Plasmonic Copper Sulfide-Based Materials: A Brief Introduction to

May 17, 2017 - The above discussion provides basic understanding and guidance about the dependence of plasmonic properties of copper sulfide based NCs...
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Plasmonic Copper Sulfide-Based Materials: A Brief Introduction to Their Synthesis, Doping, Alloying, and Applications Yang Liu,† Maixian Liu,§ and Mark T. Swihart*,† †

Department of Chemical and Biological Engineering and §Department of Pharmaceutical Sciences, University at Buffalo, The State University of New York, Buffalo, New York 14260, United States ABSTRACT: Plasmonic copper sulfide-based colloidal nanocrystals (NCs) have attracted considerable attention due to their unique and versatile optical and electronic properties. In this Feature Article, we first introduce the optical properties of these p-type semiconductor nanostructures, particularly localized surface plasmon resonance (LSPR). We then discuss nanostructures of copper sulfides [CuS and Cu2−xS, (0 ≤ x < 1)] with different crystal structures and optical properties. In addition to the synthesis and transformation between these copper sulfide phases, we review their doping or alloying with extrinsic cations, which can produce homogeneous alloy nanostructures, new crystal phases, and multidomain (e.g., core−shell or dimer) NCs. In general, divalent cations (e.g., Zn, Hg, Cd) do not form a homogeneous phase upon incorporation into copper sulfide NCs, but trivalent and tetravalent cations (e.g., In, Sn, Ga) can do so. Filling of the Cu vacancies responsible for p-type doping results in red-shifting and damping of the LSPR upon incorporation of extrinsic elements. Finally, we present some emerging applications of copper sulfide-based nanomaterials. generate and control the LSPR in NCs,10−12 and the range of nanomaterials used for their LSPR properties has expanded from noble metals to include self-doped copper chalcogenides and doped metal oxides.11,12 Copper chalcogenide NCs, with and without LSPR, have been explored for diverse applications. The LSPR in plasmonic copper chalcogenide NCs can be tuned across the NIR region of the spectrum by varying their composition and crystal structure. In contrast, the free carrier concentration of a given noble metal is fixed, and the LSPR can only be tuned by varying the NC morphology. Because there is a biological window in the NIR region in which penetration of light through tissue is maximized, tunable NIR LSPR is of great value in bioimaging13 and photothermal therapy.14 Colloidal Cu2−xTe NCs of tunable morphology including spheres, rods, and tetrapods have been synthesized by controlling the time, temperature, and concentration of cations and anions in colloidal reactions, illustrating the dependence of LSPR on morphology. 15 Beyond these applications, Cu2Se has been studied as an absorber in photovoltaic devices because its band gap (1.1−1.5 eV) is well matched to the solar spectrum.16 More complex metal chalcogenides, such as Cu2ZnSnSxSe4−x and CuInxGa1−xSe2 have been even more intensely studied for photovoltaic applications.17,18

1. INTRODUCTION A diverse array of colloidal nanomaterials has been synthesized and studied over the past few decades. These nanomaterials can be classified based upon their composition and properties into categories such as noble metal nanocrystals (NCs),1 metal oxide NCs,2 metal carbide NCs,3 and semiconductor quantum dots (QDs).4,5 They are being explored or used in emerging applications based on their unique physical, chemical, and biological properties. Metal chalcogenide NCs have been the basis for most research in colloidal QDs, which are of interest for their strong size-dependent photoluminescence (PL). The II−VI semiconductors (e.g., CdS, CdSe, ZnS) and IV−VI semiconductors (e.g., PbS, PbSe) have been the most-studied QD materials,4,5 but new Pb- and Cd-free alternatives, such as CuInS2, are emerging.6 The efficient and stable size-dependent PL of QDs is the basis for applications ranging from bioimaging and sensing to displays and LEDs. In addition to PL, localized surface plasmon resonance (LSPR) is an especially important optical phenomenon with rapidly growing applications in theranostics,7 nanophotonics,8 and nanoelectronics.9 LSPR is manifested in electrically conductive structures that are much smaller than the wavelength of the incident light, in which free charge carriers (i.e., free electrons or holes) can collectively oscillate in resonance with incoming light (Figure 1a). Gold (Au) NCs with LSPR have been used or tested in applications including bioimaging, sensing, cancer therapy, and surfaceenhanced Raman scattering.1 These applications all depend on the LSPR of Au NCs, which produces very strong optical absorbance. Recent studies have revealed many possible ways to © 2017 American Chemical Society

