Highly Conducting, n-Type Bi12O15Cl6 Nanosheets with Superlattice

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Highly conducting, n-type Bi12O15Cl6 nanosheets with superlattice-like structure Yoon Myung, Fei Wu, Sriya Banerjee, Andreea Stoica, Hongxia Zhong, Seung Soo Lee, John Fortner, Li Yang, and Parag Banerjee Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b03345 • Publication Date (Web): 27 Oct 2015 Downloaded from http://pubs.acs.org on November 1, 2015

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Chemistry of Materials

Highly conducting, n-type Bi12O15Cl6 nanosheets with superlattice-like structure Yoon Myung¹, Fei Wu¹, Sriya Banerjee¹, Andreea Stoica1, Hongxia Zhong3, Seung-Soo Lee2, John Fortner2, Li Yang3,4, and Parag Banerjee1,4,* 1 2

Department of Mechanical Engineering and Materials Science

Department of Energy, Environmental and Chemical Engineering, 3 4

Department of Physics,

Institute of Materials Science and Engineering

Washington University in St. Louis, St. Louis, MO 63130, USA * Corresponding author, E-mail: [email protected]

Abstract Modulating the type and magnitude of electrical conductivity remains a basic requirement for a semiconductor’s widespread acceptability and use. Here, we convert nanosheets of BiOCl, a V-VI-VII ternary semiconductor, to an oxygen-rich Bi12O15Cl6 phase. In the process, the intrinsic conductivity switches from p-type to n-type. The phase change is achieved using a vacuum annealing step at 500 oC for 1 hour. BiOCl nanosheets convert to Bi12O15Cl6 phase via volatilization of BiCl3 resulting in a unique superlattice like structure with a periodicity of 1.48 nm. Correspondingly, the band gap decreases from 3.41 eV to 2.48 eV from the raising of the valence band edge. Activation energy for electrical conductivity reduces from 862 meV for BiOCl to 778 meV for Bi12O15Cl6 and a corresponding photoconductivity increase of 80× is observed. Density functional theory calculations predict changes to the valence band and increase in the Fermi level towards the conduction band edge for the Bi12O15Cl6 nanosheets – in accordance with experimental data. The availability of both p and n-type ternary semiconducting systems widen the application base for Bi-O-Cl based materials.

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Introduction Bismuth oxychloride (BiOCl) is a V-VI-VII ternary, p-type semiconductor with many attractive properties that stem from its unique, layered atomic structure.1 BiOCl consists of 2D sheets of [Cl-Bi-O-Bi-Cl] vertically aligned in the [001] direction through non-bonding interaction between adjacent Cl--Cl- layers.2 Strong internal electric fields are generated along [001] due to the coulombic interaction of atomically adjacent [Bi2O2]2+ and Cl- layers. Historically, this effect has been exploited in BiOCl to develop photocatalysts.3 The perpendicular electric field is shown to aid charge separation and transport of electrons and holes produced via light absorption, so much so, that the (001) plane is five times more catalytically active than (100) or (010) plane.4, 5 The potential of BiOCl as a 2D semiconductor for a wide ranging set of applications such as photocathodes, sensors and nanoelectronic devices has not been fully exploited yet. Besides some of the properties described above, these applications specifically require that material systems first demonstrate an ability to tailor conductivity – its type and magnitude via simple but effective doping strategies. In the context of BiOCl nanostructuring,6 doping7-9 and coupling BiOCl with nanoparticles10 have been attempted. While these strategies aim at improving photocatalyst performance, intrinsic properties of BiOCl (its p-type behavior and low conductivity) have not been tailored or enhanced. Therefore, the first step to widen 2D BiOCl based applications is to explore within the Bi-O-Cl system, phases and polymorphs which can compliment or enhance properties of this unique 2D ternary semiconducting system. In this regards, only a few BiOCl phases have been explored. For example, Bi3O4Cl nanosheets have been reportedly synthesized using a hydrothermal method. These sheets show a low photocurrent density of 15 µA/cm2 under visible light illumination.11 Dye-sensitized solar cells using Bi24O31Cl10 with controlled Eg of 2.8 eV and power conversion efficiency of 1.5% have been reported also.12 2 ACS Paragon Plus Environment

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Herein we report on the structure and properties of vertically oriented, n-type Bi12O15Cl6 nanosheet arrays obtained from the parent, p-type BiOCl nanosheets by a simple vacuum annealing step. The discovery of this n-type phase opens up the possibility of a system of homogeneous Bi-O-Cl based structures for V-VI-VII nanoelectronic devices such as pn junctions and 2D field effect transistors. The Bi12O15Cl6 is O-rich and possesses a suite of attractive properties including lower activation energy for electrical conduction and an 80× higher photoconductivity than its parent BiOCl phase. The Bi12O15Cl6 phase has an Eg of 2.48 eV thus, making it suitable for visible light absorption and solar applications as well. An intriguing aspect of our work is that the structural modification resulting from the atomic rearrangement of the Bi3+, O2- and Cl- ions shows the formation of a periodic superlattice structure of the Bi12O15Cl6 nanosheets that has not been previously reported for the Bi-O-Cl system. As described in this paper, superlattice periodicity and the resulting properties can be tailored by control of the stoichiometry in the Bi-O-Cl system.

