Tetragonal–Orthorhombic–Cubic Phase Transitions in Ag2Se

Sep 23, 2014 - ABSTRACT: First-order solid−solid phase transition of crystalline solids ... nanoscale has attracted an increasing interest in solid-...
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Tetragonal−Orthorhombic−Cubic Phase Transitions in Ag2Se Nanocrystals Junli Wang,*,†,‡ Weiling Fan,† Juan Yang,‡ Zulin Da,† Xiaofei Yang,‡ Kangmin Chen,‡ Huan Yu,† and Xiaonong Cheng*,†,‡ †

Scientific Research Academy and School of Chemistry & Chemical Engineering, and ‡School of Materials Science & Engineering, Jiangsu University, Zhenjiang 212013, P. R. China S Supporting Information *

ABSTRACT: First-order solid−solid phase transition of crystalline solids at the nanoscale has attracted an increasing interest in solid-state physics and chemistry, which can be used to alter the properties of materials without changing chemical compositions. Herein, we report the results of our comparative studies on phase transitions between tetragonal (t), orthorhombic (β), and cubic (α) polymorphs in Ag2Se nanocrystals. A significant discrepancy in stability and phase transition behavior is determined for t-Ag2Se nanocrystals, which were prepared separately by two different methods. Differential scanning calorimetry (DSC) and variable-temperature XRD studies reveal that the t-Ag2Se nanocrystals prepared by the oleylamine (OLA)-mediated method show a highly temperature- and time-sensitive metastability and undergo a t → β → α → β phase transition during the thermal cycling, in which the t → β transition is exothermic and irreversible, whereas the β → α transition is reversible. Similarly, the reversible β → α structure transition is detected in the β-Ag2Se nanocrystals, which were also prepared using the OLA-mediated method with different post-treatment manners and stabilized conditions. In contrast, the t-Ag2Se nanocrystals prepared by the PVP-assisted solvothermal method are more stable and exhibit a direct, reversible t → α phase transition without undergoing the β phase; however, when heated to a high temperature, for example, ≥250 °C, the stability of the t phase and the reversibility of the t → α transition will be destroyed due to the sintering and size increase of the sample, which is confirmed by the determination of the t → α → β phase transition in the DSC cycle. The formation of the t phase is attributed to the α → t structure transformation with the temperature cooled from synthetic temperatures (160−220 °C) to room temperature. Moreover, the reasons for the difference in the stabilities and phase transitions of t-Ag2Se nanocrystals prepared in our two methods are discussed based on the influences of size, surface (or shape), and defects on the thermodynamics and kinetics of a solid−solid structure transformation.

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Besides the β phase, Ag2Se can stably exist in another lowtemperature polymorphic structure, namely, the tetragonal phase (t-Ag2Se), which has been usually found within nanosized Ag2Se grains (nanocrystals) or the thin films composed of small polycrystalline Ag2Se.16−23 However, the lattice parameters or interplanar spacings (d values) for t-Ag2Se, characterized by various techniques, such as X-ray diffraction (XRD), electron diffraction (ED), and high-resolution transmission electron microscopy (HRTEM), are controversial or even contradictory. For instance, Boettcher et al. reported that there are four tetragonal phases with different lattice parameters in the Ag2Se thin films.16 Meanwhile, tetragonal t-Ag2Se was mistaken for the orthorhombic β-Ag2Se or indexed to a superlattice structure of β-Ag2Se (this point will be discussed later).24,25 Moreover, the difficulty in the synthesis and the lack of clarity in the stabilized conditions of pure t-Ag2Se, as well as

INTRODUCTION

In recent years, the first-order solid−solid phase transition of crystalline solids at the nanoscale has drawn growing interest in solid-state physics and chemistry1−7 and is of much importance to the studies on the phase-dependent physical and chemical properties.5−7 Silver selenide (Ag2Se) is known for two kinds of stable crystalline polymorphs, i.e., the narrow band gap semiconducting orthorhombic phase (β-Ag2Se) and the superionic conducting cubic phase (α-Ag2Se). β-Ag2Se is stable at low temperature and will transform to the high-temperature α-Ag2Se with a transition temperature around 133−140 °C.6−9 This is a reversible first-order phase (or structure) transition, which will lead to an abrupt change in a series of physical and chemical properties and thus can be used to adjust these properties without changing chemical compositions, such as electronic/electrical conductivity (or resistance), thermal conductivity, and ionic conductivity. Therefore, in terms of its different structural phases, Ag2Se can be selected as a good candidate for resistance switching/nonvolatile memory,6,10,11 thermoelectric materials,7,12,13 and solid electrolyte.9,14,15 © 2014 American Chemical Society

