Vertically Aligned Nanosheets-Array-like BiOI Homojunction: Three-in

Research Article pubs.acs.org/journal/ascecg

Vertically Aligned Nanosheets-Array-like BiOI Homojunction: Threein-One Promoting Photocatalytic Oxidation and Reduction Abilities Hongwei Huang,*,† Ke Xiao,† Xin Du,‡ and 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 ‡ Research Center for Bioengineering and Sensing Technology, Department of Chemistry & Biological Engineering, University of Science & Technology Beijing, Beijing 100083, China ABSTRACT: An unprecedented BiOI homojunction constructed by BiOI nanosheets and BiOI microplates was developed through charge-induced in situ precipitation assembly under ambient conditions. BiOI nanosheets are found vertically assembling on the surface of BiOI microplates to form a nanosheets-array-like hierarchical architecture due to electrostatic interaction. This BiOI homojunction casts not only increased adsorption, but also profoundly boosted photocatalytic performance for decomposition of multiplicate organic contaminants and water splitting into H2 evolution, contrasting with the individual nanosheets or microplates. The pronounced performance is ascribed to the synergistic effect from the following three aspects: (1) nanosheets-array-like hierarchical architecture endues BiOI homojunction increased surface area, which favors adsorption of reactants and generation of reactive sites; (2) the BiOI nanosheets forest induces an enhanced multiple reflection and scattering effect of light, engendering more photoinduced charge carriers; (3) more critically, band alignment in the BiOI homojunction impels the electron and hole separately to BiOI nanosheets and BiOI microplates, drastically promoting surface charge transfer efficiency and elevating the carrier density. The present work may furnish a new concept of designing efficient homojunctions with multiple advantageous factors and unique architectures in solar energy conversion. KEYWORDS: Homojunction, Nanosheets-array, BiOI, Photodegradation, H2 evolution

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into H2 and O2.13 Yu et al. reported that the well-engineered {101}/{001} facets coexposed TiO2 crystal demonstrates enhanced photocatalytic performance toward CO2 reduction into CH4.14 A crystal facet-based CeO2 homojunction composed of a {100} facets exposed hexahedron prism and a {111} facets exposed octahedron displays much enhanced CH4 generation performance from CO2 reduction.15 Pan et al. very recently synthesized a TiO2 p-n homojunction and employ it for high photoelectrochemical and photocatalytic hydrogen generation.16 Dong et al. in situ constructed the g-C3N4/g-C3N4 homojunction by calcination of a mixed precursor of urea and thiourea to achieve enhanced photocatalytic activity for NO removal.17 Another g-C3N4 homojunction example is the fabrication of acidified g-C3N4 and g-C3N4 composites by ultrasonic dispersion assisted electrostatic self-assembly, which shows higher photocatalytic activity for dye degradation.18 Layered bismuth-based photocatalytic materials, such as the bismuth halides BiOX (X = Cl, Br, I) series, project huge prospects in environmental remediation due to their strong photo-oxidation ability.19−26 As a typical member, BiOI has been paid much attention for its very narrow band gap (∼1.8 eV), with intense light-harvesting in the visible region.27,28 Very

INTRODUCTION Energy crisis and environmental deterioration issues enormously urge the development of semiconductor photocatalysis, as it was considered to be an efficient and green technology to dispose of these troubles through solar energy conversion.1−6 The photocatalytic activity of a semiconductor is intently associated with its surface physical and chemical properties. Heterojunction fabrication, which effectively facilitates surface charge separation and improves photocatalytic performance, thus attracts enormous efforts.7−12 Nevertheless, construction of a heterojunction is closely correlated to the crystal structures, energy band configurations, and surface/interfacial properties of constituting components, which makes it difficult to operate in practical applications. Homojunctions, constructed by the same semiconductor materials with different crystal phases, exposing facets or semiconductor types, etc., have recently aroused considerable interests.13−17 In comparison with heterojunctions built from different components, the homojunction displays many advantages in the above-mentioned respects, and it also introduces an internal field between the two parts for retarding the recombination of photoinduced carriers and keeping the movement of charge carriers across the interface. For instance, the formation of α-Ga2O3/β-Ga2O3 homojunction significantly promotes the charge separation transfer across the α−β phase interface and improves the photocatalytic overall water splitting © 2017 American Chemical Society

