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silver nanoparticle composite as a catalyst for reduction of 4-nitrophenol
Carbohydrate Polymers 156 (2017) 253–258
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Carbohydrate Polymers journal homepage: www.elsevier.com/locate/carbpol
Cellulose nanocrystal/hexadecyltrimethylammonium bromide/silver nanoparticle composite as a catalyst for reduction of 4-nitrophenol Xingye An a,b , Yunduo Long b,∗ , Yonghao Ni a,b,∗ a b
Tianjin Key Laboratory of Pulp and Paper, Tianjin University of Science and Technology, Tianjin 300457, China Department of Chemical Engineering, University of New Brunswick, Fredericton, New Brunswick, E3B 5A3, Canada
a r t i c l e
i n f o
Article history: Received 1 August 2016 Received in revised form 27 August 2016 Accepted 30 August 2016 Available online 13 September 2016 Keywords: Nanocatalyst Silver nanoparticles (Ag NPs) Cellulose nano-crystals (CNC) Support/carrier Hexadecyltrimethylammonium bromide (CTAB) 4-Nitrophenol (4-NP)
a b s t r a c t The application of metal nanoparticles (NPs) in catalysis has drawn significant research interest, including in their applications to the wastewater treatment. Nanocellulose can be a versatile support/carrier in stabilizing metal NPs catalysts, due to its unique properties. In this paper, hexadecyl-trimethylammonium bromide (CTAB) surfactant-adsorbed cellulose nanocrystals (CNC) was employed as a support for silver nanoparticles (Ag NPs). The obtained CNC/CTAB/Ag nanohybrid composite was characterized by ultraviolet–visible (UV–vis) spectroscopy, transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDX). The results of the catalytic reaction experiments showed that Ag NPs immobilized on CTAB-adsorbed CNC displayed higher catalytic efficiency towards the reduction of 4-nitrophenol to 4-aminophenol, compared to the control Ag NPs and CNC/Ag samples. The enhanced catalytic performance of Ag NPs was attributed to the improved dispersity and narrow distribution of silver nanoparticles in the catalytic system. This work provides a novel catalyst based on CNC-CTAB stabilized Ag NPs which would have promising applications towards catalytic treatment of wastewater. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Nanoscience and nanotechnology offer new opportunities for making superior materials for use in industrial, health, energy and environmental applications (HPS et al., 2016; Khademhosseini, Parak, & Weiss, 2016; Mulvaney, 2015; Murriello, Contier, & Knobel, 2016; Shen, Song, Qian, Yang, & Kong, 2010). Nanostructured particles, especially metal nanoparticles (NPs), have attracted much attention over the last decade (Djerahov, Vasileva, Karadjova, Kurakalva, & Aradhi, 2016; Kang et al., 2016; Nazirov et al., 2016; Sadanand, Rajini, Rajulu, & Satyanarayana, 2016), and have also been widely studied in different fields from materials science and engineering to biomedical applications (Li et al., 2012; Tada, Kiyonaga, & Naya, 2009; Walekar et al., 2014). Among these applications, the use of catalytic metal NPs in waste water treatment is of particular interest because of their unique electronic properties and versatile catalytic activities (Daniel & Astruc, 2004; Narayanan & El-Sayed, 2005; Zheng & Stucky, 2006). For example, 4-nitrophenol (4-NP), which is a pollutant compound in both industrial and agri-
∗ Corresponding authors at: Department of Chemical Engineering, University of New Brunswick, Fredericton, New Brunswick, E3B 5A3, Canada. E-mail addresses: [email protected] (Y. Long), [email protected] (Y. Ni). http://dx.doi.org/10.1016/j.carbpol.2016.08.099 0144-8617/© 2016 Elsevier Ltd. All rights reserved.
cultural wastewater (Murugan & Jebaranjitham, 2012; Ofomaja & Unuabonah, 2011), can be reduced by metal NPs catalysts to 4aminophenol (4-AP) (Alshehri et al., 2016; Narayanan & Sakthivel, 2011; Torkamani & Azizian, 2016). However, metal NPs are thermodynamically unstable and tend to aggregate to minimize their surface area, thus resulting in the significant loss of catalytic activities (Kaushik & Moores, 2016; Koga et al., 2010; Maity et al., 2013). The concept of immobilization or stabilization of metal NPs is one of the potential methods to overcome the aggregation issue (Cao, Tang, Liu, Nie, & Zhao, 2010; Liu et al., 2015a; Mahmoud, Lam, Hrapovic, & Luong, 2013). There are many types of matrix used to support/immobilize metal NPs in catalysis, such as carbon materials (Daio et al., 2015; Duman, Tunc¸, Polat, & Bozo˘glan, 2016; Sun and Li, 2004), metal oxides (Mogyorósi et al., 2009; Zhang, Liu, Guo, Wu, & Wang, 2010), and polymers (Kuroda, Ishida, & Haruta, 2009). A numerous variety of NPs can be synthesized through different methods (onto different supports/substrates) to give tailored sizes, shapes and distributions, which have shown a positive impact on the subsequent catalytic processes (Campelo, Luna, Luque, Marinas, & Romero, 2009). Nanocelluloses, including cellulose nanocrystal (CNC), nanofibrillated cellulose (NFC) and bacterial nanocellulose (BNC), have potential applications in paper, cosmetics, food, pharmaceuticals,
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and biomedical industries (An, Wen, Cheng, Zhu, & Ni, 2016; Jahan, Saeed, He, & Ni, 2010; Nath, Chaliha, Kalita, & Kalita, 2016; Zaman, Liu, Xiao, Chibante, & Ni, 2013). Among the many applications of nanocelluloses, their use in the synthesis of metal NPs catalysts is appealing due to their high surface area, thermal stability, functionalized surface and environmental benefits (Kaushik & Moores, 2016; Liu et al., 2016). Based on the unique features mentioned above, CNC can play an important role in synthesizing metal NPs catalyst and may have a noticeable impact in terms of the efficiency of wastewater treatment. Generally, the deposition of metal NPs on the surface of CNC can be readily achieved by using a reducing agent, e.g., sodium borohydride (NaBH4 ). In these cases, the in-situ formed nano-metal particles are then stabilized due to the complexation with CNC via ion-dipole interactions (He, Kunitake, & Nakao, 2003). Hexadecyltrimethylammonium bromide (CTAB), a cationic surfactant with an alkyl chain (hydrophobic tail) and quaternary ammonium group (hydrophilic head), has been used to stabilize inorganic NPs (Cai, Kimura, Wada, & Kuga, 2008; Sun, Yin, Mayers, Herricks, & Xia, 2002). Recently, Padalkar et al. have successfully synthesized the well dispersed Ag, Cu, Au, and Pt NPs decorated tunicate CNC, respectively, with the help of multifunctional CTAB (Padalkar et al., 2010). It is noted that CTAB was not only a stabilizer of metallic NPs but also a vehicle for positioning of these particles on the CNC surface by non-covalent interactions with hydroxyl groups. Herein, we put out a facile synthesis of CNC/CTAB/Ag nanohybrid composite toward the catalytic reduction of 4-nitrophenol to 4-aminophenol. In this study, CNC was prepared from dissolving pulp by traditional 64 wt% sulfuric acid hydrolysis. It was concluded that Ag NPs loaded on the surface of CNC substrate can be stabilized and in a narrow particle size distribution with the help of CTAB, due to: 1) the steric effect from CTAB binding onto Ag NPs via electrostatic interactions between the hydroxyl groups in Ag NPs and hydrophilic heads of CTAB, and 2) the increased negative charges from the exposed CTAB’s quaternary ammonium ions, thus enhancing the dispersity of Ag NPs. The catalytic activity of CNC/CTAB/Ag nanohybrid composite was compared with those of Ag NPs and CNC/Ag nanohybrid composite. The as-prepared nanocatalyst were characterized by ultraviolet–visible (UV–vis) spectroscopy, transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDX). The catalytic activities of the nanohybrid composite were evaluated by the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) with an excess amount of NaBH4 .
