Journal of Colloid and Interface Science 468 (2016) 34–41
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Nickel nanoparticles with hcp structure: Preparation, deposition as thin films and application as electrochemical sensor Eduardo G.C. Neiva a, Marcela M. Oliveira b, Luiz H. Marcolino Jr. a, Aldo J.G. Zarbin a,⇑ a b
Departamento de Química, Universidade Federal do Paraná (UFPR), CP 19081, CEP 81531-990, Curitiba, PR, Brazil Departamento de Química e Biologia, Universidade Tecnológica Federal do Paraná (UTFPR), Curitiba, PR, Brazil
g r a p h i c a l a b s t r a c t
a r t i c l e
i n f o
Article history: Received 19 August 2015 Revised 14 January 2016 Accepted 18 January 2016 Available online 18 January 2016 Keywords: Nickel nanoparticles Metal nanoparticles Amperometric sensor Thin films Liquid–liquid interfaces
a b s t r a c t Hexagonal close packed (hcp) nickel nanoparticles stabilized by polyvinylpyrrolidone (PVP) were synthesized through the thermal treatment of face centered cubic (fcc) nickel nanoparticles. Controlling both the temperature of the heat treatment and the amount of PVP was possible the control of the hcp/fcc rate in the samples, where the higher Ni/PVP ratio produces only the hcp-nickel phase (average size of 8.9 nm) highly stable in air. The crystalline structure, the presence of PVP, the size of the nanoparticles and the stability of the hcp-nickel were confirmed using X-ray diffractometry, Fourier transform infrared spectroscopy, transmission electron microscopy, Raman spectroscopy, scanning electron microscopy and thermogravimetric analysis. Thin films of hcp and fcc nickel nanoparticles were prepared through a biphasic system and deposited over indium-doped tin oxide (ITO) substrates, which were electrochemically characterized and applied as glycerol amperometric sensors in NaOH medium. Parameters as the number of cycles applied and the scan rate were evaluated and indicate that hcp nickel nanoparticles are more reactive to form Ni(OH)2 and lead to more electroactive Ni(OH)2 structure. The hcp nickel nanoparticles-modified electrode showed the best sensitivity (0.258 lA L lmol 1) and detection limit (2.4 lmol L 1) toward glycerol. Ó 2016 Elsevier Inc. All rights reserved.
1. Introduction ⇑ Corresponding author. E-mail address: [email protected] (A.J.G. Zarbin). http://dx.doi.org/10.1016/j.jcis.2016.01.036 0021-9797/Ó 2016 Elsevier Inc. All rights reserved.
Nickel nanoparticles have been extensively studied due their potential application in several fields like magnetic devices [1,2],
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catalysts [3,4], batteries [5] and fuel cells [6,7]. Although the face centered cubic (fcc) is the most stable crystalline structure adopted by Ni metal, the metastable hexagonal closed packed (hcp) structure has been obtained in nanometric scale through specific synthetic conditions and stabilizers [8–10]. The most used routes to hcp-Ni production are based on the polyol method [11], thermal decomposition [12] and epitaxial growth [13]. There are, however, some contrarieties in the literature about the real occurrence of hcp-nickel in these reported samples [14], because that crystalline phase has lattice parameters very similar to the Ni3C, which has a rhombohedral structure and can be describe as a hcp-nickel structure with an interstitial carbon superstructure [15]. The contamination with carbon happens because most of the preparative routes use organic molecules as stabilizing or solvent and temperatures higher than 200 °C [9,16]. The main property affected by the phase transition in metallic nickel reported in the literature is related to the magnetism. The fcc-Ni to hcp-Ni phase transition led to an increase in the atomic distance from 2.499 to 2.665 Å and decreases the spin interaction between the atoms, decreasing the magnetization of the material [17]. Besides the magnetic property, other properties also change as the reactivity of the material, since hcp-Ni is metastable. This material is also employed for hydrogen storage [18], methanol oxidation [6] and as electrochemical sensor [19]. In the electrochemical applications, nickel nanoparticles should be electrochemically converted to the electroactive species Ni(OH)2 and NiOOH, in which the last one is the responsible for the electrocatalytic performance of the electrode [20]. However, the electrochemical behavior of these species depends of the morphology and the structure of these compounds (a-Ni(OH)2/ b-Ni(OH)2 and c-NiOOH/b-NiOOH), which will depend of the features of the metallic nickel nanoparticles employed as precursor [21–23]. Here we report a new route to prepare high air-stable hcp-Ni nanoparticles, as well as to prepare materials controlling hcp/fcc rate in the nickel nanoparticles, through the heat treatment of fcc-Ni nanoparticles stabilized by polyvinylpyrrolidone (PVP). The further preparation and deposition of thin films of these nanoparticles over indium-doped tin oxide (ITO) electrodes, and the application of these films as glycerol sensor in alkaline medium, are also reported. Glycerol is a very interesting analyte since it is a sideproduct in the production of the biodiesel, it used as a raw material in pharmaceutical industry and it is employed as sweetener in the food industry [24,25]. 2. Experimental The fcc-Ni nanoparticles were prepared by a modification in the polyol process described by our group [20], in which Ni(OCH3COO)24H2O (Vetec) was used as precursor, PVP (Vetec, 10,000 g mol 1 average molecular weight) as stabilizing agent, NaBH4 (Merck) as reducing agent and ethylene glycol (Merck) as solvent. Summarizing, 99.5 10 3 g of Ni(CH3COO)24H2O was dissolved in 20.0 mL of ethylene glycol in a 250 mL round flask. To prepare samples with different Ni/PVP ratio, different amount of PVP were added to the nickel solution (no PVP, 23.5 10 3, 47 10 3, 94 10 3, 141 10 3 or 188 10 3 g of PVP, leading to the samples named here as non-PVP–Ni, Ni-1/1, Ni-1/2, Ni-1/4, Ni-1/6 and Ni-1/8, respectively. The numbers in the nomenclature are related to the Ni/PVP molar ratio in each sample). Then, the system was maintained under magnetic stirring until the completely dissolution of the polymer. Next, the flask was heated to 140 °C and, at this temperature, 60.5 10 3 g of NaBH4 was added. The system was maintained under strong magnetic stirring at 140 °C for 2 h, and then the heating and magnetic stirring were turned off. To isolate the dispersed nickel nanoparticles, 50 mL of
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acetone was added to the dispersion and the material was separated by centrifugation, washed several times with acetone and dried at room atmosphere. Nickel nanoparticles with hcp structure were prepared by the controlled thermal treatment of fcc-Ni obtained according described before. In a typical experiment, the Ni-1/6 sample was placed in an alumina substrate and kept under an argon flux (75 mL min 1) for 20 min in a tubular oven (EDGCON 5P). After that, the sample was heated at 200, 250, 300, 350, 400, 450, 500, 600 or 700 °C for 1 min after reach the desired temperature, using a heating rate of 10 °C min 1, and then allowed to cool spontaneously to room temperature. Fcc-Ni nanoparticles containing different Ni/PVP ratios were also used to the heat treatment at 350 °C, leading to the samples non-PVP–Ni-ht, Ni-1/1-ht, Ni-1/2-ht, Ni-1/4-ht, Ni-1/6-ht and Ni-1/8-ht. The time of heating was also evaluated at 300 and 350 °C using the Ni-1/8 sample and 60 and 180 min after reach the desired temperature. To the electrochemical characterization and application as glycerol sensors, the samples Ni-1/1-ht and Ni-1/8-ht were deposited as thin films over ITO electrodes through a biphasic route described by our