Received: February 26, 2017 Revised: April 19, 2017 Published: April 25, 2017 5253

DOI: 10.1021/acssuschemeng.7b00599 ACS Sustainable Chem. Eng. 2017, 5, 5253−5264

Research Article

ACS Sustainable Chemistry & Engineering Scheme 1. Formation Diagram of the BiOI Nanosheets@BiOI Microplates Homojunction

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recently, the photocatalytic performance of BiOI for CO2 reduction into CO and CH4 was also demonstrated, which helps to make it more attractive.23,29,30 To further improve its photoreactivity, numerous attempts have been made toward construction of BiOI heterojunctions, such as ZnO/BiOI,31 TiO2/BiOI,32 BiOCl/BiOI,33 BiOBr/BiOI,34 BiPO4/BiOI,35 BiVO4/BiOI,36,37 Bi12O17Cl2/BiOI,38 and BiOIO3/BiOI.39 Though certain successes have been achieved by these works, the utilization efficiency of the solar spectrum of the above heterojunctions is decreased more or less. Given the narrow band gap of BiOI and the above-mentioned advantages of homojunctions, crafty design of a BiOI homojunction may simultaneously realize high visible-light harvesting and efficient charge separation, which makes it highly alluring and full of expectation. In this work, we report an unprecedented size-based BiOI homojunction composed of BiOI nanosheets and microplates through in situ precipitation under ambient conditions. In this BiOI homojunction, BiOI nanosheets are all vertically assembled on the surface of BiOI microplates to form a nanosheets-array-like hierarchical architecture as a result of charge interaction. To systematically survey the advantages of this hierarchical homojunction, we first choose a charged dye model Rhodamine B (RhB) to inspect its adsorption ability, and then employed colorless bisphenol A (BPA) degradation and H2 production experiments to assess the photocatalytic oxidation and reduction capabilities, respectively. It is manifested that the BiOI homojunction shows not only enhanced adsorption ability, but also more importantly remarkably promoted all-round photocatalytic performance in contrast to the nanosheets and microplates individuals. A series of in-depth photoelectrochemical measurements disclose that the significantly elevated surface charge transfer efficiency and charge density derived from the band aligned homojunction take prominent roles in strengthening the photocatalytic activity in addition to increased surface area and photoabsorption. This study may further our understanding of fabrication of homojunctions with unique architectures for enhancing photocatalytic activity.

EXPERIMENTAL SECTION

All the chemicals are from commercial sources: Bi(NO3)3·5H2O (Sigma-Aldrich), KI (Aldrich), Rhodamine B (Sigma), bisphenol A (Aldrich), and methylviologen dichloride (Sigma-Aldrich) are of analytical purity grade, and are used without further purification. Synthesis of BiOI Microplates (MPs). BiOI microplates are synthesized by a precipitation method in aqueous solution at room temperature. 2 mmol of Bi(NO3)3·5H2O (0.970g) was put into 25 mL of deionized water under strong stirring, and then this suspension was dropwise added into 25 mL of water solution containing stoichiometric KI (2 mmol, 0.332g), and was kept stirring for 5 h. Afterward, the as-obtained products were collected by filtration and washed alternatingly with ethanol and deionized water, and dried at 60 °C for 10 h to obtain the BiOI microplates. Synthesis of BiOI@BiOI Homojunctions. The BiOI nanosheet@ BiOI microplate (NS@MP) homojunctions were synthesized via an ethylene glycol (EG)-assisted precipitation method at room temperature. First, 2 mmol of Bi(NO3)3·5H2O (0.970g) was dissolved in 25 mL of EG solution to a homogeneous solution, which includes a certain quantity of BiOI microplates. Then, the stoichiometric KI solution (containing 0.332 g of KI) to Bi(NO3)3·5H2O was dropped into the above suspension to obtain BiOI@BiOI homojunctions, as illustrated in Scheme 1. The collection process of products is the same as preparation of BiOI microplates. The BiOI@BiOI homojunctions obtained with molar ratios of BiOI microplates and Bi(NO3)3·5H2O of 1:1, 1:2, 1:3.5, and 1:5 (determined by their mass and molecular weight) are denoted as NS@MP-1, NS@MP-2, NS@MP-3, and NS@ MP-4, respectively. Synthesis of BiOI Microspheres (MSs). BiOI microspheres constructed by nanosheets were prepared by the same synthesis procedure with BiOI homojunctions without adding BiOI microplates according to the previous reference.40,41

