Construction of porous diamond film with enhanced electric double-layer capacitance via regrowth of diamond nanoplatelets

Abstract The engineering of porous boron-doped diamond (BDD) film has sparked significant interest in improving electric double-layer (EDL) capacitance. However, the presence of disordered and tortuous pores in the BDD film hinders the accessibility of the bottom pore surface area, leading to a decrease in specific EDL capacitance. Herein, a novel porous BDD film with vertically open pore channels is constructed through the overcoating of diamond nanoplatelet template with a BDD layer using chemical vapor deposition. Electrochemical investigations manifest that the specific areal EDL capacitance of the porous BDD is ∼428 times greater than that of the planar BDD film. More impressively, the porous BDD film demonstrates a higher specific volumetric EDL capacitance (39.30 F/cm3) and superior long-term stability (100% capacitance retention after 12,000 cycles), surpassing the performance of most developed porous BDD electrodes. The exceptional performance of the porous BDD film was attributed to the abundant vertically open pore channels, good hydrophilic property, maximized electrochemical available area, and good chemical/mechanical stability. This work benefits the development of the BDD film as an EDL micro-capacitor electrode, and the methodology proposed here shows great potentials to fabricate an ordered porous BDD nanostructure with large specific area for more electrochemical applications.


Introduction
Conductive boron doped diamond (BDD) film, which possesses high conductivity, wide potential window, sustained mechanical stability, and inert chemical property granted by the sp 3 -hybridized carbon bonding, has attracted much research attentions in electrochemical micro-capacitor, electrochemical degradation of organic contamination, and electrochemical sensing [1][2][3][4].For the development of electric double-layer (EDL) micro-capacitor involving storing charges by ion adsorption/desorption at electrode/electrolyte interfaces, construction of porous BDD film with large specific area is highly expected, because it would decrease the ion transmitting distance and increase the active sites for the adsorption of species, thus, contributing to an enhanced specific areal/volumetric capacitance [5][6][7].
The top-down method through etching of the diamond was proposed to construct the porous BDD film.By using Ni particle and anodic alumina as shadow masks, nanohoneycomb diamond structure and aligned diamond nanowires were prepared through reactive ion etching [8,9].The diamond nanowire film displayed ~10 times greater of EDL capacitance than that of planar diamond.With the help of metal (e.g., Ni, Co, Pt) nanoparticles as catalyst, nanopores were constructed on the diamond film during the annealing, leading to an enhancement of the areal EDL capacitance up to 15 times as much as the pristine BDD film [10,11].Porous BDD texture could also be synthesized using mask-free etching approach.Ohashi fabricated a highly porous texture through the steam-activated corrosion of BDD film at high temperature [12].Kondo developed a two-step thermal treatment method to produce dense nanopores on the BDD film surface.And the areal EDL capacitance was calculated to be 0.14 mF/cm 2 , approximately 30 times larger than that of as-deposited BDD film [13].Nevertheless, the efficiency of these top-down etching method to enhance the EDL capacitance was very limited.In addition, problem of the contamination from the metal mask/catalyst is required to be handled, because it would trigger the pseudo capacitance behavior, thus greatly restricting the long-term stability [14].
Compared with the top-down method, deposition of the BDD on a nanostructured template is a more efficient method to prepare the porous BDD film with high EDL capacitance.Highly-ordered porous BDD matrix has been developed by coating of BDD crystals on vertically aligned carbon nanotubes (CNTs) and silicon pillar array.The measured areal EDL capacitance up to 2.83 mF/cm 2 exhibited a significant enhancement [15,16].Apart from array templates, more complex interconnected templates, such as polymers, SiO 2 spheres, SiO 2 fibers, and SiO 2 fibers/CNTs, have been employed for the conformal growth of BDD, so as to obtain the porous nanostructure [7,[17][18][19][20][21]. Hebert et al. prepared porous polypyrrole film as a conductive template and then deposited nanocrystalline BDD layer with a few hundred nanometers over the template [17].Zivcova et al. fabricated porous BDD film electrode using a template made of SiO 2 fibers [21].Hydrofluoric acid was further employed to remove SiO 2 fibers in order to attain a highly porous BDD film with high areal EDL capacitance (7 mF/cm 2 ) [20].Notably, through facile repetition of the template deposition and the diamond coating, a much larger areal EDL capacitance (10.28 mF/cm 2 ) was reported by Ashcheulov et al. [7].Recently, a technique involving drop-casting of diamond colloid onto a substrate has been employed to obtain a porous template for the regrowth of BDD [22,23].This approach resulted in the formation of a three-dimensional porous BDD structure with a thickness of ~8 μm.The areal EDL capacitance was measured to be 17.18 mF/cm 2 , while the volumetric capacitance was calculated to be 21.48F/cm 3 [22].Despite the achievement of a high areal capacitance using the complex template, the pores within the BDD film are disordered and tortuous.This impedes the diffusion of the electrolyte into the bottom pores, leading to a significant reduction in the availability of the active area of these pores in the electrochemical process.As a result, the achieved volumetric capacitance still falls short of the expectation.
In this work, we constructed a novel porous BDD film electrode with vertically open pore channels throughout the film via overcoating of the diamond nanoplatelet template with a BDD layer, and demonstrated its exceptional volumetric performance (39.30F/cm 3 ) and great long-term stability.The dependence of microstructure on the EDL capacitance of porous BDD film is revealed by high-resolution transmission electron microscopy (TEM) and X-ray photo-electron spectroscopy (XPS).This work presents a promising approach to construct the porous BDD film with ordered pore structure, greatly benefitting the development of high-performance EDL micro-capacitor based on the BDD film.

