Communication—Texture and Bandgap Tuning of Phase Pure Cu2O Thin Films Grown by a Simple Potentiostatic Electrodeposition Technique

Highly textured phase pure Cu2O thin films have been grown by a simple electrodeposition technique with varying deposition voltages (−0.3 to −1.0 V). The surface morphology characterized by Scanning Electron Microscopy (SEM) revealed that the deposited thin films coherently carpet the underlying substrate and are composed of sharp faceted well-defined grains of 0.5–1.0 μm sizes. XRD analyses showed that all films are composed of polycrystalline cubic Cu2O phase only and have average crystalline domain size in the range of 30–73 nm. The preferred crystalline orientation of phase pure Cu2O films was found to be changing from (200) to (111) with increasing cathodic voltages and showed the highest (111) and (200) crystalline texture coefficient while growing at −1.0 and −0.8 V respectively. The optical bandgap of the as-grown samples was calculated in the range of 1.95–2.20 eV using UV–vis Transmission data. The performance of Cu2O/FTO photocathodes was tested by estimating LED “ON/OFF” modulated surface photovoltage into a photoelectrochemical cell at a zero bias.

Cuprous oxide (Cu 2 O) is one of the most desirable p-type semiconducting metal oxides used as absorber materials for ZnObased all-oxide solar cell [1][2][3] because of its reported direct bandgap (∼2.17 eV), a suitable band alignment with n-type ZnO electrodes, and its environmentally benign nature. 1,4One of the two major difficulties to realizing ZnO/Cu 2 O -based optoelectronic devices are: (1) single phase Cu 2 O synthesis at low processing temperature to avoid formation of interfacial defects at the ZnO/Cu 2 O junction; (2)  desired crystallite orientation of cubic Cu 2 O phase for achieving heteroepitaxy with hexagonal closed packed ZnO wurtzite structure. 1,4,5To this end, Akimoto et al. 1 demonstrated that solar cells involving the growth sequence of ZnO/Cu 2 O exhibited better performance compared to those involving the Cu 2 O/ZnO growth sequence and attributed the improved cell performance to the interface with low defects due to the fact of similar atomic structure in ZnO(0001)/Cu 2 O(111) stacks.In this direction, we demonstrated that defect-free single-crystal like ZnO(0001) NRs could be grown on textured ZnO seeding layers by using hydrothermal method 5 useful for realizing efficient ZnO(0001)/Cu 2 O(111) based radial junctions.
7][8] It is well known that Cu (I) is less stable than Cu (II) and Cu (0) both in air and in aqueous conditions 4,9,10 and the synthesis of phase pure Cu 2 O is very cumbersome to achieve.Besides, CuO and/or Cu impurities has been proved to be detrimental 4,8 for devising highperformance ZnO/Cu 2 O junction based optoelectronic devices.Therefore, highly pure Cu 2 O with optimal physicochemical and optoelectronic properties has been an active area of research for past six decades. 1 Among the wet-chemical techniques, electrodeposition (ED) is simple, economical, and offers good control over large deposition parameters for producing single phase Cu 2 O films with controlled texture, thickness, and surface morphology without the need for additives.Furthermore, one of the intriguing features of electrodeposition is its ability to control oxidation state of Cu (i.e., Cu 2+ , Cu 1+ , and Cu 0 ) simply by applying appropriate potential/ current on the working electrode of electrochemical cell. 2,6,7Both, 2-electrode galvanostatic 6 and 3-electrode potentiostatic 2,7 mode ED of Cu 2 O thin films were reported.In this study, we report a very simple 2-electrode potentiostatic ED technique for tuning texture and optical bandgap of the single phase Cu 2 O films with distinct morphologies and discussed their physical properties as a function of deposition voltages.

