A perspective on diamond composites and their electrochemical applications

Diamond composites have gained increasing interests because of their outstanding properties (e.g. robust mechanical properties, high conductivity and activity, good chemical and thermal stability, as well as excellent electrochemical properties) and their promising applications in a wide range of different fields. In this perspective, recent advances on the synthesis of diamond composites are summarized together with state-of-the-art progress in their electrochemical applications. Metal-diamond, alloy–diamond, oxide/carbide/nitride-diamond, sp² carbon/sp³ diamond, and organic-diamond composites are covered in the context of enhancing their performance of electrocatalysis, sensing, water treatment, supercapacitor, and photoelectrochemistry. Ongoing challenges and future perspectives of the synthesis and electrochemical applications of diamond composites are outlined and discussed.     

Electron transfer kinetics on composite diamond (sp3)–graphite (sp2) electrodes

The concept of non-diamond sp² impurity states as charge transfer mediators on boron-doped diamond (BDD) surface was suggested as an explanation for the electrochemical behavior of synthetic diamond based electrodes. In order to verify this concept, graphite particles (sp²) were deposited on diamond electrodes (sp³) by mechanical abrasion. The behavior of the so prepared diamond–graphite composite electrodes were compared with those of as-grown (BDD_ag) and those after mild anodic polarization (BDD_mild).Outer-sphere electron transfer processes such as ferri/ferrocyanide (Fe(CN)₆III/II) and inner-sphere charge transfer reactions such as 1,4-benzoquinone/hydroquinone (Q/H₂Q) were chosen in order to investigate the electrochemical properties of these composite electrodes. Both redox systems became more reversible as the graphite (sp²) loading increased. A strong analogy existed between as-grown diamond electrodes and diamond–graphite composite electrodes.Finally a model is proposed which describes the BDD electrode surface as a diamond matrix in which non-diamond (sp²) impurity states are dispersed. These non-diamond sp² states on BDD surface acts as charge mediators for both inner-sphere and outer-sphere reactions.     

Nanodiamond Thin Film Electrodes: Metal Electro‐Deposition and Stripping Processes

The properties of a nanodiamond thin film deposit formed on titanium substrates in a microwave‐plasma enhanced CVD process, are investigated for applications in electroanalysis. The nanodiamond deposit consists of intergrown nano‐sized platelets of diamond with a high sp² carbon content giving it high electrical conductivity and electrochemical reactivity. Nanodiamond thin film electrodes (of approximately 2 μm thickness) are characterized by electron microscopy and electrochemical methods. First, for a reversible one electron redox system, Ru(NH₃)₆^3+/2+, nanodiamond is shown to give well‐defined diffusion controlled voltammetric responses. Next, metal deposition processes are shown to proceed on nanodiamond with high reversibility and high efficiency compared to processes reported on boron‐doped diamond. The nucleation of gold is shown to be facile at edge sites, which are abundant on the nanodiamond surface. For the deposition and stripping of both gold and copper, a stripping efficiency (the ratio of electro‐dissolution charge to electro‐deposition charge) of close to unity is detected even at low concentrations of analyte. The effect of thermal annealing in air is shown to drastically modify the electrode characteristics probably due to interfacial oxidation, loss of active sp² sites, and loss of conductivity.     

Structural and electrochemical heterogeneities of boron-doped diamond surfaces

This brief review is focussed on the recent progress in studies of the heterogeneous electrochemical behaviour of various boron-doped materials extending from zero-dimensional particles through polycrystalline or nanostructured three-dimensional surfaces. A boron-doped diamond reveals large heterogeneities induced by numerous factors, inter alia multi-faceted crystallinity, inhomogeneous boron concentration, sp²/sp³-carbon ratio, surface terminations and grain size distribution. We also present single nanodiamond particles and a nanostructured diamond, which are fabricated by either a top-down or a bottom-up procedure. Nanoarchitectured surfaces allow high areas and large aspect ratios to be achieved, exhibiting highly heterogeneous charge-transfer performance for catalytic, sensing and energy applications. We have anticipated multi-factor-originated heterogeneities of various boron-doped diamond surfaces displaying the essential fabrication and diagnostic methodologies and critically reviewing their benefits and drawbacks.     

