PbS Quantum Dots Decorating TiO2 Nanocrystals: Synthesis, Topology, and Optical Properties of the Colloidal Hybrid Architecture

Abstract

Fabrication of heterostructures by merging two or more materials in a single object. The domains at the nanoscale represent a viable strategy to purposely address materials' properties for applications in several fields such as catalysis, biomedicine, and energy conversion. In this case, solution-phase seeded growth and the hot-injection method are ingeniously combined to fabricate TiO₂/PbS heterostructures. The interest in such hybrid nanostructures arises from their absorption properties that make them advantageous candidates as solar cell materials for more efficient solar light harvesting and improved light conversion. Due to the strong lattice mismatch between TiO₂ and PbS, the yield of the hybrid structure and the control over its properties are challenging. In this study, a systematic investigation of the heterostructure synthesis as a function of the experimental conditions (such as seeds' surface chemistry, reaction temperature, and precursor concentration), its topology, structural properties, and optical properties are carried out. The morphological and chemical characterizations confirm the formation of small dots of PbS by decorating the oleylamine surface capped TiO₂ nanocrystals under temperature control. Remarkably, structural characterization points out that the formation of heterostructures is accompanied by modification of the crystallinity of the TiO₂ domain, which is mainly ascribed to lattice distortion. This result is also confirmed by photoluminescence spectroscopy, which shows intense emission in the visible range. This originated from self-trapped excitons, defects, and trap emissive states.

Keywords: PbS/TiO2 heterostructure; TiO2 nanocrystal defects; colloidal heterostructures; heterogeneous nucleation; seed mediated growth.

Figure 1

Seeded-growth under high (A) and low (B) supersaturation condition. Homogenous nucleation represents the upper-limit to heterogeneous nucleation. At low supersaturation, the overall interfacial energy (Δγ = γ_{nuclei/solution} + γ_i – γ_{seed/solution}, with γ_i = interfacial tension) defines the topology of the hybrid structure that, under high interfacial strain, may result in segregated phases (Δγ < 0) or island-like structures (Δγ > 0, Volmer-Weber growth mode). The grey dodecahedral shape NP represents the TiO₂ NC seeds while the orange structures represent the PbS NCs.

Figure 2

TEM micrographs (scale bar 20 nm) of oleic acid (OA)-capped TiO₂ NCs before (A) and after (B-C) injection of (B) [Pb²⁺] = 0.01M, [HMDS] = 0.002 M at 120 °C and (C) [Pb²⁺] = 0.005M, [HMDS] = 0.001 M at 100 °C. (D–F) oleylamine (Olam)-capped TiO₂ NCs after injection of [Pb²⁺] = 0.005M, [HMDS] = 0.001 M at 120 °C (D), 100 °C (E), and 80 °C (F). Dashed red and white circles in the picture used to highlight homogenously nucleated PbS NCs and TiO₂/PbS hybrid NCs, respectively.

Figure 3

FTIR spectra in the ATR mode of oleic acid (OA)-capped (a–a1) and oleylamine (Olam)-capped TiO₂ NCs (b–b1).

Figure 4

HAADF-STEM images (scale bar 10 nm) of (A) TiO₂ seed NCs and (B) TiO₂/PbS hybrid nanostructures. (C,D) XPS spectrum of TiO₂/PbS hybrid NCs in the 130–170 eV range: raw data (scattered plot) and fitting curve of the S 2p (C, light blue line) and of the Pb 4f (D), based on two contributions Pb1 (D, green line) and Pb2 (D, violet line). (E) XRD spectra of TiO₂ NCs (red line) and TiO₂/PbS hybrid (blue line) together with the Bragg hkl reflections positions for TiO₂ anatase (bottom markers, crystal system: tetragonal, PDF2-ICDD code: 842186) and PbS (upper panel, crystal system: cubic, PDF2-ICDD code: 00-005-0592) crystal structures. The XRD spectra of the two samples are reported, and shifted for the sake of clarity. Filled square and empty circle symbols are ascribed to TiO₂ anatase and PbS peaks, respectively.

Figure 5

(A) UV-Vis-NIR absorbance spectra and (B) steady state emission spectra (λ_ex= 375 nm) in the visible spectral range of the Olam-capped TiO₂ seeds (red line) and TiO₂/PbS hybrid structures (blue line). Each sample has been suitably diluted in order to show the same absorbance value at 375 nm.