In this work, the use of nanodiamonds (NDs; 2-10 nm) in the seeding of diamond films grown by chemical vapor deposition (CVD) and in the synthesis of composite TiO₂-nanodiamond photocatalysts is discussed. First, enhanced growth of faceted, nanocrystalline diamond (NCD) films by hot filament CVD at moderate substrate temperature (650°C) and carbon-lean gas chemistry (1.0% CH₄ in H₂) is demonstrated. The enhancement of diamond nucleation and growth on Si substrates is achieved by using a sputter deposition of different metallic (Cr, Nb, Ti, V, and W) nanolayers and an ultrasonic seeding with NDs. We conclude that the kinetics of diamond nucleation in the NCD film growth is determined by the number density of NDs embedded on the nanorough metallic surfaces after ultrasonic pretreatment and by the specific surface chemistry (i.e., carbon diffusivity and carburization) during diamond growth. Second., the synthesis and application of composites based on microdiamonds and NDs for the photocatalytic degradation of diphenhydramine pharmaceutical water pollutant is demonstrated. Micro- A nd nanodiamond powders were combined with TiO₂, varying the carbon phase content and tested as composite photocatalysts under near UV-Vis irradiation. The thus prepared composites exhibited higher photocatalytic activity than the respective bare materials. Best photocatalytic performance was observed in the case of using composites with 15 wt.% of NDs oxidized in air at 703 K.