Received: June 26, 2014 Revised: September 20, 2014 Published: September 23, 2014 5647

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sealed, and heated at 160−220 °C for 3−12 h. The precipitates of tAg2Se nanocrystals were collected by centrifugation/washing with ethanol four times and then dried in vacuum at 45 °C for further characterization. Characterization. Room-temperature X-ray powder diffraction (XRD) was conducted on a Bruker D8 Advance X-ray diffractometer with Cu Kα radiation (λ = 1.54184 A) or on an X’pert Pro MFD X-ray diffractometer (PANalytical) with Cu Kα radiation (λ = 1.5418 A). The variable-temperature XRD data were collected using an X’pert Pro MFD X-ray diffractometer in the temperature range of 25−175 °C and the 2θ range of 10−80°, which was carried out under a voltage/current of 40 kV/40 mA, a step size of 0.02°, and a scan rate of 3°(2θ)/min for a specific temperature point. A Pt heating filament, N2 protection, and heating rate of 5 °C/min were used in the variable-temperature XRD analysis. The sample was maintained at each specific temperature for 5 min before the XRD data started to be collected. Differential scanning calorimetry (DSC) was carried out on a NETZSCH DSC 204 under a heating/cooling rate of 5 °C/min and Ar protection. The weight of each Ag2Se sample ranged from 10 to 20 mg during DSC analysis, and liquid N2 was used for the cooling process. Transmission electron microscopy (TEM) images were recorded on a JEM 2010 microscopy (JEOL, Japan) at 200 kV, equipped with an X-ray energy-dispersive spectrometer (EDS, Oxford Inca) for elemental analysis.

its metastability, boost the difficulty in studying and elucidating the phase transition behavior of t-Ag2Se with temperature. Recently, it has been reported that pure Ag2Se nanocrystals in the low-temperature t phase, typically with a set of lattice parameters of a = b = 7.06 A and c = 4.98 A,16−20 were prepared by several different methods,22,23,26−28 making it accessible to investigate phase transitions of the t phase. Our group and Norris et al. have shown that t-Ag2Se nanocrystals, which were synthesized, respectively, by a PVP-assisted solvothermal synthesis26,27 and a TOP-assisted colloidal synthesis,28 exhibit a direct t → α structure transition at a relatively low temperature of 101−107 °C,26−28 compared to the β → α phase transition temperature (133−140 °C).6−9 In our latest study, it is interesting to find that Ag2Se nanocrystals with low-temperature t and β crystal structures could be separately prepared in an oleylamine (OLA)-mediated synthesis by controlling the post-treatment manners and stabilized conditions of samples. In particular, the resultant t-Ag2Se nanocrystals were found to have the same d values (or lattice parameters) as those prepared by the PVP-assisted solvothermal method, but display an obvious metastability, which often causes researchers to ignore the existence of the t phase,23 and a significant difference in phase transition behavior. Herein, we report these findings and show our comparative studies on the first-order structure transitions between t, β, and α phases for Ag2Se nanocrystals, thus offering a valuable reference for those who investigate the polymorph, structure transition, and physical properties in the Ag2Se system, specifically of nanoscale crystallites or thin films.

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RESULTS AND DISCUSSION

The selective preparation of t-Ag2Se and β-Ag2Se nanocrystals is briefly described as follows (see details in the Experimental Section). First, Ag2Se nanocrystals (with an average diameter of ∼44 nm, extracted from TEM, and a high purity, Figure S1 in the Supporting Information) were synthesized from the reaction of AgNO3 with SeO2 in oleylamine (OLA) at 180 °C for 1 h in a two-neck flask under N2 protection, and as the reaction was naturally cooled to room temperature (RT, ≤25 °C), the precipitates of Ag2Se nanocrystals were extracted by centrifugation/washing treatment three or four times with hexane or toluene. The crystal structures of Ag2Se nanocrystals are dependent on the following post-treatments and stabilized conditions. As collected by centrifugation/washing, the precipitates of Ag2Se nanocrystals, which could be well dispersed and kept in the above organic solvents at RT, are tetragonal (t-Ag2Se), whereas Ag2Se nanocrystals are converted to be orthorhombic (β-Ag2Se) after a drying treatment at 40− 60 °C for 8−24 h. Such a structure change was confirmed by RT powder XRD, and as shown in Figure 1, two different diffraction patterns are detected and are well indexed to the β and t crystal phases of Ag2Se, respectively. From the d values (see Figures S2 and S3, Supporting Information), β-Ag2Se has lattice parameters consistent with the standard JCPDS card (No. 24-1041, a = 4.333 Å, b = 7.062 Å, and c = 7.764 Å) and literature values;7,8,24 t-Ag2Se shows a set of lattice parameters that are almost identical to the previously reported data of a = b = 7.06 Å and c = 4.98 Å.16−18,23 As compared, it is found that, although the intensities of some peaks may be slightly different, the XRD pattern of these t-Ag2Se nanocrystals prepared in OLA matches well with that detected for t-Ag2Se nanocrystals we recently prepared by a PVP-assisted solvothermal method26,27 (average diameter: ∼125 nm, extracted from TEM, Figure S4, Supporting Information; refer to the Experimental Section). However, as shown below, the different stabilities and phase transition behavior would be detected in these t-Ag2Se nanocrystals obtained from the two such preparation methods. The change in the heat energy (latent heat) is usually involved in first-order phase transition, and differential scanning calorimetry (DSC) is used as an effective technique for the

