Pulse electrodeposited bismuth-tellurium superlattices with controllable bismuth content

Abstract Superlattice structures of (Bi2)m(Bi2Te3)n series with controllable Bi mole fraction from 0.41 to 0.71 are electrodeposited in pulse potentiostatic mode from acidic electrolytes containing Bi(NO3)3 and TeO2 as precursors. Two valence states of bismuth in superlattices are identified by X-ray photoelectron spectroscopy (XPS). One of those states is attributed to interlayered Bi0 which is present in (Bi2)m(Bi2Te3)n superlattice in the form of biatomic layers between bismuth telluride quintuples. X-ray difraction (XRD) analysis and density functional theory (DFT) calculations indicate an increase in subcell parameter asub and decrease in subcell parameter csub with the increase of Bi mole fraction. Biatomic layers of Bi0 are identified with cyclic voltammetry by characteristic anodic peak between potentials of metallic bismuth and Bi2Te3 oxidation. The selective oxidation of Bi-bilayers in (Bi2)m(Bi2Te3)n superlattice at the potential of the anodic peak results in the product corresponding to Bi2Te3 by stoichiometry, but having an expanded crystal structure. Superlattices with controllable Bi mole fraction and Bi2Te3 with “memory effect” may be of interest for design of new thermoelectric materials with controllable parameters.