Construction of porous BDD film
Construction of porous BDD film consisted of three steps: (a) growth of hybrid diamond/graphite film, (b) fabricating diamond nanoplatelets, and (c) overcoating of the diamond nanoplatelets with a BDD layer.
For the deposition of hybrid diamond/graphite film, n-type (100) Si wafers were employed as substrates.The substrate was firstly cleaned in acetone, ethanol, and water, followed by seeded in the diamond aqueous suspension (0.025 w/v%, 3.3 ± 0.5 nm in diameter) using ultrasonic method.Then the substrates were loaded onto the Al 2 O 3 pillar with a height of 15 mm in the microwave plasma chemical vapor deposition (CVD) reactor.The deposition was carried out at a microwave power of 6 kW, a gas pressure of ~32 mbar, and a gas mixture of H 2 /CH 4 with ratio of 100: 7. The substrate temperature was measured to be ~1070 °C.After 60 min growth, the hybrid diamond/graphite film was obtained.
For synthesis of diamond nanoplatelet film, the hybrid diamond/graphite film was exposed to a mixture of H 2 SO 4 (98 w/w%) and HNO 3 (69 w/w%) with a volume ratio of 3: 1.The etching process was performed in an oil bath at 150 °C for 1 h.After the etching, the sample was rinsed using deionized water and dried in an oven at 80 °C for 2 h.
During the growth of BDD, the diamond nanoplatelet film was placed into the home-made hot-filament CVD apparatus.12 tantalum filaments were placed at a height of 9 mm above the substrate and then pre-carbonized in 2.5% CH 4 /H 2 admixture at a gas pressure of 1000 Pa for 1.5 h.The following parameters were employed in the BDD deposition: a power f 7 kW, a chamber pressure of ~100 Pa, and a gas mixture composed of H 2 , CH 4 , and B(CH 3 ) 3 diluted in the hydrogen (1 v/v%) with a flow rate of 300, 3, and 35 sccm, respectively.The substrate temperature was measured to be ~870 °C.The growth time was manipulated from 10 to 40 min to engineer the microstructure of the porous BDD film.Additionally, a planar BDD film was synthesized on a n-type (100) Si substrate with a duration of 7 h.
Through the above-mentioned process, a single layered porous BDD film was prepared.Further repeating of the process on top of the porous BDD film enabled fabrication of multilayered (e.g., three-layered) porous BDD film.Besides, the as-grown porous BDD film and planar BDD film were treated in the acid mixture as just used, in order to enhance the wetting ability for electrochemical applications.

Microstructural characterizations of porous BDD film
The morphologies of pristine diamond/graphite film, diamond nanoplatelet film, and porous BDD film were studied by scanning electron microscope (SEM, Hitachi, SU-70).The microstructure of the porous BDD film was investigated by TEM (FEI, Talos F200X).The carbon phases of the porous BDD film were examined by micro-Raman spectroscope (Horiba, Labram HR Evolution) based on 532 nm laser and 1800 line/mm grating as well as micro-X-ray diffraction (XRD, Bruker, D8 Discover) with a Co Kα radiation source (λ = 0.178897 nm).Surface chemical bonding of the porous BDD film was analyzed by XPS (Thermo, ESCALAB Xi+) with an Al Kα source (hν = 1486.6eV).The wettability of the porous BDD film was examined by static contact angle measurement with the distilled water on an OCA20 instrument (Dataphysics).

