Over the last decade, nanostructured particles, especially inorganic nanoparticles (NPs), have received more attention in biomedical applications, materials science, and engineering [1-6].
Inorganic NPs, show special properties including tunable physicochemical characteristics, high surface area to volume ratio, small size, increased numbers of atoms at the edges, and the corners which are surrounded by fewer atoms and considered to be more reactive [7, 8]. Among the inorganic NPs, silver as the most frequently used noble metal NPs has attracted considerable attention because of efficiency, cost viability and the potential in various applications such as catalysis and antimicrobials [2, 3, 9, 10]. However, metal NPs tend to aggregate that minimizes their surface area and reduce their catalytic performance. One of the potential methods to overcome this problem is using hybrid inorganic-polymer nanocomposites. In this regard, environmentally benign natural polymers, like chitin, alginic acid, chitosan, and cellulose [11-14] related to hydrophilicity and multiple functionalities, having the strong ability to chelate metal ions could serve the purpose better.
NCC, as one of the natural biopolymers could be obtained from different sources such as cotton by using acid hydrolysis process as a common method . However, other modified techniques including mechanical processes compared to acid hydrolysis have been developed which are simple, low cost, and environmentally friendly .
Generally, the deposition of metal NPs on the surface of NCC can be readily achieved using a strong reducing agent like NaBH4. Recently, various methods were applied for surface functionalization of NCC by chemical bonding to control morphology and particle size of metal nanoparticles using a surfactant . Moreover, surfactant similar to other dispersants was utilized to prevent aggregation of NPs in the chemical reduction process .
Cetyltrimethylammonium bromide (CTAB), a cationic surfactant with multifunctional groups (alkyl chain and quaternary ammonium group as hydrophobic tail and hydrophilic head) with the abilities of reduction and/or coordination, has been used to stabilize inorganic NPs [18- 20].
Zaid et al. synthesized functionalized iron oxide with mercaptopropionic acid (MPA-Fe3O4) nanoparticle and commercial nanocellulose crystalline functionalized cetyl trimethyl ammonium bromide (NCC/CTAB) for DNA electrochemical biosensor. .
Manan et al. prepared commercial nanocrystalline cellulose (NCC) with cationic surfactant of cetyltriammonium bromide (CTAB) and further decorated with 3-mercaptopropionic acid (3-MPA) capped CdS QDs as biosensor for phenol determination . An et al. reported NCC/CTAB/Ag nanocomposite as a catalyst for the reduction of 4-Nitrophenol. Yalcinkaya et al. have synthesized NCC/CTAB/Ag nanocomposites for use in PLA films. NCC in all the above composites are prepared commercially or they are obtained by sulphuric acid hydrolysis [7, 19]. However, in our study, NCC was prepared from microcrystalline cellulose by a simple and facile mechanical method using ultrasonication without acid hydrolysis as a common chemical process. It was used as green support/carrier for silver nanoparticle by CTAB as stabilizer.
Organic dyes were widely used and released by many industries, which have been attracting serious public concern due to environmental contamination [23, 24]. The anionic dye of methyl orange due to intense color, toxicity, and carcinogenic effects is a pollutant compound when released into the environment. 4-Nitrophenol is another toxic and chemically hazardous compound due to solubility and stability in the ordinary and wastewater systems. Numerous methods were used for the degradation of MO and 4-NP. Catalytic degradation of MO to hydrazine derivatives and 4-NP to 4-Aminophenol (4-AP) by metal nanoparticles is an important way without the production of secondary toxic substances [25-30]. Hence, developing an effective method for the degradation of these dyes from the environment is still a challenge. Recently, we reported the synthesis of Ag/NCC and its catalytic application in degradation of toxic dyes . Here, NCC can be improved by using the surfactant for the synthesis of NCC/CTAB/Ag nanocomposite. The formation of cellulose base nanocomposite was confirmed by FTIR, FESEM, EDS, TEM and XRD characterization. This research reported an effective catalyst for highly efficient reduction of organic pollutants (MO and 4-NP) from environmental water within few minutes.
MATERIALS AND METHODS
All chemicals were purchased from Merck and Acros chemical Companies and were used without further purification. IR spectra were recorded from KBr disk using FT-IR Bruker Tensor 27 instrument. Fieldemission scanning electron microscopy (FE-SEM) with a FEI-Nanosem 450. Transmission electron microscopy (TEM) images have been taken with a Philips, model EM 208s. The energy dispersive spectroscopy (EDS) analysis was done using a SAMx-analyzer. The X-ray diffraction patterns (XRD) were obtained using a Phillips PW1730 X-ray diffractometer. The diffraction patterns were recorded using Cu-K radiation at 40 kV and 30 mA. The samples were scanned in a 2 range between 10 and 80. Ultraviolet–visible (UV–Vis) measurements were performed using Lambda-25 UV–Vis spectrometer.
