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Characterization and application of Cu based superhydrophobic catalyst
Progress in Natural Science: Materials International xxx (xxxx) xxx–xxx HOSTED BY
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Characterization and application of Cu based superhydrophobic catalyst Ramasamy Anbarasana,c,∗, Shanmugam Palanikumarb, Ayyadurai Anitha Devib, Ping-Hei Chenc, Kuo Lun Tungc,∗∗ a b c
Department of Polymer Technology, Kamaraj College of Engineering and Technology, Virudhunagar, 626 001, Tamilnadu, India Department of Mechanical Engineering, MEMS Thermal Control Lab, National Taiwan University, Taipei, 10617, Taiwan Department of Chemical Engineering, National Taiwan University, Taipei, 10617, Taiwan
A R T I C LE I N FO
A B S T R A C T
Keywords: Superhydrophobicity Cu salts Characterization Catalytic reduction Water contact angle
Superhydrophobic (SH) Copper salt was prepared with the help of three different chemical etching agents and characterized by various analytical techniques like Fourier transform infrared (FTIR) spectroscopy, UV–visible spectroscopy, X-ray diffractogram (XRD), X-ray photoelectron spectroscopy (XPS), high resolution transmission electron microscopy (HRTEM) and scanning electron microscopy (SEM). The SH nature of the material was confirmed by water contact angle (WCA) measurement. Cu surface with the etching agent was made to produce SH Cu surface. Among the chemical etching agents the system with Cl, F and Si exhibited the SH character. The SH hierarchically structured heterogeneous materials produced were used as a catalyst for the reduction of Cr (VI) and 4-nitrophenol (NiP). The catalytic activities of the materials were compared on the apparent rate constant (kapp) and induction time (Ti). Among the catalyst systems used, the Cu-Trichoropentaflourooctylsilane (TCPFOSi) system exhibited the highest kapp and Ti values.
1. Introduction
was followed for the fabrication of SH copper surface [15]. The literature analysis indicates that the fabrication of SH surface is an easy process but unfortunately the applications of side products formed during the fabrication of SH surface is not found. Hence, in the present investigation we would like to extend the application of SH material in the catalysis field as a low cost catalyst material. Due to the SH nature of the catalyst, the isolation of catalyst after the completion of the reaction is more easy and the same can be reused for many times. Above all, the hierarchical structure of the catalyst offers more surface area and the same can be effectively used for any reaction. The primary importance is given for public health and environmental safety. This is because of modernization, industrialization, increase in population and luxurious life style of human beings. Nitrophenolic compounds are widely used particularly in pharmaceutical industry as one of the starting materials for the manufacture of drug molecules. The effluents from pharmaceutical industry contains lot of NiP, a toxic pollutant and when it is discharged to open environment it pollutes water and soil and make it unfit for the domestic application. So, it is necessary to develop a novel methodology to decompose or to reduce the toxicity. The simple and easiest way to reduce the toxicity is reduction of NiP into aminophenol. Different catalytic systems were used because the reduction of NiP follows the pseudo first
A superhydrophobic (SH) surface is defined as a surface with the static water contact angle of > 150° and it is used for surface self cleaning application. High water contact angle, flower like morphology and high reflectance value are the some of the requirements of SH surface. The SH surface can be generated by various methodologies and discussed below. A facile spin coating method was used for the fabrication of SH surface [1]. In 2012, Chen et al [2] explained the surface wettability effects on critical heat flow of boiling heat transfer. An inexpensive template based SH polymer surface was reported in the literature [3]. Phase separation methodology was used to create SH polymer surface [4] Precursor method was used for the generation of SH surface [5]. Sol-gel method was adopted for SH glass substrate [6]. Etching and electro deposition methodology was followed to create SH surface [7]. Ag nanoparticles were used to tune the SH [8]. In 2013, Sribala and co-workers reported the hydrophobic copolymers [9]. Chen et al [10,11] reported about the SH Cu surface. A facile galvanic exchange reaction was followed for preparing SH nano Ag coated copper plate [12]. Chen and research team [13] fabricated the SH copper surface with anticorrosive property. SH copper hydroxide nanoneedles were synthesized by Wu et al [14]. One step solution immersion process
∗
Corresponding author. Department of Polymer Technology, Kamaraj College of Engineering and Technology, Virudhunagar, 626 001, Tamilnadu, India. Corresponding author. E-mail addresses: [email protected] (R. Anbarasan), [email protected] (K.L. Tung).
∗∗
https://doi.org/10.1016/j.pnsc.2019.08.002 Received 22 January 2019; Received in revised form 25 July 2019; Accepted 4 August 2019 1002-0071/ © 2019 Chinese Materials Research Society. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Please cite this article as: Ramasamy Anbarasan, et al., Progress in Natural Science: Materials International, https://doi.org/10.1016/j.pnsc.2019.08.002
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from the ethanolic solution and dried in an oven at 60 °C for 30 min. Weight of the dried Cu plate was noted. During the etching process the surface of the Cu plate was eroded by ethanol medium and coated with the etching agent as a micro film. This leads to the not only surface properties but also the weight of Cu plate. As a result hierarchical structured Cu plate was fabricated. In this case, a nano structured material is coated on the micro structured substrate material. From the difference in weight of the Cu plate, the chemical etching rate was calculated using the following formula [11].
order kinetics. In 2012, Maheswaran and research team [16] reported the reduction of NiP with the help of Au nanoparticles. The reduction of NiP was reported by Lin and co-workers [17]. A bimetallic layer of Pd and Ag was used for the reduction of NiP [18]. Babji and research team [19] studied the catalytic reduction of NiP by using Fe3+ and Ag+ codoped TiO2 nanoparticle. ZnO loaded Co–Se nanocomposites were used as a catalyst for the reduction of NiP [20]. Similarly, various catalyst like Ni/Al2O3 [21], fullerene nanowhiskers-Au nanoparticle [22], Fe3O4@NiCo@Ag [23], Au nanoparticles decorated ceria nanotube [24], Fe nanoparticle-aminoclay [25], ceria microspheres embedded with Pt nanoparticles [26], Cu nanoparticles [27], SnO2 coated metal nanoparticle [28], layered double hydroxides [29] and Fe nanoparticle immobilized mesoporous silica [30] were used for the reduction of NiP. By thorough literature survey, no report based on the catalytic reduction of NiP using hierarchical architectured catalyst was found. This urged to do the present investigation keeping the quality of environment in mind. Chromium compounds are used in analytical work as a primary standard, chrome plating industry and in tannery as a tanning agent. The effluent from these industries contains larger amount of Cr(VI) (highly toxic) and polluted the water and soil, affecting the human's health. In order to avoid the environmental pollution various methodologies are adopted. Photocatalytic reduction of hexavalent Cr was studied by Assadi et al [31]. TiO2 semiconductor catalyzed photoreduction of Cr(VI) was reported in the literature [32]. For the reduction of Cr(VI) into Cr(III) various catalyst and reducing agent systems such as goethite [33], Pd nanoparticle [34], cell cytoplasm [35], electrolytic reduction [36], pseudomonas sp. JF 122 [37], Fe(II) [38], Fe3O4 [39], antioxidants [40], Mn(II) [41], Kaolin immobilized ZnO nanorods [42], flavanoids [43], nano structured poly(amicacids) [44] and Pd nanoparticle supported amine functionalized SiO2 [45] were used. During the literature review process, no any report on the hierarchical structured material as a catalyst for the reduction of Cr(VI) was found. This motivated the researcher to do the present investigation. The novelty of the present investigation is effective utilization of hierarchical structured Cu salt catalyst, received as a by-product, towards the reduction of Cr(VI) and NiP in the presence of NaBH4 as a reducing agent and the study of influence of superhydrophobicity on the rate constant.
VB = Δm / 2Aρt Δm - mass difference, A - etch surface area, ρ - density of plate, t etching time. 2.3. Catalytic reduction study
2. Experimental
Before the reduction reaction, the standard solution of NiP (1.1 × 10−5 M) was prepared using DDW as a solvent taken in a 3 mL capacity cuvette reactor. 15 mg of NaBH4 was added to the cuvette reactor with the addition of 1 mg of hierarchically structured catalyst. The contents were shaken well for a second and subjected to the UV–visible spectral measurement [46]. The absorbance at 400 nm was measured at room temperature in a regular 1 min time interval. The UV–visible spectrum was recorded till the absorbance at 400 nm becomes negligible. A similar procedure was followed for the catalytic reduction of Cr(VI). For the catalytic reduction of NiP or Cr(VI), control experiment is necessary. In the absence of catalyst, NiP was not reduced. The catalytic reduction NiP follows the pseudo first order kinetics. In the case of Cr(VI), in the absence of catalyst, a control experiment was carried out and the kapp value was calculated as 1.08 × 10−4 sec−1. In the presence of catalyst also the catalytic reduction of Cr(VI) was carried out and the kapp values were calculated from which the control value was subtracted. The subtracted kapp values are already displayed here.(Scheme 1). A standard PDC (5.1 × 10−6 M) solution was prepared. From the standard solution, 2 mL PDC solution was pipette out into a 3 mL capacity cuvette reactor. Then, 15 mg of NaBH4 and 1 mg catalyst were added with a cuvette reactor under vigorous shaking. The reduction of Cr (VI) into Cr(III) was quantitatively followed with the help of a UV–visible spectrophotometer. The absorbance at 371.8 nm was followed with 30 s time interval [47].