Electrochemistry of graphene: new horizons for sensing and energy storage

Graphene is a new 2D nanomaterial with outstanding material, physical, chemical, and electrochemical properties. In this review, we first discuss the methods of preparing graphene sheets and their chemistry. Following that, the fundamental reasons governing the electrochemistry of graphene are meaningfully described. Graphene is an excellent electrode material with the advantages of conductivity and electrochemistry of sp² carbon but without the disadvantages related to carbon nanotubes, such as residual metallic impurities. We highlight important applications of graphene and graphene nanoplatelets for sensing, biosensing, and energy storage. © 2009 The Japan Chemical Journal Forum and Wiley Periodicals, Inc. Chem Rec 9: 211–223; 2009: Published online in Wiley InterScience ( www.interscience.wiley.com ) DOI 10.1002/tcr.200900008     

Electrochemical Detection of Tryptophan Metabolites via Kynurenine Pathway by Using Nanocarbon Films

We studied nanocarbon film electrodes with the aim of detecting tryptophan metabolites via the kynurenine pathway. The nanocarbon films were formed by using unbalanced magnetron sputtering, and they exhibited superior electrode properties including a wide potential window and a low background current as a result of the sp³-containing structure and ultraflat surface. These properties allowed us to detect certain tryptophan metabolites such as kynurenic acid (KYNA), which has a relatively high oxidation potential. We also investigated the effect of the sp²/sp³ ratio of the nanocarbon film as regards the electrode activity in relation to target molecules. We found that the sp²/sp³ ratio played important roles in both widening the potential window and obtaining superior electrode performance for the metabolites. The nanocarbon film with a high sp³ content was beneficial as regards the electrode performance with respect to the detection limit and sensitivity. Compared with conventional carbon-based electrodes, the nanocarbon film electrode with a high sp³ content exhibited higher electrode activity against KYNA while maintaining a low background current. Computational experiments revealed that the theoretical oxidation potential (E_ox) value for some targets coincided with that obtained in electrochemical experiments using our nanocarbon film electrode.     

Diamond Nanowires: Fabrication,Structure, Properties,and Applications

C(sp³)? C‐bonded diamond nanowires are wide band gap semiconductors that exhibit a combination of superior properties such as negative electron affinity, chemical inertness, high Young’s modulus, the highest hardness, and room‐temperature thermal conductivity. The creation of 1D diamond nanowires with their giant surface‐to‐volume ratio enhancements makes it possible to control and enhance the fundamental properties of diamond. Although theoretical comparisons with carbon nanotubes have shown that diamond nanowires are energetically and mechanically viable structures, reproducibly synthesizing the crystalline diamond nanowires has remained challenging. We present a comprehensive, up‐to‐date review of diamond nanowires, including a discussion of their synthesis along with their structures, properties, and applications.     

Opportunities and challenges of thin-film boron-doped diamond electrochemistry for valuable resources recovery from waste: Organic,inorganic, and volatile product electrosynthesis

Thin-film boron-doped diamond (BDD) electrochemistry has made a tremendous progress in electrochemical synthesis/recovery of high-added value products from aqueous and gaseous waste streams. The distinguished electrochemical characteristic of this electrode has made this material emerging and successfully used in electrosynthetic transformations, besides its destructive and powerful performance in disinfection and detoxification of wastewaters. Organic electrosynthesis is achieved by the oxyl radical oxidation formed at BDD, peroxo compounds electrosynthesis is attained by oxidation of corresponding anions at the BDD surface, whereas electrochemical conversion of SO₂, CO₂, NO₃^?, and NH₃ to value-added products occurs by BDD cathodic reduction process. There are still some challenges needed to address for seamless scale-up and translation into application of this future technology.     

Understanding active chlorine species production using boron doped diamond films with lower and higher sp3/sp2 ratio

The present study was motivated by the reports that promote the use of boron doped diamond (BDD) anode for electrochemical disinfection. The discussion about the production of undesirable active chlorine species on diamond films is still open. For this reason, the influence of sp³/sp² ratio on the performance on the evolution of chlorine-related species was investigated by polarization and electrolytic techniques in order to establish whether their formation and consumption related to either chemical or electrochemical reactions. The results demonstrated that dissolved Cl₂, ClO₂⁻, ClO₂, ClO₃⁻ and ClO₄⁻ species can be electrochemically formed at both BDD electrodes. However, the concentration trends are different, indicating that the relation of sp³/sp² ratio has a key role in the electrochemical route to produce ClO₃⁻ and ClO₄⁻.     