2. Experimental section 2.1. Materials The CNC suspension was prepared by sulfuric acid (64 wt%) hydrolysis of dissolving pulp according to the method described in the literature (Boluk, Lahiji, Zhao, & McDermott, 2011; Hu, Cranston, Ng & Pelton, 2014). Typically, 5 g of dissolving pulp was hydrolyzed by using 64 wt% sulfuric acid at 50 ◦ C for 45 min and then diluted with deionized water to stop the reaction. The suspension was then centrifuged, and dialyzed to remove the acid and other impurities until a fairly steady pH was reached. Finally, ultrasound treatment was conducted to disperse the CNC particles. The sulfate ester group content of as-prepared CNC was 325.3 mol/g based on the conductometric titration method (Abitbol, Kloser & Gray, 2013; Araki, Wada, Kuga & Okano, 1998). Silver nitrate (AgNO3 , ≥99.0%), hexadecyltrimethylammonium bromide (CTAB, ≥99.0%), sodium borohydride (NaBH4 , ≥99.0%) and 4-nitrophenol (4-NP, ≥99.0%) were all purchased from SigmaAldrich Co. Ltd. Deionized water was used to prepare all aqueous
solutions. All other chemicals were of analytical grade and used without any further purification. 2.2. Synthesis of CNC/CTAB/Ag nanohybrid composites For preparation of CNC/CTAB/Ag nanohybrid composite, the asprepared CNC suspension was diluted into 0.2 wt% followed by sonification for 10 min, and the well-dispersed CNC suspension (50 mL, 0.2 wt%) was added in the 100 mL beaker under constant magnetic stirring. A buffer solution (0.1 M HAc-NaAc) was used to adjust the pH to around 4.5. Then 30 mL CTAB solution (0.5 mM) was added into the diluted CNC suspension and stirred for 10 min, followed by the addition of dropwise silver nitrate solution (20 mL, 0.1 mM). After that, a freshly prepared aqueous sodium borohydride solution (20 mL, 10 mM) was added dropwise into the well-mixed suspension under constant magnetic stirring for 5 min, which reduced Ag+ ions into Ag NPs. The thus obtained CNC/CTAB/Ag nanohybrid composite was ready for further experiments. For comparison, Ag NPs, CNC/Ag nanohybrid composite was also synthesized using the similar procedure above. 2.3. Characterization and measurements A UV–vis spectrophotometer (Genesys 10-S, Thermo) was used to record the UV–vis adsorption spectrums of CNC-CTAB and CNC/CTAB/Ag to confirm Ag NPs in the nanohybrid composite. The morphology and structure of Ag NPs, CNC/Ag and CNC/CTAB/Ag nanohybrid composite were characterized by a transmission electron microscopy (TEM, JEOL 2010, Japan) equipped with an energy dispersive X-ray spectroscopy (EDX). 2.4. Catalytic reduction of 4-nitrophenol The reduction of 4-NP with NaBH4 solution was used as a model reaction to determine the catalytic activity of CNC/CTAB/Ag nanohybrid composite. As an excess amount of NaBH4 was present in the system, the reduction kinetics would be independent of the NaBH4 concentration. Upon the addition of NaBH4 into the 4-NP solution, its color changed immediately from light yellow to dark yellow due to the formation of 4-nitrophenolate ion (formed from the high alkalinity of NaBH4 ). Then, the dark yellow color faded with time (due to the conversion of 4-NP to 4-AP) after the addition of CNC/CTAB/Ag nanohybrid composite, which could be monitored by a UV–vis spectroscopy. Typically, an aqueous suspension of CNC/CTAB/Ag nanohybrid composite (containing 0.02 mol Ag) was added into a mixed aqueous solution of 30 mL containing 4-NP (0.1 mM) and NaBH4 (10 mM) under stirring at room temperature (293 K). At appropriate time intervals, an aliquot of reaction mixture (1.2 mL) was filtered, and the UV–vis adsorption spectrum of the filtrate was recorded by UV–vis spectrophotometer in the range of 250–550 nm. The reaction rate was followed by measuring the change in absorbance at 400 nm (the peak of 4-nitrophenolate ion) as a function of time. 3. Results and discussion 3.1. Characterization of CNC/CTAB/Ag nanohybrid composites 3.1.1. UV–vis spectra analysis UV–vis spectra technique can be applied in confirming the presence of metal NPs in the reaction system. As shown in Fig. 1, the aqueous suspension of CNC-CTAB was almost transparent and no bands appear in the spectrum (Fig. 1(A)). In contrast, the obtained CNC/CTAB/Ag nanohybrid composite suspension was light yellow, and the surface plasmon resonance (SPR) band at 410 nm confirmed the formation of silver nanoparticles.
