Band Gap Tuning in Bismuth Oxide Carbodiimide Bi2O2NCN

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Band Gap Tuning in Bismuth Oxide Carbodiimide Bi2O2NCN Alex J. Corkett,*,† Zheng Chen,†,‡ Dimitri Bogdanovski,†,‡ Adam Slabon,§ and Richard Dronskowski†,∥ †

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Chair of Solid-State and Quantum Chemistry, Institute of Inorganic Chemistry, RWTH Aachen University, 52056 Aachen, Germany ∥ Hoffmann Institute of Advanced Materials, Shenzhen Polytechnic, 7098 Liuxian Boulevard, Nanshan District, Shenzhen, China § Department of Materials and Environmental Chemistry, Stockholm University, Svante Arrhenius väg 16 C, 106 91 Stockholm, Sweden S Supporting Information *

ABSTRACT: Layered bismuth oxides exhibit a broad range of tunable physical properties as a result of their excellent structural versatility which facilitates compositional substitutions at both cationic and anionic positions. Here we expand this family in a new direction through the preparation of the first example of a bismuthcontaining oxide carbodiimide, Bi2O2NCN, which assumes an extended variant of the anti-ThCr2Si2 structure-type adopted by Bi2O2Ch (Ch = Se or Te) oxide chalcogenides. Electronic structure calculations reveal the title compound to be an indirect band gap semiconductor with a band gap of approximately 1.4 eV, in good agreement with the measured value of 1.8 eV, and intermediate between that of structurally related Bi2O2S (1.12 eV) and β-Bi2O3 (2.48 eV). Mott−Schottky experiments demonstrate Bi2O2NCN to be an n-type semiconductor with a conduction band edge position of −0.37 V vs reversible hydrogen electrode. This study highlights the pseudochalcogenide nature of the −NCN− carbodiimide anion, which may be substituted in place of oxide or chalcogenide anions in this and potentially other structural classes as an effective means of electronic tuning.

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INTRODUCTION Materials that incorporate PbO-like [Bi2O2]2+ layers have garnered much attention since they express a wealth of diverse physical properties. Perhaps most notably, BiOX oxide halides (X = Cl, Br, or I),1−3 and Bi2O2Ch oxide chalcogenides (Ch = S, Se or Te) have been exploited for their excellent photocatalytic activities,4,5 with BiOCl reported to show enhanced performance over TiO2 in the decomposition of methyl orange. In addition, ferroelectricity has been demonstrated in ABi2M2O9 Aurivillius phases (A = Ca, Sr, or Ba, M = Ta, Nb, or Ti),6−8 along with superconductivity in Bi3O2S3,9 and promising thermoelectric properties in Bi2O2Ch (Ch = Se or Te)10,11 and BiOCuCh (Ch = Se or Te).12,13 The common structural motif of all of these materials are fluorite-type layers constructed from corner-sharing [OBi4] tetrahedra, which alternate with anionic layers. In the simplest case, this involves the incorporation of charge-balancing anions, as in BiOX oxide halides, with X = F, Cl, Br, or I,14 which adopt the PbFCl structure-type (Figure 1a). Filling of the tetrahedral holes in this structure generates antifluoritetype [M2Ch2]2− layers yielding quaternary BiOMCh materials (M = Cu or Ag, Ch = S, Se, or Te) with ZrCuSiAs-like structures (Figure 1b).15,16 An alternative stacking of the [Bi2O2] layers generates an 8-fold coordination position © XXXX American Chemical Society

