Tri-n-octylphosphine: Versatile Reagent in Nanoparticle Synthesis and Catalysis

Tri-n-octylphosphine serves as a phosphorus source and used in the synthesis of metal nanoparticles. Tri-n-octylphosphine is used for coating zinc sulfide shells on cadmium-selenium quantum dot core by successive ionic layer adsorption and reaction method. It acts as a precursor to trioctylphosphine oxide. Further, it is used as a solvent for cadmium and selenium precursors. It also serves as a common reagent in the chemical synthesis of nanoparticles. It acts as a precursor to trioctylphosphine oxide. Tri-n-octylphosphine can also be used for the conversion of metal nanocrystals, bulk powders, foils, wires, thin films to metal phosphides. It can also act as a solvent and stabilizer for synthesizing cadmium sulfide nanorods from cadmium acetate and sulfur.

Tri-n-octylphosphine as Both Solvent and Stabilizer to Synthesize CdS Nanorods

Nanoscale one-dimensional semiconductor materials have drawn much attention due to their unique mechanical, optical, and electronic properties. Among metal chalcogenides, cadmium sulfide is of particular interest because of its intrinsic direct band gap (2.5 eV), which has shown great potentials in bioimaging, solar energy conversion and photocatalysis. In this paper, we describe a new synthesis that favors growth of CdS nanorods under nonextreme conditions. Only tri-n-octylphosphine (TOP) both as solvent and stabilizer was required for the synthesis. The method is simple and suitable for the large-scale preparation of high-quality CdS nanorods. In brief, cadmium acetate (40 mg) was first dissolved in 3 mL hot TOP under N2flow. Separately, elemental sulfur powder (24.0 mg) was added to tri-n-octylphosphine (1 mL) and mixed vigorously until dissolved. Next, the sulfur solution was injected to the cadmium solution and mixed thoroughly at 260 °C. The resulting yellow solution was stirred at 260 °C for reaction 6 h, cooled to 50 °C and finally methanol was added to give a fine deposit of CdS nanorods, which was separated by centrifugation and dissolved in toluene.[1]

In conclusion, we have demonstrated the formation of CdS nanorods by using tri-n-octylphosphine as single-coordinating solvent. It is worth mentioning that the growth conditions are simple and also can be easily adopted for large-scale preparations. Also, no additional solvents and stabilizers are needed. This process may bring conveniences to explore the capping mechanism of nanocrystallites surface. Attempts to grow other II–VI semiconductor nanorods with the present synthesis scheme are being pursued. Theoretical calculations are also underway to gain an insight into the mechanism of rod formation.

Role of the amine and phosphine groups in oleylamine and tri-n-octylphosphine

Colloidal nanocrystals are nanometer-sized inorganic particles that are stabilized by a layer of surfactants that are attached to their surface. Each colloidal nanocrystal is made up of few hundreds atoms and its growth can be easily controlled by varying synthetic parameters such as temperature, time of the reaction, concentration of precursors and capping molecule [2]. The capping molecules such as tri-n-octylphosphine (TOP) and oleylamine (OLA) can be used as solvents, reducing agents and stabilizers. They provide good interaction and they can be used at higher temperatures due to their high boiling points. Since OLA is liquid at room temperature, it simplifies the washing procedures that follow the chemical synthesis of nanocrystals. Tri-n-octylphosphine is a long chain alkyl phosphine (C24H51P) that uses its phosphine group (PR3) for interaction whereas oleylamine is a long-chain primary alkyl amine that uses its amine group (NH2-) for interaction. The increased proton affinity of phosphines is due to the stabilization of phenyl phosphonium ion by Л donation from phenyl group to the empty orbitals of phosphorus in the PR3 group, in contrast, the amines are rather stabilized by conjugation of nitrogen lone pair with the aromatic ring.[2]