2.1. Materials
2.4. Characterization
Potassium dichromate (PDC, Nice chemicals, India) and 4-nitrophenol (NiP, CDH chemicals, India) were purchased and used as received. Copper plate with the purity of 99.9% was used for this study. Ethanol (Himedia, India), Pentaflourobenzoicacid (PFBA, Himedia, AR), Trichoropentaflourooctylsilane (TCPFOSi, Aldrich, India), Stearic acid (SA, Himedia, AR) were used. Double distilled water (DDW) was used for the purpose of cleaning and solution preparation.
FTIR spectrum was taken using Shimadzu 8400 S, Japan model instrument from 4000 to 400 cm−1 by KBr pelletization method. Shimadzu 3600 NIR, Japan instrument was used for measuring solid state UV–visible spectrum. X-ray diffraction (XRD, XS08, Bruker, USA) was recorded with an advanced instrument, scanning from the 2θ value
2.2. Synthesis of hierarchial structured Cu plate An etching is defined as a process by which a layer of the surface of a material is removed by using a chemical (is known as etching agent) for certain period of time. As a result of etching, the surface property of a material is changed. First, the Cu plate {7.5 cm (length or height) × 2.5 cm (breadth) × 0.61 cm (thickness)} was rubbed using an emery sheet then washed with DDW and excess acetone for removing impurities present on the surface of Cu plate. It was placed in an open atmosphere for air drying. The initial exact weight of the surface cleaned Cu plate was noted. The plate was immersed into a 250 mL capacity beaker containing 200 mL ethanol mixed with 1 g of etching agent (SA or PFBA or TCPFFOSi) at room temperature for 24 h [10]. After the definite interval of etching time, the Cu plate was taken out
Scheme 1. Catalytic reduction of 4-NiP and Cr(VI). 2
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of 10–80° at a scanning rate of 2° min−1. The surface morphology of the samples was scanned by SEM (JSM 6300, JEOL, USA model) instrument. Water contact angle (WCA) was measured by Kyowa DMs-200, Japan model goniometer instrument. The WCA was measured in 10 places of the same plate and the average value was considered with 5% error. The HRTEM image was recorded for all the three systems by High resolution transmission electron microscope (HRTEM) measurements for the samples were carried out using JEM-200 CX, USA transmission electron microscope instrument. For reduction study, both PDC and NiP were dissolved in water and the catalyst was used as a heterogeneous one. The binding energy was determined by XPS, (XPS, Thermo Scientific, Theta Probe, UK).
PFBA system. This system exhibits the rod like morphology. The length of the rod was found to be 5–8 μm whereas the breadth of rod was 1–2.5 μm. Fig. 2b indicates the surface morphology of Cu-stearate system. Here also one can see the rod like morphology [10] with the length of 5–15 μm. Fig. 2c denotes the SEM image of Cu-TCPFOSi system. The present system exhibits an entirely different surface morphology with the rod shape and with the size of ∼1 μm. Among the three systems, the third system exhibits an entirely different morphology due to the absence of formation of metal salt. These distorted, agglomerated, spherical and micro sized particles are responsible for the chemically etched Cu particles. Generally, the flower like morphology alone will give the highest water contact angle. The present system inferred that the spherical shaped material can also exhibit the highest water contact angle evidenced by the water contact angle measurement. Elements present in the system can be identified by EDX spectrum. Fig. 2 (d, e, f) indicates the EDX spectrum of Cu-PFBA system, Custearate and Cu-TCPFOSi systems respectively. Fig. 2d gives the percentage of C, F, O and Cu as 49.47%, 33.95%, 12.82% and 3.77% respectively. The Cu-stearate system (Fig. 2e) exhibited the percentage of C, O and Cu as 89.4%, 8.68% and 1.84% respectively. The Cu-TCPFOSi system (Fig. 2f) gives the percentage of the elements Cu, C, Cl, F, O and Si as 62.92%, 23.59%, 6.22%, 4.16%, 1.5% and 0.81% respectively. In comparison, both PFBA and TCPFOSi system contains fluoride ions and exhibited the higher water contact angle. In the case of TCPFOSi system, in addition to F and Cl, Si was also present in considerable quantity. From the EDX analysis one can come to a conclusion that the chemically etched Cu particle which is coated on the surface of Cu plate leads to the SH nature. The HRTEM image of system-1 (Cu-PFBA) is given in Fig. 3a. One can see the agglomerated spherical nanoparticle with some layered structure (or rod shaped) corresponding to the various crystal planes of Cu nanoparticle. The length of the crystal plane was calculated as 5–7 nm with the breadth of < 1 nm. The crystal planes are arranged in various possible directions. This confirmed the multi-orientation of crystal planes. The arrow mark indicated the same. Fig. 3b represents the HRTEM image of system-2 (Cu-SA). System-2 also exhibits the rod or layer like structure with the length of 2–7 nm. But the layers are arranged in parallel and perpendicular directions. This confirmed the bi-orientation of the crystal planes. System-2 does not show the nanospheres [11]. The HRTEM image of system-3 (Cu-TCPFOSi) is shown in Fig. 3c. Here also one can see the layered structure but they are aligned in a particular direction. This declared the uni-orientation of crystal planes. In comparison, one can say that while changing the structure of the etching agent the crystal planes alignment of Cu nanoparticle is disturbed. This can be further supported with the water contact angle measurement. The hierarchical structure of the materials prepared in the present investigation was confirmed by XPS. Fig. 4 (a) indicates the XPS of system 1. The C1s, O1s and F1s of PCBA appeared at 282.9, 532.7 and 684.5 eV respectively. The Cu2p3/2 and Cu2p1/2 peaks appeared at 931.4 and 950.7 eV respectively [11]. The appearance of Cu2p3/2 and Cu2p1/2 confirmed the Cu is in zero oxidation state. But the intensity of the peaks is found to be very low. This indicates that the nano sized Cu is also present in the system-1. From this one can come to a conclusion that the synthesized material contains both nano and micro sized particles. This proved the hierarchical nature of system-1. Fig. 4 (b) represents the XPS of Cu-SA system (system-2) with peaks corresponding to C1s, O1s, Cu2p3/2 and Cu2p1/2 and hierarchical structure. Fig. 4 (c) indicates the XPS of system-3. Some new peaks corresponding to Si (Si2p-100.2 and Si2s-154.4 eV) and Cl (Cl2p-208.5 and Cl2s-274.3 eV) confirmed the hierarchical structure of system 3 from its Cu2p peak intensities. The hydrophobic nature of various chemically etched Cu plates was measured by water contact angle. The Cu-PFBA system (Fig. 5a) exhibited the water contact angle of 147.8°. The Cu stearate (Fig. 5b)
3. Results and discussion For a comparative study, the present results and discussion part are sub-divided into three parts namely, i) Characterization of hierarchical structured catalyst, ii) Catalytic reduction of Cr(VI) and iii) Catalytic reduction of NiP. 3.1. Characterization of hierarchical structured catalyst The FTIR spectrum is used to confirm the functional groups present in the Cu salts. Fig. 1a indicates the FTIR spectrum of Cu-PFBA system. A broad peak around 3406 cm−1 was due to the OH stretching of PFBA. The aromatic C]O stretching of PFBA appeared at 1670 cm−1. Due to the presence of long aliphatic methylene groups, the carbonyl stretching is blue shifted. The C–O–C stretching appeared at 1042 cm−1 [47]. Fig. 1b indicates the FTIR spectrum of Cu-SA system. The formation of Cu-stearate during the simple etching reaction was confirmed by FTIR spectroscopy. The existence of Cu co-planarity [48] appeared as a twin peak at 3362 and 3458 cm−1. This confirmed that during Cu salt formation the Cu atom was aligned on the plane. The symmetric and antisymmetric stretching of SA was noticed at 2859 and 2924 cm−1 respectively. The C]O stretching of SA appeared at 1681 cm−1. The C–H out plane bending vibration was seen at 819 cm−1. The Cu–O stretching appeared at 529 cm−1 [49]. Thus the FTIR spectrum confirmed the Cu-stearate formation. Fig. 1c represents the FTIR spectrum of Cu-TCPFOSi system. The –OH and C–H stretchings appeared at 3377 cm−1 and 2915 cm−1 respectively. The Cu–Si appeared at 1194 cm−1. Thus the FTIR spectrum confirmed the functional groups present in the systems. The surface morphology of metal salts gives an idea about the hydrophobic nature. Fig. 2a represents the surface morphology of Cu-
Fig. 1. FTIR spectrum of (a) Cu-PFBA, (b) Cu-SA and (c) Cu-TCPFOSi system. 3
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Fig. 2. SEM images of (a) Cu-PFBA, (b) Cu-SA and (c) Cu-TCPFOSi system and EDX spectra of (d) Cu-PFBA, (e) Cu-SA and (f) Cu-TCPFOSi system.
exhibited the water contact angle of 129°. The Cu-TCPFOSi system (Fig. 5c) exhibited the maximum water contact angle of 170.9°. Among the three, the later exhibited the highest water contact angle because (i) In addition to fluoride ion the chloride ion helps to make the etching agent more ionic, particularly more electro negative. (ii) Naturally, the TCPFOSi is water immiscible. (iii) The medium size length of octyl group maintains the high electro negativity unlike SA. iv) uni-orientation of various crystal planes (evidenced by HRTEM images). Even though the SA is having 17 methylene units in its backbone the electro negativity is found to be very low. Above all, the TCPFOSi system didn't form metal salt. It is not compulsory to produce the hydrophobic surface by coating the metal salt on the Cu or any other substrate. The simple chemical etching for a required time can create a hydrophobic surface. The present investigation indicates that the TCPFOSi system produced the hydrophobic Cu surface due to the simple etching reaction. Generally, a material with flower like morphology or water contact angle of greater than 160° or nanosized particle can exhibit SH character. In the present investigation, the system with high water contact angle due to the more electronegativity exhibits SH character. When compared with the literature value [10], the present system
Fig. 4. XPS of (a) Cu-PFBA, (b) Cu-SA and (c) Cu-TCPFOSi system.
