Substituting Copper with Silver in the BiMOCh Layered Compounds

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Substituting Copper by Silver in the BiMOCh Layered Compounds (M = Cu, Ag; Ch = S, Se, Te): Crystal, Electronic Structure and Optoelectronic Properties J. Gamon, D. Giaume, G. Wallez, J-B. Labégorre, O. I. Lebedev, R. Al Rahal Al Orabi, S. Haller, T. Le Mercier, E. Guilmeau, A. Maignan, and P. Barboux Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b04962 • Publication Date (Web): 20 Dec 2017 Downloaded from http://pubs.acs.org on December 28, 2017

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Chemistry of Materials

Substituting Copper by Silver in the BiMOCh Layered Compounds (M = Cu, Ag; Ch = S, Se, Te): Crystal, Electronic Structure and Optoelectronic Properties J. Gamon,1,2* D. Giaume,1 G. Wallez,1,3 J-B. Labégorre,2,4 O. I. Lebedev,4 R. Al Rahal Al Orabi,5,6 S. Haller,1,2 T. Le Mercier,2 E. Guilmeau,4 A. Maignan4, P. Barboux1 1

Chimie ParisTech, PSL ResearchUniversity, CNRS, Institut de Recherche de Chimie Paris (IRCP), F-75005 Paris, France

2

Solvay, Research and Innovation Center Paris, 52 rue de La Haie Coq, 93308 Aubervilliers Cedex, France

3

Sorbonne University, UPMC Université, Paris 06, 75005 Paris, France

4

Laboratoire CRISMAT, UMR-CNRS 6508, ENSICAEN, UNICAEN, Normandie Université, 6 bd du Maréchal Juin F-14050 CAEN Cedex 4 – France 5

Solvay, Design and Development of Functional Materials Department, Axel’One, 87 avenue des Frères Perret, 69192 Saint Fons, Cedex, France 6

Department of Physics, Central Michigan University, Mt. Pleasant, MI 48859, USA

* Corresponding author: [email protected] Abstract The synthesis of BiAgOCh (Ch = S, Se) compounds has been successfully achieved thanks to ion exchange of copper with silver in aqueous solutions, starting from the copper parent phase. Optical and electrical measurements of BiAgOCh powders confirm an increase of both the bandgap and the electrical resistivity, as compared to the copper compounds. The structure of the BiAgOS phase has been clearly examined. X-Ray diffraction synchrotron measurements coupled with advanced high resolution transmission electron microscopy analysis evidenced an Ag-deficient structure, as well as Bi-rich defects, both types of defects being oppositively charged. Silver are also found in interstial sites which explain the 2D ionic conductivity. This structural study combined with theoretical calculations explains the intrinsic conductivity behavior of these semiconductors linked to the mutual compensation of both defect types in the structure and to the increase of the hole effective mass. This study shows the feasibility of modifying the optoelectronic properties of the BiMOCh compounds, with the goal of integrating them in heterojunction solar cells. Moreover, it brings a very 1 ACS Paragon Plus Environment

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precise insight on the complexity of the relationship between structural defects and optoelectronic properties. Introduction The oxysulfide family BiCuOCh, with Ch = S, Se, Te, has been identified for the first time by Kusainova et al. in 1994.1 The related compounds crystallize in the same tetragonal space group P4/nmm. The structure consists of alternating layers of (Bi2O2)2+ and (Cu2Ch2)2-. BiCuOCh phases have been widely studied for their thermoelectric properties,2-8 owing to their intrinsically low thermal conductivity as well as to their high electronic conductivity. Indeed, they are naturally degenerated p-type semiconductors with low bandgap values (from 0.4 for BiCuOTe to 1.1 eV for BiCuOS).2, 9 Among these compounds, Ba-doped BiCuOSe exhibits the best reported figure of merit (ZT = 1.4 at 923 K).10 The p-type conductivity originates from the creation of copper vacancies2, 5, 6, 11, 12 following the Kroeger Vink’s equation: → + ℎ⦁ +

The valence band being composed of the 3d orbitals of copper and p orbitals of the chalcogenide atom, copper vacancies will induce the creation of holes delocalized in the [Cu2Ch2]2- layer. In the QCuOCh system (Q = Bi, La) the copper vacancies appear to be thermodynamically favored due to their low formation energy.13 In particularly, the copper vacancies in BiCuOS seem to be intrinsically induced by the structure, as evidenced by high resolution transmission electronic microscopy TEM analysis.6 The actual measured composition is BiCu1-δOS with δ ≈ 0.06 as shown by neutron diffraction studies,12 and confirmed by Rietveld measurements on our powders. Recently, Le Bahers et al.14, 15 also identified BiCuOS as a promising material for heterojunction photovoltaic cell. Band structure calculations showed that this compound possesses in theory all the requirements for an absorber (p-type) layer: a bandgap between 1.1 and 1.4 eV, a high mobility of charge carriers (holes effective mass mh < 0.5 m0) and a high dielectric constant (ε > 10)16. However, the carrier concentration, around 1021 cm-3 (considering that each copper vacancies induces one delocalized hole in the structure), is found too high (optimum value is between 1015-1017 cm-3), so the space charge layer length at the junction with the n-type semiconductor will be decreased, as well as the diffusion length of photogenerated electron holes. 2 ACS Paragon Plus Environment

