High-Pressure Behaviors of Ag2s Nanosheets: an in Situ High-Pressure X-Ray Diffraction Research

$High-Pressure Behaviors of Ag2s Nanosheets: an in Situ High-Pressure X-Ray Diffraction Research$

nanomaterials

Article High-Pressure Behaviors of Ag2S Nanosheets: An in Situ High-Pressure X-Ray Diﬀraction Research

Ran Liu, Bo Liu, Quan-Jun Li and Bing-Bing Liu * State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, China; [email protected] (R.L.); [email protected] (B.L.); [email protected] (Q.-J.L.) * Correspondence: [email protected]; Tel.: +86-431-8518256

Received: 25 July 2020; Accepted: 14 August 2020; Published: 21 August 2020

Abstract: An in situ high-pressure X-ray diffraction study was performed on Ag2S nanosheets, with an average lateral size of 29 nm and a relatively thin thickness. Based on the experimental data, we demonstrated that under high pressure, the samples experienced two different high-pressure structural phase transitions up to 29.4 GPa: from monoclinic P21/n structure (phase I, α-Ag2S) to orthorhombic P212121 structure (phase II) at 8.9 GPa and then to monoclinic P21/n structure (phase III) at 12.4 GPa. The critical phase transition pressures for phase II and phase III are approximately 2–3 GPa higher than that of 30 nm Ag2S nanoparticles and bulk materials. Additionally, phase III was stable up to the highest pressure of 29.4 GPa. Bulk moduli of Ag2S nanosheets were obtained as 73(6) GPa for phase I and 141(4) GPa for phase III, which indicate that the samples are more difficult to compress than their bulk counterparts and some other reported Ag2S nanoparticles. Further analysis suggested that the nanosize effect arising from the smaller thickness of Ag2S nanosheets restricts the relative position slip of the interlayer atoms during the compression, which leads to the enhancing of phase stabilities and the elevating of bulk moduli.

Keywords: Ag2S; nanosheets; high pressure; phase transitions

1. Introduction

As a well-known metal sulﬁde, Ag2S is a direct narrow band gap semiconductor (~1.5 eV), 4 1 with high absorption coeﬃcient (10 m− ), good chemical stability and optical properties [1–3]. So far, Ag2S materials have been extensively synthesized and studied due to their important applications including semiconductors, photovoltaic cells, infrared detectors, photoelectric switches and oxygen sensors [4–6]. Recently, the phase transitions of Ag2S have attracted much attention and a lot of research has been conducted around this topic. Under ambient conditions, Ag2S is stable in a monoclinic structure with P21/n space group (α-Ag2S) [7]. While heating above 450 K, Ag2S undergoes a thermo-induced phase transition and reforms into a body-centered cubic (bcc) structure (β-Ag2S, space group Im3m)[8,9]. In this high-temperature structure, silver ions are randomly distributed over the interstitial sites of a bcc sulfur lattice [10,11], leading to a favorable ionic conductivity as high as 1 1 5 Ω− cm− [12]. Thereby, β-Ag2S is considered as a fast ionic conductor (FIC), which has potential applications including in energy, analytical chemistry, biomedicine, solid-state ionic devices and so forth [13–15]. At about 860 K, Ag2S further converse into a face-centered cubic (fcc) phase (γ-Ag2S), then keeping stable up to melting temperature [9]. Instead of temperature, high pressure is an effective approach for tuning both structural phase transitions and physical properties [16,17]. Under high pressure, the atomic arrangement of materials would be dramatically changed, and thus, the electronic structures and physical properties could be improved. For bulk Ag2S, structural and optical behaviors under high pressure have been systematically studied by synchrotron X-ray diffraction and infrared spectroscopy

Nanomaterials 2020, 10, 1640; doi:10.3390/nano10091640 www.mdpi.com/journal/nanomaterials Nanomaterials 2020, 10, 1640 2 of 9

