Synthesis of diamond nanowires using atmospheric-pressure chemical vapor deposition

Improved H2O2 Electrosynthesis on S-doped Co-N-C through Cooperation of Co-S and Thiophene S

Co-N-C based catalysts have emerged as a prospective alternative for H2O2 electrosynthesis via a selective 2e- oxygen reduction reaction (ORR). However, conventional Co-N-C with Co-N4 configurations usually exhibits low selectivity toward 2e- ORR for H2O2 production. In this study, the S-doped Co-N-C (Co-N-C@S) catalysts were designed and synthesized for enhancing the electrosynthesis of H2O2, and their S doping levels and species were tuned to investigate their relationship with the H2O2 yield. The results showed that the S doping greatly enhanced the activity and selectivity of Co-N-C@S for H2O2 production. The optimal Co-N-C@S(12) displayed a high H2O2 production rate of 395 mmol gcat-1 h-1, H2O2 selectivity of 76.06%, and Faraday efficiency of 91.66% at 0.2 V, which were obviously better than those of Co-N-C (H2O2 production rate of 44 mmol gcat-1 h-1, H2O2 selectivity of 26.63%, and Faraday efficiency of 17.37%). Moreover, the Co-N-C@S(12) based electron-Fenton system displayed effective rhodamine B (RhB) removal, significantly outperforming the Co-N-C-based system. Experimental results combined with density functional theory unveiled that the enhanced performance of Co-N-C@S(12) stemmed from the combined effect of Co-S and thiophene S, which jointly enhanced electron density of the Co center, reduced the desorption energy of the *OOH intermediate, and then promoted the production of H2O2.

Flow-Through Electrochemical Membrane Reactor with a Self-Supported Carbon Membrane Electrode for Highly Efficient Synthesis of Hydrogen Peroxide

In situ electroreduction of O2 to H2O2 by using electrons as reagents is known as a green process, which is highly desirable for environmental remediation and chemical industries. However, the development of a cost-effective electrode with superior H2O2 synthesis rate and stability is challenging. A self-supported carbon membrane (CM) was prepared in this study from activated carbon and phenolic resin by carbonization under a H2 atmosphere. It was employed as the cathode to build a flow-through electrochemical membrane reactor (FT-ECMR) for electrosynthesis of H2O2. The results showed that the CM had a small pore size (34 nm), a high porosity (42.3%), and a high surface area (450.7 m2 g-1). In contrast to most of the state-of-the-art self-supported carbon electrode reported in the previous works, the FT-ECMR exhibited a high concentration of continuous and stable H2O2 electrosynthesis (1042 mg L-1) as well as a H2O2 synthesis rate of 5.21 mg h-1 cm-2. It had also demonstrated a high oxygen conversion (0.37%) and current efficiency (88%). The outstanding performance of the FT-ECMR for H2O2 synthesis was attributed to the enhanced mass transfer of the reactor, the existence of a relatively high surface area of CM, and the abundant disordered carbon structures (sp3-C, defects, and edges). In conclusion, our work highlighted using the FT-ECMR with the CM to synthesize H2O2 efficiently and cost-effectively.

Thermoplasmonic In Situ Fabrication of Nanohybrid Electrocatalysts over Gas Diffusion Electrodes for Enhanced H2O2 Electrosynthesis

Large-scale development of electrochemical cells is currently hindered by the lack of Earth-abundant electrocatalysts with high catalytic activity, product selectivity, and interfacial mass transfer. Herein, we developed an electrocatalyst fabrication approach which responds to these requirements by irradiating plasmonic titanium nitride (TiN) nanocubes self-assembled on a carbon gas diffusion layer in the presence of polymeric binders. The localized heating produced upon illumination creates unique conditions for the formation of TiN/F-doped carbon hybrids that show up to nearly 20 times the activity of the pristine electrodes. In alkaline conditions, they exhibit enhanced stability, a maximum H2O2 selectivity of 90%, and achieve a H2O2 productivity of 207 mmol gTiN-1 h-1 at 0.2 V vs RHE. A detailed electrochemical investigation with different electrode arrangements demonstrated the key role of nanocomposite formation to achieve high currents. In particular, an increased TiOxNy surface content promoted a higher H2O2 selectivity, and fluorinated nanocarbons imparted good stability to the electrodes due to their superhydrophobic properties.

