Research Article www.acsami.org
Controlled Synthesis of CuInS2/ZnS Nanocubes and Their Sensitive Photoluminescence Response toward Hydrogen Peroxide Youngsun Kim,†,‡ Ho Seong Jang,§ Hyunki Kim,† Sehoon Kim,‡ and Duk Young Jeon*,† †
Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, Republic of Korea ‡ Center for Theragnosis and §Materials Architecturing Research Center, Korea Institute of Science and Technology (KIST), Seoul 136-791, Republic of Korea S Supporting Information *
ABSTRACT: We synthesized uniform CuInS2/ZnS nanocubes by adjusting reaction parameters at the ZnS growth stage. Higher temperature and zinc concentration were shown to drive resultant crystals to have cubic morphology, which could be ascribed to the facet-dependent ligand dynamics on the crystal surface and concomitantly preferred directions of crystal growth. It was found that these nanocubes exhibit sensitive responses, as of photoluminescence quenching, toward hydrogen peroxide, compared to pyramid-shaped nanocrystals. The origin of quenching was further analyzed to be the oxidation of thiolate ligands that leaves the quenching center on the surface. It was noted that the quenched photoluminescence could be fully recovered by introducing additional ligand molecules into the system. Being adopted in the shape-controlled crystal growth, the ligand-tocrystal interaction was shown to still govern the interfacial reaction, the oxidation by hydrogen peroxide, of faceted crystals in our system. It turns out that the reactivity at the crystal surface depends on the exposed facets, especially induced by shape control, and the weak ligand-binding nature of the nanocube renders it vulnerable to the surface reaction. KEYWORDS: CuInS2/ZnS nanocrystals, shape control, crystal/ligand interface, surface chemistry, photoluminescence
■
INTRODUCTION Shape control of colloidal nanocrystals (NCs) has been a focus for feasibility in tuning physical, chemical, and optical properties of materials originating from differences in geometrical dimensions, surface nature, or energy structure. Diverse morphologies can be derived depending on crystal structures of nuclei or growing crystal species as well as synthetic conditions (e.g., temperature, type and concentration of ligands and precursors) during crystal growth, all of which are delicately designed to overcome thermodynamic driving force that supports a spherical shape.1−4 For example, by introducing phosphonic acid ligands that strongly bind to lateral facets of wurtzite crystal, rods and tetrapods of cadmium chalcogenide were successfully synthesized,5−7 and spherical zinc blende CdSe NCs were modified into cubic and tetrahedral crystals through adjustment of growth temperature.8 Morphological studies on NCs with various compositions (ZnS,9 CuInS210−12 and PbS13) even with complicated shapes © 2017 American Chemical Society
have been also conducted. With the establishment of the crystal growth mechanism, it is not too much to say that the shapecontrolled synthesis technique reaches a somewhat artistic realm. The true worth of the shape control comes when a beneficial change in the corresponding attribute is derived from the engineered morphologies. There have been several reports on applications of the shape specific properties of NCs. For examples, one-dimensional matrixes of nanorods could improve charge propagation and separation in photovoltaics,14,15 and faceted NCs were shown to have facet-dependent catalytic activity (e.g., Pt,16,17 Pd,18,19 Co3O4,20 and TiO2 NCs21). The former case utilized a spatial aspect of NCs, while the latter is on the scope of surface chemistry of crystal. Both the synthetic Received: June 29, 2017 Accepted: August 28, 2017 Published: August 28, 2017 32097
DOI: 10.1021/acsami.7b09388 ACS Appl. Mater. Interfaces 2017, 9, 32097−32105
Research Article
ACS Applied Materials & Interfaces
Shimadzu UV-3101 spectrophotometer. The PL quantum yields were calculated with Rhodamine 6G as a reference dye. Transmission electron spectroscopy (TEM) images were obtained using a JEMARM200F. Powder X-ray diffraction patterns (XRD) were obtained with a D/MAX-RB (12 kW) from RIGAKU. Size distribution of NCs was traced by the dynamic light scattering (DLS) method at 25 °C (Zen 3600 Nano-ZS, Malvern Instruments). X-ray photoelectron spectroscopy (XPS) spectra were acquired using a PHI 500 VersaProbe (ULVAC PHI) with a monochromator Al Ka (1486.6 eV) anode (24.5 W, 15 kV) for X-ray source. The spectra were calibrated with the adventitious C 1s peak at 284.6 eV. 1H NMR spectra were collected in CDCl3 at 25 °C on a Bruker AV-300 spectrometer (400 MHz). Time Correlated Single Photon Counting (TCSPC) was performed with an FL920 from Edinburgh Instruments. The PL decay times of nanopyramids and nanocubes were obtained at 650 and 640 nm, respectively, with the excitation source of 470 nm (picosecond pulsed diode laser, EPL-470) and calculated by the F900 program with an instrument response function (IRF). Synthesis of CIS and CIS/ZnS NCs. CIS core NCs were synthesized according to previously reported procedures.35 0.2 mmol of CuI, 0.2 mmol of In(OAc)3, and 8.3 mmol of DDT were mixed with 16 mL of ODE in a three-neck flask and degassed under vacuum at 60 °C for 60 min. The mixture was then heated to 230 °C and kept for 30 min under an argon atmosphere. The resultant NCs were purified by precipitation with excess acetone and redispersed in chloroform. Subsequently, 40 mg of as-prepared CIS NCs (vacuum-dried) and 0.5 mmol of Zn(OAc)2 were mixed with 41.7 mmol of DDT and degassed under vacuum at 60 °C for 60 min. The mixture was heated to various temperatures (230−280 °C) and kept for 180 min. Aliquots were sampled for investigating the evolution of NCs, and the amount of Zn precursor was varied to 2 mmol for acquiring nanocubes with different emission wavelengths. The reaction medium was then cooled to room temperature and centrifuged to remove any aggregates. NCs were purified three times by precipitation in an excess amount of acetone and redispersed in chloroform for further use. Photoluminescence Quenching of CIS/ZnS NCs with H2O2. To observe the fluorescence quenching behaviors of NCs, 999 μL of NC dispersion in chloroform (∼1 μM) and 1 μL of H2O2 diluted to various concentrations by tetrahydrofuran were mixed. The molar concentration of NCs was estimated by calculation based on UV absorption and PL peak position as reported earlier.36