Experimental All reagents were purchased from Sigma-Aldrich (ACS quality), and were used as received without further purification. Before the deposition of BiOCl nanosheets, the ITO glass was cleaned by distilled water (D.I. water), ethanol, and acetone. The BiOCl nanosheets were deposited on ITO electrode using the successive ionic layer adsorption and reaction (SILAR) method.13 Typically, aqueous 10 mM of Bi(NO3)3·5H2O and 10 mM KCl solutions were prepared in separate glass beaker. In each cycle, the ITO substrate was immersed in Bi(NO3)3·5H2O solution for 30 s to ensure that a layer of Bi3+ ions was bonded on the surface of substrate. The substrate was then rinsed with D.I. water for 30 s to remove the weakly adsorbed ions and molecules. Afterwards, the substrate was immersed in KCl solution for 30 s, leading to the production of BiOCl, followed by rinsing in water. This process was 3 ACS Paragon Plus Environment


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repeated in the range of 10-60 cycles. The post-deposition thermal annealing processes were performed in a horizontal tube furnace in the temperature range of 100-500 oC under air and vacuum, respectively. The surface morphology was characterized by field-emission scanning electron microscopy (FE-SEM, JEOL-7001LVF) and field-emission transmission electron microscopy (FE-TEM, JEOL-2100F). X-ray diffraction (XRD) was performed on a Rigaku Geigerflex DMAX/A diffractometer. Raman spectra of the samples were obtained using a Renishaw® In Via Raman Microscope with a spot size < 1 µm2. The objective of the microscope was 50× with a numerical aperture (N.A.) of 0.75. The wavelength of the laser was 514 nm. The UVVis spectrophotometer (Varian, Cary 1000) was used to measure the optical properties. X-ray photoelectron spectroscopy (XPS) was measured using the PHI Versa Probe II spectrometer (Physical Electronics, Inc.) with a photon energy of 1486.6 eV (Al Kα). Photolithography was used to pattern a two electrode structure consisting of thermally evaporated metallic Al on a thermally grown SiO2 on Si substrate. The gap distance between the two electrodes was 20 µm. To collect Bi12O15Cl6 powder, 10 substrates were immersed in isopropanol solution and ultrasonication 1 hour. The collected powder was washed using an ultracentrifuge. A drop of 20 mg Bi12O15Cl6 nanosheets dispersed in 1 mL isopropanol solution was placed on the Al patterned electrode and the solvent was evaporated to form a uniform film covering the electrode. All electronic transport property measurements were carried out in a commercial probe station (Janis ST500-1-2CX) with Cu-Be probe tips, 50 µm in tip diameter. Temperature dependent current-voltage (I-V) tests were done with temperature varying from 295 K to 425 K, with a step of 5 K. The pressure in the chamber was maintained at atmosphere (1000 mbar). I-V measurements were made using a Keithley 2400 Source


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Measure Unit coupled to a Labview control program. The applied voltage was increased from 0 to 30 V, at a rate of 0.05 V per step. The electrochemical analysis was performed using a standard three electrode cell in combination with Biologic potentiostat (EC-LAB). The nanosheet arrays grown on the ITO substrate was used as the working electrode, a Pt foil auxiliary electrode as the counter electrode and Ag/0.01 M AgNO3 + 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) acetonitrile reference electrode (Bas Inc.). The conduction band (CB) energy was calculated from the onset of reduction potential (Ered) values, assuming the energy level of ferrocene/ferrocenium (Fc/Fc+) to be -4.8 eV below the vacuum level. The formal potential of Fc/Fc+ was measured by 0.075 V against an Ag/Ag+ reference electrode. Therefore, ECB (ELUMO) = -(Ered + 4.725) eV, where the onset potential values are relative to the Ag/Ag+ reference electrode. The valence band (VB) energy, EVB (EHOMO) was calculated with band gap from UV-Vis spectra, EVB (EHOMO) = ECB-Eg (indirect). The photoelectrochemical properties of nanosheet arrays were measured as follows. For the working electrode, we choose 30 cycle SILAR substrate within nanosheet thickness dependence as shown in Figure S1. The standard three electrode cell was used consisting of the working electrode of nanosheets arrays having a surface area of 0.25 cm2, a Pt foil as the counter electrode, and a saturated Ag/AgCl reference electrode. A 500 W Xe arc lamp was utilized as a light source. A 0.5 M Na2SO4 aqueous solution was used as the electrolyte. Mott-Schottky measurements were made with SILAR films deposited on ITO glass electrodes. A small signal ac amplitude of 10 mV at 1 KHz was used while the dc bias was swept from -1.0 V to + 1.0 V. First-principles density functional theory (DFT) simulations were made to study the electronic structure of BiOCl. The DFT calculations with the Perdew-Burke-Ernzerh (PBE)14 functional have been carried out by using the Vienna Ab initio Simulation Package (VASP)15, 5 ACS Paragon Plus Environment


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16

with a plane wave basis set and the projector-augmented wave method.17 The force and

stress are fully relaxed within DFT/PBE.

