Electrodeposition of cuprous oxide on boron doped diamond electrodes

Nowadays, Cu2O is very promising electrode material for photoelectrochemical applications. In this paper, we report on the controllable synthesis of Cu2O single particles as well as compact layers on Boron Doped Diamond (BDD) electrodes using potentiostatic deposition in continuous and pulse mode. The BDD layers were prepared with different B/C ratios in the gas phase in order to investigate boron doping level influence on the Cu2O properties. The effect of electrodeposition conditions such as deposition regime and pulse duration was investigated as well. The Cu2O covered BDD electrodes were analysed by Scanning Electron Microscopy (SEM) and Raman spectroscopy. Improvement in the homogeneity of the electrodeposit and removal of clusters were achieved when the pulse potentiostatic regime was used. Using the same pulse electrodeposition parameters, we confirmed the possibility of controlling the deposition rate of Cu2O by varying the BDD conductivity. Finally, we were able to scale the size of Cu2O particles by changing the number of deposition pulses. The obtained results have shown a great potential of controlling the morphology, amount, size and distribution of Cu2O films on BDD substrates by changing the boron doping level and electrodeposition conditions as well. The investigations reported herein allowed us to better understand the deposition mechanism of Cu2O on BDD electrodes which could then be used for preparation of active layers for electrochemical applications and in optoelectronic devices such as solar cells and photodetectors.


Introduction
Diamond possesses many exceptional properties such as the highest hardness and thermal conductivity, excellent wear resistance, chemical inertness and the lowest friction coefficient.Therefore, diamond films grown by Chemical Vapor Deposition (CVD) found wide range of applications in electronics, optics, electrochemistry and other fields [1], [2], [3] and [4].In addition, controlled boron doping enables to adjust conductivity which has a positive impact mainly on the sensitivity and selectivity of BDD electrodes used for electrochemical applications [5].Electrochemical reactions perform mostly at the interface between electrolyte solutions and the electrodes surfaces.Therefore, modification of the electrode surface morphology is a key factor of tuning the electrode performance [6] and [7].Electrodeposition of cuprous oxide (Cu 2 O) microcrystals may be used to control surface properties for biosensing or biocatalytic applications.Cu 2 O is well known as one of the stable oxide forms of copper which has a brownish-red color and a bandgap of 2.0-2.2eV [8].Cuprous oxide is an interesting substitute for noble metal catalysts owing to its significant catalytic activity [9].Other advantages of Cu 2 O include the abundance of the precursor materials, nontoxic nature and cost effective synthesis routes.Cu 2 O exhibits several attractive characteristics for optoelectronic devices such as solar cells, lasers and photodetectors due to its high absorption coefficient and good majority carrier mobility [10] and [11].BDD is very promising candidate as a substrate for Cu 2 O based photoelectrodes due to chemical inertness, mechanical stability, and it supports the charge separation within the semiconductor and facilitate the photocatalytic activities in general [12] and [13].Cu 2 O thin films can be prepared using various methods including chemical deposition, reactive evaporation, Chemical Vapor Deposition (CVD), thermal oxidation and sol-gel [8].In contrast to other synthesis processes electrosynthesis is a relatively simple and versatile technique that is performed at atmospheric pressure and does not require high temperatures.Moreover, it allows a good control of the products by controlling parameters such as potential, current densities, pH or precursor concentration in the electrolyte.This work is aimed to demonstrate the possibility of electrodeposition of Cu 2 O particles and full layers on the BDD electrodes.For the first time, the influence of boron doping level on the Cu 2 O morphology is investigated.