EXPERIMENTAL SECTION

Synthesis of Tetragonal (t) or Orthorhombic (β) Ag2Se Nanocrystals by Oleylamine-Mediated Method. All the reagents were of analytic grade purity and used as received without further purification. Ag2Se nanocrystals were prepared by an oleylamine (OLA)-mediated route. Typically, 8 mL of OLA (Aladdin, 80−90%, technical grade) and 0.030 g of SeO2 (0.27 mmol) were loaded into a two-neck flask. The flask was heated to 180 °C and kept at this temperature for 5 min under magnetically stirring and N2 protection. This process can dissolve and reduce SeO2 (to Se) in the OLA solution. Then, 0.034 g of AgNO3 (0.2 mmol) was quickly added into the flask and maintained at 180 °C for 1 h. As the reaction was naturally cooled to room temperature (RT), the precipitates of Ag2Se nanocrystals were extracted by centrifugation/washing treatment three or four times with small molecular organic solvents, such as hexane and toluene. The synthetic temperature could be fixed at 160−220 °C with a reaction time ranging from 3 to 1 h in our work. The above-obtained precipitates of Ag2Se nanocrystals are in the tetragonal (t) crystal structure, which could be stably dispersed and kept in the above organic solvents at room temperature for several days (at least 3 days) or could be kept unchanged after they naturally dried at RT for at least 8 h. For the XRD measurement, the t-Ag2Se nanocrystal dispersion was dropped onto a sample holder or a thin glass slide and then naturally dried at RT. The t crystal structure is very sensitive to temperature and time. After the precipitates of Ag2Se nanocrystals were dried at 40−60 °C for 8−24 h, t-Ag2Se nanocrystals were converted to orthorhombic β-Ag2Se nanocrystals. Synthesis of Tetragonal (t) Ag2Se Nanocrystals by PVPAssisted Solvothermal Method. This synthesis was conducted by using a modification of the solvothermal method proposed by our group.26,27 In a typical procedure, 0.25 g of polyvinyl pyrrolidone (PVP K-30, average MW = 40 000), 0.030 g of SeO2 (0.27 mmol), and 0.034 g of AgNO3 (0.2 mmol) were successively added and dissolved into 10 mL of dimethyl formamide (DMF) under stirring. Then, 5 mL of oleic acid was added. With 5−10 min of stirring, the solution was loaded into a Teflon-lined stainless-steel autoclave of 18 mL capacity, 5648

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Figure 1. Room-temperature XRD patterns of (a) β-Ag2Se nanocrystals and (b) t-Ag2Se nanocrystals. The two samples are both prepared by the OLA-mediated method with different post-treatments and stabilized conditions (see the Experimental Section).

