Research progress on electrochemical property and surface modifications of nanodiamond powders

Abstract Nanodiamond (ND) has strong chemical stability, the initial oxidation temperature of ND is above 500 °C. A variety of oxygen-containing functional groups are adsorbed on the surface of ND, which makes ND has certain conductivity. Then ND can be used as highly stable catalyst or ideal support material. This paper reviews the properties, functionalization and electrochemical applications of ND. In this review, the catalytic activity and stability of diamond-based catalysts can be further improved by appropriately functionalizing ND, and the research progress in the field of electrochemistry can be increased.


Introduction
Diamond, as an allotrope of carbon, has excellent chemical stability and high thermal stability due to its unique sp 3 hybrid bonding [1][2][3], which can be made into electrodes to meet the new requirements of electrochemical research.Compared with traditional glassy carbon electrodes, diamond electrodes have three major advantages: (1) high chemical and electrochemical stability, low adsorption capacity for organic and biological compounds [4,5], and electrochemical response remain stable over a long period of time [6][7][8]; (2) unparalleled physicochemical properties, such as high corrosion resistance, high hardness, high thermal conductivity and high hole mobility [9][10][11]; (3) wide potential window and low background current in aqueous [12][13][14][15] and non-aqueous solutions [16,17].The above-mentioned unique characteristics of the diamond electrodes enable them to have good electrochemical performance in the fields of wastewater treatment, electrolytic analysis, fuel cells and electrosynthesis processes.
Nanodiamond (ND) is a diamond particle with a size of nanometer and exhibits different properties from those of large particles or bulk materials.In addition to the above properties of diamond, ND also has a larger specific surface area, more structural defects and higher electrical conductivity [18][19][20].Additionally, ND has both excellent physicochemical properties and extraordinary surface effect and volume effect [21][22][23][24], so its high chemical stability, high surface activity and excellent catalytic performance, making it have broad application prospects in the field of electrochemistry [25][26][27][28][29][30].When exploiting certain properties of nanomaterials, it is often affected by some subsidiary properties.Generally, the surface of nanomaterials needs to be modified to improve the structural characteristics and application effects of nanomaterials.In addition, the electrical conductivity of ND is not strong, which undoubtedly restricts its development and application in the field of electrochemistry.
The research content of ND can be roughly divided into several parts: synthesis, functionalization, surface modification, performance and applications in different fields such as energy storage, environmental engineering, electronic and electrical fields and biomedicine.For example, ND hold potential applications in the medical and biomedical fields, such as diagnostics, therapeutics, imaging and drug delivery systems.NDs can be functionalized with various polymers, peptides, amino acids, and genetic material (DNA, RNA) through physical and chemical reactions to satisfy the desired characteristics.
In our research group, a lot of research work on the surface modification of ND was carried out, and they were made into electrode materials in the field of electrochemistry, and some research progress was made.In this paper, graphite layers, functional groups, polyaniline, metal compounds and noble metals were introduced onto the surface of ND to improve its electrochemical activity, and the results of ND-based catalysts in detection and fuel cells were briefly described (Figure 1).The specific work content is described as follows.

Physiochemical properties of ND particles
At present, the ND can be produced in diverse ways, but the detonation and mechanical crushing method are widely used.It is worth nothing that the properties of ND change based on the method employed for the production.Figure 2a,b shows the transmission electron microscopy (TEM) images of ND prepared by the detonation method and the mechanical crushing method.The particles of ND synthesized by the detonation method were spherical or quasi-spherical, with good dispersibility and uniform particle size of about 5 nm (named as ND5).On the other hand, the ND prepared by mechanical crushing method had flake-like particles with sizes of about 100 nm (named as ND100), with smooth surfaces and distinct edges.
Both ND5 and ND100 (Figure 2c) had three diffraction peaks at 2θ of 43.98°, 75.28° and 91.68°, corresponding to the (111), ( 220) and (311) reflection planes of diamond, respectively [33].However, with the decrease of ND particle size, the diffraction peaks began to broaden.In addition, the ND5 prepared by detonation method absorbed with a large number of functional groups.In the infrared spectrum (Figure 2d), the absorption peak near 1130 cm −1 was caused by the C − O stretching vibration [34], and the absorption band at 1730-1790 cm −1 corresponded to the C = O bond stretching vibration in the carbonyl or carboxyl group [35].The peaks appearing at 1620-1640 cm −1 and 3430-3450 cm −1 were ascribed to the − OH vibration of adsorbed water, while peaks at 2927 and 2854 cm −1 were assigned to the asymmetric and symmetric vibrations of the C − H bond, respectively [36].
The ND powder electrode was fabricated by filling in the front end of a plastic hose with ND powder within a diameter of 1 mm and then inserting a Pt wire into the ND.The ND powder electrode had a very low background current about 10 −7 A in 0.1 mol/L KCl solution.In addition, the cyclic voltammetry (CV) curves of ND5 powder electrode in 0.1 mol/L KCl solution containing 0.01 mol/L [Fe(CN) 6 ] 3−/4− redox couple was also measured.In Figure 2e, the oxidation peaks and reduction peaks were basically symmetrical, and the potential difference (ΔE p ) between them was 99 mV at a scan rate of 0.01 V/s, which was very close to the reversible reaction (ΔE p = 59 mV) [37].The ΔE p increased as the scanning speed increased, making the electrode reversibility worse.Furthermore, the oxidation peak current (I pa ) had a linear relationship with the square root of the corresponding scan rate (v) (the inset of Figure 2e).
In addition, the electrochemical performance of ND5 powder electrode in non-aqueous electrolyte was also investigated.It can be seen from Figure 2f that a pair of symmetrical redox peaks appeared corresponding to the oxidation of ferrocene and the reduction of ferrocene ions.With the increase of scanning rate, ΔE p gradually increased from 96 to 150 mV, and the ratio of the oxidation peak current to the reduction peak current was close to 1, and the I pa had a linear relationship with the square root of the corresponding v (the inset of Figure 2f) [39].Up until now, the conduction mechanism of ND in solution needs to be further studied, but the properties and states of its surface should be the main factors affecting its electrochemical activity.