measurements [15,18,19]. It is demonstrated that Ag2S bulk material experiences a series of phase transitions up to 40 GPa. At 5.1 GPa, bulk Ag2S transforms from the P21/n structure (α-Ag2S, phase I) to an orthorhombic P212121 structure (phase II). Elevating the pressure up to 8.8 GPa, Ag2S undergoes the second phase transition, from phase II to a monoclinic P21/n structure (phase III), which is isosymmetric to phase I. At 28.4 GPa, phase III further transforms into a Pnma structure (phase IV). Accompanied by structural transformations, the band gap of Ag2S is narrowed gradually by increasing the pressure. Beyond 22 GPa, the pressure effectively tunes the semiconducting Ag2S into a metal [19]. Up to now, with the rapid development of synthesis technology, many kinds of nanostructured Ag2S have been prepared, such as quantum dots [20], nanoparticles [21,22], nanotubes [23], nanowires [24], nanorods [25], sheet-like and cube-like nanocrystals [26] and so forth. This progress inspired researchers to begin to focus on the pressure-induced phase transitions of Ag2S nanomaterials, where they expected to obtain novel properties under pressure. Thus far, there has been one report on the high-pressure behaviors of pure and Y-doped Ag2S nanoparticles [27]. The particle sizes of the two samples were estimated to be about 30 nm. Indeed, Wang et al. have found that both of the samples exhibit slightly higher transition pressure values and obviously increased bulk moduli than that of the corresponding bulk materials. We note that previous studies have been focused on Ag2S nanocrystals with random geometric shapes. It is still unclear what behaviors of Ag2S nanocrystals with special morphologies will actually be induced by high pressure. In particular, it is still unknown whether or not they will choose the known structural evolution laws as well as the nanoparticles. These require further studies. In this work, we investigate the high-pressure behaviors of Ag2S nanomaterials with sheet-like morphologies using the in situ high-pressure X-ray diffraction up to about 30 GPa. A set of phase transitions of Ag2S nanosheets were observed. Interestingly, the Ag2S nanosheets exhibit even higher structural stability and lower compressibility under high pressure, remarkably different from the Ag2S bulk materials and nanoparticles. Further analysis suggested that the stronger nanosize effect arising from the smaller thickness of Ag2S nanosheets effectively restricts the relative position slip of the interlayer atoms during the compression, which leads to the enhancement of phase stabilities and the elevation of bulk moduli. Our findings give a further insight into high-pressure behaviors of Ag2S nanomaterials.

2. Materials and Methods

In this study, Ag2S nanosheet was provided by Tongshun Wu, Ph. D. of State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University (Changchun, China), which was prepared by solvothermal method. The initial sample was dispersed in ethanol, with no substrate or coating. A HITACHI H-8100 (Hitachi, Ltd., Tokyo, Japan) transmission electron microscope (TEM) with accelerating voltage of 200 kV was employed to analyze the typical product. X-ray powder diffraction (XRD) was used to characterize the product and the XRD patterns at ambient condition were collected by a Rigaku D/max-rA XRD (Rigaku Co., Tokyo, Japan) spectrometer (Cu, Kα, λ = 1.5406 Å). A symmetric diamond anvil cell (DAC) with flat diamond culets of 400 µm in diameter was used to generate high pressure. The DAC allowed access to the X-ray diffraction angular range 4θ = 50◦. A sheet of T301 stainless steel was selected as the gasket and pre-indented in the DAC to an initial thickness of about 40 µm. Then, a hole of 150 µm in diameter was drilled manually by a tungsten carbide needle in the middle of the dent, which served as a sample chamber. The Ag2S nanosheet samples along with the dispersant were loaded simultaneously into the sample chamber to keep the dispersity. A mixture of methanol and ethanol at a ratio of 4:1 was used as the pressure transmitting medium. A tiny ruby ball ~5 µm in diameter was loaded in the sample chamber, and pressure was determined by the frequency shift of the ruby R1 fluorescence line. High-pressure X-ray diffraction experiments were carried out up to 30 GPa using a synchrotron X-ray source (λ = 0.386 Å) of the beamline X17C of National Synchrotron Light Source (NSLS), Brookhaven National Lab (BNL). The monochromatic X-ray beam was focused down to 25 µm 25 µm using Kirkpatrick-Baez mirrors. For filtering out the tail of × Nanomaterials 2020, 10, 1640 3 of 9

