Polymer Encapsulated Self-Assemblies of Ultrasmall Rhenium

Research Article pubs.acs.org/journal/ascecg

Cite This: ACS Sustainable Chem. Eng. 2017, 5, 10186-10198

Polymer Encapsulated Self-Assemblies of Ultrasmall Rhenium Nanoparticles: Catalysis and SERS Applications Subrata Kundu,*,†,‡ Lian Ma,§ Wei Dai,§ Yunyun Chen,† Alexander M. Sinyukov,∥ and Hong Liang*,†

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Department of Materials Science and Engineering, Texas A&M University, 201D Doherty Building, College Station, Texas 77843, United States ‡ Electrochemical Materials Science (ECMS) Division, CSIR-Central Electrochemical Research Institute (CECRI), Karaikudi 630006, Tamil Nadu, India § Department of Mechanical Engineering, Texas A&M University, 3123 Engineering Physics (ENPH) Building, College Station, Texas 77843, United States ∥ Department of Physics and Astronomy, Texas A&M University, College Station, Texas 77843, United States S Supporting Information *

ABSTRACT: Ultrafast formation of stable and self-assembled rhenium (Re) nanoparticles (NPs) using a poly allylamine hydrochloride (PAH) scaffold within 120 s of wet-chemical reaction at room temperature in aqueous solution has been reported. The average diameters of the two different sets of Re NPs synthesized are ∼0.7 ± 0.25 and ∼1.7 ± 0.3 nm, which can be easily achieved by controlling the polymer to Re7+ molar ratio. The small-size Re NPs are formed in solution, self-assembled together to form the chain-like or necklace-like structure. The synthesized Re NPs were used in two different potential applications, such as in catalysis and in surface-enhanced Raman scattering (SERS) studies. Catalysis study was done for 4-nitroaniline (4-NA) reduction with excess NaBH4 taking two different sets of Re NPs as catalyst. The highest catalytic rate for nitroaromatics reduction ever reported of ∼1.52 × 10−1 min−1 has been observed with large-size Re NPs as catalyst. In SERS, methylene blue (MB) was used as a Raman probe molecule. Strong SERS enhancements were observed with both sets of Re NPs due to their ultrasmall size, narrow interparticle gap, and self-assembled structure in PAH scaffold. These closely tethered and self-assembled Re NPs generated more surface active “hot spots” that resulted in good SERS enhancement. The present synthesis route is easy, cost effective, and fast and can generate stable Re NPs which could further be applied in interdisciplinary fields other than catalysis and SERS in the near future. KEYWORDS: Rhenium nanoparticles, Self-assemblies, PAH, Chain-like, Catalysis, Surface-enhanced Raman scattering

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except W, which has a melting point of 3422 °C.10 The high melting point of Re makes it a very promising material for different practical applications which require high-temperature withstanding ability such as jet engine components and to improve the life and performance of several other electronic gadgets used in high-temperature environments.11 Moreover, due to its low standard reduction potential of 0.3 V, it can be used for the preparation of alloy NPs by galvanic replacement reactions.10 Further, Re NPs can be used in medicinal fields for magnetically targeted radiotherapy,12 complex catalytic processes such as glycerol reforming, hydrocarbon transformation, hydrogenation of difficult functional groups,13 and aqueousphase reforming of biomass materials14 and also for different reduction reactions such as removal of perchlorate from water.15

INTRODUCTION Research on nanomaterials focusing on their preparation and characterization has been experiencing significant progress over the past two decades due to their unique physical and chemical properties. Due to their size and shape-dependent unique physicochemical properties, metallic nanostructures show a variety of unique optical,1 electronic,2 magnetic,3 and catalytic4 properties that make them suitable for application in many interdisciplinary fields of research. Among the different metallic nanostructures, noble metal nanoparticles (NPs) are found to be more interesting due to their close-lying valence and conduction bands, which makes them promising materials for nanoelectronics,2 catalysis,4−6 and surface-enhanced Raman scattering (SERS)7−9 studies. As their properties vary with their morphology, tremendous efforts have been made in studying the synthesis of size- and shape-controlled metal NPs. Among different metal NPs, the most focused metals are gold, silver, palladium, and platinum while the synthesis of zerovalent rhenium (Re0) has been passed over for a decade. Re has the second highest melting point of 3185 °C among all metals © 2017 American Chemical Society

Received: July 1, 2017 Revised: August 18, 2017 Published: September 11, 2017 10186

DOI: 10.1021/acssuschemeng.7b02175 ACS Sustainable Chem. Eng. 2017, 5, 10186−10198

Research Article

ACS Sustainable Chemistry & Engineering

Table 1. Final Concentration of All Reagents, Resultant Particle Size, and Shapes for the Synthesis of Self-Assembled Re NPs on PAH Scaffold set no.

conc. of stock PAH (wt %)

final conc. of PAH (wt %)

final conc. of NH4ReO4 solution (M)

1 2 3 4

1 0.75 0.5 0.1

0.076 0.057 0.038 0.0076

3.84 3.84 3.84 3.84

5

0.01

0.00076

3.84 × 10−3

× × × ×

10−3 10−3 10−3 10−3

final conc. of NaBH4 (M)

stirring time by magnetic stirrer (s)

average size (nm), morphology, and particles distribution

10−2 10−2 10−2 10−2

120 120 120 120

1.53 × 10−2

120

0.7 ± 0.25, chain like, spherical, 100% 0.8 ± 0.2, chain like, spherical, 100% 1.7 ± 0.3, chain like, spherical, 100% NPs formed but precipitated with an hour after synthesis NPs formed but precipitated with 20 min of synthesis