Surface modification of ND
Previous studies have shown that modifying the surface of ND can improve its electrochemical performance.Our research group had carried out graphitization treatment, modification of functional groups, deposition of polyaniline conductive films, metal compounds or noble metals on the surface of ND, and their electrochemical properties were preliminarily investigated.

Graphitization treatment
In order to improve the conductivity of ND, the surface of ND was graphitized by heating it in a vacuum environment.Theoretically, graphitized ND has higher electrical conductivity and electrochemical activity than pristine ND.The fringe spacing of 0.206 nm in Figure 3a corresponded to the (111) plane of diamond [40], and no evidence graphite layer was found on the surface of pristine ND50 (uniform particle size of about 50 nm).Figure 3b and c show the surface "graphitization" after annealing in a 10 −3 Pa vacuum at 1200 (ND50/G-1200) and 1500 °C (ND50/G-1500) for 1 h.
After vacuum annealing at 1200 °C, a thin graphene shell with two to three graphite layers were formed parallel to the (111) plane of ND.When the annealing temperature increased to 1500 °C, a continuous graphene shell containing 10-14 layers covered the core of ND.As shown in the inset of Figure 3b, it was found that the interlayer spacing of the ND crystal gradually increased from 0.206 to 0.333 nm in the graphene shell [41], that is, the graphitization of ND proceeded from surface to bulk.An interfacial layer with a distance of less than 0.333 nm was observed, indicating that there was an interaction between the ND and graphene layers.[31,32].copyright 2020, Elsevier.(c) X-Ray powder diffraction (XRD) patterns of nD5 and nD100 [33].(d) the infrared spectrum of nD5.Reproduced with permission from Ref. [33].copyright 2011, Yanshan university.cV curves of nD5 powder electrode in (e) 0.1 mol/l Kcl solution containing 0.01 mol/l [Fe(cn) 6 ] 3−/4− and (f) 0.01 mol/l ferrocene + 0.5 mol/l tetrabutylammonium fluoroborate + acetonitrile non-aqueous electrolyte, v = 0.5, 0.2, 0.1, 0.05, 0.02, 0.01 V/s (from outside to inside); the illustration is the relationship curves of I pa and v. Reproduced with permission from Ref. [38].copyright 2011, Yanshan university.
The electrochemical oxidation of ND50/G and ND50 were tested by CVs.In Figure 3d and e, there was little change in CV area before and after 500 cycles, indicating that both ND50/G-1200 and ND were highly resistant to electrochemical oxidation.In Figure 3f, two small peaks at 0.43 and 1.07 V appeared in the first cycle, and a small area difference was observed after 100-500 cycles, indicating a slight oxidation of the thick graphite layer.It should also be noted that the background current of the ND50/G-1200 and ND50/G-1500 was an order of magnitude larger than that of ND, which was attributed to the formation of graphene shells.The above results indicated that the electrical conductivity of ND/G was higher than that of ND while maintaining high electrochemical stability.

Modified functional group
To improve the electrochemical activity of ND, the surface of ND5 was modified by fluorination or amination.The method of fluorination treatment of ND5 was to fully fluoride ND5 in fluorine gas to obtain fluorinated ND5.The fluorinated ND5 was then azeotroped in ethylenediamine to prepare aminated ND5.
As can be seen from Figure 4a, the peak at 3450 cm −1 of pristine ND5 was the stretching vibration peak of − C− OH, and the peak located at 1630 cm −1 was the bending vibration peak of − OH in H 2 O, which was caused by the water adsorbed on the surface.The peaks at 2923, 1384 and 1462 cm −1 were the asymmetric stretching vibration peak, symmetric bending vibration peak and asymmetric bending vibration peak of − CH 3 [44]; the 2850 cm −1 peak was the symmetric stretching vibration peak of − CH 2 .The 1729, 1262 and 1128 cm −1 peaks were the stretching vibration peaks of − C=O, −C = C and − C−O − C, respectively.Compared with pristine ND, in the range of 1000-1400 cm −1 of fluorinated ND5, some new peaks appeared at 1049, 1096, 1245 and 1328 cm −1 , which were assigned to the stretching vibration peaks of − C−F x (x = 1, 2, 3) [45], and it can also be seen that the intensities of − C−O and − C=O as well as some − CH 3 , −CH 2 peaks decreased or even disappeared, indicating that some functional groups on the surface of ND were replaced by fluorine after fluorination.It can be seen from aminated ND5 that there were two peaks near 3430 and 1633 cm −1 , which can be assigned to the − NH stretching vibration peak and the − NH 2 bending vibration peak, respectively [46,47].The intensity of some peaks of the − C−F x (x = 1, 2, 3) decreased or even disappeared, indicating that some fluorine groups had been replaced by amine groups.
The pristine ND5, fluorinated ND5 and aminated ND5 electrodes all had a pair of symmetrical redox peaks (Figure 4b), corresponding to the redox reaction of Fe(CN) 6 3−/4− redox pair at the interface between ND electrode and electrolyte.The ΔE p was 80 mV for the pristine ND5 electrode.However, when compared with the pristine ND5 electrode, the oxidation and reduction peak currents of fluorinated ND5 electrode significantly decreased, and the ΔE p widened to 140 mV, indicating that the reversibility of electrode reaction became worse.For the aminated ND5 electrode, the peak currents increased significantly, while the ΔE p did not change.A , (e) nD50/G-1200 and (f) nD50/G-1500 in 0.5 m H 2 So 4 before and after 500 cycles between 0 and 1.2 V with a scan rate of 0.05 V/s.Reproduced with permission from Ref. [42].copyright 2012, Elsevier.postulated mechanism to explain the current enhancement for reduction of Fe(CN) 6 3− in the presence of ND is shown in Figure 4c.
It is worth noting that after the fluorination or amination modification, only the groups on the surface of ND were modified, and the surface morphology did not change.Surface modification changed the surface charge properties of ND, we have verified that the fluorinated ND was negatively charged, while the aminated ND was slightly positively charged.In Fe(CN) 6 3−/4− redox system with negative charge, the fluorinated ND electrode generated electrostatic repulsion with it, which made the charge transfer more difficult, resulting in the ΔE p became wider, and the reversibility of the reaction became worse.