group [26–29]. For this purpose, 0.30 mg of sample were added to a 10 mL round flask with 4 mL of milli-Q water and 4 mL of toluene (Carlo Erba) and then sonicated in a ultrasound bath (Unique USC 1880 – 37 kHz) for 40 min. Subsequently, the flask was magnetically stirred for 40 min. (2500 rpm), leading to a self-assembled thin film at the liquid–liquid interface. After that, the entire system was transferred to a 50 mL beaker containing indium-doped tin oxide (ITO) substrates (0.7 2.5 cm). Then, the substrate was lifted in the direction of the film using tweezers, resulting in transference of the film from the liquid interface to the substrate. The films were subsequently dried at 90 °C for 30 min. The electrochemical measurements were performed in a MicroAutolab potentiostat, in a conventional three electrodes cell consisting of the thin film-modified ITO as working electrode, a platinum wire as counter electrode and an Ag/AgCl (KCl saturated solution) as reference electrode, in 1 mol L 1 NaOH solution. To the glycerol detection, the thin films were first submitted to 150 voltammetric cycles at 50 mV s 1 in the absence of the analyte. The chronoamperometric measurements were performed maintaining the solution under constant stirring at 0.46 V and using a glycerol concentration range of 1–500 lmol L 1. The X-ray diffraction (XRD) patterns were collected on a Shimadzu XRD-6000 diffractometer using Cu Ka radiation, with 40 kV and 30 mA, at 0.02° scan rate in a 2h range of 35–80°. The FT-IR spectra of the samples were carried out in a Bio-Rad FTS 3500 GX spectrophotometer in the 4000–400 cm 1 range with 32 scans, using KBr to prepare the pellets. UV–Vis spectra were collected in a Shimadzu UV-2450 spectrophotometer in transmittance mode in the 800–300 nm range. The thin films were deposited on quartz substrates and the quartz signal was discounted. The thermogravimetric analyses (TGA) were done in TGA/SDT Q 600 (TA instruments), from room temperature to 900 °C at a heating rate of 10 °C min 1 in N2 atmosphere. Raman spectra were obtained using a Renishaw Raman-Image spectrophotometer coupled with an optical microscope employing a laser emitting at 514.5 nm, from 200 to 1800 cm 1. Several spectra were recorded from different points on the samples. The transmission electron microscopy (TEM) measurements were performed in a Jeol JEM 1200. The samples were prepared by evaporation of one drop of an iso-propanol dispersion of the nanoparticles on holey copper grids covered with carbon films. The scanning electron microscopy (SEM) images were obtained from the thin films deposited on ITO in a Tescan SEM-FEG microscope.
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3. Results and discussion The X-ray diffraction patterns of the sample Ni-1/6 before and after the heat treatment from 250 to 700 °C are shown in the Fig. 1A. As we described before [20], the Ni nanoparticles obtained by the modified polyol route possess a fcc structure (JCPDS 87-0712), presenting nanometric size as evidenced by the broad peaks at the XRD patterns. After the heat treatment, temperatures of 200 and 250 °C did not lead to differences in the XRD patterns. However, increasing the temperature a narrowing in the XRD peaks is observed, indicating an annealing or a particle growth process. Other interesting change is the appearance of new peaks at d = 0.230, 0.218, 0.204, 0.158, 0.133 and 0.123 nm, as observed in the XRD pattern of the sample heated at 300 °C. These peaks are attributed to the planes (0 1 0), (0 0 2), (0 1 1), (0 1 2), (1 1 0) and (1 0 3), respectively, of the hcp structure of metallic nickel (JCPDS 45-1027), indicating that the fcc/hcp phase transition occurs at 300 °C. It is clearly seen that the ratio between these phases is not the same at the different temperature used in the thermal treatment, where the ratio between the intensities of the peaks at d = 0.177 nm (attributed to (2 0 0) plane of fcc phase) and d = 0.218 nm (attributed to the (0 0 2) plane in the hcp phase) is 0.27, 0.20 and 0.55 to the temperatures 300, 350 and 400 °C, respectively, indicating the higher proportion of hcp phase at 350 °C. At temperatures higher than 400 °C, the peaks attributed to hcp phase completely disappear and other peaks at 41.9° and 47.7° (not yet attributed) rise at 450 °C, and almost disappear at 700 °C, leading just to fcc Ni. Fig. 1B shows the FT-IR spectra of these samples. Typical bands related to PVP at 3422 (m OAH), 2953/2918/2870 (m CAH), 1652 (m C@O), 1495/1462/1441/1423 (d CAH ring), 1375 (d CAC ring) and 1290 cm 1 (m CAN tertiary amide) were detected for the samples before and after the heat treatment till 350 °C [30]. At 400 °C and higher temperatures these bands disappear, indicating the degradation of PVP. This feature is corroborated by the TGA measurements of PVP and Ni-1/6 sample (Fig. S1, Supplementary Material), which shows that the degradation of the polymer starts at around 380 °C. Correlating XRD and FT-IR data, it is clearly seen the crucial role of the PVP-stabilizer to the metastable hcp phase
occurrence, where the degradation of the PVP lead to fcc phase. Despite at 400 °C the PVP was degraded and hcp-Ni phase is still present, it completely disappears with higher times of heat treatment at that temperature (data not showed). The degradation of PVP in Ar atmosphere led to amorphous carbon, as seen in the Raman spectra of the samples heated from 400 to 700 °C (Fig. S2), which exhibit the typical D (1365 cm 1) and G (1575 cm 1) bands. However, there is a narrowing and a shift of these bands with increasing of the temperature, from 1365 to 1335 cm 1 and from 1575 to 1590 cm 1, for the D and G bands, respectively. This behavior is related with a graphitization process induced by higher temperatures [31]. The combination of the XRD, FT-IR and Raman data indicate that hcp-Ni phase can be obtained at controlled thermal treatment of the fcc-Ni phase, and that the presence of PVP is essential to stabilize the metastable hcp-Ni phase. Also, at thermal treatment above 400 °C, a fcc-Ni/ amorphous carbon composite material is obtained. As the presence of PVP is essential to obtain stable hcp-Ni nanoparticles, samples with different Ni/PVP ratio have been heat-treated at 350 °C. It is important to notice here that all the experiments have been prepared with PVP keeping exactly the same molecular weight. Although the effect of the molecular weight of the PVP was not evaluated here, it probably could have influence on the fcc/hcp ratio of the obtained samples. Before the heat-treatment, all samples presented the same XRD profile, indicating that the amount of PVP do not change significantly the size and the structure of the Ni nanoparticles (Fig. 2A). However, the absence of PVP in the sample led to peaks related to NiO, as described elsewhere [20]. The XRD patterns for these samples after the heat treatment are shown in the Fig. 2B, showing the evident influence of the stabilizer in the structure of the Ni nanoparticles, in which increasing amounts of PVP produces samples with higher amount of hcp-Ni. These data indicate that not only the presence of PVP is important to stabilize the hcp phase, but also the amount of the polymer. The inset of the Fig. 2B shows the percentage of hcpNi obtained in function of the proportion of PVP in the precursor sample, in which the lower amount of fcc-Ni is found in the more-PVP containing Ni-1/8-ht sample. The stability of the hcpNi presented in the sample Ni-1/8-ht was evaluated, keeping the
Fig. 1. (A) X-ray diffractograms and (B) FTIR spectra of Ni-1/6 sample, before and after the heat treatment (from 250 to 700 °C). The FT-IR spectra for the samples heat-treated at 600 and 700 °C (not shown) were similar to the sample heated at 500 °C.