study of phase transitions in Ag2Se nanocrystals between different polymorphic structures. The DSC thermal cycling in this work was carried out in the temperature range of 0−180 °C, unless otherwise noted. For the β-Ag2Se nanocrystals prepared by the OLA-mediated method, the DSC cycle shows an endothermic peak at 137.1 °C (latent heat: −22.24 J/g) during heating and an exothermic peak at 106.5 °C (24.58 J/g) during cooling (Figure 2a), which indicates that the crystal structure of Ag2Se nanocrystals converts from β to α phase at 137.1 °C and then returns to the β phase at 106.5 °C. Along with the DSC results taken on four additional samples, the asprepared β-Ag2Se nanocrystals undergo the β → α structure transition around 135−139 °C and the α → β structure transition around 102−110 °C. These as-measured transition temperatures are in good agreement with the reported values for bulk or nanoscale Ag2Se.7−9 As expected, the β−α transition is reversible, which is conveyed in the second DSC cycle for βAg2Se nanocrystals (Figure 2a). Figure 2b shows the results of DSC analyses taken on the tAg2Se nanocrystals prepared by the OLA-mediated method. In the heating scan, an exothermic peak at 64.1 °C (11.15 J/g) (our DSC studies, along with the results from four additional samples, show that the position for this exothermic peak ranges from 60 to 67 °C with a latent heat of 9.91−12.84 J/g) and an endothermic peak at 135.9 °C (−21.80 J/g) are detected, while only one exothermic peak at 104.3 °C is detected in the cooling scan. Surprisingly, the exothermic peak at 64.1 °C disappears from the heating scan in the second DSC cycle, but the other two peaks are kept with a small shift in the locations (136.0 and 105.7 °C). It is considered that the peak at 64.1 °C is caused by the t → β structure transformation, which is not reversible (irreversible), and that the other two peaks are due to the reversible transitions between β and α phases. By contrast, the DSC heating/cooling curves recorded on the t-Ag2Se nanocrystals prepared by the PVP-assisted solvothermal method are shown in Figure 2c. The endothermic peak at 107.6 °C (−11.73 J/g) is due to the direct t → α phase transformation, and the two exothermic peaks at 87.8 °C (strong, 11.50 J/g) and 61.5 °C (weak, 0.536 J/g) are due to the transitions of α to t phase, as our group26,27 and Norris et al.28 reported recently. Almost identical endothermic and exothermic peaks are measured in the second DSC cycle,

Figure 2. DSC heating/cooling cycles of (a) β-Ag2Se nanocrystals prepared by the OLA-mediated method, (b) t-Ag2Se nanocrystals prepared by the OLA-mediated method, and (c) t-Ag2Se nanocrystals prepared by the PVP-assisted solvothermal method.

indicating that a reversible t−α phase transition occurs in the tAg2Se nanocrystals prepared by the PVP-assisted solvothermal method. The more detailed temperature-dependent structure transformations in t-Ag2Se nanocrystals prepared by the OLAmediated method were characterized by variable-temperature XRD. As displayed in Figure 3, the changes and assignments of the XRD patterns obtained at different temperatures confirm that, with the temperature increasing, the t-Ag2Se nanocrystals convert to the β phase and then to the high-temperature α phase (body-centered cubic (bcc) structure, a = 4.98 A, JCPDS No. 27-0619),16−20 and also that, as the temperature decreased to RT, α-Ag2Se transforms to β-Ag2Se, not to the initial tAg2Se. Clearly, the results from variable-temperature XRD and DSC measurements are consistent, revealing that, during the thermal cycle, a t → β → α → β structure transformation process occurs in the t-Ag2Se nanocrystals prepared by the OLA-mediated method. The t phase in Ag2Se nanocrystals prepared by the OLAmediated method is typically metastable, which is sensitive to temperature and time. Our studies show that it can be maintained at RT for a couple of days when dispersed in small molecular organic solvents, such as hexane and toluene, or for at least 8 h after it naturally dried at RT. When dried at 40−60 °C for 8−24 h, the t structure easily, completely transforms to 5649

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phases and hence the XRD patterns of Ag2Se nanocrystals. We noted that the as-obtained Ag2Se nanocrystals were dried in a vacuum at 60 °C for 6 h before XRD was carried out in Xie’s work and displayed the low-temperature β phase.7 The above drying treatment at 60 °C, which is not mentioned in Li’s work,29 may induce the t → β phase transition and finally produce the β-Ag2Se nanocrystals. For the t-Ag2Se nanocrystals prepared by the PVP-assisted solvothermal method (e.g., the sample prepared at 200 °C for 10 h), we also found that heating up to a high temperature (for example, 250 °C) in the DSC heating process will make the t−α phase transition irreversible; that is, the nanocrystals undergo a t → α → β structure transformation. Figure 4 shows

Figure 3. Temperature-dependent XRD patterns of t-Ag2Se nanocrystals prepared by the OLA-mediated method.

