Continuous synthesis of highly uniform noble metal nanoparticles over

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Continuous synthesis of highly uniform noble metal nanoparticles over reduced graphene oxide using microreactor technology Sha Tao, Mei Yang, Huihui Chen, and Guangwen Chen ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01032 • Publication Date (Web): 21 May 2018 Downloaded from http://pubs.acs.org on May 21, 2018

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ACS Sustainable Chemistry & Engineering

Continuous synthesis of highly uniform noble metal nanoparticles over reduced graphene oxide using microreactor technology Sha Tao†, ‡, Mei Yang†, *, Huihui Chen†, ‡, Guangwen Chen†, * † Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, 116023, China ‡ University of Chinese Academy of Sciences, Beijing, 100049, China

Corresponding authors Mei Yang: +86-411-8437-9816 (phone), +86-411-8437-9327 (fax), [email protected] (e-mail) Guangwen Chen: +86-411-8437-9031 (phone), +86-411-8437-9327 (fax), [email protected] (e-mail)

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Abstract Batch reactors always suffer from inefficient transport properties, discontinuity and scale-up effect, challenging the particle size control, reproducibility and large-scale production of noble metal-reduced graphene oxide composites. To address these issues, a microfluidic-based strategy for the continuous synthesis of highly uniform Ag nanoparticles (NPs) over reduced graphene oxide (Ag-rGO composites) was developed in this study. Ag-rGO composites were formed by the co-reduction of AgNO3 and GO with NaBH4, which was confined inside the dispersed aqueous plugs segmented by octane. By virtue of enhanced mixing and precise control of reaction parameters in the plugs, ultrafine Ag NPs with controlled particle size (1.5-5.6 nm) and narrow particle size distribution (PSD) were evenly deposited on rGO. The average particle size of Ag NPs and relative standard deviation of particle size in Ag-rGO composites synthesized via microfluidic-based strategy were smaller than those via batch method. Moreover, the versatility of this microfluidic-based strategy was further demonstrated in the continuous synthesis of Pt-rGO and Pd-rGO composites.

Keywords: microfluidic; microchannel; continuous; segmented flow; mixing

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Introduction Graphene, a two-dimensional layer of sp2-hybridized carbon atoms arranged in a honeycomb pattern, has attracted numerous attentions owing to its exceptional physical properties including large specific surface area, high electronic and thermal conductivity, and excellent mechanical strength, etc.1,2 Unfortunately, the application of graphene is greatly hindered by the inherent characteristics such as zero band gap, inert surface and hydrophobic nature. In addition, graphene is liable to aggregate or even restack due to strong van der Waals interaction. To solve the aforementioned problems and extend the applications of graphene, many efforts have been made to graphene functionalization via covalent/noncovalent attachments of functionalities (e.g. aminopropyltriethoxysilane, polyglycerol and polyaniline),3,4 substitutional doping (e.g. N, B and P atoms),5 and deposition of nanoparticles (e.g. noble metals, metal oxides and quantum dots).6,7 Among these strategies, decorating graphene with noble metal nanoparticles (NPs) can endow graphene new optical, electrochemical and catalytic properties.8,9 For instance, Zhang et al. fabricated an Ag nanocube-reduced graphene oxide sponge. The rGO sheets formed a porous scaffold and physically held Ag nanocubes, leading to the formation of “hot spots” for surface-enhanced Raman scattering (SERS) signal amplification.10 Jeong et al. found that the deposition of Ru NPs on rGO greatly lowered the charge overpotential of rGO for oxygen evolution reaction by controlling the nature of discharge product.11 The performances of noble metal-based composites are found to be remarkably influenced by the size of noble metal NPs.12,13 For example, Chen et al. found that Pt/CNT exhibited a size-dependent activity in the hydrolytic dehydrogenation of ammonia borane, and the catalyst with Pt size of ca. 1.8 nm was optimum.14 Isaifan et al. reported that the smallest Pt nanoparticles over yttria-stabilized zirconia (1.9 ± 0.4 nm) showed the highest catalytic activity for CO and C2H4 oxidation without and in the presence of molecular O2.15 These studies undoubtedly demonstrate that the precise control over the size of noble metal NPs is of vital importance for modulating the performances of noble metal-based composites. As a new class of noble metal-based composites, noble metal-graphene composites are traditionally synthesized through reduction-deposition method involving the reduction of noble metal ions in the presence of graphene.16,17 This process is usually carried out in the batch reactor such as flask and beaker. However, it is difficult for the batch reactor to offer a homogeneous nucleation and growth environment because of poor transport performances. Therefore, the broad PSD, low dispersity and bad reproducibility may be 3