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Fig. 2. (A) X-ray diffractograms of non-PVP–Ni, Ni-1/1, Ni-1/2, Ni-1/4, Ni-1/6 and Ni-1/8, and (B) non-PVP–Ni-ht, Ni-1/1-ht, Ni-1/2-ht, Ni-1/4-ht, Ni-1/6-ht and Ni-1/ 8-ht obtained after the heat treatment at 350 °C. The inset in (B) shows the percentage of hcp Ni with the Ni/PVP ratio (obtained by the ratio between the peaks at 41.5° and 51.7° attributed to hcp and fcc Ni, respectively).
sample for 7 months stored under air atmosphere, and it was confirmed by XRD, in which no detectable amounts of other species (as fcc-Ni or NiO) were detected, as seeing in Fig. S3. Fig. 3 shows the transmission electron microscopy images of the samples Ni-1/1, Ni-1/8, Ni-1/1-ht and Ni-1/8-ht. It is clearly seen the presence of Ni nanoparticles (higher contrast regions in Fig. 3A, C, E and G) surrounded by the polymer (regions of less contrast) in all the samples. The presence of the crystalline nanoparticles is more evident by the bright regions in the dark field images (Fig. 3B, D, F and H). The size distribution histograms for the nanoparticles in each sample reveal a growth for both samples after the heat treatment, from 2.6 to 9.5 nm and 2.7 to 8.9 nm for the Ni-1/1-ht and Ni-1/8-ht, respectively. The sample Ni-1/8 was also submitted to different periods of heat treatment at 300 and 350 °C. The XRD patterns of the samples after the heat treatment at 300 °C (Fig. S4A) show that the heating time did not affect the hcp/fcc Ni ratio. Opposite behavior was observed at thermal treatment of 350 °C (Fig. S4B), in which higher thermal treatment times led to increasing of the fcc-Ni amount.
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These behaviors are related with the degradation of PVP, as seen in the FT-IR spectra of these samples (Fig. S5). At higher times of heat-treatment at 300 °C the PVP was not completely degraded, but the increase of the time of heat-treatment at 350 °C led to the disappearance of the polymer bands, confirming again the important role of PVP on the stabilization of the hcp-Ni. Fig. 4C shows a photography of the thin films of the samples Ni-1/1-ht and Ni-1/8-ht prepared through a modification of the liquid–liquid interfacial route developed in our group [26–29]. This is a very interesting route to processes different kind of materials as thin films, in which the solid sample originally dispersed in one of the immiscible liquids tends to spontaneously migrate to the liquid–liquid interface in order to minimize the interfacial tension, leading to self-assembled thin films easily transferable to several substrates. As can be seen in the Fig. 4C, the hcp-Ni thin film possesses the higher transmittance, which is confirmed by the UV–Vis spectra of these samples (Fig. S6), in which the hcp and fcc Ni thin films presented 96% and 77.8% of transmittance at 550 nm, respectively. This feature can be associated to the morphology of the films. Fig. 4A, B, D and E shows the SEM images of these thin films, demonstrating that hcp-Ni thin film is homogeneously distributed into the substrate, while the fcc-Ni film presents a large amount of agglomerated nanoparticles, decreasing its transmittance. Also, the higher metal/PVP amount in the fccNi samples should contribute to the decrease in its transmittance. The electrochemical characterization and the sensor application of the hcp-Ni and fcc-Ni thin films were done in alkaline medium, leading to the development of the Ni(OH)2/NiOOH redox pair. Fig. 5A shows the 150° voltammetric cyclic for both thin films, cycled in a 1 mol L 1 aqueous solution of NaOH. Both thin films presented the Ni(OH)2/NiOOH redox pair, indicating that the presence and the amount of PVP does not prevent the occurrence of these electroactive species [20]. These electrodes also showed the same current peak intensity, suggesting the same amount of active sites. Fig. 5B and C shows both the current of the anodic peak and the half-wave anodic potential in function of the number of cycles, respectively. It is clearly seen in the Fig. 5B the continuous formation of Ni(OH)2 for both films during the cycling. However, the hcpNi film produces a higher amount of active sites in the first cycles and reaches a stable current peak after 70 cycles, while the fcc-Ni film leads to a lower amount of these active sites in the first cycles and does not present a stable current peak until the 150° cycle. This behavior suggest that the hcp-Ni thin film has a higher reactivity to produce Ni(OH)2, while the fcc-Ni film take more time to produce Ni(OH)2 and it should lead to a higher amount of active sites as it possess a higher amount of Ni. For both films it is also seen a higher half-wave anodic potential for the first cycles (Fig. 5C), which can be associated to the presence of the PVP on the surface of the nanoparticles, making the oxidation process difficult in the beginning of cycling. In addition, there is a shift for higher potentials after the tenth cycle (10 and 21 mV for fcc-Ni and hcp-Ni thin films, respectively). This phenomena indicate a modification of Ni(OH)2 for a more organized structure. The structure modification is described by the Bode diagram, which relate all the Ni(OH)2 and NiOOH structures and reports an organization of the electroactive species with the aging of the electrode (Fig. S7) [21]. Other interesting information is the lower anodic peak potential for the hcp-Ni film (422 and 438 mV for hcp-Ni and fcc-Ni thin films, respectively). All these data together indicate that the Ni(OH)2 obtained from the hcp-Ni film possess a structure more electroactive, like the a-structure, which is more disorganized and have a higher distance between the Ni(OH)2 layers than the b structure. Both the thin films were previously cycled 150 times at 50 mV s 1 in 1 mol L 1 NaOH, and further cycled at different scan rate (from 5 to 300 mV s 1). The data are presented in Fig. S8. An
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Fig. 3. TEM images of Ni-1/1 (A, B), Ni-1/1-ht (E, F), Ni-1/8 (C, D) and Ni-1/8-ht (G, H). The images in dark field (B, D, F, H) correspond to the same region of the images in bright field (A, C, E, G). The insets in A, C, E and G show the size distribution histograms of the samples, where 300 nanoparticles were individually measured for each sample.