Templated Synthesis of Diamond Nanopillar Arrays Using Porous Anodic Aluminium Oxide (AAO) Membranes

Diamond nanostructures are mostly produced from bulk diamond (single- or polycrystalline) by using time-consuming and/or costly subtractive manufacturing methods. In this study, we report the bottom-up synthesis of ordered diamond nanopillar arrays by using porous anodic aluminium oxide (AAO). Commercial ultrathin AAO membranes were adopted as the growth template in a straightforward, three-step fabrication process involving chemical vapor deposition (CVD) and the transfer and removal of the alumina foils. Two types of AAO membranes with distinct nominal pore size were employed and transferred onto the nucleation side of CVD diamond sheets. Subsequently, diamond nanopillars were grown directly on these sheets. After removal of the AAO template by chemical etching, ordered arrays of submicron and nanoscale diamond pillars with ~325 nm and ~85 nm diameters were successfully released.

Bottom-Up Evolution of Diamond-Graphite Hybrid Two-Dimensional Nanostructure: Underlying Picture and Electrochemical Activity

The diamond-graphite hybrid thin film with low-dimensional nanostructure (e.g., nitrogen-included ultrananocrystalline diamond (N-UNCD) or the alike), has been employed in many impactful breakthrough applications. However, the detailed picture behind the bottom-up evolution of such intriguing carbon nanostructure is far from clarified yet. Here, the authors clarify it, through the concerted efforts of microscopic, physical, and electrochemical analyses for a series of samples synthesized by hot-filament chemical vapor deposition using methane-hydrogen precursor gas, based on the hydrogen-dependent surface reconstruction of nanodiamond and on the substrate-temperature-dependent variation of the growth species (atomic hydrogen and methyl radical) concentration near substrate. The clarified picture provides insights for a drastic enhancement in the electrochemical activities of the hybrid thin film, concerning the detection of important biomolecule, that is, ascorbic acid, uric acid, and dopamine: their limits of detections are 490, 35, and 25 nm, respectively, which are among the best of the all-carbon thin film electrodes in the literature. This work also enables a simple and effective way of strongly enhancing AA detection.

Recent Advances in Single Crystal Diamond Device Fabrication for Photonics, Sensing and Nanomechanics

In the last two decades, the use of diamond as a material for applications in nanophotonics, optomechanics, quantum information, and sensors tremendously increased due to its outstanding mechanical properties, wide optical transparency, and biocompatibility. This has been possible owing to advances in methods for growth of high-quality single crystal diamond (SCD), nanofabrication methods and controlled incorporation of optically active point defects (e.g., nitrogen vacancy centers) in SCD. This paper reviews the recent advances in SCD nano-structuring methods for realization of micro- and nano-structures. Novel fabrication methods are discussed and the different nano-structures realized for a wide range of applications are summarized. Moreover, the methods for color center incorporation in SCD and surface treatment methods to enhance their properties are described. Challenges in the upscaling of SCD nano-structure fabrication, their commercial applications and future prospects are discussed.