approach and the study on shape-dependent behavior of relevant aspects have their importance in providing fundamental clues for unraveling the intricate nature of NCs’ surface/ interface as well as in developing appropriate application models. Being of specific interest, luminescent colloidal semiconductor NCs, called quantum dots (QDs), are what we focus on in this study. Several nanometer-sized QDs would be understood as the combinatorial system of a crystalline inorganic core and coordinating organic ligands. Such a structure possesses a heterogeneous interface between a crystal edge and a capping corona of ligands, which would be the first station at which external molecules arrive for probable chemical reactions with QDs. Due to the presence of passivating ligands, participation of the crystalline part in chemical reactions occurs likely when ligand molecules are stripped off. This interfacial site and constituting interaction are, indeed, the main concern for shape-controlled synthesis of NCs. On the other hand, the close relation of luminescent properties with the surface conditions leads to a sensitive response of photoluminescence (PL) to surface deterioration: for example, ligand detachment. It was reported that PL of several types of QDs is quenched with a blue shift under a photooxidative environment (CdSe22 and CuInS2/ZnS23). PL quenching of QDs was also observed in the presence of external oxidants such as hypochlorous acid22 and hydrogen peroxide (H2O2).24−27 Under those oxidative stresses, capping ligands28 or crystal-constructing atoms22,23,29 were found to be responsible species that undergo oxidation. In this context, PL of QDs can be considered as a window through which a nanoscopic subject of crystal-to-ligand interactions is looked up, as being a candidate for the shape-derived varying property. In addition, H2O2, a major reactive oxygen species with relatively high stability, plays crucial roles in a wide variety of physiological processes from basic cellular signal transductions to disease-specific pathological effects. Thus, highly H2O2-sensitive materials are one of the fascinating tools for diagnosing relevant diseases as well as for understanding dynamic metabolism.30−32 In this research, shape-controlled synthesis and shapedependent PL quenching behavior by H2O2 were studied. We report a facile heating-up route to synthesize uniform CuInS2/ ZnS (CIS/ZnS) NCs, a potential nontoxic candidate for substituting Cd-based QDs,33,34 through designing a reaction scheme for ZnS growth, which involves temperature-dependent growth kinetics along with precursor supply. It was found that relatively high temperature as well as high zinc and sulfur feed in a coordinating solvent leads to the formation of cube morphology. In addition, resulting nanocubes exhibited more sensitive PL responses to H2O2 than pyramid-shaped counterparts (i.e., trigonal pyramids) did. Bringing the morphological difference into line with crystal-to-ligand interaction, we found that the strategy used for shape-controlled synthesis concerning the interface is still valid in interpreting the shape-dependent response to H2O2.
■
■
RESULTS AND DISCUSSION Synthesis and Characterization of CIS/ZnS Nanocubes. Figure 1 displays transmission electron microscopy (TEM) images of CIS/ZnS NCs in the course of reaction time of ZnS growth at 280 °C. The ZnS growth was conducted from CIS NCs of a pyramidal shape with a size of 3.3 nm (an average of three edges, Figure 1A). These pyramidal CIS NCs are enveloped by one {112} and three {114} facets as reported earlier for chalcopyrite CuInS237,38 or CuInSe239 NCs, that is, other than tetrahedra with four {112} facets (for crystallographic characterization of CIS NCs, see Figure S1 in the Supporting Information). Here, {112} and {114} planes are analogous to {111} and {112} planes of the zinc blende structure. After the reaction temperature reaches 280 °C, the shape of crystals does not show significant change while the average size is reduced from 3.3 to 3.1 nm, which implies that the surface of core seeds is partially etched to some degree (Figure 1B). As the reaction proceeds at 280 °C, crystals start to grow. Cube-shaped crystals appeared at 30 min after start (Figure 1C), and the mixture of cubes and smaller pyramids were observed by this time (Figure 1D,E). The presence of smaller pyramids during these stages indicates that there exists deviation in a shell growth rate among the nanocrystal ensemble (for less magnified TEM images, see Figure S2 in the Supporting Information). This deviation might be partially
EXPERIMENTAL SECTION
Materials and Instruments. Copper(I) iodide (CuI, 99.995%), indium(II) acetate (In(OAc)3, 99.99%), 1-dodecanethiol (DDT, 97%), zinc acetate (Zn(OAc)2, 99.99%), 1-octadecene (ODE, 90%), and hydrogen peroxide (H2O2, 30 wt %) were purchased from SigmaAldrich and used as received. PL spectra were recorded using a Hitachi F-7000 Luminescence Spectrometer, and UV−vis absorbance spectra were recorded using a 32098
DOI: 10.1021/acsami.7b09388 ACS Appl. Mater. Interfaces 2017, 9, 32097−32105
Research Article
ACS Applied Materials & Interfaces