Fig. 4. SEM images of the hcp-Ni Ni-1/8-ht (A, B) and fcc-Ni Ni-1/1-ht (D, E) thin films deposited over ITO substrates. (C) Photograph images of the hcp-Ni (left) and fcc-Ni (right) thin films, covering the upper-half of the electrodes.
increase of the peak current and DE value with the increase of the scan rate was observed in both films, as expected. The linear dependence between the current peak and v1/2 indicate a diffusion-controlled process for both films in the whole scan rate range analyzed. The electrocatalytic behavior for the glycerol oxidation was evaluated for hcp-Ni and fcc-Ni thin films (Fig. 6A and B, respectively) in the absence and presence of 1, 2 and 3 mmol L 1 of glycerol after the pretreatment cited before. Both materials presented an electrocatalytic behavior for this analyte demonstrated by the anodic peak increment and the cathodic peak decrease. This phenomena happens because the catalytic species (NiOOH) oxidizes
the glycerol and regenerate the Ni(OH)2 (chemical step). This Ni(OH)2 is re-oxidized by the anodic potential that is applied when the chemical step happen, leading to a current increment. The decrease of current occurs because the NiOOH was consumed by the chemical step. However, the hcp-Ni thin film showed higher current increments for all amount of glycerol added to the system (105, 95 and 97 lA, for the first, second and third additions, respectively) than the fcc-Ni thin film (76, 70 and 75 lA, for the first, second and third additions, respectively). This higher performance of the hcp-Ni thin film could be related to a more electroactive structure formed in this electrode. The appearance of a peak at about 0.5 V can be associated to the oxidation of the products
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Fig. 5. (A) 150° cyclic voltammograms for the (a) fcc-Ni and (b) hcp-Ni thin films in 1 mol L 1 of NaOH. (B) Current of the anodic peak and (C) half-wave anodic potential in function of the number of cycles for both thin films. The cyclic voltammograms were collected at 50 mV s 1.
Fig. 6. Cyclic voltammograms for hcp-Ni (A) and fcc-Ni (B) thin films in the (a) absence of glycerol and in the presence of 1 mmol L 1 (b), 2 mmol L 1 (c) and 3 mmol L 1 (d) of glycerol in 1 mol L 1 of NaOH. The inset in (B) corresponds to the cyclic voltammogram of bare ITO electrode in the presence of 3 mmol L 1 of glycerol in 1 mol L 1 of NaOH. The cyclic voltammograms were obtained using 50 mV s 1.
originated from the first glycerol oxidation [32]. In the inset of Fig. 6B is presented the cyclic voltammograms of bare ITO in the presence of 3 mmol L 1 of glycerol, in which no electrocatalytic behavior is observed in the absence of the Ni nanomaterials. Both thin films were applied as chronoamperometric sensors to glycerol. For that, the electrodes were first submitted to the pretreatment cited before and then a chronoamperometry using 0.46 V was carried out with additions of glycerol in the concentration range from 1 to 500 lmol L 1 (Fig. 7A and B). In the whole concentration range can be seen an increment of current with the glycerol additions for both thin films. In the Fig. 7C is showed the analytical curves constructed from the current increment vs. the glycerol concentration for these thin films. Both materials exhibited two linear regions with different sensitivities: the first
from 1 to 20 lmol L 1, with 0.258 and 0.186 lA L lmol 1, and the second from 25 to 500 lmol L 1, with 0.187 and 0.146 lA L lmol 1 to hcp-Ni and fcc-Ni thin films, respectively. This decrease in the sensitivity suggests a little saturation of the electroactive sites with the increase of the glycerol concentration. However, the good correlation coefficients of 0.998 and 0.995, for the first region, and 0.997 and 0.997, for the second region, for hcp-Ni and fcc-Ni thin films, respectively, allow the use of both regions for glycerol detection. The detection limit calculated from the first region for both electrodes were 2.4 and 4.7 lmol L 1, using the standard deviation from the blank of 0.20 and 0.29 lA, to hcpNi and fcc-Ni thin films, respectively. The higher performance of the hcp-Ni compared to the fcc-Ni suggest again a Ni(OH)2 phase more electroactive. These low detection limits associated with
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Fig. 7. Chronoamperograms obtained for (A) hcp-Ni and (B) fcc-Ni thin films using a glycerol concentration range from 1 to 500 lmol L Analytical curves constructed from the chronoamperograms of (a) hcp-Ni and (b) fcc-Ni thin films.
the wide detection range are ones of the best values described for the electrochemical detection of glycerol. 4. Conclusions In conclusion, this work reports the preparation of PVPstabilized hcp-Ni and fcc-Ni nanoparticles through the polyol method, in which both the temperature of the heat treatment and the PVP ratio in the nanoparticles were the main parameters to control the hcp/fcc rate in the samples. The hcp-Ni nanoparticles showed a high stability in air atmosphere. Thin films of these materials were prepared through a biphasic system and electrochemically converted to Ni(OH)2, where the results suggest a more electroactive Ni(OH)2 phase from the hcp-Ni nanoparticlesmodified electrode. Both hcp-Ni and fcc-Ni nanoparticlesmodified electrodes were applied as amperometric sensors for glycerol, where the best result were achieved with the hcp-Ni with a detection limit of 2.4 lmol L 1 and a glycerol detection range of 1–500 lmol L 1. Acknowledgements The authors acknowledge the financial support by CNPq, CAPES, NENNAM (PRONEX, Fund. Araucária/CNPq), National Institute of Science and Technology of Carbon Nanomaterials (INCT – Nanocarbon), and the CME-UFPR by the TEM images. E.G.C.N. thanks CAPES for a research fellowship. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2016.01.036. References [1] A.P.R. Mary, C.S.S. Sandeep, T.N. Narayanan, R. Philip, P. Moloney, P.M. Ajayan, M.R. Anantharaman, Nonlinear and magneto-optical transmission studies on magnetic nanofluids of non-interacting metallic nickel nanoparticles, Nanotechnology 22 (2011) 375702. [2] O. Pascu, J.M. Caicedo, J. Fontcuberta, G. Herranz, A. Roig, Magneto-optical characterization of colloidal dispersions. Application to nickel nanoparticles, Langmuir 26 (2010) 12548–12552. [3] Ö. Metin, V. Mazumder, S. Özkar, S. Sun, Monodisperse nickel nanoparticles and their catalysis in hydrolytic dehydrogenation of ammonia borane, J. Am. Chem. Soc. 132 (2010) 1468–1469. [4] J. Aguilhon, C. Boissière, O. Durupthy, C. Thomazeau, C. Sanchez, Nickel nanoparticles with controlled morphologies application in selective hydrogenation catalysis, Stud. Surf. Sci. Catal. 175 (2010) 521–524. [5] Y.J. Mai, J.P. Tu, C.D. Gu, X.L. Wang, Graphene anchored with nickel nanoparticles as a high-performance anode material for lithium ion batteries, J. Power Sources 209 (2012) 1–6.
