Salt-assisted Hydrothermal Synthesis at 190°C Produces Nanodiamonds with Defined Sp3 Lattice Planes

The creation of nanodiamonds, materials with potential in diverse fields from biomedicine to catalysis, traditionally requires extreme conditions of high pressure and temperature, limiting scalability and sustainability. Soumya Pratap Tripathy, Sayan Saha, Saurabh Kumar Gupta, and colleagues at the National Institute of Technology Rourkela, along with Sirsendu Sekhar Ray from NEHU, now demonstrate a more accessible route using hydrothermal synthesis, a process conducted in aqueous solutions under milder conditions. This research systematically investigates how different carbohydrate-based precursors influence nanodiamond formation, revealing crucial links between precursor type, solution chemistry, and the resulting nanodiamond structure. By employing advanced microscopy and spectroscopy, the team not only confirms the creation of nanodiamonds but also elucidates the atomic-scale mechanisms driving the transformation from graphite to diamond, paving the way for a tunable and sustainable approach to nanodiamond production.

Hydrothermal Synthesis and Nanodiamond Defect Analysis

Experiments successfully created nanodiamonds from diverse carbon sources, including starch, cellulose, and dextrose, using hydrothermal treatment in alkaline conditions. Characterization using high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy revealed common defects and characteristics within the resulting materials. A prevalent defect observed was stacking faults, indicated by deviations from ideal angles between crystallographic planes, and many nanodiamond particles exhibited a coating of graphitic carbon on their surface. Some particles displayed evidence of twinning, and both cubic and hexagonal nanodiamond phases were observed.

Consistent lattice spacings indicative of nanodiamonds included 2. 05-2. 12 Å, 1. 80-1. 81 Å, 1.

30 Å, 1. 10 Å, and 1. 05-1. 06 Å. These findings demonstrate that hydrothermal treatment can effectively create nanodiamonds, although defects and coatings are important considerations for potential applications.

Mild Hydrothermal Synthesis of Defect Nanodiamonds

Scientists developed a hydrothermal synthesis method for producing nanodiamonds under mild conditions, utilizing ten different protocols with CHO-based molecular precursors. These precursors, encompassing organic acids, polyols, sugars, and polysaccharides, reacted at 190 degrees Celsius within chlorinated, strongly alkaline aqueous solutions containing alkali and alkaline-earth metal ions. By systematically varying precursor type, molecular size, and ionic composition, the team investigated their influence on defect patterns and polymorph distribution. High-resolution transmission electron microscopy and X-ray photoelectron spectroscopy confirmed the presence of diamond-specific lattice planes and sp3-hybridized carbon structures, directly demonstrating nanodiamond formation. Analysis revealed heterogeneous morphologies, including spherical, annular, and spike-like features, containing nanocrystalline domains ranging from 4 to 15 nanometers embedded within amorphous matrices. Detailed imaging identified lattice spacings consistent with twinned or faulted nanodiamond polymorphs, revealing the influence of reaction conditions on nanodiamond structure.

Hydrothermal Synthesis Creates Tunable Nanodiamonds

Scientists have demonstrated a sustainable and tunable method for creating nanodiamonds using hydrothermal synthesis. The research team designed ten distinct hydrothermal protocols, each utilizing different CHO-based molecular precursors, including organic acids, polyols, sugars, and polysaccharides, to explore their impact on nanodiamond formation. Reactions were carried out at 190 degrees Celsius in strongly alkaline aqueous solutions containing alkali and alkaline-earth metal ions, allowing for precise control over the chemical environment. Using high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy, the team confirmed the presence of diamond-specific lattice planes and sp3-hybridized carbon structures within the resulting materials.

Results show that the type of precursor, its molecular size, and the ionic composition of the solution are key determinants of the defect patterns and polymorph distribution observed. Precursors rich in carboxylic acid groups, low-molecular-weight polyols, and hydroxyl-rich compounds all yielded nanodiamonds under standardized conditions. Atomic-scale imaging revealed both coherent and incoherent transitions from graphite to diamond, demonstrating the dynamic restructuring of carbon networks during the synthesis process.

Low Temperature Aqueous Nanodiamond Synthesis Demonstrated

This study demonstrates that nanodiamonds and their polymorphs can form at relatively low temperatures, 190 degrees Celsius, under aqueous and low-pressure conditions, challenging the belief that their creation requires extreme high-pressure, high-temperature environments. Researchers successfully synthesized nanodiamonds using ten different hydrothermal protocols, each employing varying CHO-based molecular precursors, such as organic acids, polyols, sugars, and polysaccharides, in strongly alkaline aqueous solutions. High-resolution imaging confirmed the presence of diamond-specific lattice structures and sp3-hybridized carbon, establishing nanodiamond formation. The research reveals that the type of precursor molecule, its size, and the ionic composition of the solution significantly influence the resulting nanodiamonds’ defect patterns and polymorph distribution. Through atomic-scale observation, scientists tracked the transformation from graphite to diamond, noting lattice compression and complex twinning patterns, providing mechanistic insights into chemically induced nanodiamond nucleation. This work establishes hydrothermal synthesis as a viable, energy-efficient alternative to conventional methods, offering a pathway towards scalable and sustainable nanodiamond production.