Electrochemical characterizations of porous BDD film
The electrochemical measurements of the porous BDD film were conducted in 1 M Na 2 SO 4 at room temperature using an electrochemical workstation (Autolab, PGSTAT302N).A three-electrode system was employed, consisting of porous BDD films as the working electrodes, a platinum net as the counter electrode, and Ag/AgCl (saturated KCl) as the reference electrode.Cyclic voltammetry (CV) tests were performed from −0.3 to 1.3 V at different scan rate from 10 to 100 mV/s.Galvanostatic charging-discharging (GCD) tests were carried out from −0.3 to 1.3 V at a current density of 0.1 mA/cm 2 .Electrochemical impedance spectroscopy (EIS) tests were conducted at open circuit potential (OCP) in the frequency range from 100 kHz to 0.1 Hz.The long-term stability of the porous BDD film was assessed by measuring the capacitance variation in 12,000 cycles using the GCD method with a current density of 4 mA/cm 2 .

Microstructure transformation during the construction of porous BDD film
The porous BDD film was fabricated via a well-designed three-step process, as illustrated in Figure 1a.In brief, the hybrid diamond/graphite was firstly deposited by microwave plasma CVD, and then treated in the boiling acid mixture to remove the graphite.Afterwards, the rest diamond nanoplatelet was used as a template for the coating of BDD through hot-filament CVD technique.Through adjusting the growth time, we could engineer the porous BDD structure to have tunable surface and interior pores with a large specific area.
Figure 1b depicts the morphology of the hybrid diamond/graphite film after the first CVD stage.The Si substrate is covered by vertically aligned nanoplatelets.They interconnect with each other to create a porous maze-like structure.High-magnification SEM image demonstrates that the curved nanowalls grow on the sides of the nanoplatelet (see the inset in Figure 1b).The higher density of nanoplatelets compared to nanowalls is clearly evident in the images, which is distinct from the previously reported structure where more nanowalls sandwich fewer nanoplatelets [24,25].This featured diamond/graphite will endow the obtained porous BDD with a large specific area and accessible open surface.After the treatment in the acid mixture, the SEM image (Figure 1c) depicts that nanowalls have been removed, leaving intersected robust nanoplatelets on the substrate.Figure 1d shows the morphology of the sample after the second CVD stage.The morphology is clearly different from the pristine diamond/graphite and the diamond nanoplatelet.Nanocrystals grow on the surface of nanoplatelets, leading to a much larger thickness than before.There are some nanoplatelets bounding each other, thus, porous surface structure is controllably obtained.In Figure 1e, the cross-sectional morphology of the porous BDD film was recorded.The nanoplatelets almost perpendicularly stand on the substrate, and the vertically aligned pore channels designated by the arrows are distinctly observed.As the nanoplatelets thicken towards the top surface, they tend to clump together, resulting into a gradual reduction in pore size from the bottom to the top of the film.A great interior pore volume is observed within the structure.From Figure 1d and e, one could deduce that the interior pores are well connected with the surface pores.The porous BDD film demonstrates vertically continuous pore channels that resemble an inverted taper funnel-like shape.
To reveal the constituent variation of the film during the preparation, Raman and XRD measurements were carried out.As shown in Figure 1f, for the pristine diamond/graphite film, two dominant peaks positioning at ~1345 and ~1580 cm −1 are corresponding to the D band of disorder carbon and G band of graphite.Besides, a shoulder peak is observed at ~1620 cm −1 , designated as the D′ band.These features are very similar with those of carbon nanowalls reported previously [26,27].The diamond signal could not be recognized owing to the limited amount of diamond in the pristine film.After etching in the acid mixture, the sharp T 2g peak of diamond was clearly seen at 1332 cm −1 , verifying the nanoplatelets shown in Figure 1c are constituted by diamond phase.The peaks located at ~1289 and ~1553 cm −1 are derived from the D band and G band.The large full width at half maximum (FWHM) and the outstanding red-shift of the G band indicate the residual graphite has been pulverized into nanocrystalline graphite or amorphous carbon [28].Through the coating of BDD on the diamond nanoplatelets, the Raman spectrum dramatically changes.The T 2g peak of diamond shows a great red-shift and could be observed as a shoulder at 1283 cm −1 , due to the Fano effect [29,30].Moreover, there emerge two broad asymmetric peaks positioning at 452 and 1189 cm −1 , corresponding to the maximum of the phonon density of states in diamond and vibration modes of the disordered diamond lattice [31,32].These peaks are considered to be the result of the disorder generated by great boron impurities.From Bernard et al., the boron concentration could be determined from the wavenumber of the Lorentzian component in the "~ 452 cm −1 " wide peak [33].It is calculated to be 8.74 × 10 21 cm −3 , suggesting