1
in 1 mol L
1
of NaOH at 0.46 V. (C)
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Journal of Colloid and Interface Science journal homepage: www.elsevier.com/locate/jcis
Nickel nanoparticles with hcp structure: Preparation, deposition as thin films and application as electrochemical sensor Eduardo G.C. Neiva a, Marcela M. Oliveira b, Luiz H. Marcolino Jr. a, Aldo J.G. Zarbin a,⇑ a b
Departamento de Química, Universidade Federal do Paraná (UFPR), CP 19081, CEP 81531-990, Curitiba, PR, Brazil Departamento de Química e Biologia, Universidade Tecnológica Federal do Paraná (UTFPR), Curitiba, PR, Brazil
g r a p h i c a l a b s t r a c t
a r t i c l e
i n f o
Article history: Received 19 August 2015 Revised 14 January 2016 Accepted 18 January 2016 Available online 18 January 2016 Keywords: Nickel nanoparticles Metal nanoparticles Amperometric sensor Thin films Liquid–liquid interfaces
a b s t r a c t Hexagonal close packed (hcp) nickel nanoparticles stabilized by polyvinylpyrrolidone (PVP) were synthesized through the thermal treatment of face centered cubic (fcc) nickel nanoparticles. Controlling both the temperature of the heat treatment and the amount of PVP was possible the control of the hcp/fcc rate in the samples, where the higher Ni/PVP ratio produces only the hcp-nickel phase (average size of 8.9 nm) highly stable in air. The crystalline structure, the presence of PVP, the size of the nanoparticles and the stability of the hcp-nickel were confirmed using X-ray diffractometry, Fourier transform infrared spectroscopy, transmission electron microscopy, Raman spectroscopy, scanning electron microscopy and thermogravimetric analysis. Thin films of hcp and fcc nickel nanoparticles were prepared through a biphasic system and deposited over indium-doped tin oxide (ITO) substrates, which were electrochemically characterized and applied as glycerol amperometric sensors in NaOH medium. Parameters as the number of cycles applied and the scan rate were evaluated and indicate that hcp nickel nanoparticles are more reactive to form Ni(OH)2 and lead to more electroactive Ni(OH)2 structure. The hcp nickel nanoparticles-modified electrode showed the best sensitivity (0.258 lA L lmol 1) and detection limit (2.4 lmol L 1) toward glycerol. Ó 2016 Elsevier Inc. All rights reserved.
1. Introduction ⇑ Corresponding author. E-mail address: [email protected] (A.J.G. Zarbin). http://dx.doi.org/10.1016/j.jcis.2016.01.036 0021-9797/Ó 2016 Elsevier Inc. All rights reserved.
Nickel nanoparticles have been extensively studied due their potential application in several fields like magnetic devices [1,2],
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catalysts [3,4], batteries [5] and fuel cells [6,7]. Although the face centered cubic (fcc) is the most stable crystalline structure adopted by Ni metal, the metastable hexagonal closed packed (hcp) structure has been obtained in nanometric scale through specific synthetic conditions and stabilizers [8–10]. The most used routes to hcp-Ni production are based on the polyol method [11], thermal decomposition [12] and epitaxial growth [13]. There are, however, some contrarieties in the literature about the real occurrence of hcp-nickel in these reported samples [14], because that crystalline phase has lattice parameters very similar to the Ni3C, which has a rhombohedral structure and can be describe as a hcp-nickel structure with an interstitial carbon superstructure [15]. The contamination with carbon happens because most of the preparative routes use organic molecules as stabilizing or solvent and temperatures higher than 200 °C [9,16]. The main property affected by the phase transition in metallic nickel reported in the literature is related to the magnetism. The fcc-Ni to hcp-Ni phase transition led to an increase in the atomic distance from 2.499 to 2.665 Å and decreases the spin interaction between the atoms, decreasing the magnetization of the material [17]. Besides the magnetic property, other properties also change as the reactivity of the material, since hcp-Ni is metastable. This material is also employed for hydrogen storage [18], methanol oxidation [6] and as electrochemical sensor [19]. In the electrochemical applications, nickel nanoparticles should be electrochemically converted to the electroactive species Ni(OH)2 and NiOOH, in which the last one is the responsible for the electrocatalytic performance of the electrode [20]. However, the electrochemical behavior of these species depends of the morphology and the structure of these compounds (a-Ni(OH)2/ b-Ni(OH)2 and c-NiOOH/b-NiOOH), which will depend of the features of the metallic nickel nanoparticles employed as precursor [21–23]. Here we report a new route to prepare high air-stable hcp-Ni nanoparticles, as well as to prepare materials controlling hcp/fcc rate in the nickel nanoparticles, through the heat treatment of fcc-Ni nanoparticles stabilized by polyvinylpyrrolidone (PVP). The further preparation and deposition of thin films of these nanoparticles over indium-doped tin oxide (ITO) electrodes, and the application of these films as glycerol sensor in alkaline medium, are also reported. Glycerol is a very interesting analyte since it is a sideproduct in the production of the biodiesel, it used as a raw material in pharmaceutical industry and it is employed as sweetener in the food industry [24,25]. 2. Experimental The fcc-Ni nanoparticles were prepared by a modification in the polyol process described by our group [20], in which Ni(OCH3COO)24H2O (Vetec) was used as precursor, PVP (Vetec, 10,000 g mol 1 average molecular weight) as stabilizing agent, NaBH4 (Merck) as reducing agent and ethylene glycol (Merck) as solvent. Summarizing, 99.5 10 3 g of Ni(CH3COO)24H2O was dissolved in 20.0 mL of ethylene glycol in a 250 mL round flask. To prepare samples with different Ni/PVP ratio, different amount of PVP were added to the nickel solution (no PVP, 23.5 10 3, 47 10 3, 94 10 3, 141 10 3 or 188 10 3 g of PVP, leading to the samples named here as non-PVP–Ni, Ni-1/1, Ni-1/2, Ni-1/4, Ni-1/6 and Ni-1/8, respectively. The numbers in the nomenclature are related to the Ni/PVP molar ratio in each sample). Then, the system was maintained under magnetic stirring until the completely dissolution of the polymer. Next, the flask was heated to 140 °C and, at this temperature, 60.5 10 3 g of NaBH4 was added. The system was maintained under strong magnetic stirring at 140 °C for 2 h, and then the heating and magnetic stirring were turned off. To isolate the dispersed nickel nanoparticles, 50 mL of
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acetone was added to the dispersion and the material was separated by centrifugation, washed several times with acetone and dried at room atmosphere. Nickel nanoparticles with hcp structure were prepared by the controlled thermal treatment of fcc-Ni obtained according described before. In a typical experiment, the Ni-1/6 sample was placed in an alumina substrate and kept under an argon flux (75 mL min 1) for 20 min in a tubular oven (EDGCON 5P). After that, the sample was heated at 200, 250, 300, 350, 400, 450, 500, 600 or 700 °C for 1 min after reach the desired temperature, using a heating rate of 10 °C min 1, and then allowed to cool spontaneously to room temperature. Fcc-Ni nanoparticles containing different Ni/PVP ratios were also used to the heat treatment at 350 °C, leading to the samples non-PVP–Ni-ht, Ni-1/1-ht, Ni-1/2-ht, Ni-1/4-ht, Ni-1/6-ht and Ni-1/8-ht. The time of heating was also evaluated at 300 and 350 °C using the Ni-1/8 sample and 60 and 180 min after reach the desired temperature. To the electrochemical characterization and application as glycerol sensors, the samples Ni-1/1-ht and Ni-1/8-ht were deposited as thin films over ITO electrodes through a