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corresponding EDX spectra of CNC/CTAB/Ag nanohybrid composite confirmed the presence of Ag and Br elements. The improved results stated above can be attributed to the stabilizing function of CTAB surfactant due to the fact that: 1) when binding onto Ag NPs via electrostatic interactions, CTAB surfactants can serve as steric barriers; 2) the exposed CTAB’s quaternary ammonium ions, which are much stronger than the hydroxyl groups on the original Ag NPs, increased the charge density of the resultant Ag NPs, enhancing the dispersity of Ag NPs. 3.2. Proposed function of CTAB in the preparation of CNC/CTAB/Ag nanohybrid composite
Fig. 1. UV–vis adsorption spectra of an aqueous suspension of (A): CNC-CTAB; and (B): CNC/CTAB/Ag nanohybrid composite. The SPR band at 410 nm confirms the presence of silver nanoparticles(For interpretation of the references to colour in text, the reader is referred to the web version of this article.). Table 1 Comparison of catalytic performance of catalytic converting 4-NP to 4-AP, using different metal nanocatalysts. Sample
Temperature/K
4-NP/Ag [mol/mol]
TOF/h−1
Ag NPs CNC/Ag CNC/CTAB/Ag CSNFs/Au (Koga et al., 2010)
293 293 293 298
150/1 150/1 150/1 150/1
54 474 545 563
Shown in Fig. 3 is the concept of preparing the CNC/CTAB/Ag nanohybrid composite. The rich hydroxyl and sulfate ester groups presented on the CNC surface (Fig. 3(a)) can interact with the hydrophilic (cationic) head of CTAB, as shown in Fig. 3(b). Under the conditions as such, a bilayer CTAB structure (hydrophobic interactions between the two CTAB molecules) (Walekar et al., 2014) can be formed in the system. The Ag NPs, in situ synthesized by the reduction of AgNO3 by NaBH4 , can then be in complexation with CTAB via the hydrophilic interactions, shown in Fig. 3(c). Here, CTAB had the steric effect from CTAB binding onto Ag NPs via electrostatic interactions between the hydroxyl groups in Ag NPs and hydrophilic heads of CTAB; in addition, the exposed CTAB’s hydrophilic heads (quaternary ammonium ions) would create strong electron repulsion, both of which contributed to the improved Ag NPs stability and dispersity. 3.3. Catalytic reduction of 4-nitrophenol
In the literature, the band at 410 nm has been used by others to support the presence of Ag NPs in the nano-composite catalyst. For example, Awazu et al. (2008) prepared a photocatalyst consisting of Ag NPs embedded in TiO2 and SiO2 , and found a localized surface plasmon (LSP) UV–vis resonance peak at 410 nm, which was attributed to the existence of Ag NPs in the TiO2 /SiO2 /Ag NPs composite (Awazu et al., 2008). 3.1.2. TEM analyses and Ag NPs size/distribution analyses Shown in Fig. 2 are the TEM images of pristine CNC, unsupported Ag NPs, CNC/Ag and CNC/CTAB/Ag nanohybrid composite. The pristine CNC (Fig. 2(a)) had a length of ca. 200–250 nm and a width of 15–20 nm, which were typical for CNC (Wang et al., 2012). Fig. 2(b) illustrates that unsupported silver particles aggregated severely in the absence of CNC and CTAB. In contrast, Ag NPs shown in Fig. 2(c) were well dispersed in the presence of CNC substrate, which supported the conclusion that CNC can act as a good dispersant/support of nanoparticles, preventing the NPs aggregation due to the excellent properties of CNC. Liu et al. also prepared well dispersed Fe3 O4 NPs in the presence of CNC, which functioned as a dispersant (Liu, Nasrallah, Chen, Huang, & Ni, 2015b). In addition, the corresponding EDX spectra of CNC/Ag composite (Fig. 2(d)) also supported the conclusion that Ag NPs were embedded in the CNC substrate, forming the CNC/Ag composite. Comparison of Fig. 2(c) and (e) showed that Ag NPs were more efficiently dispersed in the presence of CTAB surfactant, and that fine Ag NPs were evident and in a smaller particle size and narrower particle size distribution in the case of CNC/CTAB/Ag nanohybrid composite. In the absence of CTAB (Fig. 2(c)), a greater proportion of the particles was in large aggregates (>15 nm). In contrast, in the presence of CTAB (Fig. 2(e)), the fraction of large aggregates decreased significantly and 84% of the Ag NPs was below 10 nm. The improved Ag NPs characteristics were expected to improve the catalytic activity/efficiency as nano-catalyst. Furthermore, the
Shown in Fig. 4(a) are the results on the catalytic reduction of 4-NP to 4-AP in the presence of NaBH4 and the CNC/CTAB/Ag nanohybrid composite. The reduction process was monitored based on the UV–vis spectrophotometry. It was found that the absorbance at 400 nm (due to 4-NP) gradually decreased as a function of time, while the absorbance at 290 nm (due to 4-AP) increased, indicating that the catalytic reduction of 4-NP to 4-AP, occurred successfully (Esumi, Isono & Yoshimura, 2004). The catalytic reduction was almost completed within 20 min at room temperature. The reaction did not occur at all in the presence of CNC alone or CTAB (data not shown). Similar report can also be found in the literature. Liu and Zhao found that using Ag NPs (with about 10 nm diameter) that were immobilized onto different substrates (the halloysite nanotubes (HNTs)), the catalytic reduction of 4-NP to 4-AP was completed between 25 and 50 min, depending on the amount of NaBH4 present in the reaction system (Liu & Zhao, 2009). The kinetics of the Ag NPs, CNC/Ag and CNC/CTAB/Ag systems are shown in Fig. 4(b). In each case, a pseudo-first order kinetic was evident. The pseudo-first order rate constant (k) for Ag NPs, CNC/Ag and CNC/CTAB/Ag were 1.5 × 10−4 s−1 , 1.2 × 10−3 s−1 and 1.6 × 10−3 s−1 , respectively. In case of Ag NPs, the catalytic activity was the lowest due to the aggregation of Ag nanoparticles, while it was the highest for the CNC/CTAB/Ag composite sample. The increased catalytic activity of the CNC/CTAB/Ag system was due to the well dispersed silver nanoparticles, allowing effective contact with the reactants and catalysts. Wu et al. synthesized CNC-supported palladium nanoparticles (Pd NPs) to reduce methylene blue (MB) and 4-NP (Wu et al., 2013). It was found that the obtained CNC-supported Pd NPs exhibited much higher activities than the unsupported Pd NPs in the catalyzed reduction of MB and 4-NP. Fenger et al. reported that gold nanoparticles prepared by CTAB-stabilized-supports exhibited efficient catalytic conversion of 4-NP to 4-AP (Fenger, Fertitta, Kirmse, Thünemann & Rademann, 2012).
