DOI: 10.1002/chem.201304518
Full Paper
& Conducting Materials
Mn Oxide-Silver Composite Nanowires for Improved Thermal Stability, SERS and Electrical Conductivity Mukul Pradhan, Arun Kumar Sinha, and Tarasankar Pal*[a]
Abstract: Redox transformation reaction between aqueous AgNO3 and Mn(CH3COO)2 at low temperature (~ 80 8C) has been adopted for industrial-scale production of uniform Ag– MnOOH composite nanowires for the first time. Varying amounts of incorporated Ag in the composite retain the 1D morphology of the composite. Nanowires upon annealing evolve Ag–MnO2 nanocomposites, once again with the retention of the parental morphology. Just 4 % of silver incorporation in the composite demonstrates metal-like conducting performance from the corresponding semiconducting material. Transition of MnO2 to Mn2O3 to Mn3O4 takes place upon heat treatment in relation to successive increase in Ag concentrations in the nanowires. The composites offer resist-
Introduction Surface enhanced Raman scattering (SERS) spectroscopy has a variety of applications in diverse fields.[1, 2] It has become an analytical tool with the opportunity of single molecule detection sensitivity.[3, 4] Generally Raman signal enhancement is observed by a factor of 106 to 108 depending on the size and structure of Ag and Au[3, 5] nanostructures. In a recent report, we have shown that Au-flowers with closely spaced sharp tips perform as excellent SERS substrates for detection of probe molecule down to the single-molecular level due to massive electromagnetic field enhancement.[6] Recently semiconductor nanostructures have been found to show the potential of SERS enhancement.[7] Synthesis of semiconductor–noble metal nanocomposites to study improved SERS enhancement[8–14] is a recent trend. Noticeable increase in the Raman activities of molecules adsorbed on Au–SiO2,[15] Au– TiO2,[16] and Ag–CuO[17] has been observed. Leaving aside the enhancement, the application of SERS as a general analytical tool requires reproducibility of cost-effective substrate preparation. Generally the enhancement factor of semiconductorbased SERS substrates is considerably low. Previously SERS studies had been restricted to the coinage metals (Au, Ag, and Cu) and a few transition metals (Pt, Pd, Ru, Rh, and Ni).[18–20] [a] M. Pradhan, A. K. Sinha, Prof. T. Pal Department of Chemistry, Indian Institute of Technology Kharagpur, Kharagpur-721302 (India) E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201304518. Chem. Eur. J. 2014, 20, 9111 – 9119
ance to the observed oxide transformation. This is evidenced from the progressive increase in transition temperature. In situ Raman, ex situ thermal and XRD analysis corroborate the fact. The composite with 12 % Ag offers resistance to the transformation of MnO2, which is also verified from laser heating. Importantly, Ag nanoparticle incorporation is proved to offer a thermally stable and better surface enhanced Raman scattering (SERS) platform than the individual components. Both the Ag–MnOOH and Ag–MnO2 nanocomposites with 8 atomic % Ag show the best SERS enhancement (enhancement factor ~ 1010). The observed enhancement relates to charge transfer as well as electromagnetic effects.
This provoked us to develop novel composite materials as high SERS-active substrates. The property of metal oxides (MOs) is modified by incorporating metal nanoparticles (NPs) to realise speciality applications and SERS signal enhancement to a great extent.[21] It is highly desirable to develop 1D nanomaterials that have not only controlled shapes and crystal structure but also designed electrical and optical properties for applications as sensors, field-emitters, p-n diodes, and diluted magnetic semiconductors (DMS) for spintronics.[22–27] Metal-doped metal oxide nanowires have been adopted to improve the electrical conductivity.[28] Metallic coatings on fibres have also been studied. Nylon fibre mats coated with Ag using a commercial electroless plating solution exceed about 1800 S cm1 when loaded with approximately 17 wt % Ag.[29] The chemistry of MOs is modified by incorporating metal NPs for achieving specialty applications. This has become a new trend in materials research. Often, the MO is taken to the nanodimension at which the bulk property of the MO disappears completely and new properties emerge.[30] Due to the possible charge separation at the metal–semiconductor interface, doped materials may be applied in photocatalytic processes.[31–33] On this plea, metal–semiconductor composite nanostructures have been synthesised from various combinations, such as PbSe/Au, Ag or Pd,[34] CdSe/Au, CdS/Au,[35] CdSe/ Co,[36] InAs/Au,[37] and TiO2/Co.[38] Furthermore, the size of nanoparticles as well as the mode of deposition of metal nanoparticles plays a crucial role while tailoring the properties of semiconductor nanomaterials.
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Full Paper The most common method of producing Ag–MnO2 nanoparticle is cathodic electrodeposition[39] of Ag + species from aqueous KMnO4 solution containing AgNO3 or by a reassembling reaction between Ag + ions and delaminated manganese oxide nanosheets, followed by a reduction process.[40] It is thought that improvement of the electrical property of MnO2 is sure to be achieved if Ag (the best electrical conducting metal) NPs can be incorporated in the MnO2 composite. There are several novel methods for the synthesis of 1D MnO2 nanowires.[41–43] However, in most cases Ag NPs are deposited on the surface of the already synthesised MnO2 nanoparticles.[44] But we have employed a solution-phase synthetic procedure involving wet chemical redox reaction between MnII and AgI precursor salt. We chose Mn(CH3COO)2 and AgNO3 to obtain 1D Ag–MnO2 nanomaterials. Interestingly, it has been observed that during the course of the proposed reaction Ag NPs are incorporated in situ in the MnOOH matrix. Again the amount of Ag incorporation can be increased in the Ag–MnOOH composite just by increasing the amount of silver nitrate in the reaction medium, which is easily transformed into Ag–MnO2 nanomaterial. In this context, here we report the synthesis of 1D morphology Ag–MnO2 nanocomposites with varying amount of incorporated Ag by redox transformation reaction between aqueous AgI and MnII salt. This method gives rise to a new redoxmediated synthesis for developing metal nanoparticle loaded 1D semiconductor composites. Ag–MnO2 nanowires with 12 % Ag show metal-like electrical conductivity. Ag–MnO2 composite nanowires act as a better thermally stable SERS substrate compare to individual components due to conjugate electromagnetic enhancement and charge-transfer effect. Raman, thermogravimetry and XRD analyses established Ag concentrationdependent increased transition temperature of manganese oxides (MnO2, Mn2O3 and Mn3O4) in the composite, that is, thermal stability in comparison to the pure material.
Figure 1. A) Stepwise synthetic procedure of gram-level Ag–MnO2 nanowires on a water bath at 80 8C by mixing AgNO3 and Mn(CH3COO)2 followed by annealing at 350 8C. B) Conducting interlaced Ag–MnO2 sheet formation by simple vacuum filtration.
Results and Discussion We have chosen a facile redox reaction to deliver a composite nanowire as conducting as well as high SERS active substrate in gram level. Stepwise synthetic procedure for gram-level production of Ag–MnOOH on a water bath at 80 8C by mixing AgNO3 and Mn(CH3COO)2 is shown in Figure 1. Ag–MnO2 nanowire was obtained by annealing the product at 350 8C. The digital image in Figure 1 shows interlaced Ag–MnO2 nanosheets prepared by vacuum filtration. Details of XRD (Figures S1 and S2), XPS (Figure S3), FTIR (Figure S4), DRS (Figure S5) and optical property (Figure S6) are provided in the Supporting Information. Transmission electron microscopy (TEM), high-resolution TEM (HRTEM) and energy-dispersive X-ray spectroscopy (EDX) analysis
Figure 2. A), B) TEM image of Ag–MnO2 nanowires with 4 and 12 % Ag, respectively. Inset B: corresponding SAED pattern. C), D) Fringe spacing.
The TEM image of wet, chemically synthesised nanowires is presented in Figure 1 B and Figure 2 A. From the critical fringe spacing analysis we can identify the interplanar spacing of both the MnO2 as well as Ag in the composite materials. The
fringes (Figure 2 D) with approximately 0.23 nm interplanar separation parallel to the rod axis correspond to the (111) planes of Ag, which indicates that the growth direction of the Ag is along (111) within the nanocomposite. The interplanar
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Full Paper distance of the fringes is 0.315 nm, corresponding to the (110) planes of tetragonal b-MnO2 in the composite materials. Figure 2 B (inset) represents the SAED pattern of the Ag–MnO2 composite nanowires. We observed the lattice spacing due to Ag and MnO2 in the composite until 12 % of Ag in the nanocomposite was reached. After that, Ag (> 12 %) comes out and becomes deposited outside the composite matrix surface (Figure S7 in the Supporting Information). There, the lattice spacing of the segregated particle matches well with Ag and confirms surface decoration by Ag. The composition of the synthesised product has also been observed from EDX analysis. The EDX patterns of the Ag–MnO2 nanowires are presented in Figure S8 in the Supporting Information and Figure 3. The EDX analysis confirms the presence
ter of the nanowires becomes wider (Figure S9 in the Supporting Information).