a heavily boron doping level in the BDD film.Besides, a week peak observed at 1530 cm −1 is assigned to be from the sp 2 -bonded carbon or boron [34][35][36].However, the fraction of sp 2 -bonded carbon is determined to be very low, due to the fact that the scattering cross section of sp 2 -bonded carbon is ~55 times higher than that of diamond using a 532 nm line [37].The XRD results further confirm this issue.As presented in Figure 1g, the pristine film consists of diamond and graphite.Following the etching process, the spectrum shows only a feeble diamond signal, suggesting that the graphite phase has been eliminated.The diamond peak becomes much higher for the porous BDD film, also, the graphite signal cannot be distinguished.The Raman and XRD results reveal that the high-quality, metallic, and heavily BDD film was prepared in this work.
TEM characterizations were carried out at the cross section of the porous BDD film prepared with focused ion beam method.Figure 2a shows the overall lowmagnification TEM image.There are loose diamond nanoplatelets almost vertically growing on the Si substrate.In Figure 2b, high-magnification TEM image presents that the thickness of the nanoplatelet gradually increases in the upper part, as compared to the lower part of the film.Some nanoplatelets bound with each other on the top surface.It is worth noting that the vertically oriented pore channels, with a width greater than 50 nm, are readily accessible for electrolyte from the exterior.This open pore structure possesses large specific area, promising a good performance of the EDL capacitance.To better understand the microstructure of the nanoplatelet, two distinct regions were chosen along the thickness of the film for investigation.Figure 2c presents the high-resolution TEM image corresponding to the lower part.The nanoplatelet has a thickness of ~14 nm.The interlayer spacing is measured to be 0.21 nm, well in accordance with the theoretical d-spacings of {111} planes in the diamond.The inset shows a fast Fourier transformation (FFT) pattern, which reveals a mirror relation between matrix (M) and twin (T) spots and indicates the presence of twin structures in the diamond nanoplatelet.In addition, stacking faults (SF) have also been identified in the Figure 2c.As discussed in our previous work, these planar defects (stacking faults and twins) would accelerate the lateral growth of (111) plane, contributing to the formation of diamond nanoplatelet at the first CVD stage [38].Figure 2d depicts the high-magnification TEM image, which corresponds to the dashed rectangle in Figure 2b.It is revealed that the upper part of the film is constituted by nanograins.Furthermore, high-resolution TEM images depicted in Figure 2e and f provide further insight into the microstructure of the interface between the pristine diamond nanoplatelet and the subsequent growing BDD.The FFT patterns align well with the expected diffracted spots of diamond along the [011] zone axis, confirming that the deposited material is indeed diamond during the second CVD stage.It is noticed that the BDD epitaxially grows on the nanoplatelet surface.Additionally, there are plenty of planar defects present in the BDD, as illustrated in Figure 2e.This is in contrast to the top region depicted in Figure 2f, where perfect crystalline quality is observed.It is speculated that the higher atomic hydrogen density could decrease the defect abundance and enhance the diamond quality at the top.
The microstructure of the BDD particularly in regions far from the interface was further investigated in Figure 3.As shown in Figure 3a and b, the size of grain 1 and 2 is larger than 100 nm.The distance between adjacent lattice planes, measured to be 0.21 nm, is in line with the {111} plane of diamond.Importantly, no void or nondiamond phase is surveyed at grain boundaries.Moreover, we examined the lower region of the BDD, which displays a crystalline size smaller than 50 nm.As depicted in Figure 3c and d, BDD grains tightly bound with each other.The grain boundary contains only a limited amount of amorphous carbon (a-C).
Besides, no void is detected.The above results demonstrate the high quality and structural integrity of BDD, despite the fact that the sample has been treated in the boiling acid to enhance the wettability.
The surface property of porous BDD film plays a crucial role in determining its performance of EDL capacitance.Figure 4a showcases that the very hydrophilic property (16 °) is obtained on the porous BDD film after the acid treatment.To shed deep light into the surficial chemical bonding of the porous BDD film, XPS measurements were carried out.In Figure 4b  the insets in panel (c,e,f) present the corresponding FFt patterns.[35,41].The total abundance of B-C bond is 1.8%, which is higher than that of B-O bond (0.8%).Besides, it was proposed that the BC 3 bond, characterized by B atoms in a sp 2 bonding configuration, induces the Raman peak observed at ~1530 cm −1 in the porous BDD film, as depicted in Figure 1f [35].Notably, the XPS data implies that both B and C atoms on the porous BDD film were oxidized after the acid treatment.This oxygen-terminated surface contributes to the highly hydrophilic property of the porous BDD.