Received: January 27, 2017 Revised: April 20, 2017 Published: May 17, 2017 13435

DOI: 10.1021/acs.jpcc.7b00894 J. Phys. Chem. C 2017, 121, 13435−13447

Feature Article

The Journal of Physical Chemistry C

In this Feature Article, we discuss three types of thermodynamically stable copper sulfides [CuS, Cu2−xS (0 ≤ x < 1), and Cu2S], transformations among them, and some ternary and quaternary materials derived from these copper sulfides. The LSPR in this class of materials can be broadly tuned by changing the composition (x value in Cu2−xS) or the aspect ratio of anisotropic NCs and by doping or alloying with other cations. CuS and Cu2−xS are discussed separately because of their distinctly different crystal structures. We classify extrinsically doped or alloyed copper sulfide NCs based upon the valence of the extrinsic cation. A review of published results shows that trivalent and tetravalent cations can generally be incorporated to produce ternary metal sulfides. However, divalent cations are generally not incorporated; introducing divalent cations usually results in production of a separate metal sulfide phase. In addition, we highlight a few selected emerging applications of copper sulfide-based nanomaterials. Understanding of LSPR in NCs is facilitated by considering the Drude model of the dielectric function: ε = ε∞ −

ωp2 ω 2 + iγω

(1)

where ωp is the plasma frequency, γ is the damping parameter, and ε∞ is the background dielectric constant. The plasma frequency is given by

ωp2 =

Figure 1. (a) A schematic illustration of the interaction between the oscillating electric field of light and free charge carriers that produces LSPR in NPs. (b) Simplified schematic of the band structure of a typical p-type plasmonic semiconductor highlighting the empty states (holes) in the valence band. (c) Unit cell of covellite CuS.

nee 2 mh ε0

(2)

where ne is the free carrier density, mh is the carrier effective mass, and ε0 is the permittivity of free space. In spherical NPs much smaller than the wavelength of incident light, the NP polarizability, α, is given by ε − εm α = 3ε0V ε + 2εm (3)

Among copper chalcogenide NCs, those based upon copper sulfide have captured researchers’ attention because of their promise in thermoelectric,19 electrocatalytic,20 and photovoltaic applications, as well as possible biological applications. Copper sulfide-related nanomaterials have been studied for over 25 years.21−23 For example, in an early study, Wozniak et al. synthesized polymer-capped covellite CuS clusters from elemental Cu and S powders.23 Over the years, copper sulfide has generated increasing interest based upon its tunable optical properties, particularly once the origin of LSPR in Cu2−xS became understood based on the report of Luther et al.11 The composition, crystal structure, and size- and shapedependent properties of colloidal Cu2−xS NCs have been well studied. A wide range of stable and metastable stoichiometries, including Cu2S,24 Cu31S16 (Cu1.96S),25 Cu9S5 (Cu1.8S),26 Cu7S4 (Cu1.75S),27 Cu9S8 (Cu1.12S),27 and CuS28 have been observed. Covellite (CuS) has recently attracted considerable attention because it has the highest concentration of free carriers in the copper sulfide class of materials. Among thermally and air-stable copper−sulfur phases, CuS has the lowest Cu to S ratio. In the covellite crystal lattice, disulfide bonds are formed to balance the coordination and occupancy of lattice sites at this low Cu to S ratio. The resulting electronic structure gives covellite its p-type metallic character. At the other extreme, pure high chalcocite (Cu2S) has the highest Cu to S ratio. Because the free holes in Cu2−xS materials arise from copper deficiency, Cu2S (with x = 0) has few free carriers and does not exhibit LSPR at NIR wavelengths. However, upon air exposure, Cu2S can transform to the djurleite (Cu1.97S) phase, which has sufficient free holes to exhibit NIR LSPR.24

where V is the volume of the NP and εm is the dielectric constant of the surroundings. The LSPR occurs near the frequency at which ε = −2εm. The key implication of eqs 1−3 is that the LSPR energy is approximately proportional to the square root of the free carrier density, ne. The effect of NP geometry on the LSPR is illustrated by the equation for the polarizability of an ellipsoidal particle, in which eq 3 is modified with shape factors Li, to yield eq 4. ε − εm αi = 3ε0V 3εm + 3Li(ε − εm) (4) This produces different plasmon resonance energies corresponding to charge carrier oscillations along the principle axes (denoted by i = 1, 2, or 3) in an anisotropic particle. The transverse and longitudinal resonances in gold nanorods are a prototypical example. In a system of variable composition, increasing the free hole concentration or decreasing the effective mass increases the plasma frequency and therefore the LSPR energy. In contrast, for a system with fixed free carrier concentration and dielectric background, LSPR frequency is mainly dependent upon the damping parameter and shape. A key advantage of plasmonic semiconductors relative to noble metals is that their free carrier concentration can be tuned. Copper-deficient copper chalcogenides are p-type semiconductors in which the highest energy states in the valence band are empty. The free holes associated 13436