Nanoarchitectonics for Wide Bandgap Semiconductor Nanowires: Toward the Next Generation of Nanoelectromechanical Systems for Environmental Monitoring

Semiconductor nanowires are widely considered as the building blocks that revolutionized many areas of nanosciences and nanotechnologies. The unique features in nanowires, including high electron transport, excellent mechanical robustness, large surface area, and capability to engineer their intrinsic properties, enable new classes of nanoelectromechanical systems (NEMS). Wide bandgap (WBG) semiconductors in the form of nanowires are a hot spot of research owing to the tremendous possibilities in NEMS, particularly for environmental monitoring and energy harvesting. This article presents a comprehensive overview of the recent progress on the growth, properties and applications of silicon carbide (SiC), group III-nitrides, and diamond nanowires as the materials of choice for NEMS. It begins with a snapshot on material developments and fabrication technologies, covering both bottom-up and top-down approaches. A discussion on the mechanical, electrical, optical, and thermal properties is provided detailing the fundamental physics of WBG nanowires along with their potential for NEMS. A series of sensing and electronic devices particularly for environmental monitoring is reviewed, which further extend the capability in industrial applications. The article concludes with the merits and shortcomings of environmental monitoring applications based on these classes of nanowires, providing a roadmap for future development in this fast-emerging research field.

Direct conversion of carbon nanofibers into diamond nanofibers using nanosecond pulsed laser annealing

Here, we show the direct conversion of carbon nanofibers (CNFs) into diamond nanofibers (DNFs) by irradiating CNFs with an ArF nanosecond laser at room temperature and atmospheric pressure. The nanosecond laser pulses melt the tips of CNFs into a highly undercooled state, and their subsequent quenching results in the formation of DNFs. This formation of DNFs is dependent on the degree of undercooling which is controlled by nanosecond laser energy density and one-dimensional heat flow characteristics in CNFs. The conversion process starts at the top and extends with the number of pulses. Therefore, our highly non-equilibrium nanosecond laser processing opens a new avenue for the synthesis of exciting pure and doped diamond structures at ambient temperatures and pressures for a variety of applications.

Effect of Metal Ions on Hybrid Graphite-Diamond Nanowire Growth: Conductivity Measurements from a Single Nanowire Device

Novel Cd²⁺ ions mediated reproducible hybrid graphite-diamond nanowire (G-DNWs; Cd²⁺-NDS1 NW) growth from 4-Amino-5-phenyl-4H-1,2,4-triazole-3-thiol (S1) functionalized diamond nanoparticles (NDS1) via supramolecular assembly is reported and demonstrated through TEM and AFM images. FTIR, EDX and XPS studies reveal the supramolecular coordination between functional units of NDS1 and Cd²⁺ ions towards NWs growth. Investigations of XPS, XRD and Raman data show the covering of graphite sheath over DNWs. Moreover, HR-TEM studies on Cd²⁺-NDS1 NW confirm the coexistence of less perfect sp² graphite layer and sp³ diamond carbon along with impurity channels and flatten surface morphology. Possible mechanisms behind the G-DNWs growth are proposed and clarified. Subsequently, conductivity of the as-grown G-DNWs is determined through the fabrication of a single Cd²⁺-NDS1 NW device, in which the G-DNW portion L2 demonstrates a better conductivity of 2.31 × 10⁻⁴ mS/cm. In addition, we investigate the temperature-dependent carrier transport mechanisms and the corresponding activation energy in details. Finally, comparisons in electrical resistivities with other carbon-based materials are made to validate the importance of our conductivity measurements.

Direct conversion of carbon nanofibers and nanotubes into diamond nanofibers and the subsequent growth of large-sized diamonds

We report a pulsed laser annealing method to convert carbon fibers and nanotubes into diamond fibers at ambient temperature and pressure in air. The conversion of carbon nanofibers and nanotubes into diamond nanofibers involves melting in a super undercooled state using nanosecond laser pulses, and quenching rapidly to convert into phase-pure diamond. The conversion process occurs at ambient temperature and pressure, and can be carried out in air. The structure of diamond fibers has been confirmed by selected-area electron diffraction in transmission electron microscopy, electron-back-scatter-diffraction in high-resolution scanning electron microscopy, all showing characteristic diffraction lines for the diamond structure. The bonding characteristics were determined by Raman spectroscopy with a strong peak near 1332 cm-1, and high-resolution electron-energy-loss spectroscopy in transmission electron microscopy with a characteristic peak at 292 eV for σ* for sp3 bonding and the absence of π* for sp2 bonding. The Raman peak at 1332 cm-1 downshifts to 1321 cm-1 for diamond nanofibers due to the phonon confinement in nanodiamonds. These laser-treated carbon fibers with diamond seeds are used to grow larger diamond crystallites further by using standard hot-filament chemical vapor deposition (HFCVD). We compare these results with those obtained without laser treating the carbon fibers. The details of diamond conversion and HFCVD growth are presented in this paper.