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Fig. 2. TEM images of (a) pristine CNC, (b) unsupported Ag NPs, (c) CNC/Ag, and (e) CNC/CTAB/Ag nanohybrid composite, (Scale bars of (a)–(c) and (e) were 0.5 m, respectively, while the scale bars of inserted magnified images of (c) and (e) were 200 nm, respectively), EDX spectra of (d) CNC/Ag nanohybrid composite and (f) CNC/CTAB/Ag nanohybrid composite; the inserted histograms in (c) and (e) were Ag NPs size distributions of CNC/Ag and CNC/CTAB/Ag nanohybrid composite.
Fig. 3. Schematic illustration for the synthesis of CNC/CTAB/Ag nanohybrid composite: (a) hydroxyl and sulfate ester groups on the surface of pristine CNC; (b) hydroxyl and sulfate ester groups are modified by bi-layer CTAB surfactant; (c) synthesized in-situ Ag NPs was loaded on the surface of CNC substrate. Ag NPs are stabilized due to 1): the steric barrier function on the CNC/CTAB/Ag nanohybrid composite, and 2) the increased charges from the exposed CTAB’s quaternary ammonium ions, thus enhancing the stability and dispersity of Ag NPs.
Fig. 4. (a): UV–vis absorption spectra of the catalytic reduction of 4-nitrophenol by NaBH4 in the presence of CNC/CTAB/Ag nanohybrid composite at different reaction times (0, 5, 10, 15 and 20 min); (b): corresponding first-order kinetic plotting (absorbance at 400 nm, (ln(At /A0 )) versus reaction time for the reduction of 4-nitrophenol, At and A0 represented the absorbance values of 4-NP at 400 nm at designated time t and t = 0, respectively.
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Table 1 shows the comparison of catalytic activities of the asprepared catalysts with the Au NPs previously reported in the literature. The turnover frequency (TOF), which is defined as the number of moles of reduced 4-NP per mole of catalyst per hour when the conversion reaches 90%, is summarized in Table 1. The results supported the conclusion that CNC/CTAB/Ag nanohybrid composite exhibited the highest catalytic activity (545 h−1 ), which was comparable to that of the gold nanocatalysts (Koga et al., 2010). Thanks to the highly dispersed Ag NPs in the CNC/CTAB/Ag nanohybrid composite. 4. Conclusions The CNC/CTAB/Ag nanohybrid composite was synthesized using a facile process. Herein, CNC acted as an effective support/carrier during in-situ preparation of Ag NPs, while the surfactant CTAB acted an important role in stabilizing and dispersing Ag NPs in the system via steric effect and electrostatic repulsion. The UV–vis adsorption spectra confirmed that the as-prepared Ag NPs were embedded in the CNC/CTAB/Ag nanohybrid composite, which was further supported by the EDX analysis. The TEM results indicated that Ag NPs were well dispersed in the nanohybrid composite and they were in a narrow particle size distribution. The catalytic activity of CNC/CTAB/Ag nanohybrid composite was investigated. The results showed that CNC/CTAB/Ag nanohybrid possessed the highest catalytic activity for the reduction of 4-nitrophenol, compared with those of unsupported Ag NPs and CNC/Ag nanohybrid, thanks to the uniformly dispersed Ag NPs with narrow size distribution. The catalytic activity (545 h−1 ) and pseudo-first order rate constant (k = 1.6 × 10−3 s−1 ), were determined for the CNC/CTAB/Ag nanohybrid composite. Overall, the results indicated that CNC/CTAB/Ag nanohybrid composite has great potential in catalytic treatment of wastewater. Conflict of interest The authors declare no competing financial interest. Acknowledgements The authors thank Microscopy and Microanalysis Facility of UNB, for the TEM images, Canada Research Chairs program for the financial support. References Abitbol, T., Kloser, E., & Gray, D. G. (2013). Estimation of the surface sulfur content of cellulose nanocrystals prepared by sulfuric acid hydrolysis. Cellulose, 20(2), 785–794. Alshehri, S. M., Almuqati, T., Almuqati, N., Al-Farraj, E., Alhokbany, N., & Ahamad, T. (2016). Chitosan based polymer matrix with silver nanoparticles decorated multiwalled carbon nanotubes for catalytic reduction of 4-nitrophenol. Carbohydrate Polymers, 151, 135–143. An, X., Wen, Y., Cheng, D., Zhu, X., & Ni, Y. (2016). Preparation of cellulose nano-crystals through a sequential process of cellulase pretreatment and acid hydrolysis. Cellulose, 23(4), 2409–2420. Araki, J., Wada, M., Kuga, S., & Okano, T. (1998). Flow properties of microcrystalline cellulose suspension prepared by acid treatment of native cellulose. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 142(1), 75–82. Awazu, K., Fujimaki, M., Rockstuhl, C., Tominaga, J., Murakami, H., Ohki, Y., et al. (2008). A plasmonic photocatalyst consisting of silver nanoparticles embedded in titanium dioxide. Journal of the American Chemical Society, 130(5), 1676–1680. Boluk, Y., Lahiji, R., Zhao, L., & McDermott, M. T. (2011). Suspension viscosities and shape parameter of cellulose nanocrystals (CNC). Colloids and Surfaces A: Physicochemical and Engineering Aspects, 377(1–3), 297–303. Cai, J., Kimura, S., Wada, M., & Kuga, S. (2008). Nanoporous cellulose as metal nanoparticles support. Biomacromolecules, 10(1), 87–94. Campelo, J. M., Luna, D., Luque, R., Marinas, J. M., & Romero, A. A. (2009). Sustainable preparation of supported metal nanoparticles and their applications in catalysis. ChemSusChem, 2(1), 18–45.
