Physical Study of the Primary and Secondary Photothermal Events in

Research Article pubs.acs.org/journal/ascecg

Physical Study of the Primary and Secondary Photothermal Events in Gold/Cellulose Nanocrystals (AuNP/CNC) Nanocomposites Embedded in PVA Matrices Zhijuan Hu,† Qijun Meng,† Rui Liu, Shiyu Fu,*,† and Lucian A. Lucia‡

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†

State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Wushan road, Tianhe district, Guangzhou 510640, China ‡ The Laboratory of Soft Materials & Green Chemistry, Department of Chemistry, College of Natural Resources, North Carolina State University, 2820 Faucette Drive, Raleigh, North Carolina 27695, United States ABSTRACT: Inorganic/organic nanocomposites are a class of materials that have garnered a flurry of research because of their unusually independent tandem or synergistic physical and chemical properties. A unique class of nanocomposites has been prepared that is composed of tunable diameter (24−45 nm) gold nanoparticles (AuNPs) from cellulose nanocrystals (CNCs) by a one-step protocol at room temperature. The CNCs behave as reducing, capping, and stabilizing agents during the in situ synthesis. The morphology and diameter of the AuNPs were controlled by the number of CNCs. It was serendipitously discovered that the AuNPs displayed a photothermal effect upon laser irradiation at the AuNPs SPR frequency (532 nm). The as-generated AuNPs/CNC were blended as types of bifunctional nanofillers within a poly(vinyl alcohol) (PVA) film to provide a unique and desirable combination of high tensile strength and photothermal chemistry. The CNCs played a significant role in the AuNPs/CNC/PVA film by scattering incident light in the film as a secondary light source for AuNP absorbance. Remarkably, the photothermal efficiency of AuNPs in film improved ∼50% because of CNC, a phenomenon that was enhanced by lower transparency CNCs (higher light scattering point sources). KEYWORDS: Gold nanoparticles, Cellulose nanocrystals, In situ reduction, Photothermal effect, Light scattering

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INTRODUCTION

host matrices, whereas multifunctional nanofillers can provide multiple properties for the resultant nanocomposites. Gold nanoparticles (AuNPs) are a unique class of nanometric materials that have been the subject of intense interest recently because they display great potential applications, inter alia, in diagnosis and therapy,9−11 catalysis,12 drug release,13 and biosensing.14 One of the most intriguing of the physicochemical properties that they exhibit is the photothermal effect (i.e., the conversion of light energy to heat), a phenomenon predicated by surface plasmon resonance (SRP) that is typically triggered by photoexcitation through nonradiative relaxation pathways.15,16 Figure 1 elaborates the mechanism, in which AuNPs can absorb light strongly in a specific region because of the coherent oscillations of the surface metal conduction band electrons that are in strong resonance with specific visible frequencies of light. The photoexcitation of metal atomic lattice contributes to the formation of a heated electron gas that can exchange energy

Cellulose nanocrystals (CNCs), typically isolated from natural cellulose sources by acid hydrolysis, have rod-like shapes whose aspect ratios can be ∼15 (widths of 5−70 nm and average lengths of several hundred nanometers).1,2 Because CNCs possess outstanding mechanical strength, are lightweight, have a large specific surface area, and show high crystallinity and biocompatibility, they have long been acclaimed as superior biobased physical reinforcement agents for a variety of natural or synthetic polymer matrices.3−6 In the microelectronic device area, a flexible screen with superior transparency can be made with CNCs.7 The inclusion of CNCs in polymeric matrices or films provides a high Young’s modulus in addition to the natural flexibility and transparency of the formed films. Further efforts have focused on using CNCs as a platform for inorganic nanoparticles such as observed by ferrite nanoparticles that are assembled in situ on CNCs.8 This inorganic nanocomposite system that employs CNC as the backbone is endowed with the unique function of magnetic susceptibility, an example that is characteristic of such assemblies. Nevertheless, monofunctional nanofillers only confer or improve upon a specific property for © 2016 American Chemical Society