Growth mechanism The growth demonstrates the formation of Ag–MnOOH nanowires from aqueous solution on water bath at 80 8C. The growth of Ag–MnOOH nanowires occurs via a series of intermediates, which are characterised by FESEM, EDX, TEM and HRTEM analysis. The brown solid precipitated out slowly, which was collected at different time intervals (12 to 42 h). The time-dependent growth of Ag–MnOOH nanowires was shown by TEM images (Figure 4). Finally Ag–MnO2 is obtained after annealing at 350 8C.
Figure 4. A)–E) Growth of Ag–MnOOH composite nanowires with 8 % Ag, after 5 min, 10 min, 48 h and 4 days of heating on a water bath. F) Ag–MnO2 nanowires obtained after annealing at 350 8C. Scale bars: A, 200 nm; B, 0.2 mm; C, 50 nm; D, 50 nm; E, 100 nm; F, 50 nm.
Figure 3. A) FFESEM image shows the uniformity of the synthesised Ag– MnO2 nanowires. B) Elemental mapping shows the 1D signature for each element within the composite.
of Mn, O, and Ag in all the synthesised products. We can vary the amount of Ag concentration in the composite nanowire easily. The amount of silver in different nanowires of a particular lot is very consistent. We observed a homogeneous distribution of Ag, in the product and no sign of a lateral or longitudinal concentration gradient in individual nanowires was observed. This is due to the Ag–MnO2 nanowire formation from AgI and MnII ions through solution-phase redox transformation. The nanowires are 10 mm in length and 50–500 nm in diameter. We observed that by increasing Ag proportion the diameChem. Eur. J. 2014, 20, 9111 – 9119
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Generally, controlled growth of seed particles is altogether an art of manipulating spheres into rods or faceting wires out of metallic NPs. We presume the same strategy holds good here also for composite nanowire formation. When we introduced AgNO3 solution into a solution containing Mn(CH3COO)2, formation of a bluish-brown solution was observed. This is due to Mn + + /Ag + redox-mediated silver-seed formation. The Ag seeds serve as nucleation sites for MnOOH nanowire growth under the experimental condition. During slow heat treatment the Ag and MnOOH nanomaterials coalesce to form nanowires through the shape-transformation mechanism. Why 1D growth occurs is yet to be understood fully. In the successive step of synthesis, the Ag and MnOOH nanomaterials come closer for 1D alignment through oriented attachment producing nanowire structures. Murphy et al. observed that the average aspect ratio of the Au nanorod increased with decreasing amount of seed as the spheroidal particles gradually grew into rod-like structures.[45, 46] From the FESEM images we observed that Ag–MnOOH nanowires with a higher percentage of Ag, grow nanowires with larger diameters leading to a decrease the aspect ratios compared to the Ag–MnOOH nanowires having a lower percentage of Ag. The reasons why the wire morphology is formed with different diameters is not yet
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Full Paper completely understood. Presumably, higher amounts of silver nitrate produce larger amounts of silver seed leading to decreased aspect ratio through faster growth as the spheroidal particles gradually grew into rod. However, aspect ratio of MnOOH nanostructure is dopant dependent (here Ag) but 1D growth is surely habitual for MnOOH. With the progress of the reaction the nanospheres coalesce and the diameter of the nanowires increases with time. Finally, the as-obtained product was annealed at 350 8C to obtain Ag–MnO2 composite. We observed no noticeable detachment of the Ag particle until 12 % Ag was reached (Figure 2). Then progressively, aspect ratio of nanowire decreases (Figure 2 and Figure S9 in the Supporting Information). With further increase in Ag content (> 12 %), the synthetic protocol gives rise to the formation of a mixture of nanoparticles and nanowires. For higher amounts of Ag (~ 15 %) in the composite we observed lattice spacing at 0.23 (for 111 plane of Ag) and 0.305 nm (for 110 plane of MnO2) from the same HRTEM image due to formation of segregated Ag nanoparticles on the surface of the composite nanowire (Figure S7 in the Supporting Information). So, noticeable detachment[47] of Ag nanoparticles from nanowires was observed at much higher Ag concentration but not below 12 %. FTIR spectra indicate that the prepared Ag–MnO2 shows a clean surface without any absorbed organic or hydroxyl species. The growth mechanism is also investigated by TEM (Figure 4) analysis in an ex situ fashion and is shown in Figure 4. At the initial stage (at 12 h), Ag nanospheres are formed and they coalesce and gradually change to nanowire morphology. The overall crystallisation process is shown in Figure 4. Enhanced thermal stability of Ag–MnOOH/Ag–MnO2 composite nanowires: Raman study Normal Raman scattering (NRS) studies help to understand the stability of the composite in which Ag is incorporated in MnOOH in a straight forward manner. For all the Ag–MnOOH nanocomposites we observed a broad peak at approximately 600 cm1. As for Ag–MnOOH nanocomposites with lower percentages of Ag (4 and 8 %) small peaks at approximately 360, 388, 558, 622 cm1 were observed at low laser power in our spectral window corresponding to the stretching modes of MnO6 octahedra for pure MnOOH.[48] A small peak at 534 cm1 (at lower laser power for 8 % Ag in the composite) is observed for Ag–MnOOH nanocomposites due to the formation of bMnO2.[48] The band at 648 cm1 is assigned to Mn3O4 formation during the acquisition of the spectrum at high laser power because of the local heating of the samples by laser irradiation, which is consistent with reported Mn3O4 samples (650– 660 cm1). Just like the band at 648 cm1, weak peaks at approximately 308, 362, 420 and 648 cm1 are observed in Ag– MnOOH nanowires; these originate from the formation of Mn2O3 or Mn3O4. The transformation from MnOOH to the possible Mn2O3[48] or Mn3O4 takes place by the incident laser heating.[49] In our case, Raman study qualifies as a sensitive tool to conclusively prove the stability of the as-synthesised compoChem. Eur. J. 2014, 20, 9111 – 9119
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Figure 5. Effect of laser heating during Raman analysis with variable laser power (lp) and acquisition time (aq) on the Ag–MnOOH composite nanowires. The results suggest enhanced thermal stability of the composites with increasing amount of Ag.
site with higher dosage of Ag incorporation. It requires successively higher laser power as well as longer laser exposure time to obtain Mn2O3 or Mn3O4 for Ag incorporated MnOOH samples (Figure 5). The transformation from MnOOH to Mn2O3 or Mn3O4 takes place during the acquisition of Raman spectra but
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Full Paper with higher laser power presumably via b-MnO2. This laser heating study speaks for Ag-concentration-dependent stability of the Ag–MnOOH/Ag–MnO2 composite nanowires, which is supported by thermal as well as temperature-dependent XRD analysis.
Thermal gravimetric analysis (TGA) Thermal stability of manganese oxides (MnO2, Mn2O3 and Mn3O4) in the composite materials with various amounts of Ag NP doping (4, 8 and 12 %) is ascertained from careful, critical and programmed heat-treatment experiments. Thermal stability is reported for such Ag–MnO2 composite materials for the first time. The nature of TGA and differential thermal analysis (DTA) thermograms of Ag–MnO2 samples (12 % Ag, sample A; 8 % Ag, sample B; 4 % Ag, sample C) together with TGA and DTA curves of Ag–MnO2 are presented for comparison in Figure 6. In the TGA thermogram all the Ag–MnO2 samples
Figure 6. Ag concentration-dependent TGA and DTG curves in the same window show the gradual shift of manganese oxide transition temperature towards a higher value in the Ag–MnO2 composite.
show distinct weight losses below 450 8C, which corresponds to the removal of water molecules, organic reactants, and trace amount of lattice oxygen. Sample C (with lowest Ag (4 %), Figure 6) shows weight loss at around 627 8C that corresponds to the loss of oxygen from the MnO2 lattice resulting in the transformation to Mn2O3. Another weight loss at 714 8C corresponds to further loss of oxygen resulting in the transformation from Mn2O3 to Mn3O4 (Figure 6).[50] Sample A with the highest amount of silver (12 %) shows weight loss at around 756 8C for loss of oxygen from the MnO2 lattice resulting in the transformation of MnO2 to Mn2O3 (Figure 6) and then remains stable as indicated by the horizontal plateau. In case of sample B with the intermediate amount of Ag (8 %), weight loss at around 716 8C corresponds to the loss of oxygen from the MnO2 lattice resulting in the transformation to Mn2O3 (Figure 6). The MnO2 to Mn2O3 transformation temperature has been observed to be shifted towards a higher temperature range due to the incorporation of progressively higher amount of Ag NPs in the MnO2 matrix. Still higher amounts of silver Chem. Eur. J. 2014, 20, 9111 – 9119
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doping resist oxygen loss to a noticeable extent. Thus, sample A with the highest amount of Ag (12 %), remains as Mn2O3 even at 850 8C. It can be recalled that pure MnO2 goes to Mn2O3 just at 550 8C (at a much lower temperature than reported for the as-prepared Ag–MnO2 composites) and then to Mn3O4 at 750 8C. Thermal stability of the different Mn-oxides in the composites is authenticated, which is definitely administrated by the amount of Ag NP doping. So, silver nanoparticles in the manganese oxide matrix assist nanowire growth, resist the usual transformation of MnO2 to a great extent and make the composite a stable platform for further studies.