Results and Discussion SILAR of BiOCl samples: Morphology of the as-synthesized BiOCl was confirmed by FESEM. Figure 1 shows the variation of BiOCl morphology with an increase in cycle numbers. The SEM image confirms that BiOCl nanosheet arrays were successfully deposited on ITO substrate. Initially, BiOCl was found to form on the ITO substrate in the morphology of nanosheet arrays within 10 cycles (Figure 1a) with an increase in nanosheet density observed with additional cycles (up to 30 cycles) (Figure 1b and 1c). The average sheet thickness was found to be ~ 30 nm. When the SILAR cycles were increased to 40 cycles and higher, BiOCl flakes aggregated into spheres and were deposited over the nanosheet arrays. (Figure 1d). Figure 1e inset images shows the thickness of these flakes is ~ 60 nm after 50 cycles of SILAR. These flakes covered the entire ITO surface after 60 cycles (Figure 1f). Additionally, the chemical composition and phase of the BiOCl was confirmed using Raman spectroscopy on all the substrates (Figure S2). The main advantage of the SILAR method over other deposition processes is that it offers a simple and low-cost fabrication methodology while possessing many of the characteristics of atomic layer deposition (ALD) including a ‘pseudo’ layer-by-layer deposition of the material.18,

19

During SILAR deposition, cationic and anionic solutions

alternately react, typically via a precursor decomposition process on the surface of the immersed substrate to yield a heterogeneous nucleation and growth mechanism of the desired film.20 To control nanosheet size and density, Bi(NO)3 and KCl were used as precursors. Generally, Bi(NO3)3 in D.I. water, which serves as the cationic solution, dissociates to give 6 ACS Paragon Plus Environment

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Bi3+ and NO3- ions; KCl dissolved in D.I. water dissociates to give K+ and Cl-. Bi3+ ion readily hydrolyzes to form BiO+ which are unstable in the presence of Cl- anions, and precipitate as crystalline BiOCl. The entire reaction mechanism is as follows21 Bi3+ + 2H2O ↔ Bi(OH)2+ + H3O+ BiOH2+ + H2O ↔ BiO+ + H3O+ BiO+ + Cl- ↔BiOCl (s) During the first dip, BiO+ adsorbs to the substrate surface. The DI water rinse removes unbound ions, and the second dip in Cl- facilitates the reaction with the surface bound BiO+ to yield BiOCl at BiO+ sites. The rinsing step between successive precursor dips in the SILAR deposition scheme promotes heterogeneous growth because unbound ions are washed away, resulting in a BiOCl film in intimate contact with substrate. However, after 40 cycles nanoflakes aggregate together as spherical particles and deposit above the BiOCl nanoflakes. These results indicate that residual BiO+ and Cl- ions may be left in the narrow spaces between the nanoflakes and unwanted cross reactions may promote nucleation and growth of additional spherical, nanoflower-like aggregates.22 In any case, our results show the necessity of optimizing nanostructure assemblies in an orderly manner. We choose 30 cycles SILAR (Figure 1c) as our standard nanosheet array for studying the effect of annealing on BiOCl nanosheets. Effect of annealing: As stated before, 30 cycle SILAR BiOCl nanosheets were subjected to annealing in the temperature range of 100-500 oC under air and vacuum. In this section, we look at the structure of the nanosheets which underwent phase transformation using TEM. The typical morphology of BiOCl nanosheets is shown in Figure 2a. The low magnification TEM image exhibits individual nanosheet has triangular shape with an average diameter ~ 1 7 ACS Paragon Plus Environment


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µm. Figure 2b shows the lattice resolved image of the nanosheet. Clear lattice fringes with a d-spacing of 0.28 nm is noted. This is consistent with BiOCl (110) planes of reference (JCPDS No. 85-0861; a=3.890 Å, c=7.370 Å). A corresponding FFT-ED pattern (Figure 2b, inset) generated by high resolution (HR) TEM image could be indexed to the (110) and (200) planes of the tetragonal system of BiOCl. SEM evaluation of 30 cycle SILAR BiOCl sample subjected to 500 oC vacuum annealing (Figure S3) confirms the overall nanosheet morphology does not change even after the heat treatment. Figure 2c shows HR-TEM image of vacuum annealed at 400 oC nanosheet. Although the domains are not uniform in size, two phases can be observed. Generally, BiOCl phase and growth direction has not changed in the middle of the nanosheet. However, a dspacing of 0.51 nm is observed which are consistent with (800) plane of the Bi12O15Cl6 orthorhombic crystal structure (JCPDS No. 70-0249; a=40.530 Å, b=3.868, c=15.487 Å). Additionally, the edge of the nanosheet contains 0.23 nm d-spacing which is indexed for Bi12O15Cl6 (615). Another 400 oC vacuum annealed nanosheet is observed to be composed of mixed phases of BiOCl and Bi12O15Cl6 (Figure 2e). The Bi12O15Cl6 has a superlattice structure that consists of periodic 1.11 nm long slabs along the [12,00] direction. This superlattice like formation consists of two Bi12O15Cl6 (211) planes and one (12,00) in one period. In the FFTED pattern, both [211] and [12,00] spots (d=0.37 nm and d=0.34 nm, respectively) are subdivided into 3 parts, due to the superlattice structure (Figure 2d, inset). Moreover, BiOCl (200) spot overlaps with the Bi12O15Cl6 (422). In Figure 2e the HR-TEM image and FFT ED pattern of vacuum annealed 500 oC nanosheet shows the presence of uniform superlattice of the Bi12O15Cl6 phase. In our TEM scanning we could not find any evidence of the BiOCl phase in this sample. Also, the 8 ACS Paragon Plus Environment