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In order to reduce the concentration of p-type carriers in the structure, we have investigated the feasibility of substituting copper by silver, this element being less likely to oxidize and to yield a p-type doping17. While LaAgOS can be synthesized by a high temperature solid-state annealing process,18, 19 this synthetic route does not allow to crystallize BiAgOCh materials. Mechanochemical synthesis17 as well as a wet chemistry route20 were reported for the preparation of BiAgOS and could therefore enable to overcome this issue. We report herein on an alternative method by ion exchange, at room temperature, for the synthesis of BiAgOCh compounds, together with their crystal structures and optical/electrical properties. Experimental Precursors The chemicals Bi2S3 (99.9% purity), Bi2O3 (99.975%), Bi (99.999%) and Cu (99.5%) were purchased from Alfa Aesar ; Cu2S (99.5%), Se (99.5%), Te (99.997%) and Na2S2O3,5H2O (99.5%) from Sigma Aldrich ; AgNO3 (99.8%) and NH4OH 28% (Normapur) from VWR. Synthesis BiCuOCh (Ch= S, Se or Te) powders were first synthesized by solid-state reaction in sealed quartz tubes. Bi2O3/Bi2S3/Cu2S, Bi2O3/Bi/Cu/Se and Bi2O3/Bi/Cu/Te were weighted in stoichiometric amounts according to the nominal BiCuOS, BiCuOSe and BiCuOTe compositions. The precursor powders were mixed and ground in an agate mortar and the mixtures were uniaxially pressed into pellets. They were placed in quartz tubes before sealing under primary vacuum, and heated at 550 °C for 48 h. The resulting black pellets were then manually ground in order to obtain a fine powder. BiAgOCh powders were synthesized by ion exchange of copper and silver in water. 2.5 mmol of BiCuOCh products were introduced in 75 mL of a 0.1 M AgNO3 (Ag/Cu = 3) aqueous solution. The suspension was maintained under magnetic stirring for 2 days, after which, the powder was filtered with a Millipore All-Glass Filter holder, washed with 3×100 mL of distilled water and dried at 65 °C. The filtered solution was pale blue while the resulting powder has a shiny grey color revealing the formation of metallic silver. In order to dissolve this impurity and isolate the BiAgOCh phases, the powder was washed in a thiosulfate aqueous solution. 1 g of the resulting powder was placed in 150 mL of a 0.3 M 3 ACS Paragon Plus Environment


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Na2S2O3, 1 M NH4OH aqueous solution. The suspension was maintained under vigorous magnetic stirring for 2 days. The suspension was then let to rest, the supernatant was removed and a new thiosulfate washing solution was introduced. This procedure was repeated 5 times in order to ensure complete dissolution of metallic silver. The powder was then filtered, washed with 3×100 mL of distilled water and dried in an oven at 65 °C. Faster results could have been obtained by washing with KCN solutions but thiosulfate is considered less dangerous. ICP-OES titration Solubilization of BiAgOCh samples were performed in concentrated aqueous HNO3 in a microwave oven. The titration was performed by induced coupled plasma with an optical emission spectroscopy detector (ICP-OES) with a PQ9000 Plasma Quant apparatus. Rays used for the optical detection were: S: 180.676 nm - 181.978 nm; Bi: 190.178 nm; Ag: 328.068 nm - 338.289 nm; Cu: 324.754 nm - 327.396 nm; Se: 196.026 nm. X-ray and synchrotron diffraction Preliminary X-ray diffraction (XRD) experiments were carried out in Bragg-Brentano geometry on a Panalytical X’Pert Pro apparatus with monochromatized CuKα1 beam in the 8 °≤ 2θ ≤ 140 ° scan range, step δ(2θ) = 0.013°, with a total counting time of 13 h. Synchrotron diffraction (SD), implemented to extract high-precision structural data from the thiosulfate-washed sample, was performed at the European Synchrotron Radiation Facility (ERSF), Grenoble, France, on the high-resolution beamline ID22, at λ = 0.40013 Å. The sample was introduced into a 500 µm diameter glass capillary to record the pattern in transmission mode (2 °≤ 2θ ≤ 40 °, δ(2θ) = 0.002 °) with a 12 rps spinning, giving 425 reflections. All experiments were performed at room temperature. Optical properties The optical properties of the powders were investigated by diffuse reflectance spectroscopy with a Cary 5000 UV-Vis-NIR spectrophotometer (Agilent Technologies) equipped with an integrating sphere. Reflection data have been treated following a combined approach of the Tauc21 and the Kumar method.22 Indeed, the absorbance of the material is proportional to ln

where is the diffuse reflexion intensity,

!"

and

!#$

the

maximum and minimum reflectances. As for an indirect bandgap material, one can write following Tauc’s formula21 ℎ% = ' (ℎ% − *+ )- . Therefore, if we plot the square root of 4 ACS Paragon Plus Environment

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ℎ%./

vs. ℎ%, the bandgap of the material can be obtained as the extrapolation of a

straight line to the zero value of the ordinates. Transport properties Resistivity measurements were performed with the four probes method on a pelletized sample (300 MPa, relative density 70 %). Contacts were made with a silver paint (purchased from SPI supplies). Transmission electron microscopy Electron diffraction (ED) studies were carried out on a FEI Tecnai G2 30 UT (LaB6) microscope operated at 300 kV with 0.17 nm point resolution and equipped with EDAX EDX detector. High angle annular dark field scanning transmission electron microscopy (HAADFSTEM) together with EDX elemental mapping were performed on a JEOL ARM-200F cold FEG double aberration corrected electron microscope operated at 200 kV and equipped with a large solid-angle CENTURIO EDX detector and the Quantum GIF system. TEM samples were prepared by crushing the material with butanol in an agate mortar and the resulting dispersion was transferred to holey carbon films fixed on 3 mm copper grids. Computational procedures Our calculations are based on density functional theory (DFT). We used the fullpotential linearized augmented plane wave (FLAPW) approach, as implemented in the WIEN2K code23. A plane-wave cutoff corresponding to RMTKmax= 7 was used in all calculations. The radial wave functions inside the non-overlapping muffin-tin spheres were expanded up to lm = 12. The charge density was Fourier expanded up to Gmax = 16 Å-1. Total energy convergence was achieved with respect to the Brillouin zone (BZ) integration mesh with 10 x 10 x 4 k-points. The atomic positions were relaxed (with the CASTEP code)24 by using a set of ultrasoft pseudopotentials25,

26

with the PBEsol exchange–correlation

functional27 until all atomic forces were less than 0.02 eV·Å-1. The lattice parameters were fixed to experimental values (Table S1). The kinetic energy cutoff and the charge density cutoff were set to 40 Ry and 480 Ry, respectively. The modified Becke-Johnson (mBJ) functional was used to calculate the electronic band structures. This leads to an excellent agreement with the experimental values for the energy separation between the highest occupied crystalline orbital (HOCO) and the lowest 5 ACS Paragon Plus Environment