Modified polyaniline
As a conductive polymer, polyaniline (PANI) processes high electrical conductivity and electrochemical reversibility, and has a wide application prospect in electrochemistry [48,49].In the field of supercapacitors, carbon-based electrode materials coated with PANI exhibit both electric double-layer capacitance and Faraday pseudocapacitance [50,51].The composite materials described above have the advantages of a high active surface area and low impedance [52] and are expected to be new types of electrocatalytic support materials.
Figure 5a shows the CV curves of PANI film on the surface of ND100 powder electrode during the growth process.There were three pairs of redox peaks in Figure 5a, corresponding to three different redox processes of aniline [53].The surface of ND100 particles was uniformly coated with PANI (Figure 5b), and the PANI with the structure of spherical particles and fibrous formed a rough and porous morphology.Such structure can further increase the surface area and electric double-layer capacitance, and PANI has Faraday reaction and Faraday pseudocapacitance in sulfuric acid solution, which makes the composite has very high capacitance [53].
Compared with the ND100, the infrared absorption peaks of PANI/ND100 in Figure 5c at 1483 and 1566 cm −1 corresponded to the stretching vibration peaks of the deformed benzene ring and quinone ring [55], respectively, which were the characteristic infrared peaks of PANI.The peaks located at 1305, 1244 and 1139 cm −1 were ascribed to the C − N stretching, C − N + stretching and − NH + = stretching vibration, respectively [56].Moreover, two peaks at 818 and 880 cm −1 were the C − H out-of-plane bending vibration peaks on the double-substituted polyaramide ring [57].
The corresponding peak positions of the PANI/ ND100 had a slightly red shift from the infrared absorption peaks of the pure PANI described in the literatures, which may be due to the high surface activity of ND100 particles.In addition, it also made the ND particles bonded with PANI in the PANI/ND100 composite material, which reduced the electron cloud density on the polymer molecular chain and force constant between atoms, causing the absorption peak move to a lower frequency.
The composite electrode of PANI/ND100 showed a high background current and an intrinsic Faradaic current of PANI in 0.5 mol/L sulfuric acid solution, reflecting a high capacitance.In addition, through the AC impedance analysis, it was found that the PANI/ND100 electrode had nearly ideal capacitance performance, resulting in a good application prospect in the electrochemical field.

Modified metal compounds
Nanodiamond has not only excellent properties and wide applications as an electrode material, but also becomes an ideal support and template due to its unique structure, stability and physicochemical properties.Furthermore, surface modification of nanometallic compounds on ND is expected to obtain better practical performance in many functional fields.Among them, transition metal compounds have attracted much attention because of their structure and performance characteristics, which have promoted the development of sound, optical, electrical, magnetic and other industries.

Oxides
Titanium dioxide (TiO 2 ) is a kind of non-toxic, non-irritating and chemically stable inorganic functional material with unique electrochemical and optoelectronic properties, high catalytic activity and stability in acidic or alkaline environments [58][59][60].Based on the excellent properties of TiO 2 , the production of TiO 2 composite catalytic electrodes has become a hot research field.
In our research group, Ti coated ND100 (Ti/ND100) was prepared by circulating vacuum gas-filled vapor deposition.The Ti/ND100 was then exposed to air for gradual oxidation to form anatase TiO 2 coating, thus TiO 2 /ND100 was obtained [32].It can be seen that the surface of ND100 (Figure 6a) was coated by a continuous coating with a thickness of 4 nm after vapor-deposited Ti coating (Figure 6b).After oxidation treatment in air, the Ti coating was oxidized to TiO 2 , and the morphology of coating was significantly different from that before treatment, in which the regular geometric particles disappeared, and the spherical particles appeared with uniform sizes (Figure 6c).In Figure 6d, the current of redox reaction of Fe(CN) 6 3−/4− in TiO 2 /ND100 electrode was higher than that of the ND100 electrode, indicating that the electric conductivity and catalytic performance of ND were significantly improved after being modified by TiO 2 coating.
Transmission electron microscopy and HRTEM images were used to observe the deposition state of TiO 2 on the surface of ND.In Figure 6f and g, the pristine ND with sizes of 50-100 nm had sharp edges and clear crystal planes, and 0.206 nm corresponded to the (111) crystal plane of diamond.As can be seen from Figure 6h, TiO 2 particles with sizes of 4-8 nm were uniformly deposited on the surface of ND50, and the (101) plane of anatase TiO 2 corresponded to a spacing of 0.352 nm [63].
The oxidation mechanism of aniline on PbO 2 /ND5 composite electrodes was basically the same as that on ND powder electrodes, in which aniline firstly oxidized to generate free radical cation, and then formed dimer through coupling reaction [66].In Figure 7b, the oxidation peak of aniline on ND5 powder electrodes was at 1.03 V, and the peak current gradually decreased with the increase of scanning cycles.While the oxidation peak of aniline on the PbO 2 /ND5 composite electrodes was at 0.93 V, and disappeared immediately in the second cycle.After stabilization, the oxidation peak current of  dimer was much higher than that of ND5 powder electrodes, indicating that the oxidation reaction of oligomer produced by aniline oxidation was more likely to occur on a PbO 2 /ND5 composite electrode.