unavoidably induced, leading to poor tunability of the size-dependent performance. Meanwhile, the batch method always suffers from discontinuity and scale-up effect, which puts forward a severe challenge on the large-scale production. Hence, it is very urgent to develop a new strategy to achieve precise control over the size and dispersity of noble metal NPs and continuous operation. As a promising process intensification technology, microreactor technology can offer improved transport properties, accurate control over experimental conditions, continuous operation and easy scale-up, to name a few.18 Thus, many issues of batch reactors can be addressed by microreactor technology. To date, many nano/micro materials (e.g. noble metals, metal oxides, quantum dots and composites) with narrow PSD have been continuously synthesized in the microreactor.19-21 Although great progress has been made in the continuous synthesis of nano/micro materials, little attention has been paid to preparing noble metal-graphene composites in the microreactor. In this work, we proposed a microfluidic-based strategy for the continuous synthesis of noble metal nanoparticles with narrow PSD over reduced graphene oxide. Ag was employed as a representative noble metal to demonstrate the versatility of this approach. Ag-rGO composites were formed in the dispersed aqueous plugs segmented by octane. Spherical-like Ag NPs with the average size ranging from 1.5 ± 0.6-5.6 ± 2.5 nm were uniformly distributed on rGO sheets. In addition, this approach was successfully applied to the deposition of Pt (1.7 ± 1.0 nm) and Pd (3.2 ± 1.1 nm) NPs on rGO sheets.

Experimental Section Chemicals Silver nitrate (AgNO3), sodium dodecyl sulfate (C12H25SO4Na, SDS), trisodium citrate (Na3C6H5O7·2H2O), sodium borohydride (NaBH4), octane (CH3(CH2)6CH3), palladium nitrate (Pd(NO3)2), hydrochloroplatinic acid (H2PtCl6), sulphuric acid (H2SO4), boric acid (H3BO3), potassium iodide (KI), sodium hydrate (NaOH) and potassium iodate (KIO3) were purchased with analytical grade. GO suspension (0.7-1.2 nm in sheet thickness, 2 mg/mL) prepared by the modified Hummers method was obtained from Suzhou Tanfeng Tech. Inc. The BET specific surface area of corresponding GO power was 1000-1217 m2/g. No further treatment was needed for all reagents. Deionized water was employed in this work. Materials and geometries of microreactors 4