Received: October 3, 2016 Revised: November 25, 2016 Published: December 28, 2016 1601

DOI: 10.1021/acssuschemeng.6b02380 ACS Sustainable Chem. Eng. 2017, 5, 1601−1609

Research Article

ACS Sustainable Chemistry & Engineering

multistep, multisolvent, energy intensive manufacturing processes for nanocomposite synthesis.29 Recently, Wu et al. obtained 30 nm AuNP by mixing the CNCs and gold(III) chloride trihydrate solutions under a hydrothermal synthesis reaction kettle at 120 °C for 10 h, and found that not only the particle size decreased with increase of CNCs concentration, but also its formation was affected by the alkaline environment.30 However, the specific influence of CNC concentration on the size and morphology of AuNPs have not been studied comprehensively. Meanwhile, the harsh temperature condition makes it difficult to control the uniformity of the AuNPs. Herein, a one-pot synthesis of AuNP/CNC nanocomposites was provided containing only one reagent (sodium hydroxide) at ambient conditions (∼30 °C). Compared to the mentioned high temperature methods, the mild reaction condition can provide an opportunity to study and explore the synthesis process. In addition, it is likely to control the size and morphology of AuNP, as well as to reduce the energy consumption. Moreover, due to the AuNPs easy-aggregation in solid matrixes, its in situ photothermic responsivity has received little investigation. This work utilizes CNCs as an efficient template to scaffold AuNPs as and prevent their aggregation in the PVA matrix. Meanwhile, the dynamics and rate of temperature change within the PVA matrix were key features for use in multifunction composites. CNC benefited formation of AuNP/CNC nanocomposites because not only did they encourage good dispersibility and controlled size distribution, but they also enhanced the photothermic effect, a result not previously observed. In addition, it has been shown that cellulose nanopaper that has a degree of opacity is a valuable substrate for solar cells because the high haze over the cell may improve the absorbance of light energy.31 Likewise, the manufacture of AuNP/CNC nanohybrids based on CNCs were optimized to better understand the photothermal effect.

Figure 1. Dynamic of the photothermal effect due to SRP of AuNPs.

within the nanoparticle lattice very rapidly. Thus, the AuNPs can act as a local heater by the mechanism of photoexcitation, but without the need for conduction, a very important distinction that lends its importance to many applications. For example, this nonconductively generated heat (nonradiative energy) can be used to treat blood vessel lesions, laser resurfacing, laser hair removal, and laser surgery. The photothermal effect is in principle a photophysical event whose overall behavior is strongly dependent on AuNP’s extensive properties, viz, shape and size.16−18Controlling such properties can be managed by an appropriate reducing agent and its concomitant consumption during synthesis of AuNP from nonzero valent gold. Specifically, precise tuning of the physical properties of AuNPs is critical to the dynamics of the photothermal effect. Interestingly, dispersion of AuNPs in matrices is a nontrivial issue because they aggregate because of van der Waals attractive forces that tend to be greater than the electrostatic repulsions arising from NP surfaces;19,20 thus, stabilizers are often a prerequisite for successful dispersion. The use of nanocellulose as a potential stabilizer has already attracted attention because it possesses the necessary colloidal characteristics to avoid aggregation of AuNPs and thus improve their stability. For example, Shin et al. successfully prepared Au−Ag alloy nanoparticles on CNCs via chemical reduction with NaBH4.21 In the reaction, CNCs were able to serve the dual roles of stabilizer and matrix. Similarly, Koga et al. synthesized highly dispersed AuNPs in situ with NaBH4 reduction at the surfaces of 2,2,6,6-tetramethylpiperidinyl-1oxyl radical (TEMPO)-oxidized cellulose nanofibers. In these latter highly oxidized systems, the carboxylate groups on the surface of cellulose nanofibers offered preferential sites for AuNP’s production and retention.22 In fact, without the matrix of nanocellulose, significant aggregation of as-prepared AuNPs occurred. Indeed, nanocellulose is able to trigger AuNP synthesis because its surface-rich hydroxyl groups provide the sufficient reducing power needed.23−25 This latter approach is rather simple as well as being green because it does not require any extra reductant or dispersing agent, and the as-prepared AuNP/nanocellulose composites display potentially useful catalysis applications.23−26 PVA is a common, biocompatible, and versatile synthetic polymer for use, inter alia, films, adhesives, textile sizing, and paper coatings.27,28 It is an excellent polymeric host for a number of guest molecules for numerous functional applications. Thus, it was deemed as a sufficiently useful substrate for incorporation of multifunctional nanofillers to expand its utility. It was used to prepare AuNP/CNC nanocomposites for the express purpose of probing the photothermal event in the resultant films. In modern chemistry, the sustainable and green concept commands to consider the environmental impacts of

