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

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Vertically Aligned Nanosheets-array-like BiOI Homojunction: Threein-one Promoting Photocatalytic Oxidation and Reduction Abilities Hongwei Huang, Ke Xiao, Xin Du, and Yihe Zhang ACS Sustainable Chem. Eng., Just Accepted Manuscript • Publication Date (Web): 25 Apr 2017 Downloaded from http://pubs.acs.org on April 29, 2017

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ACS Sustainable Chemistry & Engineering

Vertically

Aligned

Nanosheets-array-like

BiOI

Homojunction: Three-in-one Promoting Photocatalytic Oxidation and Reduction Abilities Hongwei Huang,*,† Ke Xiao,† Xin Du,‡ 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 *Corresponding author. e-mail: [email protected] (Hongwei Huang); [email protected] (Yihe Zhang)

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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 enhanced multiple reflection and scattering effect of light, engendering more photoinduced charges carriers; (3) more critically, band alignment in 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 on 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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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 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 the photocatalytic performance, thus attracts enormous efforts.7-12 Nevertheless, construction of heterojunction is closely correlated to crystal structures, energy band configurations and surface/interfacial properties of constituting components, which makes it difficult to operate in practical applications. Homojunction, constructed by the same semiconductor materials with different crystal phases, exposing facets or semiconductor types, etc., has 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 into H2 and O2.13 Yu et al. reported that the well-engineered {101}/{001} facets co-exposed TiO2 crystal demonstrates enhanced photocatalytic performance toward CO2 reduction into CH4.14 Crystal facet-based CeO2 homojunction 3



composed of {100} facets exposed hexahedron prism and {111} facets exposed octahedron displays much enhanced CH4 generation performance from CO2 reduction.15 Pan et al. very lately synthesized TiO2 p-n homojunction and employ it for high photoelectrochemical and photocatalytic hydrogen generation.16 Dong, et al. in situ construct the g C3N4/g C3N4 homojunction by calcination of a mixed precursors 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 bismuth halides BiOX (X=Cl, Br, I) series, project huge prospect 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 visible region.27,28 Very lately, the photocatalytic performance of BiOI for CO2 reduction into CO and CH4 was also demonstrated, which helps to make it more attractive.29,23 To further improve its photo-reactivity, 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 work, the utilization efficiency on solar spectrum of above heterojunctions is decreased more or less. Given the narrow band gap of BiOI and above-mentioned advantages of homojunctions, crafty design of a BiOI 4


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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 employ colorless bisphenol A (BPA) degradation and H2 production experiments to assess the photocatalytic oxidation and reduction capabilities, respectively. It is manifested that 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 on fabrication of homojunction with unique architectures for enhancing photocatalytic activity.

EXPERIMENTAL SECTION 5



All the chemicals are from commercial source: Bi(NO3)3·5H2O (Sigma-Aldrich), KI (Aldrich), Rhodamine B (Sigma), bisphenol A (Aldrich) and methylviologen dichloride (Sigma-Aldrich) are of analytical purity grade, which are used without further purification. Synthesis of BiOI microplates (MP) BiOI microplates are synthesized by a precipitation method in aqueous solution at room temperature. 2mmol of Bi(NO3)3·5H2O (0.970g) was put into 25 mL deionized water under strong stirring, and then this suspension was dropwise added into 25 mL water solution containing stoichiometric KI (2mmol, 0.332g), and was kept stirring for 5 h. Afterwards, 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, 2mmol of Bi(NO3)3·5H2O (0.970g) was dissolved in 25 mL EG solution to be homogeneous solution, which includes a certain quantity of BiOI microplates. Then, the stoichiometric KI solution (containing 0.332g 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 same to preparation of BiOI microplates. The BiOI@BiOI homojunctions obtained with molar ratios of BiOI microplates and 6


