Uniform, Scalable, High-Temperature Microwave Shock for Nanoparticle Synthesis through Defect Engineering

Abstract

•A facile, rapid, and universal method for synthesis of nanoparticles•Ultrahigh temperature of 1,600 K can be achieved in less than 100 ms•The number of defects is a key parameter for achieving high temperature•A unique self-quenching mechanism for nanosynthesis is proposed Despite the simplicity of conventional wet chemistry for nanosynthesis, nanoparticles fabricated by this route tend to agglomerate over the long term, particularly at high temperature and pressure, leading to a gradual loss of activity. Here we demonstrate a facile and scalable thermal shock synthesis method based on microwave irradiation for the rapid synthesis of nanoparticles on a reduced graphene oxide (RGO) substrate. Reduced at proper temperature, RGO with medium amount of defects can efficiently absorb microwaves, which leads to the rapid rise of temperature to over 1,600 K in just 100 ms. The pre-loaded precursors were easily decomposed under such high temperature and then reconstructed into nanoparticles during the subsequent rapid quenching process. Various nanoparticles were synthesized to demonstrate the feasibility and universality of this thermal shock technique. This facile, rapid, and universal synthesis method has the potential to contribute to large-scale production of nanomaterials. Here we demonstrate a thermal shock synthesis method triggered by microwave irradiation for the rapid synthesis of nanoparticles on reduced graphene oxide (RGO) substrate. With properly controlled reduction, RGO has high electrical conductivity while maintaining functional groups, leading to an extremely efficient microwave absorption of ∼70%. The high utilization of microwaves results in the ability to raise the temperature to 1,600 K in just 100 ms, which is followed by rapid quenching to room temperature. The defects on the RGO are crucial for achieving this record-high microwave-induced temperature as these defects play a fundamental role in absorbing the radiation as well as the self-quenching mechanism. By loading precursors onto RGO, we can utilize rapid temperature change to synthesize nanoparticles. The nanoparticles are ∼10 nm with uniform distribution. This facile, rapid, and universal synthesis technique has the potential to be employed in large-scale production of nanomaterials and suggests a new direction for nanosynthesis. Here we demonstrate a thermal shock synthesis method triggered by microwave irradiation for the rapid synthesis of nanoparticles on reduced graphene oxide (RGO) substrate. With properly controlled reduction, RGO has high electrical conductivity while maintaining functional groups, leading to an extremely efficient microwave absorption of ∼70%. The high utilization of microwaves results in the ability to raise the temperature to 1,600 K in just 100 ms, which is followed by rapid quenching to room temperature. The defects on the RGO are crucial for achieving this record-high microwave-induced temperature as these defects play a fundamental role in absorbing the radiation as well as the self-quenching mechanism. By loading precursors onto RGO, we can utilize rapid temperature change to synthesize nanoparticles. The nanoparticles are ∼10 nm with uniform distribution. This facile, rapid, and universal synthesis technique has the potential to be employed in large-scale production of nanomaterials and suggests a new direction for nanosynthesis. Nanoparticles are among the most widely studied nanomaterials and have been employed in various applications including catalysis,1Liu L. Corma A. Metal catalysts for heterogeneous catalysis: from single atoms to nanoclusters and nanoparticles.Chem. Rev. 2018; 118: 4981-5079Crossref PubMed Scopus (1930) Google Scholar, 2Zhou Y. Jin C. Li Y. Shen W. Dynamic behavior of metal nanoparticles for catalysis.Nano Today. 