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CHARACTERIZATION The crystalline phase of samples was examined by X-ray diffraction (XRD) on a Bruker D8 Advance diffractometer with Cu Kα radiation. A Hitachi S-4800 field emission scanning electron microscope (SEM) and a JEM-2100F transmission electron microscope (TEM) were used to characterize the microstructure and morphology. The zeta potential of samples was measured on a 90Plus Zeta potential instrument. Specific surface areas were determined on a 3020 Micromeritics instrument by the nitrogen adsorption−desorption method. Valence band (VB) X-ray photoelectron spectroscopy (XPS) was studied with 150 W Al Kα X-ray irradiation. A Varian Cary 5254


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ACS Sustainable Chemistry & Engineering 5000 UV−vis spectrophotometer was employed to record the UV−vis diffuse reflectance spectra (DRS). The fluorescence emission spectra were measured with irradiation of 250 nm light on a fluorescence spectrophotometer (Hitachi F-4600). Photocatalytic Degradation of Contaminants. The photocatalytic performance of BiOI series samples was first studied by degradation of Rhodamine B (RhB, 0.02 mM) and bisphenol A (BPA, 10 mg/L) with visible light illumination (500 W Xe lamp, λ > 420 nm) on a photochemical reactor (Bilon, Tianjin, China). Photocatalyst (50 mg) was put in 50 mL of RhB or BPA solutions with ultrasonic mixing. Before turning the light on, the mixture was stirred in darkness for 1 h to get an adsorption−desorption equilibrium between photocatalyst and contaminants. Then, they were exposed to visible light, and 2−3 mL of solutions was taken every 0.5 h. The supernatant liquid was extracted and centrifuged from the solutions, and then analyzed on a Cary 5000 UV−vis spectrophotometer to record the change of the characteristic absorbance bands of RhB or BPA. Photocatalytic H2 Evolution. The photocatalytic H2 production experiment was conducted in a 150 mL quartz reactor with a 300 W Xe lamp, and the reaction temperature was kept at 20 °C with cooling equipment. Typically, 50 mg of photocatalyst was suspended in 100 mL of deionized water with 20 mL of methanol as sacrificial agent and 1 wt % Pt as cocatalyst. Loading of 1 wt % Pt was carried out via photodeposition. H2PtCl6 was first dissolved in the abovementioned suspension. Then, this mixture was strongly stirred and illuminated by UV light for 30 min at room temperature to deposit Pt. Before starting the photocatalytic H2 production reaction, the suspensions were stirred in an ultrasonic bath, and nitrogen was bubbled into the reaction mixture for 0.5 h to exhaust the dissolved oxygen and to achieve anaerobic conditions in the reaction system. Then, the photocatalytic H2 evolution test was performed on an online photocatalytic reaction system (Labsolar-IIIAG system, Beijing Perfectlight Technology Co., Ltd., China). Hydrogen was detected with a gas chromatograph (GC7900, Tianmei Scientific Instrument Co., Ltd., China; TCD, nitrogen as a carrier gas and 5 Å molecular sieve column) at the given time intervals (1 h). Photoelectrochemical Tests. Photoelectrochemical data, such as photocurrent density and electrochemical impedance spectra (EIS), were recorded in a three-electrode quartz cell of a CHI-660E electrochemical system (Shanghai, China). The reference electrode, counter electrode, and working electrode are saturated calomel electrode (SCE), platinum wires, and photocatalyst films coated on ITO glasses, respectively. 0.1 M Na2SO4 was used as the electrolyte, and a 300 W Xe lamp with a 420 nm filter was used as the light source to provide visible light. Methylviologen dichloride (MVCl2) was added in the photocurrent onset scans to ensure the fast reaction kinetics at the electrode surface. The transient photocurrent was measured at 0.0 V. The working electrode was sampled by a dip-coating method: 30 mg of photocatalyst powder was dispersed in 2 mL of ethanol to be a homogeneous slurry, and then the suspension was dropped on a 10 mm × 20 mm indium−tin oxide (ITO) glass. The working electrode was dried at 373 K for 10 h to eliminate ethanol.