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unoccupied crystalline orbital (LUCO).28 Since the Bi atoms in oxychalcogenide compounds have fairly high atomic number and mass, the relativistic effect cannot be neglected, and we have included the spin-orbit coupling for Bi atoms in order to elucidate realistic electronic structure. We used 35 x 35 x 15 k-points in the BZ to compute the band derivatives and the density of states. Vibrational properties have been obtained by using density functional perturbation theory (DFPT)29 as implemented in the CASTEP code24. The calculation has been performed by using a set of ultrasoft pseudopotentials26 with the PBEsol exchange-correlation functional27. The kinetic energy cutoff and the charge density cutoff are set to be 40 Ry and 480 Ry, respectively. A 16 x 16 x 4 k-point mesh is used to sample the Brillouin zone. The static dielectric constant (εr) is divided in two contributions: (1) the contribution from the electronic density (ε∞) and (2) the contribution from the ionic vibration (εvib). The details of the calculation for these too later parameters were presented in the article from Le Bahers et al.15. We used BoltzTraP code30 to calculate the electronic transport coefficients within the Boltzmann Transport Equation (BTE) with constant relaxation time 0e. The effective masses were determined by calculating the direction averaged 1/0 at 300 K and the Fermi level set to the band edge. The mass is then given by the ratio of the number of carrier concentration, n, over 1/0 (note that 1/0 is proportional to the square plasma frequency, i.e., the optical Drude n/m, which is the quantity relevant for electrical conductivity). Results Synthesis BiCuOCh compounds can be synthesized via a solid state reaction of the precursor powders in a sealed tube.2, 6, 7 LaAgOS and CeAgOS, crystallizing with the same structure as QCuOCh (Q = Bi, La), have been synthesized in sealed tubes at 900 K as well, allowing the study of their silver ionic conductivity (10-2-10-3 S·cm-1 for LaAgOS18, 19 and 0.1 S·cm for CeAgOS31). However, we did not succeed in synthesizing the analogues BiAgOCh phases with this route, metallic silver and other impurities being always obtained. According to Charkin et al.32 interdependent distortions of the oxide and chalcogenide tetrahedra are more pronounced in BiAgOCh than in BiCuOCh and LaAgOCh, which inhibits the formation of 6 ACS Paragon Plus Environment

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the product. Moreover, silver salts tend to be reduced when heated at high temperature, favoring silver metal segregation. In order to overcome these structural and segregation issues at high temperature, we have used a room temperature alternative for BiAgOCh materials synthesis: ion exchange of Cu+ for Ag+ in the BiCuOCh structure. First, BiCuOCh phases were synthesized through a classical solid state route. The XRD patterns of the phases are shown in Figure 1a, b and c. All the peaks can be attributed to the BiCuOCh phases, attesting of their good purity. Only one minor Bi2Te3 impurity was detectable on the BiCuOTe pattern. The ion exchange from the BiCuOCh phases was then performed. The XRD diagram of the product retrieved after filtration for the sulfur compound is presented on Figure 1d. A silver metallic phase has grown during the process, alongside a structure similar to BiCuOS but with larger lattice parameters. The blue color of the filtrate indicates the presence of Cu2+ species in solution after exchange. Moreover, a kinetic study of the reaction was performed. It shows the progressive shift of the XRD diffraction peaks with the time at which BiCuOS is immersed in the silver nitrate bath (Figure S1). These results lead to the conclusion that an ion exchange between Cu+ and Ag+ in BiCuOS occurred according to the following chemical reaction equations: BiCuOS + Ag+(aq) → BiAgOS + Cu+(aq) Cu+(aq) + Ag+(aq) → Cu2+(aq) + Ag0 These reactions taking place simultaneously, at least two Ag(NO3) equivalents are needed to complete the ion exchange. These conditions are experimentally satisfied, with the introduction of a silver excess in the bath (Ag/Cu =3). An ICP-OES titration of the powder after filtration reveals, as expected, a molar ratio Bi/Ag of 2.0±0.2, Bi/S of 1.0±0.1, and a ratio Cu/Ag of less than 0.010±0.001, assessing the proposed chemical reactions. BiAgOS was successfully purified by washing with a solution containing S2O32- species, a strong complexing agent, used for gold and silver leaching through mild oxidation with air.33, 34 (Figure 1e). The same process was performed onto BiCuOSe and BiCuOTe to synthesize their silver analogues. Figure 1f and g show the XRD diagrams of the powders after ion exchange and silver leaching. As for BiAgOS, the systematic shift towards low angles of the diffraction peaks compared to their copper analogues indicates the formation of the silver phase. BiAgOSe could be thoroughly purified thanks to the thiosulfate silver leaching, whereas it

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was found impossible to fully purify the BiAgOTe phase, as this compound decomposes in the solution, forming Bi2Te3 and Ag2Te species. Therefore, we have succeeded in synthesizing BiAgOS and BiAgOSe compounds thanks to the ion exchange between Cu+ and Ag+ in solution, whereas it could not be formed by a high temperature treatment. Indeed, the energy needed for the formation of the phase through ion exchange is lower than through the chemical reaction of oxide and sulfur precursors as the phase transformation is topotactic and the crystal structure is maintained during the process. Moreover, the high ionic conductivity and thus the mobility of silver and copper in the (ab) plane is favorable to this exchange. Crystal structure of BiAgOS From the preliminary XRD pattern, BiAgOS was found to be isotypic (tetragonal, P4/nmm) with its copper homologue. The crystal structure was therefore refined by the Rietveld method from the BiCuOS data set2 using the Fullprof suite.35 Because of a marked anisotropic Scherrer broadening, the Thompson-Cox-Hastings function with spherical harmonics expansion was implemented to fit the peak profiles. For the sake of realism, all the standard uncertainties (s.u.) reported hereafter have been increased by a factor of 10 (for 2θdependent cell parameters) or by Bérar’s factor (4.4 according to the Fullprof, for intensitydependent variables like atomic positions and site occupancy factors).36 Significant differences were found between the cell parameters of the as-synthesized and the thiosulfate-washed samples, respectively a = 3.9308(2) Å – c = 9.1949(5) Å, and a = 3.9124(1) Å – c = 9.2309(4) Å. This roughly isotropic 0.50(2) % volume shrinkage subsequent to washing was ascribed to changes in the composition that called for a precise assessment. After refining the atomic positions from the XRD pattern of the washed compound, the site occupancy factors (sof) of the atoms were refined, each during an independent run. As a matter of fact, the quasi-absence of chemical impurities allows to consider a local discrepancy in the electron density as the result of vacancies or antisite defects. Only the silver atom exhibited a significant deviation, with a sof of 0.94(1) (vs. 1.01(1) for Bi, 1.06(4) for O and 0.99(3) for S). Although high compared to the s.u., the discrepancy was not considered as significant enough to prove the non-stoichiometry of the material, in particular because the preferred (001) orientation was only partly corrected despite implementing the March function.37