1.53 1.53 1.53 1.53

× × × ×

poorer compared to that of Au or Ag NPs. Kundu et al. reported the size-selective Rh NPs on DNA scaffold which acts as an excellent material both in catalysis and in SERS studies.37 Recently, Veerakumar et al. reported the dispersion of Re NPs on carbon nanostructures which act as an excellent catalyst for nitro compound reduction.38 SERS is a surface-sensitive technique where Raman signal gets enhanced a few orders of magnitude when a molecule is adsorbed on a rough metal surface. In SERS, there are two types of most accepted mechanisms to date: one is an electromagnetic effect, and the other one is a chemical effect. The electromagnetic effect deals with formation of a localized surface plasmon, whereas the chemical effect deals with formation of charge-transfer complexes. For metal NPs, the electromagnetic effect is the most predominant one whereas the chemical effect contributes to a much less extent. In SERS, the plasmonic coupling effect at the nanometric gap junctions creates an enormous electromagnetic field that generates strong SERS signals and is efficient to detect single molecules of interest. Moreover, it is also highlighted that aggregation of NPs or self-assembling of NPs on suitable supports can generate more surface active “hot spots” and subsequently leads to highly enhanced SERS signal.8,9 However, there are much less SERS studies on Re NPs. To date, there are only three reports on Re NPs for SERS study.22,28,29 To the best of our knowledge, there is no report on the uniform and fast formation of ultrasmall Re NPs in aqueous solution using a polymer scaffold and its subsequent application in catalysis and SERS studies. With this we report the synthesis of ultrasmall, self-assembled Re NPs in PAH scaffold for the first time. Due to the very small size and its self-assembly over PAH scaffolds, it has shown advantageous results when applied to catalysis and SERS studies. The catalytic activity was examined for the reduction of aromatic nitrocompounds, i.e., 4-NA in the presence of excess NaBH4. The SERS studies were done adapting MB as a Raman probe. The self-assembled Re NPs generated significantly enhanced SERS signals at various positions of MB peaks. The synthesized Re NPs were stable for a significant period of time when kept in a sealed tube and stored in the dark inside a refrigerator. The present synthesis process is easy, faster, and cost effective.

Though there are many potential applications for Re NPs as mentioned above, the preparation of Re NPs is not much studied in the literature.16−31 One of the major problems of preparing zerovalent Re NPs is its readiness for simultaneous oxidation in aqueous colloidal solution. Hence, so far, wet chemical synthesis of stable Re NPs has been a really challenging and hard task. Several synthesis methods have been reported to prepare metallic Re NPs in solution-based routes such as wet-chemical reduction,16−18,27−29 in reverse micelles,24,25 and alcohol-assisted reduction.23 The reduction of Re salts in water generates many unknown products due to its oxidation in air or water and results in oxidized products rather than metallic Re NPs.16 Mucalo et al. synthesized Re NPs by the chemical reduction of K2ReCl6 salt in water but observed a rapid oxidation of the synthesized NPs.17 Ayvali et al. prepared Re NPs by reduction of [Re2(C3H5)4] under a H2 atmosphere at 3 bar and 120 °C for 2 days in anisole solvent in the presence of hexadecylamine or polyvinylpyrrolidone (PVP) as capping agent.27 Anantharaj et al. prepared Re NPs in DNA scaffold and studied their application in catalysis and SERS.28 Sakthikumar et al. prepared Re NPs in organic solvent utilizing a two-phase extraction procedure in the presence of a phase transfer catalyst followed by reduction with a strong reducing agent.29 There are a few other reports where Re NPs have been synthesized using solid state methods such as thermal decomposition,19 impregnation followed by calcination under H 2 gas, 20 radiation,21 solid state thermolytic demixing,22 PLD (pulsed LASER deposition) assisted decomposition of Re salt,21 and thermal decomposition of Re2(CO)10.26 Most of the above reports described showed different drawbacks such as difficult synthetic protocols, polydispersity, the lack of information on the oxidation state of resulting NPs, and the need for sophisticated instruments, and in most cases, generated particles were not morphologically unique. Hence, a wetchemical-based easy and faster synthesis route for watersoluble, stable Re NPs still remains a bigger challenge to the scientific community. Nobel metal NPs, viz., Au,7 Ag,32 Pd,33 Pt,34 Os,35 and Rh,36,37 have only been used widely in catalysis and SERS studies. As these metal NPs have a large surface to volume ratio, they can readily act as excellent catalysts in various types of homogeneous and heterogeneous catalysis reactions. The chemical reduction of aromatic nitro compounds to their corresponding amino compound in the presence of excess NaBH4 has been used as a model reaction to specifically check the catalytic properties of noble metal NPs.4−6,32,35−37 Apart from catalysis, noble metal NPs such as Au and Ag have been extensively used in SERS studies too as substrates.7,8 Nowadays, other than Au and Ag, other metal NPs, such as Os,9,35 Rh,36,37 Pd,33 and Pt,34 have also been tested as suitable SERS substrates. However, their SERS enhancement factors (EF) are

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EXPERIMENTAL SECTION

Preparation of Ultrasmall Re NPs in PAH Scaffold. Information on reagents used for the synthesis of Re NPs for catalysis and SERS studies are given in the Supporting Information (SI). Selfassembled Re NPs are synthesized on PAH scaffold by the reduction of NH4ReO4 with NaBH4 and continuous stirring for 120 s. In a typical synthesis, 1 mL of 1% PAH solution was mixed with 5 mL of DI water and stirred well before adding 5 mL of 10−2 M NH4ReO4 solution and the stirring was continued for few more minutes. Then 2 mL of 0.1 M ice-cold freshly prepared NaBH4 solution was added at 10187


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ACS Sustainable Chemistry & Engineering once upon stirring during which the transparent solution turned into a light brown color initially, and with increasing time color changed to dark brown. The reaction was completed after 2 min. Completion of the reaction was confirmed by the stability of the color and from UV− vis absorption spectroscopy. The prepared solution exclusively contained self-assembled Re NPs having small-size particles with diameter ≈ 0.7 ± 0.25 nm as confirmed by TEM analysis. Another set of Re NPs was prepared just by changing the metal salt to PAH molar ratio while keeping other reaction parameters intact. The reaction time, concentrations of reagents used, particles size, and shape are given in Table 1. The stepwise formation of Re NPs by our present route is also shown in Scheme 1. The synthesized Re NPs of different

adsorption/desorption equilibrium prior to obtaining the Raman spectra.