not grow at the initial stages while being crystallized as the reaction continues due to the gradual decomposition of Znthiolate and resultant growth of ZnS crystal. The evolution of cubic morphology is what we are interested in. As displayed in Figure 2A, CIS/ZnS nanocubes have lattice fringes spacing parallel to the diagonal (∼0.19 nm) and to the edge (∼0.14 nm). The distances were measured from fast Fourier transform (FFT, Figure 2B). These two values could be assigned, respectively, to the d-spacing of (220) and (400) planes of zinc blende ZnS (PDF# 79-0043): the values, in a more strict way, lying in the middle of ZnS and chalcopyrite CIS (PDF# 75-0106). Given that two vectors directing from the center of the view to each pattern in FFT have the angle around 45°, thus, the zone axis is ⟨001⟩ and the cube presented in Figure 2A has four {100} side planes. In addition, characteristic peaks of X-ray diffraction (XRD) of the nanocubes are shifted from those of the chalcopyrite CIS core (see Figure S1 in the Supporting Information) toward zinc blende ZnS (Figure 2C). Core/shell NCs have such shifted XRD patterns with the existence of a shell;37,40,44 thus, the formation of a zinc blende ZnS shell is further confirmed. Taken altogether, the results indicate the cube is enveloped by {100} facets of zinc blende ZnS. The origin of the morphology derived could be found by considering the difference in surface properties of crystal planes along with crystal growth conditions. Regarding dynamic states of ligands during the crystal growth, ligands are frequently attached and detached at the growing edge of crystals, depending on temperature in a proportional manner.8,45 Basically, crystal growth occurs when growth species (i.e., Zn-thiolate) come in contact with the surface of seed crystals; in turn, pristine crystal surfaces have more chances to be exposed to the environment at higher temperature. In this regime, a growth trend would be predominantly determined by the reactivity of unpassivated surface facets. For the zinc blende structure, among polar planes (Zn- or S-terminated facets), {111} planes have higher affinity to ligands than {100} planes do, implying that bare {111} planes have higher surface energy than {100} planes.8 In addition, nonpolar {112} planes are strongly stabilized by coordinating ligands.37,39 It turns out that, at higher temperature where stabilizing ligands are stripped off, facets with stronger binding affinity to ligands would become more reactive binding sites that accommodate growth species. Consequently, crystals would grow at a setup temperature of 280 °C in a direction of diminishing reactive {111} and {112} planes, leaving relatively unreactive {100} facets on the surface. On the other hand, in a thermodynamic view, the excess use of coordinating ligands that act also as a monomer formulator might retard a growth reaction, retaining a high chemical potential of monomers to some extent for directing a shapecontrolled growth.46 Therefore, ZnS might evolve into forming
Figure 1. TEM images of CIS/ZnS NCs with different reaction times: (A) CIS core, (B) 0 min (after 24 min of heating-up to 280 °C from 60 °C), (C) 30 min, (D) 60 min, (E) 120 min, and (F) 180 min at 280 °C (insets in A and B correspond to magnified ones). All the scale bars are 20 nm.
ascribed to the wide size distribution of the CuInS2 core, synthesized by the usual heating-up methods, and concomitant difference in their capacity of consuming zinc by inner-core diffusion or surface reconstruction.40 After 180 min, eventually, quite uniform nanocubes with the average size of 5.7 nm (edge length) were obtained (Figure 1F). In this reaction regime, DDT plays dual roles of sulfur source and ligand, meaning that crystal-constituting sulfurs are decomposed from the zincthiolate complex (Zn-SR) by thermal energy.10,35,41 Furthermore, the crystal formation is prohibited by strongly coordinating solvent (DDT).42,43 With the observed temporal evolution of crystal size, therefore, it is evident that ZnS does
Figure 2. (A) HR-TEM image with corresponding (B) FFT diagram and (C) XRD pattern of CIS/ZnS nanocubes grown at 280 °C for 180 min. 32099
DOI: 10.1021/acsami.7b09388 ACS Appl. Mater. Interfaces 2017, 9, 32097−32105
Research Article
ACS Applied Materials & Interfaces a single facet-enveloped crystal rather than a quasi-spherical, highly faceted or epitaxially grown crystal. Optical characteristics were chased in parallel with the shape evolution. Both UV−visible absorbance and PL spectra show a similar trend of shifting to higher energy as the reaction advances (Figure 3A,B). At the beginning within 20 min of
Figure 3. Temporal evolution of (A) UV−vis absorbance, (B) PL spectra, and (C) emission peak wavelength and quantum yield during ZnS shell growth at 280 °C (excitation wavelength: 370 nm).
heating-up to 280 °C, the emission maximum of NCs swiftly moves from 752 to 665 nm, while a slowly changing regime appears at the later stage (Figure 3C). It is also noted that PL quantum yield exhibits a reversed trend when compared with the variation of PL peak wavelength within the same time duration (Figure 3C). A sharp rise, reaching the maximum of 74% after 30 min at 280 °C, was observed at the initial stage while slowly decreases to 38% after 180 min of the reaction. Taken together with the shape evolution, interestingly, significant changes in optical properties were observed at the early stage, whereas a negligible change in morphology. Such a blue shift of the PL peaks has been reported for CIS NCs in zinc-incorporated systems such as core/shell CIS/ZnS and quaternary Zn-Cu-In-S NCs47 with several proposed mechanisms: surface reconstruction,35 core size reduction by cation exchange,40 or by the formation of an alloyed interface,48 filling up intragap defect states,49 or band gap widening by the entrance of zinc ions.47 Even though the mechanisms underlying those phenomena are not clearly established, the rapid blue shift of both absorbance and PL spectra, indeed, in our system indicates that the incorporation of Zn ions into the CIS crystallite with probable surface reconstruction is a predominant process without the ZnS shell growth at the very beginning of the reaction. Further heating of the system at 280 °C, then, promotes the ZnS shell growth via the decomposition of Zn-SR monomers, resulting in the generation of nanocubes. Effect of Temperature. A hypothesis we supposed above is that setup at 280 °C is high enough for the dynamic ligand regime that directs cubic growth. In an attempt to verify it, we investigated the temperature effect on the morphology of NCs. Figure 4A−C shows morphologies of NCs synthesized at
Figure 4. (A−D) TEM images, (E) PL spectra, and (F) photo images of CIS/ZnS NCs grown at different reaction temperatures: (A) 230 °C, (B) 250 °C, and (C) 260 °C (scale bars: 20 nm). (D) HR-TEM image for 250 °C.