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The microreactor technology-based platform used for the continuous synthesis of noble metal-rGO composites consisted of consecutive cross-type mixer, capillary I, T-type mixer and capillary II. The materials and geometries of these microreactors were summarized in Table S1. Synthesis of Ag-rGO composites in two-phase segmented flow Firstly, 0.14 g of SDS, 0.03 g of trisodium citrate and a certain amount of AgNO3 were co-dissolved in 45 mL of deionized water, followed by the addition of 5 mL of GO suspension (2 mg/mL) into this solution (solution A). Subsequently, two aqueous NaBH4 solutions with different concentrations were prepared (solution B and C). The volumes of both solutions were 50 mL. The molar ratio of AgNO3 to NaBH4 in solution B and C was fixed at 1:3 and 1:20, respectively. Then, solution A, B and octane were injected into cross-type mixer at predetermined volume flow rates by syringe pumps (LongerPump LSP02-1B). The flow rates of solution A, B and octane were denoted as QA, QB and Qoctane, respectively. The reactant flowed through cross-type mixer, capillary I, T-type mixer and capillary II in turn. Solution C was injected into T-type mixer with the volume flow rate (QC) identical to solution B. Capillary I and capillary II were immersed in the same water bath (IKA C-MAG HS7 digital) to obtain desired temperature. Finally, Ag-rGO composites collected at the outlet of capillary II were centrifuged and washed with deionized water and ethanol by turn. The as-prepared Ag-rGO composites were denoted as Ag-rGO-S. Ag-rGO-S with theoretical Ag weight percentage of 11.7 wt.%, 21.3 wt.% and 28.8 wt.% were prepared. The detailed experimental conditions were summarized in Table S2 including water/oil volume flow ratio (W/O ratio), total volume flow rate (Qtotal), synthetic temperature (T) and residence time (τ). As the average particle size and PSD of Ag NPs in Ag-rGO-S were dramatically affected by the process parameters in the first stage, W/O ratio and Qtotal were calculated by the volume flow rates of solution A, B and octane in the first stage. Pt-rGO and Pd-rGO composites were synthesized under the same condition with Ag-rGO composites. Synthesis of Ag-rGO composite in the batch reactor A mixed aqueous solution (50 mL, solution A) of AgNO3, SDS, trisodium citrate and GO and aqueous NaBH4 (50 mL, solution B) were prepared. The concentrations of SDS, trisodium citrate and GO were the same as those used in segmented flow. The molar ratio of AgNO3 to NaBH4 was fixed at 1:23, and the theoretical Ag weight percentage in Ag-rGO composites was 21.3 wt.%. The synthesis of Ag-rGO composites was conducted by dropwise addition of solution A and B into a flask containing 10 mL 5



deionized water under magnetic stirring at 40 °C. The addition was completed after 50 min. After the addition was completed, the suspension was maintained at 40 oC for 10 min. Subsequently, the suspension was centrifuged and washed by the same procedure as Ag-GO-S. The as-prepared composite was denoted as Ag-rGO-B. Characterization X-ray powder diffractometer (PANalytical X'pert Pro) was employed to record the X-ray diffraction (XRD) pattern. The scanning rate and range (2θ) were 5°/min and 5°-90°, respectively. Raman spectrum was recorded using a Bruker Optics Senterra confocal Raman microscopy system with the excitation wavelength of 532 nm. X-ray photoelectron spectroscopy analysis (XPS) was carried out on an ESCALAB 250Xi system, using Al Ka radiation as the X-ray source. The UV-vis absorption spectrum was measured on a UV-vis spectrophotometer (METASH UV8000). Transmission electron microscopy (TEM, JEOL JEM-2000EX, accelerating voltage of 120 kV) was used to observe the morphologies of as-prepared products. Thermo-gravimetric analysis (TGA) was performed on a thermal gravimetric analyzer (Netzsch STA 449 F3) with a heating rate of 20 oC/min and temperature ranging from room temperature to 800 ºC. The system was purged by N2 and 6-10 mg of the sample was used.

Results and discussion Scheme 1 depicts the experimental arrangement for the continuous synthesis of Ag-rGO composites in the microreactor technology-based platform. As shown in Scheme 1, the formation of Ag-rGO composites can be divided into two stages: (1) the deposition of Ag NPs onto GO sheets (2) the reduction of Ag-GO composites to Ag-rGO composites. Two-phase segmented flow was employed to avoid the potential risk of clogging originating from the small channel diameter (i.d.=0.6 mm). Two streams of aqueous solutions (solution A: AgNO3+GO+SDS+trisodium citrate; solution B: NaBH4) were merged, and then segmented into dispersed plugs by employing octane as continuous phase. A thin octane film zone was widely found to exist between aqueous plugs and microchannel wall, which overcame the problem of microchannel clogging.22,23 More importantly, the recirculation inside the aqueous plugs led to intensified mixing and narrower residence time distribution (RTD) compared to laminar flow.24 In the first stage, the formation of Ag-GO composites through reducing Ag+ ions with NaBH4 in the presence of GO sheets was confined in the plugs (Fig. S1). It is well known that there are a large number of oxygen-containing 6