Carbides
Metal carbides are prone to agglomerate and have poor structural stability in an acidic environment, so we combined them with ND.The carbon in metal carbides can be obtained by an epitaxy of ND, and the metal carbide layer has a strong binding force with ND, thereby solving the problem of weak structural stability of pure carbide.Ti and W, with strong chemical affinity with carbon, can bond with the ND matrix to form carbides.
A simple one-step method [67] was used to vacuum deposit metallic Ti on the surface of ND50, and then ND50 was further reacted with Ti to obtain epitaxially grown TiC layer.The structural instability of TiC can be improved by means of strong chemical bonding of TiC and ND50.In addition, epitaxial WC-modified ND50 (WC/ND50) was prepared by microwave reduction of polyols and high-temperature reduction method.Except for the characteristic peaks of diamond, in TiC/ND50 (Figure 8a), the sharp diffraction peaks at 35.94°, 41.72°, 60.54°, 72.48° and 76.25° corresponded to the ( 111), ( 200), ( 220) and (311) planes of TiC [68,69].In addition, three weaker diffraction peaks at 35.08°, 38.06° and 40.00° were attributed to a small amount of crystal plane diffraction of hexagonal close-packed titanium.According to the XRD pattern of WC/ND50, the relatively weak diffraction peaks at 25.72°, 36.86°,53.02° and 59.80° corresponded to WO 2 , indicating that the content of WO 2 was very small.And the other nine diffraction peaks can be assigned to the crystal of WC [61].
In Figure 8b, the ND50 showed obvious edges and angles (the inset of Figure 8b) and the interplanar spacing of 0.206 nm corresponded to the (111) plane of diamond.For TiC/ND50 (Figure 8c), most ND particles were covered by a sedimentary layer.The surface of ND in Figure 8d was uniformly dispersed with a layer of fine particles ranging in sizes from 10 to 30 nm.Combined with the above XRD data, it showed that WC nanoparticles were uniformly deposited on the surface of ND.

Modified noble metals
The high stability and high oxidation resistance of diamond make it a popular material to replace carbon support, which can be used to prepare catalysts after surface modification of noble metals.In our research group, ND100 supported platinum (Pt/ND100) catalytic electrode was prepared by an electrodeposition method [33].In the XRD pattern of Pt/ND100 (Figure 9a), four diffraction peaks at 2θ of 39.8°, 46.2°, 67.4° and 81.2° corresponded to the characteristic diffraction of the ( 111), ( 200), ( 220) and (311) crystal planes of Pt [70].Besides, the diffraction peaks of Pt were broadened to a certain extent, which also indicated that the size of Pt particles was very small.
In addition, Pt/ND5 was synthesized by a microwave-assisted reduction method.As shown in Figure 9b, the size of ND prepared by detonation method was about 4-10 nm with some agglomeration.And Pt particles were uniformly dispersed on the surface of ND5 with sizes of 3-5 nm.Since the atomic numbers of C and Pt are very different, it is easy to distinguish them.In Figure 9c, the interplanar spacings corresponding to the (111) and (200) planes of Pt were 0.22 and 0.196 nm, respectively [62].
Furthermore, the PtRu/ND5 bimetallic catalyst was also prepared by microwave-assisted ethylene glycol reduction method using pristine ND5 as support [71].In Pt/ND5 (Figure 9d), the 2θ of 39.8°, 46.2°, 67.4°, 81.2 and 85.7° corresponded to the ( 111), ( 200), ( 220) and ( 311) and ( 222) planes of Pt crystals, respectively, indicating the existence of a face-centered cubic crystal phase of Pt [70].Compared with Pt/ND5, there were five diffraction peaks at 2θ values of 39.9°, 46.4°, 67.6°, 81.8° and 85.8° in PtRu/ND5, which had a higher angle direction than the characteristic peaks of pure Pt.And there was no characteristic peak of Ru, indicating that Ru penetrated into the lattice of Pt in the process of co-reduction, forming PtRu alloy.The peak broadening in the diffraction pattern was due to the small size of PtRu and ND.According to Scherrer's equation, the grain size of PtRu was calculated to be about 3.5 nm [72].In PtRu/ ND5 (Figure 9e), the size of ND particles prepared by detonation method was about 4-15 nm and fine particles were uniformly dispersed on the surface of ND5 with average sizes of about 3.5 nm.It can be seen from Figure 9f that the interplanar spacing of (111) plane of PtRu was 0.224 nm compared to that of the pure Pt (111) plane of 0.226 nm, which was consistent with the XRD results, again illustrating the formation of PtRu alloy [72].
In addition, other research groups have also researched functionalization of ND (Figure 10).For example, Lee and coworkers prepared hydrogenated NDs (H-NDs) via thermal treatment in a hydrogen atmosphere to improve surface conductivity [73].Compared with pristine ND, after hydrogenation, the symmetric and asymmetric C-H vibration peaks appeared at 2860 and 2968 cm −1 , respectively.With further hydrogenation, the electrical conductivity of H-ND increased gradually, corresponding to the O/C ratio, which is a quantitative index of electrical conductivity of ND-based materials.Shumaker-Parry et al. applied a new method of thiolating the surface of ND by treatment with elemental sulfur at 450 °C, followed by reduction of surface-bound sulfur moieties [74].The surfaces of ND were primarily thiols, which can react with maleimide functional groups.Moreover, Luo's research group prepared aminated ND (ND-PEI) by annealing pristine ND and further grafting ethylenediamine-branched polyethylenimide, and the photothermal property was stable for the ND-PEI [75].

Electrochemical applications of ND
Nanodiamond has a wide range of applications in the field of electrochemistry, including electrical analysis, electrochemical energy storage and electrocatalysis, etc.

