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Facile Shape-Controlled Fabrication of Copper Nanostructures on Borophosphate Glasses: Synthesis, Characterization, and Their Highly Sensitive Surface-Enhanced Raman Scattering (SERS) Properties Anderson J. Pereira,† Joaquim P. Gomes,† Guilherme F. Lenz,‡ Ricardo Schneider,*,‡ Juliano Alexandre Chaker,§ Paulo Eduardo Narciso de Souza,∥ and Jorlandio Francisco Felix*,†,∥ †

Department of Physics, Universidade Federal de Viçosa-UFV, Viçosa, MG 36570-000, Brazil Departament of Chemistry, Universidade Tecnológica Federal do Paraná, Toledo, PR 85902-490, Brazil § Institute of Chemistry, Universidade de Brasília, Brasília, DF 70904-970, Brazil ∥ Institute of Physics, Universidade de Brasília, Brasília, DF 70910-900, Brazil ‡

S Supporting Information *

ABSTRACT: We demonstrate air-stable copper-doped nanostructured borophosphate samples, which were prepared by a facile, low cost, and green synthesis method. The thermal annealing, in a reducing hydrogen atmosphere, enables the formation of metallic copper nanostructures, which was confirmed by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and optical absorption. The optical spectra show a main intense surface plasmon resonance (SPR) band centered at 579 nm. The shapes of the nanostructures, morphology, and thickness of the copper nanostructures coating are chosen to be suitable for SERS applications. These samples exhibited very high SERS enhancement factors (EF), depending on thermal annealing time, with excellent reproducibility. The estimated EFs have been found in the range between 107 and 108.

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INTRODUCTION

Additionally, Cu-based substrates are very favorable because of improved physical stability compared to Ag-based systems.5 Besides the catalytic applications, copper nanoparticles (CuNPs) have also been used as SERS-active substrates. Recently, Hamad et al.7 have used ultrafast laser ablation of copper to obtain Cu nanostructured with SERS activity. Kowalska et al.8 fabricated copper nanocrystals by means of a high pressure method, in which their SERS properties were investigated. Mao et al.9 have reported a synthetic method for monodisperse CuNPs. They have also demonstrated that the SERS enhancement factor of CuNPs is as high as 103.9 In general, colloidal solutions of metal nanoparticles, thin films formed from isolated metal islands of different shape and size, and rough metal substrates are commonly used in SERS studies. These kinds of systems have shown surprisingly large enhancement of Raman signal, capable of producing values over 108 (level of enhancement high enough for detecting single molecules). It is important to emphasize that in the literature one can find only few works dedicated to the study regarding

Many studies have been dedicated to the borophosphate-based glasses due to their unique features which enable applications from medicine to photonics materials. These classes of glasses are transparent in the UV−mid-IR region of the electromagnetic spectrum and have low dispersion, low refractive indices, and improved chemical stability compared to pure phosphate or borate glasses.1,2 Additionally, phosphate glasses are good alternatives to silicate glasses in biomedical applications because of their biocompatibility with the living body. Because of special characteristics, they are an important group of glass materials with several applications in medicine such as in bone repair and reconstruction.3 In our previous study, we have investigated the structure and SERS effect of silver-containing germanophosphate glasses and found that SERS activity of the as-produced glass−silver@core−shell fibers can be easily controlled by setting both the annealing temperature and time.4 On the other hand, it has been shown that copper nanoparticles provided an alternative to noble metals (silver, gold, and platinum nanoparticles) because of their low toxicity,5 good optical characteristics, and conducting properties.6 © XXXX American Chemical Society

Received: March 21, 2016 Revised: May 14, 2016

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DOI: 10.1021/acs.jpcc.6b02881 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. Scheme (a) shows the unannealed glass foil incorporating the copper ions, (b) copper ions migrating toward the surface of the glass foil and being reduced under the hydrogen atmosphere to form metallic copper nanosized structures, and (c) SERS effect for rhodamine B observed over copper nanofilms obtained by the bottom-up process.

