稀有金属(英文版) 2017,36(10),799-805

Controlled synthesis of silver nanoplates and nanoparticles by reducing silver nitrate with hydroxylamine hydrochloride

Zhi-Peng Cheng Xiao-Zhong Chu Xiao-Qing Wu Ji-Ming Xu Hui Zhong Jing-Zhou Yin

Jiangsu Key Laboratory for Chemistry of Low-Dimensional Materials, Huaiyin Normal University

收稿日期：19 March 2017

基金：financially supported by the National Natural Science Foundation of China (No.51676082);Qing Lan Project of Jiangsu Province;the Innovation Experiment Program for University Students of Jiangsu (201710323075X);

Controlled synthesis of silver nanoplates and nanoparticles by reducing silver nitrate with hydroxylamine hydrochloride

Zhi-Peng Cheng Xiao-Zhong Chu Xiao-Qing Wu Ji-Ming Xu Hui Zhong Jing-Zhou Yin

Jiangsu Key Laboratory for Chemistry of Low-Dimensional Materials, Huaiyin Normal University

Abstract：

An easy and effective method of silver nanoplate synthesis technique was created by reducing silver nitrate(AgNO₃) with hydroxylamine hydrochloride(NH₂ OH·HCl)at room temperature. Silver nanoplates of various shapes,including triangular, truncated triangular, hexagonal, and truncated hexagonal, exhibit an average width and thickness of approximately 1 μm and 50 nm, respectively. Silver nanoparticles were acquired by placing polyvinyl pyrrolidone(PVP) in the reaction solution. The produced silver nanoparticles are quasi-spherical in shape and¹00 nm in size. The catalytic activity in the thermal decomposition of ammonium perchlorate(AP) was distinguished by thermogravimetric(TG) analysis and differential scanning calorimetry(DSC). The outcomes reveal that the addition of silver nanoplates and nanoparticles diminishes the low decomposition temperature of AP by 7 and 14℃ and leads to a drop in the high decomposition temperature of AP by 60 and 110℃ and a rise in the total DSC heat release by 0.86 and 1.05 kJ·g^-1, respectively.

Keyword：

Silver nanoparticle; Silver nanoplates; Formation mechanism; Ammonium perchlorate;

Author： Zhi-Peng Cheng,e-mail: nanohytc@126.com;

Received： 19 March 2017

1 Introduction

Silver nanostructures have attracted a great amount of interest due to their plasmonic [ 1] ,optical [ 2] ,and thermal properties [ 3] ,in addition to their possible use in catalysts,microelectronics,optoelectronic devices,and surface-enhanced Raman scattering [ 4, 5] .The size and shape of nanostructures dictate their physical,optical,and chemical properties [ 6, 7] ,leading to the shape-controlled synthesis of silver nanostructures to be comprehensively examined during the last couple of years [ 8, 9, 10] .Previous experiments have prepared numerous silver nanostructures,such as nanoplates [ 11] ,nanoparticles [ 12] ,nano wires [ 13] ,nanobelts [ 14] ,nanocubes [ 15] ,bipyramids [ 16] ,nanochains [ 17] ,and nanoprisms [ 18] .A few reports have centered on the synthesis of silver nanoplates and nanoparticles,due to their unique structures,increased surface areas,and unique properties.For instance,Ren et al. [ 19] realized the one-pot wet chemical synthesis of hexagonal silver plates by using AgNO₃,placing polyvinyl pyrrolidone (PVP),sodium citrate,Na`H₄,and H₂O₂ as reagents at room temperature.Deng et al. [ 20] used Cu⁺as a reducing agent to synthesize ultrathin silver nanoplates in an organic solvent containing oleylamine.Meanwhile,Bastus et al.synthesized silver nanoparticles with managed sizes by adhering to a kinetic ally controlled seeded growth method with AgNO₃ reduction that integrates two reducing agents:sodium citrate and tannic acid [ 21] .Du et al.also synthesized silver nanoparticles using Penicillium oxalicum assisted with the assistance of simulated sunlight [ 22] .