different temperatures for ZnS growth. At 230 and 250 °C, pyramidal NCs with average sizes of 4.51 and 4.86 nm were formed, whereas some fraction of cubic NCs were observed at 260 °C. As shown in Figure 4D, for the NCs grown at 250 °C, the shape of CIS core seeds is maintained, over which the ZnS shell is grown in an epitaxial manner. Note that the (111) plane of zinc blende ZnS corresponds to the (112) plane of chalcopyrite CIS. When temperature was raised to 300 °C, a large amount of aggregates appeared. Considering the fractional formation of aggregates at a reaction temperature above 250 °C, there would, indeed, exist some ways to reduce the aggregation loss, for example, by introducing additional thiols of larger molecular weight into the growth reaction. The absence of cubes below 260 °C supports our assumption that there exists a temperature threshold that dictates the growth regime of crystals. At lower temperature, the surface of NCs is likely covered by ligands due to less frequent detachment than that at higher temperature. Furthermore, relatively low thermal energy contributes to rather slow decomposition of Zn-SR species. In this regime, loosely stabilized {100} planes would be no more favored for final facets and, instead, strongly stabilized one polar {111} and three nonpolar {112} zinc blende planes (analogue to {112} and {114} chalcopyrite planes) of the CIS core might remain at the surface with slow and epitaxial growth of ZnS shell. It is noted that a degree of blue shift in PL spectra decreases as growth temperature decreases: emission peaks at 32100
DOI: 10.1021/acsami.7b09388 ACS Appl. Mater. Interfaces 2017, 9, 32097−32105
Research Article
ACS Applied Materials & Interfaces 657 and 628 nm for 230 and 280 °C, respectively (Figure 4E,F). We ascribe this trend to the degree of cation exchange. Thermal energy would foster zinc ions to diffuse into deeper core regions, leading to the increased blue shift. Effect of Zinc Concentration. TEM images of CIS/ZnS NCs obtained with different amounts of the zinc precursor are presented in Figure 5A−D. For this series of synthesis, the
are, even though all the nanocubes were grown from the same as-prepared CIS core. In addition, given the thiol-rich reaction environment that holds zinc ions as a complex form and retards the crystal growth reaction, the shell thickness seems not to be proportional to zinc contents. Thus, the relationship between zinc content and cube size would be partially ascribed to the size of seeds before shell growth. For effects on optical properties, it was found that the emission wavelength of CIS/ ZnS NCs was dominantly controlled by the amount of zinc during the ZnS shell passivation. As shown in Figure 5E,F, CIS/ ZnS NCs of the emission ranging from 630 to 540 nm were obtained with respect to zinc amount from 0.25 to 2.00 mmol. The zinc amount could be considered as a factor that determines the degree of Zn diffusion into the core at the beginning of the reaction, resulting in the difference in the blue shift degree. When higher zinc concentration presents around the CIS core, a large driving force by the concentration gradient is exerted on zinc ions to diffuse into the core, thus, a larger reduction in effective core size with a higher degree of alloyed regions and concomitant higher blue shift in emission. As shown in Figure 6, diffraction peak positions move toward
Figure 6. XRD patterns of CIS/ZnS NCs grown with different Zn amounts. In the bottom row, red and blue lines indicate chalcopyrite CIS and zinc blende ZnS, respectively.
lattices of zinc blende ZnS from those of chalcopyrite CIS in accordance with the increment in the Zn amounts fed, accordingly implying an increasing trend in ZnS fractions of the NCs. Photoluminescence Quenching by H2O2. Shape-dependent chemical responses of CIS/ZnS NCs were studied with the mean of PL quenching by H2O2. Two NC groups, nanocube (5.7 nm, grown at 280 °C with 0.50 mmol of Zn) and nanopyramids (4.86 nm, grown at 250 °C with 0.50 mmol of Zn), were chosen for comparison. As shown in Figure 7A,B, both groups of NCs present quenched PL after applying H2O2 into the NC colloids. Compared to nanopyramids, nanocubes present more rapid and intense responses to H2O2 in terms of PL quenching. After 30 min with 1 mM of H2O2, PL intensity decreases to 13% of the pristine value for nanocubes, while to 70% for pyramids. Note that the molar concentration of NCs was estimated to be 1 μM. During the quenching process, the spectral shape of the emission is retained without a noticeable peak shift (Figure 7C), and meanwhile, aggregation of NCs accompanies the quenching process (Figure 7D). The hydrodynamic size, recorded with the dynamic light scattering (DLS) method, increased more than twice with a widened distribution for nanocubes upon the addition of H2O2, whereas a slight increment of 1.5 nm for nanopyramids ((Figure 7E). The
Figure 5. (A−D) TEM images, (E) PL spectra, and (F) photo images of CIS/ZnS NCs grown with different zinc contents: (A) 0.25 mmol, (B) 0.50 mmol, (C) 1.00 mmol, and (D) 2.00 mmol (scale bars: 10 nm).
growth temperature was fixed at 280 °C. Except for pyramidal NCs with 0.25 mmol of zinc, cube-shaped crystals were obtained regardless of zinc amount. The pyramidal shape in the lower level of zinc would be ascribed to lack of zinc ions required for further growth of ZnS. It is noted that NCs prepared with higher zinc contents (i.e., with 1.0 and 2.0 mmol) exhibit almost perfect face-to-face alignment, implying homogeneity of the faceted nature in the NC ensemble. On the other hands, the average edge length of nanocubes decreases from 5.70 nm (0.50 mmol Zn) to 5.09 nm (1.00 mmol Zn) and 4.65 nm (2.00 mmol Zn) as zinc contents increase (for size histograms of Figure 5B−D, see Figure S3 in the Supporting Information). As shown in the temporal evolution of morphology and PL (Figures 1 and 2), the surface of the core seed is etched by the zinc precursor, leading to the reduction in size. The cause of etching might be attributed to the presence of acetic acid that originated from applied zinc acetate.50 In this context, it would be deduced that the larger zinc precursor is applied, the smaller before-shell-growth seeds 32101
DOI: 10.1021/acsami.7b09388 ACS Appl. Mater. Interfaces 2017, 9, 32097−32105
Research Article
ACS Applied Materials & Interfaces
Figure 7. PL quenching of two types of NCs by H2O2 in the course of reaction time (A: cube, B: pyramid). I0 refers to the PL intensity before H2O2 exposure. Control groups are those treated with water at the same volume of H2O2. (C) PL spectra and (D) photographs taken under room and UV light of nanocubes (treated with 1000 μM H2O2 for 30 min). (E) Hydrodynamic sizes of H2O2-quenched NCs (upon the addition of 1000 μM H2O2).