EDL capacitance performance of the porous BDD film
To achieve a better EDL capacitance, the dependance of porous BDD structure on the EDL capacitance is firstly investigated.Figure 5 presents the SEM images of porous BDD films fabricated with different growth time.In Figure 5a-c, as extending the growth time from 10 to 30 min, the nanoplatelets constituted by BDD nanograins become thicker.Specifically, the thickness is estimated to increase from ~130 to ~330 nm, while the surface porosity, estimated from the SEM images, gradually decreases from 32.66% to 9.27%.After 40 min of growth, Figure 5d demonstrates that the nanoplatelet-like grains could no longer be distinguished, and the surface pores almost vanished.But from the cross-sectional SEM images in Figure 5e-h, the interior pores could still be observed in the film.This is ascribed to the higher growth rate at the top of the diamond nanoplatelet than at the lower location.These observations imply that facile manipulation of growth time could result in tunable pore structure and surface area in the porous BDD film, which is crucial for obtaining optimal performance in EDL capacitance.
The EDL capacitance of the porous BDD film was studied in 1 M Na 2 SO 4 under a three-electrode configuration.Figure 6a shows the CVs at the scan rate of 10 mV/s within the potential range of −0.3-1.3V (vs.Ag/AgCl).The CV curves of porous BDD films all have a quasi-rectangular shape, and no redox peak is observed, demonstrating an ideal EDL capacitance feature.It is noticed that the current density of the porous BDD film increases gradually from 0.021 to 0.030 mA/cm 2 (at 0.4 V) as the growth duration increases from 10 to 20 min.With the duration extending to 30 min, the current density slightly reduces to 0.027 mA/cm 2 (at 0.4 V).Furthermore, it distinctly decreases to 0.012 mA/cm 2 (at 0.4 V) when the duration comes to 40 min.The CV rectangular also obviously shrinks for porous BDD-40 min.The specific areal capacitance (C cv in mF/cm 2 ) could be calculated by using the following equation: Where j cv is the capacitive current density in mA/cm 2 , υ is the scan rate in V/s, and ΔV is the potential range in V.The areal capacitances are estimated to be 2.25, 3.07, 2.89, and 1.37 mF/cm 2 for the porous BDD film prepared with 10, 20, 30, and 40 min.Figure 6a also illustrates the CV curve of planar BDD film as the control.As shown, a nearly straight line is observed during the measurement, indicating that the planar BDD film has a very small capacitance.Notably, it is determined that the specific areal capacitance of the porous BDD film is two orders of magnitude higher than that of planar BDD film (0.03 mF/cm 2 ). Figure 6b presents the CV curves of porous BDD-30 min at different scan rates.With the increase of the scan rate, the capacitive current monotonously increases, and the CV curve gradually changes  from a quasi-rectangular shape to a leaf shape.The areal capacitances are calculated to be 2.89, 2.51, 2.11, 1.85, 1.66, and 1.51 mF/cm 2 at the scan rate of 10, 20, 40, 60, 80, and 100 mV/s, respectively.Such decreased capacitance with increasing the scan rate has been usually reported on BDD electrodes previously.This was thought to be caused by the sluggish ion adsorption/ desorption process and limited diffusion kinetics inside pores of the material at high-rate operation [42].
The EDL capacitance was further investigated using the GCD technique.Figure 6c shows the GCD curves of porous BDD films prepared with various growth duration at a charging/discharging current density of 0.1 mA/cm 2 .The curves are all quasi-symmetric and nearly linear, suggesting good reversibility of the electrodes in charging/ discharging process.The charging/discharging time increases from 56.7 to 81.5 s when the duration increases from 10 to 30 min.Further prolonging the duration up to 40 min, the time greatly declines to 36 s.In contrast, the planar BDD film demonstrates a perpendicular line close to the ordinate, implying the very small capacitance.The specific areal capacitance (C GCD in mF/cm 2 ) could be calculated by using the following equation: Where j gcd is the charging/discharging current density in mA/cm 2 , Δt is charging/discharging time in s, and ΔV is the potential range in V.The specific areal capacitance of planar BDD film is estimated to be 0.03 mF/cm 2 , in well line with the data derived from CV curve.The specific areal capacitances of porous BDD films are 1.77, 2.53, 2.55, and 1.13 mF/cm 2 for the growth duration of 10, 20, 30, and 40 min, respectively.Such trend of variation in capacitance with respect to growth duration is consistent with the results obtained from CV method.
Figure 6d depicts the impedance behaviors of porous BDD films.The EIS curves of porous BDD films are composed of a semicircle at high frequency, assigning to the electrolyte transfer behavior in the pores of the electrode, as well as an inclined line at low frequency, which is derived from the diffuse layer resistance [43,44].The point of intersection on the − Z''=0 axis of the impedance curve represents the combined resistance of the bulk electrolyte and the electrode.With increasing the duration, the value of the intersection point gradually decreases, suggesting a decreased electrode resistance of the porous BDD film, because of similar electrolyte employed in our measurements.The surface SEM images demonstrate that the BDD film becomes dense as increasing the growth time from 10 to 40 min (see Figure 5), hence, resulting into a reduced electrode resistance.Besides, it is observed that the semicircle radius distinctly decreases from porous BDD-10 to porous BDD-40 min.From Keiser et al., the semicircle shape at high frequency is greatly imparted by the pore structure [45][46][47].As shown in Figure 5, the size of the surface pore reduces as increasing the duration, while the interior pore volume roughly remains constant.We propose that such variation of pore structure leads to the shrink of the semicircle in Figure 6d.More importantly, the electrode resistance and pore structure play a key role in determining the EDL capacitance of porous BDD films.As demonstrated in Figure 6a,c, the specific capacitance of the porous BDD film (ranging from BDD-10 to BDD-30 min) increases as the electrode resistance decreases.However, in the case of the porous BDD-40 min, the accessible active area is greatly diminished due to the closure of surface pores.As a result, a significant reduction in capacitance is observed.