Detection
Electrochemical detection of solutions containing lead ions or phenol and the electrode reaction process was carried out, which provided a technical basis for the application of ND powder electrodes and modified ND electrodes in the field of electrochemistry.In Figure 11a, during the forward scanning, an oxidation peak appeared at 1 V, that is, the phenol underwent an oxidation reaction.While no corresponding reduction peak was found during the backscanning, indicating that the reaction of phenol on ND5 electrode was irreversible [78].
The oxidation peak potential of phenol on the PbO 2 / ND5 composite electrode was shifted to a negative direction than that on the ND5 electrode (Figure 11a), and the oxidation peak current was much higher than that of the ND5 electrode, indicating that the oxidation reaction of phenol on the PbO 2 /ND5 was more likely to occur.In summary, the PbO 2 /ND5 composite electrode exhibited better electrocatalytic activity and higher catalytic efficiency for the oxidation of phenol than the ND5 electrode.
Lead ion (Pb 2+ ) is a toxic element, and it is not easy to be excreted after entering the human body, resulting in cumulative poisoning [79,80].Therefore, it is of great significance to detect the existence and content of lead.Previous studies have found that ND powder electrode has high sensitivity to Pb 2+ in solution, so it can be used to detect trace amounts of Pb 2+ .In our group, differential pulse voltammetry (DPV) was used to detect trace Pb 2+ in water, and the effects of the dissolution time and initial concentration of Pb(NO 3 ) 2 on the oxidation peak current of Pb 2+ were investigated.
It can be seen from Figure 11b that the oxidation peak current of Pb 2+ increased with the increase of dissolution time, the oxidation peak gradually became sharper, and the background current also decreased, indicating that the reduction of lead ions was a process of enrichment [81].With the increase of dissolution time, the more zero-valent lead was enriched on the cathode, and the higher oxidation peak current obtained from the reoxidation of the reduced lead to Pb 2+ during the forward scanning.In addition, the current of oxidation peak on the anodic stripping voltammetry curve increased with the increase concentration of Pb(NO 3 ) 2 , and the oxidation peak became sharper gradually (Figure 11c).This peak was caused by the oxidation of zero-valent lead adsorbed or deposited on the surface of ND electrode.Due to the fast kinetics of oxidation reaction, a "depletion" type of oxidation peak was obtained.The catalytic oxidation behavior of ND100 and TiO 2 /ND100 powder electrodes for sodium nitrite solution was also investigated.In Figure 11d, the characteristic peak of NO 2 − appeared at 0.8 V, and the peak current values were 0.60 µA (ND100) and 2.59 µA (TiO 2 /ND100), respectively, which proved that the TiO 2 /ND100 electrode had stronger catalytic activity towards nitrite.

Direct methanol fuel cell
A direct methanol fuel cell uses methanol instead of dangerous hydrogen as fuel, which has the advantages of high energy ratio and low environmental pollution, so it has become a hot spot in fuel cell research [82][83][84].The anode and cathode reactions of direct methanol fuel cell are methanol oxidation reaction (MOR) and oxygen reduction reaction (ORR) as shown in Figure 12, respectively [85][86][87][88].The most active catalysts for both are mostly Pt group metals [89,90].However, the price of Pt is expensive, and the intermediate product CO generates during the oxidation of methanol, which inhibits the catalytic effect of Pt.Hence, improving the use efficiency of Pt, reducing the amount of Pt and enhancing the anti-toxicity of Pt have become important research directions of Pt catalysts.Therefore, the Pt supported on ND (Pt/ND) catalyst was prepared based on these research goals, and ND can also be used to develop and prepare non-Pt catalysts.Reproduced with permission from Ref. [87].copyright 2019 american chemical Society.

MOR
Pt/ND5 and Pt/GND5 electrocatalysts were prepared by microwave polyol method using ND5 and graphitized ND5 as supports [38].In the potential range of −0.3-0.2V, it is the adsorption and desorption regions of hydrogen, in which the lower part is the adsorption region of hydrogen, and the upper part is the desorption region of hydrogen [91,92].The hydrogen adsorption and desorption capacities of ND5, Pt/ND5 and Pt/GND5 modified glassy carbon electrodes were investigated in Figure 13a.It can be seen that in the adsorption and desorption region of hydrogen, Pt/ND5 and Pt/GND5 had two or three pairs of symmetrical redox peaks, which corresponded to the adsorption and desorption process of hydrogen atoms.Nevertheless, the adsorption and desorption area of Pt/GND5 was much larger than that of Pt/ND5, indicating that the Pt/GND5 had a larger effective active specific surface area.For pristine ND5, no peaks were seen in the hydrogen adsorption and desorption regions, indicating that Pt particles played the role of hydrogen adsorption and desorption.It can also be seen that in the non-hydrogen adsorption and desorption regions, the currents of Pt/ND5 and Pt/ GND5 gradually increased compared with the pristine ND5, indicating that the electrical conductivity of ND double-modified by Pt and graphitization was significantly improved.
In the catalytic performance of methanol oxidation (Figure 13b), the pristine ND5 electrode had no catalytic effect on methanol.On the Pt/ND5 electrode, two oxidation peaks appeared at 0.62 and 0.43 V, which represented methanol oxidation and subsequent oxidation reaction of intermediate product [93].For Pt/GND5, these two oxidation peaks also appeared at the same positions.Compared with Pt/ND5, the oxidation peak current was significantly larger, indicating that Pt/GND5 had higher catalytic activity.The peak current at 0.43 V was slightly higher than that at 0.62 V.This was because some intermediate products (such as CO) generated in methanol oxidation were easily adsorbed on the surface of catalyst, which hindered the oxidation of methanol and led to the reduction of catalytic efficiency [94].
To further improve the catalytic performance of methanol oxidation, Pt/TiO 2 /ND50 catalyst was prepared.For MOR (Figure 13c), the positive sweep peak currents of Pt/TiO 2 /ND50 and Pt/ND50 were 2.16 and 1.07 mA, respectively, and the forward peak current (I f )/ backward peak current (I b ) value of Pt/TiO 2 /ND50 (1.06) was slightly larger than that of Pt/ND50 (0.99), indicating that Pt/TiO 2 /ND50 had better resistance to CO poisoning.The better catalytic activity of Pt/TiO 2 / ND50 suggested that the presence of TiO 2 enhanced the oxidation activity to methanol, and the interaction between Pt and TiO 2 changed the electronic properties of Pt, making the electrode surface better contact with methanol [95].
It can be seen from Figure 13d that all three catalysts exhibited characteristic peaks of polycrystalline Pt, that is, two pairs of reversible redox peaks at −0.2-0.1 V represented the adsorption and resolution peaks of hydrogen.The calculated electrochemical surface areas (ECSAs) [96] of Pt/TiO 2 /GND50 and Pt/TiO 2 /C catalysts were 123.4 and 113.0 m 2 /g, respectively, which were higher than that of Pt/C (97.9 m 2 /g).These results indicated that Pt/TiO 2 /GND50 and Pt/TiO 2 /C had more active sites, which may be due to the increased surface area of Pt by the secondary support TiO 2 nanoparticles.In order to investigate the true catalytic activity of the three catalysts, the current had been divided by the respective ECSA value (Figure 13e).It can be seen that the current density of the forward oxidation peak of Pt/ TiO 2 /GND50 (1.58 mA/cm 2 ) catalyst was higher than those of Pt/TiO 2 /C (1.40 mA/cm 2 ) and Pt/C (1.05 mA/ cm 2 ), which was caused by the synergistic effect between Pt and TiO 2 .
In addition, as an intermediate layer, the carbide obtained from diamond epitaxy can strongly anchor Pt with the metal bond, thereby obtaining a composite electrocatalyst with high activity and high stability.As can be seen from Figure 13f, the Pt/TiC/ND50 catalyst exhibited the largest current density of forward oxidation peak (57.5 mA/cm 2 ), which was much larger than that of the Pt/ND50 catalyst (12.0 mA/cm 2 ) and Pt/C catalyst (50.3 mA/cm 2 ), which can be attributed the relatively high electrical conductivity of TiC/ND50 and the synergistic effect of Pt particles and TiC layer.
In general, the Pt/TiO 2 /ND50, Pt/TiO 2 /GND50 and Pt/TiC/ND50 all exhibited higher MOR catalytic activity than Pt/ND50 or even Pt/C.This was mainly because Ti can form Ti − OH groups, which can help remove the intermediate products generated during methanol oxidation and improve the CO poisoning resistance of Pt nanoparticles.