Figure 8. (A) PL spectra of pristine, quenched (5 × 10−4 M H2O2), and recovered (5 μL DDT) CIS/ZnS nanocubes and (B) recovered PL intensities of nanocubes, pretreated with varying concentrations of H2O2, by excess thiol ligands.
shortened lifetime might stem from two reasons; one would be variations in contribution ratio between comprising radiative transitions, and the other is promotion of nonradiative transitions. Considering PL quenching with the maintained spectral shape in a steady-state measurement, we attribute the decay variant to the rise in the nonradiative decay. In addition to observations on spectroscopic properties and colloidal states, disulfide formation was identified, after H2O2 exposure, via chemical shift of α-proton character in 1H NMR (see Figure S5 in the Supporting Information).28,55 According to literatures, another probable reason for the PL quenching should be taken into account, that is, oxidation of surface selenium or sulfur, often observed in photoassisted PL quenching of QDs.22,23 A narrow X-ray photoelectron spectroscopy scan of the S 2p regions was conducted to check this point, and as a result, there exists no trace of oxygen-bound surface sulfur species such as
aggregates of NCs were precipitated down when stirring is halted, whose fluorescence is, indeed, quenched; it means that the probable loss of PL records by precipitated mass is likely excluded. In order to further investigate PL quenching phenomena, time-correlated single-photon counting (TCSPC) spectroscopy was conducted. Both types of NC samples were shown to have quite long PL lifetimes in the order of several hundred nanoseconds with multiexponential decay character as reported earlier on CIS/ZnS NCs for trapinvolved radiative transitions (e.g., free-to-bound transitions51,52 or donor−acceptor recombination53,54). When the NCs are exposed to 1 mM of H2O2 for 30 min, the average PL lifetime of nanocubes decreases from 408 to 331 ns, while the nanopyramid counterpart shows only 25 ns shortening (for TCSPC decay curves and corresponding fitting details, see Figure S4 and Table S1 in the Supporting Information). The 32102
DOI: 10.1021/acsami.7b09388 ACS Appl. Mater. Interfaces 2017, 9, 32097−32105
Research Article
ACS Applied Materials & Interfaces
oxidation to make a disulfide bond with an adjacent ligand molecule, relatively weak binding affinity of ligands to the given facet would be favorable for the oxidative attack. Therefore, {100}-faceted nanocubes, obtained at the high temperature regime, whose ligand-to-crystal interaction is expected to be more feeble than that of nanopyramid counterparts (i.e., {111} and {112}), are capable of responding toward H2O2 sensitvely.
sulfite or sulfate whose characteristic bands would be laid on the region of 167−169 eV (see Figure S6 in the Supporting Information). Taken altogether, the PL quenching by H2O2 seems to originate from the oxidation of thiolate ligands and resultant formation of surface defects that play a role of quenching centers. This interpretation was further supported by tracing PL recovery of the quenched NCs with additional thiol ligands. Treated with different concentrations of H2O2, all the NCs have similar PL intensity when DDT is introduced in excess, albeit with varying quenching degree along them (Figure 8). It implies that the oxidative quenching happens in a way of consuming pristine ligands from the crystal surface, and that ligand-vacant surface sites are responsible for PL quenching. Moreover, surface vacancies stemming from ligand removal can be reversely filled up by thiol ligands, leading to PL recovery. Interestingly, the control sample with minimal quenching (treated with H2O) also shows PL recovery, indicating the pristine presence of ligand-absent surface sites. Thus, oxidative PL quenching of CIS/ZnS NCs by H2O2 at the given concentration is accompanied by oxidation and detachment of pristine thiolate ligands, which is directly relevant to nonradiative decay pathways. Due to the difference in surface area of the two types of NCs, it could be claimed that this discrepancy could affect PL response toward H2O2. Assuming that a nanocube and a nanopyramid are enveloped by, respectively, six squares and four equilateral triangles, the ratio of surface area of a nanocube to a nanopyramid is approximated to be 4.8. When the concentration of nanopyramid sample was adjusted to have a total surface area similar to that of a nanocube, its PL quenching degree was somewhat diminished (for PL response with respect to NC concentration, see Figure S7 in the Supporting Information). The increase in remnant PL intensity at high concentrations of NCs indicates that, at the examined conditions, the increment of NC concentration might mean the fractional increase of luminescent centers (i.e., unreacted surface states) in the sample. After all, at this surface areacalibrated condition, the difference in sensitivity toward H2O2 became more enlarged in comparison to the particle molar concentration-calibrated condition shown in Figure 7A,B; it implies that PL response toward H2O2 is, indeed, affected significantly by the crystal-to-ligand coordination nature. Going back to the main observation of our study, sensitive responses to H2O2 of nanocubes could be understood by scrutinizing the interface between ligand and crystal surface. As adopted for the crystal growth of shaped NCs, nanocubes are faceted with a {100} plane family whose binding affinity to ligands is relatively weak, compared to a pyramid counterpart (i.e., {111} and {112}). Note that {112} planes of the zinc blende structure are nonpolar surfaces, strongly stabilized by coordinating ligands.37,39 On the basis of our analysis, it was shown that the PL quenching process originates from the oxidation of surface-bound thiolate ligands, and concomitant detachment of those ligands from the crystal surface on which dangling bonds as quenching sites are introduced. From the dramatic difference in quenching behavior between nanocubes and nanopyramids, both in intensity and rate, it would be deduced that a chemical reaction of thiolate-capped NCs with H2O2 occurs at the crystal/ligand interface, and the reaction rate relies on how strongly ligands bound to the crystallite. Because ligands bind to surface cations via coordination bonds and one thiolate ligand should be detached after one-electron
■
CONCLUSIONS We studied how thermal energy exerted at the ZnS growth stage affects the morphology of resulting CIS/ZnS NCs. With considerations of the ligand-to-crystallite interactions, highly uniform CIS/ZnS nanocubes were grown at a reaction regime of relatively higher temperature and Zn concentration for the dynamic nature of ligands on the crystal surface. In the presence of H2O2, the PL of colloidal CIS/ZnS NCs was quenched with accompanying agglomeration, which was attributed to the oxidation of pristine thiolate ligands and simultaneous detachment of these ligands from the crystal surface. It was shown that cube-shaped NCs have a more sensitive response to H2O2 than pyramid-shaped ones do. We assigned the varying trend to a relatively low binding affinity of ligands on the {100} zinc blende facet. At the same time, the PL of CIS/ZnS NCs is shown to be directly pertinent to surface passivation of ligands, which is quenched and recovered by feeding H2O2 and thiol ligands, respectively, with a preserved emission profile. We believe that these well-defined NCs will be beneficially applied for fabricating credible NC films of largearea ordering for optoelectronics, and the switching PL phenomena with the combination of H2O2 and thiol will be further utilized as a platform for molecular sensing.