In order to further enhance the EDL capacitance, multilayered porous BDD film was constructed through repeating the CVD of the diamond/graphite, acid-etching graphite, and consequent overcoating of the diamond template with a BDD layer.Every layer of porous BDD was fabricated with the same experimental parameters as a single layer porous BDD-30 min.This ensures the vertically aligned pore channels from the bottom to the top in a single layer porous BDD.Thus, the bottom pores of multilayered porous BDD film are accessible for the electrolyte in the electrochemical process.Figure 7a and b display the surface morphology of three layers of porous BDD film.Diamond nanoplatelets measured to be ~330 nm are intertwined, leading to plenty of pores on the surface.From the cross-sectional SEM image depicted in Figure 7c, three layers of porous BDD structure could be clearly distinguished and the thickness is estimated to be 3.27 μm.As indicated by the arrows, the vertically pore channels within the three layers are connected to each other, forming a continuous pathway for ion transfer throughout the film.To examine the capacitance performance of three-layered porous BDD film, Figure 7d and e recorded the CV curve at the scan rate of 10 mV/s and the GCD curve at the current density of 0.1 mA/cm 2 .The result of a single-layered porous BDD film was also included as the control.The CV curve of three-layered porous BDD film displays a quasi-rectangle shape without noticeable redox peaks.Notably, the capacitive current significantly increases as the number of layers increases.The specific areal capacitances are calculated to be 2.89 and 10.5 mF/cm 2 for one-and three-layered film.Besides, the GCD curve of the three-layered porous BDD film demonstrates a much longer charging/discharging time compared to that of one-layered BDD film.Specifically, the specific areal capacitance for the three layers of porous BDD film is determined to be 12.85 mF/cm 2 , which is ~5 times greater than the value obtained for a single layer (2.55 mF/cm 2 ).This value is also ~428 times higher than that of the planar BDD film.Meanwhile, the volumetric capacitance of the three-layered porous BDD film is calculated to be 39.30F/cm 3 , even higher than a single layered porous BDD (24.3 F/cm 3 ), which suggests that bottom pores in the multilayered BDD are participating in capacitance performance.Furthermore, the long-term stability is evaluated using 12,000 cycles of GCD tests at a current density of 4 mA/cm 2 , and the results are shown in Figure 7f.It is observed that the GCD curves maintain a stable quasi-symmetric shape with minimal variation from the first to the 12,000th cycle.Figure 7g shows that the specific capacitance could retain 100% of the initial value after charging/discharging 12,000 cycles, suggesting the excellent stability of the three-layered porous BDD film electrode.The specific EDL capacitance of the porous BDD film herein is compared with the nanostructured BDD electrodes prepared with top-down etching method, template method and template-less method, as summarized in Figure 7h and Table 2.This porous BDD film demonstrates a remarkably higher areal capacitance in contrast to previously reported BDD electrodes [7,10,12,13,[15][16][17][18]20,21,48].Significantly, it is even more impressive that the specific volumetric capacitance of our porous BDD film surpasses that of most BDD electrodes [7,17,18,[20][21][22]48].
The above discussions indicate that the porous BDD film possesses high specific EDL capacitance as well as outstanding long-term stability.The excellent performance can be ascribed to the well-constructed microstructure, superior surface property, and stable diamond nature.Through manipulating the growth duration, the coating of BDD on the diamond nanoplatelet makes the nanoplatelet be thicker but not clumped, leading to vertically open pore channels throughout the film.Additionally, the presence of B-O and C-O groups favors a hydrophilic surface of the porous BDD film, allowing for electrolyte penetration into the porous structure.Such porous structure provides a large specific area and maximizes the available sites for the absorption/desorption of electrolyte ions, thus resulting into a high EDL capacitance.Moreover, the enhanced interaction between the hydrophilic surface and the electrolyte ions promotes the EDL capacitance performance.Notably, compared to the unavailability of the bottom pore for the electrochemical process in the case of BDD film with unordered and tortuous pores, the bottom of our porous BDD film is well accessible to the electrolyte due to the vertically open pore channels, consequently, leading to a much outstanding volumetric capacitance performance, as exhibited in Figure 7h.Besides, TEM analysis reveals that negligible a-C phase could be found except sp 3 -bonded diamond.Because the diamond possesses high mechanical stability and good resistance to the ion corrosion, the excellent capacitance retention is verified in the long-term operation.Last but not least, the preparation method, employing the diamond nanoplatelet as the template, would avoid the impurity pollution from the foreign template or mask, hence, contributing an ideal EDL capacitance feature and superior stability.This method is also simple and reproducible, endowing the repeating growth for the construction of a multilayered porous BDD film with higher EDL capacitance.More promisingly, the developed porous BDD exhibits the capacity to confine the redox electrolytes within its structure, which would benefit the diffusion-controlled faradaic reactions of redox electrolytes on the pore surface.As a result, the porous BDD demonstrates great potential with much higher capacitance than EDL capacitance [50].