ORR 4.2.2.1. Pt-based catalysts.
In order to obtain the catalytic performance of Pt/TiN/ND50 catalyst for ORR, the linear sweep voltammetry (LSV) curves of Pt/ TiN/ND50 modified glassy carbon electrode were tested.As can be seen from Figure 14a, LSV curves of different rotational speeds maintained good isometric property.And the relationship between the reciprocal square root of rotation speed and reciprocal current, that is, the K − L curve [97] is shown in Figure 14b.It can be seen that the two were basically in a linear relationship and the reaction electron number [98] of Pt/TiN/ND50 to ORR was calculated to be about 3.99 at 0.52 V, indicating that the ORR of Pt/TiN/ND50 catalyst at high potential was still a four-electron reaction.
Figure 14c compares the ORR catalytic activities of Pt/TiN/ND50, Pt/C and Pt/ND50 catalysts, including three important indicators (onset potential, half-wave potential and limiting current density).Among them, the Pt/TiN/ND50 showed much greater advantages than Pt/ND50, and a little advantage over traditional Pt/C catalysts.The current density in the mixed control region was corrected and converted into kinetic current density, and the Tafel curves of Pt/TiN/ND50, Pt/C and Pt/ND50 can be obtained, as shown in Figure 14d.
The Tafel slopes [99] of Pt/TiN/ND50, Pt/C and Pt/ ND50 in the low overpotential region were −58.3, −61.4 and −62.1 mV/dec, respectively, which were close to the theoretical Tafel slope of Pt electrode of −60 mV/dec [100].This also indicated that the oxygen adsorption in this region was Temkin adsorption [101], which refered to the single atomic layer adsorption of oxygen molecules on the surface of Pt.
In the high overpotential region, the Tafel slopes of Pt/TiN/ND50, Pt/C and Pt/ND50 were −122.3,−120.6 and −120.9 mV/dec, respectively, indicating that the oxygen adsorption in this region was Langmuir adsorption [102].As reported, the slope value of −120 mV/dec indicated that the step determining of reaction rate in this region was the process of oxygen gaining an electron to obtain the OOH* intermediate, with the corresponding transfer coefficient β of 0.5 [103].For the change of slope in Tafel curve, Wang et al. believed that the main reason was that a large number of oxides were adsorbed on the surface of Pt, which affected the adsorption of oxygen and intermediate products on the surface of electrode [104].
TEM images of Pt/TiC/ND50, Pt/ND50 and Pt/C catalysts are shown in Figure 15a-c.It can be seen that Pt nanoparticles with sizes of 3-5 nm were deposited on the surfaces of ND50, TiC/ND50 and Vulcan XC-72 supports, in which the Pt nanoparticles of Pt/TiC/ND50 and Pt/C had good dispersion.
It can be seen from Figure 15d that the onset potential and limiting current density of the Pt/TiC/ND50 catalyst for ORR were 0.692 V and 6.14 mA/cm 2 , respectively, which were significantly better than those of the Pt/ ND50 catalyst (0.577 V and 1.94 mA/cm 2 ).This was due to the enhanced conductivity of support and the synergistic effect of Pt particles and epitaxial titanium carbide layer.Compared with Pt/TiC/ND50, Pt/C had a similar limiting current density (6.19 mA/cm 2 ), but exhibited a relatively left-shifted onset potential (0.671 V).All these indicated that Pt/TiC/ND50 not only had much higher catalytic activity for ORR than Pt/ND50 catalyst, but also exhibited a higher catalytic activity than Pt/C.In addition, in the hybrid region, the half-wave potential of Pt/ TiC/ND50 had an obvious right shift relative to that of Pt/C, which further indicated that Pt/TiC/ND50 had a higher ORR catalytic activity than Pt/C.Furthermore, accelerated durability tests (ADT) experiments were performed in the potential range of −0.3 to 1.2 V (vs.Ag/AgCl) for 800 cycles of consecutive CV scanning, and the selection of a high potential of 1.2 V further accelerated the oxidation corrosion of sp 2 carbon support.By calculating the ECSAof different cycles, we normalized the initial ECSA of each catalyst, and plotted the relationship between the percentage of remaining ECSA and the number of scanning turns, as shown in Figure 15e.The Pt/TiC/ND50 had 70.2% ECSA remaining after 800 cycles of ADT, while Pt/C and Pt/ND50 had only 17.1% and 9.5% ECSA remaining.The reason for the decrease of ECSA may be caused by the migration and agglomeration of Pt particles or the oxidative collapse of support.Furthermore, strong interactions between TiC and Pt NPs made Pt NPs anchored on the supports, leading to improved durability of Pt/TiC/ND.
It can be seen from Figure 15f that the onset potential of ORR of Pt/TiO 2 /GND50 and Pt/TiO 2 /C catalysts were both higher than that of Pt/C, and the limiting current densities of the three catalysts were similar.In the mixed control region, the half-wave potential of Pt/TiO 2 / GND50 was 599 mV, which was larger than those of Pt/ TiO 2 /C (572 mV) and Pt/C (556 mV), indicating that TiO 2 /GND50 used as catalyst support can improve the electrode kinetics of ORR.It has been reported in literature that well-crystallized graphite has a certain catalytic activity for ORR [105].Moreover, the synergistic effect of Pt and TiO 2 as well as the Pt and graphite layers make Pt/TiO 2 /GND50 with good catalytic performance for ORR.