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EXPERIMENTAL SECTION

Materials. Cotton linters with a moisture content of 8% were supplied by Fumin Chemical Fiber Co. Ltd. (Shandong, China). Sulfuric acid (98 wt %), sodium hydroxide, Gold(III) chloride trihydrate, sodium citrate, and poly(vinyl alcohol) (n-1788, alcoholysis degree 87.0−89.0 mol/mol) were of analytical grade and purchased from Guangzhou Chemical Reagent Factory (Guangzhou, China). All these chemicals were used without further purification. Preparation of CNCs. Cellulose nanocrystals were prepared according to a previously reported method,32 using 64 wt % H2SO4 hydrolysis at 45 °C for 1 h under continuous mechanical stirring. The obtained suspension was diluted 10-fold by deionized water to quench hydrolysis and thoroughly washed via repeated centrifugation (10000 rpm, 10 min) at least 3×. The solid remaining was dialyzed against deionized water until neutral to remove residual acid. Finally, the suspension was ultrasonicated for 5 min at room temperature to obtain a uniform CNC dispersion that was subsequently stored at 4 °C for further experiments. Preparation of AuNPs Colloidal and AuNP/CNC Nanocomposites. AuNPs were in situ synthesized in the presence of CNCs according to an established method23 with some modifications. Herein, we adopted experimental conditions that included CNCs quantities, HAuCl4 consumption, and pH. In a typical reaction, HAuCl4 (0.01 M, 0.8 mL) was added to a 40 mL CNC (0.05−0.5 wt %) suspension under constant magnetic stirring (600 rpm) for 15 min. The pH of reaction system was adjusted to 12.40 using 0.5 M NaOH, which can weaken the hydrogen bonding of cellulose and activate the hydroxyl groups of CNCs. Then the mixed solution was continuously magnetic stirred for 48 h at room temperature to obtain the stable suspension of AuNPs/CNC nanocomposites. AuNPs with size of ∼30 1602