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observed periodicity increases from 1.11 nm for the 400 oC annealed sample to 1.48 nm for the 500 oC vacuum annealed sample. To further understand the periodicity observed, we have reconstructed the superlattice image obtained in Figure 2e in Figure 2f. This reconstruction is done with the aid of Gatan® image analysis software. This superlattice like formation consists of two Bi12O15Cl6 (211) planes and two (12,00) in one period. Therefore, we propose a relationship between superlattice periodicity and the planes involved as follows: d(superlattice periodicity) = x∙d(211) + y∙d(12,00) Here d(211) = 0.37 nm and d(12,00) = 0.34 nm. For the vacuum 400 oC sample, x=2 and y=1 for a superlattice periodicity of 1.1 nm. For vacuum 500 oC sample x=2 and y=2 for a superlattice periodicity of 1.48 nm. A higher Cl loss at 500 oC can explain this increase in the periodicity of the Bi12O15Cl6. XRD Characterization: Figure 3 shows the XRD patterns of all the thermally annealed, 30 cycle SILAR substrates tested. Figure 3a describes the thermally annealed substrates in air. All the samples showed the tetragonal BiOCl structure. Thus, the 30 cycle SILAR BiOCl samples are stable in air annealing at temperatures up to 500 oC 1 hour. Figure 3b describes the effect of vacuum annealing on the 30 cycle SILAR samples. Phase stability is maintained when annealing up to 300 oC. For 400 oC, a mixed BiOCl and Bi12O15Cl6 structure is observed. This is in line with the HRTEM observation (Figure 2c and 2d). For 500 oC annealed substrate, only Bi12O15Cl6 phase is observed. Again, the XRD data matches well with the HRTEM data in Figure 2e. For indexing the XRD peaks, the tetragonal BiOCl and orthorhombic Bi12O15Cl6 structure pattern at the bottom of both graphs is also provided. (JCPDS No. 85-0861; a=3.890 Å, c=7.370 Å and JCPDS No. 70-0249; a=40.530 Å, b=3.868, c=15.487 Å). In addition, we observe increasing 9 ACS Paragon Plus Environment


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peak intensity of (12,00) plane as a function of increasing superlattice periodicity in line with the presence of this plane in HR TEM data. Raman characterization: Figure 4a and b shows Raman spectra of the annealed substrate in air and vacuum, respectively. Figure 4a shows air annealed substrates do not undergo any changes to the peak intensity or position when compared to the as-deposited, 30 cycle SILAR BiOCl nanosheet arrays. Three peaks located at 144, 199, and 398 cm-1 are observed which are related to the A1g internal Bi-Cl stretching mode, Eg internal Bi-Cl mode and Eg and B1g modes involving the motion of oxygen atoms, respectively.23 In Figure 4b, the samples do not show any difference in peak position or intensity until 300 oC vacuum annealing. For 400 oC and 500 oC vacuum annealed substrates, the A1g peak starts to shift to higher wavenumbers (blue shift) due to the presence of Bi12O15Cl6 and lower densities of Bi-Cl bonds in the nanosheet arrays. Moreover, Eg and B1g bands have increased peak intensity due to the increase freedom of motion of oxygen atoms in Bi12O15Cl6 as compared with BiOCl.24, 25 These structural changes observed by Raman spectroscopy support previously discussed TEM and XRD results. Mechanism of formation of superlattices of Bi12O15Cl6: The general method for the synthesis of Bi12O15Cl6 is based on mixing Bi2O3 with BiOCl in an 1:2 ratio.26 However, in our work we find that Bi12O15Cl6 can be formed under vacuum annealing conditions > 400 oC 1 hour. Here we note that while BiOCl has melting point of 500 oC, BiCl3 has a melting point of 227 o

C while a boiling point at 447 oC only. Thus, we hypothesize that the volatilization of BiCl3

from the BiOCl nanosheets is responsible for Bi12O15Cl6 phase formation. This reaction is given as: 15BiOCl (s) → BiCl3 (g) + Bi12O15Cl6 (s)