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Contents lists available at ScienceDirect
Carbohydrate Polymers journal homepage: www.elsevier.com/locate/carbpol
Cellulose nanocrystal/hexadecyltrimethylammonium bromide/silver nanoparticle composite as a catalyst for reduction of 4-nitrophenol Xingye An a,b , Yunduo Long b,∗ , Yonghao Ni a,b,∗ a b
Tianjin Key Laboratory of Pulp and Paper, Tianjin University of Science and Technology, Tianjin 300457, China Department of Chemical Engineering, University of New Brunswick, Fredericton, New Brunswick, E3B 5A3, Canada
a r t i c l e
i n f o
Article history: Received 1 August 2016 Received in revised form 27 August 2016 Accepted 30 August 2016 Available online 13 September 2016 Keywords: Nanocatalyst Silver nanoparticles (Ag NPs) Cellulose nano-crystals (CNC) Support/carrier Hexadecyltrimethylammonium bromide (CTAB) 4-Nitrophenol (4-NP)
a b s t r a c t The application of metal nanoparticles (NPs) in catalysis has drawn significant research interest, including in their applications to the wastewater treatment. Nanocellulose can be a versatile support/carrier in stabilizing metal NPs catalysts, due to its unique properties. In this paper, hexadecyl-trimethylammonium bromide (CTAB) surfactant-adsorbed cellulose nanocrystals (CNC) was employed as a support for silver nanoparticles (Ag NPs). The obtained CNC/CTAB/Ag nanohybrid composite was characterized by ultraviolet–visible (UV–vis) spectroscopy, transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDX). The results of the catalytic reaction experiments showed that Ag NPs immobilized on CTAB-adsorbed CNC displayed higher catalytic efficiency towards the reduction of 4-nitrophenol to 4-aminophenol, compared to the control Ag NPs and CNC/Ag samples. The enhanced catalytic performance of Ag NPs was attributed to the improved dispersity and narrow distribution of silver nanoparticles in the catalytic system. This work provides a novel catalyst based on CNC-CTAB stabilized Ag NPs which would have promising applications towards catalytic treatment of wastewater. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Nanoscience and nanotechnology offer new opportunities for making superior materials for use in industrial, health, energy and environmental applications (HPS et al., 2016; Khademhosseini, Parak, & Weiss, 2016; Mulvaney, 2015; Murriello, Contier, & Knobel, 2016; Shen, Song, Qian, Yang, & Kong, 2010). Nanostructured particles, especially metal nanoparticles (NPs), have attracted much attention over the last decade (Djerahov, Vasileva, Karadjova, Kurakalva, & Aradhi, 2016; Kang et al., 2016; Nazirov et al., 2016; Sadanand, Rajini, Rajulu, & Satyanarayana, 2016), and have also been widely studied in different fields from materials science and engineering to biomedical applications (Li et al., 2012; Tada, Kiyonaga, & Naya, 2009; Walekar et al., 2014). Among these applications, the use of catalytic metal NPs in waste water treatment is of particular interest because of their unique electronic properties and versatile catalytic activities (Daniel & Astruc, 2004; Narayanan & El-Sayed, 2005; Zheng & Stucky, 2006). For example, 4-nitrophenol (4-NP), which is a pollutant compound in both industrial and agri-
∗ Corresponding authors at: Department of Chemical Engineering, University of New Brunswick, Fredericton, New Brunswick, E3B 5A3, Canada. E-mail addresses: [email protected] (Y. Long), [email protected] (Y. Ni). http://dx.doi.org/10.1016/j.carbpol.2016.08.099 0144-8617/© 2016 Elsevier Ltd. All rights reserved.
cultural wastewater (Murugan & Jebaranjitham, 2012; Ofomaja & Unuabonah, 2011), can be reduced by metal NPs catalysts to 4aminophenol (4-AP) (Alshehri et al., 2016; Narayanan & Sakthivel, 2011; Torkamani & Azizian, 2016). However, metal NPs are thermodynamically unstable and tend to aggregate to minimize their surface area, thus resulting in the significant loss of catalytic activities (Kaushik & Moores, 2016; Koga et al., 2010; Maity et al., 2013). The concept of immobilization or stabilization of metal NPs is one of the potential methods to overcome the aggregation issue (Cao, Tang, Liu, Nie, & Zhao, 2010; Liu et al., 2015a; Mahmoud, Lam, Hrapovic, & Luong, 2013). There are many types of matrix used to support/immobilize metal NPs in catalysis, such as carbon materials (Daio et al., 2015; Duman, Tunc¸, Polat, & Bozo˘glan, 2016; Sun and Li, 2004), metal oxides (Mogyorósi et al., 2009; Zhang, Liu, Guo, Wu, & Wang, 2010), and polymers (Kuroda, Ishida, & Haruta, 2009). A numerous variety of NPs can be synthesized through different methods (onto different supports/substrates) to give tailored sizes, shapes and distributions, which have shown a positive impact on the subsequent catalytic processes (Campelo, Luna, Luque, Marinas, & Romero, 2009). Nanocelluloses, including cellulose nanocrystal (CNC), nanofibrillated cellulose (NFC) and bacterial nanocellulose (BNC), have potential applications in paper, cosmetics, food, pharmaceuticals,
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and biomedical industries (An, Wen, Cheng, Zhu, & Ni, 2016; Jahan, Saeed, He, & Ni, 2010; Nath, Chaliha, Kalita, & Kalita, 2016; Zaman, Liu, Xiao, Chibante, & Ni, 2013). Among the many applications of nanocelluloses, their use in the synthesis of metal NPs catalysts is appealing due to their high surface area, thermal stability, functionalized surface and environmental benefits (Kaushik & Moores, 2016; Liu et al., 2016). Based on the unique features mentioned above, CNC can play an important role in synthesizing metal NPs catalyst and may have a noticeable impact in terms of the efficiency of wastewater treatment. Generally, the deposition of metal NPs on the surface of CNC can be readily achieved by using a reducing agent, e.g., sodium borohydride (NaBH4 ). In these cases, the in-situ formed nano-metal particles are then stabilized due to the complexation with CNC via ion-dipole interactions (He, Kunitake, & Nakao, 2003). Hexadecyltrimethylammonium bromide (CTAB), a cationic surfactant with an alkyl chain (hydrophobic tail) and quaternary ammonium group (hydrophilic head), has been used to stabilize inorganic NPs (Cai, Kimura, Wada, & Kuga, 2008; Sun, Yin, Mayers, Herricks, & Xia, 2002). Recently, Padalkar et al. have successfully synthesized the well dispersed Ag, Cu, Au, and Pt NPs decorated tunicate CNC, respectively, with the help of multifunctional CTAB (Padalkar et al., 2010). It is noted that CTAB was not only a stabilizer of metallic NPs but also a vehicle for positioning of these particles on the CNC surface by non-covalent interactions with hydroxyl groups. Herein, we put out a facile synthesis of CNC/CTAB/Ag nanohybrid composite toward the catalytic reduction of 4-nitrophenol to 4-aminophenol. In this study, CNC was prepared from dissolving pulp by traditional 64 wt% sulfuric acid hydrolysis. It was concluded that Ag NPs loaded on the surface of CNC substrate can be stabilized and in a narrow particle size distribution with the help of CTAB, due to: 1) the steric effect from CTAB binding onto Ag NPs via electrostatic interactions between the hydroxyl groups in Ag NPs and hydrophilic heads of CTAB, and 2) the increased negative charges from the exposed CTAB’s quaternary ammonium ions, thus enhancing the dispersity of Ag NPs. The catalytic activity of CNC/CTAB/Ag nanohybrid composite was compared with those of Ag NPs and CNC/Ag nanohybrid composite. The as-prepared nanocatalyst were characterized by ultraviolet–visible (UV–vis) spectroscopy, transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDX). The catalytic activities of the nanohybrid composite were evaluated by the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) with an excess amount of NaBH4 .