The band edge values suggest that tri-n-octylphosphine -capped particles were smaller in comparison to those capped with OLA. This could be due to TOP being a stronger coordinating agent than OLA. The absorption curves of the two materials showed different line shapes predicting different optical properties and different crystalline phases. The absorption band edges of OLA and TOP-capped copper oxide nanocrystals were observed at 492 nm and 528 nm, respectively. Xiong et al. 2017 also reported on PbS quantum dots which produced an anti-Stokes shift photoluminescence spectrum and observed a gradual shift towards shorter wavelength when the excitation intensity was increased. This phenomenon was also observed when the excitation of tri-n-octylphosphine and OLA-capped material was increased (400–450 nm) and (300–350 nm), respectively. Their intensities were drastically reduced. Copper chalcogenides (CuSe, CuS and CuO) nanomaterials were synthesized by colloidal hot injection method in both OLA and TOP as capping agents. The phosphorus atoms in TOP provided good stabilization during nanoparticles synthesis to produce monodispersed particles. This contrasts with the amine group on OLA which produced low yield of mixed shapes for CuSe and CuS nanocrystals due to its mild reducing ability. When combined with tri-n-octylphosphine better control of particles and shapes were obtained.

Synthesis of Amorphous and Various Phase-Pure Nanoparticles

Nickel phosphides are nonprecious metal compounds that are becoming a promising group of highly active and stable catalysts in the petroleum refining industry for hydrodesulfurization (HDS) and hydrodenitrogenation (HDN), in biorefinery for hydrodeoxygenation (HDO),3 and in energy conversion and storage. Among these methods, the wet chemical approach using tri-n-octylphosphine (TOP) as the phosphorus source is an effective means of synthesizing nickel phosphide nanoparticles. The general strategies often involve the thermal decomposition of metal–phosphine intermediates, thermal conversion of metals to phosphides, or a one-pot synthesis, combing the metal reduction and phosphorus incorporation. For example, Hyeon and co-workers reported that a series of metal–tri-n-octylphosphine intermediates could be thermally decomposed, leading to the formation of metal phosphide nanoparticles, including solid Ni2P particles.[3]

At a reaction temperature of 215 °C, the P incorporation into metallic Ni nanoparticles resulted in the formation of amorphous nickel phosphides. The reaction involved two competing processes: the addition of Ni(0) forming metallic Ni and the incorporation of P to the metallic Ni, yielding disordered nickel phosphides. The kinetics of these two processes depended on the ratio of tri-n-octylphosphine /Ni(acac)2 used in the synthesis. At low ratios (e.g., TOP/Ni(acac)2 = 0.5 and 2.5), the formation of metallic Ni dominated, while at high ratios (e.g., TOP/Ni(acac)2 = 11), uniform amorphous nickel phosphide nanoparticles were formed. A UV–vis study coupled with a DFT calculation verified the presence of Ni(0)-TOPx complexes as intermediates in the synthesis.

References

[1]Chen S, Zhang X, Zhang Q, Tan W. Trioctylphosphine as Both Solvent and Stabilizer to Synthesize CdS Nanorods. Nanoscale Res Lett. 2009 Jun 17;4(10):1159-1165. doi: 10.1007/s11671-009-9375-x. PMID: 20596487; PMCID: PMC2893852.

[2]Mbewana-Ntshanka NG, Moloto MJ, Mubiayi PK. Role of the amine and phosphine groups in oleylamine and trioctylphosphine in the synthesis of copper chalcogenide nanoparticles. Heliyon. 2020 Nov 12;6(11):e05130. doi: 10.1016/j.heliyon.2020.e05130. PMID: 33241131; PMCID: PMC7672287.

[3]Thompson D, Hoffman AS, Mansley ZR, York S, Wang F, Zhu Y, Bare SR, Chen J. Synthesis of Amorphous and Various Phase-Pure Nanoparticles of Nickel Phosphide with Uniform Sizes via a Trioctylphosphine-Mediated Pathway. Inorg Chem. 2024 Oct 7;63(40):18981-18991. doi: 10.1021/acs.inorgchem.4c03334. Epub 2024 Sep 27. PMID: 39328180; PMCID: PMC11462502.