DOI: 10.1021/acs.jpcc.7b00894 J. Phys. Chem. C 2017, 121, 13435−13447

Feature Article

The Journal of Physical Chemistry C with these unoccupied states are responsible for LSPR. Moreover, the energy of the highest occupied states in the valence band is lower than it would be in stoichiometric Cu2S, which leads to an increase of the optical bandgap (Figure 1b). The above discussion provides basic understanding and guidance about the dependence of plasmonic properties of copper sulfide based NCs on composition and geometry. Some dependences, such as the effect of NC geometry and the dielectric constant of the surrounding medium are understood quantitatively. Other key trends, such as the increase in LSPR energy resulting from increased free carrier concentration are captured by the simple Drude model of dielectric function and Mie−Gans theory of eqs 1−4. However, parameters of these models, including the free carrier concentration and carrier effective mass, background dielectric constant, and damping parameter, cannot yet be obtained quantitatively from the composition and crystal structure of the material. Moreover, the simple model described above does not take into account effects such as anisotropic electronic properties. Damping may be frequency dependent, and some localization of free carriers may prevent the Drude model and other simple models of the dielectric function from providing quantitative predictions of LSPR absorbance, as demonstrated by Kriegel et al.15 Numerous computational studies of the band structure of the various copper sulfide phases have appeared in recent years.29−33 These provide insights and some predictive capability but do not yet allow quantitative prediction of all needed parameters. Most theoretical studies are limited to treatment of ideal, defect-free crystals. However, defects, such as copper vacancies and nonstoichiometric surface composition, can strongly influence the LSPR. Nanocrystals in the copper sulfide family of materials are also known for PL, particularly in QDs prepared from CuInS2 and related ternary and quaternary materials. However, here we are focusing mainly on plasmonic copper-sulfide based materials. In general, the high charge carrier density required for NCs to exhibit near-IR LSPR is incompatible with efficient PL.34 The states responsible for these free charge carriers also allow rapid nonradiative recombination of excitons. While the plasmon frequency and intensity increase with increasing free charge carrier density (x in Cu2−xS), this is accompanied by PL quenching, generally due to an Auger-type recombination. Therefore, the plasmonic copper sulfide-based NCs that exhibit strong absorbance in the NIR region to not exhibit efficient PL. Photoluminescent QDs of CuInS2 and related materials have been reviewed elsewhere.6

Figure 2. (a) A typical TEM image of covellite CuS NPls, with an edge view of NPls shown in the inset. (b) High-resolution TEM (HRTEM) image of CuS NPls. Insets show a sketch of a platelet and corresponding indexed fast Fourier transform (FFT) of the HRTEM. (c) NIR absorbance spectra of CuS NPls of varying aspect ratio. (d) Illustration of in-plane and out-of-plane LSPR modes. Panels a and b are adapted with permission from ref 35. Copyright 2013 American Chemical Society. Panels c and d from ref 28. Copyright 2015 American Chemical Society.