Experimental
Polycrystalline BDD electrodes were prepared by plasma enhanced Hot Filament Chemical Vapor Deposition (HF CVD) [14].Before the BDD deposition the silicon substrates were seeded in an ultrasonic bath in suspension of 50 mg nanodiamond powder (diameter < 10 nm, CAS No. 7782-40-3, Sigma Aldrich) in 1 l of demineralized water (18 MΩ) for 40 minutes.The BDD layers with thickness of 300±50 nm were deposited for 2 hours in a CH 4 /H 2 gas mixture with an addition of trimethylboron at process pressure of 3 000 Pa.The B/C concentration in the gas phase was set up to 0, 2 000, 4 000, 8 000, 10 000, 15 000 and 20 000 ppm and the concentration of CH 4 in H 2 to 1 %.The substrate holder temperature 700 • C was monitored during the growth process by a thermocouple.Finally, BDD samples were fixed into printed circuit board support, electrically connected by a silver paste and isolated by a polymer paste (ESL 240-SB) (Fig. 1).Solution containing 100 mM sodium acetate, C

Results and Discussion
First, we have investigated the difference between pulse and continuous deposition.For this purpose, the BDD electrode with concentration B/C = 10 000 ppm and CH 4 /H 2 = 1 % was used (Fig. 3(a)).By applying the pulse deposition (Fig. 2), Cu 2 O grains doubled in size compared to the normal potentiostatic deposition were obtained along with the removal of clusters and improved homogeneity of the deposit (Fig. 3(c)).We suppose that it is because each of the pulses initiates a new nucleation processes on the surface site with the  best conditions for electrodeposition which results in a more uniform deposition.This is first represented by the more conductive BDD layer and after a homogeneous covering also by the formed Cu 2 O grains.This behaviour causes a more uniform and faster deposition.
On the other hand, in the case of continuous deposition sharper Cu 2 O crystals were observed, which might be more favourable for electrochemical reactions in some cases (Fig. 3(b)).
The prepared samples of continuously and pulse deposited Cu 2 O BDD layers were analyzed by Raman spectroscopy (Fig. 4).The boron doping is represented in Raman spectra by two broad bands at approximately 500 cm −1 and 1220 cm −1 , which are associated with incorporation of boron into the diamond lattice.The 500 cm −1 maximum is attributed to the local vibrational modes of the boron pairs and the small shoulder at 1320 cm −1 is associated with polycrystalline diamond in highly boron doped films [15] and [16].Ra- In the next part, we investigate the influence of boron doping level on the electrodeposition of Cu 2 O. Figure 5 shows evolution of the deposition current densities during the pulse electrodeposition of Cu 2 O on the BDD samples with various boron doping levels.In our previous study, Svorc et al reported a significant decrease of sheet resistance with increase of boron content  in diamond [18].Thus the increase of boron concentration and related higher electrode conductivity and higher amount of surface active sites accelerated the electrodeposition of Cu 2 O on the BDD surface.The enhanced BDD conductivity enables an increase of the total electric charge passed through the substance and, following the Faraday's law, a consequent increase of the mass deposited on the electrode surface.In the case of fully covered BDD substrates with Cu 2 O layer, 8-11 % decrease of current density was observed during the pulse deposition.This phenomenon is caused by covering the more conductive BDD layer with the less conductive Cu 2 O layer.
Figure 6 shows the scanning electron micrographs of Cu 2 O crystals deposited on BDD electrodes with various boron concentrations after pulse deposition shown in Fig. 5. On the electrode with B/C = 0 ppm no deposited Cu 2 O was observed, which is probably due to very low BDD conductivity and insufficient charge transfer.On the surface of electrodes with B/C = 2 000 ppm and 4 000 ppm, homogenously distributed Cu 2 O crystals with diameters in the range of 1-3 µm were observed.The electrodes with B/C = 8 000 and 10 000 ppm were almost continuously covered by Cu 2 O layer, and on the electrodes with ratio B/C = 15 000 ppm and higher, the covering was even denser and spherical clusters on top of a compact Cu 2 O layer were observed in addition.These results indicate a direct influence of the electrode conductivity determined by boron doping level on the total current density and consequent mass transport, thus influencing density and morphology of Cu 2 O on the BDD surface providing an effective tool of surface properties controlling.The Raman spectra (Fig. 7) show increased Cu 2 O peaks at 152 cm −1 and at 216 cm −1 in the case of electrodes with higher boron doping level [8].
This indicates the presence of thicker layers caused by faster growth on more conductive BDD substrate.The increase in intensity of 500 cm −1 and 1 220 cm −1 maxima follows the increase in B/C ratio in the gas mixture during deposition of BDD films evidencing the increasing doping level.A peak at 1 332 cm −1 corresponding to the polycrystalline diamond was also recorded in spectra of the films with lower and no boron concentrations [15].Finally, we tried to control the size of the Cu 2 O layer grains by varying the duration and number of the pulses maintaining the same charge.For this purpose, we choose the electrode with B/C = 8 000 ppm where no Cu 2 O clusters were observed.Three different depositions consisting of (1) five pulses with a duration of 400 ms and duty cycle of 10 % (Fig. 8(a)), (2) ten pulses with a duration of 200 ms and duty cycle of 5 % (Fig. 8(b)), and (3) twenty pulses with a duration of 100 ms duty cycle of 2.5 % (Fig. 8(c)) were performed.SEM micrographs have shown that the size of Cu 2 O grains was successfully scaled down by applying higher number of pulses with shorter duration.This phenomenon is caused by a termination of particle growth and nucleation of new particles during next pulse.Similar influence of deposition pulses on size of Cu 2 O particles was reported by Liau et al [19].Based on the experimental results and theoretical knowledge we can assume that the number and size of grains can be directly affected by the number and duration of the pulses, which together with the boron doping level give a great potential to control the amount, size and distribution of Cu 2 O on BDD substrate.