■
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b09388. XRD pattern, HRTEM image, FFT diagram of CIS core, temporal change of transient PL of nanocubes and nanopyramids with H2O2 and corresponding fitting details, 1H NMR and XPS spectra of nanocubes before and after H2O2 treatment, PL quenching of NCs with respect to NC concentration (PDF)
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Youngsun Kim: 0000-0001-5219-8408 Sehoon Kim: 0000-0002-8074-1006 Duk Young Jeon: 0000-0002-9224-7769 Author Contributions
The manuscript was written through contributions of all authors. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This research was supported by the Creative Materials Discovery Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and 32103
DOI: 10.1021/acsami.7b09388 ACS Appl. Mater. Interfaces 2017, 9, 32097−32105
Research Article
ACS Applied Materials & Interfaces
(22) Derfus, A. M.; Chan, W. C. W.; Bhatia, S. N. Probing the Cytotoxicity of Semiconductor Quantum Dots. Nano Lett. 2004, 4, 11−18. (23) Kim, H.; Kwon, B.-H.; Suh, M.; Kang, D. S.; Kim, Y.; Jeon, D. Y. Degradation Characteristics of Red Light-Emitting CuInS2/ZnS Quantum Dots as a Wavelength Converter for LEDs. Electrochem. Solid-State Lett. 2011, 14, K55−K57. (24) Mancini, M. C.; Kairdolf, B. A.; Smith, A. M.; Nie, S. Oxidative Quenching and Degradation of Polymer-Encapsulated Quantum Dots: New Insights into the Long-Term Fate and Toxicity of Nanocrystals in Vivo. J. Am. Chem. Soc. 2008, 130, 10836−10837. (25) Hu, M.; Tian, J.; Lu, H.-T.; Weng, L.-X.; Wang, L.-H. H2O2Sensitive Quantum Dots for the Label-Free Detection of Glucose. Talanta 2010, 82, 997−1002. (26) Li, Y.; Li, B.; Zhang, J. H2O2- and pH-Sensitive CdTe Quantum Dots as Fluorescence Probes for the Detection of Glucose. Luminescence 2013, 28, 667−672. (27) Rodrigues, S. S. M.; Ribeiro, D. S. M.; Molina-Garcia, L.; Ruiz Medina, A.; Prior, J. A. V; Santos, J. L. M. Fluorescence Enhancement of CdTe MPA-Capped Quantum Dots by Glutathione for Hydrogen Peroxide Determination. Talanta 2014, 122, 157−165. (28) Aldana, J.; Wang, Y. A.; Peng, X. Photochemical Instability of CdSe Nanocrystals Coated by Hydrophilic Thiols. J. Am. Chem. Soc. 2001, 123, 8844−8850. (29) Manner, V. W.; Koposov, A. Y.; Szymanski, P.; Klimov, V. I.; Sykora, M. Role of Solvent-Oxygen Ion Pairs in Photooxidation of CdSe Nanocrystal Quantum Dots. ACS Nano 2012, 6, 2371−2377. (30) Miller, E. W.; Tulyathan, O.; Isacoff, E. Y.; Chang, C. J. Molecular imaging of hydrogen peroxide produced for cell signaling. Nat. Chem. Biol. 2007, 3, 263−267. (31) Ramanathan, K.; Jönsson, B. R.; Danielsson, B. Thermometric sensing of peroxide in organic media. Application to monitor the stability of RBP-retinol-HRP complex. Anal. Chem. 2000, 72, 3443− 3448. (32) Dane, E. L.; King, S. B.; Swager, T. M. Conjugated Polymers That Respond to Oxidation with Increased Emission. J. Am. Chem. Soc. 2010, 132, 7758−7768. (33) Zhao, C.; Bai, Z.; Liu, X.; Zhang, Y.; Zou, B.; Zhong, H. Small GSH-Capped CuInS2 Quantum Dots: MPA-Assisted Aqueous Phase Transfer and Bioimaging Applications. ACS Appl. Mater. Interfaces 2015, 7, 17623−17629. (34) Foda, M. F.; Huang, L.; Shao, F.; Han, H.-Y. Small GSHCapped CuInS2 Quantum Dots: MPA-Assisted Aqueous Phase Transfer and Bioimaging Applications. ACS Appl. Mater. Interfaces 2014, 6, 2011−2017. (35) Li, L.; Daou, T. J.; Texier, I.; Kim Chi, T. T.; Liem, N. Q.; Reiss, P. Highly Luminescent CuInS2/ZnS Core/Shell Nanocrystals: Cadmium-Free Quantum Dots for In Vivo Imaging. Chem. Mater. 2009, 21, 2422−2429. (36) Booth, M.; Brown, A. P.; Evans, S. D.; Critchley, K. Determining the Concentration of CuInS 2 Quantum Dots from the SizeDependent Molar Extinction Coefficient. Chem. Mater. 