Electrochemical deposition of GaTe thin films for photoelectrochemical applications

GaTe thin films are electrochemically deposited on indium tin oxide for photoelectrochemical applications. The electrochemical deposition behavior of GaTe thin films in acidic solution of HTeO2+ with Ga3+ is studied with cyclic voltammetry combining with operando transmittance spectroscopy. Underpotential deposition of Ga on Te starts at potential of − 0.35 V. The presence of Ga3+ in the solution can strongly suppressed the formation of H2Te. XPS analysis reveals that Ga-rich GaTe is deposited over a wide potential range. The photoelectrochemical performance of the thin films as photocathodes is strongly dependent on the deposition potential. The GaTe films deposited at − 1.0 V produced the highest photocurrent of about − 0.03 mA cm−2. Meanwhile the film deposited at − 0.35 V shows improved performance during photoelectrochemical measurement, which can be ascribed to the increased GaTe content during photoelectrochemical measurements, as confirmed by XPS analysis.


Introduction
Photoelectrochemical (PEC) water splitting as a promising routine for producing clean fuel of hydrogen using solar energy has attracted extended investigation [1].Solar to hydrogen conversion efficiency of PEC cells is primarily determined by the photoelectrodes [2].There are many intrinsic n-type semiconductors such as TiO 2 [3], BiVO 4 [4], WO 3 [5] and ZnO [6] that can be used as photoanodes.In contrast, only a few native p-type semiconductors can be used as photocathodes.Cu 2 O as one of most representative photocathodes have been widely investigated [7,8].Recently, tellurium-based compound semiconductors such as CdTe [9] and ZnTe [10,11] exhibiting potential application as photocathodes have attracted research interest.
Gallium tellurium (GaTe) is a layered III-VI van der Waals semiconductor, having high carrier mobility and long carrier lifetime [12,13].Undoped GaTe is a native p-type semiconductor with a direct bandgap around 1.7 eV and a high optical absorption coefficient [13].GaTe has been investigated for potential applications in optoelectronic devices, such as radiation detectors [13], THz sources and sensors [14], solar cells [15,16].Recently, Li et al. demonstrated the potential application of GaTe for PEC hydrogen production [17].Photocatalytic degradation of methyl blue with GaTe nanowires and nanosheets was achieved by Tien et al. [12].However, the performance of the GaTe still does not satisfy photoelectrochemical applications, required further study.
Chemical vapor deposition (CVD) and physical vapor deposition (PVD) are usually used for synthesis of GaTe nanomaterials [12,16,17] that require high cost instruments and are time consuming.Alternatively, electrochemical deposition is a promising and facile method to prepare semiconductor thin films due to its simplicity, low cost, ease of scale-up and environmental compatibility, which has been used for deposition of Te [18,19], CdTe [9,20], Bi 2 Te 3 [21], ZnTe [22,23] thin films.However, electrochemical deposition of GaTe is still rarely reported [24].Up to now, electrochemical growth behavior of GaTe thin film is required further investigation.In this paper, we systematically study the electrochemical deposition behavior of Te and GaTe with cyclic voltammetry and operando transmittance spectroscopy.It is found out that the presence of Ga 3+ results in the growth of GaTe that strongly suppresses the formation of H 2 Te.Coexistence of Ga and Te in the films deposited at a wide potential range from − 0.35 to − 1.0 V, revealing that GaTe can be deposited at potential as positive as − 0.35 V.All the GaTe thin films can be used as photocathodes and the one deposited at − 1.0 V achieves the highest photocurrent of − 0.03 mA cm −2 .

