Boron-doped diamond (BDD) is deemed to be a prominent electrode material in the area of electrochemistry, owing to its superior electrochemical performance, such as wide potential window, unmatched chemical inertness, low background current, and long lifetime [[1], [2], [3]]. The conventional BDD electrode growth is processed with boroethane (B₂H₆) or trimethyl borate (B(OCH₃)₃) [4] as boron source in chemical vapor deposition (CVD) reactor. However, B₂H₆ is poisonous and B(OCH₃)₃ is corrosive, which leads to the experiment in low security. While boric oxide (B₂O₃) as an alternative solid boron source, is innoxious and non-corrosive [5]. With B₂O₃ as the dopant, any noxious materials are dispensable [6]. B₂O₃ could be effectively doped into the structure of diamond.

The mixture of methane (CH₄), hydrogen (H₂) is the most commonly utilized in the growth of BDD films. But recently other carbon-containing substances, such as graphite are employed for synthesizing diamond [7,8]. More specifically, graphite as the source of carbon will be etched by atom hydrogen to form high concentration of hydrocarbon precursors around the substrate for diamond deposition. Hydrocarbon precursors are only within limited regions, which can avoid carbon soot depositing on the CVD cavity [9,10]. With this mind, graphite and B₂O₃ are used as carbon source and boron source, respectively, to synthesize CVD BDD electrodes in the current work.

The BDD has been reported to possess good electrochemical performance. But the pristine BDD electrodes are not sensitive enough to determine dopamine with extremely low concentration. Such electrocatalytically active property can be improved by several approaches. One of the most useful methods is to add metal, which is more electrocatalytically active than BDD electrode. Antimony, bismuth, cobalt, copper, nickel, silver, gold (Au) etc. were employed to modify BDD electrodes for electroanalytical applications [11,12]. For instance, Mei et al. employed Au on BDD surface to improve the DA detection sensitivity and limit of detection (LOD) [13]. However, in previous reports, the doping source of BDD was still B₂H₆ or B(OCH₃)₃, which has been evidenced poison or mordant.

In the present work, BDD samples were prepared with a new carbon source and dopant, the mixture of graphite and B₂O₃. The synthesis process was safe. Also, optical emission spectroscopy (OES) was employed to monitor the species during BDD growth. The synthesized BDD electrode was employed to detect dopamine (DA), an important neurotransmitter in mammals, which directly affects human behavior. Moreover, Au nanospheres were sputtered on the BDD surface to enhance the electrochemical performance for DA. To our knowledge, this is the first time to utilize pure solid materials for synthesizing BDD film and decorating Au on the BDD film grown with this method.

BDD films were synthesized on (100) silicon (Si) substrates measuring 4 mm × 6 mm × 0.5 mm in the microwave plasma CVD reactor which was described elsewhere [14]. Prior to the deposition, the Si wafers were seeded with nanodiamond water suspension for 30 min (Guoruisheng Co.) and cleaned with deionized water. Subsequently, the solid doping source was manufactured, as shown in Fig. 1(a, b). At first, 0.2 g graphite powders (99.9 %) and 0.0057 g 99.9 % boric oxide powders (B₂O₃) were blended in a mortar. The mixed powders were then pressed into a round slice with a diameter of ~12.6 mm and a thickness of 1 mm. Eight pieces of the mixed slice and a seeded Si substrate were placed in the way shown in Fig. 1(c) to synthesize a BDD film. The pressure was 160 mbar, growth time was 10 h, and the substrate temperature was ~850 °C, which was measured by a two-wavelength parameter [15]. The nucleation and growth process of BDD films are demonstrated in Fig. 1(d, e).

The Au (99.99 %) was sputtered on the BDD film via a magnetron sputtering equipment (HYC-450) under 120 W for 90 s. Then the BDD with Au film on it was annealed at 900 °C for 1 h in the atmosphere of argon. The process is depicted in Fig. 1(f, g).

During the deposition process, OES of plasma was monitored to analyze the precursors of BDD growth for the first time. The detailed information about the OES method can be found elsewhere [16]. The morphologies and ingredients of BDD and Au/BDD films were captured by a field emission scanning electron microscopy (FE-SEM) and Energy Disperse Spectroscopy (EDS) (Helios Nanolab 600i). Raman spectra of the pure BDD film and the Au nanoparticles modified BDD film were obtained with a 532 nm Jobin Yvon LabRAM.

Electrochemical characterization was performed via an electrochemical workstation (Bio-logic SAS, VMP-3, France) with a three-electrode system. The BDD films and Au nanoparticles modified BDD films were used as the working electrode, a platinum plate with size of 10 mm × 10 mm as the counter electrode, and a silver/silver chloride (Ag/AgCl) electrode as the reference electrode. Cyclic voltammograms (CV) and differential pulse voltammetry (DPV) were used to investigate the electrochemical performance of these electrodes. DA and phosphate butter solutions (PBS, PH: 7.2-7.4) with analytical grade were purchased from Aladdin (China), which were used without further purification.

Fig. 2(a, b) demonstrates the morphology of the pristine BDD films with graphite and B₂O₃ as the mixed doping source. The BDD shows a pinhole free diamond film and well-faceted crystallites. The magnified BDD crystallites are smooth. By contrast, SEM images of Au nanospheres modified BDD films are exhibited in Fig. 2(c, d). The Au nanospheres can be clearly observed with the magnified view. The size of Au particles seems to be in the range of 20-40 nm, please see the supplementary information, Fig. S1 in Electronic Supplementary Information. They are distributed evenly on the surface of BDD. Furthermore, the content of Au is ~15.65 % in atom of all estimated by EDS. The O atom accounting for 7.07 % may be from oxygen in air.

