Electrophoretic Deposition of Antimonene for Photoelectrochemical Applications

Antimonene is a recently developed two-dimensional material with outstanding expected physical properties based on theoretical calculations. Liquid phase exfoliation has become the most straight forward preparation method to produce stable antimonene suspensions. However, the processing and deposition on substrates of antimonene that is still required towards its exploitation in various fields, as current challenges in this research area. Despite the high current research interest in antimonene, the fabrication of Sb-films and its utilization in photoelectrochemical devices remains still unexplored. Herein, the electrophoretic deposition of antimonene on different substrates and its activity as absorber and hole acceptor layer in photoelectrochemical cell (PEC) is reported. The obtained results confirm that the photoelectrochemical performance of the antimonene films electrophoretically deposited on titanium dioxide exhibits an enhanced optical absorption and charge separation properties, compared to pristine TiO2 films. Furthermore, electrochemical measurements reveal that the antimonene films acts as a hole acceptor layer, enabling better PEC performance.


Introduction
Two-dimensional (2D) materials have gathered a lot of attention during the last decade since the isolation and exploitation of graphene due to the peculiar properties derived from nanosized effects [1]. A wide variety of 2D materials have been reported and applied in different fields e.g. transition metal dichalcogenides (TMDCS), metal carbides (MXenes) [2,3], metal-free carbon-and boron nitride [4,5], and monoelemental ones (Xenes) such like graphene or phosphorene to name a few [6,7]. In this context, a new monoelemental 2D material that has recently gained a lot of attention is the so-called antimonene (2D-Sb). As phosphorene, this 2D material is another pnictogen that has been isolated as monolayer or few-layers (FL) by several methods including mechanical [8], liquid-phase (LPE) or electrochemical exfoliation, [9] as well as epitaxial growth [10,11]. Antimonene has been widely investigated due to its high stability under ambient conditions and their predicted physical properties such as strong spinorbit coupling, topological properties or its low band gap suitable for optoelectronic applications [12,13]. However, despite the recent progress in the theoretical and experimental investigations of 2D-Sb [14], and its application as supercapacitor [15], catalyst for organic reactions [16], electrocatalyst for hydrogen evolution reaction (HER) [17,18], or biosensing [19,20], the fabrication of Sb-films and its utilization in photoelectrochemical devices remain still unexplored. Here, we demonstrate the deposition of 2D-Sb layers over conductive substrates by electrophoretic deposition (EPD) and its activity as absorber and hole acceptor layer in a photoelectrochemical cell (PEC). The morphology of the 2D flakes is thoroughly characterized as well as the physical and electronic features of the films. We further explored the deposition of 2D-Sb on mesoporous TiO2 as well as its photoelectrochemical performance.
The new Sb-TiO2 film exhibits an enhanced optical absorption and charge separation properties, compared to pristine TiO2 films. Electrochemical measurements reveal that the 2D-Sb mainly acts as a hole acceptor layer, enabling better PEC performance.

Preparation of 2D-Sb
This procedure involves a pre-grinding process of the Sb crystals (Smart Elements, 99.9999% purity) with an agate mortar resulting in so-called ground Sb. After the grinding process, a stainless-steel reactor with a volume of 5 mL (Retsch 1.4112) was filled under ambient conditions with 300 mg of ground Sb powder, 3 stainless steel balls of 4.74 mm diameter and 0.5 mL of butan-2-ol (99.5 %, Sigma Aldrich). Subsequently, the samples were milled for 120 min at 30 Hz in a Retsch MM 400 mixer mill. After milling, the reactors were washed with butan-2-ol to obtain all the grey metallic Sb paste, which was then centrifuged at 13000 rpm for 30 min. The deposited Sb was dried on a hot plate at 100 °C for 12 hours and for another 24 hours in a drying oven at 75 °C and a few mbar. A colloidal dispersion of 2D-Sb was prepared by sonication of 10 mg of ball-milled Sb in 10 mL of toluene for 30 min, 400 W, 24 kHz and at an amplitude of 40 % with a sonication tip. Then, the resulting black Sb suspension was centrifuged at 3000 rpm (746 g) for 3 min, in order to eliminate the non-exfoliated crystals, and the clear supernatant was recovered.