Introduction
Synthesis of superlattice structures of the infinitely adaptive series (Bi 2 ) m (Bi 2 Te 3 ) n with variable bismuth content is of special interest due to the emerging opportunity of controllable amount of metal introduction at van der Waals planes between Te-Bi-Te-Bi-Te quintuples in the layered structure of bismuth telluride [1] and thus controlling anisotropic electron and heat transport, which is essential in thermoelectric material and topological insulator design [2][3][4][5]. Bi 0 is present in (Bi 2 ) m (Bi 2 Te 3 ) n superlattice in the form of biatomic layers between bismuth telluride quintuples, so the variability of bismuth content is provided by variable distribution of bismuth biatomic layers in the parent Bi 2 Te 3 layered structure. Previously, (Bi 2 ) m (Bi 2 Te 3 ) n superlattices were obtained by synthesis in the solid state [1,6,7] and sputtering [8].
On the contrary to high temperature solid-state synthesis which controls the target composition by temperature, the electrochemical synthesis controls the composition by electrode potential. The electrode potential drives the solid-state synthesis at mild conditions in aqueous medium, which allows preparation of various, even metastable, structures. Electrochemical equipment is simpler and more common than vacuum deposition techniques, which makes electrochemical deposition cheap and easy-scalable for practical application [9,10]. Pulse periodic potential control was used in this work to maintain uniform mass transport conditions over the whole deposition period and to control metallic bismuth oxidation, while preventing bilayers (Bi 2 ) anodic oxidation. Additionally, pulse deposition favors high substrate-film adhesion and proper film solidity [11][12][13][14][15][16]. Bi-Te binary phase diagram [17] (Figure 1) predicts the formation of a series of stoichiometric compounds. Any composition between vertical lines corresponding to Bi 2 Te 3 and Bi 7 Te 3 is possible, according to Anderson [18], and represents a superlattice of (Bi 2 ) m (Bi 2 Te 3 ) n which lacks translational symmetry in the direction of Bi 2 Te 3 quintuple and Bi-bilayer stacking at high bismuth content ( Figure 1).
For this reason, special method of XRD to proof the variable superstructure based on analysis of subcell parameters of the supelattice was applied in this work, following the approach used earlier in [1,6,19].
Cyclic voltammetry was used to distinguish the interlayer Bi 0 from metallic Bi 0 by their potentials of anodic oxidation and to set parameters of pulse electrodeposition. Bismuth in different oxidation states in the superlattices was distinguished by X-ray photoelectron spectroscopy. Elemental composition was derived with atomic absorption spectroscopy analysis, energy-dispersive X-ray analysis and mapping of elements in high-resolution transmission electron microscopy. The High resolution transmission electron microscopy (HRTEM) provided direct observation of change in parameter a with increase of bismuth mole fraction which was consistent with XRD data.
Electrodeposition was carried out in the periodic pulse potential control mode in a three-electrode cell with working electrode, Ag|AgCl|KCl(sat) reference, and Pt counter electrode. Gamry Series G300 potentiostat was used for pulse potentiostatic deposition and cyclic voltammetry (CV). The cathodic deposition at −0.1 V was followed by the ten times longer anodic phase at the potentials derived from CV for electrolytes with various concentrations. The anodic potential was set in the range of metallic bismuth anodic oxidation which prevented the superlattice growth from disturbance by 3D bismuth particles nucleation. Pulse frequency was 1 Hz. The films were deposited for 120 minutes with stirring of the electrolyte solution, rinsed with distilled water and afterwards annealed at 250 °C in nitrogen for 2 hours.
XPS analysis of samples was performed with ESCALAB MKII (VG Scientific) spectrophotometer. XPS data were analyzed using XPSPeak41 program. NIST XPS database [21] was used for reference data.
Atomic absorption spectroscopy was performed with PerkinElmer AAnalist 400 spectrometer. Both bismuth and tellurium were analyzed in the solution obtained by dissolution of Bi x Te y films in nitric acid and dilution to the concentration of the calibration curve. Calibration curves for bismuth and tellurium were recorded for individual solutions of Bi(III) and Te(IV) which were prepared by dissolution of elemental Bi and Te in concentrated nitric acid followed by dilution. Bismuth was quantitatively analyzed at wavelength 214.28 nm (calibration concentration range 5 ppm -35 ppm). Tellurium was quantitatively analyzed at wavelength 223.06 nm (calibration concentration range 5 ppm -35 ppm).
Density functional theory calculations were carried out using the projector-augmented plane-wave (PAW) method implemented in the Vienna Ab Initio Simulation Package (VASP) [22].
The generalized gradient approximation (GGA) exchange-correlation functional proposed by Perdew, Burke, and Ernzerhof (PBE) [23] together with the D3 dispersion correction proposed by Grimme [24] was applied. To model bulk tellurides we used 1×1×1 unit cells, a Monkhorst-Pack k-point mesh of 6×6×1, 6×6×1, and 6×6×2 for 17-1 (Bi 2 Te 3 ), 52 (Bi 4 Te 3 ), and 72 (Bi 2 Te) and an energy cut off of 400 eV. and no further significant current above 0.1 V. Tellurium cathodic deposition (blue curve) starts in the region of bismuth cathodic deposition and shows a sluggish cathodic current continuing through the region of bismuth anodic peak in the reverse scan. The electrochemically irreversible cathodic deposition turns into the anodic oxidation close to 0.5 V with the peak current at approx. 0.6 V (peak D).
The potential of peak B and the potential of the transition from cathodic deposition to anodic oxidation ( Figure 2 b, insert), shift negatively with Bi 3+ content in the electrolyte solution, i.e. in the opposite direction to the Nernstian shift of the reversible redox potential of Bi 3+ /Bi 0 (bulk) redox system. Also, the potential of peak B is significantly higher than that for bulk metallic bismuth anodic oxidation (peak A), thus peak B was unlikely to be attributed to metallic bismuth. Also, charges corresponding to peak B were too high for surface limited reactions; neither anodic oxidation of surface layer of Bi atoms of Bi 2 Te 3 [28], nor anodic oxidation of Bi adatoms on Bi 2 Te 3 which we described earlier [29,30], could explain the magnitude and behavior of this peak; on the contrary, the behavior of peak B was consistent with our expectation of Bi 2 Te 3 contamination by (Bi 2 ) m (Bi 2 Te 3 ) n at high с(Bi 3+ )/с(TeO 2 ) in Bi 2 Te 3 deposition bath, with attribution of peak B to anodic oxidation of bismuth biatomic layers. In order to obtain films of individual (Bi 2 ) m (Bi 2 Te 3 ) n , the conditions of the cathodic deposition were tuned in favor of its formation by using periodic pulse potential mode with separately controlled potentials of the cathodic and anodic reactions. The anodic potential was set in the region of metallic bismuth oxidation, though keeping it low enough to prevent anodic oxidation of bilayers (Bi 2 ). The formation of the bilayers and their stability at the potential of metallic bismuth anodic oxidation may be related to Bi adatoms which are formed on bismuth telluride surface in underpotential range [29]. The cathodic deposition of bismuth atomic monolayer on bismuth telluride is more favorable thermodynamically than the cathodic deposition of metallic bismuth (the deposition of adlayer proceeds at more positive electrode potential [29]).
Bismuth bilayer which is formed at van der Waals plane between bismuth telluride quintuple is a product of joining two bismuth atomic layers. Both atomic layers in the bilayer are stabilized by interaction with bismuth telluride, so the redox potential of Bi bilayer also appears in the underpotential region, which provides a route for the superlattice formation omitting formation of bismuth metallic phase. Besides favoring the bismuth bilayer electrodeposition and preventing the nucleation of metallic Bi, the periodic switching between the cathodic and anodic potentials under the pulse deposition mode helped to avoid depletion of precursors in the electrochemical reaction area. Thus, controlled uniform electrodeposition resulted in the formation of compact films with good adhesion to the substrate.