Materials and methods
Te and GaTe thin films were grown by electrochemical deposition that was carried out with the experimental setup sketched in Fig. 1.Briefly, a three-electrode cell was mounted at an optical microscope to monitor the growth of the Te and GaTe thin films with operando transmission spectroscopy.In the three-electrode cell, an indium tin oxide (ITO) covered glass, a Pt wire and an Ag/AgCl (saturated KCl) were used as the working electrode, counter electrode and reference electrode, respectively.Before deposition, the ITO substrates were cleaned in acetone, ethanol and water, sequentially.The electrode area of the ITO glass was confined to 0.2 cm 2 by a rubber O-ring.Solution was prepared by dissolution of tellurium oxide (TeO 2 , 99.99%, Aladdin) in 0.1 M H 2 SO 4 .Then 0.05 M Ga(NO 3 ) 3 (99.99%,Aladdin) was added to the 0.1 M H 2 SO 4 + 1 mM TeO 2 solution to form 0.1 M H 2 SO 4 + 1 mM TeO 2 + 0.05 M Ga (NO 3 ) 3 solution.All the electrochemical experiments were conducted with an electrochemical workstation of CHI660D.Cyclic voltammetry (CV) was performed to study the deposition behavior of Te and GaTe.During CV measurements, the potential was scanned negatively from 0.3 to − 0.9 V (or − 1.1 V) and then reversed to 0.7 V at a rate of 10 mV s −1 .Transmission of white light emitted from a LED was simultaneously recorded with a Si diode detector (Thorlabs, PAD 100 A-FS).After studying on the deposition behavior with CV, the thin films were deposited with chronoamperometry at different potentials of − 0.35 V, − 0.5 V, − 0.6 V, − 0.7 V, − 0.8 V, − 0.9 V and − 1.0 V for 300 s.
Morphology of the thin films was imaged with a Hitachi S4800 scanning electron microscope (SEM).Relative diffuse reflectance spectra of the thin films in the range of 200 to 1000 nm were measured with a Shimadzu UV-3600 Plus UV-VIS-NIR spectrophotometer.X-ray photoelectron spectroscopy (XPS) was conducted with a Thermo Nexsa G2 equipped with the Al Ka (hn = 1486.60eV) to characterize the surface composition of the thin films.All the XPS results were calibrated with the binding energy of C 1s photoelectrons of 284.8 eV.
Photoelectrochemical measurements were conducted with the same setup as the thin film preparation.0.1 M H 2 SO 4 was used as the electrolyte.During PEC measurements, the thin films deposited at different potentials were illuminated from backside with the white light LED (400-700 nm, 10 mW cm −2 ).All the photoelectrochemical results are referred to reversible hydrogen electrode (RHE), calculated with the relation of V RHE =(V Ag/AgCl + 0.197 + 0.059 × pH) V.For linear sweep voltammetry measurements, the potential of the thin films was scanned negatively from 0.25 to − 0.44 V RHE with at a rate of 10 mV s −1 under periodic illumination.Transient photocurrent of the thin films was measured by controlling the electrode potential at − 0.36 V RHE .

Electrochemical deposition behavior of Te and GaTe
Figure 2a shows the cyclic voltammograms (CVs) of the ITO substrate in the solutions without and with Ga 3+ .The scanning direction of the CVs is instructed by the arrows.The simultaneously recorded transmission spectra are shown in Fig. 2b.HTeO 2 + formed by dissolution of TeO 2 in acidic solution is the electroactive species for deposition of Te and GaTe [18].In the solution of 0.1 M H 2 SO 4 + 1 mM TeO 2 , a cathodic peak at − 0.36 V is observed, which Upon the deposition of Te, the normalized transmittance decreases with the negatively scanning potential until the potential reaches − 0.71 V.There is another cathodic peak at around − 0.77 V observed, which is corresponding to the reduction of Te to H 2 Te [19,26], as described by Eq. ( 2): Consumption of the pre-deposited Te for producing gaseous H 2 Te is proven by the slightly increased transmittance at around − 0.77 V. Dennison et al. illustrated that the generated H 2 Te would further react with HTeO 2 + to form a fine, densely packed Te film, as described by Eq. ( 3) [18]. (1) The Gibbs energy of reaction (3) is − 500 kJ mol −1 , pointing to a strong driving force for the spontaneous formation of the densely packed film [18], which is also indicated by the decreased transmittance.Hydrogen evolution occurred when the potential was more negative than − 0.8 V which is corresponding to the rapidly increase photocurrent.
At the reverse scan, the transmittance slightly increased at the potential range from − 0.7 to − 0.36 V, indicating the reduction of Te to H 2 Te.At the potential range from − 0.36 to − 0.1 V, the transmittance decreased but without current detected, which probably could be attributed to decomposition of H 2 Te to Te [26], following the Eq. ( 4): One anodic peak at 0.45 V can be seen, which is due to the re-oxidation of Te to HTeO 2 + [18] and resulted in the rapidly increased transmittance.
(3) As shown by the CV curve (in red) in Fig. 2a, the present of Ga 3+ resulted in slightly positive shift of the first cathodic peak and higher cathodic current.Coexistence of Ga and Te of the film deposited at − 0.35 V is evidenced by the X-ray photoelectron spectroscopic analysis (Fig. 4), indicating the co-deposition of Ga and Te.Similar behavior of deposition of CdTe over a wide potential range has also been observed by Dennison et al. [20].In the range from − 0.8 to − 0.9 V, cathodic current significantly increases in the present of Ga 3+ but the transmittance of the film just slightly decreases, indicating that the high cathodic current is corresponding to hydrogen evolution catalyzed by GaTe.At the reverse scan, the transmittance keeps decreasing until the potential reached 0.35 V, demonstrating that the deposited Te would react with Ga 3+ to form GaTe rather than reduced to H 2 Te.The existence of the anodic peak at 0.45 V demonstrates that the deposited Te cannot be completely converted to GaTe even though the concentration of Ga 3+ is excess.The coexistence of GaTe and Te does not affect the stability of the Te, as indicated by the unchanged anodic peak potential.
In order to assess the effect of hydrogen evolution on the stability of the thin films at more negative potential, the cathodic potential is extended to − 1.1 V.As revealed by Fig. 2c, as the cathodic potential scans to − 1.1 V, the transmittance rapidly decreases upon the formation of a thick Te film.The anodic peak at − 0.36 V can be attributed to the oxidation of H 2 Te to Te.This reaction makes the Te film become thicker rapidly, crack and peeling off finally.Differently, as indicated by the transmittance curve (in red), formation of GaTe upon the present of Ga 3+ can efficiently suppress the formation of H 2 Te even when the potential is extended to − 1.1 V.The much higher cathodic current corresponding to hydrogen evolution also demonstrates the better catalytic performance of the GaTe than Te.As indicated by the transmittance, due to the presence of Ga 3+ , the deposited thin film become solid which was not destroyed by hydrogen evolution.
Chronoamperometry was conducted for deposition of thin films at different potentials.Figure 3a shows the j-t curves at different potentials and Fig. 3b shows the corresponding transmittance spectra.The corresponding top-view SEM images of the thin films are shown in Fig. 3c-i.Deposition currents at − 0.35 V, − 0.5 V, − 0.6 V and − 0.7 V are comparable.However, the morphology of the thin films is very different.At − 0.35 V, granule morphology with many small nanoparticles is obtained (Fig. 3c).The granule morphology changes to feather-like structures at the potential of − 0.5 V (Fig. 3d).For the thin films deposited at − 0.6 and − 0.7 V, there are many congregated flocs on the top.The thin films deposited at − 0.9 V (Fig. 3h) exhibits a striped surface.The striped surface grows more homogeneously as the deposited potential changed to − 1.0 V (Fig. 3i).It is worth to notice that there is no change of the morphology for the sample after photoelectrochemical measurements (Fig. 3j).