2018; 20: 101-120Crossref Scopus (66) Google Scholar, 3Biffis A. Centomo P. Del Zotto A. Zecca M. Pd metal catalysts for cross-couplings and related reactions in the 21st century: a critical review.Chem. Rev. 2018; 118: 2249-2295Crossref PubMed Scopus (633) Google Scholar, 4Li Z. Xu Q. Metal-nanoparticle-catalyzed hydrogen generation from formic acid.Acc. Chem. Res. 2017; 50: 1449-1458Crossref PubMed Scopus (199) Google Scholar, 5Gewirth A.A. Varnll J.A. DiAscro A.M. Nonprecious metal catalysts for oxygen reduction in heterogeneous aqueous systems.Chem. Rev. 2018; 118: 2313-2339Crossref PubMed Scopus (463) Google Scholar photovoltaics,6Hasobe T. Imahori H. Kamat P.V. Ahn T.K. Kim S.K. Kim D. Fujimoto A. Hirakawa T. Fukuzumi S. Photovoltaic cells using composite nanoclusters of porphyrins and fullerenes with gold nanoparticles.J. Am. Chem. Soc. 2005; 127: 1216-1228Crossref PubMed Scopus (456) Google Scholar, 7Ma W. Luther J.M. Zheng H. Wu Y. Alivisatos A.P. Photovoltaic devices employing ternary PbSxSe1-x nanocrystals.Nano Lett. 2009; 9: 1699-1703Crossref PubMed Scopus (426) Google Scholar, 8Liang Z. Dzienis K.L. Xu J. Wang Q. Covalent layer-by-layer assembly of conjugated polymers and CdSe nanoparticles: multilayer structure and photovoltaic properties.Adv. Funct. Mater. 2006; 16: 542-548Crossref Scopus (128) Google Scholar, 9Paci B. Spyropoulos G.D. Generosi A. Bailo D. Albertini V.R. Stratakis E. Kymakis E. Enhanced structural stability and performance durability of bulk heterojunction photovoltaic devices incorporating metallic nanoparticles.Adv. Funct. Mater. 2011; 21: 3573-3582Crossref Scopus (100) Google Scholar, 10Beek W.J.E. Wienk M.M. Janssen R.A.J. Efficient hybrid solar cells from zinc oxide nanoparticles and a conjugated polymer.Adv. Mater. 2004; 16: 1009-1013Crossref Scopus (902) Google Scholar energy storage,11Shelke M.V. Gullapalli H. Kalaga K. Rodrigues M.-T.F. Devarapalli R.R. Vajtai R. Ajayan P.M. Facile synthesis of 3D anode assembly with Si nanoparticles sealed in highly pure few layer graphene deposited on porous current collector for long life Li-ion battery.Adv. Mater. 2017; 4: 1601043Google Scholar, 12Yang S. Qiao Y. He P. Liu Y. Cheng Z. Zhu J. Zhou H. A reversible lithium–CO2 battery with Ru nanoparticles as a cathode catalyst.Energy Environ. Sci. 2017; 10: 972-978Crossref Google Scholar, 13Tabassum H. Zou R. Mahmood A. Liang Z. Wang Q. Zhang H. Gao S. Qu C. Guo W. Guo S. A universal strategy for hollow metal oxide nanoparticles encapsulated into B/N co-doped graphitic nanotubes as high-performance lithium-ion battery anodes.Adv. Mater. 2018; 30: 1705441Crossref Scopus (273) Google Scholar, 14Chang W.-C. Tseng K.-W. Tuan H.-Y. Solution synthesis of iodine-doped red phosphorus nanoparticles for lithium-ion battery anodes.Nano Lett. 2017; 17: 1240-1247Crossref PubMed Scopus (99) Google Scholar, 15Xia G. Gao Q. Sun D. Yu X. Porous carbon nanofibers encapsulated with peapod-like hematite nanoparticles for high-rate and long-life battery anodes.Small. 2017; 13: 1701561Crossref Scopus (49) Google Scholar lubricants,16Rapoport L. Fleischer N. Tenne R. Fullerene-like WS2 nanoparticles: superior lubricants for harsh conditions.Adv. Mater. 2003; 15: 651-655Crossref Scopus (207) Google Scholar, 17Tevet O. Von-Huth P. Popovitz-Biro R. Rosentsveig R. Wagner H.D. Tenne R. Friction mechanism of individual multilayered nanoparticles.Proc. Natl. Acad. Sci. U S A. 2011; 108: 19901-19906Crossref PubMed Scopus (141) Google Scholar food processing,18Chaudhry Q. Scotter M. Blackburn J. Ross B. Boxall A. Castle L. Aitken R. Watkins R. Applications and implications of nanotechnologies for the food sector.Food Addit. Contam. Part A. 2008; 25: 241-258Crossref PubMed Scopus (886) Google Scholar and drug delivery,19Arruebo M. Fernandez-Pacheco R. Ibarra M.R. Santamaria J. Magnetic nanoparticles for drug delivery.Nano Today. 2007; 2: 22-32Crossref Scopus (1314) Google Scholar, 20Liong M. Lu J. Kovochich M. Xia T. Ruehm S.G. Nel A.E. Tamano F. Zink J.I. Multifunctional inorganic nanoparticles for imaging, targeting, and drug delivery.ACS Nano. 2008; 2: 889-896Crossref PubMed Scopus (1633) Google Scholar, 21Kim J. Kim H.S. Lee N. Kim T. Kim H. Yu T. Song I.C. Moon W.K. Hyeon T. Multifunctional uniform nanoparticles composed of a magnetite nanocrystal core and a mesoporous silica shell for magnetic resonance and fluorescence imaging and for drug delivery.Angew. Chem. Int. Ed. 2008; 47: 8438-8441Crossref PubMed Scopus (1142) Google Scholar, 22Tang F. Li L. Chen D. Mesoporous silica nanoparticles: synthesis, biocompatibility and drug delivery.Adv. Mater. 2012; 24: 1504-1534Crossref PubMed Scopus (1986) Google Scholar, 23Slowing I.I. Trewyn B.G. Giri S. Lin V.S.-Y. Mesoporous silica nanoparticles for drug delivery and biosensing applications.Adv. Funct. Mater. 