SERS activity obtained for copper platforms.8 Moreover, Cubased platform fabrication is commonly obtained by thermal evaporation using ultrahigh vacuum (UHV) apparatus, hydrothermal method, lithography, electrochemical deposition, and reverse micelle synthesis.10−12 Nevertheless, these methods are expensive and time-consuming and employed harsh reducing and organic solvent.13 This study explores borophosphate melts to form an active copper-doped glass substrate for supported nanostructures growth using a bottom-up process. Additionally, single-step synthesis and immobilization of nanoparticles are desired characteristics in some materials because they do not result in additional costs and surface contamination.14 Therefore, the developments of active materials that enable the single-step synthesis are very attractive for production on an industrial scale.4,15 Additionally, we show that this innovative simple method can control the dimensions of the CuNPs as a function of the annealing time. The sample characterization was performed by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and optical absorption (OA). Additionally, the presence of copper in the form of Cu2+ species has been studied and confirmed from electron paramagnetic resonance (EPR).

The crystallographic structures of the CuNPs borophosphate glasses were assessed by X-ray powder diffraction (XRD) measurements. XRD patterns were recorded using the Bruker D8 Discover diffractometer equipped with the Cu Kα radiation (λ = 1.5418 Å). The diffraction patterns were obtained at angles between 25° and 80° (θ − 2θ). The CuNPs morphology was investigated by field-emission scanning electron microscopy (FEG-SEM). The EPR spectra were recorded with a Bruker EMX Plus spectrometer in the X-band (9.45 GHz) at room temperature. The EPR parameters used were microwave power at 50 mW, field modulating frequency at 100 kHz, and field modulation of 0.2 mT. Raman spectra have been recorded at room temperature using a Renishaw InVia micro-Raman System with 514.5 nm excitation source, and spectra were collected through a CCD camera. In order to increase the exposed surface sufficiently to obtain adequate sensitivity, because of hygroscopic nature of glass, the glasses were crushed in an agate mortar and passed through a No. 325 sieve. A specially designed and constructed measuring chamber was applied to control the relative humidity (RH %) of the atmosphere. A known quantity, approximately 0.2 g, of this powder was then exposed in a shallow weighing bottle in controlled humidity conditions, and the increase in weight was obtained. The mass was measured in predetermined times for 10 h. All determinations were carried out in triplicate for each sample. SERS Sample Preparation Details. Measurements with the 514.5 nm laser were made at 0.03 mW, whereas for the 632.8 and 785 nm laser they were performed at 0.04 and 0.04 mW, respectively. The light from the laser was guided through a line filter and focused with a 50× microscope objective (resulting in a laser spot of around 1 and 0.8 μm in diameter in the focal plane, for the 514.5 and 632.8 nm laser, respectively) on a sample mounted on an X-Y-Z translation stage. Several measurements were made to reach an average for each sample. The Raman spectra presented in this work were obtained after subtracting the background baseline using cubic spline interpolation and were normalized to the acquisition time and power. The rhodamine B (RB), chosen as a standard, was

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EXPERIMENTAL SECTION Syntheses. The glasses were fabricated with a mixture of NaH2PO4−H3BO3−Al2O3 raw reagents with NaH2PO4 and H3BO3 ratio set at (in mol) 2, 3%, or 10% of Al2O3 and doped with 3.0%, 1.5%, or 0.5% of Cu2O using high purity reagents (Aldrich Co.). A typical synthesis run used 2 g of these compounds homogenized in an agate mortar. The mixture was transferred to a covered Pt/Au crucible and fused for 1 h in a preheated resistive oven at 1050 °C. The glass samples were obtained by quenching the molten mixture on a graphite mold at room temperature. The Cu0 nanofilms were obtained by thermal post-treatment of the glass template at 400 °C at constant flow (100 mL/min) of high purity hydrogen gas for 0, 15, 30, 45, and 60 min. Sample Characterization. The absorption spectra of the samples were recorded using a T80+ spectrometer from PG instruments with 0.1 nm of step and air as baseline. B


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Figure 2. SEM images of copper doped glasses annealed at 400 °C for (a) 0 (unannealed), (b) 5, (c) 10, (d) 15, (e) 20, and (f) 30 min. The inset figures shows a high magnification of the SEM image from which the grown copper nanoparticles can be seen (scale bar = 500 nm).