Fig. 3. HRTEM image of (a) Cu-PFBA, (b) Cu-SA and (c) Cu-TCPFOSi system. 4
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Fig. 5. Water contact angle of (a) Cu-PFBA, (b) Cu-SA and (c) Cu-TCPFOSi system.
Chen et al [10]. The UV–visible reflectance spectra of the Cu salts catalyst are given in Fig. S4. The reflectance spectrum of Cu-PFBA system is given in (Fig. S4) with the maximum reflectance value of 17%. This indicates the hydrophilic nature of Cu-PFBA system. This is further evidenced by the water contact angle measurement. Fig. S4b indicates the UV–visible reflectance spectrum of Cu-stearate system with the maximum reflectance value of 87.8%, due to the hydrophobic nature of the system. Fig. S4c represents the reflectance spectrum of Cu-TCPFOSi system with the maximum value of 62.6%. Among these three systems the chemical etching due to the SA exhibited the highest reflectance due to the formation of hydrophobic Cu salt. The recent literature49 indicates that a system with maximum reflectance value can exhibit the SH character. It can be further confirmed by the water contact angle measurement.
exhibited higher water contact angle value due to the high electro negativity. The XRD pattern of various Cu salts denoted the various crystalline planes of corresponding Cu salts. Fig. S1a indicates the XRD of Cu-PFBA system. The system exhibited many crystalline peaks from 15 to 50°. This confirmed the crystalline nature of Cu-PFBA system. A peak corresponding to d111 crystal plane of Cu appeared at 42.8° in accordance to the literature report [50]. The appearance of other crystal planes confirmed the crystalline nature of Cu-PFBA system. Fig. S1b indicates the XRD pattern of Cu-stearate system. The system exhibited no more peaks between 10 and 80° and confirmed the amorphous nature of Custearate system. Fig. S1c indicates the XRD pattern of Cu-TCPFOSi system. The present system exhibits more sharp crystalline peaks corresponding to d110 (32.2°), d202 (49.1°) and d200 (51.3°). This proves that Cu is present in salt form. Among the three systems the Cu-stearate system exhibited the lowest crystallinity due to the amorphous nature. The amorphous or crystalline nature of Cu salt depends on the nature of the etching agent used for chemical etching reaction. The results of XRD coincide with the results of Chen et al [51]. The influence of three different chemical etching agents and surface of the Cu plate at different time interval are represented in Fig. S2. Fig. S2 shows the plot of difference in mass Vs time. Fig. S2a indicates the bulk etching rate of Cu-PFBA system. For 175 h of etching, the system exhibited the maximum weight loss of 48.2 mg. This can be explained as follows. (i) The presence of five fluorine atom makes the metal salt more electro negativity. (ii) The presence of five fluorine atoms leads to the formation of CO2− ion. These negative ions can readily interact with the Cu2+ of Cu metal plate to form the Cu-PFBA salt and so this system yields more weight loss. Fig. S2b indicates the bulk etching rate plot of Cu-stearate system. This system exhibits 25.9 mg weight loss at 175 h. This can be explained as follows. SA is having 16 methylene repeating units in its structure and exhibits poor electro negativity. Hence, the formation of CO2− ion is difficult and there is a moderate bulk etching rate. Fig. S2c represents the bulk etching rate plot corresponding to Cu-TCPFOSi system. This system exhibits 18.6 mg weight loss at 175 h of chemical etching. Among the three systems, the CuTCPFOSi system exhibited the lowest weight loss. Even though the TCPFOSi contains five fluorine atoms in its structure there is no Cu salt formation. Instead of metal salt formation, the TCPFOSi simply deposited on the surface of Cu plate. Due to the five electro negativity fluoride ion, the etching also occurred at a mild rate [11]. Due to the absence of metal salt formation, the chemical etching was very low. This inferred that the chemical etching is not only accelerated by the electro negativity but also by the metal salt formation. In 1995, Bryce and co-workers [52] explained the chemical etching of Cu in FeCl3 solution. When compared with the literature report, the present system yielded a better result. The UV–visible spectrum is used to explain the various possible electronic transitions present in the Cu salts catalyst. A peak at 265.1 nm (Fig. S3a) represents the π→ π* transition of the phenyl ring of PFBA. Fig. S3b indicates the UV–visible spectrum of Cu-stearate system and is found no peak. Fig. S3c represents the UV–visible absorption spectrum of Cu-TCPFOSi system. A broad peak at 273.6 nm represents the n→ π* transition of TCPFOSi. This is in accordance with
3.2. Catalytic reduction of Cr(VI) The Cr(VI) is a toxic and hazardous and hence some action has to be taken against Cr(VI) pollution. The better way is to convert the Cr(VI) into Cr(III) with the help of a suitable catalyst. In the present investigation, the above synthesized SH, bio-compatible and eco-friendly Cu salts catalyst was used for the reduction purpose. The primary aim of the present investigation is conversion of waste side product into a useful catalyst. 2 mL of PDC solution was pipeted out in to a cuvette nano reactor, with this 1 mg of system 1 and 15 mg of NaBH4 were added and stirred well. Now the Cr(VI) is reduced into Cr(III) and the reaction is followed with the help of UV–visible spectrophotometer. The UV–visible spectrum was recorded for 30 s time interval (Fig. 6a-l). The peak absorbance was found at 371.8 nm corresponding to decrease of Cr(VI) concentration. This was due to the conversion of Cr(VI) into Cr(III). The decrease in absorbance at 371.8 nm confirmed the catalytic reduction of Cr(VI) into Cr(III) in the presence of NaBH4. The spectrum indicated that within 2.5 min, 98% of Cr(VI) (calculated from the calibration curve) was converted into Cr(III). In order to find out the apparent rate constant (kapp), the plot of time against ln(A/Ao) (Fig. 6m) was made. The plot showed a decreasing trend. From the slope value (linear region) the kapp value was calculated as 0.58 × 10−2 sec−1 (Table 1). From the intercept value the induction time (Ti) was determined as 0.179 s. The present system yielded higher kapp value while using aminoclay supported Pd nanoparticle as a catalyst for the reduction of Cr(VI) (kapp = 3.30 × 10−3 sec−1) [45]. A plausible mechanism for the reduction of Cr(VI) is explained here. It is well known that NaBH4 is a good reducing agent. During the reduction of Cr(VI), three electrons are transferred from NaBH4 to Cr(VI) and leads to the formation of Cr(III) with simultaneous reduction of Cu(II) into Cu(0), because of its hierarchic nature. This suggests that 2 mol of NaBH4 is required to reduce 2 mol of Cr(VI), {into Cr(III)} and 1 mol of Cu salt {Cu(II) into Cu(0)}. The catalytic reduction of Cr(VI) in the presence of system 2 and 3 was also tested. From Fig. S5a-o it was found that the absorbance decreased slowly at 371.8 nm. This confirmed the catalytic reduction of Cr (VI) into Cr(III) in the presence of system 2 as a heterogeneous catalyst. The kapp was determined from the plot of time vs ln(A/Ao) (Fig. S5p) as 0.68 × 10−2 sec−1. The Ti was determined as 0.308 s. In comparison, 5
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Fig. 6. UV–visible spectrum of PDC taken at 1min time interval in the presence of Cu-PFBA hierarchical structured catalyst and NaBH4 (a–l) and plot of time against ln(A/Ao) (m).