Nanomaterials 2020, 10, x FOR PEER REVIEW 3 of 10 the X-ray beam, a pinhole was placed before the sample position as a cleanup aperture. The X-ray diffractionmirrors. images For filtering were out collected the tail of by the a MAR345X-ray beam, image a pinhole plate, was located placed ~350 before mm the behind sample theposition sample. Exposureas a cleanup times wereaperture. typically The X-ray 300~600 diffraction s. Geometric images were correction collected and by radial a MAR345 integration image plate, was donelocated using the Fit2D~350 mm software behind (v12.077, the sample. GRENOBLE, Exposure times France). were typically The observed 300~600 intensities s. Geometric were correction integrated and as a functionradial of integration 2θ in order was to givedone conventional,using the Fit2D one-dimensional software (v12.077, di ffGRENOBLE,raction profiles. France). Rietveld The observed refinement wasintensities performed were by GSAS-EXPGUI integrated as packagea function (v1.80, of 2θ Gaithersburg, in order to give Maryland, conventional, USA). Aone-dimensional Birch-Murnaghan (BM)diffraction equation ofprofiles. state (EOS) Rietveld was employedrefinement towas fit theperformed experimental by GSAS-EXPGUIP-V relationship, package using Origin(v1.80, Pro softwareGaithersburg, (v8.0, Northampton, Maryland, USA). MA, A USA). Birch-Murnaghan (BM) equation of state (EOS) was employed to fit the experimental P-V relationship, using Origin Pro software (v8.0, Northampton, MA, USA). 3. Results and Discussion 3. Results and Discussion To investigate the morphology and structure of the Ag2S samples, TEM and selected area electron To investigate the morphology and structure of the Ag2S samples, TEM and selected area diffraction (SAED) analysis was performed. Figure1a shows the TEM image and SAED pattern of electron diffraction (SAED) analysis was performed. Figure 1a shows the TEM image and SAED the Ag S samples. It can be clearly seen that all Ag S nanocrystals are well dispersed and possess pattern2 of the Ag2S samples. It can be clearly seen that2 all Ag2S nanocrystals are well dispersed and sheet-likepossess morphology; sheet-like morphology; no other shapes no other can shapes be observed. can be observed. SAED pattern SAED (inset pattern in Figure(inset in1a) Figure determines 1a) the samplesdetermines are the well samples crystalized are well in crystalized a monoclinic in a Pmonoclinic21/n structure. P21/n structure. Figure1b Figure shows 1b the shows lateral the size distributionlateral size of distribution Ag2S nanosheets. of Ag2S The nanosheets. distribution The distribution is relatively is narrow, relatively and narrow, the average and the lateral average size is 29 lateral2 nm. Insize principle, is 29 ± 2 for nm. the In TEM principle, image, for the the contrast TEM ofimage, the image the contrast normally of representsthe image thenormally thickness ± of samples.represents The the smooth thickness contrast of sample of eachs. The Ag smooth2S nanosheet contrast grain of each proves Ag2S thatnanosheet the samples grain proves have uniformthat 2 thickness.the samples Interestingly, have uniform the Ag 2thicS nanosheetskness. Interestingly, are approximately the Ag S nanosheets semitransparent are approximately (see the sample overlapssemitransparent in TEM image), (see the which sample demonstrates overlaps in thatTEM the image), thickness which of demonstrates samples is relatively that the thickness thin, far belowof samples is relatively thin, far below 29 nm. All the analyses show that Ag2S samples are well 29 nm. All the analyses show that Ag2S samples are well crystallized and have an almost-uniform crystallized and have an almost-uniform morphology as nanosheets, providing ideal samples for morphology as nanosheets, providing ideal samples for high-pressure research. high-pressure research.

FigureFigure 1. Typical 1. Typical transmission transmission electron electron microscope microscope (TEM) (TEM images) images and and particle particle size size distribution: distribution: (a ()a TEM) imagesTEM and images selected and area selected electron area dielectronﬀraction diffra (SAED)ction image (SAED) (inset) image of (inset) the samples. of the samples. (b) Size distribution(b) Size distribution of Ag2S nanosheets. of Ag2S nanosheets.

In order to further study the crystal structure of Ag2S nanosheets, XRD analysis was carried In order to further study the crystal structure of Ag2S nanosheets, XRD analysis was carried out. out. Figure 2 shows the typical XRD pattern of the samples and its Rietveld refinement under Figure2 shows the typical XRD pattern of the samples and its Rietveld refinement under ambient ambient conditions. It can be seen that the diffraction pattern can be indexed into α-Ag2S phase conditions. It can be seen that the diffraction pattern can be indexed into α-Ag S phase (Joint Committee (Joint Committee on Powder Diffraction Standards (JCPDS) card no. 14-0072)2 (monoclinic structure, on Powder Diffraction Standards (JCPDS) card no. 14-0072) (monoclinic structure, space group P2 /n). space group P21/n). Vertical markers indicate the Bragg reflections of the P21/n monoclinic structure.1 VerticalIt can markers be seen indicate that no theother Bragg diffraction reflections peaks of exist, the Pwhich21/n monoclinic means that structure.no other phases It can beor seenotherthat no otherimpurities diffraction are mixing peaks in exist, the whichsample. means The Rietve that nold otherrefinement phases of or the other XRD impurities pattern at are ambient mixing in the sample.pressure Theyields Rietveld a = 4.214 refinement ± 0.001 Å, of b the= 6.912 XRD ± pattern0.001 Å, atc = ambient 7.853 ± 0.001 pressure Å, β yields = 99.580°a = ±4.214 0.010°, 0.001V0 = Å, 3 ± b = 6.912225.55 ±0.001 0.034 Å, Åc3= for7.853 Ag2S 0.001nanosheets, Å, β = 99.580slightly smaller0.010 ,thanV = 30225.55 nm Ag0.0342S nanoparticles Å for Ag Sand nanosheets, bulk ± ± ◦ ± ◦ 0 ± 2 slightlymaterials smaller [27,28]. than The 30 nm existence Ag2S nanoparticlesof a lattice shrinking and bulk phenomenon materials [ 27of, 28Ag].2S The nanosheets existence could of a latticebe shrinkingattributed phenomenon to the stronger of Ag nanosize2S nanosheets effect as a could result beof their attributed smaller to thickness. the stronger nanosize effect as a result of their smaller thickness. Nanomaterials 2020, 10, 1640 4 of 9 Nanomaterials 2020, 10, x FOR PEER REVIEW 4 of 10