Electrochemical Conversion of Magnetic Nanoparticles Using Disposable Working Electrode in a 3D‐Printed Electrochemical Cell

Exploration of new property/function of nanomaterials is always a strong impetus in the nanoscience field. Here, a new method of electrochemical conversion (ECC) of magnetic nanoparticles (MNPs) is proposed to endow MNPs with signal generation ability for sensing. Briefly, high potential was applied to split H₂O to generate acid, while Fe₃O₄ MNPs reacted with H⁺ and produce ferric/ferrous ions, which further reacted with K₄Fe(CN)₆ to yield Prussian blue (PB) through potential cycling. The ECC method worked well on both home‐made and commercial MNPs with different sizes. The generated PB possessed strong electrochemical activity for further applications. Interestingly, an uneven deposition of PB on working electrode and undesired contamination of the reference and counter electrodes were found when using commercial integrated three‐electrode chip. A 3D‐printed electrochemical cell was designed to facilitate the ECC and avoid drawbacks of commercial integrated electrode. The 3D‐printed electrochemical cell was proven to solve the problem above through spatial separation of electrodes and thus facilitated the ECC process. An electrochemical sensor for H₂O₂ detection based on the catalysis ability of ECC‐based PB exhibited a linear response from 5 μM to 1 mM, a high sensitivity of 269 μA mM^?1 cm^?2 and a low detection limit of 0.16 μM (S/N=3), which suggests its promising application prospect in electrochemistry‐related analysis.     

High‐Yield Electrochemical Production of Formaldehyde from CO2 and Seawater

The catalytic, electrocatalytic, or photocatalytic conversion of CO₂ into useful chemicals in high yield for industrial applications has so far proven difficult. Herein, we present our work on the electrochemical reduction of CO₂ in seawater using a boron‐doped diamond (BDD) electrode under ambient conditions to produce formaldehyde. This method overcomes the usual limitation of the low yield of higher‐order products, and also reduces the generation of H₂. In comparison with other electrode materials, BDD electrodes have a wide potential window and high electrochemical stability, and, moreover, exhibit very high Faradaic efficiency (74 %) for the production of formaldehyde, using either methanol, aqueous NaCl, or seawater as the electrolyte. The high Faradaic efficiency is attributed to the sp³‐bonded carbon of the BDD. Our results have wide ranging implications for the efficient and cost‐effective conversion of CO₂.     

Effect of Power Density on the Electrochemical Properties of Undoped Amorphous Carbon (a‐C) Thin Films

Undoped a‐C thin films were deposited with varying power density from 10 to 25 W/cm² using unbalanced closed‐field magnetron sputtering (CFUBMS). The effect of power density on the physical and electrochemical properties was investigated by experimental characterization methods and atomistic simulations. XPS indicated that the films were composed mostly of sp²‐bonded carbon (55–58 at.%) with a small amount of oxygen (8–9 at.%) in the surface region. The films appeared completely amorphous in XRD. The I_D/I_G ratio obtained by Raman spectroscopy indicated an increase from 1.76 to 2.34 with power density. The experimental and simulated data suggested a possible ordering and/or clustering of the sp² phase with power density as the cause of the improved electrical properties of the a‐C films. The electrochemical properties of a‐C were between those of glassy carbon and tetrahedral amorphous carbon with potential windows ranging from 2.77 to 2.93 V and double‐layer capacitance values around 0.90 μF cm^?2. Electron transfer for Ru(NH₃)₆^3+/2+ and FcMeOH^+1/0 was reversible whereas that for IrCl₆^2?/3? was quasi‐reversible. Peak potential separation of dopamine and oxidation potential of ascorbic acid decreased with power density, correlating with the structural and electrical changes of the films. The a‐C thin films deposited by CFUBMS are inherently conductive and their physical properties can be adjusted by varying the deposition parameters to a wide range of electrochemical applications.     