Conclusions
In summary, a novel porous BDD film has been successfully constructed through a three-step method.This method involves the initial deposition of hybrid diamond/ graphite, followed by chemical etching to create a template of diamond nanoplatelets, and finally the overcoating of BDD onto the templates.Microstructure investigations reveal that the porous BDD film consists of thickened nanoplatelets constituted by diamond grains.These vertically aligned nanoplatelets create vertically open pore channels, endowing a large specific area in the porous BDD film.Through facilely manipulating the regrowth duration and repetitions, the porous BDD film is capable of possessing an adjustable pore structure and porosity, thereby demonstrating a tunable EDL capacitance.
Electrochemical investigations indicate the specific areal capacitance of the three-layered porous film is 12.85 mF/ cm 2 , which is ~428 times higher than that of planar BDD film.Significantly, in comparison to most previously reported nanostructured BDD electrodes, our fabricated porous BDD film possesses a higher specific volumetric capacitance of 39.30 F/cm 3

Figure 1 .
Figure 1.(a) Schematic illustration of the preparation routine for the porous BDD film.(b-d) Surface SEm images, (e) cross-sectional SEm image, (f) Raman spectra, and (g) XRD patterns of the (b) pristine diamond/graphite film, (c) diamond nanoplatelet film obtained by acid etching, and (d,e) porous BDD film.
, the signals of C, O and B elements are detected, and C is dominant (85.9%) on the surface.Furthermore, the C1s spectrum is deconvoluted into five components in Figure 4c.The peaks centered at 288.4, 286.5, 285.1, 284.4 and 283.1 eV are derived from the C = O, C-O, sp 3 -C, sp 2 -C, and C-B bonds[39,40].As listed in Table1, sp3 -C takes a much large proportion (60.98%) while sp 2 -C makes up only 1.26%, hinting a high level of quality of the porous BDD film.Importantly, the small shoulder with the contribution from C-B bond confirms that B atoms have been incorporated into the diamond lattice.In Figure4d, the deconvolution of O 1s spectrum gives three components derived from B-O-C (533.7 eV), C-O-C/C-O-H (533.0 eV), and B-O-B (532 eV) bonds[39,41].The abundance of the oxygen is estimated to be 11.5% on the porous BDD film, and the main peak at 533.0 eV originates from the carbon-oxygen functional groups with 5.62%.In Figure4e, the B 1s spectrum shows three components positioned at 192.5, 188.4,and 186.6 eV, which are ascribed to B-O, BC 3 , and BC 4 bonds in the diamond