Figure 2. The typical X-ray powder diffraction (XRD) pattern of Ag2S nanosheets under ambient Figure 2. The typical X-ray powder diﬀraction (XRD) pattern of Ag2S nanosheets under ambient conditions;conditions;Rwp Rwp= 0.3133, = 0.3133,Rp Rp= =0.2028, 0.2028, and and chiˆ2chi^2 == 84.89.

To systematically explore the high-pressure behavior of Ag2S nanosheets, we carried out an in To systematically explore the high-pressure behavior of Ag2S nanosheets, we carried out an in situ high-pressuresitu high-pressure X-ray X-ray di ffdiffractionraction study. study. Figure Figure3 3 shows shows synchrotronsynchrotron X-ra X-rayy diffraction di ffraction patterns patterns of of Ag2S nanosheets under different pressures. It is clear that under high pressures, two phase Ag S nanosheets under different pressures. It is clear that under high pressures, two phase transitions 2 transitions for the samples are observed up to 29.4 GPa. With pressure increasing, all the peaks of for the samples are observed up to 29.4 GPa. With pressure increasing, all the peaks of α-Ag2S α-Ag2S phase shift to smaller d-spacing, indicating a pressure-induced reduction of d-spacing and phaseshrinkage shift to smallerof the unitd-spacing, cell. The initial indicating phase a can pressure-induced be maintained up reduction to 8.9 GPa, of muchd-spacing more andstable shrinkage than of thethat unit in Ag cell.2S nanoparticles The initial phaseand bulk can materials be maintained [19,27]. At up 5.4 to GPa 8.9 (the GPa, first much phase more transition stable point than of that in Agbulk2S nanoparticlesAg2S), the measured and bulk XRD materials pattern is [19cons,27istent]. At with 5.4 GPathe starting (the first monoclinic phase transition P21/n structure point of bulk(phase Ag2S), I).the Themeasured mutual agreements XRD pattern encourage is consistent us to perform with subsequent the starting fine monoclinic Rietveld analysisP21/n onstructure our (phaseexperimental I). The mutual data by agreements using the P encourage21/n structure. us to As perform shown in subsequent Figure 4, the fine fitting Rietveld gives analysisa successful on our experimentalresult to the data experimental by using thedata.P 2The1/n latticestructure. constants As shown of Ag2S innanosheets Figure4, theat 5.38 fitting GPa givesare as afollows: successful a 0 3 result= 4.107 to the ± experimental0.002 Å, b = 6.659 data. ± The0.003 lattice Å, c = constants 7.616 ± 0.004 of Ag Å,2S βnanosheets = 99.62° ± 0.03, at 5.38 V = GPa 205.3 are ± as0.1 follows: Å . According to the fitting data, all of the diffraction peaks can be indexed to the initial monoclinic 3 a = 4.107 0.002 Å, b = 6.659 0.003 Å, c = 7.616 0.004 Å, β = 99.62◦ 0.03, V0 = 205.3 0.1 Å . P21/n structure.± No new phases± were observed at this± pressure. ± ± According to the fitting data, all of the diffraction peaks can be indexed to the initial monoclinic P21/n While compressing up to 8.9 GPa, several new peaks (marked as asterisks) start to emerge structure. No new phases were observed at this pressure. obviously and their intensity grows with pressure increasing (see Figure 3), indicating that α-Ag2S While compressing up to 8.9 GPa, several new peaks (marked as asterisks) start to emerge undergoes a phase transition. According to previous reports, these new peaks located at 8.96°, 9.18°, obviously9.39°, 10.90° and their and intensity11.05° can grows be indexed with pressureinto the (120), increasing (112), (121), (see Figure (113) and3), indicating (201) diffractions that α -Agof 2S undergoesorthorhombic a phase P transition.212121 structure According (phase toII).previous Both phase reports, I andthese phase new II of peaks Ag2S locatedcoexist atunder 8.96 ◦this, 9.18 ◦, 9.39◦pressure., 10.90◦ Noand pure 11.05 phase◦ can II bestructure indexed could into be the obta (120),ined in (112), our work. (121), With (113) continuous and (201) compression, diffractions of orthorhombicall the peaksP2 1of2 1ambient21 structure phase (phase I became II). Bothweak phasegradually. I and phase II of Ag2S coexist under this pressure. No pure phase II structure could be obtained in our work. With continuous compression, all the peaks of ambient phase I became weak gradually. Further compressed to 12.4 GPa, it can be clearly seen that a set of new diffraction peaks (2θ = 9.39◦, 9.78◦, 10.87◦, 11.30◦) emerged (marked as arrows in Figure3), indicating that Ag 2S nanosheets undergo the second phase transition. These new diffraction peaks can be indexed into (022), (12-1), (200) and (014) diffractions of monoclinic P21/n structure (phase III), isosymmetric to the ambient phase. Both phase II and phase III coexist under this pressure. However, the peaks corresponding to phase II lost a lot of intensity and then completely disappeared at the pressure of 16.5 GPa. Under this pressure, all of the remaining peaks can be indexed to phase III, indicating Ag2S nanosheets crystallized in monoclinic P21/n structure completely. Typical Rietveld refinement for phase III is plotted in Figure5, yielding a = 4.079 0.003 Å, b = 5.800 0.006 Å, c = 7.948 0.008 Å and β = 93.15 0.07 (V = 187.7 0.2 Å3). ± ± ± ◦ ± ± Compressing the samples up to 29.4 GPa, the highest pressure in our study, no new diffraction peaks were observed, Ag2S nanosheets are mainly still maintaining in phase III. Nanomaterials 2020, 10, 1640 5 of 9 NanomaterialsNanomaterials 2020 2020, 10, 10, x, xFOR FOR PEER PEER REVIEW REVIEW 5 5of of 10 10