2007; 17: 1225-1236Crossref Scopus (1453) Google Scholar since they can drastically lower the total mass of material required while achieving the same active surface area. Wet chemistry is the most widely used method for nanoparticle synthesis due to its simplicity, inexpensive processing, and scalable nature.24Zhang P. Shao C. Zhang Z. Zhang M. Mu J. Guo Z. Liu Y. In situ assembly of well-dispersed Ag nanoparticles (AgNPs) on electrospun carbon nanofibers (CNFs) for catalyticreduction of 4-nitrophenol.Nanoscale. 2011; 3: 3357-3363Crossref PubMed Scopus (512) Google Scholar, 25Liu L. Gao F. Concepción P. Corma A. A new strategy to transform mono and bimetallic non-noble metal nanoparticles into highly active and chemoselective hydrogenation catalysts.J. Catal. 2017; 350: 218-225Crossref Scopus (72) Google Scholar However, nanoparticles made by wet chemistry tend to agglomerate easily, leading to a gradual loss of activity.26Jin Z. Nackashi D. Lu W. Kittrell C. Tour J.M. Decoration, migration, and aggregation of palladium nanoparticles on graphene sheets.Chem. Mater. 2010; 22: 5695-5699Crossref Scopus (186) Google Scholar, 27Chen Y. Li C.W. Kanan M.W. Aqueous CO2 reduction at very low overpotential on oxide-derived au nanoparticles.J. Am. Chem. Soc. 2012; 134: 19969-19972Crossref PubMed Scopus (1183) Google Scholar, 28Chung D.Y. Jun S.W. Yoon G. Kwon S.G. Shin D.Y. Seo P. Yoo J.M. Shin H. Chung Y.-H. Kim H. et al.Highly durable and active PtFe nanocatalyst for electrochemical oxygen reduction reaction.J. Am. Chem. Soc. 2015; 137: 15478-15485Crossref PubMed Scopus (379) Google Scholar Recently, we developed a transient high-temperature pulse method for the synthesis of nanoparticles on carbon substrates and demonstrated that these materials feature superior catalytic activity and stability.29Yao Y. Huang Z. Xie P. Lacey S.D. Jacob R.J. Xie H. Chen F. Nie A. Pu T. Rehwoldt M. et al.Carbothermal shock synthesis of high-entropy-alloy nanoparticles.Science. 2018; 359: 1489-1494Crossref PubMed Scopus (556) Google Scholar, 30Li T. Pickel A.D. Yao Y. Chen Y. Zeng Y. Lacey S.D. Li Y. Wang Y. Dai J. Wang Y. et al.Thermoelectric properties and performance of flexible reduced graphene oxide films up to 3,000 K.Nat. Energy. 2018; 3: 148-156Crossref Scopus (66) Google Scholar The key feature was the rapid heating/quenching leading to small, narrow-size distribution, unagglomerated nanoparticles. Nevertheless, with the limitation of the Joule heating procedure, it is more difficult to implement for scale-up. Thus, the focus in this work is to develop a heating method that can expand the use of the transient high-temperature pulse technique for the production of nanoparticles on a larger scale. Microwave irradiation has been adopted as a cost-effective heating method for several decades and has been widely employed in different applications.31Jones D.A. Lelyveld T.P. Mavrofidis S.D. Kingman S.W. Miles N.J. Microwave heating applications in environmental engineering—a review.Resour. Conserv. Recycl. 2002; 34: 75-90Crossref Scopus (779) Google Scholar, 32Faraji S. Ani F.N. Microwave-assisted synthesis of metal oxide/hydroxide composite electrodes for high power supercapacitors—a review.J. Power Sources. 2014; 263: 338-360Crossref Scopus (317) Google Scholar, 33Roy R. Agrawal D. Cheng J. Gedevanishvili S. Full sintering of powdered-metal bodies in a microwave field.Nature. 1999; 399: 668-670Crossref Scopus (787) Google Scholar Numerous studies have employed microwave irradiation in nanoparticle synthesis, but the majority of these syntheses have been conducted in liquids, which share many of the same pitfalls as conventional wet chemistry.34Liu Z. Guo B. Hong L. Lim T.H. Microwave heated polyol synthesis of carbon-supported PtSn nanoparticles for methanol electrooxidation.Electrochem. Commun. 2006; 8: 83-90Crossref Scopus (149) Google Scholar, 35Bilecka I. Elser P. Niederberger M. Kinetic and thermodynamic aspects in the microwave-assisted synthesis of ZnO nanoparticles in benzyl alcohol.ACS Nano. 2009; 3: 467-477Crossref PubMed Scopus (203) Google Scholar, 36Vollmer C. Redel E. Abu-Shandi K. Thomann R. Manyar H. Hardacre C. Janiak C. Microwave irradiation for the facile synthesis of transition-metal nanoparticles (NPs) in ionic liquids (ILs) from metal-carbonyl precursors and Ru-, Rh-, and Ir-NP/IL dispersions as biphasic liquid-liquid hydrogenation nanocatalysts for cyclohexene.Chem. Eur. J. 2010; 16: 3849-3858Crossref PubMed Scopus (168) Google Scholar, 37Fuku K. Hayashi R. Takakura S. Kamegawa T. Mori K. Yamashita H. The synthesis of size- and color-controlled silver nanoparticles by using microwave heating and their enhanced catalytic activity by localized surface plasmon resonance.Angew. Chem. Int. Ed. 2013; 52: 7446-7450Crossref PubMed Scopus (203) Google Scholar, 38Liao X. Zhu J. Zhong W. Chen H.