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RESULTS AND DISCUSSION UV−vis Spectroscopic Studies. Self-assembled, ultrasmall Re NPs were synthesized using a PAH scaffold in the presence of NaBH4 as reducing agent under continuous stirring for 120 s. The ultrasmall Re NPs were grown on a PAH scaffold and generate self-assembled superstructures in nanoscale dimension. The UV−vis spectra (Figure 1) of the different precursor

Scheme 1. Schematic Presentation of the Synthesis of SelfAssembled Re NPs on PAH Scaffold

Figure 1. UV−vis spectra of the different solution mixtures for the formation of self-assembled Re NPs in PAH scaffold. (a) Absorption spectra of only aqueous PAH solution; (b) absorption spectra of colorless ammonium perrhenate salt solution solution; (c) absorption band of a mixture of PAH and ammonium perrhenate solution; (d, e, and f) absorption bands of aqueous Re NPs solution where the stock PAH solution concentration was 0.5%, 0.75%, and 1%, respectively. (Inset) Three different Re NPs solutions corresponding to d, e, and f.

average particle sizes were characterized using UV−vis, TEM, EDS, XRD, and XPS analyses, and details of their sample preparation are given the the SI. Preparation of Samples for Catalysis and SERS Studies. The catalytic reduction of 4-NA was tested taking positively charged Re nanostructures. For a typical catalysis reaction, 4 mL of DI water was mixed with 600 μL of (10−3 M) stock 4-NA solution and stirred for 5 min. Then 600 μL of 0.1 M ice-cold NaBH4 solution was added and shaken well by hand. Then 100 μL of Re NPs solution was added, and the reaction progress was monitored in situ by an UV−vis spectrophotometer. The corresponding absorption spectra were recorded at regular time intervals until completion of the reduction. The total time required for the reduction was ∼14 min for large-size PAH-capped Re NPs during which the light yellowish 4-NA solution became colorless due to formation of the reduced product pphenylenediamine (p-PDA). Completion of the reaction was confirmed by decoloration of the reaction mixture from yellow to colorless and from the absorption spectra acquired using the UV−vis spectrophotometer. Another set of prepared Re NPs was also tested for comparison purposes. MB dye was used as the probe for SERS studies. Samples for SERS studies were prepared as follows. Several standard solutions of MB with concentrations varying as 10−3, 10−4, 10−5, 10−6, and 10−8 M were prepared in DI water. Then 200 μL of each of those MB stock solutions was separately mixed with 200 μL of the Re NPs solution and shaken well to ensure homogeneous mixing. After ∼10−15 min, about 20 μL of the solution mixture was placed over clean glass substrates and dried in air. After drying, the samples were ready for SERS studies. During SERS studies, the laser beam was placed directly to the sample under darkness to establish an

solution used for the synthesis of Re NPs shows that the absorption spectrum of aqueous PAH solution (curve a) has no specific peak, but a small hump near 240 nm matches with an earlier report.39 Curve b in Figure 1 is the absorption spectra of aqueous ammonium perrhenate solution which has a strong peak at 227 nm, indicating ligand to metal charge transfer (LMCT) spectra. The LMCT band of ReO4− appears due to the transition from O2− ion to Re7+ ion at the center of the complex. A similar type of LMCT band for Re salt was reported.28 Curve c in Figure 1 is the absorption feature of a mixture of PAH and ammonium perrhenate solutions which has no specific peak but a small hump near 242 nm due to either formation of a complex between both or due to adsorption of ReO4− ions on PAH. Shifting of peak positions or the appearance of a new peak due to adsorption of metal ions with different scaffolds like cetyltrimethylammonium bromide (CTAB) or deoxyribonucleic acid (DNA) was discussed before.4,7−9 After the addition of NaBH4 solution, the perrhenate solution was reduced, and the color of the solution mixture changed to light brown initially and finally dark brown after 120 s, indicating the formation of Re NPs in the solution. Curves d, e, and f show the absorption features of aqueous Re NPs solution in which the PAH solution concentration was varied as 0.5%, 0.75%, and 1%, respectively. From curves d, e, 10188


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Figure 2. Transmission electron microscopic (TEM) and high-resolution TEM micrographs of the self-assembled Re NPs on PAH scaffold. (A−D) Low- and high-magnification TEM imagse of the Re NPs obtained with a stock PAH solution concentration of 0.5 wt %. (E−H) Low- and highmagnification TEM images of Re NPs with a stock PAH solution concentration 1 wt %. (Inset of D and H) Corresponding SAED pattern, respectively. 10189


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Figure 3. X-ray photoelectron spectra (XPS) of the synthesized self-assembled Re NPs on PAH scaffold: (A) survey spectrum; (B) high-resolution spectrum of Re 4f; (C) high-resolution spectrum of Re 4d; (D) high-resolution spectra of O (1s).

and f, no peaks for perrhenate were observed, which indicates all the perrhenate ions were reduced to the Re(0) state. From curves d, e, and f, we did not see any characteristic absorption maxima for Re NPs, which might be due to its very low value of dielectric constant. Similar types of absorption features without any characteristic SPR maximum were observed for other metal NPs such as Pd,40 Os,9,35 etc. From curve d, e, and f we can see that with the increase in PAH concentration the curve becomes narrower and shifted toward the lower wavelength side which could probably be due to formation of small-size Re NPs. Similar types of absorption features for Re NPs were reported.21,28,29 The inset of Figure 1 shows the camera image of three different sets of Re NPs solution corresponding to curve d, e, and f. Transmission Electron Microscopic (TEM) Analysis. Figure 2 shows the TEM and HR-TEM micrographs of the synthesized Re NPs on PAH scaffold which revealed that 1 and 0.75 wt % PAH solutions led to the formation of Re NPs of almost similar size and morphology, whereas 0.5 wt % PAH solution generated significantly different Re NPs assemblies with different particle size. Hence, we are showing the TEM and HR-TEM micrographs obtained for the Re NPs using 0.5 and 1 wt % PAH solutions. Figure 2A−D shows the low- and high-magnification TEM micrographs of the Re NPs obtained with 0.5 wt % PAH solution. From Figure 2A and 2B we can