salt with Cu whereas the TCPFOSi decorates the central Cu metal ion. Hence, the system forms core-shell like structure. Here, the main aim is not proving the core-shell like structure. Here all the systems are SH in nature with slight difference in WCA. Here the WCA depends not only on the surface roughness and morphology of Cu metal but also depends on the nature (structure) of the etching agent. In fact, F is smaller in size with high electro negativity and produces high WCA. Even then the system 3 produces high kapp when compared to the system-1. Both the systems are having F atoms in its backbone. Still system-3 has 3 more Cl atoms in its structure. Hence, the system produced high kapp value. Apart from WCA, resonance stabilization plays a vital role. This leads to high electrostatic interaction. As a result of high electrostatic interaction and resonance stabilization, system 3 exhibited high kapp with high Ti. From the present investigation, the following queue was made on the performance towards the reduction reaction. Based on kapp,
Table 1 Systems with kapp and Ti values. System
Cu-PFBA Cu-SA Cu-TCPFOSi
Code
1 2 3
kapp (sec−1)
Ti (sec)
PDC
NiP
PDC
NiP
0.58 × 10−2 0.68 × 10−2 1.60 × 10−2
2.80 × 10−2 3.93 × 10−2 5.89 × 10−2
0.179 0.308 0.319
0.427 0.532 0.821
the system 1 exhibited the lower kapp due to the low WCA. The high catalytic activity of system 2 was further confirmed by Ti value. System 1 avails 0.179 s for its activation whereas system 2 avails 0.308 s (Table 1). The higher Ti value can be explained on the basis of higher WCA. The WCA measurement was already discussed. The catalytic reduction activity of system 3 was tested. Fig. 7a–f indicates the UV–visible spectrum of Cr(VI) taken at 30 s time interval. Here also one can observe the decrease in Cr(VI) concentration at 371.8 nm. As usual, the kapp is determined from the plot of time vs ln(A/ Ao) (Fig. 7g) as 1.68 × 10−2 sec−1 (Table 1). The Ti was determined to be 0.319 s. When compared with system 2, the system 3 yielded higher kapp with slight increase in Ti. While comparing the kapp value with the literature (kapp = 3.30 × 10−3 sec−1) [45] the system-3 yielded a good value but systems-1 and 2 yielded somewhat lower value in a given experimental conditions. This indicates that the catalytic activity depends not only on the nature of the central metal core but also on the WCA, electronegativity and resonance stabilized structure of the etching agent. In overall comparison, system 3 yielded better results towards the reduction of Cr(VI). Cu metal was used as a core. The PFBA and SA form
TCPFOSi > SA > PFBA Based on Ti, TCPFOSi > SA > PFBA
3.3. Catalytic reduction of NiP The catalytic activities of Cu salts were also confirmed by the reduction of NiP. Moreover, the reduction of NiP is a standard model as it follows the pseudo first order kinetics. 2 mL of NiP solution was taken in a micro reactor followed by the addition of 15 mg of NaBH4 and 1 mg of Cu salt, system-1. The reduction study was made with the aid of UV–visible spectrophotometer quantitatively. Figs. S6a–f shows the
Fig. 7. UV–visible spectrum of PDC taken at 30 s time interval in the presence of Cu-TCPFOSi hierarchical structured catalyst and NaBH4 (a–f) and plot of time against ln(A/Ao) (g). 6
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results in a given experimental conditions explained on the basis of SH character (WCA) and electronegativity of the etching agent. From the reduction reaction of NiP in the presence of NaBH4, the following queue was made on the kapp.
UV–visible spectrum of NiP taken at 30 s time interval. The spectrum exhibits a peak at 400.7 nm corresponding to the nitro group of NiP. During the course of the reaction, the absorbance decreased at 400.7 nm and the same confirmed the reduction of NiP into aminophenol. The UV–visible spectrum confirmed that within 3 min 100% NiP was reduced into aminophenol in the presence of hierarchically structured catalyst system-1. The % conversion was determined from the calibration curve. From the absorbance value the kapp value can be determined from the plot of time vs ln(A/Ao) (Fig. S6g). The plot was found to be a line with decreasing trend. Here, the plot exhibited two regions. Up to 2 min, the reduction reaction is normal. After that the reaction was very fast due to the simultaneous reduction of Cu salt, system-1. As a result of reduction reaction, the surface area of the Cu increased and provided a surface for the reduction reaction. From the slope value the kapp value was calculated as 2.81 × 10−2 sec−1 (Table 1). The Ti was taken from the intercept value as 0.42 s. When compared with the literature [53] the present investigation yielded ∼10 times higher kapp value. The catalytic reduction of NiP into aminophenol (AP) is a well explained one since it follows the pseudo first order kinetics. During the reduction of NiP, two hydrogen atoms are transferred from NaBH4 to nitro group of NiP. Here one functional group (-NO2) is converted into another one functional group (-NH2) [53]. While using Au [54] or Ni–Pd nano dimer [55] as a catalyst for the catalytic reduction of NiP, the kapp values were determined as 3.0 × 10−3 sec−1 and 2.5 × 10−2 sec−1 respectively. Hence, when compared with the literature values, the present investigation yielded an excellent kapp value. The catalytic activity of system-2 towards the reduction of NiP was tested in the presence of NaBH4. Fig. 8a–f indicates the same. The UV–visible spectrum was taken at 30 s time interval. The absorbance decreased at 400.7 nm during the course of the reduction reaction. The kapp value was determined by plotting time vs ln(A/Ao) (Fig. 8g) as 3.93 × 10−2 sec−1 (Table 1) with the Ti value of 0.53 s. When compared with system-1, system-2 yielded high value due to the high WCA and resonance stabilization effect. Pd nanoparticles [56] and amino functionalized mesoporous silica [57] nanoparticles showed the kapp value of 2.33 × 10−3 sec−1 towards the catalytic reduction of NiP. When compared with the literature reports, the present system yielded an excellent value. Figs. S7a–f indicates the UV–visible spectrum of NiP taken at 30 s time interval in the presence of NaBH4 and system-3 as a catalyst. The reduction reaction was completed within 3 min, 99.7% of NiP converted into aminophenol. The kapp value was determined from the plot of time vs ln(A/Ao) (Fig. S7g) as 5.89 × 10−2 sec−1 (Table 1) with the Ti value of 0.82 s. The plot was found to be a straight line with decreasing trend. When compared with system 1 and 2, the current system yielded higher kapp value. This is due to the highest WCA. When compared with the literature [29,53], the present systems yielded good
TCPFOSi > SA > PFBA Based on the Ti value the queue follows: TCPFOSi > SA > PFBA In overall comparison, the NiP system yielded higher kapp value than the PDC system due to the resonance stabilization in the NiP system. The Ti value also reflected the same. Among the catalysts used, system-3 exhibited higher kapp value for both PDC and NiP systems because of higher WCA. NiP exhibits resonance stabilization through the phenyl ring and –NO2 group. The above results and discussion proved that the system-3 is a most suitable catalyst for the reduction of Cr(VI) and NiP. The efficiency and recycling stability of the catalyst plays a vital in the catalysis field. In the present investigation, the efficiency is very high when compared with the other literature reports. Moreover, the catalyst is used for the reduction purpose and hence during the course of the reduction reaction there is a chance for the reduction of hierarchic structure into the complete nano structure. During the recycling process, the nanostructure was not disturbed and the same was confirmed by measuring the kapp of the system. Up to the fifth cycle, the efficiency was not found to be reduced (Fig. 9 a, b, c). In over all comparison, the present systems yielded the highest kapp and exhibited the high recycling stability. In the present investigation, the influence of WCA on the kapp value was studied. Fig. 9 (d,e) indicates the plot of WCA against kapp for PDC and NiP systems respectively. The plot was found to be a line with increasing trend. It means, while increasing the WCA the kapp value is proportionally increased. The Ti value is also followed the same trend. Due to the hydrophobic nature of the catalyst, the induction time is found to be high, but due to the nano size of the catalyst the reduction was very fast. This is the novelty of the present investigation.
4. Conclusions The niche points are presented as a conclusion. The FTIR spectrum confirmed the C–Si stretching at 1198 cm−1. The n→ π* transition in Cu-TCPFOSi system was confirmed by UV–visible spectroscopy. The Custearate system exhibited the maximum reflectance value. The CuTCPFOSi system exhibited the maximum crystalline peaks of Cu with minimum noise. The Cu-TCPFOSi system exhibited minimum bulk etching rate due to the absence of functional groups. The TCPFOSi system exhibited highest water contact angle of 170.9°. The EDX analysis had the maximum content of Cu, F, Cl and Si content in the Cu-
Fig. 8. UV–visible spectrum of NiP taken at 30 s time interval in the presence of Cu-SA hierarchical structured catalyst and NaBH4 (a–f) and plot of time against ln(A/ Ao) (g). 7
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Fig. 9. Plot of % efficiency against number of cycles of (a) Cu-PFBA, (b) Cu-SA and (c) Cu-TCPFOSi systems and plot of WCA against kapp for (d) PDC system and (e) NiP system.
TCPFOSi system that led to the high water contact angle value. System3 showed the uni-orientation of crystal planes. System-3 exhibited high kapp value towards the reduction of both Cr(VI) and NiP due to WCA. While increasing the WCA, the kapp value was proportionally increased. The present catalyst system exhibited higher stability towards the experimental conditions and stable up to the fifth repeating cycles. The present study on reduction confirmed the Cu based hierarchical structured catalysts were suitable for the reduction of NiP and Cr(VI) due to high WCA, stability towards the experimental conditions, kapp value and economically cheaper.
[16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28]
Conflicts of interest
[29] [30] [31]
There is no conflict of interest among the authors. Acknowledgements
[32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42]
The authors sincerely acknowledge the DRDO, New Delhi for the financial assistance (DRDO/ERIPR/ER/1104580/M/01/1445, 2012). Dr.N.Sundararajan, Associate Professor, Department of English is gratefully acknowledged for his valuable help in manuscript preparation. Appendix A. Supplementary data
[43]
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.pnsc.2019.08.002.