Surface composition of the thin films
The surface compositions of the thin films deposited with different potentials were studied with XPS.The first column in Fig. 4 shows the high resolution XPS spectra of the Ga 3d of the as-prepared thin films deposited at − 0.35 V, − 0.9 V and − 1.0 V.According to Bondino, binding energy (BE) of Ga 3d of the defect-free GaTe is around 19 eV, but the BE peak would shift to lower energy for the Ga-rich GaTe 1−x [27].Therefore, the BE peak at 18.3 eV in Fig. 4a of the film deposited at − 0.35 V is assigned to the Ga-rich GaTe.The growth of the GaTe is further demonstrated by the high resolution XPS spectra of Te 3d in Fig. 4d, where the BE peaks at 572.8 and 583.2 eV can be assigned to 3d 5/2 and 3d 3/2 of the Te 2− , respectively [13].In Fig. 4a, the BE peak at 17.7 eV is contributed by Ga 0 [28], indicating the existence of metallic Ga and demonstrating the underpotential deposition of Ga at − 0.35 V. Existence of Ga 2 Te 3 phase is evidenced by the BE peak at 20.1 eV [4,29].In Fig. 4d, The BE peaks at 576.1 and 586.5 eV are corresponding to the 3d 5/2 and 3d 3/2 of Te 4+ , indicating the present of TeO 2 [13].The existence of Te 0 is evidenced by the BE peaks at round 574.0 and 584.3 eV [13].
The BE peaks at 576.5 eV and 586.8 eV can be assigned to TeO 3 [27].Figure 4b and e shows the XPS of Ga 3d and Te 3d of the thin film deposited at − 0.9 V. Similarly, GaTe, Ga, Ga 2 Te 3 , Ga 2 O 3 , Te, TeO 2 , TeO 3 are found.In comparison with that deposited at − 0.35 V, the relative intensity of Te 2− significantly increases, indicating the higher content of GaTe. Figure 4c shows the XPS of Ga 3d of the film deposited at − 1.0 V, where the signal of Ga 0 is highest.It is revealed that a certain amount of metallic Ga has been deposited at this potential.
The third and fourth column in Fig. 4 show the XPS spectra of the films after conducting PEC measurements.The chemical composition and phase have not strongly changed after PEC measurements, however, the content changed.Obviously, as shown by Fig. 4j, the signals of Te 2− strongly increases after PEC measurement, which reveals the content of GaTe increased.In contrast, as shown by Fig. 4h and k, after PEC measurements, the content of GaTe of the film deposited at − 0.9 V slightly decreases, and the content of Ga increases, which reveals that GaTe could be reduced by the photocurrent.Indeed, anti-photocorrosion treatment is further required for long-term application.