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Bi(NO3)3·5H2O of 1:1, 1:2, 1:3.5 and 1:5 (determined by their mass and molecular weight) are donated as NS@MP-1, NS@MP-2, NS@MP-3 and NS@MP-4, respectively. Synthesis of BiOI microspheres (MS) Synthesis of BiOI microspheres (MS): 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 Characterization 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. 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 5000 UV-vis spectrophotometer was employed to record the UV-vis diffuse reflectance spectra (DRS). The fluorescence emission spectra were measured with irradiation of 250nm light on a fluorescence spectrophotometer (Hitachi F-4600). Photocatalytic degradation of contaminants The photocatalytic performance of BiOI series samples is first studied by degradation 7



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). 50 mg of photocatalyst was put in 50 mL of RhB or BPA solutions with ultrasonic. 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 are exposed to visible light, and 2-3 mL of solutions was taken every 0.5h. The supernatant liquid was extracted and centrifuged from the solutions, and then analyzed on a Cary 5000 UV−vis spectrophotometry 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 by a cooling equipment. Typically, 50 mg of photocatalyst was suspended in 100 mL of deionized water with 20 mL methanol as sacrificial agent and 1 wt% Pt as co-catalyst. Loading of 1 wt% Pt was carried out via photodeposition. H2PtCl6 was first dissolved in the above-mentioned 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 8


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photocatalytic H2 evolution test was performed on an online photocatalytic reaction system (Labsolar-IIIAG system, Beijing Perfectlight Technology Co., Ltd., China). Hydrogen was detected by 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, like 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 insure 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 10h to eliminate ethanol.

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RESULTS AND DISCUSSION Phase structure, microstructure and investigation on formation of BiOI homojunction Figure 1 shows the XRD patterns of BiOI series samples. All the diffraction peaks of BiOI samples can be assigned to the tetragonal BiOI phase (JCPDS #10-0445). Compared to BiOI microplates (MP), the BiOI microspheres (MS) constructed by nanosheets show 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 reduce, 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 composition and bonding environment of related atoms. Fig. 2a and b display the binding energies of bismuth and iodine elements. As seen from Fig. 2a, the binding energies of Bi 4f5/2 and Bi 4f7/2 of BiOI microplates are 164.8 eV and 159.5 eV, respectively.37 Comparatively, 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). Similar phenomenon is also observed for the characteristic peaks of I 3d3/2 and I 3d5/2 (Fig. 2b), which right-shift from 631.0 eV and 10


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619.5 eV to 630.8 eV and 619.3 eV, respectively.38 The movement of 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. Fig. 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 be resulted 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 homogenously 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 as a result of reducing surface energy (Fig. 3f). Thus, one can speculate that the BiOI nanosheets may be evenly distributed if some appropriate substrates are provided. Fig. 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 11



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 (Fig. 3e). Notably, NS@MP-3 possesses the most uniformly arranged nanosheets structure among these samples (Fig. 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. Fig. 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 {110} facet of BiOI. Thus, the exposed facet of BiOI microplates can be identified to be {001} facets of tetragonal BiOI. The fast Fourier transform (FFT) pattern (inset of Fig. 4a) also confirms this result. With respect to BiOI nanosheets@BiOI microplates (Fig. 4b), one can found numerous dispersed lattice fringes with small area, verifying the tightly distributed BiOI nanosheets on BiOI microplates. The broad lattice fringes almost all possess an 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 {110} facet. It confirms the crossing coupling between BiOI nanosheets and BiOI microplates, as depicted in Fig. 4c. 12


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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 MP, NS@MP-1, NS@MP-2, NS@MP-3, NS@MP-4 and MS, respectively (Fig. 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 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 N2 adsorption-desorption method. As shown in Fig. 5, the specific surface areas of MP, NS@MP-1, NS@MP-2, NS@MP-3, NS@MP-4 and MS 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 NS@MP architecture. NS@MP-3 possesses the largest surface area, which may be resulted 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 13



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. Fig. 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 Fig. 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 Fig. 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 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 is much lower than that of MS. It indicates that other reasons also 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 14