Thermal stability of Mn oxide–Ag composites from XRD analysis To understand the thermal stability of the synthesised composite materials we carried out the XRD analysis in the ex situ fashion. Each time a fresh batch of as-prepared sample with a different amount of sliver (4, 8 and 12 %) was separately annealed at different temperature and then XRD analysis was performed. A representative XRD result, from which we clearly observed that the Mn oxide sample with 4 % Ag is quantitatively converted to Ag–Mn3O4 for the 1000 8C annealed sample, is shown in Figure 7. However, the 1000 8C annealed samples with 8 and 12 % Ag distinctly show peaks for Mn2O3 + Mn3O4 and Mn2O3, respectively. Again, the sample annealed at 850 8C (Figure S10 in the Supporting Information) reveals the exclusive presence of Mn2O3 with 12 % Ag. It may be recalled that annealing pure MnO2 sample at 750 8C exclusively produces Mn3O4. So, it is conclusively spelt out that in terms of thermal stability Ag stimulates manganese oxides enhancing the transition temperature for Mn oxides and that is related to the Ag concentration. It seems that the presence of Ag (high density and boiling point) helps to retain the tunnel structure[51] of b-MnO2 even at high temperature.
SERS study The Ag surface in the as-synthesised composite is modified greatly because of the presence of MnOOH/MnO2. Then the acquired stability of the modified surface readily helps the chemisorption process. Though it is well known that Ag interacts with N-containing ligand and Au with S-containing ligand according to HSAB principle, we observed weak SERS signal enhancement of 1,10-phenanthroline (Figure S11 in the Supporting Information) and moderate SERS signal enhancement taking p-amino thiolphenol as a probe molecules on Ag– MnOOH composite material (Figure S11). Whereas very high SERS signal enhancement occurs from thiophenol as probe on the Ag–MnOOH composite material (Figure 8 and Figures S11– S14 in the Supporting Information). We have used Ag–MnOOH composite material as a SERS substrate, not pure Ag. Moreover, the SH stretching and bending mode of benzenethiol[52, 53] at 2600 and 918 cm1, respectively, were not observed in the measured SERS spectra (Figure S12 in the Supporting Information). This further confirms that the probe molecules are chem-
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Full Paper ry in order to obtain the maximum SERS enhancement. For example, 500 nm Ag nanoshells show a much sharper SPR at approximately 400 nm.[55] In the case of the Ag–MnO2 nanowires, however, the very broad SPR allows us to obtain maximum SERS intensity in a much broader laser energy range. Moreover, we have used 633 nm laser for SERS measurement. Again we observed that the absorbance of MnO2 gradually increases after approximately 600 nm by increasing the amount of Ag into the MnO2 matrix. Taking TP as the probe molecule we explored the SERS activity involving Ag–MnOOH compoFigure 7. XRD pattern of annealed Ag–MnO2 composite nanowires with various amounts of Ag at 1000 8C. The site 8 % Ag. The diameter of composition with the highest amount of Ag (12 %) remained stable as Mn2O3 even after annealing at 1000 8C Ag–MnOOH nanowire increases whereas the composition having the lowest amount of Ag (4 %) converted to Mn3O4, but the composition with progressively with increasing moderate Ag (8 %) evolved a mixture of Mn2O3 and Mn3O4. For pure Mn2O3 the oxide transition temperature amount of Ag in the composite. (Mn2O3 to Mn3O4) is only 750 8C. Again the increased amount of silver in the 1D structure lowers the aspect ratio of the nanowires. Thus, there arises a possibility of the reduction of hot spots after drop casting due to lower probability of interlacing of nanowires in a small area. So, we found that 8 % Ag-containing composites are comparatively better SERS substrates than 12 % Ag-containing composites. The SERS spectra of TP, adsorbed onto the Ag–MnOOH composite nanowire at various concentrations, are shown in Figure 8. An excitation wavelength of 632.8 nm laser is used for SERS studies. By comparing the SERS spectra (Figure 8 and Figure S13 in the Supporting Information) and the normal Raman spectrum of TP (Figure 8), we observed the obvious changes in terms of signal intensity and shift of several SERS bands of TP. The peak at 1092 cm1 due to CS stretching mode moved to 1073 cm1 in the SERS spectrum and the intensity is also increased. Similarly, another peak at 1583 cm1, Figure 8. NRS spectrum of 0.1 m TP in aqueous solution and SERS spectra of contribution from CC stretching, moves to 1575 cm1. The TP adsorbed on Ag–MnOOH composite nanowires at various concentrations of the adsorbate for lexc = 632.8 nm. peak at 420 cm1 is due to the contribution from the CS stretching vibration (nCS). The SH bending mode in the Raman spectrum disappeared in the SERS spectra of TP. All of these alterations can be interpreted to imply that TP remains isorbed. Hence, we obtained huge SERS signal enhancement on Ag–MnOOH composite through chemisorption, resulting in taking TP as probe molecule. changes in its structure.[56] In order to compliment the composite effect for the observed SERS signal enhancement, we conducted diffuse reflecWe calculated the enhancement factor (EF) by comparing tance spectroscopy (DRS) with MnO2 and plotted the results in the intensity of all the high intensity peaks in the SERS spectrum with those peaks obtained in the normal Raman specsame window with the Ag–MnO2 composite with different trum by using a technique similar to that reported previousamounts of Ag. An interesting feature of the Ag–MnO2 nanoly.[57] The lowest detection limit for TP was observed to be as wires is their broad surface plasmon resonance (SPR), as shown in Figure S5 in the Supporting Information. Generally sharp low as 1010 m. The metal–semiconductor hybrid nanomaterial [54] SPRs have been reported in SERS-active materials, in such as a uniform SERS substrate shows huge SERS signal enhancement in comparison to the individual components (Figure 9 situations careful tuning of laser wavelength becomes necessaChem. Eur. J. 2014, 20, 9111 – 9119
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Figure 10. Comparative SERS spectra of TP (108 m) adsorbed on MnOOH, Creighton sol, 55 nm Au nanoparticle and Ag–MnOOH composite nanowires for lexc = 632.8 nm.
The situation became reversed (Figure 10) when we carried out SERS after drop casting. The observed enhanced SERS intensity may be due to the presence of hotspots created due to the drop casted solution. The as-synthesised composite affects the Ag surface because of the presence of MnOOH. The modified surface helps better thiophenol chemisorption. We estimated the apparent enhancement factors (AEFs) of some selected Raman bands using the relation we reported previously:[7]
AEF ¼ s SERS ½C NRS =sNRS ½C SERS
Figure 9. Area mapping (SERS, considering the selected bands) on Ag– MnOOH composite taking TP (105 m) as a probe molecule.
and Figure 10). We compared the SERS spectra of thiophenol obtained from the Ag–MnOOH composite nanowire with 55 nm Au nanoparticle[58] as well as Creighton sol[59] and pure MnOOH (Figure 10). We observed huge SERS enhancement from the Ag–MnOOH composite nanowire having 8 % Ag with 108 m thiophenol as probe molecule (Figures 8 and 10). We observed comparable SERS enhancement from both the 55 nm Au nanoparticle and Ag–MnOOH composite nanowire having 8 % Ag with 108 m thiophenol as probe molecule (Figure 10). Comparative SERS spectra of thiophenol on Ag–MnOOH composite nanowire and Au nanoparticle may be due to the strong AuS interaction, that is, “soft–soft” interaction. Thus, the importance of Ag–MnOOH composite nanowire has been validated from the comparative SERS account. Nanowires, in dispersion in the present case, have a natural tendency to precipitate out from the solution. Hence, the SERS signal lowers in solution phase while Ag–MnOOH nanowire dispersion is taken as substrate. A comparative SERS of Ag– MnOOH nanowire dispersion and Creighton sol[59] indicate the superiority of the latter. The result shows that stable Creighton sol gives higher SERS intensity (Figure S14 in the Supporting Information) in solution phase in comparison to nanowires dispersed in water due to precipitation of Ag–MnOOH nanowires. Chem. Eur. J. 2014, 20, 9111 – 9119
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ð1Þ
where C and s are the concentration and the peak area, respectively, of the Raman bands measured from baseline. The AEF values of the selected enhanced Raman bands of thiophenol[60] at various concentrations of the adsorbate are shown in Table 1.