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A more precise insight of the crystal structure was expected by using synchrotron diffraction. The SD pattern of the washed sample was analyzed in the same way as the XRD ones. In the final data set, 14 intensity-dependent parameters (including anisotropic thermal displacement factors for all atoms) and 13 profile-dependent parameters were refined. No correction for preferred orientation was necessary. The results obtained for the cell parameters are a = 3.91175(5)(2) Å – c = 9.2334(1) Å, which is in good agreement with previous values reported in the literature17, 20. These values as well as the bond lengths are listed in the supplementary information (Table S1a and b). The crystal structure of BiAgOS (Figure 2a) can be described as a stacking following the c-axis of S-Bi-O2-Bi-S-Ag2 layers, with an atomic density of the O and Ag layers twice that of the Bi and S ones. Bismuth has a highly asymmetric 4mm coordination polyhedron made of 4 close oxygen atoms (0.52 valence units (v.u.) per bond 38) and 4 distant sulfur ones (0.20 v.u. per bond), that probably accounts for a marked stereochemical activity of the 6s2 lone pair towards the wide holes of the S layer. In the ideal structure, silver only occupies a 4sulfur tetrahedral site in the -4m2 symmetry, like copper in the isotype. However, the AgS4 tetrahedron appears strongly stretched along the c-axis, with angles S-Ag-S’ = 118.6(3) ° (where S and S’ belong to different layers). As a consequence, the S-S’ edges are much longer than the S-S (001)-coplanar ones (respectively 4.66 and 3.91 Å), to be compared to those of BiCuOS (3.87 and 3.99 Å from our refinements). The SD analysis pointed out a marked anomaly of the atomic displacement parameters of the silver atom, as shown by the equatorial/axial ratio: B11/B33 = 12, vs. 2 for Bi. For comparison, B11/B33 = 1.1 for Cu and 0.82 for Bi in BiCuOS. Furthermore, the Fourier map of the residual electron density exhibited local maxima on the Ag site (3.6 e·Å-3), but also on the mid-S-S’ edge position (3.5 e·Å-3, in the same (001) plane as the regular Ag site) that ruled out the harmonic model of a thermal ellipsoid. A second silver atom was introduced in the new mid-S-S’ site (Ag2 in the following, the regular site being now termed Ag1) and its sof was also refined in the final run, yielding an Ag1:Ag2 = 81:19(1) distribution between the two sites. As several attempts to refine the anisotropic thermal displacement of Ag2 led to divergence, only the average Biso parameter was measured. In these conditions, a more realistic B11/B33 = 7.8 ratio was measured for Ag1. Besides, the material was confirmed to be Ag-deficient (0.94(1) per formula). For comparison, the 9 ACS Paragon Plus Environment


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occupancies of the other atoms sites were found to be 1.000(3) for Bi, 1.003(8) for S and 1.00(1) for O during independent refinements, thus allowing to conclude in the absence of deficiencies in these regular sites. The conventional reliability factors converged to Rp = 0.032, Rwp = 0.043, RBragg = 0.023 and χ2 = 3.9; the residual electron density (e·Å-3) on Ag sites were +0.6 on Ag1 and -0.3 on Ag2; the extreme values were +1.6 and -0.8, respectively, in and around the Bi site. See also Figure 2b for the final Rietveld plot, and Table 1 for the crystallographic data. Indeed, the two-fold linear coordination of the Ag2 site reminds the regular cation environments in the M2O (M = Cu, Ag) cuprite-type structure and in some M+ complexes, as well as their ability to build Frenkel defects as in AgBr. According to Orgel, this particular coordination of some species with ground state d10s0 like Cu+ and Ag+ results from the low energy level of their d9s1 excited state, in which the dz2 orbital exhibits an axial compression.39 To the best of our knowledge, the occupation of this interstitial site has never been reported in this family of compounds, in the same way as the marked stretching of the SS’ edges of BiAgOS seems to be unique. These two peculiarities are probably correlated, but call for an explanation. Cell parameter a only increases by 1.1 % when Ag+ (r = 1.14 Å 40) substitutes for Cu+ (r = 0.74 Å), probably because the (001) face is sized by the [Bi2O2]2+ framework rather than by the poorly compact S layer or by the Ag one (r(Ag+) < r(O2-)). As the S-S edges in the (001) planes are compelled to remain invariant, the S4 tetrahedron can only accommodate Ag+ by stretching following the c-axis, as shown by the 7.8 % lengthening of the cell edge. Thanks to this distortion, the distance between the mid-S-S’ position and the sulfur anions (2.33 Å) becomes

suitable

for

a

partial

occupation

by

an

interstitial

silver

cation

(r(Ag+) + r(S2-) = 2.51 Å). Electronic transmission microscopy In order to complement these results, TEM studies, including electron diffraction and HAADF-STEM together with EDX, were undertaken, as displayed in Figure 3. The layered tetragonal structure consisting of alternatively stacked Ag-S and Bi-O layers can be seen from HAADF-STEM images, where the brightest white dots correspond to the heavy Bi atoms (Z = 83) and the less brighter dots are the Ag atoms. EDX elemental mapping shows a uniform distribution of Bi and Ag atoms. On most places, the rows of Ag atoms appear as a 10 ACS Paragon Plus Environment