Deposition of 2D 2D-Sb on FTO substrates
Electrophoretic deposition (EPD) was carried out with an ENDURO TM Power supplies system operated at 300 V and different deposition times (1 to 3 min), and a colloidal dispersion of Sb (1 mg mL -1 ) in tholuene previously sonicated for 30 min at 40% amplitude with a sonication tip. Following the EPD, the electrodes were annealed for 1 h at 300 °C for improving the contact between the material and the substrate. TiO2-coated FTO electrodes were prepared by doctor blading a transparent paste of TiO2, with 20 nm particle size over clean FTO electrodes, followed by annealing at 450 °C for 30 min under air. Sb was then deposited over TiO2 electrodes by EPD during 3 min at 300 V followed by thermal annealing under air for 1 h at 300 °C.

Characterization
AFM measurements were carried out using a Cervantes Fullmode AFM from Nanotec Electronica SL. WSxM software (www.wsxmsolutions.com) was employed both for data acquisition and image processing [21]. PPP-NCHR cantilevers (nanosensors.com) with a nominal spring constant of 42 N·m -1 and tip radius of less than 7 nm were employed. The tapping mode was used for imaging to ensure that the nanolayers would not be damaged by the tip [22]. Raman spectra were acquired on a LabRam HR Evolution confocal Raman microscope (Horiba) equipped with an automated XYZ table using 0.80 NA objectives. All measurements were conducted using an excitation wavelength of 532 nm, with an acquisition time of 5 s and a grating of 1800 grooves per mm. To minimize photo-induced laser oxidation of the samples, the laser intensity was maintained at 10 % (1.6 mW). TEM images were obtained on a JEOL JEM 2100 FX TEM system with an accelerating voltage of 200 kV. The microscope has a multiscan charge-coupled device (CCD) camera (ORIUS SC1000) and an OXFORD INCA Xray XEDS microanalysis system. SEM analysis of the 2D-Sb nanolayers was performed using a Philips XL 30 S-FEG microscope operating at an accelerating voltage of 10 kV. X-ray diffraction patterns (XRD) of the synthesized powders were obtained using an Empyrean powder diffractometer (Panalytical). Ultraviolet-visible spectroscopy (UV-Vis) spectra were collected using a Cary 100 spectrophotometer. XPS data were obtained with an X-ray photoelectron spectrometer ESCALAB 250 ultrahigh vacuum (1 × 10 −9 bar) device with an Al Kα X-ray source and a monochromator. The X-ray beam size was 500 μm, survey spectra were recorded with a pass energy (PE) of 150 eV and high energy resolution spectra were recorded with a PE of 20 eV. To correct for charging effects, all spectra were calibrated relative to a carbon C 1s peak, positioned at 284.  V is the applied voltage, VFB the flat band potential, ND the donor density, e is the elementary charge, ε0 is the permittivity in vacuum, εr is the relative permittivity of TiO2, (taken as 50) [23], k is the Boltzmann constant and T is the absolute temperature, taken as 298 K. From this analysis, the values of and N D were extracted.