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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 Fig. 6c, BPA is almost not adsorbed by on MP, MS and NS@MP-3. In contrast, their photodegradation performance displays large difference, and the BPA degradation efficiencies are 21.9%, 37.3% and 77.2% for MP, NS and NS@MP-3, respectively, under 1 h visible-light irradiation (Fig. 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 photodegradation process. Photocatalytic hydrogen production experiment is also conducted to assess the photocatalytic reduction capability of BiOI microplates, BiOI microsphere and BiOI homojunction. Methanol was used as a sacrificial reagent in water, and Pt was deposited on BiOI as a co-catalyst. As seen from Fig. 7a, BiOI homojunction shows an obviously enhanced H2 evolution activity compared to the two individuals, and the corresponding H2 production rates of MP, MS and NS@MP-3 are 1.07, 1.51 and 2.40 µmol h-1 g-1, respectively (Fig. 7b). It demonstrates that the photoreduction ability of BiOI can be also largely strengthened by fabricating homojunctional architecture. Analyses on Enhanced Photocatalytic activity Generally, different size and microstructure of photocatalysts can cause some 15



differences in their optical absorption and band gap.14-17 Photoabsorption of BiOI microplates, BiOI microspheres and BiOI nanosheets@BiOI microplates architecture is studied via diffuse reflectance spectra (DRS). The digital images of MS, MP and NS@MP-3 show that there is difference in their color (Fig. 8c). As seen from Fig. 8a, MS shows a shorter absorption edge than MP, which is due to the nano-size (NS) effect.42 It is interesting to find that all the BiOI nanosheets@BiOI microplates composites demonstrate extended absorption edge and enhanced photoabsorption in UV-visible light region (e.g. 300-600 nm) compared to MS and MP (Fig. 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 photo-induced carrier separation mechanism, the band structures of MS and MP, including band gap, conduction band (CB) and valence band (VB) levels are determined by Mott–Schottky method and VB XPS. Fig. 8d shows that the band gaps of MP and MS are separately estimated to be 1.77 eV and 1.80 eV, in which the larger band gap of MS confirms the nano-size (NS) effect. Mott-Schottky plots (Fig. 8e) reveal that the flat potentials of MP and MS are calculated to be -0.83 and -0.54 V versus the saturated calomel electrode (SCE) electrode, respectively, which are equivalent to -0.59 and -0.30 V versus the normal hydrogen electrode (NHE).11 The VB XPS spectra of MP and MS are shown in Figure 8f, which indicates that the energy gap between the Fermi 16


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level (Evf) and valence band are 1.63 and 1.50 eV for pure MP and MS, respectively.44 As the flat potential is approximately equal to Fermi level, the VB positions of MP and MS are estimated to be 1.04 and 1.20 eV, respectively. According to their band gaps, the CB positions of MP and MS are -0.73 and -0.60 eV, respectively. The diagram of the band structures of MP and MS is displayed in Fig. 8g. It is evident that MP and MS 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 ŋtrans can be determined through addition of a fast electrons scavenger methylviologen dichloride (MVCl2) in the electrolyte.45,46 Based on the literatures, the photocurrent can be expressed as Equation (1): JH2O=Jmax·ŋabs·ŋsep·ŋtrans

(1)

herein JH2O and Jmax represent the recorded and the maximum theoretical photocurrent with absence of electrons scavenger, respectively; ŋabs indicates the light absorption efficiency; ŋsep is the charge separation efficiency inside the photoanode; ŋtrans represents surface charge transfer efficiency of the photoanode. When the electron scavenger MVCl2 was added, the surface charge transfer is very rapid and ŋtrans is approximately100%. The photocurrent in the presence of MVCl2 can be expressed in the following: 17



JMV2+= Jmax·ŋabs·ŋsep

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(2)

As adding MV2+ did not alter the light absorption, pH and flat potential of photoanodes, Jmax, ŋabs and ŋsep 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 ŋtrans= JH2O/JMV2+

(3)