Non-noble metal catalysts.
Studies have shown that the degradation rate of carbon is accelerated by the metal species supported on carbon materials, leading to the shedding of metal particles from carbon and affecting the performance (Figure 16a) [106].In view of the above reason, the research on ND catalysts of nonnoble metals has attracted the attention of researchers.Among them, Dong et al. [107] carbonize ND100 to form a graphene layer by vacuum heat treatment, and heat treatment with melamine to achieve nitrogen doping of graphene layer on the surface of ND100 (N-ND100@G).It can be seen from Figure 16b that nitrogen doping for N-ND100@G can increase the conductivity of carbon materials, especially the graphitic N can provide a lone pair of electrons to π bond [108].The catalytic activity of N-ND100@G, ND100@G and Pt/C (Pt loading of 20 wt%) for ORR in alkaline solution can be analyzed according to the LSV curve in Figure 16c.
The onset potential and half-wave potential of N-ND100@G catalytic ORR was significantly higher than those of ND100@G, indicating that the catalytic activity of ND100@G for ORR can be significantly improved by nitrogen doping treatment (Figure 16d) [110].Although the ORR catalytic activity of N-ND100@G was still lower than that of Pt/C, the halfwave potential difference between the two was only 68 mV, and N-ND100@G had a larger limiting current value.
Furthermore, Wu et al. [111] prepared cobalt-embedded nitrogen-doped graphitized carbon shell covering a ND core (Co-N-C/ND100) non-noble metal catalysts (Figure 17a).The surface of Co-N-C/ND100 composite electrocatalyst was further analyzed by XPS.In Figure 17b, the binding energies of 397.7, 399.1, 400.6, 401.9 and 404.3 eV corresponded to pyridine N (16.4%),N-Co bond (35.4%), pyrrole N (31.5%),graphite N (12.6%) and oxidized N (4.1%), respectively [112][113][114].In Figure 17c, the binding energy of 780.8 eV corresponded to Co-N bond, and the positions with binding energies of 782.8 and 803.8 eV can be attributed to the peaks of divalent cobalt, and the 787.3 and 797.2 eV corresponded to trivalent cobalt [115].The appearance of Co 2+ and Co 3+ was due to the oxide layer on the surface of cobalt particles.According to the XPS results, it can be concluded that the graphite layer of Co-N-C/ND100 was doped with N atoms, and the combination of Co-N provided more active sites for the catalyst [116].
Through the LSV curves (Figure 17d), it can be seen that the ORR catalytic activity of Co-N-C/ND100 was relatively higher than that of C/ND100, N-C/ND100 and Co-C/ND100, which was similar to Pt/C.The above results showed that the addition of nitrogen and cobalt had synergistic effect on the activity of ORR, and the generated Co-N bond increased the active site of catalyst and further improved the performance of catalyst [117].In order to investigate the stability of Co-N-C/ND100 composite catalyst in alkaline environment, the current-time (i-t) curve of catalyst was tested (Figure 17e).After 80,000 s of test, the initial current of Co-N-C/ ND100 only decreased by 14.4%, while that of Co-N-C decreased by 48.6%, indicating that Co-N-C/ND100 had higher stability than Co-N-C catalyst.The initial current of commercial Pt/C had been reported in literature to decreases by 16% after 8000 s test, which further indicated that Co-N-C/ND100 had higher stability than commercial Pt/C.This was mainly because C/ND100 had a highly stable ND core, and the graphitized shell layer on the surface also had a stronger oxidation resistance than the carbon layer of XC-72.Shell/core structure B and N co-doped graphite carbon/nanodiamond (BN-C/ND60) non-noble metal catalysts were synthesized by a simple one-step heat treatment of the mixture ND60, melamine, boric acid and FeCl 3 [118].In N-C/ND60 (Figure 18a), N existed in the form of pyridine N (398.6 eV), pyrrolic N (399.7 eV) and graphitic N (401 eV), and their proportions in the graphitic carbon layer were 26.6%, 43.2% and 30.2%, respectively.After additional doping of B (Figure 18c), the percentages of both graphitic N and pyrrolic N decreased, especially graphitic N decreased even from 30.2% to 7%.
On the other hand, the percentage of pyridine N ranged from 26.6% to 52.3%, becoming the dominant doping configuration in the graphitic carbon layer.We all know that N can easily form pyridine N with C at low temperature, and then, when the temperature increases, the pyridine N will change into graphitic N [119].The above results indicated that the addition of B prevented the transformation of pyridine N to graphitic N in the process of heating.In addition, a small amount of N-B configuration (397.9 eV) appeared.In B-C/ND60 (Figure 18b), B existed in the form of BC 3 (188.4eV), BC 2 O (190.5 eV), BCO 2 (192.3 eV) and B − O bond (193.6 eV), and their proportions in the graphitic carbon layer were 11.3%, 9.3%, 31.8% and 47.6%, respectively.Among them, the proportion of B − O bond was the highest, which indicated that most of B in the sample was not doped into the graphitic lattice [120].After additional N doping (Figure 18d), the oxidized B decreased from 47.6% to 26.9%, while BCO 2 increased substantially from 31.8% to 63.3%.It is shown that the addition of N promoted the separation of B and O, as well as the doping of B into the graphitic lattice.In addition, compared with the B-C/ND60, there was a small deviation in the position of each B − C peak, possibly due to the introduction of N.
As shown in Figure 18e, the onset potential of BN-C/ ND60 electrode (−0.05 V vs. Hg/HgO) was much more positive than that of N-C/ND60 electrode (0.1 V vs. Hg/ HgO), as well as the B-C/ND60 electrode (−0.12 V vs. Hg/HgO) or undoped C/ND60 electrodes (−0.2 V vs. Hg/ HgO).In addition, the BN-C/ND60 electrode had much higher diffusion current density than B-C/ND60, N-C/ ND60 and undoped C/ND60 electrodes.These results indicated that B and N co-doped C/ND60 electrocatalysts showed the highest ORR electrocatalytic activity among all C/ND60 electrocatalysts, demonstrating a synergistic effect was produced by the co-doping of C/NDs with B and N atoms.However, the potential of BN-C/ND60 electrode was still slightly lower than that of the Pt/C electrode.In Figure 18f, the n value of BN-C/ND60 in ORR was 3.8, which was much higher than that of B-C/ ND60 (n = 2.56) or N-C/ND60 (n = 2.87).It showed that the B-C/ND60 and N-C/ND60 conducted unwanted two-electron domination pathways for ORR, but not through the four-electron dominant pathway on BN-C/ ND60 electrodes, which was the most efficient ORR process.Finally, the applications of diamond in electrochemistry are summarized, as shown in Table 1.