Electrical property Metal–semiconductor contact is an obvious component of any semiconductor device. In extrinsic (doped) semiconductors, dopant atoms increase the majority charge carrier concentration by donating electrons to the conduction band or accepting holes in the valence band. However, such contacts cannot be assumed to have a resistance as low as that of two connected metals. In particular, a large mismatch between the Fermi energy of the metal and semiconductor can result in a high-resistance rectifying contact. A proper choice of materials can provide a low resistance Ohmic contact. For both types of donor or acceptor atoms, increasing the dopant density leads to a reduction in the resistance, hence highly doped semiconductors behave metallically. Keeping this idea in mind we chose the highly conducting Ag as a dopant. We plotted the I–V characteristics for Ag–MnO2 composites with variable amount of Ag (Figure 11). The results
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Full Paper Table 1. Assignment of vibrational modes for thiophenol Raman peaks. AEF 104 SERS (105 m) [cm]
AEF 105 SERS (106 m) [cm]
AEF 106 SERS (107 m) [cm]
AEF 107 SERS (1010 m) [cm]
AEF 1010
420
0.87
3.3
420
1.52
420
0.49
420
0.29
1001 1024 1074
0.22 0.39 0.79
1000 1023 1073
0.95 1.78 3.28
1001 1023.5 1074
0.41 0.78 1.61
1001 1024 1076
0.19 0.33 0.54
1000 1024 1074
0.54 0.10 0.21
1576
0.98
1575
3.93
1575
2.23
1577
1
1575
0.23
NRS, 0.1 m Symmetry Assignment SERS (104 m) [cm1] [cm1] 418 (w) 616 (ms) 698 1000 (vvw) 1025 1072 1157 1578
(a1, a2) b1 a1 a1 a1 b1 b1 a1 and b1
bCCC + nCS bCCC + nCS bCCC + nCS bCCC bCH bCH bCH nCC
419.5
Conclusion The studies reported are gifted with low cost industry scale-up preparation strategy for Ag–MnOOH and Ag–MnO2 nanowire composites. It has been described that even after redox transformation between AgI and MnII salts at low temperature the habitual 1D growth of MnOOH is not disturbed with incorporated Ag NPs. Silver incorporation successively enhances the stability of the composite as evidenced from the progressively increased oxide transition temperature. Again, optical response (red shift) and XRD analysis portray the incorporation of Ag into the annealed MnO2 matrix. Interestingly nanowire morphology of manganese oxide is uniquely retained after annealing, but with changes in band-gap energy and drastic electrical conductivity. Finally, the synthesised composite nanomaterial shows high SERS sensitivity with an enhancement factor of approximately 1010 taking thiophenol as a probe molecule, which makes it a stable deliverable SERS substrate. Figure 11. The current–voltage characteristics of synthesised Ag–MnOOH nanocomposites.
Experimental Section show linear dependence of current with voltage. For I–V characteristic we used linear four-probe method on top of the pellet. We applied current on the outer electrodes and measured voltage in the inner electrodes. From this we measured resistivity of the samples and from there we calculated the conductivity. Here, the measured resistivity of the obtained Ag–MnO2 nanowires are 0.875 107, 4.4 104 and 2.9 103 Wm for 12, 8, 4 %, and Ag, respectively, by using standard four-probe method. Resistivity for all the composite materials is much lower than that of pure MnO2 (~ 0.02 Wm). Composite nanowires with only 4 % Ag show metal-like electrical conductivity (Figure S15 in the Supporting Information). These results show that the Ag-loaded manganese oxide can significantly improve the electrical performance, that is, the increase in conductivity like the doped materials as stated in the preceding section. It may be recalled that Ag not only increases the conductivity but also improved the thermal stability to a great extent. Finally, the nanowire composites have been proved to be a good deliverable as a stable SERS substrate.
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Synthesis of rod-shaped Ag–MnOOH/Ag–MnO2 First, aqueous solutions of three different reaction mixtures of variable compositions were prepared separately in three different round-bottom flasks. Various aliquots (50, 25, 12 mL) of 0.1 m silver nitrate (AgNO3, 5 102 m) were introduced separately drop by drop into the set containing 500 mL aqueous manganese(II) acetate (Mn(CH3COO)2·4 H2O, 5 102 m) solution under magnetic stirring at room temperature in about 15 min. The colourless reaction mixture turned initially to bluish black and finally to light brown, which indicated the onset of the evolution of Ag–MnOOH nanoparticles. Then the brown solutions were kept for 98 h on a water bath for the evolution of uniform Ag–MnOOH nanowires. Finally, Ag–MnO2 composite materials were obtained by annealing the asobtained Ag–MnOOH at 350 8C.
Procedure for SERS measurement Fresh stock solutions of thiophenol were prepared regularly in ethanol with variable concentrations (103–1010 mol dm3). To test the performance of our nanostructured SERS substrates, a dilute dispersion of Ag–MnOOH (100 mmol L1 in ethanol) and 1–50 nm Ag nanosphere (0.25 103 m in water) were separately incubated for 4 h. All SERS spectra were recorded dispensing 30 mL of the incubated solution on aluminium foil.
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Full Paper Acknowledgements We thank the BRNS Bombay, CSIR, DST New Delhi for financial assistance and Indian Institute of Technology, Kharagpur for research facilities. Keywords: conducting materials · Mn oxide–Ag composite · nanowires · SERS · thermal stability [1] I. T. Shadi, B. Z. Chowdhry, M. J. Snowden, R. Withnall, Chem. Commun. 2004, 12, 1436 – 1437. [2] Y. Borodko, S. M. Humphrey, T. D. Tilley, H. Frei, G. A. Somorjai, J. Phys. Chem. C 2007, 111, 6288 – 6295. [3] S. Nie, S. R. Emory, Science 1997, 275, 1102 – 1106. [4] A. M. Michaels, M. Nirmal, L. E. Brus, J. Am. Chem. Soc. 1999, 121, 9932 – 9939. [5] K. Kneipp, Y. Wang, H. Kneipp, L. T. Perelman, I. Itzkan, R. R. Dasari, M. S. Feld, Phys. Rev. Lett. 1997, 78, 1667 – 1670. [6] M. Pradhan, J. Chowdhury, S. Sarkar, A. K. Sinha, T. Pal, J. Phys. Chem. C 2012, 116, 24301 – 24313. [7] A. Musumeci, D. Gosztola, T. Schiller, N. M. Dimitrijevic, V. Mujica, D. Martin, T. Rajh, J. Am. Chem. Soc. 2009, 131, 6040 – 6041. [8] L. Yang, X. Jiang, W. Ruan, J. Yang, B. Zhao, W. Xu, J. R. Lombardi, J. Phys. Chem. C 2009, 113, 16226 – 16231. [9] C. Cheng, B. Yan, S. M. Wong, X. Li, W. Zhou, T. Yu, Z. Shen, H. Yu, H. J. Fan, ACS Appl. Mater. Interfaces 2010, 2, 1824 – 1828. [10] X. T. Wang, W. S. Shi, G. W. She, L. X. Mu, S. T. Lee, Appl. Phys. Lett. 2010, 96, 053104. [11] S. Deng, H. M. Fan, X. Zhang, K. P. Loh, C. L. Cheng, C. H. Sow, Y. L. Foo, Nanotechnology 2009, 20, 175705. [12] I. Alessandri, J. Colloid Interface Sci. 2010, 351, 576 – 579. [13] I. Alessandri, L. E. Depero, Chem. Commun. 2009, 17, 2359 – 2361. [14] I. Alessandri, M. Ferroni, L. E. Depero, ChemPhysChem 2009, 10, 1017 – 1022. [15] M. Roca, A. J. Haes, J. Am. Chem. Soc. 2008, 130, 14273 – 14279. [16] X. Li, H. Hu, D. Li, Z. Shen, Q. Xiong, S. Li, H. J. Fan, ACS Appl. Mater. Interfaces 2012, 4, 2180 – 2185. [17] Y. Wang, W. Song, W. Ruan, J. Yang, B. Zhao, J. R. Lombardi, J. Phys. Chem. C 2009, 113, 8065 – 8069. [18] Z. Q. Tian, B. Ren, D. Y. Wu, J. Phys. Chem. B 2002, 106, 9463 – 9483. [19] B. Ren, X. F. Lin, Z. L. Yang, G. K. Liu, R. F. Aroca, B. W. Mao, Z. Q. Tian, J. Am. Chem. Soc. 2003, 125, 9598 – 9599. [20] J. A. Dieringer, A. D. McFarland, N. C. Shah, D. A. Stuart, A. V. Whitney, C. R. Yonzon, M. A. Young, X. Zhang, Faraday Discuss. 2006, 132, 9 – 26. [21] M. W. Shao, L. Lu, H. Wang, S. Wang, M. L. Zhang, D. D. D. Ma, S. T. Lee, Chem. Commun. 2008, 2310 – 2312. [22] J. Zhong, S. Muthukumar, Y. Chen, Y. Lua, H. M. Ng, W. Jiang, E. L. Garfunkel, Appl. Phys. Lett. 2003, 83, 3401 – 3403. [23] H. J. Choi, H. K. Seong, J. Y. Chang, K. Lee, Y. J. Park, J. J. Kim, S. K. Lee, R. R. He, T. Kuykendall, P. D. Yang, Adv. Mater. 2005, 17, 1351 – 1356. [24] G. F. Zheng, W. Lu, S. Jin, C. M. Lieber, Adv. Mater. 2004, 16, 1890 – 1893. [25] Z. H. Zhong, Q. Qian, D. L. Wang, C. M. Lieber, Nano Lett. 2003, 3, 343 – 346. [26] J. Y. Lao, J. G. Wen, Z. F. Ren, Nano Lett. 2002, 2, 1287 – 1291. [27] P. V. Radovanovic, C. J. Barrelet, S. Gradecak, F. Qian, C. M. Lieber, Nano Lett. 2005, 5, 1407 – 1411. [28] J. H. He, C. S. Lao, L. J. Chen, D. Davidovic, Z. L. Wang, J. Am. Chem. Soc. 2005, 127, 16376 – 16377. [29] J. S. Jur, W. J. Sweet, C. J. Oldham, G. N. Parsons, Adv. Funct. Mater. 2011, 21, 1993.