Preparation of NCC
The mixture of microcrystalline cellulose in water (3 wt.%) put into ice/water bath and set in the ultrasonicator for 15 min at an output power of 1200 W. The obtained colloidal suspension was centrifuged, the supernatant was dried and stored at 5 ℃ .
Preparation of Ag /CTAB/ NCC nanocomposite
Synthesis of Ag nanocomposite was carried out in a typical experiment with some modification,  in which NCC suspension was diluted into 0.2 wt.% followed by sonification for 10 min. Then, in the 100 mL beaker in pH ~ 4.5, CTAB solution (30 mL, 0.5 mM) was added into the NCC suspension followed by the addition of a dropwise silver nitrate solution (20 mL, 0.1 mM). After that, aqueous sodium borohydride solution (20 mL, 10 mM) was added dropwise into the well-mixed suspension under constant magnetic stirring for 5 min the solution was then washed with distilled water and dried in an oven at 60 ℃.
Catalytic removal of dye
The catalytic reduction of methyl orange was carried out in UV cuvette using methyl orange (0.05 mM, 2.5 mL), and sodium borohydride solution (0.1 M, 0.5 mL) in presence of 5 mg of the dried composite as a catalyst. The color change of the solution was observed and the absorbance of resulting solutions was monitored using UV–Vis spectrophotometer.
An aqueous suspension of 42 mg of the dried composite was added into a mixed aqueous solution containing methyl orange or 4- nitrophenol (0.25 ml, 20 mM), sodium borohydride solution (0.25 ml, 5M), and 19.5 ml DI water under stirring at room temperature. Then, an aliquot of the reaction mixture (0.5 mL) at the appropriate intervals was filtered, the absorbance of the filtrate was recorded using a UV-Vis spectrophotometer in the wavelength range of 250-550 nm. The reaction rate was measured as a function of time  .
RESULTS AND DISCUSSION
Characterization of catalyst
In the present work, NCC prepared from MCC, NCC analysis and the difference with MCC were discussed in our previous work . Here we describe the characterization of the as- prepared composite.
The FTIR spectra of NCC and Ag /CTAB/ NCC composite are shown in (Fig. 1). NCC showed broad band at 3450 cm-1, which can be assigned to the stretching vibration of hydroxyl groups. The peaks at 2850 and 2900 cm-1 were related to the stretching of C-H. The bands at 1645, 1420, and 1010 cm-1 were attributed to water absorption, -CH2 groups, and the C-O-C stretching, respectively. For the Ag /CTAB/ NCC composite, a decreased intensity of the bands at 3450 cm-1 and 1010 cm-1 corresponding to the O-H and C-O stretching confirms the grafting of AgNPs to the O-H groups on the NCC surfaces was effective .
Fig. 2a, b showed the FE-SEM image of the Ag /CTAB/ NCC nanocomposite. In SEM images, a large number of spherical NPs were distributed on the surface of nanocellulose indicating that the formation of Ag nanoparticles. The EDS spectrum of nanocomposite is presented in (Fig. 2c). It exhibits the presence of C, O, and Ag elements. The intense peak at around 3 keV related to the Ag signal. Indicating the Ag had been successfully loaded on the surface of nanocellulose.
The TEM images of Ag/ CTAB/NCC nanocomposite with different magnification revealed the formation of spherical Ag nanoparticles with an average size between 20 and 50 nm and good dispersity on the surface of NCC. The images showed the loading of small particle size of spherical AgNPs with high dispersity and low aggregation that can be attributed to the binding of Ag nanoparticles to the ammonium ions of the CTAB as a surfactant .
The crystalline structure of the as-prepared product was determined using XRD (Fig. 4). X-ray diffractogram of Ag/ CTAB/NCC was attributed to the characteristic peaks at 38.17°, 44.37°, 64.52°, 77.42° indexed as (111), (200), (220) and (311) crystallographic planes of the face-centered cubic (fcc) structure of Ag NPs on the NCC support (JCPDS 4-783) . The sharp peaks of materials without reflections related to impurities phases show that the nanoparticles have the perfect phase purity and high crystallinity. Crystallite size of the silver nanoparticles was calculated about 30 nm through XRD analysis using Scherrer’s formula (Eq. (1):
Where d is the average particle size, is X-ray wavelength (0.154 nm), is FWHM of the diffraction line (radian), the diffraction angle, and K constant, generally assumed as 0.9.