biphasic route described by our group [26–29]. For this purpose, 0.30 mg of sample were added to a 10 mL round flask with 4 mL of milli-Q water and 4 mL of toluene (Carlo Erba) and then sonicated in a ultrasound bath (Unique USC 1880 – 37 kHz) for 40 min. Subsequently, the flask was magnetically stirred for 40 min. (2500 rpm), leading to a self-assembled thin film at the liquid–liquid interface. After that, the entire system was transferred to a 50 mL beaker containing indium-doped tin oxide (ITO) substrates (0.7 2.5 cm). Then, the substrate was lifted in the direction of the film using tweezers, resulting in transference of the film from the liquid interface to the substrate. The films were subsequently dried at 90 °C for 30 min. The electrochemical measurements were performed in a MicroAutolab potentiostat, in a conventional three electrodes cell consisting of the thin film-modified ITO as working electrode, a platinum wire as counter electrode and an Ag/AgCl (KCl saturated solution) as reference electrode, in 1 mol L 1 NaOH solution. To the glycerol detection, the thin films were first submitted to 150 voltammetric cycles at 50 mV s 1 in the absence of the analyte. The chronoamperometric measurements were performed maintaining the solution under constant stirring at 0.46 V and using a glycerol concentration range of 1–500 lmol L 1. The X-ray diffraction (XRD) patterns were collected on a Shimadzu XRD-6000 diffractometer using Cu Ka radiation, with 40 kV and 30 mA, at 0.02° scan rate in a 2h range of 35–80°. The FT-IR spectra of the samples were carried out in a Bio-Rad FTS 3500 GX spectrophotometer in the 4000–400 cm 1 range with 32 scans, using KBr to prepare the pellets. UV–Vis spectra were collected in a Shimadzu UV-2450 spectrophotometer in transmittance mode in the 800–300 nm range. The thin films were deposited on quartz substrates and the quartz signal was discounted. The thermogravimetric analyses (TGA) were done in TGA/SDT Q 600 (TA instruments), from room temperature to 900 °C at a heating rate of 10 °C min 1 in N2 atmosphere. Raman spectra were obtained using a Renishaw Raman-Image spectrophotometer coupled with an optical microscope employing a laser emitting at 514.5 nm, from 200 to 1800 cm 1. Several spectra were recorded from different points on the samples. The transmission electron microscopy (TEM) measurements were performed in a Jeol JEM 1200. The samples were prepared by evaporation of one drop of an iso-propanol dispersion of the nanoparticles on holey copper grids covered with carbon films. The scanning electron microscopy (SEM) images were obtained from the thin films deposited on ITO in a Tescan SEM-FEG microscope.
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3. Results and discussion The X-ray diffraction patterns of the sample Ni-1/6 before and after the heat treatment from 250 to 700 °C are shown in the Fig. 1A. As we described before [20], the Ni nanoparticles obtained by the modified polyol route possess a fcc structure (JCPDS 87-0712), presenting nanometric size as evidenced by the broad peaks at the XRD patterns. After the heat treatment, temperatures of 200 and 250 °C did not lead to differences in the XRD patterns. However, increasing the temperature a narrowing in the XRD peaks is observed, indicating an annealing or a particle growth process. Other interesting change is the appearance of new peaks at d = 0.230, 0.218, 0.204, 0.158, 0.133 and 0.123 nm, as observed in the XRD pattern of the sample heated at 300 °C. These peaks are attributed to the planes (0 1 0), (0 0 2), (0 1 1), (0 1 2), (1 1 0) and (1 0 3), respectively, of the hcp structure of metallic nickel (JCPDS 45-1027), indicating that the fcc/hcp phase transition occurs at 300 °C. It is clearly seen that the ratio between these phases is not the same at the different temperature used in the thermal treatment, where the ratio between the intensities of the peaks at d = 0.177 nm (attributed to (2 0 0) plane of fcc phase) and d = 0.218 nm (attributed to the (0 0 2) plane in the hcp phase) is 0.27, 0.20 and 0.55 to the temperatures 300, 350 and 400 °C, respectively, indicating the higher proportion of hcp phase at 350 °C. At temperatures higher than 400 °C, the peaks attributed to hcp phase completely disappear and other peaks at 41.9° and 47.7° (not yet attributed) rise at 450 °C, and almost disappear at 700 °C, leading just to fcc Ni. Fig. 1B shows the FT-IR spectra of these samples. Typical bands related to PVP at 3422 (m OAH), 2953/2918/2870 (m CAH), 1652 (m C@O), 1495/1462/1441/1423 (d CAH ring), 1375 (d CAC ring) and 1290 cm 1 (m CAN tertiary amide) were detected for the samples before and after the heat treatment till 350 °C [30]. At 400 °C and higher temperatures these bands disappear, indicating the degradation of PVP. This feature is corroborated by the TGA measurements of PVP and Ni-1/6 sample (Fig. S1, Supplementary Material), which shows that the degradation of the polymer starts at around 380 °C. Correlating XRD and FT-IR data, it is clearly seen the crucial role of the PVP-stabilizer to the metastable hcp phase
occurrence, where the degradation of the PVP lead to fcc phase. Despite at 400 °C the PVP was degraded and hcp-Ni phase is still present, it completely disappears with higher times of heat treatment at that temperature (data not showed). The degradation of PVP in Ar atmosphere led to amorphous carbon, as seen in the Raman spectra of the samples heated from 400 to 700 °C (Fig. S2), which exhibit the typical D (1365 cm 1) and G (1575 cm 1) bands. However, there is a narrowing and a shift of these bands with increasing of the temperature, from 1365 to 1335 cm 1 and from 1575 to 1590 cm 1, for the D and G bands, respectively. This behavior is related with a graphitization process induced by higher temperatures [31]. The combination of the XRD, FT-IR and Raman data indicate that hcp-Ni phase can be obtained at controlled thermal treatment of the fcc-Ni phase, and that the presence of PVP is essential to stabilize the metastable hcp-Ni phase. Also, at thermal treatment above 400 °C, a fcc-Ni/ amorphous carbon composite material is obtained. As the presence of PVP is essential to obtain stable hcp-Ni nanoparticles, samples with different Ni/PVP ratio have been heat-treated at 350 °C. It is important to notice here that all the experiments have been prepared with PVP keeping exactly the same molecular weight. Although the effect of the molecular weight of the PVP was not evaluated here, it probably could have influence on the fcc/hcp ratio of the obtained samples. Before the heat-treatment, all samples presented the same XRD profile, indicating that the amount of PVP do not change significantly the size and the structure of the Ni nanoparticles (Fig. 2A). However, the absence of PVP in the sample led to peaks related to NiO, as described elsewhere [20]. The XRD patterns for these samples after the heat treatment are shown in the Fig. 2B, showing the evident influence of the stabilizer in the structure of the Ni nanoparticles, in which increasing amounts of PVP produces samples with higher amount of hcp-Ni. These data indicate that not only the presence of PVP is important to stabilize the hcp phase, but also the amount of the polymer. The inset of the Fig. 2B shows the percentage of hcpNi obtained in function of the proportion of PVP in the precursor sample, in which the lower amount of fcc-Ni is found in the more-PVP containing Ni-1/8-ht sample. The stability of the hcpNi presented in the sample Ni-1/8-ht was evaluated, keeping the
Fig. 1. (A) X-ray diffractograms and (B) FTIR spectra of Ni-1/6 sample, before and after the heat treatment (from 250 to 700 °C). The FT-IR spectra for the samples heat-treated at 600 and 700 °C (not shown) were similar to the sample heated at 500 °C.