Ammonium perchlorate (AP) is an important energetic material,which is typically implemented as a conventional oxidizer in composite solid propellants.The thermal decomposition characteristics of AP directly influence the combustion behavior of the propellant.Numerous nanometal and metal oxides have been evaluated for the decomposition of AP,such as CuO [ 23] ,MnO₂ [ 24] ,and Pt [ 25] .Up to now,to the best of our knowledge the catalytic behavior of silver nanostructures for the thermal decomposition of AP has not been reported.

In the current study,we show,for the first time,the facile-controlled synthesis of silver nanoplates and nanoparticles by reducing AgNO₃ with NH₂OH·HCl in the presence or lack of PVP.The reaction was executed at room temperature and under normal pressure.The as-prepared silver nanostructures revealed exceptional catalytic activity for the thermal decomposition of AP.

2 Experimental

In the typical synthetic process,0.017 g AgNO₃ and 0.03 g NH₂OH·HCl independent of each other were dissolved in10 ml distilled water at room temperature to create uniform solutions.The NH₂OH·HCl solution was quickly placed in AgNO₃ solution with aggressive stirring with or without0.005 g·L^-1 PVP at room temperature to synthesize AgCl colloids.Subsequently,1 ml ammonium hydroxide(NH₃·H₂O) solution was titrated into the AgCl colloids.The resulting products were gathered after being stirred for20 min,washed five times in distilled water,and air-dried.

The crystalline nature of the synthesized samples was identified using X-ray diffractometer (XRD,Bruker D8).The morphology of the samples was examined with scanning electron microscope (SEM,FEI Quanta 450 FEG) and transmission electron microscope (TEM,Philips Tecnai12).The elemental compositions of the samples were obtained by energy-dispersive spectroscopy (EDS,DX-4Philips).The catalytic activity of the samples for thermal decomposition of AP was executed by differential scanning calorimetry (DSC) and thermogravimetric analysis (TG) in the STA-780 thermal analysis system at a heating rate of20℃·min^-1 in N₂ atmosphere,with a temperature ranging from 100 to 500℃.The sample mass of～20 mg was used.

3 Results and discussion

Figure 1a,a'shows low-and high-magnified SEM images of the silver products readied without PVP,respectively.The sample primarily contained silver nanoplates,regular triangular,truncated triangular,hexagonal,and truncated hexagonal shapes with the average width and thickness of approximately 1μm and 100 nm,respectively.Numerous tiny nanoparticles were attached on the surface and edge of the silver nanoplates.Figure 1b,b'displays TEM images of a single triangular nanoplate and hexagonal silver nanoplate,respectively.Electron beam hardly penetrated these samples because of their thickness,making the procurement of detailed surface information impossible.Figure 1c shows high-resolution transmission electron microscopy (HRTEM) image of the single hexagonal nanoplate.The lattice spacing is 0.23 nm,which is in agreement with the space of (111) lattice planes [ 26] .The corresponding selected area electron diffraction (SAED)spots (Fig.1d) are acquired by concentrating the electron beam on the hexagonal silver nanoplate,confirming that the silver nanoplate is an inpidual crystal.The diffraction spots are indexed to (202),(220),and (022) of fcc silver.

Fig.1 SEM and TEM images of silver nanoplates prepared without PVP:a,a'SEM images,b,b'TEM images,c HRTEM,and d SAED pattern

Figure 2a,a'displays low-and high-magnified SEMimages of the silver products readied with 0.005 g·L^-1PVP,respectively.These silver nanoparticles,quasispherical in shape and～100 nm in size,are morphologically distinct from those synthesized without PV P.They are essentially spherical in shape,although a few slightly anisotropic particles appear.TEM images in Fig.2b,b'indicate similar findings.HRTEM and SAED patterns(Fig.2c,d) also reveal that the obtained silver nanoparticles are polycrystals.

The shapes of the silver nanostructures are quite different with 0.005 g.L^-1 PVP and without PVP.Hence,experiments were performed to investigate the influence of PVP concentration on the silver nanostructures while other experimental conditions were unchanged.Figure 3 shows TEM images of silver products at different PVP concentrations.When PVP concentration decreases to0.003 g·L^-1,the mixture of silver nanoparticles and nanoplates is produced.When PVP concentration increases to 0.01 g·L^-1,the shape and size of the obtained silver product are similar to the product obtained with0.005 g·L^-PVP.However,more PVP molecules are absorbed on the silver surface because of the high PVP concentration,thereby affecting the purity of the silver product.Thus,the optimal PVP concentration for synthesis of silver nanoparticle is 0.005 g·L^-1.