2012, 24, 2064−2070. (37) Zhong, H.; Lo, S. S.; Mirkovic, T.; Li, Y.; Ding, Y.; Li, Y.; Scholes, G. D. Noninjection Gram-Scale Synthesis of Monodisperse Pyramidal CuInS2 Nanocrystals and their Size-Dependent Properties. ACS Nano 2010, 4, 5253−5262. (38) Gong, F.; Tian, S.; Liu, B.; Xiong, D.; Zhao, X. Oleic Acid Assisted Formation Mechanism of CuInS2 Nanocrystals with Tunable Structures. RSC Adv. 2014, 4, 36875−36881. (39) Koo, B.; Patel, R. N.; Korgel, B. A. Synthesis of CuInSe2 Nanocrystals with Trigonal Pyramidal Shape. J. Am. Chem. Soc. 2009, 131, 3134−3135. (40) Park, J.; Kim, S.-W. CuInS2/ZnS Core/Shell Quantum Dots by Cation Exchange and their Blue-Shifted Photoluminescence. J. Mater. Chem. 2011, 21, 3745−3750. (41) Deng, D.; Chen, Y.; Cao, J.; Tian, J.; Qian, Z.; Achilefu, S.; Gu, Y. High-Quality CuInS2/ZnS Quantum Dots for In vitro and In vivo Bioimaging. Chem. Mater. 2012, 24, 3029−3037.
Future Planning (NRF-2016M3D1A1900035) and the Intramural Research Program of KIST.
■
REFERENCES
(1) Lee, S. M.; Cho, S. N.; Cheon, J. Anisotropic Shape Control of Colloidal Inorganic Nanocrystals. Adv. Mater. 2003, 15, 441−444. (2) Yin, Y.; Alivisatos, A. P. Colloidal Nanocrystal Synthesis and the Organic−Inorganic Interface. Nature 2005, 437, 664−670. (3) Yong, K.-T.; Sahoo, Y.; Swihart, M. T.; Prasad, P. N. Shape Control of CdS Nanocrystals in One-Pot Synthesis. J. Phys. Chem. C 2007, 111, 2447−2458. (4) Xia, Y.; Xiong, Y.; Lim, B.; Skrabalak, S. E. Shape-Controlled Synthesis of Metal Nanocrystals: Simple Chemistry Meets Complex Physics? Angew. Chem., Int. Ed. 2009, 48, 60−103. (5) Peng, X.; Manna, L.; Yang, W.; Wickham, J.; Scher, E.; Kadavanich, A.; Alivisatos, A. P. Shape Control of CdSe Nanocrystals. Nature 2000, 404, 59−61. (6) Manna, L.; Scher, E. C.; Alivisatos, A. P. Synthesis of Soluble and Processable Rod-, Arrow-, Teardrop-, and Tetrapod-Shaped CdSe Nanocrystals. J. Am. Chem. Soc. 2000, 122, 12700−12706. (7) Manna, L.; Scher, E. C.; Alivisatos, A. P. Shape Control of Colloidal Semiconductor Nanocrystals. J. Cluster Sci. 2002, 13, 521− 532. (8) Liu, L.; Zhuang, Z.; Xie, T.; Wang, Y.-G.; Li, J.; Peng, Q.; Li, Y. Shape Control of CdSe Nanocrystals with Zinc Blende Structure. J. Am. Chem. Soc. 2009, 131, 16423−16429. (9) Barnard, A. S.; Feigl, C. A.; Russo, S. P. Morphological and Phase Stability of Zinc Blende, Amorphous and Mixed Core−Shell ZnS Nanoparticles. Nanoscale 2010, 2, 2294−2301. (10) Kruszynska, M.; Borchert, H.; Parisi, J.; Kolny-Olesiak, J. Synthesis and Shape Control of CuInS2 Nanoparticles. J. Am. Chem. Soc. 2010, 132, 15976−15986. (11) Zhong, H.; Zhou, Y.; Ye, M.; He, Y.; Ye, J.; He, C.; Yang, C.; Li, Y. Controlled Synthesis and Optical Properties of Colloidal Ternary Chalcogenide CuInS2 Nanocrystals. Chem. Mater. 2008, 20, 6434− 6443. (12) Li, J.; Bloemen, M.; Parisi, J.; Kolny-Olesiak, J. Role of Copper Sulfide Seeds in the Growth Process of CuInS2 Nanorods and Networks. ACS Appl. Mater. Interfaces 2014, 6, 20535−20543. (13) Warner, J. H.; Cao, H. Shape Control of PbS Nanocrystals Using Multiple Surfactants. Nanotechnology 2008, 19, 305605. (14) Huynh, W. U.; Dittmer, J. J.; Alivisatos, A. P. Hybrid NanorodPolymer Solar Cells. Science 2002, 295, 2425−2427. (15) Han, L.; Liu, J.; Yu, N.; Liu, Z.; Gu, J.; Lu, J.; Ma, W. Facile Synthesis of Ultra-Small PbSe Nanorods for Photovoltaic Application. Nanoscale 2015, 7, 2461−2470. (16) Narayanan, R.; El-Sayed, M. A. Changing Catalytic Activity during Colloidal Platinum Nanocatalysis Due to Shape Changes: Electron-Transfer Reaction. J. Am. Chem. Soc. 2004, 126, 7194−7195. (17) Tian, N.; Zhou, Z.-Y.; Sun, S.-G.; Ding, Y.; Wang, Z. L. Synthesis of Tetrahexahedral Platinum Nanocrystals with High-Index Facets and High Electro-Oxidation Activity. Science 2007, 316, 732− 735. (18) Kim, M.; Kim, Y.; Hong, J. W.; Ahn, S.; Kim, W. Y.; Han, S. W. The Facet-Dependent Enhanced Catalytic Activity of Pd Nanocrystals. Chem. Commun. 