2. Experimental section 2.1. Materials The CNC suspension was prepared by sulfuric acid (64 wt%) hydrolysis of dissolving pulp according to the method described in the literature (Boluk, Lahiji, Zhao, & McDermott, 2011; Hu, Cranston, Ng & Pelton, 2014). Typically, 5 g of dissolving pulp was hydrolyzed by using 64 wt% sulfuric acid at 50 ◦ C for 45 min and then diluted with deionized water to stop the reaction. The suspension was then centrifuged, and dialyzed to remove the acid and other impurities until a fairly steady pH was reached. Finally, ultrasound treatment was conducted to disperse the CNC particles. The sulfate ester group content of as-prepared CNC was 325.3 mol/g based on the conductometric titration method (Abitbol, Kloser & Gray, 2013; Araki, Wada, Kuga & Okano, 1998). Silver nitrate (AgNO3 , ≥99.0%), hexadecyltrimethylammonium bromide (CTAB, ≥99.0%), sodium borohydride (NaBH4 , ≥99.0%) and 4-nitrophenol (4-NP, ≥99.0%) were all purchased from SigmaAldrich Co. Ltd. Deionized water was used to prepare all aqueous
solutions. All other chemicals were of analytical grade and used without any further purification. 2.2. Synthesis of CNC/CTAB/Ag nanohybrid composites For preparation of CNC/CTAB/Ag nanohybrid composite, the asprepared CNC suspension was diluted into 0.2 wt% followed by sonification for 10 min, and the well-dispersed CNC suspension (50 mL, 0.2 wt%) was added in the 100 mL beaker under constant magnetic stirring. A buffer solution (0.1 M HAc-NaAc) was used to adjust the pH to around 4.5. Then 30 mL CTAB solution (0.5 mM) was added into the diluted CNC suspension and stirred for 10 min, followed by the addition of dropwise silver nitrate solution (20 mL, 0.1 mM). After that, a freshly prepared aqueous sodium borohydride solution (20 mL, 10 mM) was added dropwise into the well-mixed suspension under constant magnetic stirring for 5 min, which reduced Ag+ ions into Ag NPs. The thus obtained CNC/CTAB/Ag nanohybrid composite was ready for further experiments. For comparison, Ag NPs, CNC/Ag nanohybrid composite was also synthesized using the similar procedure above. 2.3. Characterization and measurements A UV–vis spectrophotometer (Genesys 10-S, Thermo) was used to record the UV–vis adsorption spectrums of CNC-CTAB and CNC/CTAB/Ag to confirm Ag NPs in the nanohybrid composite. The morphology and structure of Ag NPs, CNC/Ag and CNC/CTAB/Ag nanohybrid composite were characterized by a transmission electron microscopy (TEM, JEOL 2010, Japan) equipped with an energy dispersive X-ray spectroscopy (EDX). 2.4. Catalytic reduction of 4-nitrophenol The reduction of 4-NP with NaBH4 solution was used as a model reaction to determine the catalytic activity of CNC/CTAB/Ag nanohybrid composite. As an excess amount of NaBH4 was present in the system, the reduction kinetics would be independent of the NaBH4 concentration. Upon the addition of NaBH4 into the 4-NP solution, its color changed immediately from light yellow to dark yellow due to the formation of 4-nitrophenolate ion (formed from the high alkalinity of NaBH4 ). Then, the dark yellow color faded with time (due to the conversion of 4-NP to 4-AP) after the addition of CNC/CTAB/Ag nanohybrid composite, which could be monitored by a UV–vis spectroscopy. Typically, an aqueous suspension of CNC/CTAB/Ag nanohybrid composite (containing 0.02 mol Ag) was added into a mixed aqueous solution of 30 mL containing 4-NP (0.1 mM) and NaBH4 (10 mM) under stirring at room temperature (293 K). At appropriate time intervals, an aliquot of reaction mixture (1.2 mL) was filtered, and the UV–vis adsorption spectrum of the filtrate was recorded by UV–vis spectrophotometer in the range of 250–550 nm. The reaction rate was followed by measuring the change in absorbance at 400 nm (the peak of 4-nitrophenolate ion) as a function of time. 3. Results and discussion 3.1. Characterization of CNC/CTAB/Ag nanohybrid composites 3.1.1. UV–vis spectra analysis UV–vis spectra technique can be applied in confirming the presence of metal NPs in the reaction system. As shown in Fig. 1, the aqueous suspension of CNC-CTAB was almost transparent and no bands appear in the spectrum (Fig. 1(A)). In contrast, the obtained CNC/CTAB/Ag nanohybrid composite suspension was light yellow, and the surface plasmon resonance (SPR) band at 410 nm confirmed the formation of silver nanoparticles.