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& Conducting Materials
Mn Oxide-Silver Composite Nanowires for Improved Thermal Stability, SERS and Electrical Conductivity Mukul Pradhan, Arun Kumar Sinha, and Tarasankar Pal*[a]
Abstract: Redox transformation reaction between aqueous AgNO3 and Mn(CH3COO)2 at low temperature (~ 80 8C) has been adopted for industrial-scale production of uniform Ag– MnOOH composite nanowires for the first time. Varying amounts of incorporated Ag in the composite retain the 1D morphology of the composite. Nanowires upon annealing evolve Ag–MnO2 nanocomposites, once again with the retention of the parental morphology. Just 4 % of silver incorporation in the composite demonstrates metal-like conducting performance from the corresponding semiconducting material. Transition of MnO2 to Mn2O3 to Mn3O4 takes place upon heat treatment in relation to successive increase in Ag concentrations in the nanowires. The composites offer resist-
Introduction Surface enhanced Raman scattering (SERS) spectroscopy has a variety of applications in diverse fields.[1, 2] It has become an analytical tool with the opportunity of single molecule detection sensitivity.[3, 4] Generally Raman signal enhancement is observed by a factor of 106 to 108 depending on the size and structure of Ag and Au[3, 5] nanostructures. In a recent report, we have shown that Au-flowers with closely spaced sharp tips perform as excellent SERS substrates for detection of probe molecule down to the single-molecular level due to massive electromagnetic field enhancement.[6] Recently semiconductor nanostructures have been found to show the potential of SERS enhancement.[7] Synthesis of semiconductor–noble metal nanocomposites to study improved SERS enhancement[8–14] is a recent trend. Noticeable increase in the Raman activities of molecules adsorbed on Au–SiO2,[15] Au– TiO2,[16] and Ag–CuO[17] has been observed. Leaving aside the enhancement, the application of SERS as a general analytical tool requires reproducibility of cost-effective substrate preparation. Generally the enhancement factor of semiconductorbased SERS substrates is considerably low. Previously SERS studies had been restricted to the coinage metals (Au, Ag, and Cu) and a few transition metals (Pt, Pd, Ru, Rh, and Ni).[18–20] [a] M. Pradhan, A. K. Sinha, Prof. T. Pal Department of Chemistry, Indian Institute of Technology Kharagpur, Kharagpur-721302 (India) E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201304518. Chem. Eur. J. 2014, 20, 9111 – 9119
ance to the observed oxide transformation. This is evidenced from the progressive increase in transition temperature. In situ Raman, ex situ thermal and XRD analysis corroborate the fact. The composite with 12 % Ag offers resistance to the transformation of MnO2, which is also verified from laser heating. Importantly, Ag nanoparticle incorporation is proved to offer a thermally stable and better surface enhanced Raman scattering (SERS) platform than the individual components. Both the Ag–MnOOH and Ag–MnO2 nanocomposites with 8 atomic % Ag show the best SERS enhancement (enhancement factor ~ 1010). The observed enhancement relates to charge transfer as well as electromagnetic effects.
This provoked us to develop novel composite materials as high SERS-active substrates. The property of metal oxides (MOs) is modified by incorporating metal nanoparticles (NPs) to realise speciality applications and SERS signal enhancement to a great extent.[21] It is highly desirable to develop 1D nanomaterials that have not only controlled shapes and crystal structure but also designed electrical and optical properties for applications as sensors, field-emitters, p-n diodes, and diluted magnetic semiconductors (DMS) for spintronics.[22–27] Metal-doped metal oxide nanowires have been adopted to improve the electrical conductivity.[28] Metallic coatings on fibres have also been studied. Nylon fibre mats coated with Ag using a commercial electroless plating solution exceed about 1800 S cm1 when loaded with approximately 17 wt % Ag.[29] The chemistry of MOs is modified by incorporating metal NPs for achieving specialty applications. This has become a new trend in materials research. Often, the MO is taken to the nanodimension at which the bulk property of the MO disappears completely and new properties emerge.[30] Due to the possible charge separation at the metal–semiconductor interface, doped materials may be applied in photocatalytic processes.[31–33] On this plea, metal–semiconductor composite nanostructures have been synthesised from various combinations, such as PbSe/Au, Ag or Pd,[34] CdSe/Au, CdS/Au,[35] CdSe/ Co,[36] InAs/Au,[37] and TiO2/Co.[38] Furthermore, the size of nanoparticles as well as the mode of deposition of metal nanoparticles plays a crucial role while tailoring the properties of semiconductor nanomaterials.
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Full Paper The most common method of producing Ag–MnO2 nanoparticle is cathodic electrodeposition[39] of Ag + species from aqueous KMnO4 solution containing AgNO3 or by a reassembling reaction between Ag + ions and delaminated manganese oxide nanosheets, followed by a reduction process.[40] It is thought that improvement of the electrical property of MnO2 is sure to be achieved if Ag (the best electrical conducting metal) NPs can be incorporated in the MnO2 composite. There are several novel methods for the synthesis of 1D MnO2 nanowires.[41–43] However, in most cases Ag NPs are deposited on the surface of the already synthesised MnO2 nanoparticles.[44] But we have employed a solution-phase synthetic procedure involving wet chemical redox reaction between MnII and AgI precursor salt. We chose Mn(CH3COO)2 and AgNO3 to obtain 1D Ag–MnO2 nanomaterials. Interestingly, it has been observed that during the course of the proposed reaction Ag NPs are incorporated in situ in the MnOOH matrix. Again the amount of Ag incorporation can be increased in the Ag–MnOOH composite just by increasing the amount of silver nitrate in the reaction medium, which is easily transformed into Ag–MnO2 nanomaterial. In this context, here we report the synthesis of 1D morphology Ag–MnO2 nanocomposites with varying amount of incorporated Ag by redox transformation reaction between aqueous AgI and MnII salt. This method gives rise to a new redoxmediated synthesis for developing metal nanoparticle loaded 1D semiconductor composites. Ag–MnO2 nanowires with 12 % Ag show metal-like electrical conductivity. Ag–MnO2 composite nanowires act as a better thermally stable SERS substrate compare to individual components due to conjugate electromagnetic enhancement and charge-transfer effect. Raman, thermogravimetry and XRD analyses established Ag concentrationdependent increased transition temperature of manganese oxides (MnO2, Mn2O3 and Mn3O4) in the composite, that is, thermal stability in comparison to the pure material.