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The air annealed samples do not show the formation of Bi12O15Cl6 phase. The reason could be attributed to the high partial pressure of oxygen in the air annealing furnace. Available oxygen can have a stabilizing affect on the Cl- in the BiOCl nanosheet allowing for the immediate oxidation of any BiCl3 and thus, preventing it from volatilizing. Indeed, oxygen partial pressure based stability is shown for ZnOCl system which has a volatile ZnCl2 component as well.27 The following superlattice formation mechanism is proposed. In the 400 oC under vacuum condition, Figure 2d and 2e shows Bi12O15Cl6 (800), (615), (211), (12,00) planes which are similar to the BiCl3 (101), (311), (200), (121) of reference (JCPDS 70-1519; a=7.641, b=9.172, c=6.291, orthorhombic structure), respectively. Thus, as BiCl3 volatilizes, it could leave behind an epitaxially matched Bi12O15Cl6 phase. During such a thermal decomposition reaction, Bi2O3 and BiOCl rearrange due to their non-stoichiometric unit cell structure. As a result, vacuum annealed 500 oC annealed nanosheets have lesser Cl content and therefore longer superlattice periodicities than 400 oC annealed nanosheets. In addition, it has been suggested that the Bi12O15Cl6 contains atomically twinned structure.26, 28 In this structure, Bi3+ with the coordination number of four and five co-exist forming a network of polyhedrons of bismuth - oxygen units, i.e., [Bi12O15]6+ while in the voids, columns of chlorine ions reside in the form of trigonal prisms. The [Bi12O15]6+ polyhedrons form a zigzag, quasi periodic network parallel to the XZ plane. This network could be responsible for the superlattice periodicity observed in the HR TEM. Surface characterization using XPS: Figure 5 shows XPS spectra of selected samples for asdeposited, vacuum 400 oC and 500 oC annealed. First, survey scanned XPS spectra is shown in Figure 5a. All of the samples are composed of the elements Bi,O, Cl and C without residual contamination. The carbon peak comes from the adventitious carbon on the surface of the samples. Figure 5b shows XPS fine spectra of the Bi 4f peaks of the samples. For the 11 ACS Paragon Plus Environment


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as deposited BiOCl, it can be seen that two strong peaks at 158.9 and 164.2 eV are assigned to Bi 4f7/2 and Bi 4f5/2 which correspond to Bi3+ in BiOCl. This peak red shifts when the annealing temperature is 500 oC. This observation can be attributed to the fact that the Bi3+ in Bi12O15Cl6 has a lower charged state than the Bi3+ in BiOCl. Similar to the Bi3+ peaks, the O 1s peaks also blue-shifts by 0.2 eV, indicating a decrease in the number of oxygen related defect sites on the surface. This is shown in Figure 5c.

However, the Cl 2p peaks do not have a distinct shift (Figure 5d). To explain these observations, we note that the reported bond distance in BiOCl

consist of Bi-Bi (3.71 Å), Bi-O (3.07 Å), and Bi-Cl (3.07 Å), respectively. On the other hand, Bi12O15Cl6 possess Bi-Bi (3.553 Å ), Bi-O (2.03-2.69 Å), and Bi-Cl (3.05-3.26 Å).26 XPS results are consistent with the fact that Bi-Bi and Bi-O bond distance are shortened during the phase transformation of BiOCl to Bi12O15Cl6 and thus influences Bi 4f and O 2p fine spectra as shown in Figure 5b and Figure 5c, respectively, but does not influence Cl related binding state. Optical characterization and band edge measurements: Figure 6 shows the diffuse spectra of as deposited, vacuum 400 oC and 500 oC annealed substrate. The as deposited sample shows an absorption edge near 360 nm which is related to pure BiOCl. The residual absorption in the visible light range is related to oxygen vacancy on the nanosheet surface.29 The BiOCl/Bi12O15Cl6 sample shows a slightly red-shift absorption edge near 400 nm. However, the 500 oC annealed, pure Bi12O15Cl6 shows a significant increase in absorption with an absorption edge near 500 nm. These spectra were also used to estimate the band gap (indirect) by performing the Kubelka-Munk transformation.30 (Figure 6, inset) A plot of [αhν]1/2 (where α is the absorption coefficient) versus photon energy yields a band gap of 3.41, 3.07, and 2.48 eV, for the as deposited, 400 oC and 500 oC samples, respectively.


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For more detailed understanding of band edge positions of as-deposited BiOCl, vacuum 400 oC and 500 oC annealed samples, CV was measured using a scan rate of 20 mV/s. As shown in Figure 7, BiOCl has the reduction peak edge at -1.11 V. Thus, the BiOCl conduction band edge position was calculated as, ECB = -3.62 eV with respect to vacuum, whereas valence band position was calculated to EVB = ECB – Eg = -7.03 eV. For the 400 oC and 500 oC samples, the conduction band position was calculated to be -3.7 to -3.75 eV, respectively. However, the valence band position were calculated to be EVB = -6.77 and -6.23 eV, respectively. Therefore, while the conduction band edge maintains its position as a function of vacuum annealed temperature, the valence band increases its energy (Figure 7b). Electrical characterization: Current-voltage (I-V) characteristics of Bi12O15Cl6 nanosheets were measured under ambient atmosphere (1000 mbar) condition. Figure 8a shows the I-V of a representative 500 oC annealed (pure Bi12O15Cl6) sample. All I-V’s measured were linear in nature indicating ohmic conduction behavior. The variation of ohmic conductivity (σ) allows us to calculate the activation energy (Ea) of conduction as, σ = σo e