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corresponding EDX spectra of CNC/CTAB/Ag nanohybrid composite confirmed the presence of Ag and Br elements. The improved results stated above can be attributed to the stabilizing function of CTAB surfactant due to the fact that: 1) when binding onto Ag NPs via electrostatic interactions, CTAB surfactants can serve as steric barriers; 2) the exposed CTAB’s quaternary ammonium ions, which are much stronger than the hydroxyl groups on the original Ag NPs, increased the charge density of the resultant Ag NPs, enhancing the dispersity of Ag NPs. 3.2. Proposed function of CTAB in the preparation of CNC/CTAB/Ag nanohybrid composite
Fig. 1. UV–vis adsorption spectra of an aqueous suspension of (A): CNC-CTAB; and (B): CNC/CTAB/Ag nanohybrid composite. The SPR band at 410 nm confirms the presence of silver nanoparticles(For interpretation of the references to colour in text, the reader is referred to the web version of this article.). Table 1 Comparison of catalytic performance of catalytic converting 4-NP to 4-AP, using different metal nanocatalysts. Sample
Temperature/K
4-NP/Ag [mol/mol]
TOF/h−1
Ag NPs CNC/Ag CNC/CTAB/Ag CSNFs/Au (Koga et al., 2010)
293 293 293 298
150/1 150/1 150/1 150/1
54 474 545 563
Shown in Fig. 3 is the concept of preparing the CNC/CTAB/Ag nanohybrid composite. The rich hydroxyl and sulfate ester groups presented on the CNC surface (Fig. 3(a)) can interact with the hydrophilic (cationic) head of CTAB, as shown in Fig. 3(b). Under the conditions as such, a bilayer CTAB structure (hydrophobic interactions between the two CTAB molecules) (Walekar et al., 2014) can be formed in the system. The Ag NPs, in situ synthesized by the reduction of AgNO3 by NaBH4 , can then be in complexation with CTAB via the hydrophilic interactions, shown in Fig. 3(c). Here, CTAB had the steric effect from CTAB binding onto Ag NPs via electrostatic interactions between the hydroxyl groups in Ag NPs and hydrophilic heads of CTAB; in addition, the exposed CTAB’s hydrophilic heads (quaternary ammonium ions) would create strong electron repulsion, both of which contributed to the improved Ag NPs stability and dispersity. 3.3. Catalytic reduction of 4-nitrophenol
In the literature, the band at 410 nm has been used by others to support the presence of Ag NPs in the nano-composite catalyst. For example, Awazu et al. (2008) prepared a photocatalyst consisting of Ag NPs embedded in TiO2 and SiO2 , and found a localized surface plasmon (LSP) UV–vis resonance peak at 410 nm, which was attributed to the existence of Ag NPs in the TiO2 /SiO2 /Ag NPs composite (Awazu et al., 2008). 3.1.2. TEM analyses and Ag NPs size/distribution analyses Shown in Fig. 2 are the TEM images of pristine CNC, unsupported Ag NPs, CNC/Ag and CNC/CTAB/Ag nanohybrid composite. The pristine CNC (Fig. 2(a)) had a length of ca. 200–250 nm and a width of 15–20 nm, which were typical for CNC (Wang et al., 2012). Fig. 2(b) illustrates that unsupported silver particles aggregated severely in the absence of CNC and CTAB. In contrast, Ag NPs shown in Fig. 2(c) were well dispersed in the presence of CNC substrate, which supported the conclusion that CNC can act as a good dispersant/support of nanoparticles, preventing the NPs aggregation due to the excellent properties of CNC. Liu et al. also prepared well dispersed Fe3 O4 NPs in the presence of CNC, which functioned as a dispersant (Liu, Nasrallah, Chen, Huang, & Ni, 2015b). In addition, the corresponding EDX spectra of CNC/Ag composite (Fig. 2(d)) also supported the conclusion that Ag NPs were embedded in the CNC substrate, forming the CNC/Ag composite. Comparison of Fig. 2(c) and (e) showed that Ag NPs were more efficiently dispersed in the presence of CTAB surfactant, and that fine Ag NPs were evident and in a smaller particle size and narrower particle size distribution in the case of CNC/CTAB/Ag nanohybrid composite. In the absence of CTAB (Fig. 2(c)), a greater proportion of the particles was in large aggregates (>15 nm). In contrast, in the presence of CTAB (Fig. 2(e)), the fraction of large aggregates decreased significantly and 84% of the Ag NPs was below 10 nm. The improved Ag NPs characteristics were expected to improve the catalytic activity/efficiency as nano-catalyst. Furthermore, the
Shown in Fig. 4(a) are the results on the catalytic reduction of 4-NP to 4-AP in the presence of NaBH4 and the CNC/CTAB/Ag nanohybrid composite. The reduction process was monitored based on the UV–vis spectrophotometry. It was found that the absorbance at 400 nm (due to 4-NP) gradually decreased as a function of time, while the absorbance at 290 nm (due to 4-AP) increased, indicating that the catalytic reduction of 4-NP to 4-AP, occurred successfully (Esumi, Isono & Yoshimura, 2004). The catalytic reduction was almost completed within 20 min at room temperature. The reaction did not occur at all in the presence of CNC alone or CTAB (data not shown). Similar report can also be found in the literature. Liu and Zhao found that using Ag NPs (with about 10 nm diameter) that were immobilized onto different substrates (the halloysite nanotubes (HNTs)), the catalytic reduction of 4-NP to 4-AP was completed between 25 and 50 min, depending on the amount of NaBH4 present in the reaction system (Liu & Zhao, 2009). The kinetics of the Ag NPs, CNC/Ag and CNC/CTAB/Ag systems are shown in Fig. 4(b). In each case, a pseudo-first order kinetic was evident. The pseudo-first order rate constant (k) for Ag NPs, CNC/Ag and CNC/CTAB/Ag were 1.5 × 10−4 s−1 , 1.2 × 10−3 s−1 and 1.6 × 10−3 s−1 , respectively. In case of Ag NPs, the catalytic activity was the lowest due to the aggregation of Ag nanoparticles, while it was the highest for the CNC/CTAB/Ag composite sample. The increased catalytic activity of the CNC/CTAB/Ag system was due to the well dispersed silver nanoparticles, allowing effective contact with the reactants and catalysts. Wu et al. synthesized CNC-supported palladium nanoparticles (Pd NPs) to reduce methylene blue (MB) and 4-NP (Wu et al., 2013). It was found that the obtained CNC-supported Pd NPs exhibited much higher activities than the unsupported Pd NPs in the catalyzed reduction of MB and 4-NP. Fenger et al. reported that gold nanoparticles prepared by CTAB-stabilized-supports exhibited efficient catalytic conversion of 4-NP to 4-AP (Fenger, Fertitta, Kirmse, Thünemann & Rademann, 2012).