Reaction was carried out at 180 °C, and the product morphology was controlled by the ODE/OLAM/OA composition. Covellite CuS NPls can also be synthesized by reacting ammonium sulfide (AS) with a Cu-OLAM precursor in toluene at ambient conditions.28 In that case, the size and monodispersity were controlled by varying the manner in which the AS was injected into the Cu precursor solution. The NC thickness remained constant at ∼4.0 nm (about 2.5 unit cells along the covellite caxis), while the diameter varied from ∼4 nm to over 30 nm. The LSPR absorbance peak was tuned from 1000 to 1700 nm (Figure 2c,d) by changing the aspect ratio, red-shifting with increasing NPl diameter. In contrast to covellite CuS NCs, stoichiometric Cu2S NCs lack free charge carriers and thus do not exhibit LSPR. However, copper-deficient Cu2−xS NCs can exhibit LSPR for sufficiently large values of x. Here, we briefly consider Cu2−xS NCs, classified by their dimensionality. Ma et al. prepared ultrasmall Cu2−xS NCs, which can be considered zero-dimensional (0D) materials, by reacting a Cu-thioglycolic acid complex with thioacetamide in water.36 Similarly, Chen’s group produced 2 nm copper sulfide NCs with LSPR near 1050 nm in organic solvents by reacting CuOLAM and S-OLAM (Figure 3a).37 The aqueous NCs of Ma et al. spontaneously self-assembled into nanochains and nanoribbons of 4−20 nm width and 50−950 nm length, producing a type of one-dimensional (1D) structure. Other 1D nanostructures, including Cu2−xS nanorods, can be obtained using dodecanethiol (DDT), which not only serves as a ligand that stabilizes specific NC facets but also thermally decomposes to serve as a sulfur source (Figure 3b).38 In this case, the thiol passivation produces relatively high reactivity of (100) facets, leading to anisotropic growth of rod-like NCs. Further, the nanorod aspect ratio was tuned by varying the nucleation temperature and reaction time. Two-dimensional Cu2−xS NPls with strong NIR LSPR and controlled diameter from 2.8 to 13.5 nm were produced by Liu et al. using OA-S precursors prepared by simply dissolving sulfur

2. SELF-DOPED COPPER SULFIDE COLLOIDAL NCs WITH LSPR Covellite CuS has a simple stoichiometry but a complex structure consisting of triple layers with planar trigonal copper (CuS3) sandwiched between tetrahedral copper (CuS4) units (Figure 1c). Each CuS4−CuS3−CuS4 triple layer is linked to adjacent triple layers by disulfide (S−S) bonds. These disulfide bonds effectively contribute two free holes in the valence band per unit cell (Cu6S6) leading to metal-like character with a free carrier concentration as high as ∼1022 cm−3, which produces LSPR at NIR wavelengths.32 Cozzoli’s group reported synthesis of covellite CuS nanoplatelets (NPls) with strong NIR absorbance, produced by weak out-of-plane and dominant in-plane dipolar LSPR modes.35 The synthesis employed Cu2+ dissolved in a mixture of 1-octadecane (ODE), oleylamine (OLAM), and oleic acid (OA) and sulfur powder dissolved in OLAM (Figure 2a,b). 13437

DOI: 10.1021/acs.jpcc.7b00894 J. Phys. Chem. C 2017, 121, 13435−13447

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The Journal of Physical Chemistry C

Figure 4. (a) Vis−NIR absorbance spectra of copper sulfides upon addition of Cu+, starting from CuS. (b) XRD pattern evolution from covellite to low chalcocite upon Cu+ addition. Reprinted from ref 39. Copyright 2013 American Chemical Society.

interpret because of the numerous similar crystal structures that are possible at compositions between CuS and Cu2S. For example, djurleite Cu31S16 copper sulfide NCs were transformed from disks to tetradecahedra in the presence of Sn4+,43 and djurleite Cu31S16 was converted to roxbyite Cu7S4 by heat treatment in OLAM without changing its morphology. In the latter study, heating in OLAM made the NCs more copperdeficient (increased x in Cu2−xS), in contrast to the work of Xi’s group that produced more Cu-rich NCs by OLAM treatment.42 This difference highlights the dependence of such transformations upon the initial copper sulfide phase employed. While mechanistic details underlying this difference remain uncertain, we note that covellite CuS possesses disulfide bonds that may be reduced by OLAM at elevated temperature, while djurleite does not. Thus, it is conceivable that OLAM may extract sulfur (via disulfide bond reduction) from CuS but extract copper from djurleite Cu31S16. In a related study, Nelson et al. transformed roxbyite to djurleite through tributylphosphine etching, producing a ∼200 nm red shift in LSPR position.44 This further illustrates the complexities of the copper-deficient copper-sulfide systems. Further studies providing general pathways for, and understanding of, reversible transformations between the limiting CuS and Cu2S stoichiometries are needed.