Experimental Methods
Synthesis.-Thesynthesis of single phase cuprous oxide (Cu 2 O) thin films was reported details in our previous works. 9Briefly, the 2electrode electrodeposition setup is composed of a fluorine doped tin oxide (FTO)-coated glass substrate as working electrode, a graphite rod (dia.∼6 mm) as counter electrode, and a Keithley SMU 2450 as dc power supply for applying fixed and stable deposition voltage as shown in Fig. 1a.The plating solutions were prepared by mixing 0.2 M copper sulfate (CuSO 4 ) and 3 M lactic acid (CH 3 CH(OH)COOH) with a weight ratio of 2:1 dissolved in deionized (DI) water (∼18 MΩ.cm).The pH of this solution was adjusted to ∼9.5 by adding 4 M KOH solution dropwise.The lactic acid served as complexing agent to prevent Cu or Cu(OH) 2 precipitation 2 when KOH was added to the solution by forming a stable cupric lactate (CuLac 2 ) complex.Prior to film deposition, FTO (∼20 mm × 12 mm) substrates were cleaned by sequential ultra-sonication into acetone, isopropyl alcohol, and DI water each step for 15 min and finally blown dried by using a hot-air gun.During deposition, the solution temperature was kept at ∼60 °C.The deposition time was set to 40 min typically, if not mentioned otherwise.A number of thin films were prepared at cathodic voltages of −0.3 V, −0.5 V, −0.7 V, −0.8 V, −0.9 V, −1.0 V, and −2.0 V in order to investigate the physical properties of the as-grown electrodeposited (ED) coppor-oxide.A schematic of our homemade 2electrode deposition setup is shown in Fig. 1a.

Results and Discussion
Figure 1b shows the transient cathodic current during the electrodeposition of copper-oxide thin films at −0.8 V (black curve) and −1.5 V (green curve).The cathodic current at these two different deposition voltages are seen to be quite stable within the deposition span indicating a good and fast growth kinetic. 7The negative bias voltage on the working electrode accelerates the cathodic reduction of CuLec 2 to Cu 2 O and its subsequent deposit atop the FTO substrate due to the low solubility of Cu 1+ in water. 6he thicknesses (t) of the copper-oxide films electrodeposited at −1.0 V for 20 min (t ∼ 8.08 ± 0.19 μm), 40 min (t ∼ 8.08 ± 0.19 μm), and 80 min (t ∼ 8.08 ± 0.19 μm) are found to be almost uniform across the wide area of films as evident from Fig. 1c.The ED films are visibly compact and the thickness of the films was found to be increasing with increasing deposition time as also evident from the lighter to darker film photographs with longer deposition time given in the inset of Fig. 1c.
The surface morphology and microstructure of copper-oxide thin films deposited at −0.3 V (t ∼ 0.6 ± 0.1 μm), −0.8 V (t ∼ 3.8 ± 0.4 μm), −1.0 V (t ∼ 8.0 ± 0.5 μm), and −2.0 V (t ∼ 46 ± 1 μm) are shown in Figs.2a-2d, respectively where a magnified film-area and photographs of the respective samples are included in each figure inset.Clearly, the thicknesses of the ED films were found to be increasing with increasing deposition voltages.Below −0.3 V, no films were observed to be electrodeposited.It is seen from Fig. 2 that thin film deposited at −0.3 V is composed of isolated islands composed of grains of octahedral cuboids whereas thin films grown above this deposition voltage were found to coherently carpet the underlying substrate and composed of sharp faceted well-define grains of 0.5-1.0μm sizes.Both the surface morphology and film thickness of the electrodeposited copper-oxides can be controlled as evident from Fig. 2. The pristine copper-oxide films grown by using deposition voltages up to −1.0 V were found to be single phase Cu 2 O and those grown at −2.0 V and above (data not shown here) were found to be metallic copper (Cu) with ( 111) and ( 200) orientation (see the top panel in Fig. 3a).Additionally, the surface morphology of metallic Cu revealed that it is composed of spherical shaped grains of dia.∼1.0 ± 0.3 μm and significantly different from the morphology of Cu 2 O thin films as also seen by others. 7In fact, a coherent photoactive layer with large grains is desirable for light harvesting applications due to the possible reduction of recombination at the grain boundaries.
The XRD patterns of ED thin films at different voltages are shown in Fig. 3a.FTO substrate diffraction peaks are marked by asterisks.The XRD results confirmed the polycrystalline single phase cubic Cu 2 O structure 4,8 for all samples electrodeposited at −0.3 V to −1.0 V.It is also clear that crystal growth was largely influenced by deposited voltage.The average crystallite domain size was estimated in the range of 30-73 nm by applying Scherrer equation 10 to both (111) and ( 200) planes of the ED Cu 2 O (see Fig. 3b).Texture coefficient (TC) provides a measure of preferred orientation of polycrystalline materials 11 by comparing with a standard powder diffraction file (PDF) of same materials from XRD database (e.g., ICSD). 11However, for simplicity, the TCs of our ED Cu 2 O were calculated by using modified formula 6,8 : and shown in Fig. 3c as a function of deposition voltages.It is clear that the preferred crystalline orientation of phase pure Cu 2 O films was found to be changing from ( 200) to (111) with increasing cathodic voltages and showed the highest ( 111) and ( 200) crystalline texture coefficient while grown at −1.0 V and −0.8 V respectively.
Figure 4a shows the normalized transmission spectra of ED Cu 2 O thin films on FTO substrates as a function of deposition voltage.The absorption edge for all ED Cu 2 O films is seen at λ ≈ 475 nm, while the feature at λ ≈ 300 nm for films deposited at −0.3 V is due to the incoherent surface morphology exposing the underlying substrate 5 (cf Figs.2a and 2b).The optical bandgap (E g ) of these samples was estimated from the Tauc plot 10 generated from transmission data and was calculated in the range, E g = (1.95-2.20)eV.The E g dependence of ED Cu 2 O on deposition voltages can be attributed to the variation film thickness and grain size of the films as observed by others. 2,10rformance of an ED Cu 2 O(111)/FTO photoelectrode.-Thetype of conductivity as well as the performance of a typical ED Cu 2 O(111) film was investigated by monitoring transient surface Photovoltage (SPV) under periodic illumination of a green LED as shown in Fig. 4c.The positive value of V oc confirms the p-type conductivity of photoactive Cu 2 O layer and indicating that photogenerated minority carriers are electrons. 8Notice that SPV measurement in Fig. 4d also reveals a rapid and stable photoresponse of the ED Cu 2 O(111) in aqueous electrolyte within the investigated time frame, suggesting a good stability as well as compactness of the electrode.The open-circuit voltage (V oc ) of this film was measured to be V oc ∼ 30 ± 2 mV which might be reasonably good to integrate them into ZnO/Cu 2 O based solar cell. 2 Further experimental investigations are currently in progress to elucidate the effect of