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Fig. 2. TEM images of (a) pristine CNC, (b) unsupported Ag NPs, (c) CNC/Ag, and (e) CNC/CTAB/Ag nanohybrid composite, (Scale bars of (a)–(c) and (e) were 0.5 m, respectively, while the scale bars of inserted magnified images of (c) and (e) were 200 nm, respectively), EDX spectra of (d) CNC/Ag nanohybrid composite and (f) CNC/CTAB/Ag nanohybrid composite; the inserted histograms in (c) and (e) were Ag NPs size distributions of CNC/Ag and CNC/CTAB/Ag nanohybrid composite.
Fig. 3. Schematic illustration for the synthesis of CNC/CTAB/Ag nanohybrid composite: (a) hydroxyl and sulfate ester groups on the surface of pristine CNC; (b) hydroxyl and sulfate ester groups are modified by bi-layer CTAB surfactant; (c) synthesized in-situ Ag NPs was loaded on the surface of CNC substrate. Ag NPs are stabilized due to 1): the steric barrier function on the CNC/CTAB/Ag nanohybrid composite, and 2) the increased charges from the exposed CTAB’s quaternary ammonium ions, thus enhancing the stability and dispersity of Ag NPs.
Fig. 4. (a): UV–vis absorption spectra of the catalytic reduction of 4-nitrophenol by NaBH4 in the presence of CNC/CTAB/Ag nanohybrid composite at different reaction times (0, 5, 10, 15 and 20 min); (b): corresponding first-order kinetic plotting (absorbance at 400 nm, (ln(At /A0 )) versus reaction time for the reduction of 4-nitrophenol, At and A0 represented the absorbance values of 4-NP at 400 nm at designated time t and t = 0, respectively.
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Table 1 shows the comparison of catalytic activities of the asprepared catalysts with the Au NPs previously reported in the literature. The turnover frequency (TOF), which is defined as the number of moles of reduced 4-NP per mole of catalyst per hour when the conversion reaches 90%, is summarized in Table 1. The results supported the conclusion that CNC/CTAB/Ag nanohybrid composite exhibited the highest catalytic activity (545 h−1 ), which was comparable to that of the gold nanocatalysts (Koga et al., 2010). Thanks to the highly dispersed Ag NPs in the CNC/CTAB/Ag nanohybrid composite. 4. Conclusions The CNC/CTAB/Ag nanohybrid composite was synthesized using a facile process. Herein, CNC acted as an effective support/carrier during in-situ preparation of Ag NPs, while the surfactant CTAB acted an important role in stabilizing and dispersing Ag NPs in the system via steric effect and electrostatic repulsion. The UV–vis adsorption spectra confirmed that the as-prepared Ag NPs were embedded in the CNC/CTAB/Ag nanohybrid composite, which was further supported by the EDX analysis. The TEM results indicated that Ag NPs were well dispersed in the nanohybrid composite and they were in a narrow particle size distribution. The catalytic activity of CNC/CTAB/Ag nanohybrid composite was investigated. The results showed that CNC/CTAB/Ag nanohybrid possessed the highest catalytic activity for the reduction of 4-nitrophenol, compared with those of unsupported Ag NPs and CNC/Ag nanohybrid, thanks to the uniformly dispersed Ag NPs with narrow size distribution. The catalytic activity (545 h−1 ) and pseudo-first order rate constant (k = 1.6 × 10−3 s−1 ), were determined for the CNC/CTAB/Ag nanohybrid composite. Overall, the results indicated that CNC/CTAB/Ag nanohybrid composite has great potential in catalytic treatment of wastewater. Conflict of interest The authors declare no competing financial interest. Acknowledgements The authors thank Microscopy and Microanalysis Facility of UNB, for the TEM images, Canada Research Chairs program for the financial support. References Abitbol, T., Kloser, E., & Gray, D. G. (2013). Estimation of the surface sulfur content of cellulose nanocrystals prepared by sulfuric acid hydrolysis. Cellulose, 20(2), 785–794. Alshehri, S. M., Almuqati, T., Almuqati, N., Al-Farraj, E., Alhokbany, N., & Ahamad, T. (2016). Chitosan based polymer matrix with silver nanoparticles decorated multiwalled carbon nanotubes for catalytic reduction of 4-nitrophenol. Carbohydrate Polymers, 151, 135–143. An, X., Wen, Y., Cheng, D., Zhu, X., & Ni, Y. (2016). Preparation of cellulose nano-crystals through a sequential process of cellulase pretreatment and acid hydrolysis. Cellulose, 23(4), 2409–2420. Araki, J., Wada, M., Kuga, S., & Okano, T. (1998). Flow properties of microcrystalline cellulose suspension prepared by acid treatment of native cellulose. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 142(1), 75–82. Awazu, K., Fujimaki, M., Rockstuhl, C., Tominaga, J., Murakami, H., Ohki, Y., et al. (2008). A plasmonic photocatalyst consisting of silver nanoparticles embedded in titanium dioxide. Journal of the American Chemical Society, 130(5), 1676–1680. Boluk, Y., Lahiji, R., Zhao, L., & McDermott, M. T. (2011). Suspension viscosities and shape parameter of cellulose nanocrystals (CNC). Colloids and Surfaces A: Physicochemical and Engineering Aspects, 377(1–3), 297–303. Cai, J., Kimura, S., Wada, M., & Kuga, S. (2008). Nanoporous cellulose as metal nanoparticles support. Biomacromolecules, 10(1), 87–94. Campelo, J. M., Luna, D., Luque, R., Marinas, J. M., & Romero, A. A. (2009). Sustainable preparation of supported metal nanoparticles and their applications in catalysis. ChemSusChem, 2(1), 18–45.
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