Specific intramolecular aromatic CH insertion of diazosulfonamides

The chemoselectivity in the intramolecular CH insertion of various diazosulfonamides has been experimentally studied. The results reveal that the aliphatic 1,4-, 1,5-, or 1,6-C(sp³)?H insertions of diazosulfonamides are not accessible, while the aromatic 1,5-C(sp²)?H insertion can be realized specifically by adjusting the diazo-adjacent group. In addition, the general chemoselectivities in the intramolecular CH insertions of diazosulfonyl compounds are summarized. Generally, diazosulfones undergo both aromatic 1,5-C(sp²)?H and aliphatic 1,5- and 1,6-C(sp³)?H insertions, while diazosulfonates undergo aliphatic 1,5- and 1,6-C(sp³)?H insertions. However, diazosulfonamides only undergo aromatic 1,5-C(sp²)?H insertion.     

Hydrothermal Synthesis of a rGO Nanosheet Enwrapped NiFe Nanoalloy for Superior Electrocatalytic Oxygen Evolution Reactions

Graphene‐based hybrid nanostructures possess many advantages in the field of electrochemical energy applications. In this work, a facile and efficient hydrothermal approach has been developed for the preparation of NiFe alloy nanoparticles/rGO hybrid nanostructures, in which the nanoparticles are well combined with rGO nanosheets and the size of the nanoparticles is about 100 nm. Moreover, the electrochemical oxygen evolution reaction (OER) tests confirmed that the obtained NiFe/rGO hybrid nanostructures possess notably higher activity than both the rGO‐free NiFe nanoparticles and pure Ni/rGO hybrids, and the optimal NiFe ratio is 2:1. The OER overpotential at 20 mA cm^?1?2 with Ni₂Fe/rGO is as low as 0.285 V, which is 96 mV lower than that of pure Ni/rGO hybrids. Meanwhile, the Ni₂Fe/rGO catalyst has excellent stability. Therefore, this work contributes a facile and efficient method to prepare a NiFe alloy nanoparticles/rGO hybrid structure for potential applications in the field of electrochemical energy devices, such as electrochemical water splitting cells, rechargeable metal/air batteries, etc.     

Hybrid Nanocarbon as a Catalyst for Direct Dehydrogenation of Propane: Formation of an Active and Selective Core–Shell sp2/sp3 Nanocomposite Structure

Hybrid nanocarbon, comprised of a diamond core and a graphitic shell with a variable sp²‐/sp³‐carbon ratio, is controllably obtained through sequential annealing treatment (550–1300 °C) of nanodiamond. The formation of sp² carbon increases with annealing temperature and the nanodiamond surface is reconstructed from amorphous into a well‐ordered, onion‐like carbon structure via an intermediate composite structure—a diamond core covered by a defective, curved graphene outer shell. Direct dehydrogenation of propane shows that the sp²‐/sp³‐nanocomposite exhibits superior catalytic performance to that of individual nanodiamond and graphitic nanocarbon. The optimum catalytic activity of the diamond/graphene composite depends on the maximum structural defectiveness and high chemical reactivity of the ketone groups. Ketone‐type functional groups anchored on the defects/vacancies are active for propene formation; nevertheless, once the oxygen functional groups are desorbed, the defects/vacancies alone might be active sites responsible for the C?H bond activation of propane.     

Fluorographites (CFx)n Exhibit Improved Heterogeneous Electron‐Transfer Rates with Increasing Level of Fluorination: Towards the Sensing of Biomolecules

Halogenated sp² materials are of high interest owing to their important electronic and electrochemical properties. Although methods for graphite and graphene fluorination have been extensively researched, the fundamental electrochemical properties of fluorinated graphite are not well established. In this paper, the electrochemistry of three fluorographite materials of different carbon‐to‐fluorine ratio were studied: (CF_0.33)_n, (CF_0.47)_n, and (CF_0.75)_n. Our findings reveal that the carbon‐to‐fluorine ratio of fluorographite will impact the electrochemical performance. Faster heterogeneous electron‐transfer (HET) rates and lowered oxidation potentials for ascorbic acid and uric acid are progressively obtained with increasing fluorine content. The fluorographite (CF_0.75)_n was in fact found to exhibit the most improved electrochemical performances with the fastest HET rates and significantly lowered overpotentials in the oxidation of ascorbic acid. Analytical parameters such as sensitivity and linearity were subsequently investigated by applying the fluorographite (CF_0.75)_n in the analysis of ascorbic acid and uric acid, which can be simultaneously detected. We determined good linear responses towards the detection of both ascorbic and uric acid. Fluorographites outperform graphites in sensing applications, which will have a profound impact on applications of fluorographites and fluorographene in sensing and biosensing.     