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functional groups on the surface of GO. These oxygen-containing functional groups can act as nucleation sites for the growth of Ag nanoparticles. Benefiting from the enhanced mixing and narrow RTD, the reduction of Ag+ ions with NaBH4 and subsequent deposition of Ag NPs on GO sheets proceeded in a uniform reaction environment, avoiding concentration gradient. Considering the fact that the oxygen-containing functional groups on the surface of GO always trigger a low conductivity, an additional reduction process is required for increasing conductivity. Therefore, excessive amount of fresh aqueous NaBH4 (solution C) was introduced in the second stage to reduce GO to rGO. In a typical procedure, the volume flow rates of solution A, B, C and octane were set as 0.2, 0.2, 0.2 and 0.6 mL/min, respectively. The corresponding residence time was calculated to be 1.38 min. To demonstrate the efficiency of this microreactor technology-based strategy, the space time yields of Ag-rGO composites synthesized in continuous mode and batch mode were compared (Table S3). The space time yields of Ag-rGO composites synthesized in continuous mode and batch mode were 2.15 and 0.12 g⋅h-1⋅L-1, respectively, demonstrating that the space time yield of Ag-rGO composites was improved in continuous mode.

Scheme 1. Experimental arrangement used for the continuous synthesis of Ag-rGO composites based on microreactor technology.

Characterization of Ag-rGO composites The structure of Ag-rGO composites synthesized in segmented flow (Ag-rGO-S) was firstly characterized by XRD. As shown in Figure 1A, the diffraction peak at 11.2o in the XRD pattern of GO can be assigned to the (0 0 1) facet of GO.25 This peak completely disappears in the XRD pattern of Ag-rGO-S,

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revealing that GO is completely converted to rGO.26,27 The diffraction peaks at 38.117°, 44.279° and 64.428° can be assigned to cubic Ag (JCPDS No. 00-004-0793). Besides, no other diffraction peaks can be observed, implying there is little impurity in Ag-rGO-S. Figure 1B displays the Raman spectra of GO and Ag-rGO-S. Two characteristic peaks are observed in the Raman spectrum of GO, including D band (1349 cm-1) caused by the breathing-mode of κ-point photons of A2g symmetry, and G band (1609 cm-1) arising from E2g mode of sp2 carbon atoms.28 In comparison with GO, the D band and G band of Ag-rGO-S both exhibit obvious blue shifts of ca. 5 and 17 cm-1, respectively, further confirming the reduction of GO to rGO.29,30 Additionally, the intensities of D band and G band of Ag-rGO-S are 553% and 375% higher than those of GO. This increase is caused by SERS effect of Ag NPs.

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Figure 1. (A) XRD patterns (B) Raman spectra of (1) GO and (2) Ag-rGO-S. Ag-rGO-S was synthesized

at QA=QB=0.2 mL/min, Qoctane=0.6 mL/min, W/O ratio=2:3, Qtotal=1.0 mL/min, T=40 oC, theoretical Ag 8


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weight percentage=21.3 wt.%, τ=1.38 min.