see that the long polymer chains are connected together to form “network-like” structures and small Re NPs are embedded there. Figure 2C shows the comparatively high-magnification micrograph where the Re NPs are clearly visible and selfassembled on the PAH chains. Figure 2D shows the HR-TEM micrograph of the Re NPs where the corresponding lattice fringes are clearly visible. The average diameter of the Re NPs is ∼1.7 ± 0.3 nm, and the average PAH chain diameter is ∼75 ± 20 nm. From Figure 2D one can see that the spacing between two adjacent fringes is ∼0.264 nm. The inset of Figure 2D shows the corresponding selected area electron diffraction (SAED) pattern which shows the ring patterns probably due to small size of Re particles which could not diffract the electron beam. Similar types of SAED patterns were observed earlier for small-size Os metal NPs during TEM analysis.9,35 Figure 2E−H shows the TEM and HR-TEM micrographs of Re NPs with the 1 wt % PAH solution. Figure 2E−G shows the TEM micrographs at different magnifications, and Figure 2H is the HR-TEM micrograph of the same sample. From these micrographs it is clear that due to the high concentration of PAH, the self-assembled polymer chains are denser and aggregated together and that small-size Re NPs were grown in the polymer chain. The average diameter of the individual Re NPs is ∼0.7 ± 0.25 nm, and the average chain diameter is ∼30 ± 10 nm. Moreover, it also shown that the chains are 10190


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50.1 eV and Re 4f7/2 appears at a binding energy of 47.7 eV. Similarly, Figure 3C shows the high-resolution peak for Re 4d which was also deconvoluted where Re 4d3/2 and Re 4d5/2 peaks appear at 286.8 and 266.3 eV, respectively. In addition to these two Re 4d peaks, another satellite peak also appeared at a binding energy of 278.6 eV. Other than Re 4d and a satellite peak, two other low-intensity peaks appeared at binding energies of 271.1 and 291.04 eV, respectively, which are assigned to ReOx and might appear due to some degree of aerial oxidation of Re NPs during the entire synthesis process. It is well known that rhenium is a very oxidizable metal, and it is expected to oxidize in air and featured the oxide peaks in XPS as we also observed in our present study. The formation of a thin oxide layer on the surface of Re(0) is expected as reported before too.41 Figure 3D shows the high-resolution XPS spectra for O 1s which appeared at a binding energy of 533.5 eV. The high-resolution XPS spectra of N 1s and C 1s are shown in Figure S3A and S3B, which appear at binding energies of 401 and 286 eV, respectively. All peak positions observed are matching with the NIIST XPS data file as expected. Similar types of XPS spectral results of Re NPs were reported by Kim et al. on their study of Pt−Re bimetallic catalyst42 and by Anantharaj et al.28 and Sakthikumar et al.29 for their study on Re NPs on DNA scaffolds. Mechanism for the Formation of Self-Assembled Re NPs on PAH Scaffold. Self-assembled Re NPs have been synthesized by the reaction of NH4ReO4 salt with NaBH4 in the presence of the polyelectrolyte PAH in aqueous solution within 120 s of wet-chemical reaction at room temperature. In the present synthesis we did couple of control experiments to check the importance of each reagent. In the absence of PAH, while reducing NH4ReO4 with NaBH4, Re0 NPs were formed but immediately were precipitated due to the absence of any specific stabilizer in the medium. Similarly, the reaction did not generate any Re0 NPs without the addition of NaBH4 due to the absence of any reducing agent. Moreover, we also varied the concentration of all these reagents. We have seen that at a very high concentration of NH4ReO4 salt (≥10−2 M) but keeping other reagent concentrations the same, the Re0 NPs are formed instantly but got precipitated within a shorter time. Similarly, at a low concentration of NH4ReO4 salt (≤10−4 M), the formation of Re0 NPs takes a much longer time. We changed the concentration of PAH salt and have seen that stable Re0 NPs are formed while stock PAH concentration was between 1 and 0.5 wt %. When the PAH concentration was much less such as 0.1 or 0.01 wt %, Re0 NPs are formed and precipitated during synthesis or within a short time after synthesis. At very high PAH concentrations such as 2.5 wt % or above, Re0 NPs are not formed in the experimental time scale. Similarly, stock NaBH4 concentration was fixed to 0.1 (M), although we have seen in other concentrations such as 0.01 or 5 × 0.1 M also Re0 NPs formed but takes a longer time compared to our experimental time scale. Hence, proper concentrations and combinations of the reagents, viz., NH4ReO4 salt, PAH, and NaBH4, are essential for successful generation of Re0 NPs in solution. All details about reagents concentrations are given in Table 1. The Re salt we used was ammonium perrhenate, which is an ammonium salt of perrhenic acid and readily soluble in water having a molar mass of 268.2 g/mol and density of 3.97 g/cm3 and possesses a Scheelite crystal structure having a melting point of ∼365 °C. The polyelectrolyte PAH is a cationic polymer prepared by polymerization of allylamine and is widely used for biomedical application and surface