Optical properties of the thin films
Figure 5a shows the UV-vis-NIR diffuse absorption spectra of the thin films deposited at different potentials.Absorbance starts to decrease dramatically at around 700 nm.According to the Kubelka-Munk function: where a is absorption efficiency, hn is energy of the incident light, E g is the bandgap of the semiconductor, n is 0.5 for indirect semiconductors and is 2 for direct semiconductors, the bandgap of the thin films can be deduced.Figure 5b shows the Tauc plots of the thin films deposited at − 0.35 and − 1.0 V, derived from the diffuse reflectance UV-vis-NIR spectra via the Kubelka-Munk function.The E g of the thin films is obtained to be 1.8 eV by the intercept the linear portion of the Tauc plots with the hv-axis.

Photoelectrochemical properties of the GaTe films
Figure 6a shows the open circuit potential (OCP) of the thin films responded to periodic illumination of white light.Firstly, under dark the OCP shifts negatively as the deposition potential changed from − 0.35 to − 1.0 V, which is helpful for increasing the energy of photo-generated electrons for photoelectrochemical (PEC) hydrogen evolution by moving the Fermi level negatively.On the other hand, OCP transient of the thin films is also strongly affected by the deposition potential.OCP of the thin films deposited with − 0.35 V dose not respond to illumination at all.This indicates that the thin film deposited at these potentials have metal-like behavior initially.The OCP transient becomes obvious for the thin films deposited at − 0.7 and − 1.0 V.The positive shift of the OCP demonstrates the thin films are p-type semiconductors.Figure 6b shows the linear sweep voltammograms (LSV) of the thin films.It is consistent with the OCP results that there is not photocurrent produced by the thin film deposited at − 0.35 V.The thin films deposited at − 1.0 V produced the highest photocurrent.Figure 6c shows the transient photocurrent of the thin films measured at − 0.36 V RHE .At the first 200 s after switching on illumination, the behavior of photocurrent with deposition potential is consequent with the OCP and LSV results.After that, it is interesting to observed that activation occurred and the photocurrent of all the thin film is rapidly increased.For example, photocurrent of the thin film deposited at − 0.35 V is rapidly increased to − 0.01 mA cm −2 .By comparing the XPS results of the thin film before and after PEC measurements, it can be found out that the content of GaTe increased after PEC measurements.Thus, the activation process can be contributed to the increased content of GaTe at − 0.36 V RHE .The highest photocurrent of − 0.03 mA cm −2 is achieved by the GaTe thin films deposited at − 1.0 V.

Conclusions
In this work, GaTe thin films were electrochemically deposited with acidic solution containing HTeO + and Ga 3+ as electroactive species.As revealed by the cyclic voltammetry combining with operando transmittance spectroscopy, the presence of Ga 3+ strongly suppressed the formation of H 2 Te at the potential range from − 0.7 to − 1.1 V. On the other hand, underpotential deposition of Ga on Te occurred from the onset of deposition, as demonstrated by the coexistence of Ga and Te of the films deposited at different potentials.Photoelectrochemical measurements illustrated that the GaTe can be used as photocathodes.The photoelectrochemical performance of the thin films was strongly determined by the deposition potential.Generally, the GaTe thin films deposited at − 1.0 V generated high photocurrent.In situ conversion of the underpotential deposited Ga to GaTe was attributed to the observed activation process during PEC measurements, as revealed by the XPS analysis.However, further treatments are still required for potential application of solar hydrogen production, for example to increase its photo stability and catalysis.

Fig. 2
Fig. 2 Cyclic Voltammograms of the ITO substrates a in the solutions of 0.1 M H 2 SO 4 + 1 mM TeO 2 and 0.1 M H 2 SO 4 + 1 mM TeO 2 + 0.05 M Ga (NO 3 ) 3 ; b the simultaneously measured normalized transmittance spectra; c Cyclic Voltammograms of the ITO sub-

Fig. 6 a
Fig. 6 a Transient of open circuit potential (OCP) of the thin films deposited at various potentials responded to illumination; b LSV curves of the thin film photocathodes under periodic illumination; c Photocurrent transient of the various thin film photocathodes at − 0.36 V RHE