suitable for chalcogenide anions (Figure 1c), as observed in anti-ThCr2Si2-type Bi2O2Ch oxide chalcogenides (Ch = S, Se, or Te),17 albeit very slightly orthorhombically distorted in the case of Bi2O2S.18 Additional, more elaborate, examples include Bi2+2nO2+2nCu2−δSe2+n−δXδ homologues (X = Cl or Br)19 that incorporate both fluorite-type and chalcogenide layers, as well as Bi3O2S3, which features BiS2 bilayers and S2 dimers,9 and Aurivillius-type ABi2M2O9 (A = Ca, Sr, or Ba, M = Ta, Nb, or Ti) phases consisting of Bi2O2 layers separated by perovskitelike layers.20 This structural and compositional diversity with respect to the interlayer species has been used to selectively induce specific physical properties. In the case of Bi2+2nO2+2nCu2−δSe2+n−δXδ homologues,19 it has been demonstrated that changing the number of layers in this series is an effective means of controlling the thermal conductivity, carrier type, and band gap. Furthermore, changing the nature of the halide species in BiOX oxide halides allows band gap tuning from 3.4 eV in BiOCl,1 which is therefore only photocatalytically active under UV light, to 2.54 and 1.85 eV in BiOBr and BiOI,21,22 respectively. Unfortunately, however, BiOBr and Received: March 7, 2019

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DOI: 10.1021/acs.inorgchem.9b00670 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. Crystal structures of (a) BiOX (X = F, Cl, Br, and I), (b) BiOMCh (M = Cu or Ag, Ch = S, Se, and Te), and (c) Bi2O2Ch (Ch = S, Se, or Te). (C, N, O). A secondary phase was introduced to model additional reflections from trace amounts of the BiOCl precursor (3.12(7) wt %). Full details concerning the structure determination, including all intensity data, are available in CIF format and have been deposited under the CCDC entry number 1900547. Copies of the data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/ retrieving.html (or from the CCDC, 12 Union Road, Cambridge CB2 1EZ, U.K.; Fax: + 44 1223 336033; E-mail: [email protected]. ac.uk). Infrared Measurements. The infrared spectra of Bi2O2NCN and BiOCl were measured on a Nicolet Avatar 369 FT-IR spectrometer (Thermo Fisher Scientific, Waltham, MA, U.S.A.) in the range 400− 4000 cm−1 using KBr discs. CHN Measurements. The chemical composition of Bi2O2NCN was determined by CHN analysis using a CHN Rapid VarioEI by analyzer Heraus. Electronic Structure Calculations. The crystal structure elucidated in this work was optimized using a density-functional theory approach as implemented in the Vienna ab initio Simulation Package (VASP),31−34 employing the generalized gradient approximation (GGA) functional in the parametrization of Perdew, Burke, and Ernzerhof35 to account for exchange and correlation. The projector-augmented wave method36,37 was used for basis set representation, with a cutoff energy of 500 eV. The valence electronic configurations as given in the atomic potential files were 6s25d106p3 for Bi, 2s22p4 for O, 2s22p3 for N, and 2s22p2 for C. Brillouin zone integration was performed using Blöchl’s tetrahedron method,38 with a k-mesh generated by the Monkhorst−Pack scheme39 with dimensions of 17 × 17 × 7 to ensure energetic convergence. All calculations were performed in a non-spin-polarized way. An electronic convergence criterion of 10−7 eV was chosen, while the interatomic forces were minimized to 0.001 eV Å−1 for structural optimization. As standard DFT functionals are known to be highly prone to error in respect to excited-state properties such as band gap sizes,40−42 a follow-up static calculation of the optimized structure was performed, utilizing the strongly constrained SCAN meta-GGA functional43,44 to achieve higher accuracy in relation to the electronic structure. The output of this calculation was processed further with the LOBSTER program45−48 to obtain the band structure and both the total and numerically correct local (atom-projected) density of states (DOS).49 Optical and Electrochemical Properties. UV−Vis Spectroscopy. The UV−vis spectra of Bi2O2NCN were recorded on a Shimadzu UV-2006 spectrophotometer. The Tauc plots were calculated via the Kubelka−Munk function F(R) = (1 − R)2/2R to determine the band gap size. Mott−Schottky (MS) Experiments. The MS experiments on Bi2O2NCN electrodes were carried out in an electrochemical cell (WAT Venture) operating in a three-electrode setup. A thin film prepared by electrophoretic deposition from Bi2O2NCN powder on fluorine-doped tin oxide (FTO) glass was used as a working electrode.