The surface composition and chemical state of GO and Ag-rGO-S were studied by XPS. As depicted in Figure S2A, the deconvolution result of C 1s spectrum implies that there are three kinds of carbon species in GO, including C-C (284.6 eV), C-O (286.8 eV) and C=O (288.6 eV).31 For Ag-rGO-S, the peak area percentage of carbon species bound to oxygen is dramatically decreased, demonstrating that the oxygen-containing functional groups are mostly removed by the reduction of NaBH4 (Figure S2B). The characteristic peaks of metallic Ag located at 368.2 and 374.2 eV evidence the successful decoration of rGO sheets with Ag NPs (Figure S2C). Figure S3A shows the TGA curves of GO and Ag-rGO-S. The weight loss below 100 oC can be attributed to the evaporation of adsorbed water, while the weight loss ranging from 100 to 300 oC arises from the thermal decomposition of oxygen-containing functional groups. Ag-rGO-S loses a much smaller weight than GO, implying a smaller number of oxygen-containing functional groups on Ag-rGO-S. The optical properties of GO and Ag-rGO-S were studied by UV-vis absorption spectroscopy. As shown in Figure S3B, GO shows two distinct absorption peaks. The maximum peak centered at 229 nm corresponds to π→π* transitions of aromatic C-C bonds, while the shoulder peak centered at 303 nm is related to n→π* transitions of C=O bonds.32 In the case of Ag-rGO-S, a new absorption peak appears at 392 nm, which is attributed to the strong localized surface plasmon resonance of Ag NPs. In addition, the characteristic absorption peak of GO at 229 nm shifts to 240 nm; the one at 303 nm disappears due to the removal of oxygen-containing functional groups. TEM was performed to visually observe the morphology and size of GO and Ag-rGO-S. For comparison, Ag-rGO composites were also synthesized in the batch reactor under the same condition (Ag-rGO-B). Figure 2 shows the TEM images of GO, Ag-rGO-B and Ag-rGO-S. As shown in Figure 2A, GO is composed of thin and transparent sheets with tiny wrinkles on its surface. As for Ag-rGO-B, spherical-like Ag NPs with the particle size ranging from 1.8 to 32.4 nm are uniformly distributed on the surface of rGO sheets (Figure 2B). The average particle size is statistically calculated to be 8.3 ± 5.8 nm (Figure S4A). In the case of Ag-rGO-S, the average particle size of Ag NPs is dramatically reduced to 3.1 ± 1.3 nm (Figure S4B). No aggregated or free-standing Ag NPs are found. The relative standard deviation (RSD) of particle size is commonly used to quantify the PSD of nanoparticles with different sizes.33,34 The RSD calculated from particle size histogram of Ag NPs is 69.9% for Ag-rGO-B but only 43.0% for 9



Ag-rGO-S. This indicates the PSD of Ag NPs in Ag-rGO-S is much narrower than that of Ag-rGO-B. As expected, this extremely small particle size and narrow PSD of Ag NPs in Ag-rGO-S can be attributed to the rapid mixing and narrow RTD in the dispersed plugs.

Figure 2. TEM images of (A) GO (B) Ag-rGO-B (C) Ag-rGO-S. Ag-rGO-S was synthesized at QA=QB=0.2 mL/min, Qoctane=0.6 mL/min, W/O ratio=2:3, Qtotal=1.0 mL/min, T=40 oC, theoretical Ag

weight percentage=21.3 wt.%, τ=1.38 min.

Effects of process parameters

To achieve a better control over the continuous synthesis of Ag-rGO-S, the effects of process parameters (i.e. W/O ratio, total volume flow rate and synthetic temperature) on the average size and PSD of Ag NPs were systematically investigated. Firstly, W/O ratio was varied from 2:3 to 4:1, while the other parameters were maintained constant. Figure 3A-C show the TEM images of Ag-rGO-S obtained at diverse W/O ratios. It can be clearly seen that homogeneous dispersion of Ag NPs on rGO sheets is obtained in all samples. When W/O ratio goes up from 2:3 to 4:1, the average particle size of Ag NPs increases from 3.1 ± 1.3 to 4.1 ± 1.8 nm, with the RSD rising from 43.0% to 47.7% (Figure S5 and 3D). Evidently, increasing W/O ratio leads to a wider PSD. W/O ratio has been reported to dramatically affect the hydrodynamic characteristics of segmented flow (e.g. slug/plug length, recirculation strength and mixing efficiency).35-39 Since the average particle size and PSD of Ag NPs in Ag-rGO-S were dramatically affected by hydrodynamic characteristics in the first stage, the effects of process parameters on the plug length and mixing efficiency in the first stage were studied and reported in our previous study.40 As presented in Figure S6A, increasing W/O ratio causes an increase in the plug length. According to previous studies, this change could hinder the reactant mixing in the plugs.41,42 The effect of W/O ratio on mixing