Figure 1. A) Stepwise synthetic procedure of gram-level Ag–MnO2 nanowires on a water bath at 80 8C by mixing AgNO3 and Mn(CH3COO)2 followed by annealing at 350 8C. B) Conducting interlaced Ag–MnO2 sheet formation by simple vacuum filtration.
Results and Discussion We have chosen a facile redox reaction to deliver a composite nanowire as conducting as well as high SERS active substrate in gram level. Stepwise synthetic procedure for gram-level production of Ag–MnOOH on a water bath at 80 8C by mixing AgNO3 and Mn(CH3COO)2 is shown in Figure 1. Ag–MnO2 nanowire was obtained by annealing the product at 350 8C. The digital image in Figure 1 shows interlaced Ag–MnO2 nanosheets prepared by vacuum filtration. Details of XRD (Figures S1 and S2), XPS (Figure S3), FTIR (Figure S4), DRS (Figure S5) and optical property (Figure S6) are provided in the Supporting Information. Transmission electron microscopy (TEM), high-resolution TEM (HRTEM) and energy-dispersive X-ray spectroscopy (EDX) analysis
Figure 2. A), B) TEM image of Ag–MnO2 nanowires with 4 and 12 % Ag, respectively. Inset B: corresponding SAED pattern. C), D) Fringe spacing.
The TEM image of wet, chemically synthesised nanowires is presented in Figure 1 B and Figure 2 A. From the critical fringe spacing analysis we can identify the interplanar spacing of both the MnO2 as well as Ag in the composite materials. The
fringes (Figure 2 D) with approximately 0.23 nm interplanar separation parallel to the rod axis correspond to the (111) planes of Ag, which indicates that the growth direction of the Ag is along (111) within the nanocomposite. The interplanar
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Full Paper distance of the fringes is 0.315 nm, corresponding to the (110) planes of tetragonal b-MnO2 in the composite materials. Figure 2 B (inset) represents the SAED pattern of the Ag–MnO2 composite nanowires. We observed the lattice spacing due to Ag and MnO2 in the composite until 12 % of Ag in the nanocomposite was reached. After that, Ag (> 12 %) comes out and becomes deposited outside the composite matrix surface (Figure S7 in the Supporting Information). There, the lattice spacing of the segregated particle matches well with Ag and confirms surface decoration by Ag. The composition of the synthesised product has also been observed from EDX analysis. The EDX patterns of the Ag–MnO2 nanowires are presented in Figure S8 in the Supporting Information and Figure 3. The EDX analysis confirms the presence
ter of the nanowires becomes wider (Figure S9 in the Supporting Information).
Growth mechanism The growth demonstrates the formation of Ag–MnOOH nanowires from aqueous solution on water bath at 80 8C. The growth of Ag–MnOOH nanowires occurs via a series of intermediates, which are characterised by FESEM, EDX, TEM and HRTEM analysis. The brown solid precipitated out slowly, which was collected at different time intervals (12 to 42 h). The time-dependent growth of Ag–MnOOH nanowires was shown by TEM images (Figure 4). Finally Ag–MnO2 is obtained after annealing at 350 8C.
Figure 4. A)–E) Growth of Ag–MnOOH composite nanowires with 8 % Ag, after 5 min, 10 min, 48 h and 4 days of heating on a water bath. F) Ag–MnO2 nanowires obtained after annealing at 350 8C. Scale bars: A, 200 nm; B, 0.2 mm; C, 50 nm; D, 50 nm; E, 100 nm; F, 50 nm.
Figure 3. A) FFESEM image shows the uniformity of the synthesised Ag– MnO2 nanowires. B) Elemental mapping shows the 1D signature for each element within the composite.
of Mn, O, and Ag in all the synthesised products. We can vary the amount of Ag concentration in the composite nanowire easily. The amount of silver in different nanowires of a particular lot is very consistent. We observed a homogeneous distribution of Ag, in the product and no sign of a lateral or longitudinal concentration gradient in individual nanowires was observed. This is due to the Ag–MnO2 nanowire formation from AgI and MnII ions through solution-phase redox transformation. The nanowires are 10 mm in length and 50–500 nm in diameter. We observed that by increasing Ag proportion the diameChem. Eur. J. 2014, 20, 9111 – 9119
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Generally, controlled growth of seed particles is altogether an art of manipulating spheres into rods or faceting wires out of metallic NPs. We presume the same strategy holds good here also for composite nanowire formation. When we introduced AgNO3 solution into a solution containing Mn(CH3COO)2, formation of a bluish-brown solution was observed. This is due to Mn + + /Ag + redox-mediated silver-seed formation. The Ag seeds serve as nucleation sites for MnOOH nanowire growth under the experimental condition. During slow heat treatment the Ag and MnOOH nanomaterials coalesce to form nanowires through the shape-transformation mechanism. Why 1D growth occurs is yet to be understood fully. In the successive step of synthesis, the Ag and MnOOH nanomaterials come closer for 1D alignment through oriented attachment producing nanowire structures. Murphy et al. observed that the average aspect ratio of the Au nanorod increased with decreasing amount of seed as the spheroidal particles gradually grew into rod-like structures.[45, 46] From the FESEM images we observed that Ag–MnOOH nanowires with a higher percentage of Ag, grow nanowires with larger diameters leading to a decrease the aspect ratios compared to the Ag–MnOOH nanowires having a lower percentage of Ag. The reasons why the wire morphology is formed with different diameters is not yet
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Full Paper completely understood. Presumably, higher amounts of silver nitrate produce larger amounts of silver seed leading to decreased aspect ratio through faster growth as the spheroidal particles gradually grew into rod. However, aspect ratio of MnOOH nanostructure is dopant dependent (here Ag) but 1D growth is surely habitual for MnOOH. With the progress of the reaction the nanospheres coalesce and the diameter of the nanowires increases with time. Finally, the as-obtained product was annealed at 350 8C to obtain Ag–MnO2 composite. We observed no noticeable detachment of the Ag particle until 12 % Ag was reached (Figure 2). Then progressively, aspect ratio of nanowire decreases (Figure 2 and Figure S9 in the Supporting Information). With further increase in Ag content (> 12 %), the synthetic protocol gives rise to the formation of a mixture of nanoparticles and nanowires. For higher amounts of Ag (~ 15 %) in the composite we observed lattice spacing at 0.23 (for 111 plane of Ag) and 0.305 nm (for 110 plane of MnO2) from the same HRTEM image due to formation of segregated Ag nanoparticles on the surface of the composite nanowire (Figure S7 in the Supporting Information). So, noticeable detachment[47] of Ag nanoparticles from nanowires was observed at much higher Ag concentration but not below 12 %. FTIR spectra indicate that the prepared Ag–MnO2 shows a clean surface without any absorbed organic or hydroxyl species. The growth mechanism is also investigated by TEM (Figure 4) analysis in an ex situ fashion and is shown in Figure 4. At the initial stage (at 12 h), Ag nanospheres are formed and they coalesce and gradually change to nanowire morphology. The overall crystallisation process is shown in Figure 4. Enhanced thermal stability of Ag–MnOOH/Ag–MnO2 composite nanowires: Raman study Normal Raman scattering (NRS) studies help to understand the stability of the composite in which Ag is incorporated in MnOOH in a straight forward manner. For all the Ag–MnOOH nanocomposites we observed a broad peak at approximately 600 cm1. As for Ag–MnOOH nanocomposites with lower percentages of Ag (4 and 8 %) small peaks at approximately 360, 388, 558, 622 cm1 were observed at low laser power in our spectral window corresponding to the stretching modes of MnO6 octahedra for pure MnOOH.[48] A small peak at 534 cm1 (at lower laser power for 8 % Ag in the composite) is observed for Ag–MnOOH nanocomposites due to the formation of bMnO2.[48] The band at 648 cm1 is assigned to Mn3O4 formation during the acquisition of the spectrum at high laser power because of the local heating of the samples by laser irradiation, which is consistent with reported Mn3O4 samples (650– 660 cm1). Just like the band at 648 cm1, weak peaks at approximately 308, 362, 420 and 648 cm1 are observed in Ag– MnOOH nanowires; these originate from the formation of Mn2O3 or Mn3O4. The transformation from MnOOH to the possible Mn2O3[48] or Mn3O4 takes place by the incident laser heating.[49] In our case, Raman study qualifies as a sensitive tool to conclusively prove the stability of the as-synthesised compoChem. Eur. J. 2014, 20, 9111 – 9119
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Figure 5. Effect of laser heating during Raman analysis with variable laser power (lp) and acquisition time (aq) on the Ag–MnOOH composite nanowires. The results suggest enhanced thermal stability of the composites with increasing amount of Ag.
site with higher dosage of Ag incorporation. It requires successively higher laser power as well as longer laser exposure time to obtain Mn2O3 or Mn3O4 for Ag incorporated MnOOH samples (Figure 5). The transformation from MnOOH to Mn2O3 or Mn3O4 takes place during the acquisition of Raman spectra but
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Full Paper with higher laser power presumably via b-MnO2. This laser heating study speaks for Ag-concentration-dependent stability of the Ag–MnOOH/Ag–MnO2 composite nanowires, which is supported by thermal as well as temperature-dependent XRD analysis.