BiOI suffer considerably from photodecomposition and are therefore unsuitable for photocatalytic applications. Recently, the structurally related n-type semiconductor Bi2O2S was demonstrated to be a promising photoelectric material with a band gap (Eg = 1.12 eV) in the optimal region for use as solar absorber materials (1−1.8 eV).4,5 Thus, exploring the photoelectric properties of alternative, but topologically similar, layered bismuth oxides is an attractive proposition, in particular when modifying the anionic parts of the crystal structures. The carbodiimide anion, −NCN−, represents an encouraging prospect for such investigations, as it is well thought of as a pseudochalcogenide (“divalent nitride”) and, from an HSAB perspective, lies between oxide and sulfide anions.23 Indeed, transition metal carbodiimides have recently been reported as prospective photochemical and electrochemical materials.24−27 Here we report the synthesis and preliminary characterization of the novel bismuth oxide carbodiimide Bi2O2NCN, a direct analogue of Bi2O2Ch chalcogenides, which has been prepared for the first time by a solid-state metathesis (SSM) reaction.

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EXPERIMENTAL SECTION

Synthesis. Bi2O2NCN was prepared on the 0.5 g scale in an argon-filled glovebox by a solid-state metathesis reaction, motivated by the recent preparation of the oxide carbodiimide Sn2ONCN by such methods.28 BiOCl (prepared by the hydrolysis of BiCl3, with subsequent drying) and Na2NCN (prepared as described in ref 29) were reacted in a 2:1 stoichiometric ratio, according to eq 1. The reactants were homogenized using an agate pestle and mortar, and the reaction mixture was loaded into an open dry glass capillary (8 mm). The sample was then placed in a glass ampule and positioned in a tube furnace under flowing argon and heated to 350 °C for 2 h, with heating and cooling rates of 2 °C min−1. The resultant green powder was subsequently air-exposed and then washed to isolate Bi2O2NCN, as detailed in the Results and Discussion section. 2BiOCl + Na 2NCN → Bi 2O2 NCN + 2NaCl

(1)

PXRD Analysis. Powder X-ray diffraction (PXRD) data were recorded on as-made and washed samples of Bi2O2NCN pre- and post-air exposure (1 week) at room temperature using a calibrated STOE STADI-P powder diffractometer with a flat sample holder (Cu Kα1, linear PSD, 2θ range 5−120°, with individual steps of 0.01°). Structural refinements were performed using the Rietveld refinement suite GSAS with the EXPGUI interface.30 In the final cycles of leastsquares refinement, lattice parameters, fractional coordinates, and isotropic thermal displacement parameters of Bi2O2NCN were refined. In view of the data quality and striving for reliable thermal displacement parameters, a single Uiso was refined for all light atoms B


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Inorganic Chemistry

Figure 2. Crystalline powder samples of (a) as-made and (b) air-exposed (1 week) Bi2O2NCN. Platinum wire and a 1 M Ag/AgCl electrode were used as counter electrode and reference electrode, respectively. All current values of the electrodes were recorded versus 1 M Ag/ AgCl and converted versus the reversible hydrogen electrode (RHE) according to ERHE (V) = E1 M Ag/AgCl (V) + 0.236 (V) + [0.059 × pH] (V) at 25 °C. The electrochemical data were recorded by a potentiostat (SP-150, BioLogic). A solar light simulator (class-AAA 94023A, Newport) with an ozone-free 450 W xenon short-arc lamp was used to illuminate the photoelectrodes with 100 mW cm−2 (AM 1.5G) simulated visible light. A 0.1 M potassium/sodium phosphate (KPi) buffer at pH 7.0 was used as the electrolyte for photoelectrochemical (PEC) experiments and prepared with Milli-Q water (18.3 Ω cm) at 25 °C. The MS measurements were carried out under dark and under AM 1.5G illumination in an electromagnetically shielded box. A sinusoidal modulation of 10 mV was applied at frequencies of 10, 100, and 1000 Hz in the potential range from 0.05 to 1.23 V versus RHE with an equilibration time of 10 s.