Fig.2 SEM and TEM images of silver nanoparticles prepared with0.005 g·L^-1 PVP:a,a'SEM images,b,b'TEM images,c HRTEM,and d SAED pattern

Fig.3 TEM images of silver products at different PVP concentra-tions:a 0.003 g·L^-1 and b 0.01 g·L^-1

Figure 4 shows XRD patterns of the obtained silver nanoplates and nanoparticles.Each reflection relates to that of pure silver metal with fcc symmetry;they are indexed as(111),(200),(220),and (311) planes with the corresponding 2θvalues of 38.1°,44.3°,64.6°,and 77.4°(JCPDS No.4-783).In addition,no impurity peaks are identified,indicating the elevated purity of the silver samples.The diffraction peaks of the silver nanoparticles are wider and smoother,showing that the crystallite size of the silver nanoparticles decreases compared to that of the silver nanoplate.

Fig.4 XRD patterns of silver nanoplates and nanoparticles

Considering its group,NH₂OH·HCl was implemented as a gentle reducing agent to reduce silver ions [ 27] ,but NH₂OH·HCl contains chloride ions.The addition of NH₂OH·HCl solution into AgNO₃ solution immediately produces white AgCl colloids,and this reaction can be expressed as AgNO₃+NH₂OH·HCl→AgCl+NH₂OH+HO₃.The reducing ability of is insufficient to reduce AgCl into metallic silver.Hence,a certain concentration of NH_3·H₂O solution is necessarily introduced into the reaction system.This solution can dissolve AgCl to form a silver ammonia complex ion[Ag(NH₃)2]⁺,and this reaction can be expressed as AgCl+2NH₃·H₂O→[Ag(NH₃)₂]⁺+2H₂O+Cl^-.The latter can be easily reduced to zerovalent silver by in accordance with the following reaction:[Ag(NH₃)₂]⁺+2NH₂OH→Ag+ +N₂+2H₂O.

The controlled synthesis of two silver nanostructures(i.e.,nanoplates and nanoparticles) was performed by reducing AgNO₃ with NH₂OH·HCl without or with PVP.Figure 5 shows a schematic of the formation of silver nanostructures without and with PVP.The surfactant PVP is obviously responsible for the formation of these two silver nanostructures.The PVP structure has a polyvinyl skeleton with polar groups.The contributed solitary pairs of nitrogen and oxygen atoms in the polar groups of 1 PVP unit may populate 2 sp orbitals of silver ions to create a complex compound [ 28] .Therefore,PVP can be strongly adsorbed on the surface of the initially formed silver atoms,which hinders silver diffusion and reduces silver grain growth,leading to the production of small silver products.Further,the resistance to surface addition by the forthcoming silver atoms is nearly isotropic due to the increase in PVP coverage on the silver nucleation faces.This occurrence prompts isotropic growth and ultimately the development of silver nanoparticles [ 29] .

The silver nanoplates acquired in the PVP-free solution show a tight connection to the intrinsic silver structure.The work function variations of silver are 0.83,0.85,and0.57 eV for their{100},{110},and{111}facets,respectively [ 20] .Consequently,particle growth is expedited parallel to the twin planes,widened the lowest energy crystal facets of{111},and drove the particle toward a well-defined triangular or hexagonal nanoplate structure.These data can be verified by above HRTEM and SAED analyses.Further,the triangular or hexagonal nanoplates with sharp corners are not thermodynamically steady;subsequently,some high-energy corners are fragmented to create truncated nanoplates.