interwoven with one another. Figure 2H is the corresponding HR-TEM micrograph, and the average spacing between two individual lattice planes is ∼0.204 nm. The inset of Figure 2H is the corresponding SAED pattern which says that the particles are nearly crystalline in nature. However, we did not get any separate diffraction spots; rather, we observed simple ring-type diffuse patterns which might be due to the very small size of the individual Re NPs and due to the presence of some amorphous materials such as the PAH on the surface of Re NPs. Hence, from the TEM and HR-TEM analyses, it is clear that small-size Re NPs were formed in the solution which subsequently grew over the PAH chains, self-assembled, and generated the chainlike morphologies. Energy-Dispersive X-ray Spectroscopic (EDS) and Xray Diffraction (XRD) Analysis. Energy-dispersive X-ray spectroscopy (EDS) analysis was done to ascertain the elements present in our synthesized Re NPs solution. The EDS spectrum of the synthesized Re NPs is shown in Figure S1, SI. As both synthesized Re NPs have shown similar spectral features, the EDS spectrum of Re NP synthesized using 0.5 wt % PAH solution is only provided here. The EDS spectrum consists of different elements such as C, O, Cl, Na, N, and Re. The C, Cl, and N peaks arise due to the presence of PAH. The Na peak came as a result of the used reducing agent, NaBH4, in our synthesis. The high-intensity Re peak came from the Re NPs, and we observed the Re peak in different places respective to the shells of varying energy. Other than the expected elements, we did not find any other peaks, which confirmed the purity of our synthesized Re NPs. The X-ray diffraction (XRD) patterns of the synthesized Re NPs are shown in Figure S2, SI. The ultrasmall Re NPs were deposited on glass substrates and analyzed by XRD. Curve a of Figure S2 shows the XRD pattern obtained from small-size Re NPs prepared with 1 wt % PAH solution, whereas curve b of Figure S2 shows the XRD pattern of large-size Re NPs prepared with 0.5 wt % PAH solution. Both XRD patterns exhibited similar features, and we did not observe any strong diffraction peaks. However, the expected peak positions are indicated in the corresponding patterns. These XRD patterns are in accordance with the SAED patterns as described before in Figure 2. Similar types of XRD patterns for ultrasmall-size Re NPs and Os NPs were reported.9,28,29,35 However, few other groups, viz., Chong et al.21 and Yi et al.,23 observed some diffraction peaks for Re NPs in their work due to the larger size of the synthesized Re NPs as indexed with JCPDS card number 00-087-0599. X-ray Photoelectron Spectroscopic (XPS) Analysis. Figure 3 shows the X-ray photoelectron spectroscopic (XPS) analysis of the synthesized Re NPs which was used to identify the oxidation states of the corresponding elements present in the material. It is important to note here that as both morphologies are chemically equivalent, we are showing the detailed XPS analysis of Re NPs prepared using 0.5 wt % PAH solution. Figure 3A shows the survey spectrum which consists of different peaks corresponding elements Na 1s, C KLL, O KLL, O 1s, Na KLL, C 1s, Cl 2p, and Re 4f. From the survey spectrum the Na 1s peak appears at a binding energy of 1072 eV, C KLL at 996 eV, O KLL at 746 eV, O 1s at 533 eV, Na KLL at 305 eV, C 1s at 286 eV, Cl 2s at 265 eV, Cl 2p at 201 eV, and Re 4f at 46.8 eV. The high-resolution XPS spectra of Re 4f, Re 4d, and O 1s are shown in Figure 3B, 3C, and 3D, respectively. From Figure 3B, the high-resolution Re 4f peak was deconvoluted where Re 4f5/2 appears at a binding energy of 10191


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to the solution mixture containing PAH and NH4ReO4, the light brown color started to appear, and the color intensified with increasing reaction time, which is clearly indicating the formation of Re0 particles in the solution. We can expect the reduction of free ReO4− ions which were not adsorbed with PAH initially, and with increasing time the preformed Re0 particles acted as seed particles or as catalyst for the reduction of the remaining ReO4− ions in solution or the ReO4− ions adsorbed on the PAH self-assemblies. Within a short time all ReO4− ions were reduced, as indicated by the generation of dark brown Re0 NPs solution which simultaneously selfassembled on the PAH chains and finally formed self-assembled chain-like morphologies. The small-size Re0 particles initially formed might have aggregated together and generated the large-size Re0 NPs. From Table 1 and Scheme 2 we see that when keeping other reaction parameters fixed but just altering the PAH concentration we were able to generate different sizes of Re NPs in the chain-like morphologies. When PAH concentration was high (1 wt %), smaller Re NPs having an average diameter of ∼0.7 ± 0.25 nm were formed, while at a comparatively lower PAH concentration (0.5 wt %), larger size particles having a diameter of ∼1.7 ± 0.3 nm were formed. However, at a very low PAH concentration of 0.1 or 0.01 wt %, the particles were formed but precipitated soon after synthesis. The formation of different size particles was due to the presence of different concentrations of PAH in solution. At a lower PAH concentration (at 0.5 wt %), the Re0 particles that formed had more free space during their growth and then aggregated to generate particles of larger diameter, while at a high PAH concentration (at 1 wt %), as the available space was comparatively lower, once the particles were nucleated they did not get sufficient space to grow and become agglomerated. Hence, generated Re NPs were smaller in size as evidenced from the TEM results in Figure 2. Thus, using our present process we can easily tune the particles size, although in all cases morphologies are chain-like only with different chain diameter as shown in Table 1 and Scheme 2. It is important to note here that in our previous study on PAH-encapsulated Au NPs synthesis39 we saw that PAH acted as a reducing agent due to presence of an amino group on its structure during synthesis and after the formation of Au NPs they acted as stabilizers. However, in the present study, without the addition of NaBH4, reduction did not take place, which means that PAH was not able to reduce ReO4− ions. However, during the growth of the particles and after the formation, PAH played a major role to stabilize the colloidal solution of Re NPs. The amino group of the PAH might have coordinated with the ReO4− ions, stabilized the Re0 NPs after their formation, and finally generated the self-assembled chain-like structures. Similar types of NPs assembly on a polyelectrolyte scaffold were highlighted earlier by Minko et al.43 Then taking the two sets of different Re NPs prepared with 0.5 and 1 wt % PAH solutions, we studied their applicability in catalysis for the reduction of aromatic nitrocompound and as a substrate for SERS studies as described below in the following sections. Catalytic Reduction of 4-Nitroaniline (4-NA) Using PAH-Stabilized Re NPs as Catalyst in the Presence of NaBH4. The synthesized self-assembled Re NPs were tested for the catalytic reduction of nitroaromatic compound, i.e., 4-NA, in the presence of excess NaBH4 at room temperature under ambient condition. As discussed before in the Introduction, Re NPs have been used extensively in different types of catalysis reaction.13−17 Re NPs can also be used in other catalysis

functionalization for layer by layer assembly of nanomaterials in device-related applications. Before the synthesis and after the formation of Re NPs, we measured the pH of all intermediate and final solution mixtures. The pH of PAH (1 wt % solution) was 3.73, and for PAH (0.5 wt % solution) it was 3.79. The pH of NH4ReO4 slat solution (10−2 M) was 5.11, a mixture of PAH (1 wt % solution) with NH4ReO4 is 4.53, and a mixture of PAH (0.5 wt % solution) with NH4ReO4 had a pH of 4.57. The pH of the Re0 NPs solution after formation was 9.01 with 1 wt % PAH solution and 9.18 with 0.5 wt % solution. In our synthesis we varied the concentration of PAH to Re salt and observed that only at particular concentrations were stable Re0 NPs formed. The formation mechanism of Re0 NPs is schematically shown in Scheme 2. Scheme 1 described before showed the Scheme 2. Schematic Presentation Illustrating the Formation Mechanism of Self-Assembled Re NPs on PAH Scaffold