Figure 1. XRD patterns of BiOI microplates, BiOI microspheres, and BiOI homojunctions (NS@MP-1, NS@MP-2, NS@MP-3, and NS@ MP-4).

of BiOI samples can be assigned to the tetragonal BiOI phase (JCPDS #10-0445). Compared to BiOI microplates (MPs), the BiOI microspheres (MSs) constructed by nanosheets show an obviously broadened diffraction peak, which is attributed to the smaller crystalline size of nanosheets. It is significant to note that, with increasing the content of BiOI nanosheets, the diffraction peaks of BiOI nanosheets@BiOI microplates (NS@ MP) samples gradually widen and the peak intensity gradually reduces, confirming that more and more BiOI nanosheets are distributed on BiOI microplates. The NS@MP-3 sample shows the widest diffraction peak with lowest intensity, which suggests that it may contain the largest amount of nanosheets. XPS is performed to reveal the composition and bonding environment of related atoms. Figure 2a and b display the binding energies of bismuth and iodine elements. As seen from Figure 2a, the binding energies of Bi 4f5/2 and Bi 4f7/2 of BiOI microplates are 164.8 and 159.5 eV, respectively.37 Comparatively, a small shift to lower binding energies occurred for the two Bi3+ characteristic peaks of BiOI nanosheets (164.66 and 159.38 eV) and NS@MP-3 (164.74 and 159.44 eV). A similar phenomenon is also observed for the characteristic peaks of I 3d3/2 and I 3d5/2 (Figure 2b), which right-shift from 631.0 and 619.5 eV to 630.8 and 619.3 eV, respectively.38 The movement of the XPS peaks to lower binding energy in BiOI nanosheets and NS@MP-3 demonstrated that the coordination interaction around Bi and I atoms is weakened, which is in good accordance with the fact there are more unsaturated bond atoms in nanosheets compared to the micron-scaled counterparts.3 No big difference was found for O 1s XPS, so its data was not shown here. SEM was used here to investigate the microstructure and morphology. Figure 3a shows the SEM images of BiOI microplates synthesized by a precipitation method in aqueous solution. The products are composed of large smooth plates with size ranging from dozens of nanometers to several micrometers, which should result from the fast hydrolytic reactions. The large and flat BiOI microplates may provide a good platform for BiOI nanosheets to grow on. When ethylene glycol (EG) was introduced as the solvent to homogeneously dissolve Bi(NO3)3, EG can react with Bi3+ to form a homogeneous alkoxides Bi(OCH2CH2OH)2+ solution. When I− is introduced in this alkoxides solution, it would transform Bi(OCH2CH2OH)2+ into numerous thin nanosheets. Then, the small nanosheets self-assemble into uniform BiOI microspheres

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RESULTS AND DISCUSSION Phase Structure, Microstructure, and Investigation of Formation of the BiOI Homojunction. Figure 1 shows the XRD patterns of BiOI series samples. All the diffraction peaks 5255


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Figure 2. XPS spectra of BiOI microplates, BiOI microspheres, and the NS@MP-3 homojunction: (a) Bi 4f and (b) I 3d.