Fig.5 Schematic of formation of silver nanoplates and nanoparticles

Fig.6 TEM images of silver nanostructures obtained at different reaction time without PVP:a 1 min,b 5 min,c 10 min,and d 15 min

Time-dependent experiments were conducted to examine the comprehensive growth of the silver nanoplates and nanoparticles.The products primarily contain nonuniform～100-200 nm AgCl nanostructures after reaction for1 min during the synthesis of the silver nanoplates without PVP (Fig.6a).Silver nanoparticles developed after 5 min,and unplanned adsorption on the AgCl surface instead of homogeneous nucleation occurs in the solution to diminish the energy of the entire system (Fig.6b).This phenomenon forms silver nanoplate rudiments.The protracted reaction time of 10 min significantly increases silver nanoplates;further,countless tiny silver nanoparticles are affixed to the edge and the surface of the silver nanoplates (Fig.6c).After 15 min,these outside tiny silver nanoparticles nearly disappear,indicating that they were used as building blocks in silver nanoplate assembly.That is to say,throughout the whole reaction,silver nanoplates develop from the accumulation of tiny silver nanoparticles via the Oswald ripening mechanism.The silver nanoparticles and primary AgCl nanostructures co-occur at the inceptive reaction stage of 5 min to synthesize silver nanoparticles in the company of PVP (Fig.7a).AgCl slowly is consumed as the reaction advances,and the sizes of the formed silver particles grow proportionally (Figs.7b,c,d).

EDS examination captured the elemental composition of the silver products at various time points.Figure 8 displays EDS spectra acquired from the silver nanoplates at various stages.The molar ratio of silver and chlorine atoms is nearly 1:1 at the inceptive reaction stage of 1 min,suggesting that the product is primarily AgCl.The intensity of chlorine is notably reduced at 5 min,showing that additional metallic silver decreases.Only silver peaks are noted in the spectrum at 15 min,showing that the initial AgCl is fully diminished to silver,and the product is pure silver.The silver nanoparticles prepared at different reaction time show similar EDS spectra which thus are no longer shown here.

Fig.7 TEM images of silver nanostructures obtained at different reaction time with PVP:a 1 min,b 5 min,c 10 min,and d 15 min

Fig.8 EDS spectra of silver nanostructures obtained at different reaction time without PVP

Figure 9 displays TG curves of pure AP and AP with silver nanostructures.Thermal decomposition of pure AP occurs in two stages.The first stage,defined as low temperature decomposition (LTD),starts at～305℃and ends at 360℃;this stage results with 20%weight loss.The second stage,defined as high temperature decomposition(HTD),immediately follows the first weight loss and is completed at approximately 460℃,accounting for the remaining 80%mass loss.The thermal decomposition of AP is completed at 405 and 360℃when only 3 wt%silver nanoplates and nanoparticles are applied as catalyst,respectively.

Fig.9 TG curves of (1) pure AP,(2) silver nanoplates,and (3) silver nanoparticles for decomposition of AP

Figure 10 displays DSC curves of pure AP and AP with silver nanostructures.For pure AP,three peaks at 240,320,and 470℃are linked with the phase transition temperatures of orthorhombic AP to cubic AP,LTD of AP,and HTD of AP,respectively [ 30] .LTD has been associated with the incomplete decomposition of AP and the development of intermediate NH₃ and HCIO₄ by dissociation and sublimation.HTD is attributed to the oxidation of NH₃by in the gas phase and decomposition of AP on a firm surface [ 25, 31] .DSC curves show that the addition of silver nanoplates and nanoparticles reduces LTD peak temperature of AP by 7 and 14°C,decreases HTD peak temperature of AP by 60 and 110℃,and increases the total DSC heat release by 0.86 and 1.05 kJ·g-1,respectively.

Up to now,the thermal decomposition mechanism of AP is still unclear because the decomposition process is a complex hetero-phase process involving coupled reactions in the solid,adsorbed,and gaseous phases.Most nanocatalysts exert excellent catalytic activities on HTD of AP,but exert little influence on LTD of AP.However,the obtained silver catalysts not only promote the HTD of AP,but also exert evident catalytic activity toward LTD of AP.This occurrence may be ascribed to the ability of silver nanostructure to adsorb some NH₃ generated by the low-and high-temperature decomposition of AP to produce the complex ion[Ag(NH₃)2]⁺ [ 32] ,which is beneficial for the acceleration of AP decomposition.Silver nanoparticle exhibits higher catalytic performance for the thermal decomposition of AP than silver nanoplate.This phenomenon may be ascribed to the smaller crystallite size of silver nanoparticle.Therefore,it is speculated that silver catalysts may be used in special solid propellants in the