FigureFigureFigure 3. 3. 3. Synchrotron Synchrotron X-ray X-ray diffrac diffrac diﬀractiontiontion patterns patterns patterns of of Ag ofAg2S Ag2 Snanosheets nanosheets2S nanosheets under under underdifferent different di ﬀpressures. erentpressures. pressures. The The Theasterisksasterisks asterisks and and and arrows arrows arrows denote denote denote the the o the ccurrencesoccurrences occurrences of of new new of newpeaks peaks peaks for for phase phase for phase II II and and II phase andphase phaseIII, III, respectively. respectively. III, respectively.

Figure 4. The refinement result for the monoclinic P21/n structure (phase I) of Ag2S nanosheets at FigureFigure 4. 4.The The reﬁnement refinement resultresult forfor the monoclinic PP221/1n/n structurestructure (phase (phase I) I)of ofAg Ag2S 2nanosheetsS nanosheets at at 5.385.385.38 GPa, GPa, GPa,Rwp Rwp Rwp= =0.3011, =0.3011, 0.3011, RpRp Rp = =0.18360.1836 0.1836 and and and chi^2chiˆ2 chi^2 == =109.9. 109.9.109.9.

Nanomaterials 2020, 10, x FOR PEER REVIEW 6 of 10

Further compressed to 12.4 GPa, it can be clearly seen that a set of new diffraction peaks (2θ= 9.39°, 9.78°, 10.87°, 11.30°) emerged (marked as arrows in Figure 3), indicating that Ag2S nanosheets undergo the second phase transition. These new diffraction peaks can be indexed into (022), (12-1), (200) and (014) diffractions of monoclinic P21/n structure (phase III), isosymmetric to the ambient phase. Both phase II and phase III coexist under this pressure. However, the peaks corresponding to phase II lost a lot of intensity and then completely disappeared at the pressure of 16.5 GPa. Under this pressure, all of the remaining peaks can be indexed to phase III, indicating Ag2S nanosheets crystallized in monoclinic P21/n structure completely. Typical Rietveld refinement for phase III is plotted in Figure 5, yielding a = 4.079 ± 0.003 Å, b = 5.800 ± 0.006 Å, c = 7.948 ± 0.008 Å and β = 93.15° ± 0.07 (V = 187.7 ± 0.2 Å3). Compressing the samples up to 29.4 GPa, the highest pressure in our study, no new diffraction peaks were observed, Ag2S nanosheets are mainly still maintaining in phase III.Nanomaterials 2020, 10, 1640 6 of 9

Figure Figure5. The 5. refinementThe reﬁnement result result forfor phase phase III ofIII Ag of2S Ag nanosheets2S nanosheets at 16.5 GPa, at Rwp16.5= GPa,0.1921, RwpRp = 0.1402= 0.1921, Rp = and chiˆ2 = 67.78. 0.1402 and chi^2 = 67.78.

