Abstract
Accurate prediction of the structures, stabilities, and electronic structures of hybrid inorganic/organic systems is an essential prerequisite for tuning their electronic properties and functions. Herein, the interface chemistry between the 4-aminothiophenol (4ATP) molecule and the (001), (101), and (110) surfaces of zinc phosphide (Zn3P2) has been investigated by means of first-principles density functional theory calculation with a correction for van der Waals interactions. In particular, the atomic-level insights into the fundamental aspects of the 4ATP adsorption, including the lowest-energy adsorption configurations, binding energetics, structural parameters, and electronic properties are presented and discussed. The 4ATP molecule is demonstrated to bind most strongly onto the least stable Zn3P2(001) surface (Eads = −1.91 eV) and least strongly onto the most stable Zn3P2(101) surface (Eads = −1.21 eV). Partial density of states analysis shows that the adsorption of 4ATP on the Zn3P2 surfaces is characterized by strong hybridization between the molecule’s sulfur and nitrogen p-orbitals and the d-orbitals of the interacting surface Zn ions, which gave rise to electron density accumulation around the centers of the newly formed Zn–S and Zn–N chemical bonds. The thermodynamic crystal morphology of the nonfunctionalized and 4ATP-functionalized Zn3P2 nanoparticles was obtained using Wulff construction based on the calculated surface energies. The stronger binding of the 4ATP molecule onto the less stable (001) and (110) surfaces in preference to the most stable (101) facet resulted in the modulation of the Zn3P2 nanocrystal shape, with the reactive (001) and (110) surfaces becoming more pronounced in the equilibrium morphology.
Highlights
Zinc phosphide is an attractive earth-abundant solar absorber material for scalable thin-film photovoltaic applications owing to its direct band gap of 1.5 eV,[1] high visible-light absorption coefficient (>104 cm−1),[2,3] long minority-carrier diffusion length (∼10 μm),[4] high extinction coefficient,[5] passive grain boundaries,[6] and large range of potential doping concentrations (1013 to 1018 cm−3).[7]
Efforts have been made to passivate Zn3P2 surfaces via in situ functionalization, wherein the Zn3P2 nanoparticles of thin films are exposed to a vapor of organic functional molecules immediately after synthesis.[15−18] Functionalization of Zn3P2 nanoparticles can enhance their surface stability against temperature and possible oxidation in the presence of oxygen and moisture that could result in their degradation.[19,20]
The results demonstrate the selectivity of the 4ATP functional groups toward stabilizing the different Zn3P2 surfaces, favoring the expression of the more reactive surfaces in the particle morphology
Summary
Zinc phosphide is an attractive earth-abundant solar absorber material for scalable thin-film photovoltaic applications owing to its direct band gap of 1.5 eV,[1] high visible-light absorption coefficient (>104 cm−1),[2,3] long minority-carrier diffusion length (∼10 μm),[4] high extinction coefficient,[5] passive grain boundaries,[6] and large range of potential doping concentrations (1013 to 1018 cm−3).[7]. Zinc phosphide nanoparticles can get oxidized when in contact with water and oxygen owing to the higher specific surface area and higher reactivity relative to the bulk.[12−14] It is important to develop synthesis techniques to protect Zn3P2 surfaces against unwanted oxidation. Efforts have been made to passivate Zn3P2 surfaces via in situ functionalization, wherein the Zn3P2 nanoparticles of thin films are exposed to a vapor of organic functional molecules immediately after synthesis.[15−18] Functionalization of Zn3P2 nanoparticles can enhance their surface stability against temperature and possible oxidation in the presence of oxygen and moisture that could result in their degradation.[19,20] The binding of the organic molecules to the nanoparticle crystal facets helps to dictate the growth mechanism in terms of rate, final size, or geometric shape.[21] Various functional groups react differently with inorganic surfaces, with the common example being thiol to gold.[15,22] Strongly binding molecules can form a dense protective layer and stabilize the nanoparticles better than weakly binding ones.
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