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continuous line. This observation is in fair agreement with the structural data determined by the Rietveld refinement, in which the Ag atoms are distributed among two crystallographic sites. Figure 3 shows the overlaid structural model corresponding to a full occupation of each site (2a and 4d). Curiously, in some places the aspect of the Ag layer changes from a single line to separate dots. The white arrows in the [110] HAADF-STEM image (Figure 3) show the Ag columns where the atoms appear as separate dots instead of a continuous line. Moreover, anomalies in the contrast of [100] HAADF-STEM images (Figure 4a) as well as in the EDX mapping (Figure 4b) were found. These observations highlight the presence of defects that called for a precise assessment. Spontaneously, the identification of separate dots in the Ag column can be attributed to some particular and randomly distributed places where only the regular site is occupied, as if one out of two Ag atoms were absent. The distribution of Ag within the interstitial sites would therefore not be homogenous. Unfortunately, precisely because of a very broad distribution of Ag within its sites, it has not been possible to determine the exact occupation of each site. In some places, EDX mapping images shows Bi-rich and Ag deficient areas. These are randomly distributed over the crystallites, as pointed out by white arrows on Figure 4b. By looking closely at the HAADF-STEM images (Figure 4a,c,d), these anomalies actually correspond to those places where the Ag column appears as separate dots. The corresponding intensity plot profile through the layers shows that the intensity of these dots corresponds to that of Bi atoms (Figure 4d). Moreover, in the normal case where the Ag column appears as a continuous line, the 2D structure formed by Bi atoms from one regular Bi column to the other, is a parallelepiped (in red on Figure 4e). When looking at the geometry at the place where the Ag column appears as separate dots, the 2D structure is a rectangle (in green on Figure 4e). This defect layer created a shift of the AgBiOS the structure of ½ b along the b axis (Figure 4c). Therefore, the observation of these separate dots can be explained by a substitution of Ag atoms by Bi atoms. Conceivably, the change in the local symmetry could result from the bigger size of Bi3+ which could hardly take place in the S4 tetrahedra of the regular structure. Instead, the opposite S layers now build square-based S8 prisms. This Bi-rich defect is actually an m mirror instead of the expected n that acts as a boundary between twinned crystalline domains (Figure 4c, d). These defects are randomly distributed within the lattice. Electronically, the defect can be written (BiAg•• + VAg’). It is therefore positively charged. This part will be further discussed in the discussion section. 11 ACS Paragon Plus Environment


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To conclude on the observation made with HR-TEM, it is clear that Bi-rich defects are present. However, it is possible that the places in the Ag columns where only separate dots are observed correspond to a non-uniform distribution of Ag in the interstitial sites. Electronic structure For all compounds, the crystal structure was geometry-optimized using the PBEsol functional, known to produce high accuracy optimizations (Table S2). The average discrepancy between theoretical and experimental structures is approximately 1%. The calculated band structures of BiMOCh (M = Cu, Ag; Ch = S, Se) phases are depicted in Figure 5 and Figure S2. For all compounds, the bandgap is indirect with a valence band maximum located along the Γ-M direction and a conduction band minimum at the Z point. The calculated energy bandgaps are 1.08 eV and 0.79 eV for BiCuOS and BiCuOSe, respectively. Substitution of copper by silver leads to an increase of the band gap to 1.52 eV and 0.98 eV for BiAgOS and BiAgOSe, respectively. The increase in bandgap compared to the copper compounds can be explained by a lower contribution of the d orbitals of silver to the top of the valence band, as these orbitals are located deeper in energy (Figure S3). This interpretation is also in good agreement with previous results20, 41. For all compounds, the total static dielectric constants are higher than 10 and the value for the BiAgOS phase is in very good agreement with the previous reported data in the literature20 (Table S3). This validates the requirements needed for a photovoltaic absorber16. The calculated hole effective mass considerably changes when substituting with silver. From 1.43 and 1.02 for BiCuOS and BiCuOSe, respectively, it increases to 2.1 and 1.15 for BiAgOS and BiAgOSe, respectively. The increase in the overall hole effective mass can be sensed when looking at the nearly flat band at the top of the valence band in the Z-R direction. On the other hand, the electron effective mass increases slightly for the silver compounds compared to their copper analogues, but not in the same proportions (Table S4). Optical properties The bandgaps of BiAgOS and BiAgOSe and their copper analogues were determined experimentally from diffuse reflectance measurements thanks to the Tauc Plot3 (Figure 6a). As an indirect bandgap was determined for all compounds thanks to band structure calculation, (αhν)0.5 is plotted vs. hν in Figure 6b. The bandgap is determined by the intersection of the linear part of the curve with the x axis as indicated by the dashed line. 12 ACS Paragon Plus Environment

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Bandgaps of 1.0 eV and 0.75 eV are respectively obtained for BiCuOS and BiCuOSe, which are in perfect agreement with our band structures calculations and some other values reported in the literature.2, 3, 9, 42 The valence band being composed of Ch p and Cu 3d orbitals (Figure S3), this decrease of the bandgap when S is replaced by Se is explained by the higher energy of Se orbitals (less electronegative atom). The same trend is observed for silver compounds, with a bandgap decreasing from 1.43 eV for BiAgOS to 0.94 eV for BiAgOSe. These later values are also in very good agreement with our band structure calculation. Moreover, the bandgap increases when Cu is replaced by Ag for an identical chalcogen atom. These results validate those obtained from the band structure calculations. Transport properties BiCuOCh materials are well-known p-type conductors, as demonstrated by Hall effect and Seebeck measurements in the literature.5, 6, 43 Transport properties of the powders were assessed by four point resistance measurements on pressed pellets (Figure 7). As expected, BiCuOSe shows a metallic behavior with a low electronic resistivity (2 Ω·cm at 300 K) which increases slightly for the sulfur compound BiCuOS (30 Ω·cm at 300 K). This is due to a lower mobility caused by a reduction in the valence band dispersion3, 5 but also a lower amount of charge carriers7 induced by a larger bandgap and therefore a less favored thermal activation of the carriers. The same trend is observed when shifting from S to Se in the silver compound. Compared to their copper analogues, the resistivity of BiAgOSe and BiAgOS are considerably higher, i.e. 500 kΩ·cm and 200 MΩ·cm at 300 K, respectively. The temperature dependence of the electrical resistivity of BiAgOS could not be obtained on the full temperature range because of its too high resistance. Moreover, the activation energy was 7 determined thanks to the Arrhenius law: 3 = 3 exp 89 by the slope of the curve