Pulse potential electrodeposition
The (Bi 2 ) m (Bi 2 Te 3 ) n films were electrodeposited onto steel under control of the deposition and refinement at intermittent potentials of Bi 2 Te 3 cathodic deposition and bismuth anodic oxidation. Pulse frequency was set to 1 Hz with the deposition potential (E dep ) at −0.10 V and 10% duty cycle. The potential of the anodic phase was set from CV data for each с(Bi 3+ )/с(TeO 2 ) as the potential of the end of the cathodic current loop before the peak B (insert in Figure 2 b). The purpose of applying the anodic stage at с(Bi 3+ )/с(TeO 2 ) dependent potentials was to oxidize anodically nuclei of bulk bismuth phase with preservation of bismuth bilayers which were presumably associated with the anodic peak B.  The elemental composition of deposit is a function of с(Bi 3+ )/с(TeO 2 ) ratio in the electrolyte solution. An almost linear region of the dependence is observed for the low ratios whereas the slope of the graph decreases at high с(Bi 3+ )/с(TeO 2 ) ratio. Thus, the atomic Bi/Te ratio in the product of the bipotential controlled periodic pulse deposition can be efficiently managed by the electrolyte composition.

XRD analysis
Figure 4 a-d shows XRD patterns of Bi x Te y films deposited from electrolytes with different с(Bi 3+ )/с(TeO 2 ) ratio and annealed under N 2 atmosphere at 250 °C for 2 hours. We found that annealing did not influence on the phase composition and only improved the degree of crystallinity which allowed applying more accurately the subsequent profile fitting in the XRD patterns. The profile fitting was carried out using Jana2006 program [20]. Basis subcell of R-3m symmetry with approximate lattice parameters a sub = 4.4 Å, and c sub = 6.0 Å was used for the refinement. Modulation vector was not included in the refinement procedure, as insufficient number of satellites was registered using CuKα radiation to provide convergence of γ value.
The refined subcell parameters are in good agreement with the data obtained by Bos et al.

Scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM)
SEM images of the Bi x Te y films pulse deposited from electrolytes of different с(Bi 3+ )/с(TeO 2 ) ratio are shown in Figure 5. The films consist of big (ca 10 μm) aggregates with high surface area. The shape of the aggregates changes from roundish at stoichiometric Bi 2 Te 3 to more angular and rougher for films with high Bi content. Interestingly, small light spots appeared on SEM image (Figure 5 a), when the films were deposited with excess of Te. These spots most likely correspond to tellurium [30] which, unlike bismuth, cannot form a single phase with Bi 2 Te 3 and tend to form elemental Te phase. That latter was justified by XRD of Te-rich film. No phase separation was observable in SEM images of the deposits with Bi excess.

X-ray photoelectron spectroscopy (XPS)
Bi 0 state related to metallic bismuth was excluded by conditions of electrodeposition (sufficiently positive electrode potential applied in the anodic refinement phase of the pulsed periodic electrodeposition provided complete anodic oxidation of metallic bismuth and this was controlled by CV), therefore Bi 0 in the pulsed electrodeposited film observable with XPS could be attributed to the interlayer bismuth, the one characterized by anodic oxidation peak B in Figure 2, and that was the motif for investigation of the electrodeposited films with XPS.
X-ray photoelectron spectra of (Bi 2 ) m (Bi 2 Te 3 ) n electrodeposited films of various composition are presented in Figure 6. Analysis of the spectra has disclosed no Bi 0 in nearly stoichiometric Bi 2 Te 3 samples and the significant presence of Bi 0 at enhanced Bi content, with the over-stoichiometric bismuth corresponding to Bi 0 ( Table 1). The similar XPS proof of Bi-bilayer formation was provided in [31][32][33].
Bi III is represented in the spectra by Bi 2 Te 3 and Bi 2 O 3 joint contribution. The contribution of Bi 2 O 3 did not vanish completely with argon etching, especially at the highest used Bi content, probably, due to incompact film structure which prevented from efficient elimination of the chemically adsorbed oxygen at argon etching. purple -5d 5/2 + 5d 3/2 band of Bi 0 , red line represents sum spectrum.