unchanged (≈0%) in 10% of the Al2O3 sample for the same time. The surge in Al3+ ions improved the glass chemical stability attributed to the increased cross-linking of phosphate chains.17 Thus, due to the small water absorption (better chemical stability), the 10% Al2O3-doped glass was selected for CuNPs growth. Optical and Structural Analysis. Optical absorption of copper borophosphate glasses with 3% of Cu+ ions at different annealing times is shown in Figure 3. In this way, the copper-

purchased from Sigma-Aldrich. The CuNPs borophosphate glasses doped with 1.5% Cu2O were coated with ethanolic solution (volume = 5 μL) containing 10−9 M RB and then dried.

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RESULTS AND DISCUSSION Scanning Electron Microscopy Analysis. The samples were grown using the same procedure as reported in ref 4 but with a different glass system. Briefly, the preparation process of the borophosphate glasses SERS substrates is schematically shown in Figure 1. The process of growing the copper nanoparticles onto the surface of the as produced substrates is based on the bottom-up approach. Figure 2a shows a SEM image of the as-made glass containing the Cu2+ ions without thermal treatment. This Cu2+ ions will be further investigated using EPR experiments as described below. The Cu0 nanofilms were obtained by thermal post-treatment of the glass template at 400 °C at constant flow of hydrogen during different times (see Figure 2). In a glass substrate, the annealing at 400 °C enables the Cu2+ ions to migrate toward the surface of the glass where the Cu0-based nanostructure is self-assembled via a bottom-up process due to the employed hydrogen reducing atmosphere. Figure 2 shows the field emission scanning electron microscopic images of copper-doped glass system annealed during (a) 0 (unannealed), (b) 5, (c) 10, (d) 15, (e) 20, and (f) 30 min, and insets present the images with a highmagnification image (500 nm scale bar). Noticeable size difference in both CuNPs and distance between CuNPs can be observed, and shorter annealing time results in smaller CuNPs sizes. Copper nanoparticles have not been observed by SEM for reference sample (unannealed) (Figure 2a). For large areas, it can be seen that the samples show good uniformity (see Figure S1 in Supporting Information), where a quasi-continuous Cu0 thin film can be obtained for a long annealing time. Phosphate-based glasses are humidity sensitive. However, the additions of trivalent ions in these glasses make them more chemically stable.16,17 An increase in the Al2O3 content from 3% up to 10% resulted in a dramatic reduction of water absorption. In our case, for undoped and doped glasses, after 10 h of water exposition, a mass increase of ≈20% in the 3% Al2O3 sample was observed. On the other hand, the mass remains

Figure 3. UV−vis spectra for 10% Al2O3 Cu-doped glass (a) 15 min at 400 °C under H2 flow, (b) 5 min at 400 °C under H2 flow, (c) unannealed Cu doped glass, (d) undoped sample after 60 min at 400 °C, and (e) undoped and unannealed sample. Inset: 15 min at 400 °C; the spectra were collected every 15 min up to 105 min under laboratory air.

doped glass was prepared with Cu+ ions. One observes a broad absorption band centered at 838.2 nm, which is related to the superposition of peaks 2Eg → 2B1g, 2B1g → 2B2g, and 2A1g → 2 B1g in distorted octahedral coordination for Cu2+ ion.18,19 The thermal annealing, in reducing hydrogen atmosphere, enables the formation of metallic copper. Their optical spectra show an intense surface plasmon resonance (SPR) peak C


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The Journal of Physical Chemistry C centered at 579 nm (Figure 3a) which is absent in the untreated doped sample spectrum (Figure 3c). Additionally, we can observe an increase in the SPR peak with the annealing time from 5 min (Figure 3b) to 15 min (Figure 3b). The thermal annealing has no effect on the spectra profile for the doped samples (Figure 3e,f). The SPR of CuNPs obtained in solution is reported between 550 and 615 nm.20,21 Immobilized CuNPs in glass matrix shows a SPR peak of CuNP at ≈560 nm.22 The inset of Figure 3 shows the SPR band evolution of the doped glass annealing for 15 min at 400 °C as a function of increasing time when the CuNPs are exposed to air. This inset shows a red-shift (from 576.5 to 580.2 nm) and an increase in absorption intensity of SPR band, which has been related to the formation of copper oxide shell.23 Figure 4b shows the X-ray powder pattern of the