log(ρ) = f(1000/T) (Figure S4). For BiCuOSe, the value of the slope is negative because of its metallic behavior, whereas for BiCuOS, the activation energy is small but positive (0.03 eV). This is typical for doped semiconductors, where the conductivity is limited by scattering at impurities such as grain boundaries, charge centers, structural defects… The activation energy of BiAgOSe is close to 0.4 eV, which corresponds to half the value of its band gap. This activation energy corresponds to the behavior of an intrinsic semiconductor.44

Photoelectric properties 13 ACS Paragon Plus Environment


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Photoconductivity of BiMOCh compounds was assessed on pressed pellets by current measurement under an applied potential of 0.1 V. The sample was placed near a Xe lamp, and periods of dark and illumination were alternated. Figure 8 shows the current as a function of time and illumination. All materials present a typical photoconductivity behavior with an exponential growth and decay of the current as a function of time. In the case of BiAgOS, the photocurrent decreases with time. This may be attributed to a photo or thermal degradation. BiCuOS and BiCuOSe show a higher photocurrent, Iphoto, compared to their silver derivatives. However the sensitivity, being assessed by the ratio Iphoto/Idark, is around 2 orders of magnitude higher due to the low value of the dark current, Idark. The curves were fitted by an exponential type function, ; = '<

"⁄ =

+ ?, where 0 represents the time constant associated to

the growth (0+ ) and decay (0@ ) of the current. Values of 0+ and 0@ along with the principal characteristics of the experiments are given in Table 2. While both growth and decay constants are similar for BiCuOS (26 s vs. 30 s) and BiCuOSe (13 s vs. 10 s), 0+ is higher compared to 0@ for BiAgOCh. This could indicate a better life time of electron-hole pairs in BiCuOCh, compared to BiAgOCh. Discussion The crystal and electronic structures of BiCuOS and BiAgOS are very similar. Nevertheless, the observed differences have a huge impact on the experimental electronic properties. The BiCuOS phase presents a copper site occupancy of 0.94. The composition BiCu1-δOS is associated to the co-existence of Cu+ and Cu2+ species in the material and results in a p-type doping. The initial objective of this study was to reduce the amount of cation vacancies in order to reduce the amount of charge carriers by exchanging Cu+ and Cu2+ ions for Ag+ ions. However, after ionic exchange, the site occupation obtained by the Rietveld analysis was again 0.94 for silver and our objective seemed at first unachieved. However, this is not experimentally observed. The temperature dependence of the resistivity of BiCuOS shows the behavior of a heavily doped semiconductor, while BiAgOS is insulating. A similar trend is obtained for the selenide compounds. BiCuOSe is metallic, while BiAgOSe exhibits an intrinsic semiconductor behavior. These observations lead us to conclude that the charge carrier density or their mobility was considerably reduced in the silver compounds and that the silver vacancies could not explain the electronic properties.


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Concerning the mobility, the calculated hole effective mass of BiAgOS is higher than that of BiCuOS. The mobility of the charge carrier being inversely proportional to the effective mass, the BiAgOS phase should have a lower hole mobility than BiCuOS, of around half of its value. Its resistivity should be higher. But if a similar amount of charge carriers were present in the two compounds, the temperature dependence of their resistivity would be similar. This is not the case and the increase in resistivity cannot be only attributed to an decrease in mobility. In order to further understand these particularities and seek for hidden defects, high resolution TEM was performed. As explained in the results section, a Bi-rich defect is observed, which is positively charged, on the contrary to the silver vacancy defects revealed with Rietveld analysis. The presence of both types of defects in the structure therefore induces a charge balance. This observation can explain the origin of a high electrical resistivity in the silver compounds: the charge compensation results in a very low concentration of mobile charge carriers. This charge balance phenomenon has recently been reported for parent oxychalcogenide compounds45. Unfortunately, Hall effects measurements were unsuccessful in such compacted pellet specimens. Finally, the photoconductivity properties show that the reduced amount of charge carriers is beneficial to a higher difference between dark and photocurrents, and therefore a higher sensitivity. This result, supported by those of Baqais et al.20 who succeeded in evolving hydrogen using BiAgOS as well as a previous study from our group17 presenting the generation of a photocurrent in a photovoltaic device comprising BiAgOS, show the potential of these materials for photoconversion applications. One must still keep in mind that the charge carrier mobility is reduced in the silver compounds, which may be detrimental to the collection of charges in photovoltaic devices.

Conclusion For the first time, the synthesis of BiAgOCh (Ch = S, Se, Te) was performed through an ion exchange of copper and silver in solution at room temperature. Thereby, the present study contributes in finding low temperature synthesis alternatives for compounds unstable at high temperature. The crystallographic study of BiAgOCh reveals that this compound crystallizes in the same space group as BiCuOCh (P4/nmm) and shows the presence of silver in an interstitial site (4d) in BiAgOS, which proves the ionic conductivity ability of silver in 15 ACS Paragon Plus Environment