E − a kT

Thus, from the Arrhenius equation above, we can obtain the activation energy. For the 500 oC annealed substrates i.e., fully converted Bi12O15Cl6, Ea ~ 778 meV. This value is lower than our previous reported BiOCl nanosheet with an Ea of 862 meV by 84 meV (Figure 8b).31 The observed lower activation energy indicates that the barrier to charge conduction in the Bi12O15Cl6 phase is lower than the original BiOCl phase. Electrochemical characterization: Mott-Schottky curves (1/capacitance2 vs. voltage) were obtained for BiOCl in Figure 9a and BiOCl/Bi12O15Cl6 and Bi12O15Cl6 samples in Figure 9b, respectively. p-type conductivity is clearly observed with a negative slope of the BiOCl in Figure 9a, in line with recent findings of Bai et al.32 In contrast, n-type conductivity (i.e., 13 ACS Paragon Plus Environment


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positive slope) was detected for both BiOCl/Bi12O15Cl6 and Bi12O15Cl6 samples. Previously, Bi-rich phases have shown similar Mott-Schottky curves and a propensity to generate molecular oxygen.33 Thus, our data indicates that the superlattice structure of Bi12O15Cl6 derived by the removal of BiCl3 from BiOCl converts the p-type BiOCl to n-type Bi12O15Cl6. Dielectric constants for BiOCl and Bi12O15Cl6 have not been previously reported in literature. Therefore, at present the carrier concentrations for these samples could not be estimated. Further, it is not exactly clear why mixed BiOCl/Bi12O15Cl6 phase shows n-type behavior. This could be related to the pn junction formed in these nanosheets which creates a surface depleted p-type region or, an ‘apparent’ n-type surface. Flat-band voltages of +0.74 V, -0.90 V and -0.69 V were obtained for BiOCl, BiOCl/Bi12O15Cl6 and Bi12O15Cl6, respectively. Photoelectrochemical data: Figure 10a shows the photocurrent density J (μA/cm2) vs. applied potential V (vs. Ag/AgCl) curves using BiOCl, BiOCl/Bi12O15Cl6, and Bi12O15Cl6 samples under Xe light illumination of 100 mW/cm2. The BiOCl photoresponse (symbol, ∆) is very low and is therefore shown amplified to 50X for clarity. The behavior of the BiOCl is photocathodic with a current density of only 1.2 µA/cm2 at -0.2 V. Light response on the cathodic side is a characteristic of a p-type material – which is to be expected from BiOCl.31, 34-36

The 400 oC sample which is a mixture of BiOCl/Bi12O15Cl6 (symbol, ) is slightly

photoanodic and again, a weak light response with a current density of 6 µA/cm2 is obtained at +0.2V. As expected, the most significant effect in the photoelectrochemical data comes from the 500 oC annealed sample which is purely Bi12O15Cl6. The sample demonstrates a fully developed photoanodic behavior (symbol, O) indicating its complete conversion to an ntype material.10, 12 This is in line with data from Figure 9b. Furthermore, this sample shows significantly higher photocurrent density of 131 µA/cm2 . 14 ACS Paragon Plus Environment

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Transient photoresponse of the three samples are shown in Figure 10b. In line with the photoresponse data, Bi12O15Cl6 displays the highest photocurrent density (114 µA/cm2) which is 45× higher than mixed phase BiOCl/Bi12O15Cl6 sample and 80× higher than pure BiOCl. Fast photoresponse (time response of < 1 sec and faster than the data recording frequency) is obtained in all cases, indicating good crystalline quality of the assembled nanosheets. Taken together, the photoelectrochemical data demonstrates that the BiOCl Bi12O15Cl6 conversion causes an increase in the conductivity and its switch from p-type to ntype. DFT calculations: As shown in Figure 11a, bulk BiOCl exhibits an indirect band gap. In particular, the projected band structure shows that the main component of the top of valence band is from O and Cl atoms and the bottom of conduction band is mainly from Bi atoms. This is consistent with our measured results which show that removal of Cl from the BiOCl lattice to form Bi12O15Cl6 results in the rise of the valence band edges, while the conduction band edge remains unaffected (Figure 7b). Next, the effect of removing Cl atoms was evaluated as shown in Figure 11b, whereby simulation of a 2x2x2 supercell (including 48 atoms) with one Cl vacancy is shown. Such a situation is sufficient to provide qualitative trends; however the absolute values may not be accurate due to the intrinsic deficiency of DFT and limited-size of the supercells. From these calculations, two observations can be made. First, we observe that the Fermi level is crossing the conduction bands, indicating that a Cl depleted BiOCl system is ndoped. This is consistent with experimental measurements from Figure 9b as well as Figure 10a. Second, the band gap is not significantly perturbed. This seems to be in contradiction with our measurements. However, it is known that DFT cannot give reliable band gaps and 15 ACS Paragon Plus Environment


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many-electron interactions have to be included for correct values.37 In particular, for such a quasi-2D structure, recent studies have shown that doping can substantially reduce band gaps by a few-hundred meV.38 Therefore, it is reasonable to expect the band gap reduction in our experimental data (Figure 7b) is due to removal of Cl atoms.