Materials Preparation and Characterization
Firstly, we pre-processed the bulk antimony crystals with butan-2-ol using a ball-milling method as described in our previous work [17]. Then, we used the pre-processed antimony crystals to obtain a colloidal dispersion of 2D-Sb by LPE using toluene as solvent (For further details see Experimental Section). The final concentration of the as-prepared dispersions was 0.135 g·L -1 (measured upon vacuum drying the sample overnight and the obtained solid was weighed to know the exact amount of Sb that was in the sample). This concentration value is not as high as those previously reported in butan-2-ol, ca. 0.368 g·L -1 [17], but suitable for films preparation by LPE. Toluene was selected as solvent instead of the typical alcohols [17] because it is highly suitable for the subsequent electrophoretic deposition step. Besides, using toluene does not affect to the morphological and chemical integrity to the obtained 2D-Sb nanolayers.  Figure S1 shows a limiting step height of ca. 4 nm that corresponds with a lateral dimension of ~ 209 nm, what perfectly agrees to that previously reported [17]. The ruffled morphology observed by AFM has been further confirmed by electron microscopy measurements using transmission (TEM) and scanning (SEM) modes. To further characterize the sample quality, we have also performed Raman measurements. Figure 1f shows a single point Raman spectrum of a 2D-Sb nanolayers deposited on a SiO2/Si substrate with a thickness of ca. 12 nm, showed in Figure 1b, revealing the representative main phonon peaks, the A1g mode at 150 cm -1 and Eg mode at 110 cm -1 [10]. We also performed Xray energy dispersive spectroscopy (XEDS) measurements in the 2D-Sb nanolayer showed in Figure 1e, corroborating their composition, showing small signals of oxygen ( Figure S4).
2D-Sb films were prepared by electrophoretic deposition (EPD) of a 0.135 g·L -1 toluene suspension of 2D-Sb nanolayers at 300 V and different time ranges for achieving different coverages thickness, followed by the thermal annealing at 300 °C for 1 h. Electrophoretic deposition is a very useful technique for the fabrication of homogeneous films of a wide variety of materials, including among others graphitic carbons [24,25], metal-organic frameworks (MOFs) [26,27], covalent organic frameworks (COFs) [28], or inorganic structures [29,30]. Nevertheless, XPS depth profile up to 17.5 nm reveals that the bulk Sb is much less oxidized, showing the peak corresponding to metallic Sb (527.9 eV) in addition to the antimony oxides Sb2O3 (529.8 eV) and Sb2O5 (530.2 eV), which appear at lower binding energies compared to the surface level (530.5 and 530.9 eV, respectively). A peak corresponding to O1s was also observed at 531.9 and 531.5 eV for the surface, and the deeper level, respectively (Figure 2b).
TiO2 is a widely used semiconductor in photo-catalytic applications due to its low cost and its utilization in a photoelectrochemical cell has been widely reported [35]. Nevertheless, its wide band gap alongside its poor hole extraction kinetics hinder its capability to achieve a high current density upon illumination and therefore its optimal water splitting performance. Our research group and others showed recently the formation of 2D/2D heterojunctions between a wide band gap semiconductor and low-dimensional pnictogens like phosphorene or 2D-Sb, where the intimate contact resulted in charge transfer, the quench of the electron-hole pairs recombination and consequently significant enhancement of the photocatalytic activity [36][37][38][39][40]. Based on our previous results, and the predicted electrocatalytic activity of Sb [41], we  Figure S8). Additionally, XEDS imaging shows a homogeneous distribution of the different elements ( Figure S9, S10) along the electrodes.

Photoelectrochemical activity
The photoelectrochemical performance of the prepared electrodes was tested in a standard three electrodes photoelectrochemical cell (PEC) with an Ag/AgCl reference electrode, Pt as counter electrode, in different electrolytes and upon 1 sun illumination [44,45]. The SbFTO electrodes prepared by deposition during 3 min compared to 1, 2, and 4 min show a slightly higher photocurrent in basic media (KOH 0.1M, pH = 13) ( Figure S11a). Therefore, we decided to focus the study on electrodes prepared with deposition conditions of 3 min and 300 V. SbFTO showed a higher photoelectrochemical performance in acid electrolyte ( Figure S11b, Figure   S12), reaching 4 µA cm -2 of stable current density upon illumination at 1.23 V vs RHE for the course of measurement. This fact could be due to the higher stability of the passivation layer of antimony oxides in acidic media, and partial dissolution of the antimony oxides layer into hydroxides [46]. The deposition of Sb flakes on TiO2 dramatically enhances the photoanodic performance, reaching an initial current of more than 250 µA cm -2 (vs 20 µA cm -2 of bare TiO2 electrodes) in a basic media, 130 µA cm -2 in acidic electrolyte, and 100 µA cm -2 in neutral pH (Figure 4a, b). We want to note that, despite higher values have been reported in the state of the art of photoelectrochemical cells using materials such like perovskites, [47] bismuth vanadates (BiVO4), [48,49] metal oxides, [50,51] carbon nitrides (C3N4) [52,53] and more (Table S1)  Further mechanistic evaluation of the photoelectrodes was carried out by impedance spectroscopy (IS). We have studied the behavior of the SbTiO2 photoanode (compared to TiO2 reference) under different polarization conditions. The transport and recombination dynamics of the electrodes were determined under forward polarization (negative currents in the cyclic voltammograms, Figure S14), and the results clearly showed that at this regime, both conductivity and recombination dynamics were less favorable for PEC performance, compared to the reference TiO2 (Figure 5a, b).  On the other hand, a flat-band potential around 0.22 V vs RHE and a donor density of around 5×10 20 cm -3 were obtained for SbTiO2 doped ( Figure S15c, d). These values are in good agreement with previous studies on TiO2 photoanodes [58,59]. It is clear that the deposition of Sb flakes anodically shifts the conduction band of TiO2 and slightly increases the carrier density. The energy diagram for both TiO2 and SbTiO2 was determined from optical and electronic characterization ( Figure S16). Compared to the reference TiO2, SbTiO2 exhibits a valence band (VB) edge located at slightly lower energy respect to vacuum level, which thermodynamically favors the driving force of photo-generated holes for water oxidation.