Figure 5. Raman analysis for 10% Al2O3 Cu-doped glass (foil) (a) glass without thermal annealing and (b) 60 min (400 °C) under H2(g). Inset: zoom for sample (b). Vertical lines indicate the unchanged positions.

we can observe the characteristic bands of glass substrate and −1 two peaks at 150 cm−1 (Γ(1) (2Γ−12) 15 (LO)) and 218 cm assigned to Cu2O formed by air exposition of the CuNPs, creating a very thin native oxide layer. The phonon mode at 150 cm−1 is Raman inactive, and the peak at 218 cm−1 is a second-order overtone of inactive phonon mode Γ−12 (at 109 cm−1) not observed.28 The fraction Cu2O phase is so small that it can not detected by XRD (Figure 4), but it is detectable by Raman spectroscopy, which is a powerful technique that can be used to obtain chemical information on the surfaces.29 Electron Paramagnetic Resonance (EPR) Studies. The addition of copper ions to a borophosphate glass matrix has been investigated as a function of copper concentration by EPR spectroscopy. Probably, in these glasses matrix, copper ions exist in two stable ionic states (Cu+ and Cu2+); however, only Cu 2+ can be detected by EPR spectroscopy at room temperature. The ESR spectra of Cu2+ ions in glasses (Figure 6) are analyzed using a spin-Hamiltonian as follows:

Figure 4. Powder XRD analysis for 10% Al2O3 Cu-doped glass annealed at (a) 60 min at 400 °C under H2 (b) doped and unannealed sample. The vertical lines indicate the XRD pattern for Cu0 (blue lines). JCPDS(Cu0): 04-0836.

copper borophosphate glasses matrix after thermal annealing under hydrogen atmosphere. The presence of the metallic copper film is confirmed by the three characteristics XRD peaks associated with crystalline planes (111) (43.3°), (200) (50.4°), and (220) (74.1°). The peaks are indexed on the basis of JCPDS File 04-0836. In the unannealed sample (Figure 4a) only a characteristic broad band of glassy materials is observed.24 Raman Spectroscopy Investigations. Figure 5a shows the Raman spectra of Cu-doped glass without annealing. The band at 353 cm−1 can be assigned to in chain O−P−O bending motions.25 The broad band at ≈560 cm−1 is assigned to bending mode related to the cation motion and chain conformation.25 The addition of boron to Li2O−P2O5 glass split the P−O−P stretching band around 700 cm−1, so the peaks are attributed to P−O−B stretching of groups containing boron and phosphorus.26,27 For the sodium borophosphate glass, studied here, the band at 636 cm−1 is assigned to P−O− B, the band at 683 cm−1 is assigned to νs(P−O−P), and the shoulder at ≈741 cm−1 is ascribed to P−O−P bridges.27 The broad Raman band between 850 and 1250 cm−1 is assigned to Raman scattering of complex overlap of νs(PO3) and symmetric/asymmetric stretching of PO2 groups.25,27 Figure 5b shows the Raman spectra of Cu-doped glass after the thermal annealing for 60 min at 400 °C. From this figure,

Figure 6. EPR spectrum of Cu2+ ions in as-made borophosphate glasses with Cu2O concentration of 0.5% (1% Cu+ ions) at room temperature. D