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the (ab) plane. The X-ray diffraction studies shows massive silver vacancies but these negatively charged defects may be compensated by extended Bi-rich defects found by high resolution TEM studies. The optical and electrical properties of the two pure samples BiAgOS and BiAgOSe were measured revealing a bandgap higher than their copper parent: 1.43 eV and 0.94 eV respectively. These compounds are highly resistive. This was interpreted on the basis of a reduced concentration of charge carriers due to the presence of compensating defects in the structure. The low mobilities induced by a higher hole carrier effective mass as compared to BiCuO(S, Se), is detrimental to charge carrier collection, and this will unfortunately limit the use of these compounds as photovoltaic absorber. However, BiAgOS and BiAgOSe show a high photoelectric response (Iphoto/Idark =70 for BiAgOSe). These results show the potential of BiAgOCh compound for photoelectric applications, such as photodetection. In a larger extent, this study contributes to understanding the high complexity between structure, defects and electronic properties in semiconductors. Acknowledgment We thank the French National Research and Technology Agency (ANRT) for the funding and Andrew N. Fitch (ESRF-ID 22, European Synchrotron Radiation Facility, 71 Avenue des Martyrs, 38000 Grenoble, France) for performing the synchrotron measurements. We also thank T. Barbier, S. Goyard, S. Croyeau (Solvay, Research and Innovation Center of Paris, France) for performing ICP-OES analysis. Supporting Information. Rietveld refinement results, band structure calculations, calculated projected density of states and calculated electron and hole effective masse of BiAgOS and BiAgOSe.


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34. Puente-Siller, D. M.; Fuentes-Aceituno, J. C.; Nava-Alonso, F., A kinetic– thermodynamic study of silver leaching in thiosulfate–copper–ammonia–EDTA solutions. Hydrometallurgy 2013, 134-135, 124-131. 35. FullProf Suite - Crystallographic Tool for Rietveld, Profile Matching & Integrated Intensity Refinements of X-Ray and/or Neutron Data. In 2006. 36. Bérar, J. F.; Lelann, P., E.S.D.'s and Estimated Probable Error Obtained in Rietveld Refinements with Local Correlations. J. Appl. Crystallogr. 1991, 24, 1-5. 37. Dollase, W. A., Correction of Intensities For Preferred Orientation in Powder Diffractometry - Application of The March Model. J. Appl. Crystallogr. 1986, 19, 267-272. 38. Brese, N. E.; O'Keefe, M., Bond-Valence Parameters for Solids. Acta Crystallogr. Sec. B 1991, B47, 192-197. 39. Orgel, L., Stereochemistry of Metals of the B Sub-Groups .1. Ions With Filled dElectron Shells. J. Chem. Soc. 1958, 4186-4190. 40. Shannon, R. D., revised Effective Ionic Radii and Systematic Studies of Interatomic Distances in Halides and Chalcogenides. Acta Crystallogr. Sec. A 1976, A32, 751-767. 41. Gamon, J.; Barboux, P.; Le Mercier, T.; Giaume, D. Optimization of doping and interfaces in semiconductors : a two case study of BiCuOS and ZnO. Thesis, Institut de Recherche de Chimie de Paris (IRCP, Chimie ParisTech); Solvay, January 2017. 42. Sheets, W. C.; Stampler, E. S.; Kabbour, H.; Bertoni, M. I.; Cario, L.; Mason, T. O.; Marks, T. J.; Poeppelmeier, K. R., Facile Synthesis of BiCuOS by Hydrothermal Methods. Inorg. Chem. 2007, 46, 10741-10748. 43. Li, J.; Sui, J.; Pei, Y.; Barreteau, C.; Berardan, D.; Dragoe, N.; Cai, W.; He, J.; Zhao, L.-D., A high thermoelectric figure of merit ZT > 1 in Ba heavily doped BiCuSeO oxyselenides. Energy Environ. Sci. 2012, 5, 8543-8458. 44. Wolpert, D.; Ampadu, P., Managing Temperature Effects in Nanoscale Adaptive Systems. Springer: 2012. 45. Gibson, Q. D.; Dyer, M. S.; Whitehead, G. F. S.; Alaria, J.; Pitcher, M. J.; Edwards, H. J.; Claridge, J. B.; Zanella, M.; Dawson, K.; Manning, T. D.; Dhanak, V. R.; Rosseinsky, M. J., Bi4O4Cu1.7Se2.7Cl0.3: Intergrowth of BiOCuSe and Bi2O2Se Stabilized by the Addition of a Third Anion. J. Am. Chem. Soc. 2017, 139, 15568-15571.



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Table of content/ Graphical abstract


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Figures caption Figure 1. XRD patterns of BiCuOCh compounds synthesized in quartz sealed tubes at 550 °C for 48 h, with (a) Ch = S, (b) Ch = Se, (c) Ch = Te; and of the corresponding BiAgOCh phases obtained by copper-silver ion exchange in solution from their copper analogues, with (d,e) Ch = S, (f) Ch = Se, (g) Ch = Te. XRD patterns of BiAgOS phase before (d) and after (e) silver leaching are presented. Impurity peaks are indicated as $ for Ag, * for Ag2Te and ° for Bi2Te3. Figure 2. (a) Crystal structure of BiAg0.94OS (left) and details of the Bi and Ag layers (right). Atoms: Bi (red), Ag1 (regular site, black), Ag2 (interstitial site, grey), O (blue), S (yellow). Cell parameters: a = 3.9118(1) Å, c = 9.2335(2) Å. Bond lengths: Bi-O = 2.332(1) Å (× 4), Bi-S = 3.132(2) Å (× 4), Ag1-S = 2.711(3) Å (× 4), Ag2-S = 2.331(3) Å (× 2). Shortest Ag1Ag2 distance is 1.383(1) Å. (b) Final Rietveld plot with inset high angle zoom of the synchrotron diffraction pattern of BiAg0.94OS, with Iobs (red dots), Icalc (black line), Iobs – Icalc (blue line) and Bragg positions (bars). Figure 3. High resolution HAADF-STEM images along main zone axis of BiAgOS: [001], [100] and [110] together with EDX elemental mapping. Enlargement images with overlaid structural model are given as an insert. Appearance of Bi rich layers within the BiAgOS structure in the [110] image is marked by white arrow heads. Figure 4. [100] HAADF-STEM images of the Bi rich defect in the BiAgOS crystallite. (a) Low magnification image and corresponding ED pattern: defects in the contrast are indicated by black arrows. (b) EDX elemental mapping (Ag L – red, Bi M – green) of defect region and overlaid color images. The Bi rich layers (green line) are indicated by white arrows. (c) High resolution HAADF-STEM image of the Bi rich defect, the shift of the AgBiOS structure is highlighted by the white lines. (d) Enlargement image together with the overlaid corresponding structural model showing the replacement of Ag by Bi in the same plane. Intensity plot profile is given as an insert. (e) Enlargement image at the place where the Birich defects occurs showing the change in the 2D geometry formed by 4 regular Bi atoms, the overlaid corresponding structural model is given as an insert. Figure 5. Band structure calculations (a) BiCuOS and (b) BiAgOS. Fermi energies are referred to the top of the valence band (black dashed lines at 0 eV).