[44] [45]
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Contents lists available at ScienceDirect
Progress in Natural Science: Materials International journal homepage: www.elsevier.com/locate/pnsmi
Characterization and application of Cu based superhydrophobic catalyst Ramasamy Anbarasana,c,∗, Shanmugam Palanikumarb, Ayyadurai Anitha Devib, Ping-Hei Chenc, Kuo Lun Tungc,∗∗ a b c
Department of Polymer Technology, Kamaraj College of Engineering and Technology, Virudhunagar, 626 001, Tamilnadu, India Department of Mechanical Engineering, MEMS Thermal Control Lab, National Taiwan University, Taipei, 10617, Taiwan Department of Chemical Engineering, National Taiwan University, Taipei, 10617, Taiwan
A R T I C LE I N FO
A B S T R A C T
Keywords: Superhydrophobicity Cu salts Characterization Catalytic reduction Water contact angle
Superhydrophobic (SH) Copper salt was prepared with the help of three different chemical etching agents and characterized by various analytical techniques like Fourier transform infrared (FTIR) spectroscopy, UV–visible spectroscopy, X-ray diffractogram (XRD), X-ray photoelectron spectroscopy (XPS), high resolution transmission electron microscopy (HRTEM) and scanning electron microscopy (SEM). The SH nature of the material was confirmed by water contact angle (WCA) measurement. Cu surface with the etching agent was made to produce SH Cu surface. Among the chemical etching agents the system with Cl, F and Si exhibited the SH character. The SH hierarchically structured heterogeneous materials produced were used as a catalyst for the reduction of Cr (VI) and 4-nitrophenol (NiP). The catalytic activities of the materials were compared on the apparent rate constant (kapp) and induction time (Ti). Among the catalyst systems used, the Cu-Trichoropentaflourooctylsilane (TCPFOSi) system exhibited the highest kapp and Ti values.
1. Introduction
was followed for the fabrication of SH copper surface [15]. The literature analysis indicates that the fabrication of SH surface is an easy process but unfortunately the applications of side products formed during the fabrication of SH surface is not found. Hence, in the present investigation we would like to extend the application of SH material in the catalysis field as a low cost catalyst material. Due to the SH nature of the catalyst, the isolation of catalyst after the completion of the reaction is more easy and the same can be reused for many times. Above all, the hierarchical structure of the catalyst offers more surface area and the same can be effectively used for any reaction. The primary importance is given for public health and environmental safety. This is because of modernization, industrialization, increase in population and luxurious life style of human beings. Nitrophenolic compounds are widely used particularly in pharmaceutical industry as one of the starting materials for the manufacture of drug molecules. The effluents from pharmaceutical industry contains lot of NiP, a toxic pollutant and when it is discharged to open environment it pollutes water and soil and make it unfit for the domestic application. So, it is necessary to develop a novel methodology to decompose or to reduce the toxicity. The simple and easiest way to reduce the toxicity is reduction of NiP into aminophenol. Different catalytic systems were used because the reduction of NiP follows the pseudo first
A superhydrophobic (SH) surface is defined as a surface with the static water contact angle of > 150° and it is used for surface self cleaning application. High water contact angle, flower like morphology and high reflectance value are the some of the requirements of SH surface. The SH surface can be generated by various methodologies and discussed below. A facile spin coating method was used for the fabrication of SH surface [1]. In 2012, Chen et al [2] explained the surface wettability effects on critical heat flow of boiling heat transfer. An inexpensive template based SH polymer surface was reported in the literature [3]. Phase separation methodology was used to create SH polymer surface [4] Precursor method was used for the generation of SH surface [5]. Sol-gel method was adopted for SH glass substrate [6]. Etching and electro deposition methodology was followed to create SH surface [7]. Ag nanoparticles were used to tune the SH [8]. In 2013, Sribala and co-workers reported the hydrophobic copolymers [9]. Chen et al [10,11] reported about the SH Cu surface. A facile galvanic exchange reaction was followed for preparing SH nano Ag coated copper plate [12]. Chen and research team [13] fabricated the SH copper surface with anticorrosive property. SH copper hydroxide nanoneedles were synthesized by Wu et al [14]. One step solution immersion process
∗
Corresponding author. Department of Polymer Technology, Kamaraj College of Engineering and Technology, Virudhunagar, 626 001, Tamilnadu, India. Corresponding author. E-mail addresses: [email protected] (R. Anbarasan), [email protected] (K.L. Tung).
∗∗
https://doi.org/10.1016/j.pnsc.2019.08.002 Received 22 January 2019; Received in revised form 25 July 2019; Accepted 4 August 2019 1002-0071/ © 2019 Chinese Materials Research Society. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Please cite this article as: Ramasamy Anbarasan, et al., Progress in Natural Science: Materials International, https://doi.org/10.1016/j.pnsc.2019.08.002
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from the ethanolic solution and dried in an oven at 60 °C for 30 min. Weight of the dried Cu plate was noted. During the etching process the surface of the Cu plate was eroded by ethanol medium and coated with the etching agent as a micro film. This leads to the not only surface properties but also the weight of Cu plate. As a result hierarchical structured Cu plate was fabricated. In this case, a nano structured material is coated on the micro structured substrate material. From the difference in weight of the Cu plate, the chemical etching rate was calculated using the following formula [11].
order kinetics. In 2012, Maheswaran and research team [16] reported the reduction of NiP with the help of Au nanoparticles. The reduction of NiP was reported by Lin and co-workers [17]. A bimetallic layer of Pd and Ag was used for the reduction of NiP [18]. Babji and research team [19] studied the catalytic reduction of NiP by using Fe3+ and Ag+ codoped TiO2 nanoparticle. ZnO loaded Co–Se nanocomposites were used as a catalyst for the reduction of NiP [20]. Similarly, various catalyst like Ni/Al2O3 [21], fullerene nanowhiskers-Au nanoparticle [22], Fe3O4@NiCo@Ag [23], Au nanoparticles decorated ceria nanotube [24], Fe nanoparticle-aminoclay [25], ceria microspheres embedded with Pt nanoparticles [26], Cu nanoparticles [27], SnO2 coated metal nanoparticle [28], layered double hydroxides [29] and Fe nanoparticle immobilized mesoporous silica [30] were used for the reduction of NiP. By thorough literature survey, no report based on the catalytic reduction of NiP using hierarchical architectured catalyst was found. This urged to do the present investigation keeping the quality of environment in mind. Chromium compounds are used in analytical work as a primary standard, chrome plating industry and in tannery as a tanning agent. The effluent from these industries contains larger amount of Cr(VI) (highly toxic) and polluted the water and soil, affecting the human's health. In order to avoid the environmental pollution various methodologies are adopted. Photocatalytic reduction of hexavalent Cr was studied by Assadi et al [31]. TiO2 semiconductor catalyzed photoreduction of Cr(VI) was reported in the literature [32]. For the reduction of Cr(VI) into Cr(III) various catalyst and reducing agent systems such as goethite [33], Pd nanoparticle [34], cell cytoplasm [35], electrolytic reduction [36], pseudomonas sp. JF 122 [37], Fe(II) [38], Fe3O4 [39], antioxidants [40], Mn(II) [41], Kaolin immobilized ZnO nanorods [42], flavanoids [43], nano structured poly(amicacids) [44] and Pd nanoparticle supported amine functionalized SiO2 [45] were used. During the literature review process, no any report on the hierarchical structured material as a catalyst for the reduction of Cr(VI) was found. This motivated the researcher to do the present investigation. The novelty of the present investigation is effective utilization of hierarchical structured Cu salt catalyst, received as a by-product, towards the reduction of Cr(VI) and NiP in the presence of NaBH4 as a reducing agent and the study of influence of superhydrophobicity on the rate constant.
VB = Δm / 2Aρt Δm - mass difference, A - etch surface area, ρ - density of plate, t etching time. 2.3. Catalytic reduction study
2. Experimental
Before the reduction reaction, the standard solution of NiP (1.1 × 10−5 M) was prepared using DDW as a solvent taken in a 3 mL capacity cuvette reactor. 15 mg of NaBH4 was added to the cuvette reactor with the addition of 1 mg of hierarchically structured catalyst. The contents were shaken well for a second and subjected to the UV–visible spectral measurement [46]. The absorbance at 400 nm was measured at room temperature in a regular 1 min time interval. The UV–visible spectrum was recorded till the absorbance at 400 nm becomes negligible. A similar procedure was followed for the catalytic reduction of Cr(VI). For the catalytic reduction of NiP or Cr(VI), control experiment is necessary. In the absence of catalyst, NiP was not reduced. The catalytic reduction NiP follows the pseudo first order kinetics. In the case of Cr(VI), in the absence of catalyst, a control experiment was carried out and the kapp value was calculated as 1.08 × 10−4 sec−1. In the presence of catalyst also the catalytic reduction of Cr(VI) was carried out and the kapp values were calculated from which the control value was subtracted. The subtracted kapp values are already displayed here.(Scheme 1). A standard PDC (5.1 × 10−6 M) solution was prepared. From the standard solution, 2 mL PDC solution was pipette out into a 3 mL capacity cuvette reactor. Then, 15 mg of NaBH4 and 1 mg catalyst were added with a cuvette reactor under vigorous shaking. The reduction of Cr (VI) into Cr(III) was quantitatively followed with the help of a UV–visible spectrophotometer. The absorbance at 371.8 nm was followed with 30 s time interval [47].