excessive (e.g., for NS@MP-4). Therefore, it can be concluded that the {001} facet of BiOI is positively charged, while the {110} facet of BiOI is negatively charged. Formation of this unique hierarchical architecture, that the BiOI nanosheets are vertically assembled on the surface of BiOI microplates, is owing to charge interaction. The BET specific surface area is investigated by the N2 adsorption−desorption method. As shown in Figure 5, the specific surface areas of MPs, NS@MP-1, NS@MP-2, NS@ MP-3, NS@MP-4, and MSs are 4.5, 10.1, 18.2, 27.1, 21.6, and 15.5 m2/g, respectively. Evidently, in comparison with the BiOI microspheres or BiOI microplates, the specific surface area can be largely enhanced by construction of the NS@MP architecture. NS@MP-3 possesses the largest surface area, which may result from its most uniformly arranged nanosheets. The enhanced specific surface area is believed to benefit the photocatalytic reaction. Photocatalysis Performance Evaluation. The photocatalytic performance of BiOI microspheres, BiOI microplates, and BiOI nanosheets@BiOI microplates architecture is first evaluated by degradation of Rhodamine B (RhB) with irradiation of visible light (λ > 420 nm). Because RhB is a type of azo dye with charge, the adsorption ability of photocatalysts can also be inspected. Figure 6a and b show the adsorption and photodegradation efficiencies of RhB over different samples under 1 h adsorption in darkness and 1 h visible-light irradiation, respectively. As shown in Figure 6a, BiOI microplates exhibit almost neglectable adsorption efficiency (approximately 2.8%), and the BiOI microspheres show a much higher RhB adsorption ratio of 30.3%, which is attributed to the higher surface area. Comparatively, the adsorption is greatly enhanced for the BiOI nanosheets@ BiOI microplates architectures except NS@MP-1, and NS@ MP-3 demonstrates a maximum RhB adsorption efficiency, which is up to 74.8%. In addition to the enhanced adsorption ability, it is obvious to see from Figure 6b that the photodegradation of RhB over most of the composites is also improved. The photodegratation also shows a trend of first increase and then decrease, which should be related to the specific surface area and active site, and NS@MP-3 still possesses the highest photodegradation activity with a RhB removal ratio of 81.8%. It is worth noting that NS@MP-1 shows comparable photocatalytic activity with MS, though the BET surface area and adsorption ability of NS@MP-1 are much lower than those of MS. This indicates that other reasons also

as a result of reducing surface energy (Figure 3f). Thus, one can speculate that the BiOI nanosheets may be evenly distributed if some appropriate substrates are provided. Figure 3b−e show the SEM images of the BiOI nanosheets@BiOI microplates composites. It obviously displays that the BiOI nanosheets are successfully assembled on the surface of BiOI microplates. It is significant to note that the BiOI nanosheets are vertically inserted onto the BiOI microplates to form a BiOI nanosheets forest instead of continuing growth of already existing surfaces. This nanosheets structure can offer larger specific surface area and more reactive sites, in favor of high photocatalytic activity. As the content of BiOI nanosheets increases, the distribution of BiOI nanosheets on BiOI microplates becomes more and more dense and compact. When the BiOI nanosheets content is too high, e.g. for NS@MP-4, some of them are self-aggregated into microspheres on the BiOI nanosheets@BiOI microplates architecture (Figure 3e). Notably, NS@MP-3 possesses the most uniformly arranged nanosheets structure among these samples (Figure 3d). The difference in microstructure may be closely associated with their specific surface area and photocatalytic activity, which will be discussed later. HRTEM is performed to confirm the unique architecture of BiOI nanosheets@BiOI microplates. Figure 4a displays the HRTEM images of BiOI microplates. Two sets of perpendicularly crossing lattice fringes with interplanar spacing of 0.28 nm are found, which can be indexed into the (110) and (1−10) planes, namely the {110} facet of BiOI. Thus, the exposed facet of BiOI microplates can be identified to be the {001} facets of tetragonal BiOI. The fast Fourier transform (FFT) pattern (inset of Figure 4a) also confirms this result. With respect to BiOI nanosheets@BiOI microplates (Figure 4b), one can find numerous dispersed lattice fringes with small area, verifying the tightly distributed BiOI nanosheets on BiOI microplates. The broad lattice fringes almost all possess a uniform interplanar spacing (about 0.91 nm), which is well indexed into the (001) plane of BiOI. In other words, the crystal facet of BiOI nanosheets in top-view is the {110} facet. It confirms the crossing coupling between BiOI nanosheets and BiOI microplates, as depicted in Figure 4c. To study the reason for forming such an interesting architecture, zeta potentials of samples are measured. The zeta potentials are +23.4, +30.3, +34.6, +44.2, +39.1, and +27.9 mV for MPs, NS@MP-1, NS@MP-2, NS@ MP-3, NS@MP-4, and MS, respectively (Figure 4d). Obviously, these samples are all positively charged, and their zeta potential orderly increases with raising the amount of BiOI nanosheets, and then decreases when BiOI nanosheets are 5256


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Figure 3. SEM images of (a) BiOI microplates, and (b) NS@MP-1, (c) NS@MP-2, (d) NS@MP-3, (e) NS@MP-4, and (f) BiOI microspheres.

