Semiconductor p-n Junction and Dominantly Exposed Reactive Facets Hongwei Huang,* Ke Xiao, Kun Liu, Shixin Yu, Yihe Zhang* Beijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid Wastes, National Laboratory of Mineral Materials, School of Materials Science and Technology, China University of Geosciences, Beijing 100083, China

BiOI/BiOIO3 p-n junctions with dominantly exposed reactive facets are fabricated via an in situ reduction route by using thiourea as reducing agent. The BiOI/BiOIO3 p-n junction exhibits highly improved photocatalysis and photoelectrochemical properties.


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Heterostructure: Semiconductor pân Junction - ACS Publications

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