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parameter of Bi2O2NCN (a = 3.75867(5) Å) to that of Bi2O2Se (a = 3.88 Å), indicates that the structures may be closely related. An initial I4/mmm starting model for Bi2O2NCN was therefore generated from the structure of Bi2O2Se by replacing Se with C at the 2a site and adding a N atom at a 4e position at a distance of 1.24 Å from C, typical for carbodiimides. Rietveld refinement of this model resulted in excellent agreement between the calculated and the observed intensities, as seen in the Rietveld fit (Figure 3). Crystallographic data are reported in Table 1, with selected bond lengths and angles in Table 2.

RESULTS AND DISCUSSION

PXRD data collected on as-made Bi2O2NCN reveals the presence of the NaCl metathesis salt and trace amounts of BiOCl (3.21(7) wt %), along with reflections attributed to the title compound (Figure S1(a)). Direct washing of as-made Bi2O2NCN, to remove the coproduced NaCl, results in partial decomposition of the product (Figure S1(b)). However, prolonged air-exposure of as-made Bi2O2NCN (ca. 1 week) results in a gradual color change from green to yellow (Figure 2), with no discernible change in the PXRD pattern (Figure S1(c)). This air-exposed sample may then be washed with water to remove the NaCl and yield near-monophasic yellow Bi2O2NCN, hereafter referred to as Bi2O2NCN, without decomposition to BiOCl (Figure S1(d)). In light of this enhanced chemical stability and associated color change it is proposed that upon air exposure Bi2O2NCN undergoes a gradual passivation process which results in the incorporation of a thin layer of yellow Bi2O3 on the particle surface, similar to that postulated in the related oxide carbodiimide Sn2ONCN,28 which is undetectable by PXRD. The chemical formula of Bi2O2NCN was verified by CHN analysis, which shows good agreement between experimental and calculated C and N contents (Table S1). The PXRD pattern of Bi2O2NCN was indexed to a body-centered tetragonal cell, with a = 3.75867(5) Å and c = 14.4358(3) Å. The observed reflection conditions suggest the extinction symbol I, which is consistent with the I4/mmm space group symmetry of the oxide selenide analogue Bi2O2Se. This observation, along with the similarity of the basal lattice

Figure 3. Rietveld fit of Bi2O2NCN to PXRD data, showing observed (red), calculated (black), and difference (blue) intensities. Bragg positions of Bi2O2NCN (green) and BiOCl (pink) are denoted by vertical markers.

Table 1. Crystallographic Data and Fractional Atomic Coordinates for Bi2O2NCNa atom

Wyckoff site

x

y

z

Uiso (102 × Å2)

Bi O C N

4e 4d 2a 4e

1/2 0 0 0

1/2 1/2 0 0

0.16007(4) 1/4 0 0.0857(5)

1.91(2) 2.3(1) ″ ″

a

Standard deviations are given in parentheses. Tetragonal, I4/mmm (No. 139), Z = 2, a = 3.75867(5) Å, c = 14.4358(3) Å; Rwp = 6.76%, Rp = 5.55%, χ2 = 1.70, 33 variables. C


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Inorganic Chemistry

symmetric −NCN−, support the crystallographic evidence for D∞h symmetry carbodiimide anions. The band structure of Bi2O2NCN was calculated for the geometry-optimized crystal structure, which shows excellent agreement between the experimentally observed and calculated bond distances (Table S2), with differences well below 1% for all values. This is a remarkable fact, considering the known DFT problem of a lack of distinction between the energetics of carbodiimides and cyanamides, often resulting in large errors during structural optimization.52 The static calculation yields an indirect zero-temperature band gap of approximately 1.4 eV (Figure 5). The optical band gap was experimentally