contribute to the photocatalytic activity enhancement in addition to surface area, e.g. charge separation. In order to confirm the enhanced photocatalytic activity and simultaneously exclude the dye adsorption and photosensitization effects, colorless bisphenol A (BPA), a typical endocrine disruptor that can induce sexual and reproductive abnormalities, was utilized as a target. As seen from Figure 6c, BPA is almost not adsorbed on MPs, MSs, and NS@MP-3. In contrast, their photodegradation performance displays a large difference, and the BPA degradation efficiencies are 21.9%, 37.3%, and 77.2% for MPs, MSs, and NS@MP-3, respectively, under 1 h of visible-light irradiation (Figure 6d). This result, along with the

above RhB degradation, verified that the photocatalytic activity of BiOI microplates and BiOI microspheres can be improved by fabricating their composites, and simultaneously demonstrated that there exist some other factors other than surface area playing critical roles in the photodegradation process. A photocatalytic hydrogen production experiment is also conducted to assess the photocatalytic reduction capability of BiOI microplates, BiOI microspheres, and the BiOI homojunction. Methanol was used as a sacrificial reagent in water, and Pt was deposited on BiOI as a cocatalyst. As seen from Figure 7a, the BiOI homojunction shows an obviously enhanced H2 evolution activity compared to the two 5257


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Figure 4. HR-TEM images of (a) BiOI microplates and (b) NS@MP-3; (c) schematic diagram of NS@MP architecture; (d) zeta potentials of samples.

images of MSs, MPs, and NS@MP-3 show that there is a difference in their color (Figure 8c). As seen from Figure 8a, MSs show a shorter absorption edge than MPs, which is due to the nanosize (NS) effect.42 It is interesting to find that all the BiOI nanosheets@BiOI microplates composites demonstrate an extended absorption edge and enhanced photoabsorption in the UV−visible light region (e.g., 300−600 nm) compared to MSs and MPs (Figure 8a and b). The enhanced order of photoabsorption is NS@MP-1 < NS@MP-2 < NS@MP-4 < NS@MP-3. The enhancement in 300−600 nm is mainly owing to the multiple reflection and scattering of light in the BiOI nanosheets forest.43 To disclose the photoinduced carrier separation mechanism, the band structures of MSs and MPs, including band gap, conduction band (CB), and valence band (VB) levels, are determined by the Mott−Schottky method and VB XPS. Figure 8d shows that the band gaps of MPs and MSs are separately estimated to be 1.77 and 1.80 eV, in which the larger band gap of MS confirms the nanosize (NS) effect. Mott−Schottky plots (Figure 8e) reveal that the flat potentials of MPs and MSs are calculated to be −0.83 and −0.54 V versus the saturated calomel electrode (SCE), respectively, which are equivalent to −0.59 and −0.30 V versus the normal hydrogen electrode (NHE).11 The VB XPS spectra of MPs and MSs are shown in Figure 8f, which indicates that the energy gaps between the Fermi level (Evf) and the valence band are 1.63 and 1.50 eV for pure MPs and MSs, respectively.44 As the flat potential is approximately equal to the Fermi level, the VB positions of MPs and MSs are estimated to be 1.04 and 1.20 eV, respectively. According to their band gaps, the CB positions of MPs and MSs are −0.73 and −0.60 eV, respectively. The

Figure 5. BET surface areas of BiOI microplates, BiOI microspheres, and BiOI homojunctions (NS@MP-1, NS@MP-2, NS@MP-3, and NS@MP-4).

individuals, and the corresponding H2 production rates of MPs, MSs, and NS@MP-3 are 1.07, 1.51, and 2.40 μmol h−1 g−1, respectively (Figure 7b). This demonstrates that the photoreduction ability of BiOI can also be largely strengthened by fabricating a homojunctional architecture. Analyses on Enhanced Photocatalytic Activity. Generally, different sizes and microstructures of photocatalysts can cause some differences in their optical absorptions and band gaps.14−17 Photoabsorption of BiOI microplates, BiOI microspheres, and BiOI nanosheets@BiOI microplates architectures is studied via diffuse reflectance spectra (DRS). The digital 5258


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Figure 6. (a) Adsorption efficiencies of RhB in darkness for 1 h and (b) photocatalytic degradation efficiencies of RhB under visible light irradiation (λ > 420 nm) for 1 h over BiOI microplates, BiOI microspheres, and BiOI homojunctions (NS@MP-1, NS@MP-2, NS@MP-3, and NS@MP-4). (c) Adsorption efficiencies of BPA in darkness for 1 h and (d) photocatalytic degradation efficiencies of BPA under visible light irradiation (λ > 420 nm) for 1 h over BiOI microplates, BiOI microspheres, and NS@MP-3.