Fig.10 DSC curves of pure AP and AP with silver nanoplates and nanoparticles

4 Conclusion

In summary,silver nanoplate and nanoparticles were successfully synthesized by reduction of AgNO₃ with NH₂OH·HCl with or without PVP at room temperature.The PVP is responsible for the formation of these two silver nanostructures.Silver nanoplates exhibit an average width and thickness of approximately 1μm and 50 nm,respectively.Silver nanoparticles are quasi-spherical in shape and～100 nm in size.The inclusion of silver nanoplates and nanoparticles reduces the low decomposition temperature of AP by 7 and 14℃,decreases the high decomposition temperature of AP by 60 and 110℃,and increases the total DSC heat release by 0.86 and1.05 kJ·g^-1,respectively.The obtained silver catalysts can be implemented in unique solid-state propellants in the future.

Acknowledgements This study was financially supported by the National Natural Science Foundation of China (No.51676082),Qing Lan Project of Jiangsu Province,and the Innovation Experiment Program for University Students of Jiangsu (201710323075X).

参考文献

[1] Rycenga M,Cobley CM,Zeng J,Li WY,Moran CH,Zhang Q,Qin D,Xia YN.Controlling the synthesis and assembly of silver nanostructures for plasmonic applications.Chem Rev.2016;111(6):3669.

[2] Botelho G,Sczancoski JC,Andres J,Gracia L,Longo E.Experimental and theoretical study on the structure,optical properties,and growth of metallic silver nanostructures in Ag3P04.J Phys Chem C.2015;119(11):6293.

[3] Lewandowski W,Fruhnert M,Mieczkowski J,Rockstuhl C,Gorecka E.Dynamically self-assembled silver nanoparticles as a thermally tunable metamaterial.Nature Commun.2015;6:1.

[4] Lee D,Lee H,Ahn Y,Jeong Y,Lee DY,Lee Y.Highly stable and flexible silver nanowire-graphene hybrid transparent conducting electrodes for emerging optoelectronic devices.Nanoscale.2013;5(17):7750.

[5] Meng W,Hu F,Jiang XH,Lu LD.Preparation of silver colloids with improved uniformity and stable surface-enhanced Raman-scattering.Nanoscale Res Lett.2015;10:34.

[6] Sun J,Xiao XZ,Zheng ZJ,Fan XL,Xu CC,Liu LA,Li SQ,Chen LX.Synthesis of nanoscale CeAl_4 and its high catalytic efficiency for hydrogen storage of sodium alanate.Rare Met.2017;36(2):77.

[7] Xiao W,Wang DH.Rare metals preparation by electro-reduction of solid compounds in high-temperature molten salts.Rare Met.2016;35(8):581.

[8] Cai XH,Zhai AX.Preparation of microsized silver crystals with different morphologies by a wet-chemical method.Rare Met.2010;29(4):407.

[9] Chambers BA,Afrooz ARMN,Bae S,Aich N,Katz L,Saleh NB,Kirisits MJ.Effects of chloride and ionic strength on physical morphology,dissolution,and bacterial toxicity of silver nanoparticles.Environ Sci Technol.2014;48(1):761.

[10] Chen L,Fu XL,Lu WH,Chen LX.Highly sensitive and selective colorimetric sensing of Hg~(2+)based on the morphologytransition of silver nanoprisms.ACS Appl Mater Interfaces.2013;5(2):284.

[11] Ahn HY,Cha JR,Gong MS.Preparation of sintered silver nanosheets by coating technique using silver carbamate complex.Mater Chem Phys.2015;153:390.

[12] Alimohammadi F,Gashti MP,Sharmei A,Kiumarsi A.Deposition of silver nanoparticles on carbon nanotube by chemical reduction method:evaluation of surface,thermal and optical properties.Superlatt Microstruct.2012;52(1):50.

[13] Xiang XZ,Gong WY,Kuang MS,Wang L.Progress in application and preparation of silver nanowires.Rare Met.2016;35(4):289.

[14] Liu LC,Yoo SH,Lee SA,Park S.Electrochemical growth of silver nanobelts in cylindrical alumina nanochannels.Cryst Growth Des.2011;11(9):3731.