Thermal gravimetric analysis (TGA) Thermal stability of manganese oxides (MnO2, Mn2O3 and Mn3O4) in the composite materials with various amounts of Ag NP doping (4, 8 and 12 %) is ascertained from careful, critical and programmed heat-treatment experiments. Thermal stability is reported for such Ag–MnO2 composite materials for the first time. The nature of TGA and differential thermal analysis (DTA) thermograms of Ag–MnO2 samples (12 % Ag, sample A; 8 % Ag, sample B; 4 % Ag, sample C) together with TGA and DTA curves of Ag–MnO2 are presented for comparison in Figure 6. In the TGA thermogram all the Ag–MnO2 samples
Figure 6. Ag concentration-dependent TGA and DTG curves in the same window show the gradual shift of manganese oxide transition temperature towards a higher value in the Ag–MnO2 composite.
show distinct weight losses below 450 8C, which corresponds to the removal of water molecules, organic reactants, and trace amount of lattice oxygen. Sample C (with lowest Ag (4 %), Figure 6) shows weight loss at around 627 8C that corresponds to the loss of oxygen from the MnO2 lattice resulting in the transformation to Mn2O3. Another weight loss at 714 8C corresponds to further loss of oxygen resulting in the transformation from Mn2O3 to Mn3O4 (Figure 6).[50] Sample A with the highest amount of silver (12 %) shows weight loss at around 756 8C for loss of oxygen from the MnO2 lattice resulting in the transformation of MnO2 to Mn2O3 (Figure 6) and then remains stable as indicated by the horizontal plateau. In case of sample B with the intermediate amount of Ag (8 %), weight loss at around 716 8C corresponds to the loss of oxygen from the MnO2 lattice resulting in the transformation to Mn2O3 (Figure 6). The MnO2 to Mn2O3 transformation temperature has been observed to be shifted towards a higher temperature range due to the incorporation of progressively higher amount of Ag NPs in the MnO2 matrix. Still higher amounts of silver Chem. Eur. J. 2014, 20, 9111 – 9119
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doping resist oxygen loss to a noticeable extent. Thus, sample A with the highest amount of Ag (12 %), remains as Mn2O3 even at 850 8C. It can be recalled that pure MnO2 goes to Mn2O3 just at 550 8C (at a much lower temperature than reported for the as-prepared Ag–MnO2 composites) and then to Mn3O4 at 750 8C. Thermal stability of the different Mn-oxides in the composites is authenticated, which is definitely administrated by the amount of Ag NP doping. So, silver nanoparticles in the manganese oxide matrix assist nanowire growth, resist the usual transformation of MnO2 to a great extent and make the composite a stable platform for further studies.
Thermal stability of Mn oxide–Ag composites from XRD analysis To understand the thermal stability of the synthesised composite materials we carried out the XRD analysis in the ex situ fashion. Each time a fresh batch of as-prepared sample with a different amount of sliver (4, 8 and 12 %) was separately annealed at different temperature and then XRD analysis was performed. A representative XRD result, from which we clearly observed that the Mn oxide sample with 4 % Ag is quantitatively converted to Ag–Mn3O4 for the 1000 8C annealed sample, is shown in Figure 7. However, the 1000 8C annealed samples with 8 and 12 % Ag distinctly show peaks for Mn2O3 + Mn3O4 and Mn2O3, respectively. Again, the sample annealed at 850 8C (Figure S10 in the Supporting Information) reveals the exclusive presence of Mn2O3 with 12 % Ag. It may be recalled that annealing pure MnO2 sample at 750 8C exclusively produces Mn3O4. So, it is conclusively spelt out that in terms of thermal stability Ag stimulates manganese oxides enhancing the transition temperature for Mn oxides and that is related to the Ag concentration. It seems that the presence of Ag (high density and boiling point) helps to retain the tunnel structure[51] of b-MnO2 even at high temperature.
SERS study The Ag surface in the as-synthesised composite is modified greatly because of the presence of MnOOH/MnO2. Then the acquired stability of the modified surface readily helps the chemisorption process. Though it is well known that Ag interacts with N-containing ligand and Au with S-containing ligand according to HSAB principle, we observed weak SERS signal enhancement of 1,10-phenanthroline (Figure S11 in the Supporting Information) and moderate SERS signal enhancement taking p-amino thiolphenol as a probe molecules on Ag– MnOOH composite material (Figure S11). Whereas very high SERS signal enhancement occurs from thiophenol as probe on the Ag–MnOOH composite material (Figure 8 and Figures S11– S14 in the Supporting Information). We have used Ag–MnOOH composite material as a SERS substrate, not pure Ag. Moreover, the SH stretching and bending mode of benzenethiol[52, 53] at 2600 and 918 cm1, respectively, were not observed in the measured SERS spectra (Figure S12 in the Supporting Information). This further confirms that the probe molecules are chem-
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Full Paper ry in order to obtain the maximum SERS enhancement. For example, 500 nm Ag nanoshells show a much sharper SPR at approximately 400 nm.[55] In the case of the Ag–MnO2 nanowires, however, the very broad SPR allows us to obtain maximum SERS intensity in a much broader laser energy range. Moreover, we have used 633 nm laser for SERS measurement. Again we observed that the absorbance of MnO2 gradually increases after approximately 600 nm by increasing the amount of Ag into the MnO2 matrix. Taking TP as the probe molecule we explored the SERS activity involving Ag–MnOOH compoFigure 7. XRD pattern of annealed Ag–MnO2 composite nanowires with various amounts of Ag at 1000 8C. The site 8 % Ag. The diameter of composition with the highest amount of Ag (12 %) remained stable as Mn2O3 even after annealing at 1000 8C Ag–MnOOH nanowire increases whereas the composition having the lowest amount of Ag (4 %) converted to Mn3O4, but the composition with progressively with increasing moderate Ag (8 %) evolved a mixture of Mn2O3 and Mn3O4. For pure Mn2O3 the oxide transition temperature amount of Ag in the composite. (Mn2O3 to Mn3O4) is only 750 8C. Again the increased amount of silver in the 1D structure lowers the aspect ratio of the nanowires. Thus, there arises a possibility of the reduction of hot spots after drop casting due to lower probability of interlacing of nanowires in a small area. So, we found that 8 % Ag-containing composites are comparatively better SERS substrates than 12 % Ag-containing composites. The SERS spectra of TP, adsorbed onto the Ag–MnOOH composite nanowire at various concentrations, are shown in Figure 8. An excitation wavelength of 632.8 nm laser is used for SERS studies. By comparing the SERS spectra (Figure 8 and Figure S13 in the Supporting Information) and the normal Raman spectrum of TP (Figure 8), we observed the obvious changes in terms of signal intensity and shift of several SERS bands of TP. The peak at 1092 cm1 due to CS stretching mode moved to 1073 cm1 in the SERS spectrum and the intensity is also increased. Similarly, another peak at 1583 cm1, Figure 8. NRS spectrum of 0.1 m TP in aqueous solution and SERS spectra of contribution from CC stretching, moves to 1575 cm1. The TP adsorbed on Ag–MnOOH composite nanowires at various concentrations of the adsorbate for lexc = 632.8 nm. peak at 420 cm1 is due to the contribution from the CS stretching vibration (nCS). The SH bending mode in the Raman spectrum disappeared in the SERS spectra of TP. All of these alterations can be interpreted to imply that TP remains isorbed. Hence, we obtained huge SERS signal enhancement on Ag–MnOOH composite through chemisorption, resulting in taking TP as probe molecule. changes in its structure.[56] In order to compliment the composite effect for the observed SERS signal enhancement, we conducted diffuse reflecWe calculated the enhancement factor (EF) by comparing tance spectroscopy (DRS) with MnO2 and plotted the results in the intensity of all the high intensity peaks in the SERS spectrum with those peaks obtained in the normal Raman specsame window with the Ag–MnO2 composite with different trum by using a technique similar to that reported previousamounts of Ag. An interesting feature of the Ag–MnO2 nanoly.[57] The lowest detection limit for TP was observed to be as wires is their broad surface plasmon resonance (SPR), as shown in Figure S5 in the Supporting Information. Generally sharp low as 1010 m. The metal–semiconductor hybrid nanomaterial [54] SPRs have been reported in SERS-active materials, in such as a uniform SERS substrate shows huge SERS signal enhancement in comparison to the individual components (Figure 9 situations careful tuning of laser wavelength becomes necessaChem. Eur. J. 2014, 20, 9111 – 9119
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Figure 10. Comparative SERS spectra of TP (108 m) adsorbed on MnOOH, Creighton sol, 55 nm Au nanoparticle and Ag–MnOOH composite nanowires for lexc = 632.8 nm.