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efficiency inside the plugs was studied via Villermaux/Dushman method. Segregating index (XS), an indicator of mixing efficiency, was calculated based on experimental results (the experimental details and calculation process of XS were stated in the supporting information). XS equals to 0 for ideal mixing, while XS equals to 1 for total segregation. The value of XS between 0 and 1 represents partial segregation. It can

be seen that XS increases with the increase in W/O ratio (Figure S6A). The phenomenon implied that the mixing performance in the plugs got worse as W/O ratio increased. Therefore, the larger average particle size and wider PSD of Ag NPs obtained at evaluated W/O ratio could be ascribed to the insufficient mixing. A broader PSD was also caused by low mixing efficiency in the slugs in the study of Cabeza et al.43 Subsequently, the effect of total volume flow rate on plug length, XS, average particle size and PSD of Ag NPs was discussed. As shown in Figure S6B, the plug length shortens as the total volume flow rate ascends. As stated above, the reduction in the plug length gave rise to a better mixing performance. Additionally, it was reported that the gradient of viscous force became stronger with the increase in total volume flow rate, triggering enhanced recirculation as well as reactant mixing efficiency.44,45 Hence, we could deduce that the increasing total volume flow rate might cause high mixing efficiency and thus narrower PSD of Ag NPs. As expected, XS declines with the increase in total volume flow rate (Figure S6B). Figure 4A-C show the TEM images of Ag-rGO-S synthesized at different total volume flow rates. When the total volume flow rate goes up from 0.5 to 1.5 mL/min, the average particle size of Ag NPs decreases from 3.4 ± 1.7 to 2.0 ± 0.7 nm (Figure 4D and S7). Ag-rGO-S prepared at 1.5 mL/min processes the narrowest PSD of Ag NPs (RSD=35.9%, Figure 4D). Furthermore, Figure 5A-C show the TEM images of Ag-rGO-S prepared at diverse synthetic temperatures. The average particle sizes of Ag NPs for Ag-rGO-S synthesized at 20, 40 and 60 oC are 3.0 ± 1.4, 3.1 ± 1.3 and 5.6 ± 2.5 nm, respectively (Fig. 5D and S8). The increase in the particle size can be attributed to the faster growth rate of Ag NPs at evaluated synthetic temperature. In addition, the synthetic temperature shows no obvious effect on the RSD (Figure 5D).

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Figure 3. TEM images of Ag-rGO-S synthesized at different W/O ratios (A) W/O ratio=2:3, QA=QB=0.2

mL/min, Qoctane=0.6 mL/min, τ=1.38 min (B) W/O ratio=3:2, QA=QB=0.3 mL/min, Qoctane=0.4 mL/min, τ=1.36 min (C) W/O ratio=4:1, QA=QB=0.4 mL/min, Qoctane=0.2 mL/min, τ=1.35 min; (D) average particle

size and RSD of Ag NPs in Ag-rGO-S. Qtotal=1.0 mL/min, T=40 oC, theoretical Ag weight percentage=21.3 wt.%.

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min; (D) average particle size and RSD of Ag NPs in Ag-rGO-S. W/O ratio=2:3, T=40 oC, theoretical Ag weight percentage=21.3 wt.%.

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Figure 5. TEM images of Ag-rGO-S synthesized at different synthetic temperatures (A) 20 oC (B) 40 oC

(C) 60 oC; (D) average particle size and RSD of Ag NPs in Ag-rGO-S. QA=QB=0.2 mL/min, Qoctane=0.6 mL/min, W/O ratio=2:3, Qtotal=1.0 mL/min, theoretical Ag weight percentage=21.3 wt.%, τ=1.38 min.