Prospects
Introducing new surface modification layers or surface functional groups to further deepen the research on surface modification of diamond, and improving the catalytic activity and stability of diamond-based electrocatalysts will still be the focus of electrochemical field, especially the modification technologies with strong controllability, a simple and green preparation method will attract more attention.In addition, due to the large number of functional groups adsorb on its surface, the surface potential of ND is changed, and it is easy to cause the agglomerate ND particles.As a result, the excellent properties of ND cannot be given full play, and the application of ND is severely limited.In order to solve this problem, the surface modification of ND should strive to achieve the effect of disaggregation and stable dispersion of nanoparticles in different systems.The research on diamond-based electrocatalysts has been carried out for a short period of time, and the entire field is still in its infancy.
There is great research potential in terms of electrocatalyst mechanism, stability mechanism and preparation technology.The development of new electrocatalysts may also be a hot research direction when they are compounded with other materials.In summary, many exciting achievements have been achieved in the related research of ND electrode materials.With the development of future research, ND will be widely used in the field of electrochemistry.

Figure 1 .
Figure 1.Schematic illustration of different topics in this review based on nD.

Figure 7 .
Figure 7. (a) XRD pattern of Pbo 2 /nD5; (b) cV curves of Pbo 2 /nD5 and nD5 electrodes in aniline solution with a scan rate of 0.1 V/s. the inset is the cV curves of aniline on the nD5 electrode for the first two turns.Reproduced with permission from Ref. [65].copyright 2010, Yanshan university.

[ 76 ,
77].Our research group mainly introduces it from the following aspects.

Figure 15 .
Figure 15.tEm images of (a) Pt/tic/nD, (b) Pt/nD and (c) Pt/c; (d) lSV curves of Pt/tic/nD, Pt/c and Pt/nD in o 2 -saturated 0.5 mol/l H 2 So 4 solution with the electrode speed of 1600 rpm and the scan rate of 10 mV/s; (e) comparison of the changes of EcSa of each catalyst before and after aDt.(f) lSV curves of Pt/tio 2 /GnD50, Pt/tio 2 /c and Pt/c electrode measured in o 2 -saturated 0.5 mol/l H 2 So 4 at a rotation rate of 1600 rpm.Reproduced with permission from Ref. [61].copyright 2015, Yanshan university.

Figure 17 .
Figure 17.(a) Formation process of co-n-c/nD composites.High-resolution XPS spectra of (b) n1s and (c) co 2p of co-n-c/nD100.(d) lSV curves of c/nD100, n-c/nD100, co-c/nD100, co-n-c/nD100 and Pt/c in 0.1 mol/l KoH o 2 -saturated solution at scanning speed of 10 mV/s and speed of 1600 rpm; (e) i-t comparison curves of co-c/nD100 and co-n-c at −0.3 V in o 2 -saturated 0.1 mol/l KoH solution at 80,000 s.Reproduced with permission from Ref.[111].copyright 2016, Elsevier.

Figure 16 .
Figure16.(a) a Schematic illustration of Pt particle detachment from carbon support in an aqueous alkaline electrolyte.Reproduced with permission from Ref.[106].copyright 2022, Elsevier.(b) cV curves of n-nD100@G and nD100@G measured in 0.1 mol/l KoH solution with a scan rate of 0.1 V/s; (c) lSV curves of n-nD100@G, nD100@G and Pt/c in o 2 -saturated 0.1 mol/l KoH solution at sweep speed of 0.01 V/s and rotation speed of 1600 rpm.Reproduced with permission from Ref.[109].copyright 2014, Yanshan university.(d) Schematic pathway for the oxygen reduction reaction on nitrogen-doped carbon materials.Reproduced with permission from Ref.[110].copyright 2018, royal Society of chemistry.

Figure 18 .
Figure 18.High-resolution XPS spectra of (a) n in n-c/nD60, (b) B in B-c/nD60 (c) n and (d) B in Bn-c/nD60; (e) lSVs of different electrodes in o 2 -saturated 0.1 m KoH electrolyte at a scan rate of 10 mV/s, and at a rotation rate of 1600 rpm.(f) K-l plots and the dependence of n on the potential of −0.6 V (vs.Hg/Hgo) for B-c/nD60, n-c/nD60 and Bn-c/nD60 electrodes [118].

Table 1 .
Summary for electrochemical applications of nD.