the β form. Such a structure transformation enables the production of β-Ag2Se nanocrystals. However, the t phase in Ag2Se nanocrystals prepared by the PVP-assisted solvothermal method could be stable for a longer time (typically longer than 1 week) at RT after it naturally dried at RT or even dried at 40−60 °C for 5 h; after a much longer duration (i.e., 3 weeks), part of them will transform to β-Ag2Se (Figure S5, Supporting Information). It is shown in Figure 3 that a mixture of t and β phases at 45 °C and a mixture of β and α phases at 125 °C are, respectively, detected in the variable-temperature XRD studies. This indicates that part of t-Ag2Se has converted to β-Ag2Se at 45 °C and part of β-Ag2Se to α-Ag2Se at 125 °C. These transition temperatures are slightly lower than the values measured by DSC (60−67 °C and 135−139 °C). Such a discrepancy is also reported by some previous studies7,26−28 and may be due to the longer time required in the XRD analyses28 (28 min for each temperature point in our work). The mixture of low-temperature t and β phases will increase the difficulty and complexity or even cause confusion or mistakes in the characterization of crystal structures of Ag2Se. For example, the XRD lines from t-Ag2Se were mistaken for those from the β phase25 or indexed to a superlattice structure of the β phase (discussed later).24 Similarly, the high metastability and flexibility in t-Ag2Se may also lead to confusion or incorrectness in indexing crystal structures of Ag2Se. For example, Ag2Se nanocrystals could be synthesized using an octadecylamine (ODA)-mediated approach and were reported separately by Xie’s and Li’s groups to show the lowtemperature β phase.7,29 Then, Norris et al.23 pointed out that the XRD pattern in Li’s work matched better with lowtemperature t-Ag2Se (a = b = 0.706 nm and c = 0.498 nm) than with β-Ag2Se. It is believed that different post-treatment methods and stabilized conditions (e.g., time and temperature) in these two reports caused the discrepancy in the crystal

Figure 4. DSC heating/cooling cycles of t-Ag2Se nanocrystals prepared by the PVP-assisted solvothermal method. Both thermal cycles were shown between 20 and 240 °C, but actually performed between 0 and 250 °C.

this result. In the first thermal cycle (in which the temperature range is shown in 20−240−20 °C, but actually measured in 0− 250−0 °C), the endothermic peak appearing in the heating scan is loaded at 107.8 °C, which is consistent with the values for the t → α phase transition in Figure 2c. During the cooling scan, only one exothermic peak with a high temperature position (104.7 °C) is detected, instead of two peaks at 87.8 and 61.5 °C. The temperature value for this peak is well comparable to the α → β transition temperatures as measured in Figure 2a,b. At the same time, this exothermic peak possesses a larger peak area (i.e., latent heat: 24.78 J/g) than the endothermic peak (−11.62 J/g) appearing in the heating scan, which is also comparable to the value measured for the α → β structure transition. Therefore, we assign the exothermic peak loaded at 104.7 °C in the cooling scan to the α → β phase transition, which means that, during cooling from 250 to 0 °C, the high-temperature α phase converts to the β form, not the pristine t form. As displayed in the second DSC thermal cycle (0−250−0 °C, Figure 4), an endothermic peak at 139.3 °C during heating and an exothermic peak at 106.2 °C during cooling are, respectively, detected, which are in good agreement with the characteristic features of phase transition between β and α structures of Ag2Se and thus further confirm the t → α → β structure transition occurring in the first DSC cycle. The formation of the low-temperature t phase in Ag2Se nanocrystals is here attributed to the structure transition of high-temperature α phase with the temperature decreasing to 5650

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that a much smaller size is probably formed in the heat-treated coprecipitation-grown Ag2Se sample and then plays an important part in the formation of the superlattice (tetragonal) structure. Obviously, the melt-grown Ag2Se is a bulk, extended solid with a large size and tends to be stable in the β phase. This size-related structural stability can be used to interpret the t → α → β structure transformation of t-Ag2Se nanocrystals prepared from the PVP-assisted solvothermal method (Figure 4). Upon heating the sample to a temperature ≥ 250 °C in the DSC cycle, the small size and good dispersity of t-Ag2Se nanocrystals will be damaged and a sintered (or agglomerated) Ag2Se of an increased size is yielded. This is confirmed in our work by the fact that the sintered Ag2Se sample cannot be redispersed in hexane by sonication, even with a long-time sonication. The sintering and size increase in Ag2Se make the α phase convert to the β phase, rather than to the initial t phase, upon the temperature decreasing. In comparison with Norris’s study,28 the t-Ag2Se nanocrystals (44 nm) prepared in OLA in our work have an average diameter much larger than the t-Ag2Se nanocrystals, respectively, prepared in octadecylamine (ODA, 10.8 nm), hexadecylamine (HDA, 9.4 nm), and a mixture of OLA, TOP, and TOPO (8.6 nm) in Norris’s work. Surprisingly, the 10.8, 9.4, and 8.6 nm t-Ag2Se nanocrystals exhibited the stability and phase transition behavior (based on the DSC results in Norris’s work28) similar to the 125 nm t-Ag2Se nanocrystals prepared in the PVP-assisted solvothermal synthesis, but greatly varied from the 44 nm t-Ag2Se nanocrystals prepared in OLA; that is, they are more stable than the t-Ag2Se nanocrystals prepared in OLA and undergo the t → α phase transition at a higher temperature around 101−104 °C.28 These temperatures are a little lower than the values (104−109 °C) we measured for the t → α phase transition in t-Ag2Se nanocrystals synthesized by the PVP-assisted solvothermal method. In all of these syntheses of t-Ag2Se nanocrystals, capping agents were used. On the one hand, they ensure the small nanoparticle size and thus stabilize Ag2Se nanocrystals in the t structure. On the other hand, they can exhibit different binding ability between the t-Ag2Se surface and ligand atoms due to their different ligand atoms (e.g., O, N, P) and spatial structures. A strong binding capability may hinder atomic displacement to some degree and enhance the stability of a certain crystal phase. Maybe, OLA has a binding ability weaker than PVP, ODA, HDA, TOP and/or TOPO, which could explain why t-Ag2Se nanocrystals prepared by the OLA method show a weaker stability than those prepared by the PVP-assisted solvothermal method and Norris’s method using ODA, HDA, TOP, and/or TOPO. However, to our knowledge, no direct evidence or study has been reported to show that the capping ability of PVP, ODA, HDA, TOP, or TOPO is stronger than that of OLA. Furthermore, Norris et al. pointed out that the phase transition did not depend on variations of the surface ligands (at least for the molecules that they studied).28 Phase transition is closely associated with the total energy (including internal energy and surface energy) and the energy barrier for a structural transition in a nanocrystalline solid.1,30,34−36 For the 44 nm t-Ag2Se nanocrystals produced in OLA, it is considered that the total energy in them is comparable to or higher than the energy barrier of the t → β phase transition in Ag2Se at RT or the temperatures slightly higher than RT (40−60 °C or 60−67 °C detected by DSC; a relatively high temperature is detected in DSC for the phase transition due to the shorter time), and thus they easily