The radical species in the BDD synthesis atmosphere were detected, as shown in Fig. 3. The hydrocarbon species, including CH radical at 430 nm, C₂ radicals at 471, 514, and 561 nm; the BH emission peak at 433 nm, and other species H_γ at 434 nm, H_β at 486 nm, H₂ at 587 nm, H_α at 656 nm from atom hydrogen were observed in the OES spectroscopy. These species are favorable for the growth of BDD film [17].

Fig. 4 exhibits the Raman spectra of the BDD film grown via B₂O₃ as doping source and Au nanoparticles modified BDD film. There are several bands at ~500, 1200, and 1323.3 cm^-1 in Fig. 4. The peak at 1323.3 cm^-1 represents diamond band [18]. The deviation of diamond peak in Fig. 4 from the typical diamond Raman peak of 1332 cm^-1 is caused by the residual stress in the BDD films. The other bands at 500 and 1200 cm^-1 are ascribed to the locally disordered structures induced by the doped boron, indicating that boron was successfully incorporated into the diamond crystal lattice [13,19]. Also, the boron concentration in the BDD film can be calculated via an equation:

where W is the wavenumber of Lorentzian component of the 500 cm^-1 broad peak [20]. The wavenumber of Lorentzian component in the spectrum is approximately 479.7 cm^-1. Hence, the boron doping level [B] was calculated to be 8.44 × 10²⁰ cm^-3. After decorating Au nanospheres on BDD surface, one more peak at ~1600 cm^-1 was observed, which belonged to sp² carbon. The increase of sp² carbon in Au nanoparticles modified BDD film can be explained by the Au catalysis [21]. Besides, the high boron concentration in the BDD film may be the second reason. The high boron concentration destroys the crystal structure of diamond and makes it easier to transfer the diamond phase to graphite phase under thermal treatment than the BDD film with low boron concentration. Another reason could be the oxygen participation during the thermal treatment of Au/BDD preparation process. After sputtering Au films were annealed at 900 °C for 1 h in the atmosphere of argon. But there is still some oxygen remained in tube furnace, causing the generation of graphite phase.

Fig. 5 shows the CV curves of Au nanoparticles modified BDD electrode and its relationship with scan rates. The electrode has a good response to the DA, exhibiting significant redox currents. With the increase of scan rates from 0.02 to 0.2 V s^-1, the oxidation peak currents increase, while the cathodic peak currents decrease. As a result, the peak separation (△Ep) is enhanced. Furthermore, the redox peak currents show a linear relationship with the square root of scan rate (Fig. 5(b)). The specific equation can be written as I(pa) = 1.15v^1/2 - 0.14 (R² = 0.977) for the anodic peak currents; and I(pc)= -1.79 v^1/2 + 0.16 (R² = 0.985) for the cathodic peak currents. The consequence indicates that the Au nanoparticles modified BDD electrode is a typical diffusion-controlled process. Additionally, the successive CV investigation of the Au/BDD electrode was conducted to assess the adhesion strength of the Au particles on the BDD film and stability of Au/BDD electrodes. After 50 cycles, the peak currents of DA oxidation changes slightly with a small decrease of 0.46 % of initial current value. The result suggests that the adhesion between the BDD surface and Au nanoparticles is strong and the stability of the Au/BDD electrode is good (Fig. S2 in Electronic Supplementary Information).

Fig. 6(a) demonstrates the DPV curves of Au nanoparticles modified BDD electrode in different DA concentrations. With increasing of DA concentration from 5, 10, 20, 50, 100, 200, and 500 μM to 1 mM, the oxidation peak current significantly increases. The specific equation can be written as I (μA) = 0.03 C(μM) + 0.031 (R² = 0.998), as shown in Fig. 6(b). The sensitivity of Au nanoparticles modified BDD electrode is ~125 μA m M^-1 cm^-2, and its limit of detection is ~ 0.83 μM (S/N = 3). The performance parameters of Au/BDD sensor were compared with those of other BDD-based DA sensors, as Table. S1 summarized in Electronic supplementary information. The results imply that the BDD electrode synthesized with solid boron source and decorated with Au nonaspheres is efficient for detecting DA in real-world samples.

In summary, the mixture of solid boron oxide and graphite was employed as doping source to synthesize CVD BDD films. The new boron source is innoxious and non-corrosive compared to those traditional dopants. The BDD films are pinhole free and with well crystal morphology. The Raman peak at 500 and 1200 cm^-1 indicates that boron atoms are successfully doped into diamond film, and boron concentration in BDD film is calculated to 8.44 × 10²⁰ cm^-3. The BDD film is a promising material in DA detection. In order to improve the detection sensitivity of BDD film, Au nanospheres are decorated on BDD surface. The Au nanoparticles modified BDD electrode possesses high sensitivity (in the range of 5 μM-1 mM), and a low detection limit (~0.8 μM) for detecting DA. Hence, the Au nanoparticles modified BDD electrode based on the mixture of solid boron oxide and graphite as dopants will be a promising candidate for efficient DA detection.

This work was financially supported by the National Science Fund for Distinguished Young Scholars (No. 51625201), the National Natural Science Foundation of China No. 51,702,066, the National Key Research and Development Program of China (No. 2016YFE0201600), the Key Laboratory of Micro-systems and Micro-structures Manufacturing, Ministry of Education, Harbin Institute of Technology (No. 2016KM001) and the Innovative research group of NSFC11421091.

Supplementary material related to this article can be found, inthe online version, at doi:https://doi.org/10.1016/j.jmst.2020.02.003.