As shown in Fig. 9a, the photocurrent densities of MP, NS 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 (Fig. 9b). Therefore, the surface charge transfer efficiency ŋtrans is 15.8%, 20.3% and 31.6% for MP, MS and NS@MP-3, respectively (Fig. 9d). This result strongly verifies the significantly promoted surface charge transfer of NS@MP-3 compared to the MP and MS, which stems from the fabrication of energy-level-matched BiOI homojunction. Besides, density of charge carrier produced by photocatalysts was also investigated as an important parameter. It is reported that the onset potential of photocurrent in a voltammograms can indicate the quasi Fermi level of majority carriers in the presence of fast electron acceptor.46 Since there are almost not any overpotentials for the reduction of fast electron acceptor MVCl2, charge carrier can transfer to the external circuit to generate photocurrent as soon as the applied bias reaches the quasi Fermi level. The relationship between carrier density and quasi Fermi level of MP, MS and NS@MP-3 can 18


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be elucidated based on the Nernst equation: 45,47 Ef1-Ef2= kTIn(Nf1-Nf2)/e

(4)

where Ef1 and Ef2 are the quasi Fermi level of sample 1 and sample 2, Nf1 and Nf2 are their carrier density, k is the Boltzmann’s Constant, T the temperature and e the elementary charge. As displayed in Fig. 9c, the potential is -0.25, -0.15 and -0.08 eV for MP, MS and NS@MP-3 respectively. According to equation 4, the carrier densities of NS@MP-3 and MS are 718 and 48 times that of MP (Fig. 9d). In other words, the carrier density of NS@MP-3 is determined to be 718 and 15 times higher than that of MP and MS. Additionally, electrochemical impedance spectra (EIS) of MP, MS and NS@MP-3 are also measured to confirm the above conclusion.48,49 As shown in Fig. 9e, it is obvious the arc slope of MP, MS, and NS@MP-3 is 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 all the BiOI homojunctions show reduced PL emission intensity (Fig. 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 are significantly facilitated in the BiOI@BiOI homojunctions, which play critical role in advancing the photocatalytic activity. On that basis, the enhanced photocatalytic performance of BiOI nanosheets@BiOI microplates composites are summarized as illustrated by Fig. 10: First, by construction of 19



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. Secondly,

BiOI nanosheets@BiOI microplates

architecture, particularly NS@MP-3, possesses compact BiOI nanosheets forest. It results in multiple reflection and scattering effect of light and photoabsorption of longer wavelength, strengthening the generation of photoinduced charges carriers. Additionally and more importantly, the staggered band energy levels of MP and MS 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 fast 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.

CONCLUSIONS In conclusion, 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 20


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microplates to form a nanosheets-array-like hierarchical architecture due to charge inducement. Owing to this unique structure, 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 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 co-promotes the prominent photocatalytic performance of BiOI homojunction. We believe that our findings present a promising protocol for construction of homojunctional architectures for more efficient solar energy conversion applications.

AUSTHOR INFORMATION Corresponding Author *E-mail: [email protected];

[email protected]

Phone: +86-010-82332247

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was jointly supported by the National Natural Science Foundations of 21



China (No. 51672258, 51302251 and 51572246), the Fundamental Research Funds for the Central Universities (2652015296).

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29



Scheme 1. Formation diagram of the BiOI nanosheets@BiOI microplates homojunction.

30


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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).

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

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

33



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

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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).

35



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.

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Figure 7. Photocatalytic H2 production cure (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.

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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 MP, MS 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.

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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 MP, MS and NS@MP-3.

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

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Vertically

Aligned

Nanosheets-array-like

BiOI

Homojunction: Three-in-one Promoting Photocatalytic Oxidation and Reduction Abilities Hongwei Huang,*,† Ke Xiao,† Xin Du,‡ Yihe Zhang*,†

BiOI homojunction constructed by nanosheets assembling on microplates was developed, which presents high photocatalytic performance for multiplicate contaminants degradation and H2 evolution.

41


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

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