Figure 7. Photocatalytic H2 production curve (a) and the H2 production rates (b) of BiOI microplates, BiOI microspheres, and NS@MP-3 samples under irradiation of a 300 W Xe lamp.

scavenger methylviologen dichloride (MVCl2) in the electrolyte.45,46 Based on the literature, the photocurrent can be expressed as eq 1:

diagram of the band structures of MPs and MSs is displayed in Figure 8g. It is evident that MPs and MSs possess staggered band energy levels, which are matchable for forming a homojunction in favor of separating the photogenerated charge carriers. For the sake of evidencing the enhanced charge separation of BiOI homojunctions, systematical photoelectrochemical experiments are conducted. The surface charge transfer efficiency ηtrans can be determined through addition of the fast electrons

JH2O = Jmax ·ηabs ·ηsep ·ηtrans

(1)

herein JH2O and Jmax represent the recorded and the maximum theoretical photocurrent with an absence of electron scavenger, respectively; ηabs indicates the light absorption efficiency; ηsep is the charge separation efficiency inside the photoanode; ηtrans 5259


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Figure 8. (a) UV−vis diffuse reflectance spectra (DRS) and (b) enlarged DRS of BiOI microplates, BiOI microspheres, and BiOI homojunctions. (c) Digital images of MPs, MSs, and NS@MP-3. (d) Band gaps, (e) Mott−Schottky plots, (f) VB XPS, and (g) schematic band structures of BiOI microplates and BiOI microspheres.

(Figure 9b). Therefore, the surface charge transfer efficiency ηtrans is 15.8%, 20.3%, and 31.6% for MPs, MSs, and NS@MP-3, respectively (Figure 9d). This result strongly verifies the significantly promoted surface charge transfer of NS@MP-3 compared to the MPs and MSs, which stems from the fabrication of the energy-level-matched BiOI homojunction. Besides, the density of the charge carrier produced by photocatalysts was also investigated as an important parameter. It is reported that the onset potential of the photocurrent in a voltammograms can indicate the quasi Fermi level of majority carriers in the presence of a fast electron acceptor.46 Since there are almost not any overpotentials for the reduction of the fast electron acceptor MVCl2, the charge carrier can transfer to the external circuit to generate a photocurrent as soon as the applied bias reaches the quasi Fermi level. The relationship between the carrier density and the quasi Fermi level of MPs, MSs, and NS@MP-3 can be elucidated based on the Nernst equation:45,47

represents the surface charge transfer efficiency of the photoanode. When the electron scavenger MVCl2 was added, the surface charge transfer was very rapid and ηtrans was approximately 100%. The photocurrent in the presence of MVCl2 can be expressed in the following: JMV2+ = Jmax ·ηabs ·ηsep

(2)

As adding MV2+ did not alter the light absorption, pH, and flat potential of photoanodes, Jmax, ηabs, and ηsep are all the same for JH2O and JMV2+. Thus, through comparison of the photocurrent from water and MV2+ reduction, the surface transfer efficiency can be determined:45,46 ηtrans = JH2O /JMV2+

(3)

As shown in Figure 9a, the photocurrent densities of MPs, MSs, and NS@MP-3 are 0.38, 1.42, and 3.01 mA/cm2, respectively, in water. With adding MVCl2, their photocurrent densities enhance to 2.40, 6.98, and 9.53 mA/cm2, respectively 5260


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Figure 9. Photocurrent density of BiOI microplates, BiOI microspheres, and NS@MP-3 under visible light irradiation (λ > 420 nm) ([Na2SO4] = 0.1 M) (a) with and (b) without methylviologen dichloride (MVCl2). (c) Voltammograms of BiOI microplates, BiOI microspheres, and NS@MP-3 under visible light irradiation (λ > 420 nm) with MVCl2. (d) Calculated surface charge transfer efficiency and relative charge density. (e) EIS and (f) PL spectra of MPs, MSs, and NS@MP-3.