-Y. Synthesis of amorphous Fe2O3 nanoparticles by microwave irradiation.Mater. Lett. 2001; 50: 341-346Crossref Scopus (86) Google Scholar, 39Hu Y. Liu Y. Qian H. Li Z. Chen J. Coating colloidal carbon spheres with CdS nanoparticles: microwave-assisted synthesis and enhanced photocatalytic activity.Langmuir. 2010; 36: 18570-18575Crossref Scopus (153) Google Scholar, 40Tsuji M. Hashimoto M. Nishizawa Y. Kubokawa M. Tsuji T. Microwave-assisted synthesis of metallic nanostructures in solution.Chem. Eur. J. 2005; 11: 440-452Crossref PubMed Scopus (660) Google Scholar, 41Hu H. Yang H. Huang P. Cui D. Peng Y. Zhang J. Lu F. Lian J. Shi D. Unique role of ionic liquid in microwave-assisted synthesis of monodisperse magnetite nanoparticles.Chem. Commun. (Camb.). 2010; 46: 3866-3868Crossref PubMed Scopus (101) Google Scholar Carbon substrates and polymers have been used as microwave absorber to enable microwave syntheses of nanomaterials under solid-state conditions.42Lin Y. Baggett D.W. Kim J.-W. Siochi E.J. Connell J.W. Instantaneous formation of metal and metal oxide nanoparticles on carbon nanotubes and graphene via solvent-free microwave heating.ACS Appl. Mater. Interfaces. 2011; 3: 1652-1664Crossref PubMed Scopus (88) Google Scholar, 43Zhang X. Manohar S.K. Microwave synthesis of nanocarbons from conducting polymers.Chem. Commun. (Camb.). 2006; 0: 2477-2479Crossref Scopus (71) Google Scholar, 44Liu Z. Zhang X. Poyraz S. Surwade S.P. Manohar S.K. Oxidative template for conducting polymer nanoclips.J. Am. Chem. Soc. 2010; 132: 13158-13159Crossref PubMed Scopus (119) Google Scholar, 45Liu Z. Wang J. Kushvaha V. Poyraz S. Tippur H. Park S. Kim M. Liu Y. Bar J. Chen H. Zhang X. Poptube approach for ultrafast carbon nanotube growth.Chem. Commun. (Camb.). 2011; 47: 9912-9914Crossref PubMed Scopus (101) Google Scholar, 46Poyraz S. Zhang L. Schroder A. Zhang X. Ultrafast microwave welding/reinforcing approach at the interface of thermoplastic materials.ACS Appl. Mater. Interfaces. 2015; 7: 22469-22477Crossref PubMed Scopus (40) Google Scholar, 47Liu Z. Zhang L. Wang R. Poyraz S. Cook J. Bozack M.J. Das S. Zhang X. Hu L. Ultrafast microwave nano-manufacturing of fullerene-like metal chalcogenides.Sci. Rep. 2016; 6: 22503Crossref PubMed Scopus (31) Google Scholar, 48Bi Y. Nautiyal A. Zhang H. Luo J. Zhang X. One-pot microwave synthesis of NiO/MnO2 composite as a high-performance electrode material for supercapacitors.Electrochim. Acta. 2018; 260: 952-958Crossref Scopus (52) Google Scholar, 49Tian Y. Yang X. Nautiyal A. Zheng Y. Guo Q. Luo J. Zhang X. One-step microwave synthesis of MoS2/MoO3@graphite nanocomposite as an excellent electrode material for supercapacitors.Adv. Compos. Hybrid Mater. 2019; 2: 151-161Crossref Scopus (54) Google Scholar, 50Liu Y. Zhang X. Poyraz S. Zhang C. Xin J.H. One-step synthesis of multifunctional zinc-iron-oxide hybrid carbon nanowires by chemical fusion for supercapacitors and interfacial water marbles.ChemNanoMat. 2018; 4: 546-556Crossref Scopus (11) Google Scholar These techniques often require relatively long heating times (>1 min) and the peak temperatures tend to be relatively low (<1,000°C). As a result, precursor materials do not undergo ultrafast melting or reconstruction, causing the resulting particles to suffer from the aforementioned disadvantages often associated with wet chemical processes. In this work, we develop a rapid thermal shock method for nanoparticle synthesis using microwave irradiation. Reduced graphene oxide (RGO) has been demonstrated as an excellent carbon support for nanoparticles and an efficient microwave absorber.51Xu S. Chen Y. Li Y. Lu A. Yao Y. Dai J. Wang Y. Liu B. Lacey S.D. Pastel G.R. et al.Universal, in situ transformation of bulky compounds into nanoscale catalysts by high-temperature pulse.Nano Lett. 2017; 17: 5817-5822Crossref PubMed Scopus (21) Google Scholar, 52Voiry D. Yang J. Kupferberg J. Fullon R. Lee C. Jeong H.Y. Shin H.S. Chhowalla M. High-quality graphene via microwave reduction of solution-exfoliated graphene oxide.Science. 2016; 353: 1413-1416Crossref PubMed Scopus (542) Google Scholar, 53Fei H. Dong J. Wan C. Zhao Z. Xu X. Lin Z. Wang Y. Liu H. Zang K. Luo J. et al.Microwave-assisted rapid synthesis of graphene-supported single atomic metals.Adv. Mater. 