Controlled Synthesis and Exploration of CuxFeS4 Bornite Nanocrystals

Abstract

Plasmonic semiconductor nanocrystals (NCs) are a new and exciting class of materials that enable higher control of their localized surface plasmon resonance (LSPR) than metallic counterparts. Additionally, earth-abundant and non-toxic materials such as copper iron sulfides are gaining interest as alternatives to heavy metal-based semiconductor materials. Colloidal bornite (Cu₅FeS₄) is an interesting but underexplored example of a heavy metal-free plasmonic semiconductor. This report details the hot-injection synthesis of bornite yielding NCs ranging from 2.7 to 6.1 nm in diameter with stoichiometric control of the copper and iron content. The absorbance spectra of bornite NCs with different Cu:Fe ratios change at different rates as the particles oxidize and develop LSPR in the near-infrared region. X-ray photoelectron spectroscopy results indicate that oxidation produces sulfates rather than metal oxides as well as a decrease in the iron content within the NCs. Additionally, increasing iron content leads to decreases in carrier density and effective mass of the carrier, as determined by the Drude model. This controlled synthesis, combined with a further understanding of the relationship between the particle structure and optical properties, will enable the continued development and application of these fascinating heavy metal-free plasmonic semiconductor nanoparticles.

Figure 1.

Structural, chemical, and optical characterization of bornite NCs. (A) Representative TEM of CFS-5:1 NCs, with sizing in the Supporting Information (Figure S2). The inset (white box) is a higher magnification image showing lattice fringes. Both scale bars are 10 nm. (B) SAED image from bornite NCs. (C) XRD profiles of different bornite NCs drop-cast on the substrate, along with the bornite American Mineralogist Crystal Structure Database standard (AMCSD 0000048) in black. (D) Iron content of NCs compared to feed ratio, demonstrating stoichiometric control. Data included from refs and . (E) Absorbance of unoxidized bornite NCs normalized to 400 nm, with the inset of band gaps (indirect = triangles, direct = circles) calculated from Tauc plots (Figure S4). (F) Absorbance of unoxidized time points drawn from CFS-5:1 synthesis, normalized to 400 nm. The inset shows estimated diameter from XRD (via the Scherrer Equation) plotted against reaction time, along with a logarithmic fit.

Figure 2.

Oxidation of Bornite NCs. Oxidation of CFS-7:1 (A), CFS-5:1 (B), and CFS-3:1 (C) bornite NCs over time. All curves normalized to 1.5 eV, which was the position of a near isosbestic point in the raw data. (D) Oxidation of CFS-5:1 NCs of different sizes over time.

Figure 3.

(A) Titration of CFSox 5:1 with the cobaltocene reductant. The inset is the integrated plasmon area at different titrations. (B) Frequency-independent Drude fits for various bornite samples. Experimental data were fit using the MATLAB code from ref .

Figure 4.

XPS of bornite NCs before and after oxidation. XPS for (A) fresh and (B) oxidized CFS-3:1 in the sulfur 2p regime as well as iron 2p spectra for (C) fresh and (D) oxidized samples. (E) Cu 2p peaks of fresh and oxidized CFS-3:1. The offset is for clarity only. (F) Sulfur quantification from spectra in (A) and (B). (G) Atomic ratio of sulfur, copper, and iron, normalized to the average copper signal per sample with two measurements per sample.

Figure 5.

Summary schematic. Unoxidized NCs (left) are n-type doped semiconductors with a visible LSPR that can be lost with confinement effects at smaller sizes. Oxidation causes the loss of iron content (middle), leading to p-type doping and NIR LSPR arising from the excess holes for all NC sizes. Subsequent reduction of the NCs (right) by cobaltocene quenches the NIR LSPR.