Conclusions In summary, BiOCl nanosheet arrays were synthesized by successive ionic layer adsorption and reaction (SILAR) onto a transparent ITO electrode. Results indicate that 30 cycle SILAR deposited BiOCl nanosheets effectively cover the entire surface of ITO electrode. A vacuum assisted annealing step at 500 oC can volatilize BiCl3 from the single crystalline BiOCl nanosheets, resulting in O-rich, superlattice Bi12O15Cl6 phase structures. Removal of Cl from BiOCl occurs in a systematic manner and depending on the amount of Cl removed, the Bi12O15Cl6 superlattice periodicity varies between 1.11 nm to 1.48 nm. While exact mechanisms of the observed systematic removal of Cl remain unclear, the periodicity clearly plays a role in modulating the optical and electronic properties of the Bi12O15Cl6 phase. The Bi12O15Cl6 is an n-type semiconductor. This is in contrast to BiOCl which is ptype. The band gap of the Bi12O15Cl6 phase is 2.48 eV as opposed to BiOCl, which is 3.41 eV. The reduction in band gap is due to the Cl removal, resulting in the raising of the valence band edge. This occurs primarily due to the O and Cl character of the valence band. Further, the Bi12O15Cl6 phase shows lower activation energy (by 84 meV) for electronic conductivity and 80× higher photoconductivity when compared to pure BiOCl nanosheets. Experimental results are supported by first principles DFT calculations for the electronic structure of BiOCl and O-rich BiOCl. The calculations predict that Cl removal leads to n-type behavior in BiOCl. The discovery of an n-type phase in the Bi-O-Cl system 16 ACS Paragon Plus Environment

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underpins the possibility of future nanoelectronic devices for this versatile and diverse V-VIVII ternary semiconductor. Supporting Information Thickness variation and Raman of SILAR substrates, SEM image of 30 cycle 500 oC sample. The supporting Information is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgements Start up funds from Washington University is acknowledged. Partial support for this work was provided under the US-India Partnership to Advance Clean Energy-Research (PACE-R) for the Solar Energy Research Institute for India and the United States (SERIIUS), funded jointly by the U.S. Department of Energy (Office of Science, Office of Basic Energy Sciences, and Energy Efficiency and Renewable Energy, Solar Energy Technology Program, under Subcontract DE-AC36-08GO28308 to the National Renewable Energy Laboratory, Golden, Colorado and the Government of India, through the Department of Science and Technology under Subcontract IUSSTF/JCERDC-SERIIUS/2012 is acknowledged. Partial support for this work comes from the International Center for Advanced Renewable Energy & Sustainability (ICARES) at Washington University. XPS support was provided through CBET NSF MRI Grant No. 1337374. Support from the instrumentation facilities of the Institute of Materials Science and Engineering, Professor Singamaneni and Biswas Labs for Raman and photoelectrochemical measurements are acknowledged.



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LIST OF FIGURES

Figure 1. SEM micrograph of BiOCl nanosheet arrays on ITO substrates after (a) 10, (b) 20, (c) 30, (d) 40, (e) 50, and (f) 60 cycles.


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Figure 2. (a) TEM image showing a general morphology of BiOCl nanosheet. (b) HRTEM and FFT ED patterns of BiOCl nanosheet. (c) HRTEM image of 400 oC vacuum annealed nanosheet consists of BiOCl and Bi12O15Cl6 separate domains. (d) Another 400 oC nanosheet shows superlattice with BiOCl. (e) 500 oC shows Bi12O15Cl6 superlattice structure and (f) reconstructed image of the marked area in (e) produced by Gatan digital micrograph GMS 1.4 software (Gatan Inc.).



o

400 C

o

300 C

o

200 C

o

300 C

20

30

40

(113)

(200)

(112)

(003)

BiOCl 85-0861 Bi12O15Cl6 70-0249

BiOCl 85-0861

10

(102)

(110)

(101)

(001)

(113)

(200)

(112)

(102)

(003)

(110)

(101)

(002)

RT

(002)

o

200 C

RT

(405), (413)

o

400 C

(001)

500 C

(b) Vacuum (20 02) (101) (617)

500 C

o

(912) (414) (913) (714)

o

(211)

(202), (501)

(1200)

(615) (715)

(a) Air

Intensity (arb. units)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 33

50 10

Degree (2θ)

20

30

40

50

Degree (2θ)

Figure 3. The XRD patterns of of different temperature annealed nanosheet arrays in the (a) air and (b) vacuum.


Page 25 of 33

(a) Air

(b) Vacuum

o

500 C

o

o

400 C

o

300 C

o

200 C

100 C

o

100 C

RT

RT

500 C


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


o

400 C

o

300 C

o

200 C

100

200

300

o

400

500 100

-1

200

300

400

500

-1

Raman Shift (cm )

Raman Shift (cm )

Figure 4. Raman spectra of of different temperature annealed nanosheet arrays in the (a) air and (b) vacuum.



(b) Bi 4f

Bi 5d

BiOCl/ Bi12O15Cl6


Bi 4f C 1s Cl 2s Cl 2p

Bi12O15Cl6

Bi 4d

O 1s

Bi 4p


(a) survey

∆=0.1 eV

Bi12O15Cl6

BiOCl/ Bi12O15Cl6

BiOCl BiOCl

1000

800

600

400

200

0

168

166

Binding Energy (eV)

162

160

158

156

(d) Cl 2p

∆=0.2 eV

Bi12O15Cl6 ∆=0.1 eV

BiOCl/ Bi12O15Cl6

BiOCl

532

164

Binding Energy (eV)


(c) O 1s Intensity (arb. units)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 33

Bi12O15Cl6

BiOCl/ Bi12O15Cl6

BiOCl

531

530

529

528

202

Binding Energy (eV)

200

198

196

Binding Energy (eV)

Figure 5. (a) XPS survey spectra of nanosheet arrays and their fine-scanned (b) Bi 4f and (c) O 1s and (d) Cl 2p XPS spectra.