Table 2. Selected Bond Lengths and Angles in Bi2O2NCN bond lengths (Å) C−N Bi−O Bi−N

1.239(8) 2.2841(4) 2.866(3)

bond angle (°) Bi−O−Bi

108.85(3)

Our refinement results indicate that Bi2O2NCN adopts a crystal structure akin to that of Bi2O2Ch (Ch = Se, Te) oxide chalcogenides, best described as an extended anti-ThCr2Si2 structure-type. It comprises fluorite-type [Bi2O2]2+ layers which alternate along the c axis with a square net of rod-like − NCN− carbodiimide anions that are orientated parallel to the c axis (Figure 4a). This results in significantly greater

Figure 5. Calculated band structure of Bi2O2NCN with indicated valence band maximum (VBmax) and conduction band minimum (CBmin), showing the indirect character of the band gap.

determined by UV−vis spectroscopy, yielding a value of 1.8 eV (Figure 7). The difference of 0.4 eV in comparison to the calculated value is attributed to the generally known underestimation of the band gap from DFT calculations that persists even with the improved functional chosen here. The local DOS analyses (Figure 6) reveal the states occupied by the 6s electrons of Bi to be the dominant contribution in the region between approximately −11.5 and −8.5 eV, where 0 eV signifies the Fermi level. The broad region between −7 and −2 eV is mainly comprised of O 2p and Bi 6p states. However, in the vicinity of the valence band maximum (VBmax), that is, between −2 and 0 eV, N 2p states dominate, while the conduction band minimum (CBmin) mainly contains Bi 6p states. This dispersed nature of the 6s and 6p contributions of Bi is characteristic of layered bismuth oxides and is proposed to provide high mobility for lightinduced electron−hole separation, which facilitates the excellent photocatalytic properties of such materials.53 In the case of Bi2O2NCN, the predominance of N 2p states around VBmax is very similar to that of the S 3s and 3p states in Bi2O2S. However, the slightly higher electronegativity of NCN2− (3.36) compared with sulfide (2.89) results in a band gap for Bi2O2NCN that is larger than that of Bi2O2S (1.12 eV), but smaller than that of structurally related fluorite-like β-Bi2O3 (2.48 eV).4,54 In order to determine the valence band edge (VBE) and conduction band edge (CBE) positions as well as the semiconductor type for Bi2O2NCN, we performed MS experiments on electrodes prepared from powder. Figure 8a

Figure 4. Crystal structure of Bi2O2NCN (a) and coordination environments of Bi (b) and carbodiimide (c).

interlayer separation in Bi2O2NCN (c/2 = 7.2179(2) Å) compared with Bi2O2Se (c/2 = 6.080(15) Å).17 The oxide carbodiimide, La2O2NCN,50 crystallizes in a related structure, but a disordered arrangement of carbodiimide anions, orientated in the basal plane, generate a unit cell that is extended within the layers (a = 4.096 Å) and compressed along the 4-fold axis (c/2 = 6.166 Å) compared with the title compound. The oxygen anions in Bi2 O2 NCN are tetrahedrally coordinated by four bismuth cations at a distance of 2.2841(4) Å, congruent with literature examples (Bi−OBiOCuS = 2.3138 Å, Bi−OBiOCl = 2.311 Å).15,51 Bismuth is coordinated by four O anions and four terminal N anions of the carbodiimide in a square antiprismatic environment with a Bi−N bond distance of 2.866(3) Å (Figure 4b). The carbodiimide units are orientated strictly along the crystallographic c axis and constrained to reflect a D∞h symmetry with a CN bond distance of 1.239(8) Å, typical for metal carbodiimides and characteristic of CN double bonds (Figure 4c). This finding is consistent with IR data, which clearly show that Bi2O2NCN expresses asymmetric vibrations (νas(NCN) = 2032 cm−1) and deformation vibrations (δ(NCN) = 642 cm−1) characteristic of metal carbodiimides (Figure S2). Furthermore, the absence of symmetric stretching modes around 1250 cm−1, which are IR-forbidden for D


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Inorganic Chemistry

Figure 6. Total density of states (DOS) and the contributing local DOS for all atoms. The local DOS is orbital-resolved for Bi and N, showing contributions from the respective s and p valence orbitals. The contribution of the s orbital is negligible and thus omitted for O and C.