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The Journal of Physical Chemistry C / = g βHzSz + g⊥β(HxSx + HySy) + A SzIz + A⊥(SxIx + SyIy)

concentration of copper ion is increased. It is worthwhile to mention that the EPR signals are not observed in the spectra of undoped glasses. The EPR spectra are composed by a superposition of two bands. One due to isolated Cu2+ ions in axial symmetry which shows hyperfine structure with g∥ = 2.4. The other broader signal with g⊥= 2.08, also cited by other works, could result from the interaction between neighboring Cu2+ ions.35,37,38 The signal broadening increases due to the increase in the Cu2+ contents (see Figure 7a−c). The first band shows a poorly resolved hyperfine structure for the g∥ component and an unresolved band for the g⊥ component. As cited by Ciorcas et al.,38 the anisotropic hyperfine structure could be due to pronounced ligand field fluctuations present in the vitreous matrices that broaden the line. Over this anisotropy a broadening of the spectral lines appears due to the long-range interactions that become stronger with increasing CuO concentration.38 While no significant changes were found in the relative EPR signal intensity after the annealing at 400 °C (see Figure 7a−c). Thus, as Dehelean et al.35 we conclude that either the neighborhood symmetry of the isolated ions and the interacting ions (forming a cluster of mutually interacting ions) is basically the same or not all the ions are in some cluster, and there are always some isolated ions. It is clear that the

(1)

where the elements are defined by Ciorcas et al.30 The observed values of g∥ > g⊥ > 2.0023 for all samples, as shown in Table 1, allow us to conclude that Cu2+ ions are coordinated by Table 1. EPR Fit Data for Copper-Doped Glass Samples sample Cu2Oa

annealing

g∥

g⊥

A∥ × 104 cm−1

A⊥ × 104 cm−1

0.5 1.5 3.0 0.5 1.5 3.0

no no no 400 °C, 30 min 400 °C, 30 min 400 °C, 30 min

2.080 2.077 2.077 2.080 2.080 2.135

2.404 2.404 2.410 2.410 2.404 2.404

17 17 50 17 17 65

127 127 127 127 127 127

a

In mol %.

six ligand atoms in an octahedral environment elongated along one of the cube axis, and the ground state of the Cu2+ is dx2−y2.31 Our results are in good agreement with those presented in the literature.32−36 Additionally, it was observed that there is no significant change in spin-Hamiltonian parameters as the

Figure 7. EPR spectrum of Cu2+ ions in annealed borophosphate glasses with different Cu2O concentrations at room temperature. E


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Figure 8. (a) SERS spectra of rhodamine B on CuNPs borophosphate glasses with 514.5 nm laser excitation. (b) SERS spectra of rhodamine B on CuNPs borophosphate glasses with 632.8 nm excitation source.

Figure 9. SERS intensity versus annealing time for 1645 cm−1 mode of RB on the copper active borophosphate substrates using (a) 514.5 nm and (b) 632.8 nm laser lines.

population density of the isolated ions decreases with the increasing concentration of Cu2O. Surface-Enhanced Raman Spectroscopy (SERS): Applications. Rhodamine B (RB), a synthetic dye, was used as a standard in order to study SERS activity. It has been identified as an illegal additive in food because of its carcinogenic properties, neurotoxicity, and chronic toxicity to humans and animals.39,40 In order to determine optimum laser excitation lines for SERS activity, the RB solution was deposited on CuNPs borophosphate glasses (Figure 1c), and the Raman spectra were carried out with the wavelengths of 514.5 nm (green), 632.8 nm (red), and 785 nm (NIR). However, the excitation at 785 nm did not result in any detectable SERS signal. On the other hand, Figures 8a and 8b show SERS activity using CuNPs borophosphate glasses and 514.5 and 632.8 nm excitation, respectively. Our results show that 632.8 nm excitation results in much stronger SERS signals than 514.5 nm excitation. As can be seen in Figure 8, the SERS intensity increases as the annealing time increases up to 20 min and then decreases to further increases. This effect can be explained by the SPR (Figure 3a) that shows a red-shift with annealing time and particularly due to ideal narrow nanogaps between sharp corners and edges of CuNPs for some annealing time (see Figure 2). The inset of Figure 8b shows the Raman spectra for RB on two different substrates: conventional glass substrate and as-made borophosphate glass (unannealed). For these substrates we did not observe any SERS activity.