2014, 50, 9454−9457. (19) Zhang, Y.; Zhu, X.; Guo, J.; Huang, X. Controlling Palladium Nanocrystals by Solvent-Induced Strategy for Efficient Multiple Liquid Fuels Electrooxidation. ACS Appl. Mater. Interfaces 2016, 8, 20642− 20649. (20) Yu, X.-Y.; Meng, Q.-Q.; Luo, T.; Jia, Y.; Sun, B.; Li, Q.-X.; Liu, J.-H.; Huang, X.-J. Facet-Dependent Electrochemical Properties of Co3O4 Nanocrystals toward Heavy Metal Ions. Sci. Rep. 2013, 3, 2886. (21) Maisano, M.; Dozzi, M. V.; Coduri, M.; Artiglia, L.; Granozzi, G.; Selli, E. Unraveling the Multiple Effects Originating the Increased Oxidative Photoactivity of {001}-Facet Enriched Anatase TiO2. ACS Appl. Mater. Interfaces 2016, 8, 9745−9754. 32104
DOI: 10.1021/acsami.7b09388 ACS Appl. Mater. Interfaces 2017, 9, 32097−32105
Research Article
ACS Applied Materials & Interfaces (42) Yu, W. W.; Peng, X. Formation of High-Quality CdS Other IIVI Semiconductor Nanocrystals in Noncoordinating Solvents: Tunable Reactivity of Monomers. Angew. Chem., Int. Ed. 2002, 41, 2368−2371. (43) Yu, W. W.; Falkner, J. C.; Yavuz, C. T.; Colvin, V. L. Synthesis of Monodisperse Iron Oxide Nanocrystals by Thermal Decomposition of Iron Carboxylate Salts. Chem. Commun. 2004, 2306−2307. (44) Li, L.; Reiss, P. One-Pot Synthesis of Highly Luminescent InP/ ZnS Nanocrystals without Precursor Injection. J. Am. Chem. Soc. 2008, 130, 11588−11589. (45) Pradhan, N.; Reifsnyder, D.; Xie, R.; Aldana, J.; Peng, X. Surface Ligand Dynamics in Growth of Nanocrystals. J. Am. Chem. Soc. 2007, 129, 9500−9509. (46) Yang, Y. A.; Wu, H.; Williams, K. R.; Cao, Y. C. Synthesis of CdSe and CdTe Nanocrystals without Precursor Injection. Angew. Chem., Int. Ed. 2005, 44, 6712−6715. (47) De Trizio, L.; Prato, M.; Genovese, A.; Casu, A.; Povia, M.; Simonutti, R.; Alcocer, M. J. P.; D’Andrea, C.; Tassone, F.; Manna, L. Strongly Fluorescent Quaternary Cu−In−Zn−S Nanocrystals Prepared from Cu1‑xInS2 Nanocrystals by Partial Cation Exchange. Chem. Mater. 2012, 24, 2400−2406. (48) Nam, D.-E.; Song, W.-S.; Yang, H. Facile, Air-Insensitive Solvothermal Synthesis of Emission-Tunable CuInS2/ZnS Quantum Dots with High Quantum Yields. J. Mater. Chem. 2011, 21, 18220− 18226. (49) Nam, D. E.; Song, W. S.; Yang, H. Noninjection, One-Pot Synthesis of Cu-Deficient CuInS2/ZnS Core/Shell Quantum Dots and their Fluorescent Properties. J. Colloid Interface Sci. 2011, 361, 491− 496. (50) Ryu, E.; Kim, S.; Jang, E.; Jun, S.; Jang, H.; Kim, B.; Kim, S.-W. Step-Wise Synthesis of InP/ZnS Core−Shell Quantum Dots and the Role of Zinc Acetate. Chem. Mater. 2009, 21, 573−575. (51) Li, L.; Pandey, A.; Werder, D. J.; Khanal, B. P.; Pietryga, J. M.; Klimov, V. I. Efficient Synthesis of Highly Luminescent Copper Indium Sulfide-Based Core/Shell Nanocrystals with Surprisingly Long-Lived Emission. J. Am. Chem. Soc. 2011, 133, 1176−1179. (52) Knowles, K. E.; Hartstein, K. H.; Kilburn, T. B.; Marchioro, A.; Nelson, H. D.; Whitham, P. J.; Gamelin, D. R. Luminescent Colloidal Semiconductor Nanocrystals Containing Copper: Synthesis, Photophysics, and Applications. Chem. Rev. 2016, 116, 10820−10851. (53) Tran, T. K. C.; Le, Q. P.; Nguyen, Q. L.; Li, L.; Reiss, P. TimeResolved Photoluminescence Study of CuInS2/ZnS Nanocrystals. Adv. Nat. Sci.: Nanosci. Nanotechnol. 2010, 1, 025007. (54) Kim, H.; Suh, M.; Kwon, B. H.; Jang, D. S.; Kim, S. W.; Jeon, D. Y. In Situ Ligand Exchange of Thiol-Capped CuInS2/ZnS Quantum Dots at Growth Stage without Affecting Luminescent Characteristics. J. Colloid Interface Sci. 2011, 363, 703−706. (55) Dasog, M.; Scott, R. W. J. Understanding the Oxidative Stability of Gold Monolayer-Protected Clusters in the Presence of Halide Ions under Ambient Conditions. Langmuir 2007, 23, 3381−3387.
32105
DOI: 10.1021/acsami.7b09388 ACS Appl. Mater. Interfaces 2017, 9, 32097−32105