stepwise formation of Re0 NPs within 120 s of reaction. From Scheme 1 we can see that the solution mixture of PAH and NH4ReO4 was colorless. Then once NaBH4 was added, within 10−20 s the colorless solution turned a light brown color, which indicates nucleation and reduction of the Re salt. With increasing time, the color of the solution became dark after 120 s, indicating completion of the reaction. Scheme 2 proposes that Re NPs were initially formed in all the concentrations of PAH such as 1, 0.75, 0.5, 0.1, and 0.01 wt %. However, the particles were stable only at high concentrations of PAH such as 1, 0.75, and 0.5 wt %. In the case of 0.1 and 0.01 wt % PAH, the Re NPs were formed initially but got precipitated within a short time as depicted in Scheme 1 and Table 1. In our synthesis, after the addition of NH4ReO4 with PAH solution, the negatively charged ReO4− ions might have been adsorbed first with positively charged polymer chain self-assemblies, which was confirmed from the UV−vis spectra where the peak of the NH4ReO4 solution disappeared and the nature of the peak was also significantly altered, indicating that certain types of interactions existed between them. Then NaBH4 was added 10192


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Figure 4. (A and B) UV−vis spectra for the successive reduction of 4-NA taking large-size and small-size Re0 NPs, respectively. (C and D) Corresponding ln(Abs) vs time (T, min) plot taking large- and small-size Re NPs as catalyst, respectively.

reactions such as hydrogenation of succinic acid,44 isomerization of fatty alcohol, and alcohol dehydrogenation.21 It is important to note here that different types of metal NPs such as Au,4,5 Ag,32 Pd,45 Pt,46 Os,9,35 Cu,47 etc. have been used exclusively for testing the catalytic reduction of aromatic nitrocompounds due to their industrial importance. However, there is only one report by Anantharaj et al. for the catalytic reduction of nitrocompounds using DNA-encapsulated Re NPs as catalyst.28 Moreover, this catalysis reaction will also confirm the formation of Re0 NPs in our synthesis. The reduction of 4-NA with only NaBH4 solution is very slow, and only 6.02% reduction took place after keeping the reaction mixture for 5 days as seen from the UV−vis spectrum in Figure S4, SI . In Figure S4 curve a is for only aqueous 4-NA solution and curve b is for the mixture of 4-NA and NaBH4 solution after keeping them for 5 days. The inset shows two camera images of 4-NA solution corresponding to curve a and curve b. Hence, this experiment proved that the 4-NA reduction was not taking place significantly in the absence of a catalyst which might be due to the presence of some kinetic barriers that prevent the electron transfer from BH4− to the nitrocompound. We also conducted two more control experiment in our catalysis reaction. We tested the catalysis reaction with PAH alone and Re0 NPs solution without the addition of NaBH4. The catalysis reaction did not take place at all after waiting for more than 2−3 days in either case and signifies that a proper mixture of all three reagents such as 4-

NA, Re0 NPs, and NaBH4 was important for the catalysis reduction to take place. The reduced product of the 4-NA is pphenylenediamine (p-PDA), which is an important material used extensively in dyeing industry for making different azo and fur dyes. In our present catalysis reaction 4 mL of DI water was mixed with 600 μL of 10−2 M 4-NA solution, and then 600 μL of ice-cold freshly prepared NaBH4 was added. Finally, 100 μL of Re0 NPs solution was added, and the reaction was monitored using a UV−vis spectrophotometer. We measured the pH of the 4-NA solution (10−3 M), which was 5.64, the pH of NaBH4 solution (0.1 M) was 10.66, and the pH of the reduced solution mixture was 10.01. The successive reduction of the 4-NA peak can be easily monitored in situ using a UV−vis spectrophotometer with changing reaction time. The 380 nm peak for 4NA gradually reduced down with time, and a new peak at 238 nm had begun to appear due to formation of p-PDA in solution. Figure 4A and 4B shows the UV−vis spectra for the successive reduction of 4-NA taking large-size and small-size Re0 NPs, respectively. From Figure 4A we can see that the reaction was completed after 14 min, and from Figure 4B the same reaction was completed after 25 min. The insets of both figures show the yellowish color 4-NA solution before reduction and colorless p-PDA after reduction. Figure 4C and 4D shows the corresponding ln(Abs) vs time (T, min) plots for the catalysis reaction taking large and small Re NPs as catalysts, respectively. The ln(Abs) vs T (time, min) plots show a linear relationship with rate constant values of 1.52 × 10−1 and 7.58 × 10193


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ACS Sustainable Chemistry & Engineering 10−2 min−1, respectively, for large- and small-size Re NPs. Table S1 (SI) shows the observed rate constant values with % of yield and product selectivity for the catalysis reaction. The catalytic rate observed in this reaction was found to be faster than reported ones for Re0 NPs. A previous report by Anantharaj et al. showed a catalytic rate of 6 × 10−2 min−1, which is much lower than our present results taking self-assembled Re NPs in PAH scaffold as scaffolds.28 It is interesting to note that the average diameter of the individual particles was ∼1.1 ± 0.1 and 0.7 ± 0.1 nm in the DNA scaffold having aggregated chain-like morphology which is comparable to the Re NPs size in our present study. The observed catalytic rate is higher compared to the reported ones which could be probably due to the specific morphology, ultrasmall particles size in the PAH scaffolds. Highly stable rhenium organosol on DNA scaffold was reported by Sakthikumar et al., and they studied their application for catalytic reduction of hexavalent chromium and in SERS study.29 As there is only one report of a nitrocompound reduction taking Re NPs as catalyst, we compared our present catalysis results with other metal NPs such as Au,4,5,48−51 Ag,32 Rh,36 and Os9,35 NPs and observed that our results are better than many of them. FOr all other catalysts used, their catalytic rate and name of the capping agent used for the synthesis are tabulated in Table S2 (SI). From Table S2 it is also confirmed that Re NPs had the highest rate constant value when compared to others. The formation of the reduced product was further confirmed by 1H NMR and HPLC studies (results are not shown here). Apart from this, we checked the formation of reduced product by comparing the Raman spectra of 4-NA with the reduced product as seen in Figure S5, SI. In Figure S5 curve a is the Raman spectrum for 4-NA and curve b is the Raman spectrum for p-PDA solution where a clear difference between the two spectra with new additional peaks is observed, confirming the conversion of 4-NA to p-PDA. To check the versatility of our reaction, we tested the same catalysis reaction taking few other nitrocompounds such as 2-nitrophenol (2NP), 4-nitrophenol (4-NP), etc. and observed that the catalysis reaction took place and completed within a short time scale. In our present catalysis reaction, the reducing agent BH4− transferred an electron via the self-assembled Re NPs in the PAH scaffold to the nitrocompounds and nitrocompounds were reduced. Scheme S1, SI, shows the electron transfer pathways for the catalysis reaction taking two different sizes of Re NPs as catalyst. From Table S1 (SI) we can see that the catalytic reaction rate was faster in the case of larger size Re NPs compared to smaller Re NPs. For any homogeneous catalysis reaction, the catalytic rate depends upon two important parameters: one is the number of catalyst particles in the solution, and other is the availability of the active surface area. In the present study we tried to fix the number of catalyst particles by adding different volumes of catalyst solution. As the quantity of particles or the loading of particles is approximately the same, the observed difference in the catalytic rate was mainly due to the difference in their surface area. As in both cases the NPs are spherical in morphology and the amounts of particles are the same, the available active surface area of NPs having a small diameter will be higher compared to the larger diameter particles and the catalytic rate is also expected to be higher in the case of smaller diameter Re NPs. However, , in our present study we observed the opposite trend and the order of the catalytic rate is larger diameter Re NPs > smaller diameter Re NPs. Although, as the particle size difference for two morphologies is much less, the reason for the difference in