Substrate-orientation dependent epitaxial growth of highly ordered diamond nanosheet arrays by chemical vapor deposition

Three-dimensional ordering of two-dimensional nanomaterials has long been a challenge. Simultaneously, diamond nanomaterials are difficult to synthesize due to the harsh synthesizing conditions required. Here, we report substrate-crystal-orientation dependent growth of diamond nanosheets (DNSs) by chemical vapor deposition, which generates different DNS arrays on different substrates. The DNSs are grown by the in-plane epitaxy of the diamond {111} planes. So the arrays are highly ordered and solely determined by the spatial orientation of the {111} planes in the diamond FCC structure. The DNSs grown on the {110}, {111}, {001}, and {113} oriented substrates show inclination angles ranging from 90 to 29.5°. The DNSs with larger inclination angles grow preferentially, forming parallelogram arrays with inclination angles of 90° on the {110} substrates and parallel-line arrays with inclination angles of 80° on the {113} substrates. The density, thickness, size, and morphology of the DNSs have been well controlled. The present understanding and materials are highly promising for many applications such as sensors, catalysis, photonics, thermal management, and electronics.

An Affordable Wet Chemical Route to Grow Conducting Hybrid Graphite-Diamond Nanowires: Demonstration by A Single Nanowire Device

We report an affordable wet chemical route for the reproducible hybrid graphite-diamond nanowires (G-DNWs) growth from cysteamine functionalized diamond nanoparticles (ND-Cys) via pH induced self-assembly, which has been visualized through SEM and TEM images. Interestingly, the mechanistic aspects behind that self-assembly directed G-DNWs formation was discussed in details. Notably, above self-assembly was validated by AFM and TEM data. Further interrogations by XRD and Raman data were revealed the possible graphite sheath wrapping over DNWs. Moreover, the HR-TEM studies also verified the coexistence of less perfect sp² graphite layer wrapped over the sp³ diamond carbon and the impurity channels as well. Very importantly, conductivity of hybrid G-DNWs was verified via fabrication of a single G-DNW. Wherein, the better conductivity of G-DNW portion L2 was found as 2.4 ± 1.92 × 10⁻⁶ mS/cm and revealed its effective applicability in near future. In addition to note, temperature dependent carrier transport mechanisms and activation energy calculations were reported in details in this work. Ultimately, to demonstrate the importance of our conductivity measurements, the possible mechanism behind the electrical transport and the comparative account on electrical resistivities of carbon based materials were provided.

Recognition of diamond with phage display peptides

We show for the first time the identification of diamond-binding peptide motifs for diamond with phage display.

Single-Crystal Diamond Nanowire Tips for Ultrasensitive Force Microscopy

We report the fabrication, integration, and assessment of sharp diamond tips for ultrasensitive force microscopy experiments. Two types of tips, corresponding to the upper and lower halves of a diamond nanowire, were fabricated by top-down plasma etching from a single-crystalline substrate. The lower, surface-attached halves can be directly integrated into lithographically defined nanostructures, like cantilevers. The upper, detachable halves result in diamond nanowires with a tunable diameter (50-500 nm) and lengths of a few microns. Tip radii were around 10 nm and tip apex angles around 15°. We demonstrate the integration of diamond nanowires for use as scanning tips onto ultrasensitive pendulum-style silicon cantilevers. We find the noncontact friction and frequency jitter to be exceptionally low, with no degradation in the intrinsic mechanical quality factor (Q ≈ 130,000) down to tip-to-surface distances of about 10 nm. Our results are an encouraging step toward further improvement of the sensitivity and resolution of force-detected magnetic resonance imaging.