Site-selective coupling of remote C(sp3)–H/meta-C(sp2)–H bonds enabled by Ru/photoredox dual catalysis and mechanistic studies

Construction of C(sp²)–C(sp³) bonds via regioselective coupling of C(sp²)–H/C(sp³)–H bonds is challenging due to the low reactivity and regioselectivity of C–H bonds. Here, a novel photoinduced Ru/photocatalyst-cocatalyzed regioselective cross-dehydrogenative coupling of dual remote C–H bonds, including inert γ-C(sp³)–H bonds in amides and meta-C(sp²)–H bonds in arenes, to construct meta-alkylated arenes has been accomplished. This metallaphotoredox-enabled site-selective coupling between remote inert C(sp³)–H bonds and meta-C(sp²)–H bonds is characterized by its unique site-selectivity, redox-neutral conditions, broad substrate scope and wide use of late-stage functionalization of bioactive molecules. Moreover, this reaction represents a novel case of regioselective cross-dehydrogenative coupling of unactivated alkanes and arenes via a new catalytic process and provides a new strategy for meta-functionalized arenes under mild reaction conditions. Density functional theory (DFT) calculations and control experiments explained the site-selectivity and the detailed mechanism of this reaction.

A novel photoinduced Ru/photocatalyst-cocatalyzed regioselective cross-dehydrogenative coupling of dual remote C–H bonds, including inert γ-C(sp³)–H bonds in amides and meta-C(sp²)–H bonds in arenes, to construct meta-alkylated arenes has been accomplished.     

Electrochemical Properties of Highly Sensitive and Selective CuO Nanostructures Based Neurotransmitter Dopamine Sensor

In this article, electrochemical properties of CuO nanostructures based dopamine (DA) sensor was investigated. The morphology, structure, optical, and compositional properties of the CuO nanostructures were characterized by using SEM, XRD, UV‐Vis, and XPS techniques. The electrochemical properties were investigated using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) techniques. The CV results indicate that biosensors based on CuO nanostructures exhibit a high selectivity and sensitivity of 0.1975 μA μM^–1 toward DA and effectively avoids the interference of ascorbic acid (AA) and uric acid (UA). The obtained EIS spectra for CuO sensors were analysed using an electrical equivalent circuit to understand the bulk and surface response via the capacitive and resistive parameters. The EIS measurement also leads to the direct determination of parameters like series resistance and ion diffusion phenomena at electrode‐electrolyte interface. The experimental CV and EIS results along with their analysis will have a significant impact on understanding the mechanism of high sensitivity and selectivity performance of CuO based sensors. This study may also lay the basis for efficient characterization of biosensors by coupling both the CV and EIS characterization techniques.     

Superlight and Superflexible Three‐Dimensional Semiconductor Frameworks A(X≡Y)4 (A=Si,Ge; X/Y=C,B, N) with Tunable Optoelectronic and Mechanical Properties from First‐Principles

Silicon carbide materials, as leading wide band gap semiconductors, hold significant importance in semiconductor technologies. Herein, diamond‐like 3D materials with low density, but high elasticity properties, have been designed from first‐principles calculations. They are porous single‐crystalline materials composed of sp³‐hybridized silicon (or germanium) and sp‐type C≡C (or B≡N) linear moieties; their stabilities are comparable to those of recently prepared SiC₄ materials. Moreover, such wide band gap semiconductors have strong absorption over a wide UV range and exhibit superlight, superflexible, and incompressible mechanical properties, and their optoelectronic and mechanical properties can be well tuned through structural modifications. Such features provide high potential for practicable application under extreme conditions, and suggest promising applications for the design of UV optoelectronic devices.     

Wall‐ and Hybridisation‐Selective Synthesis of Nitrogen‐Doped Double‐Walled Carbon Nanotubes

Controlled nitrogen‐doping is a powerful methodology to modify the properties of carbon nanostructures and produce functional materials for electrocatalysis, energy conversion and storage, and sensing, among others. Herein, we report a wall‐ and hybridisation‐selective synthetic methodology to produce double‐walled carbon nanotubes with an inner tube doped exclusively with graphitic sp²‐nitrogen atoms. Our measurements shed light on the fundamental properties of nitrogen‐doped nanocarbons opening the door for developing their potential applications.