To investigate the catalytic performance of the Ag /CTAB/ NCC nanocatalyst in the presence of NaBH4, methyl orange dye was used. Fig. 5a, b show a typical evolution of the UV–Vis spectra of methyl orange during catalytic reduction after the addition of NaBH4 via method (1) and 2, respectively. It is well known that the spectrum of methyl orange shows a peak at 465 nm . After the addition of newly synthesized nanocomposite, the spectral band of MO is disappeared at 465 nm after 180 s in method 1 (Fig. 5a) and 150 s in method 2 (Fig. 5b). A new band appears at 250 nm which is assigned to hydrazine derivatives by changing the color from orange to colorless . When the nanocatalyst was added for the reduction of 4-NP, the peak of 4- nitrophenolate ion (~400 nm) faded gradually. Meanwhile, a new absorption peak at 298 nm increased intensity with reaction time. The appearance of such peak was taken as evidence for the formation of 4-AP . The catalytic degradation at ambient conditions was complete within 180 s (Fig. 5c).
The linear plots of ln (At/ A0) versus reduction time for the MO catalytic reduction over the Ag/ CTAB/NCC composite were shown in Fig. 6. Reaction rates are determined by measuring the absorbance values at 465 nm as a function of time. Generally, the reduction reaction can be assumed as pseudo-first-order kinetics due to excess concentration of NaBH4 compared to MO dye. It can be described by the Langmuir-Hinshelwood equation . The rate constant of the catalyst was estimated directly by the slope of the linear graph 4.8 × 10-3 s-1 by method 1(Fig. 6a). and 14.2× 10-3 s-1 by method 2 (Fig. 6b) for MO and 5.4 × 10-3 s-1 for 4-NP (Fig. 6c).
To compare the catalytic activity of the nanocomposite with the other reported catalysts including Ag NPs used for the reduction of MO and 4-NP, the rate constants, k, were summarized in Table 1 and Table 2, respectively. The catalytic activity of this catalyst is comparable with other listed catalysts in Tables 1 and 2. As shown in Table 2, the differences between the reaction rate constants of NCC/CTAB/ Ag (Ref. ) (1.6 × 10-3 s-1) and Ag/ CTAB/NCC in this work (5.4 ×10-3 s-1), are as follows: addition to difference in NCC preparation method as support, the concentration of 4-NP and reducing agent (NaBH4) also the amount of catalyst are different. In ref , the molar ratio of sodium borohydride to 4-nitrophenol (mmol NaBH4 / mmol 4-Np) is 100 and the aqueous suspension of CNC/ CTAB/ Ag nanocatalyst contains 0.02 µmol Ag. While in this work, the molar ratio (mmol NaBH4 / mmol 4-Np) increased to 250 and 42 mg of composite was used. Due to the aforementioned differences, the reaction rate constant in this work is almost 3 times higher than that mentioned in ref .
The nanohybrid (Ag/CTAB/NCC), consists of environmentally benign components Ag and NCC as support for Ag nanoparticles in which NCC was prepared using the sonication process without acid hydrolysis. Ag NPs was also stabilized on NCC surface using CTAB. The nanocomposite demonstrated high catalytic efficiency in reduction of MO and 4-NP by NaBH4. The effect of the catalyst amount in enhancing the reduction rate was shown by comparing the reduction process in the two methods, which is about three times higher for that of employing higher amount of catalyst. Increasing of catalyst provides more active sites for binding of reducing agent and organic dye.The corresponding pseudo-first-order rate constants for reduction of MO increased with the method (2) (14.2 × 10-3 s-1) as compared to those exhibited by the method (1) (4.8 × 10-3 s-1). Besides, this catalyst reduced 4-NP to 4-AP via the method (2) by a rate constant of 5.4× 10-3 s-1. The results showed that Ag /CTAB/ NCC nanocatalyst has strong catalytic properties compared to other reported catalysts. This green and cost-effective nanocatalyst expands the application of inorganic nano-metal composites for use in environmental protection.
The authors gratefully acknowledge from the Research Council of Alzahra University for financial support.
CONFLICTS OF INTEREST
The authors declare that there are no conflicts of interest regarding the publication of this manuscript.