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Figure 6. (A) Diffuse reflectance and (B) Tauc plot of an indirect band gap for (a) BiCuOS, (b) BiCuOSe, (c) BiAgOS and (d) BiAgOSe. α is the absorption coefficient calculated thanks to the Kumar expression, and hν is the energy of the incident radiation. Figure 7. Electrical resistivity as a function of temperature of (a) BiCuOS, (b) BiCuOSe, (c) BiAgOS and (d) BiAgOSe compounds. Electrical resistivity of BiAgOS too high to be measured properly. Figure 8. Photoconductivity measurement of a pellet of (a) BiCuOS, (b) BiCuOSe, (c) BiAgOS, (d) BiAgOSe) under an applied voltage of 0.1 V between two linear electrodes on the top surface at a distance of 0.6 cm from each other. Illuminated surface area of 0.75 cm2. Table 1. Atomic positions in space group P4/nmm (Z setting), thermal displacement parameters and site occupancy factors for BiAg0.94OS. Table 2. Photoconductivity properties of Bi(Cu,Ag)O(S,Se) pressed powders under a 300 W visible Xe lamp. Iphoto and Idark are the dark and photo current respectively,0+ and 0@ are the growth and decay constant of the current respectively.


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Figure 1. XRD patterns of BiCuOCh compounds synthesized in quartz sealed tubes at 550 °C for 48 h, with (a) Ch = S, (b) Ch = Se, (c) Ch = Te; and of the corresponding BiAgOCh phases obtained by copper-silver ion exchange in solution from their copper analogues, with (d,e) Ch = S, (f) Ch = Se, (g) Ch = Te. XRD patterns of BiAgOS phase before (d) and after (e) silver leaching are presented. Impurity peaks are indicated as $ for Ag, * for Ag2Te and ° for Bi2Te3.



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Figure 2. (a) Crystal structure of BiAg0.94OS (left) and details of the Bi and Ag layers (right). Atoms: Bi (red), Ag1 (regular site, black), Ag2 (interstitial site, grey), O (blue), S (yellow). Cell parameters: a = 3.9118(1) Å, c = 9.2335(2) Å. Bond lengths: Bi-O = 2.332(1) Å (× 4), Bi-S = 3.132(2) Å (× 4), Ag1-S = 2.711(3) Å (× 4), Ag2-S = 2.331(3) Å (× 2). Shortest Ag1Ag2 distance is 1.383(1) Å. (b) Final Rietveld plot with inset high angle zoom of the synchrotron diffraction pattern of BiAg0.94OS, with Iobs (red dots), Icalc (black line), Iobs – Icalc (blue line) and Bragg positions (bars).


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Figure 3. High resolution HAADF-STEM images along main zone axis of BiAgOS: [001], [100] and [110] together with EDX elemental mapping. Enlargement images with overlaid structural model are given as an insert (balls and stick representation: Bi, dark blue; Ag, orange; O, light blue; S, yellow). Appearance of Bi rich layers within the BiAgOS structure in the [110] image is marked by white arrow heads.



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Figure 4. [100] HAADF-STEM images of the Bi rich defect in the BiAgOS crystallite. (a) Low magnification image and corresponding ED pattern: defects in the contrast are indicated by black arrows. (b) EDX elemental mapping (Ag L – red, Bi M – green) of defect region and overlaid color images. The Bi rich layers (green line) are indicated by white arrows. (c) High resolution HAADF-STEM image of the Bi rich defect, the shift of the AgBiOS structure is highlighted by the white lines. (d) Enlargement image together with the overlaid corresponding structural model showing the replacement of Ag by Bi in the same plane (balls and stick representation: Bi, dark blue; Ag, orange; O, light blue; S, yellow). Intensity plot profile is given as an insert. (e) Enlargement image at the place where the Bi-rich defects occurs showing the change in the 2D geometry formed by 4 regular Bi atoms, the overlaid corresponding structural model is given as an insert.


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Figure 5. Band structure calculations (a) BiCuOS and (b) BiAgOS. Fermi energies are referred to the top of the valence band (black dashed lines at 0 eV).



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Figure 6. (A) Diffuse reflectance and (B) Tauc plot of an indirect band gap for (a) BiCuOS, (b) BiCuOSe, (c) BiAgOS and (d) BiAgOSe. α is the absorption coefficient calculated thanks to the Kumar expression, and hν is the energy of the incident radiation.


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Figure 7. Electrical resistivity as a function of temperature of (a) BiCuOS, (b) BiCuOSe, (c) BiAgOS and (d) BiAgOSe compounds. Electrical resistivity of BiAgOS too high to be measured properly.



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Figure 8. Photoconductivity measurement of a pellet of (a) BiCuOS, (b) BiCuOSe, (c) BiAgOS, (d) BiAgOSe) under an applied voltage of 0.1 V between two linear electrodes on the top surface at a distance of 0.6 cm from each other. Illuminated surface area of 0.75 cm2.


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Table 1. Atomic positions in space group P4/nmm (Z setting), thermal displacement parameters and site occupancy factors for BiAg0.94OS.

Table 2. Photoconductivity properties of Bi(Cu,Ag)O(S,Se) pressed powders under a 300 W visible Xe lamp. Iphoto and Idark are the dark and photo current respectively,0+ and 0@ are the growth and decay constant of the current respectively.


Substituting Copper with Silver in the BiMOCh Layered Compounds

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