Photoelectrodes stability
The stability of the photoelectrodes was evaluated by performing photoelectrochemical measurements for a prolonged period of time in both basic and acidic media. Despite the high initial photocurrent density, it decreases quickly with time, until reaching 40-50 and 30 µA cm -S17a, b). The decay in the photoelectrochemical performance is attributed to the partial dissolution of the Sb layer in the electrolyte forming antimony hydroxides in basic media, as confirmed by the XRD patterns of the recycled electrode, where all the contributions corresponding to the -antimony layer almost vanished (Figure 17c).
In the case of the measurement performed in acid electrolyte, where the system produced 0.03 µmol H2 after 1 h ( Figure S18) the 2D-Sb layer suffers less losses and still shows remaining XRD diffraction peaks of the initial -antimony. Additionally, XPS in depth profile confirmed the presence of the same chemical states shown before PEC measurements ( Figure S17d, S19).

Conclusions
Liquid phase exfoliation of antimony in toluene allows the formation of a suspension containing 2D-Sb nanolayers (200-400 nm in lateral dimensions) with a thickness of few nanometers (ca. 4 nm). This suspension is highly suitable to produce homogeneous and well-defined films with thickness of antimonene on FTO and TiO2-coated FTO using electrophoretic deposition.
The SbFTO electrodes prepared by deposition during 3 min and 300 V showed a higher photoelectrochemical performance in acid electrolyte, reaching 4 µA cm -2 of stable current density upon illumination at 1.23 V vs RHE for the course of measurement. This is attributable to the higher stability of the passivation layer of antimony oxides in acidic media, and partial dissolution of the antimony oxides layer into hydroxides. Thus, the deposition of Sb flakes on TiO2 dramatically enhances the photoanodic performance, reaching an initial current of more than 250 µA cm -2 (vs 20 µA cm -2 of bare TiO2 electrodes) in a basic medium, 130 µA cm -2 in acidic electrolyte, and 100 µA cm -2 in neutral pH with remarkable hole extraction kinetics as shown by the measurements performed in the presence of TEOA. Therefore, the so-formed TiO2-coated FTO films have shown promising activity as absorber and hole acceptor layer in a photoelectrochemical cell. The mechanistic evaluation of the TiO2-coated FTO photoelectrodes carried out by impedance spectroscopy shows that both conductivity and recombination dynamics in the cathodic region are less favorable for PEC performance, compared to the reference TiO2. Nevertheless, the deposition of Sb flakes anodically shifts the conduction band of TiO2 and slightly increases the carrier density resulting in improved optical absorption and charge separation properties.
The energy diagram for both TiO2 and SbTiO2 determined from optical and electronic characterization shows a valence band edge located at slightly lower energy respect to vacuum level, which thermodynamically favors the driving force of photo-generated holes for water oxidation.