1/2

(αhν)

eV

3.2

2.48

3.6

eV

4.0

3.07

400

3.41 eV

Cl 6 Bi 2O 15 1 l O C6 /Bi 12 15

300

l BiOC

BiOCl

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Absorption (%)

Page 27 of 33

2.8

2.4

2.0

Energy (eV)

500

600

700

800

Wavelength (nm) Figure 6. UV-visible absorption spectra of the nanosheet arrays samples, The inset shows plot of [ahv]1/2 versus photon energy (eV).



(b)

Ered: -0.98 V

-0.02

Ered: -1.03 V

-0.04

-0.06

-0.08

BiOCl

-3

CB

-3.62

Bi12O15Cl6 -3.7

-4 -5 -6

-3.75

2.48 eV

Ered: -1.11 V

3.07 eV

0.00

(a) (b) (c)

3.41 eV

0.02

Energy vs. Vacuum Level (eV)

(a)

Current (mA)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 33

-6.23 -7

VB -7.03

-6.77

-8

-1.8 -1.6 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 +

Potential (V vs. Ag/Ag )

Figure 7. (a) CV curve (vs. Ag/Ag+ reference electrode) of the (a) as deposited BiOCl, vacuum annealed samples in the temperature of (b) 400 oC, and (c) 500 oC. (b) The scheme of the band position of nanosheet arrays.


Page 29 of 33

60n

(a)

(b) -10

ln (conductivity) S/m

50n 295K - 425 K

40n

Current (A)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


30n 20n 10n 5

10

15

20

25

Bi12O15Cl6

-12

Ea=778 meV

-14

BiOCl (Ref. 31) -16 Ea=862 meV

30

2.2 2.4 2.6 2.8 3.0 3.2 3.4 -1

Voltage (V)

1000/T (K )

Figure 8. (a) Temperature dependent I-V characteristics of 500 oC annealed, pure Bi12O15Cl6 nanosheets between 295 to 425 K under ambient atmosphere. The linear I-V indicates ohmic behavior. (b) Arrhenius plot of conductivity versus reciprocal temperature yields the activation energy, Ea = 778 meV for the Bi12O15Cl6 phase as compared to 862 meV for the BiOCl phase. Data for BiOCl obtained from Myung et al.31 Also, notice the conductivity for BiOCl is lower than the Bi12O15Cl6 phase in the temperature range tested.



13

1.5x10

(a)

11

8x10

(b)

11

7x10

13

1.2x10

Bi12O15Cl6

11

6x10

BiOCl -2

5x10

-2

11

9.0x10

4x10

-2

C /F

-2

12

C /F

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 33

12

6.0x10

11

11

3x10

BiOCl/Bi12O15Cl6

11

2x10

12

3.0x10

11

1x10

0 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4

0.0 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0

Potential (V vs. Ag/AgCl)


Figure 9. (a) Mott-Schottky of BiOCl films showing p-type conductivity and a flat-band voltage of +0.74 V. (b) Mott-Schottky of BiOCl/Bi12O15Cl6 and Bi12O15Cl6 films showing ntype behavior and a flat-band voltage of -0.90 V and -0.69 V, respectively.


Page 31 of 33

300

(a)

100

∆ Current Density (µA/cm )

2

2

BiOCl/ 100 Bi12O15Cl6 0 -100 -200

On

(b)

Bi12O15Cl6

200

Current Density (µA/cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


BiOCl x50

-300 -400 -0.4 -0.3 -0.2 -0.1 0.0

0.1

0.2

0.3

0.4

10

1

Bi12O15Cl6 Off BiOCl/ Bi12O15Cl6 BiOCl

-100

-50

0


50

100

150

200

250

Time (s)

Figure 10. (a) J-V characteristic curves of BiOCl (∆), BiOCl/Bi12O15Cl6, () and Bi12O15Cl6 (O). The dark current is shown with close symbols whereas the photocurrent is shown as open symbols. Notice the BiOCl photocurrent is amplified 50x for clarity. (b) Transient photocurrent responses (in log scale) of the three samples. The applied bias was -0.2 V for BiOCl and +0.2 V for BiOCl/Bi12O15Cl6 and Bi12O15Cl6, respectively.



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Bi O

3+

Cl vacancy

2-

Cl

-

Figure 11. (a) Band structures of pristine bulk BiOCl. The top of valence bands is set to be zero. The green, blue, and red colors indicate the states contributed by O, Cl, and Bi atoms. The thickness of these colors is proportional to the weight. (b) The band structure of a supercell of Bi16O16Cl16 with one Cl vacancy. The Fermi level is set to be zero and crosses the conduction band indicating its n-type behavior.


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Table of Contents.


Highly Conducting, n-Type Bi12O15Cl6 Nanosheets with Superlattice

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