In previous studies, we noted that bulk Ag2S undergoes the phase transition sequence P21/n In (phaseprevious I) studies,P2 2 2 (phasewe noted II) thatP2 / nbulk(phase Ag III)2S undergoesPnma (phase the IV)phase at pressures transition of 5.1GPa,sequence P21/n → 1 1 1 → 1 → (phase 8.8I) → GPa P2 and1212 28.41 (phase GPa, respectively II) → P21/ [n19 (phase]. For pure III) 30 → nm Pnma Ag2S (phase nanoparticles, IV) at thepressures transition of pressures 5.1GPa, 8.8 GPa elevate slightly compared to the bulk: from phase I to phase II at about 6.83 GPa and then to phase III and 28.4 GPa, respectively [19]. For pure 30 nm Ag2S nanoparticles, the transition pressures elevate at 9.3 GPa [27]. Compared to these reports, we noted that phase transition pressure in Ag S nanosheets slightly compared to the bulk: from phase I to phase II at about 6.83 GPa and then2 to phase III at 9.3 was about 2–3 GPa higher than that in 30 nm Ag2S nanoparticles and bulk crystals. Moreover, GPa [27].phase Compared IV [19], which to these formed reports, at 28.4 GPawe no in bulkted that counterparts, phase transition was not observed pressure in ourin Ag samples.2S nanosheets was aboutThus, 2–3 our GPa work higher reveals thatthan Ag that2S nanosheets in 30 nm exhibit Ag2S a nanoparticles different high-pressure and bulk property crystals. compared Moreover, with phase IV [19],previous which formed reports. Accordingat 28.4 GPa to thein bulk analysis counterparts, in the TEM image was before,not observed it was shown in our that samples. the Ag2S Thus, our nanosheets have a relatively thin thickness, far below 29 nm (the average size of the sample). Therefore, work reveals that Ag2S nanosheets exhibit a different high-pressure property compared with we suggest that the stronger nanosize effect arising from the smaller thickness of Ag2S nanosheets previous reports. According to the analysis in the TEM image before, it was shown that the Ag2S effectively restricts the relative position slip of the interlayer atoms during compression, which leads to nanosheetsthe enhancing have a ofrelatively phase stabilities. thin thickness, far below 29 nm (the average size of the sample). Therefore, Figurewe suggest6 shows that the compressivethe stronger behavior nanosize of Ag effect2S nanosheets. arising from A third-order the smaller BM thickness EOS was of Ag2S nanosheetsintroduced effectively to fit our restricts pressure-volume the relative data. Forposition monoclinic slip Pof21 /then structure interlayer (phase atoms I), by fixingduring the compression, first which leadspressure to derivativethe enhancing of isothermal of phase bulk stabilities. modulus (B00) to 4, the bulk modulus (B0) was obtained as 73(6) GPa. The obtained bulk modulus B0 for phase I, the ambient phase, was much higher Figure 6 shows the compressive behavior of Ag2S nanosheets. A third-order BM EOS was than that of the bulk Ag2S(~34–38(2) GPa), even higher than that of the 30 nm Ag2S nanoparticles introduced to fit our pressure-volume data. For monoclinic P21/n structure (phase I), by fixing the (B0 = 57.3(6) GPa) [19,27]. For phase III, during the fitness, the values of the bulk modulus (B0) first pressureand zero derivative pressure cell of volume isothermal (V0) were bulk left tomodulus vary freely, (B and0’) to the 4, first the derivative bulk modulus of B0 with (B pressure0) was obtained as 73(6)(B GPa.00) was The fixed obtained to 4. As shownbulk modulus in Figure6 ,B the0 for fitting phase to the I, theP-V ambientcurve yields phase, a bulk was modulus muchB0 higherof than 141(4) GPa and unit cell volume V of 207(6) Å3. Compared to previous studies, the obtained bulk that of the bulk Ag2S (~34–38(2) GPa),0 even higher than that of the 30 nm Ag2S nanoparticles (B0 = modulus B0 for phase III of Ag2S nanosheets was also larger than the bulk counterparts (102(4) GPa) 57.3(6) GPa) [19,27]. For phase III, during the fitness, the values of the bulk modulus (B0) and zero and 30 nm nanoparticles (123.1(0) GPa) [19,27]. According to the above analysis, we consider that Ag2S nanosheets exhibit significant difference in compression properties under high pressure, a much lower volume compressibility than previous reports. A similar phenomenon was observed in some other nanomaterials, e.g., C60 nanosheets and ZnO nanocrystal [29,30], and has potential benefits in keeping the stability of materials. It is well known that nanosize effect can strongly influence the bulk Nanomaterials 2020, 10, x FOR PEER REVIEW 7 of 10