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ACS Sustainable Chemistry & Engineering nm were synthesized by a sodium citrate reduction method.33 Briefly, 0.45 mL of sodium citrate (0.1M) solution was added into 100 mL boiling HAuCl4 (0.2 mM) solution for 15 min. The obtained AuNP/ CNC nanocomposites and AuNPs colloidal solutions were washed via repeated centrifugation (12000 rpm, 20 min) under 4 °C to purify the AuNPs and AuNP/CNC nanocomposites. Preparation of AuNP/CNC/PVA Film. Approximately 10 wt % PVA solution was mixed with an AuNP/CNC suspension with magnetic stirring for 10 h under ambient conditions. The resulting mixtures were subsequently cast into circular polytetrafluoroethylene (PTFE) molds (6 cm in diameter) and dried in a vacuum oven at 50 °C for 8 h. A series of AuNP/CNC/PVA nanocomposite films were prepared by changing the AuNP/CNC nanohybrid contents (0, 2, 4, 8, and 10 wt %) and denoted as PVA, AuNP/CNC/PVA-2, AuNP/ CNC/PVA-4, AuNP/CNC/PVA-6, AuNP/CNC/PVA-8, and AuNP/ CNC/PVA-10, respectively. Finally, dried films with thicknesses of ∼125 μm were conditioned under ISO constant temperature (23 ± 1 °C) and humidity (50 ± 2%) for at least 72 h prior to further testing. Analyses of AuNPs. UV−visible absorption spectra from 400 to 1000 nm were obtained (HP-8453, Agilent Technologies Inc., CA, USA) using deionized water as blank. The morphologies and sizes of gold nanoparticles were determined by transmission electron microscopy (TEM) (H-7650, Hitachi, Japan) at 80 kV accelerating voltage. The nanocellulose samples stained by 3 wt % solution of phosphotungstic acid were prepared for TEM. X-ray diffraction (XRD) patterns of freeze-dried AuNP/CNCs were collected on a D8 Advance X-ray Diffractometer (Bruker Corporation, Germany) using Ni-filtered Cu KR radiation (λ = 0.154 18 nm) at 40 kV and 20 mA. Data were collected over the 2θ range of 5−90° with a scan step size of 0.04° at a rate of 1°/min. Fourier transform infrared spectroscopy (FT-IR) spectra of neat CNCs and AuNP/CNC nanohybrids in KBr discs were recorded using a Vector 33 spectrometer (Bruker, Germany) over 500 to 4000 cm−1. The ζ-potentials of CNCs and AuNP/CNC suspension (1 wt %) were measured using HORIBA Nano Particle Analyzer (SZ-100, Japan). An atomic absorption spectrometer (Z-2000, Japan) was employed to detect the contents of AuNP in the AuNP/CNC nanohybrid. The morphology and structure of a thin AuNP/CNC nanocomposites films were obtained using a high-resolution field emission gun scanning electron microscope (Nova, NanoSEM 430, FEI Company) operating at an acceleration voltage of 15 kV. AFM images of the thin AuNP/CNC nanocomposites films were recorded on a Bruker Multimode 8 (America) atomic force microscope. Characterization of AuNP/CNC/PVA Films. The mechanical strength of AuNP/CNC/PVA films was tested using a universal material testing machine (Instron 5567, USA) with a 100N load cell. The films were cut into rectangular specimens with width of 10 mm and the length of ≥40 mm. The gauge length was set to 10 mm and testing speed to 10 mm/min. Before the test, the thickness of the films was calculated from the average of six measurements. The tensile modulus and tensile strength of the samples were calculated using five measurements. Photothermal conversion properties of films were tested using a 532 nm green diode laser at a power density of 1.6 W/ cm2 (Changchun New Industries Optoelectronics Technology Co., Ltd., China) and a thermometer (UNI-T 1310, UNI-T Electronic Corp, China).

Figure 2. X-ray diffraction patterns (a), FT-IR spectra (b) of CNCs and AuNP/CNC nanocomposites.

CNC samples, not surprisingly, showed a typical cellulose I structure with characteristic peaks at 2θ angles 14.6° (101), 16.5° (10), 22.5° (002), and 34.6° (004). Compared to CNC, the diffractogram of the AuNP/CNC nanocomposite had five (5) extra diffraction peaks of 2θ angles at 38.0°, 43.7°, 64.6°, 77.4°, and 81.7°, corresponding to the (111), (200), (220), (311), and (222) crystal planes of cubic Au, respectively.34,35 The native crystalline structure of CNC was therefore maintained during in situ preparation of AuNPs at room temperature, similar to what has been reported previously when AuNPs were prepared under high temperature (boiling) conditions.23,26 Structural changes of cellulose during AuNPs preparation were determined by FT-IR (Figure 2b). The absorbance bands ∼3200−3500 cm−1, 2850−3000 cm−1, and 1059−1162 cm−1 corresponded to the OH stretching, CH stretching, and CO/COC stretching in cellulose, respectively.36,37 The band at 1645 cm−1 was due to the bending vibration of OH in CNCs. Relative to pristine CNCs, the FT-IR spectrum of AuNP/CNC showed an additional absorbance peak at 1793 cm−1 attributable to CO stretching verifying that oxidation of CNCs occurred at primary hydroxyl groups.36,37