2018; 30: 1802146Crossref PubMed Scopus (177) Google Scholar Using RGO produced under controlled reduction conditions, we demonstrate that its temperature can be raised to over 1,600 K in just 100 ms by the absorption of microwave radiation—a temperature that is over 2-fold higher than in previously reported microwave synthesis methods.40Tsuji M. Hashimoto M. Nishizawa Y. Kubokawa M. Tsuji T. Microwave-assisted synthesis of metallic nanostructures in solution.Chem. Eur. J. 2005; 11: 440-452Crossref PubMed Scopus (660) Google Scholar, 41Hu H. Yang H. Huang P. Cui D. Peng Y. Zhang J. Lu F. Lian J. Shi D. Unique role of ionic liquid in microwave-assisted synthesis of monodisperse magnetite nanoparticles.Chem. Commun. (Camb.). 2010; 46: 3866-3868Crossref PubMed Scopus (101) Google Scholar, 42Lin Y. Baggett D.W. Kim J.-W. Siochi E.J. Connell J.W. Instantaneous formation of metal and metal oxide nanoparticles on carbon nanotubes and graphene via solvent-free microwave heating.ACS Appl. Mater. Interfaces. 2011; 3: 1652-1664Crossref PubMed Scopus (88) Google Scholar, 43Zhang X. Manohar S.K. Microwave synthesis of nanocarbons from conducting polymers.Chem. Commun. (Camb.). 2006; 0: 2477-2479Crossref Scopus (71) Google Scholar Moreover, at these high temperatures the kinetics become sufficiently fast such that reaction times are less than 1 s. This is dramatically shorter than any reported in previous studies using microwave synthesis.41Hu H. Yang H. Huang P. Cui D. Peng Y. Zhang J. Lu F. Lian J. Shi D. Unique role of ionic liquid in microwave-assisted synthesis of monodisperse magnetite nanoparticles.Chem. Commun. (Camb.). 2010; 46: 3866-3868Crossref PubMed Scopus (101) Google Scholar, 42Lin Y. Baggett D.W. Kim J.-W. Siochi E.J. Connell J.W. Instantaneous formation of metal and metal oxide nanoparticles on carbon nanotubes and graphene via solvent-free microwave heating.ACS Appl. Mater. Interfaces. 2011; 3: 1652-1664Crossref PubMed Scopus (88) Google Scholar, 43Zhang X. Manohar S.K. Microwave synthesis of nanocarbons from conducting polymers.Chem. Commun. (Camb.). 2006; 0: 2477-2479Crossref Scopus (71) Google Scholar, 44Liu Z. Zhang X. Poyraz S. Surwade S.P. Manohar S.K. Oxidative template for conducting polymer nanoclips.J. Am. Chem. Soc. 2010; 132: 13158-13159Crossref PubMed Scopus (119) Google Scholar By loading precursor materials onto the RGO, the rapid temperature increase upon microwaving causes the precursors to undergo decomposition, and the subsequent rapid cooling leads to particle nucleation. Due to this ultrafast phase transition, the resulting nanoparticles are both small in size (∼10 nm) and feature a uniform size distribution. The rapid quench also freezes any mobility, preventing agglomeration. We also show that the defect concentration in the RGO substrate is a key factor in successfully triggering this thermal shock effect, as the defects aid in the microwave absorption as well as participate in the rapid self-quenching mechanism. Additionally, with the simplicity of microwave heating process, the synthesis can be easily scaled up while maintaining the uniformity of the nanoparticle size. This scalable, high-temperature pulse generated by microwave irradiation provides a new route for the synthesis of uniform nanoparticles and can shed light on further innovation in nanosynthesis. Figure 1 illustrates the concept of this scalable thermal shock synthesis using microwave irradiation. Since the microwave is uniformly distributed in a large space, the synthesis can be easily scaled up to produce large batches of powder RGO samples decorated with nanoparticles. Initially, we loaded the RGO with salt precursors that are microscale in size (schematics in Figure 1A). After less than 1 s of microwave irradiation, during which the temperature was as high as 1,600 K, the micron-sized precursors decompose, and upon quenching nucleate to form nanoparticles decorating the RGO film. Although there was no apparent macroscopic change before and after microwave heating (photos in Figure 1A), the nanoscale morphology is drastically different. We have found that the rapid temperature change can only be achieved by carefully controlling the defect density on the RGO substrate (Figure 1B). Sufficient defects are needed first to promote high microwave absorption efficiency; however, too high a defect density leads to