[15] K(o|¨)nig TAF,Ledin PA,Russell M,Geldmeier JA,Mahmoud MA,El-Sayed MA,Tsukruk VV.Silver nanocube aggregation gradient materials in search for total internal reflection with high phase sensitivity.Nanoscale.2015;7:5230.

[16] Bordenave MD,Scarpettini AF,Roldán MV,Pellegri N,Bragas AV.Plasmon-induced photochemical synthesis of silver triangular prisms and pentagonal bipyramids by illumination with light emitting diodes.Mater Chem Phys.2013;139(1):100.

[17] Chen SL,Liu KH,Luo YF,Wei Y,Li FC,Liu L.Construction of silver nanochains on DNA template for flexible electrical conductive composites.Mater Lett.2015;147:109.

[18] Metraux GS,Mirkin CA.Rapid thermal synthesis of silver nanoprisms with chemically tailorable thickness.Adv Mater.2005;17(4):412.

[19] Ren HM,Guo Y,Huang SY,Zhang K,Yuen MMF,Fu XZ,Yu SH,Sun R,Wong CP.One-step preparation of silver hexagonal microsheets as electrically conductive adhesive fillers for printed electronics.ACS Appl Mater Interfaces.2015;7(24):13685.

[20] Deng ZT,Mansuipur M,Muscat AJ.New method to single-crystal micrometer-sized ultra-thin silver nanosheets:synthesis and characterization.J Phys Chem C.2009;113(3):867.

[21] Bastus NG,Merkoci F,Piella J,Puntes V.Synthesis of highly monodisperse citrate-stabilized silver nanoparticles of up to200 nm:kinetic control and catalytic properties.Chem Mater.2014;26(9):2836.

[22] Du LW,Xu QH,Huang MY,Xian L,Feng JX.Synthesis of small silver nanoparticles under light radiation by fungus Penicillium oxalicum and its application for the catalytic reduction of methylene blue.Mater Chem Phys.2015;160:40.

[23] Cheng ZP,Chu XZ,Xu JM,Zhong H,Zhang L.Synthesis of various CuO nanostructures via a Na_3PO_4—assisted hydrothermal route in a CuSO_4-NaOH aqueous system and their catalytic performances.Ceram Int.2016;42:3876.

[24] Chen LJ,Zhu DY.The particle dimension controlling synthesis of a-MnO_2 nanowires with enhanced catalytic activity on the thermal decomposition of ammonium perchlorate.Solid State Sci.2014;27:69.

[25] Cheng ZP,Chu XZ,Xu JM,Zhong H,Zhang L.Synthesis of flower-like and dendritic platinum nanostructures with excellent catalytic activities on thermal decomposition of ammonium perchlorate.Mater Res Bull.2016;77:54.

[26] Du JM,Han BX,Liu ZM,Liu YQ.Control synthesis of silver nanosheets,chainlike sheets,and microwires via a simple solvent-thermal method.Crystal Growth Des.2007;7(5):900.

[27] Leopold N,Lendl B.A new method for fast preparation of highly surface-enhanced raman scattering(SERS)active silver colloids at room temperature by reduction of silver nitrate with hydroxylamine hydrochloride.J Phys Chem B.2003;107(24):5723.

[28] Zhang ZT,Zhao B,Hu LM.PVP protective mechanism of ultrafine silver powder synthesized by chemical reduction processes.J Solid State Chem.1996;121(1):105.

[29] Wiley B,Sun YG,Mayers B,Xia YN.Shape-controlled synthesis of metal nanostructures:the case of silver.Chem Eur J.2005;11(2):454.

[30] Luo XL,Wang MJ,Yun L,Yang J,Chen YS.Structure-dependent activities of Cu_2O cubes in thermal decomposition of ammonium perchlorate.J Phys Chem Solids.2016;90:1.

[31] Lan YF,Jin BX,Deng JK,Luo YJ.Graphene/nickel aerogel:an effective catalyst for the thermal decomposition of ammonium perchlorate.RSC Adv.2016;6(85):82112.

[32] Galwey AK,Mohamed MA,Cromie DS.Role of silver compounds in promoting the thermal decomposition of ammonium perchlorate.React Solids.1986;1:235.