Figure 2 .
Figure 2. (a) low-and (b) high-magnification bright-field tEm images of porous BDD film.(c) High-resolution tEm image of the diamond nanoplatelet denoted by the rectangle in the bottom layer in panel (b).(d) magnified bright-field tEm image corresponding to the dashed rectangle in the upper layer in panel (b).(e,f) High-resolution tEm images corresponding to the rectangles in panel (d). the insets in panel (c,e,f) present the corresponding FFt patterns.

Figure 3 .
Figure 3. (a,c) tEm images of the upper layer in the porous BDD film.(b,d) High-resolution tEm images of corresponding regions shown in panel (a,c).

Figure 4 .
Figure 4. (a) contact angle of the porous BDD film.(b) XPS survey spectrum of the porous BDD film.High-resolution XPS (c) c 1s spectrum, (d) o 1s spectrum, and (e) B 1s spectrum of the porous BDD film.

Figure 6 .
Figure 6.Electrochemical performance of a single layer of porous BDD film prepared with varied growth time in 1 m na 2 So 4 .(a) cV curves at a scan rate of 10 mV/s.(b) cV curves of the porous BDD film prepared with the growth time of 30 min at different scan rates.(c) GcD curves at a current density of 0.1 ma/cm 2 .(d) EiS nyquist plots at ocP vs. ag/agcl.the electrochemical performance of the planar BDD film was included in panel (a,c) as a control.

Figure 7 .
Figure 7. microstructure and electrochemical performance of three layers of porous BDD film.(a) low-magnification surface, (b) high-magnification surface, and (c) cross-sectional SEm images.(d) cV curve at a scan rate of 10 mV/s.(e) GcD curve at a current density of 0.1 ma/cm 2 .(f) GcD curves of 1st, 3000st, 6000st, 9000st, and 12,000st cycles as well as (g) capacitance retention during the long-term stability measurement at the current density of 4 ma/cm 2 .(h) comparison of areal capacitance and volumetric capacitance of nanostructured BDD electrodes prepared with top-down etching, template method and template-less method.the electrochemical performance of a single layer of the porous BDD film was included in panel (d,e) as a control.

Table 1 .
Percentages of different chemical states in the porous BDD film.

Table 2 .
comparison of the EDl capacitance performance of nanostructured BDD electrodes prepared with (i) top-down etching method, (ii) template method, and (iii) template-less method.
and superior long-term stability with a capacitance retention of 100% after 12,000 cycles.The outstanding capacitance performance of the porous BDD could be ascribed to abundant vertically open pore channels, good hydrophilic property, maximized electrochemical available area, excellent chemical/ mechanical robust, and absence of impurity pollution.The above results promote the development of high-performance EDL micro-capacitor electrode based on the BDD film.Additionally, this work showcases an effective template method to prepare an ordered porous BDD nanostructure, holding good potential in various other electrochemical applications, such as electrochemical degradation and sensing.Zhaofeng Zhai is now a researcher in the Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences.He received his Ph.D degree in materials science and engineering in 2020.His research includes rational construction of diamond nanocomposite film using MP CVD and their electrochemical applications.Bin Chen is now a Ph.D candidate in the Institute of Metal Research, Chinese Academy of Sciences.He is focusing on revealing the electrochemistry energy storage mechanism of sp2-bonded carbon electrode using in-situ spectroscopy method.Chuyan Zhang is now a researcher in the Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences.She received her Ph.D degree in University of Siegen in 2023.She is focusing on the structural dependence of diamond composite on electrocatalytic performance of small molecules.LushengLiu graduated from the Shenyang Jianzhu University in 2004.He joined the Institute of Metal Research, Chinese Academy of Sciences in 2014 and dedicated to the development and design of vacuum equipment.Haozhe Song graduated from Liaoning University of Petroleum and Chemical Technology.He joined the Institute of Metal Research, Chinese Academy of Sciences in 2018 and dedicated to the optimization of equipment and production of diamond film.