It is important to emphasize that after being kept in ambient air for more than 3 months, the SERS EF and morphologic properties of CuNPs borophosphate glasses did not change. This is strong evidence that the CuNPs borophosphate glasses possess excellent air stability. Furthermore, we found no significant change in the Raman signal assigned to the RB bands when CuNPs borophosphate glasses were used; see inset of Figure 8a. This suggests that our CuNPs borophosphate glasses do not affect the molecules which are physically adsorbed on the surface of the substrates. A representative asacquired SERS spectrum of rhodamine B on CuNPs borophosphate glasses with annealing time of 20 min obtained with 632.8 nm excitation source is presented in Figure S2. In this work, we adopted the estimation of the SERS enhancement factor (EF) of the our CuNPs borophosphate substrates reported by Zhang et al.41 and Xue Liu et al.42 as EF =

ISERS/NSERS IVol /NVol

(2)

where ISERS is the SERS peak intensity, IVol is the normal Raman peak intensity, NSERS is the average number of adsorbed molecules in the scattering volume for the SERS measurements, and NVol is the average number of molecules in the Raman scattering volume. The representative band at 1645 cm−1 due to functional group CO was selected to calculate the EF. In this case, assuming that the probed molecules are distributed on F


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The Journal of Physical Chemistry C Notes

the substrates uniformly, the number of probe molecules contributing to the signal can be estimated by N = NA × C ×

Vdroplet A spot

× Alaser

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Brazilian agency CNPq, CAPES, and FAPDF (research deans number 193.001.078/2015) for financial support and the research scholarship. We thank Matheus H. Lazzarin for his careful reading of the manuscript.

(3)

where NA is the Avogadro constant, C is the concentration of the used RB, Vdroplet is the volume of the RB droplet, Aspot is the area of the spot formed by the RB droplet, and Alaser is the area of the laser spot. A sample for the normal Raman measurement was prepared by dropping 5 μL of a RB (0.1 M) ethanolic solution onto a conventional glass substrate. The estimated EFs for the 1645 cm−1 mode were ≈5.7 × 107 and ≈1.5 × 108 for 514.5 and 632.8 nm lasers, respectively. The EF for 632.8 nm laser line is 1 order of magnitude higher than for 514.5 nm. This behavior can be explained by employing SPR, which occurs in the vicinity of the metal surface, especially narrow nanogaps between sharp corners and edges of nanostructures, namely hot spots.41,42 As can be seen in the Figure 3, the SPR absorption band is centered at about 579 nm, which is closer to the 632.8 nm laser line than the 514.5 nm laser line; i.e., the wavelength of 632.8 nm is proximity to the absorbance peak of the CuNPs borophosphate substrates. Uniformity and spectral reproducibility of the prepared CuNPs substrates were studied, which is a well-established SERS probe molecule and a very efficient Raman scatterer (for its chemical structure see Figure 1). Figure 9 shows the statistical analysis of the SERS signal at 1645 cm−1 mode for all annealing times evaluated. The results demonstrated good reproducibility. The uncertainty/deviation shown in Figure 9 and in Figure S3, among other factors, can be more likely caused by a nonuniform RB deposition on the substrates surface.

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CONCLUSIONS In summary, we have proposed a novel, low-cost, and facilely synthesized Cu-doped glass system as an active SERS substrate. As a result, CuNPs borophosphate SERS substrates with hierarchical nanostructures including nanofilm as well as Cu nanoparticles have been achieved. By using CuNPs borophosphate as the SERS substrate, the SERS detection limit as low as 10−9 M has been achieved, and the SERS signals on different positions of the substrate showed good reproducibility. The estimated EFs for the 1645 cm−1 mode of the rhodamine B were ≈5.7 × 107 and ≈1.5 × 108 for 514.5 and 632.8 nm lasers, respectively. The SERS enhancement factor values are among the highest for Cu nanostructures. Additionally, from EPR measurements we show that the population density of the isolated Cu2+ ions decreases with the increasing concentration of Cu2O.

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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/acs.jpcc.6b02881. Additional SERS/Raman spectra, statistical analysis of intense Raman peaks, and large area SEM images (ZIP)

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REFERENCES

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AUTHOR INFORMATION

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*E-mail [email protected] (R.S.). *E-mail [email protected] (J.F.F.). G


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