2.1. Materials
2.4. Characterization
Potassium dichromate (PDC, Nice chemicals, India) and 4-nitrophenol (NiP, CDH chemicals, India) were purchased and used as received. Copper plate with the purity of 99.9% was used for this study. Ethanol (Himedia, India), Pentaflourobenzoicacid (PFBA, Himedia, AR), Trichoropentaflourooctylsilane (TCPFOSi, Aldrich, India), Stearic acid (SA, Himedia, AR) were used. Double distilled water (DDW) was used for the purpose of cleaning and solution preparation.
FTIR spectrum was taken using Shimadzu 8400 S, Japan model instrument from 4000 to 400 cm−1 by KBr pelletization method. Shimadzu 3600 NIR, Japan instrument was used for measuring solid state UV–visible spectrum. X-ray diffraction (XRD, XS08, Bruker, USA) was recorded with an advanced instrument, scanning from the 2θ value
2.2. Synthesis of hierarchial structured Cu plate An etching is defined as a process by which a layer of the surface of a material is removed by using a chemical (is known as etching agent) for certain period of time. As a result of etching, the surface property of a material is changed. First, the Cu plate {7.5 cm (length or height) × 2.5 cm (breadth) × 0.61 cm (thickness)} was rubbed using an emery sheet then washed with DDW and excess acetone for removing impurities present on the surface of Cu plate. It was placed in an open atmosphere for air drying. The initial exact weight of the surface cleaned Cu plate was noted. The plate was immersed into a 250 mL capacity beaker containing 200 mL ethanol mixed with 1 g of etching agent (SA or PFBA or TCPFFOSi) at room temperature for 24 h [10]. After the definite interval of etching time, the Cu plate was taken out
Scheme 1. Catalytic reduction of 4-NiP and Cr(VI). 2
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of 10–80° at a scanning rate of 2° min−1. The surface morphology of the samples was scanned by SEM (JSM 6300, JEOL, USA model) instrument. Water contact angle (WCA) was measured by Kyowa DMs-200, Japan model goniometer instrument. The WCA was measured in 10 places of the same plate and the average value was considered with 5% error. The HRTEM image was recorded for all the three systems by High resolution transmission electron microscope (HRTEM) measurements for the samples were carried out using JEM-200 CX, USA transmission electron microscope instrument. For reduction study, both PDC and NiP were dissolved in water and the catalyst was used as a heterogeneous one. The binding energy was determined by XPS, (XPS, Thermo Scientific, Theta Probe, UK).
PFBA system. This system exhibits the rod like morphology. The length of the rod was found to be 5–8 μm whereas the breadth of rod was 1–2.5 μm. Fig. 2b indicates the surface morphology of Cu-stearate system. Here also one can see the rod like morphology [10] with the length of 5–15 μm. Fig. 2c denotes the SEM image of Cu-TCPFOSi system. The present system exhibits an entirely different surface morphology with the rod shape and with the size of ∼1 μm. Among the three systems, the third system exhibits an entirely different morphology due to the absence of formation of metal salt. These distorted, agglomerated, spherical and micro sized particles are responsible for the chemically etched Cu particles. Generally, the flower like morphology alone will give the highest water contact angle. The present system inferred that the spherical shaped material can also exhibit the highest water contact angle evidenced by the water contact angle measurement. Elements present in the system can be identified by EDX spectrum. Fig. 2 (d, e, f) indicates the EDX spectrum of Cu-PFBA system, Custearate and Cu-TCPFOSi systems respectively. Fig. 2d gives the percentage of C, F, O and Cu as 49.47%, 33.95%, 12.82% and 3.77% respectively. The Cu-stearate system (Fig. 2e) exhibited the percentage of C, O and Cu as 89.4%, 8.68% and 1.84% respectively. The Cu-TCPFOSi system (Fig. 2f) gives the percentage of the elements Cu, C, Cl, F, O and Si as 62.92%, 23.59%, 6.22%, 4.16%, 1.5% and 0.81% respectively. In comparison, both PFBA and TCPFOSi system contains fluoride ions and exhibited the higher water contact angle. In the case of TCPFOSi system, in addition to F and Cl, Si was also present in considerable quantity. From the EDX analysis one can come to a conclusion that the chemically etched Cu particle which is coated on the surface of Cu plate leads to the SH nature. The HRTEM image of system-1 (Cu-PFBA) is given in Fig. 3a. One can see the agglomerated spherical nanoparticle with some layered structure (or rod shaped) corresponding to the various crystal planes of Cu nanoparticle. The length of the crystal plane was calculated as 5–7 nm with the breadth of < 1 nm. The crystal planes are arranged in various possible directions. This confirmed the multi-orientation of crystal planes. The arrow mark indicated the same. Fig. 3b represents the HRTEM image of system-2 (Cu-SA). System-2 also exhibits the rod or layer like structure with the length of 2–7 nm. But the layers are arranged in parallel and perpendicular directions. This confirmed the bi-orientation of the crystal planes. System-2 does not show the nanospheres [11]. The HRTEM image of system-3 (Cu-TCPFOSi) is shown in Fig. 3c. Here also one can see the layered structure but they are aligned in a particular direction. This declared the uni-orientation of crystal planes. In comparison, one can say that while changing the structure of the etching agent the crystal planes alignment of Cu nanoparticle is disturbed. This can be further supported with the water contact angle measurement. The hierarchical structure of the materials prepared in the present investigation was confirmed by XPS. Fig. 4 (a) indicates the XPS of system 1. The C1s, O1s and F1s of PCBA appeared at 282.9, 532.7 and 684.5 eV respectively. The Cu2p3/2 and Cu2p1/2 peaks appeared at 931.4 and 950.7 eV respectively [11]. The appearance of Cu2p3/2 and Cu2p1/2 confirmed the Cu is in zero oxidation state. But the intensity of the peaks is found to be very low. This indicates that the nano sized Cu is also present in the system-1. From this one can come to a conclusion that the synthesized material contains both nano and micro sized particles. This proved the hierarchical nature of system-1. Fig. 4 (b) represents the XPS of Cu-SA system (system-2) with peaks corresponding to C1s, O1s, Cu2p3/2 and Cu2p1/2 and hierarchical structure. Fig. 4 (c) indicates the XPS of system-3. Some new peaks corresponding to Si (Si2p-100.2 and Si2s-154.4 eV) and Cl (Cl2p-208.5 and Cl2s-274.3 eV) confirmed the hierarchical structure of system 3 from its Cu2p peak intensities. The hydrophobic nature of various chemically etched Cu plates was measured by water contact angle. The Cu-PFBA system (Fig. 5a) exhibited the water contact angle of 147.8°. The Cu stearate (Fig. 5b)
3. Results and discussion For a comparative study, the present results and discussion part are sub-divided into three parts namely, i) Characterization of hierarchical structured catalyst, ii) Catalytic reduction of Cr(VI) and iii) Catalytic reduction of NiP. 3.1. Characterization of hierarchical structured catalyst The FTIR spectrum is used to confirm the functional groups present in the Cu salts. Fig. 1a indicates the FTIR spectrum of Cu-PFBA system. A broad peak around 3406 cm−1 was due to the OH stretching of PFBA. The aromatic C]O stretching of PFBA appeared at 1670 cm−1. Due to the presence of long aliphatic methylene groups, the carbonyl stretching is blue shifted. The C–O–C stretching appeared at 1042 cm−1 [47]. Fig. 1b indicates the FTIR spectrum of Cu-SA system. The formation of Cu-stearate during the simple etching reaction was confirmed by FTIR spectroscopy. The existence of Cu co-planarity [48] appeared as a twin peak at 3362 and 3458 cm−1. This confirmed that during Cu salt formation the Cu atom was aligned on the plane. The symmetric and antisymmetric stretching of SA was noticed at 2859 and 2924 cm−1 respectively. The C]O stretching of SA appeared at 1681 cm−1. The C–H out plane bending vibration was seen at 819 cm−1. The Cu–O stretching appeared at 529 cm−1 [49]. Thus the FTIR spectrum confirmed the Cu-stearate formation. Fig. 1c represents the FTIR spectrum of Cu-TCPFOSi system. The –OH and C–H stretchings appeared at 3377 cm−1 and 2915 cm−1 respectively. The Cu–Si appeared at 1194 cm−1. Thus the FTIR spectrum confirmed the functional groups present in the systems. The surface morphology of metal salts gives an idea about the hydrophobic nature. Fig. 2a represents the surface morphology of Cu-
Fig. 1. FTIR spectrum of (a) Cu-PFBA, (b) Cu-SA and (c) Cu-TCPFOSi system. 3
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Fig. 2. SEM images of (a) Cu-PFBA, (b) Cu-SA and (c) Cu-TCPFOSi system and EDX spectra of (d) Cu-PFBA, (e) Cu-SA and (f) Cu-TCPFOSi system.
exhibited the water contact angle of 129°. The Cu-TCPFOSi system (Fig. 5c) exhibited the maximum water contact angle of 170.9°. Among the three, the later exhibited the highest water contact angle because (i) In addition to fluoride ion the chloride ion helps to make the etching agent more ionic, particularly more electro negative. (ii) Naturally, the TCPFOSi is water immiscible. (iii) The medium size length of octyl group maintains the high electro negativity unlike SA. iv) uni-orientation of various crystal planes (evidenced by HRTEM images). Even though the SA is having 17 methylene units in its backbone the electro negativity is found to be very low. Above all, the TCPFOSi system didn't form metal salt. It is not compulsory to produce the hydrophobic surface by coating the metal salt on the Cu or any other substrate. The simple chemical etching for a required time can create a hydrophobic surface. The present investigation indicates that the TCPFOSi system produced the hydrophobic Cu surface due to the simple etching reaction. Generally, a material with flower like morphology or water contact angle of greater than 160° or nanosized particle can exhibit SH character. In the present investigation, the system with high water contact angle due to the more electronegativity exhibits SH character. When compared with the literature value [10], the present system
Fig. 4. XPS of (a) Cu-PFBA, (b) Cu-SA and (c) Cu-TCPFOSi system.