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RESULTS AND DISCUSSION Preparation of AuNP/CNC Nanocomposites. There are many methods to prepare AuNPs for wide application as photothermic materials or catalysis.5−8,12 Among previously used methods, CNC was also used as the scaffold for AuNP seeding. However, the reaction was carried out at 120 °C where the size of AuNPs cannot be controlled.30 It is interesting that AuNP/CNC nanocomposites were not only obtained, but the size of AuNPs was also subject to control as discussed later in this section. The XRD interrogated the crystal structures of CNCs and AuNP/CNC nanocomposites (Figure 2a). The 1603


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Figure 3. Photographs (insets in both photomicrographs) and corresponding TEM images of pristine CNCs (a), AuNP/CNC nanocomposites (b), and AuNPs (c).

A benefit from the low temperature (about 30 °C) reaction with CNC was well dispersed AuNP on CNCs. Figure 3 showed TEM images of CNCs, AuNP/CNC nanocomposites, and AuNP without CNC. The CNCs were rod-like nanobricks with lengths of 150−250 nm and widths of 10−20 nm that were well dispersed with few aggregates. A rationale for the low aggregation is that the negatively charged sulfate groups installed onto CNCs surfaces from their preparation prevent it.38,39 These well-dispersed CNCs furnish a uniform area for the inorganic nanoparticles to form. The in situ prepared AuNPs localized on the surface of CNCs were well dispersed as shown in Figure 3b. Apparently, the AuNPs prepared by sodium citrate20 were more inclined to aggregate as shown in Figure 3c. The AuNPs result display surface plasmon resonance giving rise to an intense colorimetric behavior (red color in the small bottle in Figure 3b,c). A possible reason is that the negatively charged nanocellulose and the positively charged metal particles interact via electrostatic forces, which thereby improved the dispersibility of AuNPs.23 In fact, the solution of CNCs showed herein has a ζ-potential of −41.1 mV and the AuNP/CNC suspension was −38.8 mV, which further confirmed the explanation. Effects of CNC Concentration to the Size of AuNPs. AuNPs exhibit surface plasmon resonance (SPR) bands in the visible region40 because of the coherent interaction of free electrons in this metal conduction band with incident light.41,42 The unique plasmon resonance frequencies of AuNPs strongly depend on particle size, shape, and state of aggregation.41−44 Thus, changes in λmax and SPR bandwidth characterized by UV−vis absorbance spectroscopy provide valuable information on the geometric properties of AuNPs. Figure 4 illustrated the UV−vis spectra for the gold nanoparticles obtained at various CNC concentrations in the same dilution. Clearly, the absorbance peaks increased when CNC concentration increased from 0.05 to 0.3 wt %, viz., higher CNC concentration for more AuNPs; whereas, further increases in the CNC concentration, from 0.3 to 0.5 wt % demonstrated no significant changes in the peak intensity. It is postulated that Au(III) was completely reduced to Au(0) when CNC quantities were more than a specific level. As the CNC concentration increased, the SPR bands narrowed substantially, indicating that both the general monodispersity and homogeneity (size distribution) of AuNPs improved.40 This is likely a result of the presence of CNCs that acted as reducing agents, in addition to dispersing and stabilizing agents thus preventing aggregation. Additionally, a clear blue shift in λmax from 541 to 522 nm was indicative of a decrease in particle size. The shape evolution and size variation of AuNPs as observed by TEM (Figure 5) corroborated this latter assertion.

Figure 4. UV−vis absorption spectra of AuNPs prepared using various CNC concentrations (0.05, 0.1, 0.2, 0.3, 0.4, and 0.5 wt %) at 0.2 mM HAuCl4 concentration and pH 12.40.