RT. All the temperatures used for the Ag2Se nanocrystal syntheses in our two methods, namely, 160−220 °C, are higher than the β → α or t → α phase transition temperatures of Ag2Se (133−140 °C for the former and 101−109 °C for the latter; these data are extracted from the literature7−9,26−28 and our current work). Under these synthetic temperatures, Ag2Se crystallizes in its high-temperature α phase. Upon the temperatures decreasing to RT, α-Ag2Se will structurally convert to its low-temperature phases. Nanoscale solid materials may often exhibit crystal structures different from thermodynamically stable phases observed in their bulk counterparts.30 For the Ag2Se nanocrystals synthesized using our two different approaches, it is seen that α-Ag2Se tends to convert to the low-temperature t phase, rather than the conventional low-temperature β phase. Several reasons can be employed to explain such a structure transition. One reason is the high monodispersity and nanometer particle size of the asprepared nanocrystals, which can constrain a nanometer sized solid in a metastable phase.22,26,28,30−33 Second, t-Ag2Se nanocrystals may have a lower surface energy than β-Ag2Se nanocrystals when the particle size and surface environment are equivalent, which favors the structure transition from α to t phase and the formation of the t phase with the temperature cooled to RT, for the low surface energy can reduce the total energy of a metastable structure.30 The simpler relation in crystal lattices of α-Ag2Se with t-Ag2Se (at = 21/2aα, ct = aα)17,18 than with β-Ag2Se20 may be another reason for the α → t phase transition. In addition, the presence of capping ligands (i.e., OLA, PVP, TOP, and ODA)23,26−29 that are bound to the surface atoms of nanocrystals can also play a role in stabilizing the t phase in Ag2Se nanocrystals at RT. At the same time, it is confirmed that t-Ag2Se nanocrystals could also be synthesized at 120 °C by either the OLAmediated method or the PVP-assisted solvothermal method, which display the almost same XRD patterns and DSC curves (Figures S6 and S7, Supporting Information) as the samples were synthesized at 160−220 °C. Although this synthetic temperature (120 °C) is lower than the β → α phase transition temperatures (133−140 °C), we consider that, during growing at 120 °C, Ag2Se nanocrystals still crystallize in the α phase, not the β phase, due to the small particle size effect.22,31−33 The α phase will transform to the low-temperature t polymorph when the temperature changes to RT on the basis of the abovementioned several reasons. In a previous study, Miki reported that a superlattice structure of the β-Ag2Se phase was formed after the heat treatment of coprecipitation-grown Ag2Se at 150−300 °C.24 It is noted that the d values of the XRD lines and the transition temperature (105 °C, detected using differential thermal analysis, DTA) for the superlattice structure are in good agreement with the values we measured for the t-Ag2Se nanocrystals prepared by the PVP-assisted solvothermal method. On the basis of these similarities, it is better to index such a superlattice structure to the tetragonal (t) phase of Ag2Se (a = b = 0.706 nm and c = 0.498 nm). It is considered that the heat treatment at 150−300 °C will enable the coprecipitation-grown Ag2Se to be stable in the high-temperature α phase and that the formation of the superlattice (tetragonal) structure is the result of the α → t phase transition as the heat treatment temperature decreases to RT. Moreover, in Miki’s report, the superlattice (tetragonal) structure was only found in the heat-treated coprecipitation-grown Ag2Se, but not in the melt-grown Ag2Se prepared above 1000 °C.24 We think 5651