Fig. 3. HRTEM image of (a) Cu-PFBA, (b) Cu-SA and (c) Cu-TCPFOSi system. 4
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Fig. 5. Water contact angle of (a) Cu-PFBA, (b) Cu-SA and (c) Cu-TCPFOSi system.
Chen et al [10]. The UV–visible reflectance spectra of the Cu salts catalyst are given in Fig. S4. The reflectance spectrum of Cu-PFBA system is given in (Fig. S4) with the maximum reflectance value of 17%. This indicates the hydrophilic nature of Cu-PFBA system. This is further evidenced by the water contact angle measurement. Fig. S4b indicates the UV–visible reflectance spectrum of Cu-stearate system with the maximum reflectance value of 87.8%, due to the hydrophobic nature of the system. Fig. S4c represents the reflectance spectrum of Cu-TCPFOSi system with the maximum value of 62.6%. Among these three systems the chemical etching due to the SA exhibited the highest reflectance due to the formation of hydrophobic Cu salt. The recent literature49 indicates that a system with maximum reflectance value can exhibit the SH character. It can be further confirmed by the water contact angle measurement.
exhibited higher water contact angle value due to the high electro negativity. The XRD pattern of various Cu salts denoted the various crystalline planes of corresponding Cu salts. Fig. S1a indicates the XRD of Cu-PFBA system. The system exhibited many crystalline peaks from 15 to 50°. This confirmed the crystalline nature of Cu-PFBA system. A peak corresponding to d111 crystal plane of Cu appeared at 42.8° in accordance to the literature report [50]. The appearance of other crystal planes confirmed the crystalline nature of Cu-PFBA system. Fig. S1b indicates the XRD pattern of Cu-stearate system. The system exhibited no more peaks between 10 and 80° and confirmed the amorphous nature of Custearate system. Fig. S1c indicates the XRD pattern of Cu-TCPFOSi system. The present system exhibits more sharp crystalline peaks corresponding to d110 (32.2°), d202 (49.1°) and d200 (51.3°). This proves that Cu is present in salt form. Among the three systems the Cu-stearate system exhibited the lowest crystallinity due to the amorphous nature. The amorphous or crystalline nature of Cu salt depends on the nature of the etching agent used for chemical etching reaction. The results of XRD coincide with the results of Chen et al [51]. The influence of three different chemical etching agents and surface of the Cu plate at different time interval are represented in Fig. S2. Fig. S2 shows the plot of difference in mass Vs time. Fig. S2a indicates the bulk etching rate of Cu-PFBA system. For 175 h of etching, the system exhibited the maximum weight loss of 48.2 mg. This can be explained as follows. (i) The presence of five fluorine atom makes the metal salt more electro negativity. (ii) The presence of five fluorine atoms leads to the formation of CO2− ion. These negative ions can readily interact with the Cu2+ of Cu metal plate to form the Cu-PFBA salt and so this system yields more weight loss. Fig. S2b indicates the bulk etching rate plot of Cu-stearate system. This system exhibits 25.9 mg weight loss at 175 h. This can be explained as follows. SA is having 16 methylene repeating units in its structure and exhibits poor electro negativity. Hence, the formation of CO2− ion is difficult and there is a moderate bulk etching rate. Fig. S2c represents the bulk etching rate plot corresponding to Cu-TCPFOSi system. This system exhibits 18.6 mg weight loss at 175 h of chemical etching. Among the three systems, the CuTCPFOSi system exhibited the lowest weight loss. Even though the TCPFOSi contains five fluorine atoms in its structure there is no Cu salt formation. Instead of metal salt formation, the TCPFOSi simply deposited on the surface of Cu plate. Due to the five electro negativity fluoride ion, the etching also occurred at a mild rate [11]. Due to the absence of metal salt formation, the chemical etching was very low. This inferred that the chemical etching is not only accelerated by the electro negativity but also by the metal salt formation. In 1995, Bryce and co-workers [52] explained the chemical etching of Cu in FeCl3 solution. When compared with the literature report, the present system yielded a better result. The UV–visible spectrum is used to explain the various possible electronic transitions present in the Cu salts catalyst. A peak at 265.1 nm (Fig. S3a) represents the π→ π* transition of the phenyl ring of PFBA. Fig. S3b indicates the UV–visible spectrum of Cu-stearate system and is found no peak. Fig. S3c represents the UV–visible absorption spectrum of Cu-TCPFOSi system. A broad peak at 273.6 nm represents the n→ π* transition of TCPFOSi. This is in accordance with
3.2. Catalytic reduction of Cr(VI) The Cr(VI) is a toxic and hazardous and hence some action has to be taken against Cr(VI) pollution. The better way is to convert the Cr(VI) into Cr(III) with the help of a suitable catalyst. In the present investigation, the above synthesized SH, bio-compatible and eco-friendly Cu salts catalyst was used for the reduction purpose. The primary aim of the present investigation is conversion of waste side product into a useful catalyst. 2 mL of PDC solution was pipeted out in to a cuvette nano reactor, with this 1 mg of system 1 and 15 mg of NaBH4 were added and stirred well. Now the Cr(VI) is reduced into Cr(III) and the reaction is followed with the help of UV–visible spectrophotometer. The UV–visible spectrum was recorded for 30 s time interval (Fig. 6a-l). The peak absorbance was found at 371.8 nm corresponding to decrease of Cr(VI) concentration. This was due to the conversion of Cr(VI) into Cr(III). The decrease in absorbance at 371.8 nm confirmed the catalytic reduction of Cr(VI) into Cr(III) in the presence of NaBH4. The spectrum indicated that within 2.5 min, 98% of Cr(VI) (calculated from the calibration curve) was converted into Cr(III). In order to find out the apparent rate constant (kapp), the plot of time against ln(A/Ao) (Fig. 6m) was made. The plot showed a decreasing trend. From the slope value (linear region) the kapp value was calculated as 0.58 × 10−2 sec−1 (Table 1). From the intercept value the induction time (Ti) was determined as 0.179 s. The present system yielded higher kapp value while using aminoclay supported Pd nanoparticle as a catalyst for the reduction of Cr(VI) (kapp = 3.30 × 10−3 sec−1) [45]. A plausible mechanism for the reduction of Cr(VI) is explained here. It is well known that NaBH4 is a good reducing agent. During the reduction of Cr(VI), three electrons are transferred from NaBH4 to Cr(VI) and leads to the formation of Cr(III) with simultaneous reduction of Cu(II) into Cu(0), because of its hierarchic nature. This suggests that 2 mol of NaBH4 is required to reduce 2 mol of Cr(VI), {into Cr(III)} and 1 mol of Cu salt {Cu(II) into Cu(0)}. The catalytic reduction of Cr(VI) in the presence of system 2 and 3 was also tested. From Fig. S5a-o it was found that the absorbance decreased slowly at 371.8 nm. This confirmed the catalytic reduction of Cr (VI) into Cr(III) in the presence of system 2 as a heterogeneous catalyst. The kapp was determined from the plot of time vs ln(A/Ao) (Fig. S5p) as 0.68 × 10−2 sec−1. The Ti was determined as 0.308 s. In comparison, 5
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Fig. 6. UV–visible spectrum of PDC taken at 1min time interval in the presence of Cu-PFBA hierarchical structured catalyst and NaBH4 (a–l) and plot of time against ln(A/Ao) (m).