Figure 3. Representative TEM images of Cu2−xS (0 ≤ x < 1) NC morphologies including (a) nanodot, (b) nanorod, (c) nanoplatelet, and (d) nanododecahedron. Scale bar in panel b is 100 nm, and scale bar in panel d is 200 nm. Panel a has been adapted from ref 37, Copyright 2015 John Wiley and Sons, panel b from ref 38, Copyright 2012 American Chemical Society, panel c from ref 25, Copyright 2013 John Wiley and Sons, and panel d from ref 26, Copyright 2011 Royal Society of Chemistry.

powder in pure OA (Figure 3c).25 Interestingly, the LSPR redshifted with increasing amount of OA used in the synthesis. This phenomenon was attributed to reduced free carrier density due to trapping of free holes by deprotonated carboxyl groups of adsorbed OA. Three-dimensional (3D) copper sulfide NCs with morphologies including spheres, dimers, tetradecahedra, and dodecahedra were reported by Li et al. (Figure 3d). In general, starting from the same small copper sulfide seeds, the reaction rates of different crystal facets were promoted by varying growth conditions, finally producing dramatic differences in NC shape.26 Along with many separate studies of CuS and Cu2−xS NCs, the relationship between them has also been investigated, providing new insights into their colloidal synthesis and optical properties. Manna’s group transformed CuS to Cu2S by reaction with a Cu(I) complex ([Cu(CH3CN)4]PF6) at room temperature (Figure 4a,b).39 Starting from covellite, which has a high density of free carriers and strong NIR LSPR, the nanostructure was gradually transformed to Cu2S NCs with no LSPR, exhibiting damped NIR LSPR at intermediate states (Figure 4). Importantly, during the interconversion, the effective valency of S gradually changed from −1 to −2, while the valency of Cu remained close to +1. This is consistent with a view of the covellite structure in which all Cu atoms are in the +1 state while S atoms are in a mixed valence state, with an average oxidation state of −1.40,41 The hexagonal NPl morphology was also maintained during the transformation from covellite to chalcocite. Xi’s group achieved a similar transformation from CuS to Cu2S without adding a new cation source.42 They converted covellite CuS NPls to chalcocite Cu2S NPls by exposing them to OLAM at 220 °C. Transformations among Cu2−xS NCs of varying crystal structure and stoichiometry have also been studied, but these results are even more difficult to

3. EXTRINSICALLY DOPED OR ALLOYED COPPER SULFIDE COLLOIDAL NCs The free carrier concentration, and therefore LSPR wavelength, of Cu2−xS NCs can be tuned not only by adjusting the copper deficiency, x, but also through addition of other cations. At low levels of extrinsic cation content, this is usually called doping, while at higher levels, alloying may be a more appropriate term. Such extrinsically doped or alloyed copper sulfide NCs have been the subject of numerous recent studies, producing ternary and quaternary copper sulfide-based nanomaterials through addition of monovalent (Au+, Ag+),45,46 divalent (Zn2+, Cd2+, Hg2+),47,48 trivalent (In3+, Fe3+, Ga3+, Sb3+ and As3+),49−55 and tetravalent (Sn4+)56−59 cations. Some noble metal−copper sulfide combinations have been developed in an effort to combine the advantages of each, that is, low cost and compositional tunability of the LSPR of Cu2−xS with intense visible LSPR from noble metals. In contrast to AuCu2−xSe nanodimers,60 which have shown promise for use in photoacoustic imaging, combining Au with Cu2−xS does not produce pure Au domains. Instead, self-doped Cu2−xS NCs were converted into monodisperse Cu2−xS−Au2S NCs of tunable composition, including pure Au2S, by cation exchange (Figure 13438

DOI: 10.1021/acs.jpcc.7b00894 J. Phys. Chem. C 2017, 121, 13435−13447

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The Journal of Physical Chemistry C 5a,b).45 The LSPR of the NCs was damped and red-shifted with increasing Au incorporation (Figure 5c), as the Cu+ vacancies

addition to Au and Ag, other noble metals such as Pd have also been combined with copper sulfide to create heterogeneous NCs. For example, Cu7S4-Pd NCs were designed to combine photocatalytic properties of Pd with the LSPR of Cu7S4 to improve solar photocatalytic activity (Figure 5g−i).62 Note that although Pd can adopt several stable oxidation states and forms a stable sulfide in its +2 oxidation state, in these studies it did not form a sulfide, but remained as a zerovalent metal. This is the desired outcome for applications in catalysis. The incorporation of divalent cations such as Zn2+, Hg2+, and Cd2+ into Cu2−xS NCs by cation exchange has also been explored. Zinc was chosen based on its similar cation radius to Cu+, and a Cu1.81S template was chosen because its cation deficiency was expected to promote cation exchange. However, exposure of the Cu1.81S templates to Zn2+ created epitaxial heterostructures (Figure 6a−f) rather than doped or alloyed single-phase NCs.47 The thickness of the inner-disk layer was tuned to form thin (