Selective anodic oxidation of interlayer bismuth in (Bi 2 ) m (Bi 2 Te 3 ) n
Based on obtained evidences from XRD and XPS analysis of (Bi 2 ) m (Bi 2 Te 3 ) n structures formation, the peak B in Figure  The stripping of the deposit obtained at E dep = −0.05 V discloses a single peak C of bismuth telluride anodic oxidation (Figure 7 a) The interlayer bismuth inclusion requires the use of lower E dep (Figure 7 b,c,d). For further justification of the bismuth interlayer anodic oxidation peak B, films of different composition using various deposition potentials and different electrolyte composition were obtained and changes in subcell parameters and elemental composition were examined upon the potentiostatic treatment at 0.38 V in 3 mol·L -1 HNO 3 for 10 minutes (Figure 8). The 0.38 V potential corresponds to the end of interlayer bismuth oxidation, but is insufficient for anodic oxidation of bismuth telluride. The results presented in Figure 8 and Table 2 show that the selective anodic oxidation at 0.38 V causes a decrease in Bi/Te ratio almost to that of stoichiometric Bi 2 Te 3 which corresponds to Eq (1): (Bi 2 ) m (Bi 2 Te 3 ) n = nBi 2 Te 3 + 6ne − + 2mBi 3+ Thus, the peak B in stripping voltammograms is the convenient indicator of the interlayer bismuth in the electrodeposits, and this interlayer bismuth can be surprisingly easily removed by the anodic treatment.
The even more surprising result is the prevention of the expanded structure in the oxidized superlattice of moderate initial bismuth content (Figure 8 g, h). It appears that the greater part of interlayer bismuth was transformed into cations and evacuated from the crystals in the strongly acidic medium which was used in the experiments with selective anodic oxidation of the interlayer bismuth.  This oxidative lability of the superlattice may provide novel possibility to control structure and composition of Bi x Te y materials that may be of interest for complex thermoelectric materials design based on bismuth telluride.

Conclusions
Superlattice structures of (Bi 2 ) m (Bi 2 Te 3 ) n series with Bi mole fraction varying from 0.41 to 0.71 were synthesized by pulse potentiostatic electrodeposition with controlled potentials of cathodic and anodic phases: the lower potential was set in the region of bismuth telluride cathodic deposition, while the upper potential was set in the region of metallic bismuth anodic oxidation, where bismuth adatoms on bismuth telluride were yet stable. The molar fraction of bismuth in the superlattices was controlled by atomic ratio of Bi(III) and Te(IV) in the electrolyte solution.
The pulse potential control mode favored formation of compact film by providing improved mass transport of precursors. The presence of Bi 0 valence state of the interlayer bismuth was proved. The interlayer Bi 0 component of (Bi 2 ) m (Bi 2 Te 3 ) n superlattices was distinguished from metallic bismuth by significantly higher potential of anodic oxidation.
An increase in the subcell parameter a sub from 4.39 Å to 4.45 Å and a decrease in parameter c sub from to 6.09 Å to 5.97 Å with bismuth molar fraction increase from 0.41 to 0.71 was determined. The variation of the structure was confirmed by DFT calculations and high resolution transmission electron microscopy observation of the superlattices.
The potential range of the interlayer bismuth anodic oxidation is significantly below the range of bismuth telluride anodic oxidation. The separation of the two anodic reactions on the potential scale has provided the selective anodic oxidation and removal of the interlayer bismuth from the superlattice with formation of the product, which was very close to Bi 2 Te 3 by stoichiometry, but differed from bismuth telluride by expanded crystal structure.
Due to advantages inherent in the electodeposition technique (low temperature of solid phase formation, precise control of the film composition and thickness, economy and appropriateness for deposition on surfaces of complex topology), the new electrochemical method of (Bi 2 ) m (Bi 2 Te 3 ) n superlattice structure preparation is expected to give a stimulus in the development of thermoelectric materials with controllable electrophysical properties. The discovered effect of the expanded structure preservation during the anodic oxidation and removal of the interlayer bismuth can be also of interest for other energy conversion and storage devises development based on intercalation of metal cations.