Aside from the average particle size and PSD, the properties of Ag-rGO composites were also strongly dependent on the Ag NPs loading. The Ag NPs loading was adjusted by altering the concentration of Ag+ ions. Figure 6A-C show the TEM images of Ag-rGO-S with different theoretical Ag weight percentages. It can be seen that the density of Ag NPs decorated on rGO sheets increases with the increase in theoretical Ag weight percentage. As the theoretical Ag weight percentage increases from 11.7 wt.% to 28.8 wt.%, the average size of Ag NPs goes up from 1.5 ± 0.6 nm to 4.7 ± 2.2 nm (Figure S9 and 6D). The Ag-rGO-S with the theoretical Ag weight percentage of 28.8 wt.% shows the widest PSD (RSD=46.5%, Figure 6D), which can be attributed to the highest Ag+ concentration. Table S4 summarizes the average particle sizes of Ag NPs for Ag-rGO and Ag-graphene composites in the previous studies. Apparently, this microreactor technology-based strategy holds great promise for the continuous synthesis of ultrafine Ag NPs (< 5.6 nm) decorated rGO. This microreactor technology-based strategy was also employed to synthesize Pt-rGO and Pd-rGO composites. Similar to Ag, ultrafine Pt and Pd nanoparticles are uniformly distributed on rGO sheets without obvious aggregation, as shown in Figure S10A and S10B. The average particle sizes of Pt and Pd NPs are 1.7 ± 1.0 nm and 3.2 ± 1.1 nm, respectively, with the RSD below 40% (Figure S10C and S11).

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Figure 6. TEM images of Ag-rGO-S with different theoretical Ag weight percentages (A) 11.7 wt.% (B)

21.3 wt.% (C) 28.8 wt.%; (D) average particle size and RSD of Ag NPs in Ag-rGO-S. QA=QB=0.2 mL/min, Qoctane=0.6 mL/min, W/O ratio=2:3, Qtotal=1.0 mL/min, T=40 oC, τ=1.38 min.

Conclusion

In summary, a microreactor technology-based strategy was developed for the deposition of ultrafine Ag NPs with narrow PSD on rGO sheets in continuous mode. Octane was employed as the continuous phase to segment mixed aqueous reactions into discrete plugs, in which the deposition of Ag NPs on GO sheets occurred. It was found that the water/oil volume flow ratio, total volume flow rate, temperature had effects on the average particle size and PSD of Ag NPs in Ag-rGO composites. The average particle size of Ag NPs was readily controlled in the range of 1.5-5.6 nm. More importantly, this synthetic strategy can be extended to synthesize Pt-rGO and Pd-rGO composites in continuous mode. Evidently, this microreactor technology-based strategy paves a new route for continuous synthesis of high-quality rGO-based composites.

ASSOCIATED CONTENT Supporting information

The supporting information is available free of charge on the ACS Publication website. TGA curves and UV-vis absorption spectra of Ag-rGO-S, PSD of Ag NPs in Ag-rGO-S synthesized at different process parameters, and supplementary tables, etc.

15



Author information Corresponding authors

E-mail: [email protected], [email protected] ORCID

Guangwen Chen: 0000-0001-5290-7921 Notes

The authors declare no competing financial interest.

Nomenclature QA

the volume flow rate of solution A, mL/min

QB

the volume flow rate of solution B, mL/min

Qoctane

the volume flow rate of octane, mL/min

Qtotal

the total volume flow rate in the first stage, mL/min

W/O ratio

the water/oil volume flow ratio in the first stage

T

synthetic temperature, oC

τ

residence time, min

Acknowledgements

The authors gratefully acknowledged the financial supports from National Natural Science Foundation of China (No. 21776274), Dalian Institute of Chemical Physics Grant (No. DICP ZZBS201708), and Ministry of Science and Technology of the People's Republic of China (No. 2016RA4053).

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Abstract graphic

Synopsis

A highly efficient and easily scalable strategy based on microfluidic flow for continuous synthesis of noble metal-rGO composites was developed.

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Continuous synthesis of highly uniform noble metal nanoparticles over

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