poor thermal conductivity and inhomogeneous sample temperature. Compared with previous studies, the microwave irradiation synthesis method reported in this work has the highest temperature and the shortest heating time (Figure 1C), making it a unique thermal shock process. To demonstrate the feasibility of the thermal shock synthesis using microwave irradiation, we chose to synthesize cobalt sulfide (CoS) nanoparticles as a proof of concept. Figure 2A shows the photographs of the RGO sample during the process of microwave irradiation, captured using a high-speed video camera. The images are computer processed to capture the raw color of each pixel and enabling a spatial- and time-resolved ratio pyrometer to obtain the temperature (Figure S1). Figure 2B shows a plot of the sample light emission intensity versus time over the synthesis. By fitting the light emission to Planck's law for gray-body radiation, we were able to calculate the average temperature over all pixels (Figure 2C). Visible emission is observed approximately ∼100 ms after the microwave is turned on and once several bright spots can be seen. These hotspots are localized so that the total emission is relatively low (Figure 2B); nevertheless, the average temperature at ∼100 ms was as high as 2,000 K (Figure 2C). As the heating progressed, the initial locally generated heat spreads to the whole sample, leading to the rapid increase in light intensity, making the sample very bright. Although the light emission becomes stronger, since the whole sample is hot the temperature drops and stabilizes at ∼1,600 K. After ∼250 ms of microwave heating, the sample emits light with peak brightness. Thereafter, the light intensity starts dropping and the sample becomes visibly dimmer, until it eventually extinguishes after 600 ms and we cannot measure temperature past this point. The total effective heating time of ∼600 ms is orders of magnitude shorter than that of previous reports,41Hu H. Yang H. Huang P. Cui D. Peng Y. Zhang J. Lu F. Lian J. Shi D. Unique role of ionic liquid in microwave-assisted synthesis of monodisperse magnetite nanoparticles.Chem. Commun. (Camb.). 2010; 46: 3866-3868Crossref PubMed Scopus (101) Google Scholar, 42Lin Y. Baggett D.W. Kim J.-W. Siochi E.J. Connell J.W. Instantaneous formation of metal and metal oxide nanoparticles on carbon nanotubes and graphene via solvent-free microwave heating.ACS Appl. Mater. Interfaces. 2011; 3: 1652-1664Crossref PubMed Scopus (88) Google Scholar, 43Zhang X. Manohar S.K. Microwave synthesis of nanocarbons from conducting polymers.Chem. Commun. (Camb.). 2006; 0: 2477-2479Crossref Scopus (71) Google Scholar, 44Liu Z. Zhang X. Poyraz S. Surwade S.P. Manohar S.K. Oxidative template for conducting polymer nanoclips.J. Am. Chem. Soc. 2010; 132: 13158-13159Crossref PubMed Scopus (119) Google Scholar making this a unique thermal shock process. Although the intensity of the light emission changes constantly during the microwave synthesis (Figure 2B), the temperature is almost constant during the whole heating process, maintaining a high temperature of ∼1,600 K (Figure 2C). The temperature reported in Figure 2C is the median temperature spatially averaged over the entire sample. As such the temperature is biased toward the hotspots that emit the most light. This is the reason why the emission entity can increase while the temperature appears to be constant. Based on these results, we divide the microwave synthesis into three periods: (1) initial heating; (2) high-temperature thermal shock; and (3) rapid cooling. We employed scanning and transmission electron microscopy (SEM and TEM) to study the morphology of the nanoparticle products (Figure 3). Lower-magnification SEM (Figure 3A) and TEM (Figure S2) capture the panorama of the synthesized product, which shows that the CoS nanoparticles feature a uniform size distribution (10.8 ± 2.3 nm, Figure 3B) that densely covers the surface of the RGO. High-magnification TEM images indicate that the nanoparticles are 8–10 nm in diameter with lattice spacing of 0.19 nm, which corresponds to the (102) plane of CoS (Figure 3C). X-ray diffraction was employed to study the crystal structure of the product, which shows the characteristic peaks of CoS (Figure S3). Energy dispersive X-ray spectroscopy mapping shows the matching between the distribution of Co (Figure 3D) and S (Figure 3E), indicating that the expected product composition was achieved. Due to