Hydrothermal synthesis offers a versatile route for producing nanocrystals with controlled defect structures, including oxygen vacancies and dopant incorporation. The method leverages aqueous solutions under elevated temperatures and pressures to facilitate crystal growth while allowing precise tuning of defect chemistry. By adjusting reaction parameters such as precursor chemistry, pH, temperature, duration, and pressure, it becomes possible to engineer defects that critically influence electronic properties, including band structure, charge carrier mobility, and surface reactivity. These tailored properties are particularly relevant for applications in photocatalysis and electronic conductivity.

Precursor selection plays a fundamental role in determining defect formation during hydrothermal synthesis. Metal precursors with varying oxidation states or ligands can influence the likelihood of oxygen vacancy generation. For example, using titanium(III) precursors instead of titanium(IV) sources increases the probability of oxygen-deficient TiO₂ nanocrystals due to the inherent reduction potential of Ti³⁺. Similarly, introducing dopants via precursor mixtures allows controlled integration of foreign ions into the crystal lattice. For instance, adding ammonium metavanadate to a zinc nitrate solution during ZnO synthesis introduces vanadium dopants, which modify charge carrier concentrations and introduce mid-gap states. The precursor concentration also affects defect density; higher concentrations often lead to increased defect formation due to kinetic competition between nucleation and growth.

The pH of the hydrothermal solution is another critical parameter for defect control. Acidic conditions tend to promote oxygen vacancies in metal oxide nanocrystals by protonating surface oxygen sites, which can subsequently desorb as water molecules during crystallization. For example, ZnO nanocrystals synthesized at pH 3 exhibit higher oxygen vacancy concentrations compared to those grown at neutral or alkaline pH. Conversely, alkaline conditions favor substitutional doping by enhancing the solubility of certain dopant precursors. In the case of nitrogen doping in TiO₂, ammonia-mediated hydrothermal synthesis at pH 10 facilitates nitrogen incorporation into oxygen lattice sites, narrowing the bandgap and enhancing visible-light absorption.

Temperature and pressure govern reaction kinetics and thermodynamics, directly impacting defect formation. Higher temperatures generally reduce point defect concentrations in equilibrium conditions due to enhanced defect annihilation. However, under hydrothermal conditions, rapid crystallization at elevated temperatures can kinetically trap defects that would otherwise anneal out. For instance, CeO₂ nanocrystals synthesized at 200°C exhibit higher oxygen vacancy densities than those grown at 150°C due to faster growth rates limiting defect diffusion. Pressure influences defect formation by altering precursor solubility and reaction pathways. Increased pressure can suppress oxygen vacancy formation in some systems by promoting oxidative conditions, while in others, it enhances dopant incorporation by improving precursor dissolution.

Reaction time determines the extent of defect equilibration during nanocrystal growth. Shorter durations often yield higher defect densities due to incomplete crystallization, while prolonged reactions allow defect annealing. However, intermediate timescales can optimize defect distributions for specific applications. For example, 12-hour hydrothermal synthesis of WO₃ yields nanocrystals with an optimal balance of oxygen vacancies and crystallinity for photocatalytic water oxidation, whereas shorter or longer durations result in either excessive defects degrading charge transport or overly reduced activity due to defect depletion.

The electronic properties of hydrothermally synthesized nanocrystals are profoundly influenced by these defect-engineering strategies. Oxygen vacancies introduce donor states near the conduction band in oxides like TiO₂ and ZnO, enhancing n-type conductivity and promoting electron-hole separation in photocatalysis. Dopants can either increase carrier concentrations or introduce trap states, depending on their energy levels relative to the host band structure. For instance, manganese doping in SnO₂ nanocrystals introduces shallow acceptor levels, improving p-type conductivity, while chromium doping creates deep traps that hinder charge transport but enhance light absorption.

In photocatalysis, controlled defects can significantly improve performance. Oxygen vacancies in BiVO₄ nanocrystals synthesized hydrothermally under reducing conditions act as active sites for water oxidation by stabilizing adsorbed intermediates. Similarly, nitrogen-doped TiO₂ nanocrystals prepared in ammonia-rich solutions exhibit enhanced visible-light activity due to mid-gap states introduced by nitrogen impurities. The synergy between intrinsic vacancies and extrinsic dopants can further optimize photocatalytic efficiency. For example, simultaneous fluorine doping and oxygen vacancy generation in hydrothermally grown Nb₂O₅ nanocrystals create a gradient electronic structure that facilitates charge separation and surface reactions.

For electronic conductivity applications, defect engineering during hydrothermal synthesis enables tuning of carrier type and concentration. Oxygen-deficient SrTiO₃ nanocrystals display enhanced electronic conductivity suitable for solid oxide fuel cell anodes, while lanthanum-doped BaSnO₃ nanocrystals exhibit high electron mobility for transparent conducting oxides. The hydrothermal approach allows precise control over defect distributions, avoiding the inhomogeneities often encountered in post-synthesis treatments.

The interplay between multiple reaction parameters necessitates systematic optimization for desired defect configurations. A representative example is the hydrothermal synthesis of defective MoS₂ nanocrystals, where temperature and precursor ratio jointly determine sulfur vacancy concentrations. At 200°C with a 1:3 Mo:S precursor ratio, optimal sulfur vacancies are achieved, enhancing catalytic activity for hydrogen evolution while maintaining structural integrity. Similar parameter interdependence governs defect formation in other systems, requiring careful balancing of competing factors.

Advanced characterization techniques are essential for verifying defect structures introduced during hydrothermal synthesis. X-ray photoelectron spectroscopy quantifies oxygen vacancy concentrations through changes in metal-oxygen bonding environments. Electron paramagnetic resonance spectroscopy detects unpaired electrons associated with certain defects, such as oxygen vacancies in ZnO. Photoluminescence spectroscopy reveals defect-related emissions, providing insights into electronic states created during synthesis.

Future developments in hydrothermal defect engineering may explore machine learning-assisted parameter optimization to navigate complex multi-variable spaces. Real-time monitoring techniques could provide dynamic feedback on defect evolution during crystal growth. The continued refinement of hydrothermal methods for defect control will further expand the applicability of nanocrystals in energy conversion, electronics, and catalysis.

The hydrothermal approach stands out for its ability to produce nanocrystals with tailored defects without requiring post-synthesis treatments. By understanding and manipulating the relationships between synthesis parameters and defect formation, researchers can design materials with precisely tuned electronic properties for targeted applications. This in situ defect engineering strategy offers advantages in reproducibility, scalability, and uniformity compared to methods relying on post-growth modifications.