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(104−109 °C). The existence of a considerable number of lattice defects probably has a significant or even crucial effect on the decrease of the phase transition temperature, besides the effects arising from crystal structures.28 In a first-order solid− solid phase transition, the new phase prefers to nucleate at the defect sites and then propagates into the whole of a nanoscale or bulk solid with requiring a relatively low energy or temperature.1,34 In this respect, defects can not only stabilize the t phase but also lower the transition temperature (or energy barrier) of the t → α structure transformation. Much work is needed to investigate and elucidate the microscopic kinetic mechanism/trajectory at the atomic level and the effects of crystalline defects for the t → α phase transition in the faceted t-Ag2Se nanocrystals.

transform to the more stable, lower energy β structure through releasing heat (confirmed by DSC, Figure 2b). Such an exothermic phase transition is a thermodynamically favored process. It is known that the size, surface nature (shape), and defect are important factors that can influence the thermodynamics and kinetics of a first-order solid−solid phase transition;1,3,30,34−36 especially, the lattice defects can effectively stabilize a metastable structure.1,3,34 For the 10.8, 9.4, and 8.6 nm t-Ag2Se nanocrystals prepared in Norris’s work, the small size will lead to a large number of defects on the nanocrystal surface (e.g., the unsaturated Ag−Se bonds). For the t-Ag2Se nanocrystals prepared by the PVP-assisted solvothermal method in our work, a comparative study has indicated that the influence of high pressure, generated from the solvothermal reaction in a sealed autoclave,37 on the stability of the t phase can be negligible (Figure S8, Supporting Information); we consider that the existence of a considerable number of lattice defects influences their stability and structure transformation. Briefly, the lattice defects stabilize the t phase by kinetically inhibiting the structure transformation of t-Ag2Se to β-Ag2Se, although the total energy in these t-Ag2Se nanocrystals is probably sufficient to overcome the energy barrier of the t → β structure transition. From the view of latent heat of phase transition, these t-Ag2Se nanocrystals prepared by the PVPassisted solvothermal method may have a total energy comparable to the t-Ag2Se nanocrystals prepared in OLA.38 There are several signs that are supportive of the existence of crystalline defects in the t-Ag2Se nanocrystals prepared by the PVP-assisted solvothermal method. First, some of the lattice defects, like stacking faults, are determined directly by HRTEM analysis (Figure S4d). Second, the breaking of the stability of the t phase and its conversion to the β structure after being heated up to 250 °C, as detected by DSC analyses shown in Figure 4, may be a result of the ruling out of defects34 from tAg2Se nanocrystals in addition to the sintering and size increase of the sample, which is, in turn, indicative of the existence of lattice defects in t-Ag2Se nanocrystals. Thirdly, it is clearly observed by TEM in Figure S4 that these t-Ag2Se nanocrystals are in a faceted (or polyhedral) shape, whose formation may be often induced by lattice defects (such as twins and stacking faults).39 At the synthetic temperature of 160−220 °C (even at 120 °C; such a low temperature can still produce the faceted tAg2Se nanocrystals, Figure S9, Supporting Information), Ag2Se is in its bcc-structured α phase. Therefore, the faceted t-Ag2Se nanocrystals are derived from the faceted α-Ag2Se nanocrystals. The anisotropic faceted shape is easily formed when twins or stacking faults are preformed with the assistance of capping agents, like PVP.39 Along with the shape being transferred, these lattice defects will be partly or wholly transferred from αAg2Se to t-Ag2Se nanocrystals with the temperature falling to RT. Fourthly, some new crystalline defects (e.g., stacking faults, atomic vacancies, interstices, and dislocations) may be also introduced into the t-Ag2Se nanocrystals during the collective atomic displacements from the α to t phase. In addition, the faceted shape will lead to many interfacial boundaries (junctures) between different crystal facets, at which atomic vacancies, interstices, and/or dislocations are easily generated. On the other side, the t → α phase transition is a thermodynamically driven process through absorbing heat (Figure 2c). Compared to the common β → α phase transition (133−140 °C), the t → α phase transition of the as-prepared tAg2Se nanocrystals occurs at a relatively low temperature