The smallest gold nanosphere with a highly uniform average diameter of 24.2 ± 3.4 nm was obtained at 0.5 wt % CNC concentrations (Table 1); whereas, at very low CNC concentration (0.05 and 0.1 wt %), these gold nanoparticles formed random irregularly shaped aggregates. There was no AuNPs formed without adding CNCs as evident from the absence of the plasmon resonance peak in the UV−vis spectra (clear solution). The effect of CNC concentration on AuNPs size is similar to that reported by Wu et al.23 It seems likely that higher CNC quantities favor AuNPs formation because of larger levels of exposure of reducing hydroxyl groups to Au(III). At lower CNC levels, however, in situ accretion of AuNPs may become localized due to the relatively limited number of available sites, thereby forming larger nanoparticles and/or aggregates. Therefore, it is concluded that CNCs play a crucial role in the formation of AuNPs. Effect of CNC on Strength of AuNP/CNC/PVA Films. The purpose of preparing AuNP/CNC nanocomposites was originally for the preparations of photothermic materials. Previous research showed that CNC acted as a reducing and dispersing agent. Due to their intrinsic high Young’s modulus, CNCs may act as a reinforcement agent or nanofiller to enhance the final mechanical strength of a PVA film containing AuNP/CNC. PVA or PVA with AuNP/CNC (2, 4, 6, 8, and 10 wt % loading, in which Au is 1.5%) were casted in a round plate model to form films. The tensile modulus and tensile strength of films are recorded in Figure 6. The tensile moduli of the composites increased with an increase of nanofiller loading when fillers were no more than 1604


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Figure 5. TEM images of AuNPs prepared using various CNC concentrations. (0.05, 0.1, 0.2, 0.3, 0.4, and 0.5 wt %) at 0.2 mM HAuCl4 concentration and pH 12.40.

Table 1. Average Diameters and Maximum Absorbance Wavelength (λmax) of AuNPs Prepared at Various CNC Concentrations CNC concentration (wt %) λmax (nm) average diameter (nm)a a

0.05 543 45.8 ± 10.4

0.1 541 42.8 ± 7.9

0.2 535 35.5 ± 5.1

0.3 528 31.9 ± 4.0

0.4 523 27.9 ± 5.1

0.5 522 24.2 ± 3.4

Particle size is averaged over 100 nanoparticles on TEM images.

Figure 6. Tensile modulus (a) and tensile strength (b) of PVA or AuNP/CNC/PVA films.

properties.47 The declining trend of tensile strength at higher nanocomposite loading (8, 10 wt %) may be explained by the aggregation and the heterogeneous dispersion of nanocomposites in the PVA matrix which make local distributions of stress and tensile failure more likely.48,49 CNCs Improve the Photothermic Effect of AuNPs within Composites. AuNPs/CNC was mixed in PVA and formed films. Because of the SPR frequency of the AuNPs from exposure to visible light, they exhibit a photothermic effect. The observed profile of temperature rising with time under irradiation of green laser light (1.6W/cm2 532 nm) is depicted in Figure 7a for films blended with various amounts of AuNP/ CNC. It is evident in Figure 7a that the heating rate of films is rapid in which the elevated temperature (ΔT) reaches

10%; in fact, the tensile strength did not increase until up to 6% of fillers. The PVA film with 10 wt % of AuNP/CNC demonstrated 82.9% higher tensile moduli than pure PVA films shown in Figure 6a. The strong reinforcement may be related to the inherent chain stiffness and rigidity of nanocellulose from inter- and intramolecular hydrogen bonding.45 Compared to the tensile strength of neat PVA (22.4 MPa), the nanocomposites-reinforced film exhibited the highest tensile strength (29.3 MPa) at 6 wt % (Figure. 6b). Similar observations have been made by Uddin,46 who added cellulose nanocrystals in PVA to improve film strength. An explanation may be that homogeneous distribution of the nanocomposites in PVA, a high level of compatibility, and hydrogen bonding between the nanocellulose and matrix improved the mechanical 1605


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Figure 7. Temperature profiles (a) and temperature changes at steady-state (b) of films.