pressure cell volume (V0) were left to vary freely, and the first derivative of B0 with pressure (B0’) was fixed to 4. As shown in Figure 6, the fitting to the P-V curve yields a bulk modulus B0 of 141(4) GPa and unit cell volume V0 of 207(6) Å3. Compared to previous studies, the obtained bulk modulus B0 for phase III of Ag2S nanosheets was also larger than the bulk counterparts (102(4) GPa) and 30 nm nanoparticles (123.1(0) GPa) [19,27]. According to the above analysis, we consider that Ag2S nanosheets exhibit significant difference in compression properties under high pressure, a much Nanomaterialslower volume2020, 10 compressibility, 1640 than previous reports. A similar phenomenon was observed in some7 of 9 other nanomaterials, e.g., C60 nanosheets and ZnO nanocrystal [29,30], and has potential benefits in keeping the stability of materials. It is well known that nanosize effect can strongly influence the modulus;bulk modulus; thereby, thethereby, 30 nm the nanoparticles 30 nm nanoparticle exhibit highers exhibit bulk modulushigher bulk than modulus the bulk materials.than the bulk In our experiment,materials. theIn our Ag 2experiment,S nanosheets the have Ag2S ananosheets very small have size a in very thickness, small size much in thickness, lower than much their lower grain sizethan of 29 their nm. grain Thus, size the of stronger 29 nm. Thus, nanosize the stronger eﬀect from nanosize our samples effect from may our cause samples the bulk may modulus cause theto largelybulk elevate.modulus to largely elevate.

2 FigureFigure 6. Room-temperature 6. Room-temperature equation equation of state dataof state for Ag data2S nanosheets. for Ag S Ananosheets. third-order Birch-MurnaghanA third-order (BM)Birch-Murnaghan equation of state (BM) (EOS) equation is introduced of state (EOS) to ﬁt ouris introduced pressure-volume to fit our data. pressure-volume data.

In ourIn our high-pressure high-pressure research, research, the quenchthe quench process process is implemented. is implemented. TEM TEM images images and XRD and patternXRD pattern of Ag2S nanosheets quenched to the ambient conditions are shown in Figure 7. According to of Ag2S nanosheets quenched to the ambient conditions are shown in Figure7. According to the TEM the TEM image of the quenched sample, although Ag2S samples could not maintain their uniform image of the quenched sample, although Ag2S samples could not maintain their uniform shape because of theshape high because pressure of andthe high pressure shear stresses, and high it still shear keeps stresses, the sheet-like it still keeps morphology the sheet-like in nanoscale. morphology Based in nanoscale. Based on this result, we can reasonably draw a conclusion that during the on this result, we can reasonably draw a conclusion that during the compressing process, Ag2S samples compressing process, Ag2S samples in our experiment remain nanomaterials, and no agglomeration in our experiment remain nanomaterials, and no agglomeration took place. After decompressing took place. After decompressing to ambient pressure, the shapes of all the diffraction peaks for to ambient pressure, the shapes of all the diﬀraction peaks for Ag2S nanosheets return to the initial Ag2S nanosheets return to the initial monoclinic P21/n structure (phase I), indicating the reversibility monoclinic P2 /n structure (phase I), indicating the reversibility of the pressure-induced structural of the pressure-induced1 structural transitions. Further analysis showed that the (-112), (-103) peak transitions. Further analysis showed that the (-112), (-103) peak was enhanced after compression. was enhanced after compression. The effect of non-hydrostatic pressure and high pressure gives Thenanosheet eﬀect of non-hydrostatic samples a certain pressure orientation and and high texture. pressure gives nanosheet samples a certain orientation and texture. Nanomaterials 2020, 10, x FOR PEER REVIEW 8 of 10

Figure 7. (a) TEM images and (b) Rietveld refinement result of Ag2S nanosheets quenched to Figure 7. (a) TEM images and (b) Rietveld reﬁnement result of Ag2S nanosheets quenched to ambient ambient conditions; Rwp = 0.3982, Rp = 0.2929 and chi^2 = 160.3. conditions; Rwp = 0.3982, Rp = 0.2929 and chiˆ2 = 160.3. 4. Conclusions