Figure 7. (a) Diffuse-reflectance spectra for Bi2O2NCN and (b) Tauc plot for the electrodes showing indirect band gap.

Figure 8. (a) Mott−Schottky analysis of EIS measurements of Bi2O2NCN electrodes. The intercept of −0.37 V relates to the extrapolated values. (b) Electronic structures of Bi2O2NCN, α-Bi2O3, and β-Bi2O3. Band edge potentials are referenced to RHE.54

counterpart Bi2O2S,5 to be a n-type semiconductor and we could not observe a noticeable difference in the plots in dark and under illumination. With an electronic band gap of 1.8 eV (see above), the VBE and CBE values are 1.43 and −0.37 V versus RHE, respectively.55 Figure 8b compares these determined values with the other bismuth oxides on the

shows the MS plots for different applied frequencies of 10, 100, and 1000 Hz measured under simulated solar illumination and in the dark. Extrapolation of the measured data yield a flatband potential of −0.37 V versus RHE, which is consistent for all three frequencies. All curves exhibit a positive slope, indicating the oxide carbodiimide Bi2O2NCN, like its oxide sulfide E


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Inorganic Chemistry Author Contributions

RHE scale. The VBE position and n-type behavior would render Bi2O2NCN a possible photoanode candidate for photochemical water oxidation. However, we could not measure a photocurrent for Bi2O2NCN under operating conditions. One possible reason may be the formation of a passivating bismuth oxide layer, which may act as a holeblocking layer, because the VBE positions of both the α-Bi2O3 and β-Bi2O3 are energetically lower than for Bi2O2NCN (Figure 8b).24 Additionally, fast charge-carrier recombination in the bulk upon illumination and low electronic conductivity of the oxide carbodiimide material may limit the photoelectrochemical response.

‡

These authors contributed equally to this work (Z.C. and D.B.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Mr. B. Faßbänder for assistance with PXRD measurements and Mrs. I. Kalf for collecting IR spectra. Furthermore, we thank the IT Center of RWTH Aachen University for the provision of computing time and resources in respect to the computational part of this study. We also acknowledge the financial support of the Deutsche Forschungsgemeinschaft.

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CONCLUSION The novel bismuth oxide carbodiimide Bi2O2NCN was prepared via a solid-state metathesis reaction and found to adopt an extended anti-ThCr2Si2 structure-type similar to that in Bi2O2Ch oxide chalcogenide analogues. Electronic structure calculations indicate an indirect band gap of 1.4 eV which, although slightly underestimated, is congruent with the measured value of 1.8 eV. Mott−Schottky analysis of Bi2O2NCN reveals the title compound to be an n-type semiconductor with a VBE position theoretically suitable for application as a photoanode for photochemical water oxidation. However, no measurable photocurrent for Bi2O2NCN was detected. Nonetheless, the isovalent substitution of the extended −NCN− carbodiimide anion in place of oxide or chalcogenide anions has been demonstrated to be an effective means of control of the electronic properties of this emergent family.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b00670. Additional PXRD data, elemental analysis, IR data, and a comparison of observed and calculated structural variables (DOCX) Accession Codes

CCDC 1900547 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, U.K.; fax: +44 1223 336033.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Alex J. Corkett: 0000-0002-7725-2498 Zheng Chen: 0000-0002-2021-3332 Dimitri Bogdanovski: 0000-0001-8881-0777 Adam Slabon: 0000-0002-4452-1831 Richard Dronskowski: 0000-0002-1925-9624 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. F


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Inorganic Chemistry

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Band Gap Tuning in Bismuth Oxide Carbodiimide Bi2O2NCN

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