the catalytic rate and the exact mechanism of the electron transfer process during catalysis needs further study to get a clear picture at the nanoscale catalytic interfaces. Finally, it is important to mention that as we used all reagents in the same phase, it was not possible to remove the catalyst after reduction from the reduced product. Nevertheles, as we used a much lower concentration of catalysts, there was no impurity observed in the reaction product. Moreover, we also checked the recyclability of our catalyst by adding more 4-NA solution in the same reduced solution with the necessary addition of NaBH4 solution again and observed that the catalysis reaction takes place significantly up to 4−5 consecutive cycles. Beyond 7−8 cycles, the catalytic rate was reduced, which might be due to surface poisoning of the catalyst. Taking two sets of different Re NPs we tested their application in SERS study as discussed below. Surface-Enhanced Raman Spectroscopic (SERS) Studies with PAH-Capped Re0 NPs as Substrate with MB as Raman Probe. After the discovery of SERS in 1974 by Feildsmann, a tremendous effort has been made to get SERS active materials of high EF.52 To date, the high EF values are mostly observed with Au and Ag metal NPs as their SPR bands fall in the same region with an excitation wavelength near or in the visible region.53−55 The sensitivity of SERS depends upon several factors such as the specific size and shape of the NPs, the presence of a rough surface, aggregation of metal NPs, selfassembled metal NPs having a specific gap between two particles which can generate more surface active “hot spots”, and enormous SERS signal.56−59 Mazumdar et al. reported that aggregated Ag NPs having an interparticles distance below 2 nm gave highly enhanced SERS signal compared to monodispersed Ag NPs with no aggregation.8 Nithiyanantham et al. reported self-assembled Os nanoclusters (NCs) gave better SERS enhancement.9 Kundu reported the shape-selective SERS activity of Au NPs with different shapes before where he observed that Au nanoprisms gave better SERS signal compared to Au nanorods or Au nanospheres.7 Other than Au and Ag, metal NPs such as Os,9,35 Pd,33 Pt,34 and Re22,28,29 do not have any specific surface plasmon resonance bands in the visible region to generate highly enhanced SERS signal. Hence, they always result in low EF values. Valenzuela et al. reported high index facet SERS active Re NPs by considering phase demixing.22 In the present study we prepared ultrasmall Re NPs self-assembled on PAH scaffold and expected them to give good SERS activity due to their small size and selfassembled structure. In the present study, The ability to act as SERS substrates were checked for the synthesized two different Re NPs we prepared in PAH scaffold. We used MB as a SERS probe molecule as it has a large molecular structure and charged functional group and has been well studied before as a SERS probe molecule. The specific preparation of SERS sample is elaborated in the Experimental Section. Figure 5A shows the chemical structure of the Raman probe molecule MB at the middle and left optical image of MB solution, and the right optical image is of a mixture of MB and Re NPs solution. Figure 5B shows the UV−vis absorption spectra of the dye MB, the Re NPs solution, and the mixture of MB and Re NPs solution together. Figure 5B curve a shows the absorption spectra of MB solution, which has a prominent band at 664 nm and two other small bands at 611 and 291 nm, respectively. Curve b in Figure 5B shows the absorption band of only Re NPs solution which has no significant bands in the UV−vis 10194


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Figure 6. Raman spectra of only MB (10−3 M) (a) and SERS spectra with small-size Re NPs (b) and large-size Re NPs (c).

large-size Re NPs as substrates, respectively. From Figure 6 we can see that there is better SERS enhancement taking large-size Re NPs compared to small-size Re NPs. The enhancement factor (EF) value for SERS is calculated according to a specific equation as given in the literature.7,8 The EF value was calculated by choosing three different intensity bands at 1623, 1391, and 447 cm−1. All EF values are tabulated in Table S4, SI. From Table S4 we can see that EF values are almost comparable between the two morphologies. However, the EF value is high for large-size Re NPs at peak positions of 1623 and 1391 cm−1, whereas for small-size Re NPs the EF value is high at a peak position of 443 cm−1. The difference of EF values is less just an order of one or two. Moreover, in both cases for small- and large-size Re NPs the interparticles gap between two individual particles is low, below 2 nm, which can generate number surface active “hot spots” that in turn give better and enhanced SERS signal. The highest EF value is observed for small-size Re NPs as 5.18 × 102. The observed EF value is less compared to other metal NPs like Au7 or Ag.8 However, it is comparable with several other less studied metal NPs like Os,9,35 Re,22,28,29 Rh,36,37 etc. In the present study we tested the concentration-dependent SERS taking two different size Re NPs into consideration as shown in Figure 7. Figure 7A shows the concentration-dependent SERS spectra taking large-size Re NPs, while Figure 7B shows the small-size Re NPs as substrate. In both cases we plotted the Raman spectra of only MB for comparison purposes. From Figure 7 we can see that we were able to detect dye concentrations up to the 10−5 M level. We prepared a few other samples having a dye concentration lower than that, such as 10−6 M, 10−8 M etc. However, we were unable to detect them using our present experiment taking Re NPs as substrates. Considering Figures 6and 7 and Table S4 we can see that we got good SERS EF, and the EF values are varied for two different size Re NPs. From TEM images in Figure 2 we can see that ultrasmall-size Re NPs were self-assembled together having their individual particle size below 2 nm, and the interparticle gap was also below 2 nm. This small size of the NPs and lower interparticle gap generated more surface active “hot spots” that results in better SERS signal. Similar types of phenomena were described before for self-assembled Os and Rh NPs on DNA scaffolds.35−37 The schematic presentation of