their small size and uniform distribution, the CoS nanoparticles show excellent catalytic performance toward the hydrogen evolution reaction (HER) (Figure S4). In acidic solution, the CoS nanoparticles show a low onset overpotential of 120 mV (Figure S4A) and a Tafel slope of 102 mV/dec (Figure S4B), indicating the remarkable catalytic activity of the material. Additionally, the overpotential only showed a shift of 20 mV after 1,000 HER cycles (Figure S4C), demonstrating the excellent stability of the nanoparticles for long-term operation. At an overpotential of 180 mV, the current density remains almost unchanged for over 10 h, which indicates the catalyst's ability to survive high-stress production (Figure S4D). This superb activity and remarkable stability prove that our thermal shock synthesis using microwave irradiation is an excellent choice for the fabrication of nanocatalysts. Since this method using microwave irradiation should be easily applied for large-scale production, the uniformity of the synthesized nanoparticles in large batches is a crucial criterion for judging the technique's feasibility. To study the uniformity of the synthesis, we synthesized 100 mg of CoS/RGO samples in a single thermal shock process (Figure S5). From this batch, we imaged the material at five different positions using SEM to compare the nanomaterial morphology and distribution. Even at these different locales, the morphologies remained almost identical, with all five samples showing ∼10-nm-sized CoS nanoparticles densely decorating the surface of the RGO. These results show that thermal shock microwave heating is an exceptional choice for the scaled-up synthesis of small and uniform nanoparticles. To understand the mechanism of the thermal shock induced by microwave irradiation, we tested different RGO substrates made at different reducing temperatures. Our results showed that only RGO reduced at 300°C (RGO-300) demonstrated the thermal shock phenomenon, while pristine GO (GO) and RGO reduced at 600°C (RGO-600) did not trigger such behavior. We hypothesize that this is due to the difference in the amount of defects and functional groups between RGO with various degrees of reduction. RGO is rich in various oxygen defects and functional groups (Figure 4A ). The dipoles of these functional groups can rotate in the presence of an electromagnetic field, thus leading to frictional heating, which we believe is the main contributor to the microwave absorption. The oxygen-rich functional groups rapidly absorb the microwaves to generate heat and drastically increase the sample temperature. Another mechanism of microwave absorption involves the generation of eddy currents, which is induced when a conductive material is treated with an oscillating electromagnetic field. Due to the conducting nature of RGO, an eddy current loop can be formed when placed in a microwave field, leading to Joule heating and triggering the thermal shock. Additionally, in order to trigger this thermal shock effect, high thermal conductivity is also required to transport the absorbed heat to the whole sample. Therefore, the RGO substrate needs to meet three requirements to maximize the ability to trigger the thermal shock synthesis. (1) The substrate should have an adequate number of functional groups after reduction to provide sufficient rotating dipoles in the microwave field. (2) The substrate should have sufficient electronic conductivity to generate eddy currents in the presence of microwaves. However, the conductivity should not be so high that the substrate is almost metallic, leading to the majority of the microwaves being reflected instead of absorbed. (3) High thermal conductivity is also a prerequisite for the thermal shock effect, since the absorbed heat needs to quickly spread through the whole sample. Therefore, the degree of the RGO reduction is a crucial factor for this method of nanoparticle synthesis. To better understand the effect of defects and functional groups, we employed Fourier transform infrared spectroscopy (FTIR) (Figure 4B) to study different substrates. The results revealed that RGO-600 has a lower content of –OH, –C=O, and –CO–O–CO groups compared with GO and RGO-300. The low functional group content combined with the almost metallic properties of RGO-600, leads to a low microwave absorption of only 20% (Figure 4C), which explains the absence of the thermal shock phenomenon with the RGO-600 substrate. On