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Fig. 2. (A) X-ray diffractograms of non-PVP–Ni, Ni-1/1, Ni-1/2, Ni-1/4, Ni-1/6 and Ni-1/8, and (B) non-PVP–Ni-ht, Ni-1/1-ht, Ni-1/2-ht, Ni-1/4-ht, Ni-1/6-ht and Ni-1/ 8-ht obtained after the heat treatment at 350 °C. The inset in (B) shows the percentage of hcp Ni with the Ni/PVP ratio (obtained by the ratio between the peaks at 41.5° and 51.7° attributed to hcp and fcc Ni, respectively).
sample for 7 months stored under air atmosphere, and it was confirmed by XRD, in which no detectable amounts of other species (as fcc-Ni or NiO) were detected, as seeing in Fig. S3. Fig. 3 shows the transmission electron microscopy images of the samples Ni-1/1, Ni-1/8, Ni-1/1-ht and Ni-1/8-ht. It is clearly seen the presence of Ni nanoparticles (higher contrast regions in Fig. 3A, C, E and G) surrounded by the polymer (regions of less contrast) in all the samples. The presence of the crystalline nanoparticles is more evident by the bright regions in the dark field images (Fig. 3B, D, F and H). The size distribution histograms for the nanoparticles in each sample reveal a growth for both samples after the heat treatment, from 2.6 to 9.5 nm and 2.7 to 8.9 nm for the Ni-1/1-ht and Ni-1/8-ht, respectively. The sample Ni-1/8 was also submitted to different periods of heat treatment at 300 and 350 °C. The XRD patterns of the samples after the heat treatment at 300 °C (Fig. S4A) show that the heating time did not affect the hcp/fcc Ni ratio. Opposite behavior was observed at thermal treatment of 350 °C (Fig. S4B), in which higher thermal treatment times led to increasing of the fcc-Ni amount.
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These behaviors are related with the degradation of PVP, as seen in the FT-IR spectra of these samples (Fig. S5). At higher times of heat-treatment at 300 °C the PVP was not completely degraded, but the increase of the time of heat-treatment at 350 °C led to the disappearance of the polymer bands, confirming again the important role of PVP on the stabilization of the hcp-Ni. Fig. 4C shows a photography of the thin films of the samples Ni-1/1-ht and Ni-1/8-ht prepared through a modification of the liquid–liquid interfacial route developed in our group [26–29]. This is a very interesting route to processes different kind of materials as thin films, in which the solid sample originally dispersed in one of the immiscible liquids tends to spontaneously migrate to the liquid–liquid interface in order to minimize the interfacial tension, leading to self-assembled thin films easily transferable to several substrates. As can be seen in the Fig. 4C, the hcp-Ni thin film possesses the higher transmittance, which is confirmed by the UV–Vis spectra of these samples (Fig. S6), in which the hcp and fcc Ni thin films presented 96% and 77.8% of transmittance at 550 nm, respectively. This feature can be associated to the morphology of the films. Fig. 4A, B, D and E shows the SEM images of these thin films, demonstrating that hcp-Ni thin film is homogeneously distributed into the substrate, while the fcc-Ni film presents a large amount of agglomerated nanoparticles, decreasing its transmittance. Also, the higher metal/PVP amount in the fccNi samples should contribute to the decrease in its transmittance. The electrochemical characterization and the sensor application of the hcp-Ni and fcc-Ni thin films were done in alkaline medium, leading to the development of the Ni(OH)2/NiOOH redox pair. Fig. 5A shows the 150° voltammetric cyclic for both thin films, cycled in a 1 mol L 1 aqueous solution of NaOH. Both thin films presented the Ni(OH)2/NiOOH redox pair, indicating that the presence and the amount of PVP does not prevent the occurrence of these electroactive species [20]. These electrodes also showed the same current peak intensity, suggesting the same amount of active sites. Fig. 5B and C shows both the current of the anodic peak and the half-wave anodic potential in function of the number of cycles, respectively. It is clearly seen in the Fig. 5B the continuous formation of Ni(OH)2 for both films during the cycling. However, the hcpNi film produces a higher amount of active sites in the first cycles and reaches a stable current peak after 70 cycles, while the fcc-Ni film leads to a lower amount of these active sites in the first cycles and does not present a stable current peak until the 150° cycle. This behavior suggest that the hcp-Ni thin film has a higher reactivity to produce Ni(OH)2, while the fcc-Ni film take more time to produce Ni(OH)2 and it should lead to a higher amount of active sites as it possess a higher amount of Ni. For both films it is also seen a higher half-wave anodic potential for the first cycles (Fig. 5C), which can be associated to the presence of the PVP on the surface of the nanoparticles, making the oxidation process difficult in the beginning of cycling. In addition, there is a shift for higher potentials after the tenth cycle (10 and 21 mV for fcc-Ni and hcp-Ni thin films, respectively). This phenomena indicate a modification of Ni(OH)2 for a more organized structure. The structure modification is described by the Bode diagram, which relate all the Ni(OH)2 and NiOOH structures and reports an organization of the electroactive species with the aging of the electrode (Fig. S7) [21]. Other interesting information is the lower anodic peak potential for the hcp-Ni film (422 and 438 mV for hcp-Ni and fcc-Ni thin films, respectively). All these data together indicate that the Ni(OH)2 obtained from the hcp-Ni film possess a structure more electroactive, like the a-structure, which is more disorganized and have a higher distance between the Ni(OH)2 layers than the b structure. Both the thin films were previously cycled 150 times at 50 mV s 1 in 1 mol L 1 NaOH, and further cycled at different scan rate (from 5 to 300 mV s 1). The data are presented in Fig. S8. An
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Fig. 3. TEM images of Ni-1/1 (A, B), Ni-1/1-ht (E, F), Ni-1/8 (C, D) and Ni-1/8-ht (G, H). The images in dark field (B, D, F, H) correspond to the same region of the images in bright field (A, C, E, G). The insets in A, C, E and G show the size distribution histograms of the samples, where 300 nanoparticles were individually measured for each sample.