Table 2. ΔT and Transmittance of Various Composite Films

equilibrium within 20 s. In fact, the temperature equilibrium of AuNPs in the matrix is dominated by a thermodynamic equilibrium from heat-absorbing caused by the photothermal effect and heat-loss such as convection, conduction etc.50,51 Thus, when the two processes are unbalanced, ΔT presents increased or decreased accordingly. Figure 6b shows the relationship between the concentration of AuNPs and ΔT. Generally, ΔT can be calculated as ΔT = ϕNNPI0 3ε0 R 3 ω ϕ = NP 3K 0 8π 2ε0 + εNP

Sample PVA AuNP/PVA-4 AuNP/CNC/ PVA-2 AuNP/CNC/ PVA-4 AuNP/CNC/ PVA-6 AuNP/CNC/ PVA-8 AuNP/CNC/ PVA-10

(1) 2

Im εNP

8π c ε0

(2)

where NNP is the density of AuNPs, I0 is the light intensity inside the matrix, ϕ is function shown in eq 2 normally depending on the radius of AuNP (RNP), the thermal conductivity of the surrounding medium (K0), the angular frequency of the light wave (ω), and the dielectric constants of Au NP (εNP) and surrounding medium (ε0).52 In this experiment, the above variables (RNP, K0, ω, εNP, ε0) are constant, which means ϕ is a constant too. Therefore, the ΔT should show a linear increase with the concentration of AuNPs. As expected, the result shown in Figure 7b is approximately coincident with the general concept when the content of AuNPs is below 8 wt %. However, there is no further increase at 10 wt % loading which can be mainly ascribed to smaller inter-AuNPs spacing (D) at higher concentration;52 when D is shorter than 5 times of radius of AuNPs, the SPR of AuNPs will be weakened as well as the efficacy of the photothermic effect.41 The laser wavelength used at 532 nm has a power density of 1.6 W/cm2. Meanwhile, a distinct difference for the ΔT was found between Au/PVA-4 and Au/CNC/PVA-4 films as shown in Table 2. The presence of CNCs had a positive effect on promoting the ΔT at the same content of AuNPs in films. One reason may be that the CNCs prevent aggregation of AuNPs, and maintain the monodispersity and stability of AuNPs in the films. Moreover, in order to make sure the influence of CNCs on the uniformity of overall temperature distribution in the films, 36 temperature measurement points were uniformly selected in 3 × 3 cm square films, and a 3D temperature map was drawn within the polymer nanocomposites as shown in Figure 8. Clearly, the Au/CNC/PVA-4 film has a more uniform

Au/CNC content (wt %)

Au content (wt %)

ΔT (°C)

Transmittance (%)

0 0 2

0 0.0628 0.0314

1.2 16 12

20 8.3 5.6

4

0.0628

23

4.9

6

0.0942

28

3.6

8

0.1256

33

1.4

10

0.157

34

0.91

temperature distribution than the Au/PVA-4 film. For the Au/ PVA-4 film, the measured increasing temperature (ΔT) ranged from 12.7 to 22.1 °C, whereas it ranged from 19.4 to 25.6 °C in the Au/CNC/PVA-4 film. This observation further indicates that CNCs contribute to the dispersive uniformity of AuNPs in the PVA matrix. The other one is that CNC in the films caused more laser light scattering which led to a second photothermic effect among the AuNPs. Additionally, it is worth noticing the AuNP/ CNC/PVA-4 had a lower transmittance than the AuNP/PVA-4 shown in Table 2. It indicates that the existence of CNCs decreased the transmittance of the polymer films for a given AuNPs content. The rationale is that the thin and high aspect ratio of the CNC can form more easily a continuous network, and in turn this cellulosic scaffold lead to the decrease of transmittance in the polymer matrix by promoting light scattering.54 Similarly, Sami Bouf et al. also draw the same conclusion by studying on the optical properties of the CNCs− acrylic polymer film.53,54 In fact, as each individual nanofiber will lead to small forward scattering,54 the multiple scattering of the cellulosic scaffold in the PVA matrix lead to multiple direction change of light and increase the length of light path within the film. Therefore, the other reasonable reason causing the obvious difference of ΔT between Au/PVA-4 and Au/CNC/PVA-4 films is that CNC in the films caused more laser light scattering from a secondary scattering event, which led to a second photothermic effect among the AuNPs. This second photothermic effect of AuNPs in the film elevated the temperature (ΔT) even further. Figure 1606