salt with Cu whereas the TCPFOSi decorates the central Cu metal ion. Hence, the system forms core-shell like structure. Here, the main aim is not proving the core-shell like structure. Here all the systems are SH in nature with slight difference in WCA. Here the WCA depends not only on the surface roughness and morphology of Cu metal but also depends on the nature (structure) of the etching agent. In fact, F is smaller in size with high electro negativity and produces high WCA. Even then the system 3 produces high kapp when compared to the system-1. Both the systems are having F atoms in its backbone. Still system-3 has 3 more Cl atoms in its structure. Hence, the system produced high kapp value. Apart from WCA, resonance stabilization plays a vital role. This leads to high electrostatic interaction. As a result of high electrostatic interaction and resonance stabilization, system 3 exhibited high kapp with high Ti. From the present investigation, the following queue was made on the performance towards the reduction reaction. Based on kapp,
Table 1 Systems with kapp and Ti values. System
Cu-PFBA Cu-SA Cu-TCPFOSi
Code
1 2 3
kapp (sec−1)
Ti (sec)
PDC
NiP
PDC
NiP
0.58 × 10−2 0.68 × 10−2 1.60 × 10−2
2.80 × 10−2 3.93 × 10−2 5.89 × 10−2
0.179 0.308 0.319
0.427 0.532 0.821
the system 1 exhibited the lower kapp due to the low WCA. The high catalytic activity of system 2 was further confirmed by Ti value. System 1 avails 0.179 s for its activation whereas system 2 avails 0.308 s (Table 1). The higher Ti value can be explained on the basis of higher WCA. The WCA measurement was already discussed. The catalytic reduction activity of system 3 was tested. Fig. 7a–f indicates the UV–visible spectrum of Cr(VI) taken at 30 s time interval. Here also one can observe the decrease in Cr(VI) concentration at 371.8 nm. As usual, the kapp is determined from the plot of time vs ln(A/ Ao) (Fig. 7g) as 1.68 × 10−2 sec−1 (Table 1). The Ti was determined to be 0.319 s. When compared with system 2, the system 3 yielded higher kapp with slight increase in Ti. While comparing the kapp value with the literature (kapp = 3.30 × 10−3 sec−1) [45] the system-3 yielded a good value but systems-1 and 2 yielded somewhat lower value in a given experimental conditions. This indicates that the catalytic activity depends not only on the nature of the central metal core but also on the WCA, electronegativity and resonance stabilized structure of the etching agent. In overall comparison, system 3 yielded better results towards the reduction of Cr(VI). Cu metal was used as a core. The PFBA and SA form
TCPFOSi > SA > PFBA Based on Ti, TCPFOSi > SA > PFBA
3.3. Catalytic reduction of NiP The catalytic activities of Cu salts were also confirmed by the reduction of NiP. Moreover, the reduction of NiP is a standard model as it follows the pseudo first order kinetics. 2 mL of NiP solution was taken in a micro reactor followed by the addition of 15 mg of NaBH4 and 1 mg of Cu salt, system-1. The reduction study was made with the aid of UV–visible spectrophotometer quantitatively. Figs. S6a–f shows the
Fig. 7. UV–visible spectrum of PDC taken at 30 s time interval in the presence of Cu-TCPFOSi hierarchical structured catalyst and NaBH4 (a–f) and plot of time against ln(A/Ao) (g). 6
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results in a given experimental conditions explained on the basis of SH character (WCA) and electronegativity of the etching agent. From the reduction reaction of NiP in the presence of NaBH4, the following queue was made on the kapp.
UV–visible spectrum of NiP taken at 30 s time interval. The spectrum exhibits a peak at 400.7 nm corresponding to the nitro group of NiP. During the course of the reaction, the absorbance decreased at 400.7 nm and the same confirmed the reduction of NiP into aminophenol. The UV–visible spectrum confirmed that within 3 min 100% NiP was reduced into aminophenol in the presence of hierarchically structured catalyst system-1. The % conversion was determined from the calibration curve. From the absorbance value the kapp value can be determined from the plot of time vs ln(A/Ao) (Fig. S6g). The plot was found to be a line with decreasing trend. Here, the plot exhibited two regions. Up to 2 min, the reduction reaction is normal. After that the reaction was very fast due to the simultaneous reduction of Cu salt, system-1. As a result of reduction reaction, the surface area of the Cu increased and provided a surface for the reduction reaction. From the slope value the kapp value was calculated as 2.81 × 10−2 sec−1 (Table 1). The Ti was taken from the intercept value as 0.42 s. When compared with the literature [53] the present investigation yielded ∼10 times higher kapp value. The catalytic reduction of NiP into aminophenol (AP) is a well explained one since it follows the pseudo first order kinetics. During the reduction of NiP, two hydrogen atoms are transferred from NaBH4 to nitro group of NiP. Here one functional group (-NO2) is converted into another one functional group (-NH2) [53]. While using Au [54] or Ni–Pd nano dimer [55] as a catalyst for the catalytic reduction of NiP, the kapp values were determined as 3.0 × 10−3 sec−1 and 2.5 × 10−2 sec−1 respectively. Hence, when compared with the literature values, the present investigation yielded an excellent kapp value. The catalytic activity of system-2 towards the reduction of NiP was tested in the presence of NaBH4. Fig. 8a–f indicates the same. The UV–visible spectrum was taken at 30 s time interval. The absorbance decreased at 400.7 nm during the course of the reduction reaction. The kapp value was determined by plotting time vs ln(A/Ao) (Fig. 8g) as 3.93 × 10−2 sec−1 (Table 1) with the Ti value of 0.53 s. When compared with system-1, system-2 yielded high value due to the high WCA and resonance stabilization effect. Pd nanoparticles [56] and amino functionalized mesoporous silica [57] nanoparticles showed the kapp value of 2.33 × 10−3 sec−1 towards the catalytic reduction of NiP. When compared with the literature reports, the present system yielded an excellent value. Figs. S7a–f indicates the UV–visible spectrum of NiP taken at 30 s time interval in the presence of NaBH4 and system-3 as a catalyst. The reduction reaction was completed within 3 min, 99.7% of NiP converted into aminophenol. The kapp value was determined from the plot of time vs ln(A/Ao) (Fig. S7g) as 5.89 × 10−2 sec−1 (Table 1) with the Ti value of 0.82 s. The plot was found to be a straight line with decreasing trend. When compared with system 1 and 2, the current system yielded higher kapp value. This is due to the highest WCA. When compared with the literature [29,53], the present systems yielded good
TCPFOSi > SA > PFBA Based on the Ti value the queue follows: TCPFOSi > SA > PFBA In overall comparison, the NiP system yielded higher kapp value than the PDC system due to the resonance stabilization in the NiP system. The Ti value also reflected the same. Among the catalysts used, system-3 exhibited higher kapp value for both PDC and NiP systems because of higher WCA. NiP exhibits resonance stabilization through the phenyl ring and –NO2 group. The above results and discussion proved that the system-3 is a most suitable catalyst for the reduction of Cr(VI) and NiP. The efficiency and recycling stability of the catalyst plays a vital in the catalysis field. In the present investigation, the efficiency is very high when compared with the other literature reports. Moreover, the catalyst is used for the reduction purpose and hence during the course of the reduction reaction there is a chance for the reduction of hierarchic structure into the complete nano structure. During the recycling process, the nanostructure was not disturbed and the same was confirmed by measuring the kapp of the system. Up to the fifth cycle, the efficiency was not found to be reduced (Fig. 9 a, b, c). In over all comparison, the present systems yielded the highest kapp and exhibited the high recycling stability. In the present investigation, the influence of WCA on the kapp value was studied. Fig. 9 (d,e) indicates the plot of WCA against kapp for PDC and NiP systems respectively. The plot was found to be a line with increasing trend. It means, while increasing the WCA the kapp value is proportionally increased. The Ti value is also followed the same trend. Due to the hydrophobic nature of the catalyst, the induction time is found to be high, but due to the nano size of the catalyst the reduction was very fast. This is the novelty of the present investigation.
4. Conclusions The niche points are presented as a conclusion. The FTIR spectrum confirmed the C–Si stretching at 1198 cm−1. The n→ π* transition in Cu-TCPFOSi system was confirmed by UV–visible spectroscopy. The Custearate system exhibited the maximum reflectance value. The CuTCPFOSi system exhibited the maximum crystalline peaks of Cu with minimum noise. The Cu-TCPFOSi system exhibited minimum bulk etching rate due to the absence of functional groups. The TCPFOSi system exhibited highest water contact angle of 170.9°. The EDX analysis had the maximum content of Cu, F, Cl and Si content in the Cu-
Fig. 8. UV–visible spectrum of NiP taken at 30 s time interval in the presence of Cu-SA hierarchical structured catalyst and NaBH4 (a–f) and plot of time against ln(A/ Ao) (g). 7
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Fig. 9. Plot of % efficiency against number of cycles of (a) Cu-PFBA, (b) Cu-SA and (c) Cu-TCPFOSi systems and plot of WCA against kapp for (d) PDC system and (e) NiP system.
TCPFOSi system that led to the high water contact angle value. System3 showed the uni-orientation of crystal planes. System-3 exhibited high kapp value towards the reduction of both Cr(VI) and NiP due to WCA. While increasing the WCA, the kapp value was proportionally increased. The present catalyst system exhibited higher stability towards the experimental conditions and stable up to the fifth repeating cycles. The present study on reduction confirmed the Cu based hierarchical structured catalysts were suitable for the reduction of NiP and Cr(VI) due to high WCA, stability towards the experimental conditions, kapp value and economically cheaper.
[16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28]
Conflicts of interest
[29] [30] [31]
There is no conflict of interest among the authors. Acknowledgements
[32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42]
The authors sincerely acknowledge the DRDO, New Delhi for the financial assistance (DRDO/ERIPR/ER/1104580/M/01/1445, 2012). Dr.N.Sundararajan, Associate Professor, Department of English is gratefully acknowledged for his valuable help in manuscript preparation. Appendix A. Supplementary data
[43]
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.pnsc.2019.08.002.
[44] [45]
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