Conclusion
In this study, we demonstrated the possibility of controllable electrodeposition of Cu

2 H 3 Fig. 1 :
Fig. 1: Process of fixation of BDD electrodes into the electrode holders -(a) electrode holder, (b) connection of BDD electrode using conductive silver paste, (c) applying of polymer isolation paste, (d) photo of real electrode.

Fig. 2 :
Fig. 2: Evolution of the current density during the continuous and pulse electrodeposition of Cu 2 O.
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Fig. 7 :
Fig. 7: Raman spectra of Cu 2 O deposited on BDD electrodes with various B/C concentration and CH 4 /H 2 = 1 % using pulse deposition.

Fig. 8 :
Fig. 8: Morphology of Cu 2 O layers deposited using different pulses durations.The duration was set up to (a) 400 ms, (b) 200 ms and (c) 100 ms.
2 O particles as well as compact layers on the boron doped diamond electrodes.The BDD electrodes fabricated with various B/C concentration in gas mixture (0-20 000 ppm) were used for the depositions of Cu 2 O by potentiostatic pulse electrodeposition.Comparison of continuous electrodeposition with pulse electrodeposition showed significant enhancement of the homogeneity using a pulse electrodeposition.The investigations of boron doping level influence on the morphology of Cu 2 O on BDD substrate has shown several interesting results.In the case of non doped diamond, no Cu 2 O deposition was observed.In the case of B/C = 2 000-4 000 ppm, the Cu 2 O micro particles were observed and in the case of B/C > 8 000 ppm the BDD the surface was fully cov-ered with a Cu 2 O layer.The obtained results indicate a significant influence of the B/C ratio on the deposition rate, density and the morphology of Cu 2 O films caused by different BDD conductivity and surfaceactive sites distribution.Following the SEM investigation, we also found that the electrodeposited layers of Cu 2 O particles were homogeneously distributed over the whole BDD substrate.Finally, we successfully controlled the size of the Cu 2 O grains by varying the number and duration of the deposition pulses.The presented investigations have shown a great potential of controlling the morphology, amount, size and distribution of Cu 2 O films on BDD substrates through the change of boron doping level and electrodeposition conditions as well.