Figure 5. (A) Chemical structure of MB (middle), only MB dye (left), and mixture of MB dye and Re NPs solution (right). (B) UV−vis absorption spectra: (a) absorption band of only MB solution, (b) absorption band of only Re NPs, and (c) absorption band of the mixture of MB and Re NPs together.

region. Curve c in Figure 5B shows the absorption bands of a mixture of MB and Re NPs solution where a decrease in the absorption intensity of the peak at 664 nm but an increase in the absorption intensity of the peak at 291 nm were observed. This change in absorption intensity indicates that some physical interaction or adsorption of MB dye on the Re NPs surface took place. The Raman spectra of only aqueous MB (10−3 M) has been taken as seen in Figure S6, SI. From Figure S6 we can see several high- and low-intensity Raman bands for MB at different Raman shift values of 1624, 1430, 1392, 1328, 1147, 1073, 954, 895, 800, 660, 594, 495, 478, and 444 cm−1. The peaks of MB are assigned to different stretching and bending vibration modes of different bonds present in MB.60 In all of the observed peaks of MB in our present study, reported in the literature, the SERS bands were observed after mixing MB with Re NPs, and the corresponding band assignments are elaborated in Table S3, SI. Before doing the SERS experiment we tested the Raman spectra of the polymer PAH used to synthesized Re NPs and the Re NPs in PAH as seen in Figure S7, SI. In Figure S7 curve a shows the Raman spectra of PAH, while curve b shows the Raman spectra of PAH-Re NPs. Most of the peaks are matching with one another as PAH is present in both samples. However, for PAH-Re NPs sample, few additional peaks appeared at 1606, 1529, 1188, and 963 cm−1. Figure 6 shows the comparison SERS spectra taking two different size Re NPs. In the same picture we included the Raman spectra of only MB (10−3 M) also to make a clear comparison between the Raman spectra and the SERS spectra. In Figure 6 curve a is the Raman spectrum MB (10−3 M) and curves b and c are the SERS spectra of the same small- and 10195


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of aromatic nitroaromatics and in SERS studies. Catalysis study was done for the reduction of 4-NA in the presence of excess NaBH4 taking two different sizes of Re NPs as catalyst. The highest catalytic rate ever reported of 1.52 × 10−1 min−1 was observed taking large-size Re NPs as catalyst. The order of the catalytic rate was large-size Re NPs > small-size Re NPs. A SERS study was done taking MB as the Raman probe molecule, and a strong SERS signal was observed with both sizes of Re NPs due to their ultrasmall size, narrow interparticle gap, and close packing in the PAH scaffold. These close-packed Re NPs are self-assembled together in a PAH chain, generated more surface active “hot spots”, and ultimately resulted in good SERS signal. Overall, our present synthesis route is easy, cost effective, and ultrafast, completed within 120 s, and generated stable Re NPs which can be further applied in other interdisciplinary research fields in the near future other than catalysis and SERS.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b02175. Information on reagents used for synthesis, catalysis, and SERS studies, instrument used for different characterizations, preparation of samples for other characterization, figures related to EDS, XRD, XPS, and UV−vis spectra related to catalysis study and Raman spectra related to control experiments, tables related to catalysis and SERS, and schemes related to catalysis and SERS (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]; [email protected]. Fax: 979845-3081. Phone: 979-985-9609. *E-mail: [email protected]. Fax: 979-845-3081. Phone: 979985-9609. ORCID

Figure 7. Concentration-dependent SERS spectra with large-size Re NPs (A) and small-size Re NPs (B) in PAH scaffold as substrate.

Subrata Kundu: 0000-0002-1992-9659 Notes

The authors declare no competing financial interest.

our SERS experiment taking MB dye and Re NPs as substrate is shown in Scheme S2, SI. This strong EF values and good SERS signal observed taking Re NPs in PAH scaffold as substrate might find potential applications in the future for trace detection of biologically important molecules and various other biomedical applications.

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ACKNOWLEDGMENTS Financial support from the Bhaskara Advanced Solar Energy Fellowship (BASE) program in 2016, from the Department of Science and Technology (DST), and from the Indo US Science and Technology Forum (IUSSTF) is thankfully acknowledged. S.K. thanks Dr. Choongho Yu (Mechanical Engineering Department, TAMU) for discussion and Mr. S. R. Ede and Mr. S. Anantharaj (SRF, CSIR-CECRI) for helping to draw some illustrations and proof reading the manuscript. S.K. also acknowledges Dr. Vijayamohanan K. Pillai, Director, CSIRCECRI for his continuous support and encouragement.

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CONCLUSION In conclusion, we have highlighted an easy, ultrafast, and facile route for the formation of stable Re NPs using the polyelectrolyte PAH scaffold within 120 s of reaction at room temperature in aqueous solution. The average diameters of the Re NPs were 0.7 ± 0.25 and 1.7 ± 0.3 nm for two different morphologies. The size of the Re NPs was tuned just by controlling the polymer to metal salt molar ratio. We observed that the synthesized Re NPs were extremely stable when prepared in a PAH having concentrations of 1, 0.75, and 0.5 wt %. However, at lower PAH concentrations, viz., 0.1% or 0.01%, the formed Re NPs were precipitated within a short time after synthesis. The potentiality of the synthesized Re NPs was assessed in two different applications such as catalytic reduction

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