The situation became reversed (Figure 10) when we carried out SERS after drop casting. The observed enhanced SERS intensity may be due to the presence of hotspots created due to the drop casted solution. The as-synthesised composite affects the Ag surface because of the presence of MnOOH. The modified surface helps better thiophenol chemisorption. We estimated the apparent enhancement factors (AEFs) of some selected Raman bands using the relation we reported previously:[7]
AEF ¼ s SERS ½C NRS =sNRS ½C SERS
Figure 9. Area mapping (SERS, considering the selected bands) on Ag– MnOOH composite taking TP (105 m) as a probe molecule.
and Figure 10). We compared the SERS spectra of thiophenol obtained from the Ag–MnOOH composite nanowire with 55 nm Au nanoparticle[58] as well as Creighton sol[59] and pure MnOOH (Figure 10). We observed huge SERS enhancement from the Ag–MnOOH composite nanowire having 8 % Ag with 108 m thiophenol as probe molecule (Figures 8 and 10). We observed comparable SERS enhancement from both the 55 nm Au nanoparticle and Ag–MnOOH composite nanowire having 8 % Ag with 108 m thiophenol as probe molecule (Figure 10). Comparative SERS spectra of thiophenol on Ag–MnOOH composite nanowire and Au nanoparticle may be due to the strong AuS interaction, that is, “soft–soft” interaction. Thus, the importance of Ag–MnOOH composite nanowire has been validated from the comparative SERS account. Nanowires, in dispersion in the present case, have a natural tendency to precipitate out from the solution. Hence, the SERS signal lowers in solution phase while Ag–MnOOH nanowire dispersion is taken as substrate. A comparative SERS of Ag– MnOOH nanowire dispersion and Creighton sol[59] indicate the superiority of the latter. The result shows that stable Creighton sol gives higher SERS intensity (Figure S14 in the Supporting Information) in solution phase in comparison to nanowires dispersed in water due to precipitation of Ag–MnOOH nanowires. Chem. Eur. J. 2014, 20, 9111 – 9119
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ð1Þ
where C and s are the concentration and the peak area, respectively, of the Raman bands measured from baseline. The AEF values of the selected enhanced Raman bands of thiophenol[60] at various concentrations of the adsorbate are shown in Table 1.
Electrical property Metal–semiconductor contact is an obvious component of any semiconductor device. In extrinsic (doped) semiconductors, dopant atoms increase the majority charge carrier concentration by donating electrons to the conduction band or accepting holes in the valence band. However, such contacts cannot be assumed to have a resistance as low as that of two connected metals. In particular, a large mismatch between the Fermi energy of the metal and semiconductor can result in a high-resistance rectifying contact. A proper choice of materials can provide a low resistance Ohmic contact. For both types of donor or acceptor atoms, increasing the dopant density leads to a reduction in the resistance, hence highly doped semiconductors behave metallically. Keeping this idea in mind we chose the highly conducting Ag as a dopant. We plotted the I–V characteristics for Ag–MnO2 composites with variable amount of Ag (Figure 11). The results
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Full Paper Table 1. Assignment of vibrational modes for thiophenol Raman peaks. AEF 104 SERS (105 m) [cm]
AEF 105 SERS (106 m) [cm]
AEF 106 SERS (107 m) [cm]
AEF 107 SERS (1010 m) [cm]
AEF 1010
420
0.87
3.3
420
1.52
420
0.49
420
0.29
1001 1024 1074
0.22 0.39 0.79
1000 1023 1073
0.95 1.78 3.28
1001 1023.5 1074
0.41 0.78 1.61
1001 1024 1076
0.19 0.33 0.54
1000 1024 1074
0.54 0.10 0.21
1576
0.98
1575
3.93
1575
2.23
1577
1
1575
0.23
NRS, 0.1 m Symmetry Assignment SERS (104 m) [cm1] [cm1] 418 (w) 616 (ms) 698 1000 (vvw) 1025 1072 1157 1578
(a1, a2) b1 a1 a1 a1 b1 b1 a1 and b1
bCCC + nCS bCCC + nCS bCCC + nCS bCCC bCH bCH bCH nCC
419.5
Conclusion The studies reported are gifted with low cost industry scale-up preparation strategy for Ag–MnOOH and Ag–MnO2 nanowire composites. It has been described that even after redox transformation between AgI and MnII salts at low temperature the habitual 1D growth of MnOOH is not disturbed with incorporated Ag NPs. Silver incorporation successively enhances the stability of the composite as evidenced from the progressively increased oxide transition temperature. Again, optical response (red shift) and XRD analysis portray the incorporation of Ag into the annealed MnO2 matrix. Interestingly nanowire morphology of manganese oxide is uniquely retained after annealing, but with changes in band-gap energy and drastic electrical conductivity. Finally, the synthesised composite nanomaterial shows high SERS sensitivity with an enhancement factor of approximately 1010 taking thiophenol as a probe molecule, which makes it a stable deliverable SERS substrate. Figure 11. The current–voltage characteristics of synthesised Ag–MnOOH nanocomposites.
Experimental Section show linear dependence of current with voltage. For I–V characteristic we used linear four-probe method on top of the pellet. We applied current on the outer electrodes and measured voltage in the inner electrodes. From this we measured resistivity of the samples and from there we calculated the conductivity. Here, the measured resistivity of the obtained Ag–MnO2 nanowires are 0.875 107, 4.4 104 and 2.9 103 Wm for 12, 8, 4 %, and Ag, respectively, by using standard four-probe method. Resistivity for all the composite materials is much lower than that of pure MnO2 (~ 0.02 Wm). Composite nanowires with only 4 % Ag show metal-like electrical conductivity (Figure S15 in the Supporting Information). These results show that the Ag-loaded manganese oxide can significantly improve the electrical performance, that is, the increase in conductivity like the doped materials as stated in the preceding section. It may be recalled that Ag not only increases the conductivity but also improved the thermal stability to a great extent. Finally, the nanowire composites have been proved to be a good deliverable as a stable SERS substrate.
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Synthesis of rod-shaped Ag–MnOOH/Ag–MnO2 First, aqueous solutions of three different reaction mixtures of variable compositions were prepared separately in three different round-bottom flasks. Various aliquots (50, 25, 12 mL) of 0.1 m silver nitrate (AgNO3, 5 102 m) were introduced separately drop by drop into the set containing 500 mL aqueous manganese(II) acetate (Mn(CH3COO)2·4 H2O, 5 102 m) solution under magnetic stirring at room temperature in about 15 min. The colourless reaction mixture turned initially to bluish black and finally to light brown, which indicated the onset of the evolution of Ag–MnOOH nanoparticles. Then the brown solutions were kept for 98 h on a water bath for the evolution of uniform Ag–MnOOH nanowires. Finally, Ag–MnO2 composite materials were obtained by annealing the asobtained Ag–MnOOH at 350 8C.
Procedure for SERS measurement Fresh stock solutions of thiophenol were prepared regularly in ethanol with variable concentrations (103–1010 mol dm3). To test the performance of our nanostructured SERS substrates, a dilute dispersion of Ag–MnOOH (100 mmol L1 in ethanol) and 1–50 nm Ag nanosphere (0.25 103 m in water) were separately incubated for 4 h. All SERS spectra were recorded dispensing 30 mL of the incubated solution on aluminium foil.
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