the other hand, because RGO-300 has moderate electronic conductivity to generate eddy currents and sufficient functional groups to produce rotating dipoles, it has a high microwave absorption of over 60%, leading to its strong ability to trigger the thermal shock. It is worth noting that GO does not trigger the thermal shock either, despite its high content of functional groups, likely because its low electronic conductivity leads to the inability to form eddy current loops and thus absorb less microwave radiation. Additionally, due to the low thermal conductivity of GO, even though microwaves can be absorbed and areas of the sample can become heated, the heat is isolated and cannot easily spread to the whole sample, leading to the failure of the overall thermal shock effect. The combination of high functional group content, microwave absorption, and thermal conductivity results in RGO-300 being the best substrate for this application. After the microwave synthesis in argon, the FTIR spectrum reveals drastically lower content of –OH, –C=O, and –CO–O–CO groups (Figure 4D), indicating the consumption of the functional groups and a more graphitic structure. As a result, the substrate's ability to absorb microwave radiation is significantly attenuated, creating a self-quenching process that results in a rapid decrease in temperature (Figure 4D). This whole synthesis process occurs in just 600 ms. In summary, the electronic conductivity and functional groups enable the absorption of microwave irradiation and the initial isolated heating effect, which is followed by the conduction of heat to the whole sample. This process ultimately consumes the substrate functional groups, causing the temperature to immediately drop due to the loss of microwave absorption. The feasibility of the thermal shock synthesis using microwave irradiation is not limited to just the synthesis of CoS nanoparticles. We demonstrated the universality of this method by synthesizing other materials, including ruthenium (Ru), palladium (Pd), and iridium (Ir) nanoparticles (Figures 5 and S7). As a result of the high scalability of the microwave synthesis, we were able to produce over 100 mg of product in a single batch synthesis (Figures 5A–5C). After synthesis, the nanoparticles formed on the RGO film featured a uniform size of ∼20 nm (Figures 5D–5F). Single nanoparticle microanalysis shows lattice spacings of 0.2 nm (Ru), 0.22 nm (Pd), and 0.22 nm (Ir), corresponding to the (111) plane of zero-valent metals. This successful synthesis of metal nanoparticles demonstrates the universality of the thermal shock synthesis technique by microwave irradiation. The rapid and facile approach can be further employed in the synthesis of other various nanomaterials. In conclusion, we have developed a facile and scalable thermal shock method for nanoparticle synthesis using microwave irradiation. Using RGO reduced at 300°C as the microwave absorber, this substrate's ample functional groups lead to high microwave absorption and consequently a rapid local temperature increase to over 2,000 K in just 100 ms. The heat then spreads over the entire sample due to the good thermal conductivity of RGO, which results in a stabilized heating temperature of 1,600 K. After the consumption of the functional groups, the temperature rapidly drops in less than 300 ms, resulting in a unique self-quenching mechanism. Such rapid and extreme temperature change leads to ultrafast melting and decomposition of loaded precursor materials and their subsequent condensation into nanoparticles, which are both uniformly small (∼10 nm) and unagglomerated. Using CoS as a proof of concept, the synthesized nanoparticles densely decorate the surface of the RGO, which leads to excellent catalytic activity for HER performance. Additionally, due to the simplicity of the microwave irradiation treatment, the synthesis can be scaled up to large batch synthesis, the uniformity of which we have proved through the identical morphology of the products at different locations of the batch material. This thermal shock nanoparticle synthesis using microwave irradiation can also be applied to other materials. The rapid, facile, and scalable thermal shock method made possible by microwave heating and an electrically and thermally conductive substrate is advantageous over conventional wet chemistry techniques and has the potential to be employed for large-scale nanoparticle production.