Diamond nanowires: a novel platform for electrochemistry and matrix-free mass spectrometry

Over the last decades, carbon-based nanostructures have generated a huge interest from both fundamental and technological viewpoints owing to their physicochemical characteristics, markedly different from their corresponding bulk states. Among these nanostructured materials, carbon nanotubes (CNTs), and more recently graphene and its derivatives, hold a central position. The large amount of work devoted to these materials is driven not only by their unique mechanical and electrical properties, but also by the advances made in synthetic methods to produce these materials in large quantities with reasonably controllable morphologies. While much less studied than CNTs and graphene, diamond nanowires, the diamond analogue of CNTs, hold promise for several important applications. Diamond nanowires display several advantages such as chemical inertness, high mechanical strength, high thermal and electrical conductivity, together with proven biocompatibility and existence of various strategies to functionalize their surface. The unique physicochemical properties of diamond nanowires have generated wide interest for their use as fillers in nanocomposites, as light detectors and emitters, as substrates for nanoelectronic devices, as tips for scanning probe microscopy as well as for sensing applications. In the past few years, studies on boron-doped diamond nanowires (BDD NWs) focused on increasing their electrochemical active surface area to achieve higher sensitivity and selectivity compared to planar diamond interfaces. The first part of the present review article will cover the promising applications of BDD NWS for label-free sensing. Then, the potential use of diamond nanowires as inorganic substrates for matrix-free laser desorption/ionization mass spectrometry, a powerful label-free approach for quantification and identification of small compounds, will be discussed.

Nanodiamonds for field emission: state of the art

The aim of this review is to highlight the recent advances and the main remaining challenges related to the issue of electron field emission (FE) from nanodiamonds. The roadmap for FE vacuum microelectronic devices envisages that nanodiamonds could become very important in a short time. The intrinsic properties of the nanodiamond materials indeed meet many of the requirements of cutting-edge technologies and further benefits can be obtained by tailored improvements of processing methodologies. The current strategies used to modulate the morphological and structural features of diamond to produce highly performing emitting systems are reported and discussed. The focus is on the current understanding of the FE process from nanodiamond-based materials and on the major concepts used to improve their performance. A short survey of non-conventional microsized cold cathodes based on nanodiamonds is also reported.

Diamond nanowires: fabrication, structure, properties, and applications

C(sp(3) )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.

Synthesis and transformation of linear adamantane assemblies inside carbon nanotubes

We report the assembly and thermal transformation of linear diamondoid assemblies inside carbon nanotubes. Our calculations and observations indicate that these molecules undergo selective reactions within the narrow confining space of a carbon nanotube. Upon vacuum annealing of adamantane molecules encapsulated in a carbon nanotube, we observe a sharp Raman feature at 1857 cm(-1), which we interpret as a stretching mode of carbon chains formed by thermal conversion of adamantane inside a carbon nanotube. Introduction of pure hydrogen during thermal annealing, however, suppresses the formation of carbon chains and seems to keep adamantane intact.

Diamond nanowire--a challenge from extremes

Crystalline diamond nanowires have been grown in a chemical vapor deposition (CVD) process under 900 °C and atmospheric pressure--an extraordinary find in diamond growth. These diamond nanowires are straight, thin and long, and uniform in diameter (60-90 nm) over their entire lengths of tens of microns. Extensive characterizations including electron microscopy and Raman spectroscopy were performed to confirm that the diamond nanowire has highly crystalline cubic diamond structure encased inside a graphitic or carbonaceous shell. Such a core-shell structure suggests a potential formation mechanism in the framework of an effectively lowered Gibbs free energy due to nano-capillary and surface charge pressure. The capillary pressure (inversely proportional to the wire radius) can be sufficiently high to allow the diamond phase to be thermodynamically favorable in the inner core while the outer shell takes on the graphitic phase. The properties of diamond can manifest themselves differently in the nanowire morphology. Examples include single-photon emission of nitrogen-vacancy and electron field-emission. Whereas the former has received much attention in the literature, the latter turned out to be just as impressive and is show-cased here for the first time.