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CONCLUSIONS In conclusion, we have comparatively studied the phase transitions between tetragonal (t), orthorhombic (β), and cubic (α) polymorphic structures of Ag2Se nanocrystals. A particular attention is paid to the stabilities and structure transformations of t-Ag2Se nanocrystals, which could be separately prepared through two different approaches. The tAg2Se nanocrystals prepared by the OLA-mediated method are highly metastable and easily transform to the β phase irreversibly and exothermically, followed by a structure transition to the α phase, while the high-temperature α phase is cooled to the low-temperature stable β phase, not to the initial t-Ag2Se phase, during the DSC heating/cooling cycle. It is found that the OLA-mediated method could also be used to prepare β-Ag2Se nanocrystals, which show a reversible β−α phase transition with transition temperatures as reported in the literature. On the other hand, the t-Ag2Se nanocrystals prepared by the PVP-assisted solvothermal method are more stable and show a direct, reversible structure transformation between t and α phases, but a high temperature ≥ 250 °C will damage the stability of the t phase, leading to the occurrence of the t → α → β phase transition. The yield of the low-temperature t phase is considered to be derived from the α → t structure transition as the temperature was cooled from reaction temperatures (160−220 °C) to RT. Furthermore, from the aspects of the effects of size, surface (shape), and crystalline defects on structural transitions of a nanocrystalline solid, we discussed and analyzed the origins of the difference in the stability and phase transition of t-Ag2Se nanocrystals prepared by our two methods. It is expected that our understanding and clarification of the stabilities and structure transitions of polymorphic phases in nanoscale Ag2Se would be of much use for studying and adjusting phase-dependent physical and chemical properties.

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ASSOCIATED CONTENT

* Supporting Information S

Figures S1−S9: TEM images, EDS spectrum, XRD patterns marked with detailed d values, and additional DSC data for Ag2Se nanocrystals. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

* E-mail: [email protected], [email protected] (J.W.). *E-mail: [email protected] (X.C.). 5652

dx.doi.org/10.1021/cm502317g | Chem. Mater. 2014, 26, 5647−5653

Chemistry of Materials

Article

Notes

(31) Wang, S.; Hu, B.; Liu, C.; Yu, S. H. J. Colloid Interface Sci. 2008, 325, 351−355. (32) Tadanaga, O.; Koide, Y.; Hashimoto, K.; Oku, T.; Teraguchi, N.; Tomomura, Y.; Suzuki, A.; Murakami, M. Jpn. J. Appl. Phys. 1996, 35, 1657−1663. (33) Kozicki, M. N.; Mitkova, M.; Zhu, J.; Park, M. Microelectron. Eng. 2002, 63, 155−159. (34) Chen, C.-C.; Herhold, A. B.; Johnson, C. S.; Alivisatos, A. P. Science 1997, 276, 398−401. (35) Rivest, J. B.; Fong, L.-K.; Jain, P. K.; Toney, M. F.; Alivisatos, A. P. J. Phys. Chem. Lett. 2011, 2, 2402−2406. (36) Jacobs, K.; Zaziski, D.; Scher, E. C.; Herhold, A. B.; Alivisatos, A. P. Science 2001, 293, 1803−1806. (37) Xie, Y.; Qian, Y.; Wang, W.; Zhang, S.; Zhang, Y. Science 1996, 272, 1926−1927. (38) Latent heat detected by DSC: 9.91−12.84 J/g (releasing heat) for the t → β phase transition and −(21.55−26.92) J/g (absorbing heat) for the β → α phase transition in t-Ag2Se nanocrystals prepared by the OLA-mediated method; −(9.68−12.49) J/g (absorbing heat) for the t → α phase transition of t-Ag2Se nanocrystals prepared by the PVP-assisted solvothermal method. The total heat change in the t → β → α phase transition in t-Ag2Se nanocrystals prepared by the OLAmediated method can be roughly calculated by summing the latent heats of t → β and β → α phase transitions, and the value is −(11.64− 14.08) J/g, which is quite close to the latent heat we measured for the t → α phase transition of t-Ag2Se nanocrystals prepared by the PVPassisted solvothermal method. If the total energies in α-Ag2Se are equivalent, the total energy in t-Ag2Se nanocrystals prepared by the PVP-assisted solvothermal method is considered to be approximate to that in t-Ag2Se nanocrystals prepared by the OLA-mediated method. (39) Xia, Y.; Xiong, Y.; Lim, B.; Skrabalak, S. E. Angew. Chem., Int. Ed. 2009, 48, 60−103.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (21201086, 51272093), the China Postdoctoral Science Foundation (2014M550267), the Natural Science Foundation of Jiangsu Province (BK20141297), the Research Foundation of Jiangsu University (11JDG071), and the Cultivating Project of Young Academic Leader of Jiangsu University.

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dx.doi.org/10.1021/cm502317g | Chem. Mater. 2014, 26, 5647−5653