Ef1 − Ef2 = kT ln(Nf1 − Nf2)/e

Additionally, electrochemical impedance spectra (EIS) of MPs, MSs, and NS@MP-3 are also measured to confirm the above conclusion.48,49 As shown in Figure 9e, it is obvious the arc slopes of MPs, MSs, and NS@MP-3 are orderly reduced, which implies the gradually promoted interfacial charge transfer of these samples. Moreover, photoluminescence (PL) spectra were used to monitor charge recombination efficiency.50 It can be seen that all the BiOI homojunctions show reduced PL emission intensity (Figure 9f) compared to the two individuals, which means a retarded recombination rate of photogenerated electrons and holes. The above results corroborate that the separation and transfer of electrons and holes is significantly

(4)

where Ef1 and Ef2 are the quasi Fermi levels of sample 1 and sample 2, Nf1 and Nf2 are their carrier density, k is Boltzmann’s constant, T is the temperature, and e is the elementary charge. As displayed in Figure 9c, the potential is −0.25, −0.15, and −0.08 eV for MPs, MSs, and NS@MP-3, respectively. According to eq 4, the carrier densities of NS@MP-3 and MSs are 718 and 48 times that of MPs (Figure 9d). In other words, the carrier density of NS@MP-3 is determined to be 718 and 15 times higher than that of MPs and MSs. 5261


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Figure 10. Schematic diagrams for enhanced photocatalytic activity of BiOI@BOI homojunctions.

microplates to form a nanosheets-array-like hierarchical architecture due to charge inducement. Owing to this unique structure, the BiOI homojunction displays not only enhanced adsorption ability, but also significantly promoted photodegradation and photoreduction H2 evolution capabilities in comparison with the nanosheets and microplates. It is demonstrated that the nanosheets-array-like hierarchical homojunction induced increased surface area, strengthened photoabsorption, and more importantly highly elevated charge transfer efficiency and higher carrier density stemmed from the band alignment. The synergistic effect of the above advantages copromotes the prominent photocatalytic performance of the BiOI homojunction. We believe that our findings present a promising protocol for construction of homojunctional architectures for more efficient solar energy conversion applications.

facilitated in the BiOI@BiOI homojunctions, which play a critical role in advancing the photocatalytic activity. On that basis, the enhanced photocatalytic performance of BiOI nanosheets@BiOI microplates composites is summarized as illustrated by Figure 10: First, by construction of BiOI nanosheets@BiOI microplates architectures, the specific surface area of composite samples is increased compared to the BiOI microplates and BiOI microspheres. This is beneficial to the adsorption of organic contaminants, especially dyes (e.g., RhB), on the surface of photocatalyst as well as increasing the reactive sites, accelerating the photocatalytic reaction process. Second, the BiOI nanosheets@BiOI microplates architecture, particularly NS@MP-3, possesses a compact BiOI nanosheets forest. This results in a multiple reflection and scattering effect of light and photoabsorption of longer wavelength, strengthening the generation of photoinduced charge carriers. Additionally and more importantly, the staggered band energy levels of MPs and MSs enable them to form an effective BiOI@BiOI homojunction, which can induce an internal electrical field to reduce the barrier for charge transfer and suppress the recombination of the photogenerated electrons and holes. In that case, the photogenerated electrons produced from the CB of BiOI microplates can be quickly transferred onto that of BiOI nanosheets, and meanwhile the holes from the VB of BiOI nanosheets migrate to that of BiOI microplates, giving rise to more efficient charge separation and promoting the photocatalytic activity.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Hongwei Huang: 0000-0003-0271-1079 Notes

The authors declare no competing financial interest.

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CONCLUSIONS

In conclusion, the size-based BiOI homojunction consisting of BiOI nanosheets and microplates has been fabricated by a facile in situ precipitation strategy at room temperature. BiOI nanosheets are all vertically assembled on the surface of BiOI

ACKNOWLEDGMENTS

This work was jointly supported by the National Natural Science Foundations of China (No. 51672258, 51302251, and 51572246) and the Fundamental Research Funds for the Central Universities (2652015296). 5262


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Vertically Aligned Nanosheets-Array-like BiOI Homojunction: Three-in

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