Nanopatterning of ultrananocrystalline diamond nanowires

We report the fabrication of horizontally aligned ultrananocrystalline diamond (UNCD) nanowires (NWs) via two different approaches. First, with the top-down approach by using electron beam lithography (EBL) and reactive ion etching (RIE) with a photo resist layer as an etch mask. Using this approach, we demonstrate fabrication of 50 µm long UNCD NWs with widths as narrow as 40 nm. We further present an alternative approach to grow UNCD NWs at pre-defined positions through a selective seeding process. No RIE was needed either to etch the NWs or to remove the mask. In this case, we achieved UNCD NWs with lengths of 50 µm and smallest width of 90 nm respectively. Characterization of these nanowires by using scanning electron microscopy (SEM) and atomic force microscopy (AFM) shows that the UNCD NWs are well defined and fully released, with no indication of residual stress. Characterization using visible and ultraviolet (UV) Raman spectroscopy indicates that in both fabrication approaches, UNCD NWs maintain their intrinsic diamond structure.

Freestanding ultrananocrystalline diamond films with homojunction insulating layer on conducting layer and their high electron field emission properties

Freestanding ultrananocrystalline diamond (UNCD) films with homojunction insulating layer in situ grown on a conducting layer showed superior electron field emission (EFE) properties. The insulating layer of the films contains large dendrite type grains (400-600 nm in size), whereas the conducting layer contains nanosize equi-axed grains (5-20 nm in size) separated by grain boundaries of about 0.5-1 nm in width. The conducting layer possesses n-type (or semimetallic) conductivity of about 5.6 × 10(-3) (Ω cm)(-1), with sheet carrier concentration of about 1.4 × 10(12) cm(-2), which is ascribed to in situ doping of Li-species from LiNbO(3) substrates during growth of the films. The conducting layer intimately contacts the bottom electrodes (Cu-foil) by without forming the Schottky barrier, form homojunction with the insulating layer that facilitates injection of electrons into conduction band of diamond, and readily field emitted at low applied field. The EFE of freestanding UNCD films could be turned on at a low field of E(0) = 10.0 V/μm, attaining EFE current density of 0.2 mA/cm(2) at an applied field of 18.0 V/μm, which is superior to the EFE properties of UNCD films grown on Si substrates with the same chemical vapor deposition (CVD) process. Such an observation reveals the importance in the formation of homojunction on enhancing the EFE properties of materials. The large grain granular structure of the freestanding UNCD films is more robust against harsh environment and shows high potential toward diamond based electronic applications.

Controlled synthesis of a large fraction of metallic single-walled carbon nanotube and semiconducting carbon nanowire networks

Controlled synthesis of both single-walled carbon nanotube and carbon nanowire networks using the same CVD reactor and Fe/Al(2)O(3) catalyst by slightly altering the hydrogenation and temperature conditions is demonstrated. Structural, bonding and electrical characterization using SEM, TEM, Raman spectroscopy, and temperature-dependent resistivity measurements suggest that the nanotubes are of a high quality and a large fraction (well above the common 33% and possibly up to 75%) of them are metallic. On the other hand, the carbon nanowires are amorphous and semiconducting and feature a controlled sp(2)/sp(3) ratio. The growth mechanism which is based on the catalyst nanoisland analysis by AFM and takes into account the hydrogenation and temperature control effects explains the observed switch-over of the nanostructure growth modes. These results are important to achieve the ultimate control of chirality, structure, and conductivity of one-dimensional all-carbon networks.