Conclusions
In summary, we successfully synthesized highly textured phase pure Cu 2 O using a simple electrodeposition (ED) technique with varying deposition voltages (−0.3 to −1.0 V) using FTO substrates as working electrodes and a carbon rod as counter electrode immersed in an alkaline aqueous electrolyte held at ∼60 °C.The surface morphology, texture, and thicknesses of the ED Cu 2 O films can be controlled by varying deposition voltages and time as evident from the SEM, XRD, and stylus surface profiler data analyses.XRD analyses further showed that the preferred crystalline orientation of phase pure Cu 2 O films can be tuned from (200) to (111) with increasing cathodic voltages and showed the highest (111) and (200) crystalline texture coefficient while electrodeposited at −1.0 and −0.8 V respectively.The estimated optical bandgap of the as-grown ED Cu 2 O films was found in the range of 2.00-2.20 eV and the variation of the optical bandgap could be attributed to the variation of the film thickness.The performance of ED Cu 2 O(111) films was tested by estimating LED "ON/OFF" modulated surface photovoltage into a photoelectrochemical cell at a zero bias and was found to be reasonable to integrate them into optoelectronic devices.

Figure 1 .
Figure 1.Schematic depiction of a simple 2-electrode electrodeposition setup (a), cathodic current over elapsed deposition time of Cu 2 O on FTO substrate at two fixed deposition voltages (b), variation of film thickness with deposition time 20, 40, and 80 min electrodeposited at −1.0 V (c).The photographs of the same samples are shown in the inset of (c).

Figure 2 .
Figure 2. SEM micrographs and photographs of Cu 2 O thin films grown at −0.3 V (a), −0.8 V (b), −1.0 V (c), and −2.0 V (d).A zoomed region of the respective sample is given in each micrograph (500 nm scale bar in insets).Deposition time: 40 min.

Figure 3 .
Figure 3. XRD patterns of the electrodeposited thin films grown at −0.3 V to −2.0 V (a), Dependence of crystallite domain size (b), and Texture coefficient (c) on electrodeposition voltage.Deposition time: 40 min.