In summary, we studied the high-pressure behaviors of Ag2S nanomaterials with sheet-like morphologies using in situ high-pressure X-ray diffraction up to about 30 GPa. Under high pressure, Ag2S nanosheets undergo the phase transition sequence P21/n (phase I) → P212121 (phase II) → P21/n (phase III) at pressures of 8.9 GPa and 12.4 GPa. Interestingly, the Ag2S nanosheets exhibit a wide field of structural stability and much lower compressibility under high pressure, remarkably different from the corresponding bulk materials and 30 nm nanoparticles. Further analysis suggests that a stronger nanosize effect arising from smaller thickness in Ag2S nanosheets effectively restricts the relative position slip of the interlayer atoms during the compression, which leads to the enhancing of phase stabilities and the elevating of bulk moduli. Our findings give a further insight into high-pressure behaviors in Ag2S nanomaterials.

Funding: This research was funded by National Key R&D Program of China (2018YFA0305900) and the NSFC (11634004, 51320105007, 11604116, 11847094 and 51602124), Program for Changjiang Scholars and Innovative Research Team in University (IRT1132).

Acknowledgments: We thank Dr. Tongshun Wu (Ph. D. of State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, Changchun, China) for providing ideal Ag2S nanosheet samples, thank Dongmei Li (State Key Laboratory of Superhard Materials, Jilin University, Changchun, China) for helping us to get excellent TEM images; we also thank Zhiqiang Chen (X17C, NSLS, BNL, Upton, NY, USA) for helping us to collect perfect HP in-situ synchrotron XDR data.

Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

References

1. Yang, W.; Zhang, L.; Hu, Y.; Zhong, Y.; Wu, H.B.; Lou, X.W. Microwave-assisted synthesis of porous Ag2S-Ag hybrid nanotubes with high visible-light photocatalytic activity. Angew. Chem. 2012, 51, 11501– 11504. 2. Cui, C.; Li, X.; Liu, J.; Hou, Y.; Zhao, Y.; Zhong, G. Synthesis and Functions of Ag2S Nanostructures. Nanoscale Res. Lett. 2015, 10, 431. 3. Sadovnikov, S.I.; Gusev, A.I. Universal Approach to the Synthesis of Silver Sulfide in the Forms of Nanopowders, Quantum Dots, Core-Shell Nanoparticles, and Heteronanostructures. Eur. J. Inorg. Chem. 2016, 2016, 4944–4957.

Nanomaterials 2020, 10, 1640 8 of 9

4. Conclusions

In summary, we studied the high-pressure behaviors of Ag2S nanomaterials with sheet-like morphologies using in situ high-pressure X-ray diffraction up to about 30 GPa. Under high pressure, Ag S nanosheets undergo the phase transition sequence P2 /n (phase I) P2 2 2 (phase II) P2 /n 2 1 → 1 1 1 → 1 (phase III) at pressures of 8.9 GPa and 12.4 GPa. Interestingly, the Ag2S nanosheets exhibit a wide field of structural stability and much lower compressibility under high pressure, remarkably different from the corresponding bulk materials and 30 nm nanoparticles. Further analysis suggests that a stronger nanosize effect arising from smaller thickness in Ag2S nanosheets effectively restricts the relative position slip of the interlayer atoms during the compression, which leads to the enhancing of phase stabilities and the elevating of bulk moduli. Our findings give a further insight into high-pressure behaviors in Ag2S nanomaterials.

Author Contributions: Conceptualization, R.L. and B.-B.L.; investigation, R.L.; writing—original draft preparation, R.L. and B.L.; writing—review and editing, Q.-J.L. and B.-B.L.; project administration, B.-B.L.; funding acquisition, B.-B.L. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by National Key R&D Program of China (2018YFA0305900) and the NSFC (11634004, 51320105007, 11604116, 11847094 and 51602124), Program for Changjiang Scholars and Innovative Research Team in University (IRT1132). Acknowledgments: We thank Tongshun Wu (Ph.D. of State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, Changchun, China) for providing ideal Ag2S nanosheet samples, thank Dongmei Li (State Key Laboratory of Superhard Materials, Jilin University, Changchun, China) for helping us to get excellent TEM images; we also thank Zhiqiang Chen (X17C, NSLS, BNL, Upton, NY, USA) for helping us to collect perfect HP in-situ synchrotron XDR data. Conﬂicts of Interest: The authors declare no conﬂict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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