Fig. 4. SEM images of the hcp-Ni Ni-1/8-ht (A, B) and fcc-Ni Ni-1/1-ht (D, E) thin films deposited over ITO substrates. (C) Photograph images of the hcp-Ni (left) and fcc-Ni (right) thin films, covering the upper-half of the electrodes.
increase of the peak current and DE value with the increase of the scan rate was observed in both films, as expected. The linear dependence between the current peak and v1/2 indicate a diffusion-controlled process for both films in the whole scan rate range analyzed. The electrocatalytic behavior for the glycerol oxidation was evaluated for hcp-Ni and fcc-Ni thin films (Fig. 6A and B, respectively) in the absence and presence of 1, 2 and 3 mmol L 1 of glycerol after the pretreatment cited before. Both materials presented an electrocatalytic behavior for this analyte demonstrated by the anodic peak increment and the cathodic peak decrease. This phenomena happens because the catalytic species (NiOOH) oxidizes
the glycerol and regenerate the Ni(OH)2 (chemical step). This Ni(OH)2 is re-oxidized by the anodic potential that is applied when the chemical step happen, leading to a current increment. The decrease of current occurs because the NiOOH was consumed by the chemical step. However, the hcp-Ni thin film showed higher current increments for all amount of glycerol added to the system (105, 95 and 97 lA, for the first, second and third additions, respectively) than the fcc-Ni thin film (76, 70 and 75 lA, for the first, second and third additions, respectively). This higher performance of the hcp-Ni thin film could be related to a more electroactive structure formed in this electrode. The appearance of a peak at about 0.5 V can be associated to the oxidation of the products
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Fig. 5. (A) 150° cyclic voltammograms for the (a) fcc-Ni and (b) hcp-Ni thin films in 1 mol L 1 of NaOH. (B) Current of the anodic peak and (C) half-wave anodic potential in function of the number of cycles for both thin films. The cyclic voltammograms were collected at 50 mV s 1.
Fig. 6. Cyclic voltammograms for hcp-Ni (A) and fcc-Ni (B) thin films in the (a) absence of glycerol and in the presence of 1 mmol L 1 (b), 2 mmol L 1 (c) and 3 mmol L 1 (d) of glycerol in 1 mol L 1 of NaOH. The inset in (B) corresponds to the cyclic voltammogram of bare ITO electrode in the presence of 3 mmol L 1 of glycerol in 1 mol L 1 of NaOH. The cyclic voltammograms were obtained using 50 mV s 1.
originated from the first glycerol oxidation [32]. In the inset of Fig. 6B is presented the cyclic voltammograms of bare ITO in the presence of 3 mmol L 1 of glycerol, in which no electrocatalytic behavior is observed in the absence of the Ni nanomaterials. Both thin films were applied as chronoamperometric sensors to glycerol. For that, the electrodes were first submitted to the pretreatment cited before and then a chronoamperometry using 0.46 V was carried out with additions of glycerol in the concentration range from 1 to 500 lmol L 1 (Fig. 7A and B). In the whole concentration range can be seen an increment of current with the glycerol additions for both thin films. In the Fig. 7C is showed the analytical curves constructed from the current increment vs. the glycerol concentration for these thin films. Both materials exhibited two linear regions with different sensitivities: the first
from 1 to 20 lmol L 1, with 0.258 and 0.186 lA L lmol 1, and the second from 25 to 500 lmol L 1, with 0.187 and 0.146 lA L lmol 1 to hcp-Ni and fcc-Ni thin films, respectively. This decrease in the sensitivity suggests a little saturation of the electroactive sites with the increase of the glycerol concentration. However, the good correlation coefficients of 0.998 and 0.995, for the first region, and 0.997 and 0.997, for the second region, for hcp-Ni and fcc-Ni thin films, respectively, allow the use of both regions for glycerol detection. The detection limit calculated from the first region for both electrodes were 2.4 and 4.7 lmol L 1, using the standard deviation from the blank of 0.20 and 0.29 lA, to hcpNi and fcc-Ni thin films, respectively. The higher performance of the hcp-Ni compared to the fcc-Ni suggest again a Ni(OH)2 phase more electroactive. These low detection limits associated with
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Fig. 7. Chronoamperograms obtained for (A) hcp-Ni and (B) fcc-Ni thin films using a glycerol concentration range from 1 to 500 lmol L Analytical curves constructed from the chronoamperograms of (a) hcp-Ni and (b) fcc-Ni thin films.
the wide detection range are ones of the best values described for the electrochemical detection of glycerol. 4. Conclusions In conclusion, this work reports the preparation of PVPstabilized hcp-Ni and fcc-Ni nanoparticles through the polyol method, in which both the temperature of the heat treatment and the PVP ratio in the nanoparticles were the main parameters to control the hcp/fcc rate in the samples. The hcp-Ni nanoparticles showed a high stability in air atmosphere. Thin films of these materials were prepared through a biphasic system and electrochemically converted to Ni(OH)2, where the results suggest a more electroactive Ni(OH)2 phase from the hcp-Ni nanoparticlesmodified electrode. Both hcp-Ni and fcc-Ni nanoparticlesmodified electrodes were applied as amperometric sensors for glycerol, where the best result were achieved with the hcp-Ni with a detection limit of 2.4 lmol L 1 and a glycerol detection range of 1–500 lmol L 1. Acknowledgements The authors acknowledge the financial support by CNPq, CAPES, NENNAM (PRONEX, Fund. Araucária/CNPq), National Institute of Science and Technology of Carbon Nanomaterials (INCT – Nanocarbon), and the CME-UFPR by the TEM images. E.G.C.N. thanks CAPES for a research fellowship. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2016.01.036. References [1] A.P.R. Mary, C.S.S. Sandeep, T.N. Narayanan, R. Philip, P. Moloney, P.M. Ajayan, M.R. Anantharaman, Nonlinear and magneto-optical transmission studies on magnetic nanofluids of non-interacting metallic nickel nanoparticles, Nanotechnology 22 (2011) 375702. [2] O. Pascu, J.M. Caicedo, J. Fontcuberta, G. Herranz, A. Roig, Magneto-optical characterization of colloidal dispersions. Application to nickel nanoparticles, Langmuir 26 (2010) 12548–12552. [3] Ö. Metin, V. Mazumder, S. Özkar, S. Sun, Monodisperse nickel nanoparticles and their catalysis in hydrolytic dehydrogenation of ammonia borane, J. Am. Chem. Soc. 132 (2010) 1468–1469. [4] J. Aguilhon, C. Boissière, O. Durupthy, C. Thomazeau, C. Sanchez, Nickel nanoparticles with controlled morphologies application in selective hydrogenation catalysis, Stud. Surf. Sci. Catal. 175 (2010) 521–524. [5] Y.J. Mai, J.P. Tu, C.D. Gu, X.L. Wang, Graphene anchored with nickel nanoparticles as a high-performance anode material for lithium ion batteries, J. Power Sources 209 (2012) 1–6.
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