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Figure 8. Three-dimensional images of the temperature distribution on the surface of AuNP/PVA-4 (a) and Au/CNC/PVA-4 (b) films, respectively.

transparency CNCs, an effect which may be applied in a solar cell.18

9a provides a simple cartoon to demonstrate the physical findings related to the light scattering in the film. As shown in

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CONCLUSIONS Cellulose nanocrystals (CNC) promote AuNPs preparation by not only acting as a reducing agent but also capping and stabilizing agents. The preparation can be carried out at ambient temperatures, a novel mode of study that allows for a better understanding of the reaction process. It was found that the size and shape of AuNPs depend on concentration of CNCs. AuNP/CNC nanocomposites were successfully embedded into PVA films whose mechanical strength was improved significantly, while photothermal heating occurred using low intensity, continuous-wave visible laser irradiation. Increasing the concentration of nanoparticles proportionally increased the photothermal effects if conditions leading to NP aggregation were avoided. Remarkably, it was found that the CNC also gave rise to secondary light scattering wavelengths that augmented the overall intensity of the photothermal event. AuNP/CNC nanocomposites are therefore a novel means to endow polymer matrices with a photothermic effect for multifunctional materials. CNC acted as both reducing and dispersing agent in AuNP preparation, and an extraordinary point scatterer for the reinforcement of the photothermal event in the nanocomposites.

Figure 9. Proposed mechanism (a) for the influence of CNCs on the photothermal effect of AuNPs within the PVA films. And the SEM image (b) and AFM (b) of the AuNP/CNC nanocomposites.

Figure 9b,c, the AuNPs dispersed into the cellulosic scaffold uniformly. The CNCs cause more scattering than the other colocated materials.34 AuNPs thus absorb light to give rise to a photothermal effect and may scatter some light. In the case of films blended with AuNPs and without nanocellulose, AuNP may absorb laser light directly, and the absorbed light causes a photothermal effect. In films with AuNP and nanocellulose, AuNPs directly absorbed light, while a degree of the incident light was scattered by CNCs. The scattered light is a second light source in the films, which also can be absorbed by AuNPs to produce heat. Therefore, the composites with the same amount of AuNPs increased the temperature by 23 °C when CNC was present in the film, whereas others rose the temperature 16 °C without CNC in the film. Thus, it is clear that the heating is initiated at extremely localized spatial conditions and is controllable due to the specificity provided by the AuNP SPR or quantity of the particles. Using an appropriate level of AuNPs/CNC provides a good photothermic effect. 8% AuNPs/CNC in PVA film can yield a 33 °C temperature gain (Table 2). This impressive phenomenon is mainly ascribed to the strong light scattering ability of low

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AUTHOR INFORMATION

Corresponding Author

*Shiyu Fu, Ph.D. Email: [email protected]. Tel.: + 86 202 2236 078. ORCID

Shiyu Fu: 0000-0001-5680-7493 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was supported by State Key Laboratory of Pulp & Paper Engineering (2016PY01), the Guangdong-Hongkong Joint Innovation Program (2014B050505019), the Natural Science Foundation of Guangdong Province, China (2014A030311030), and the National Natural Science Foundation of China (31570569). In addition, L.A.L. is grateful for an honorary professorship and funding from SCUT that allowed parts of this work to be completed. 1607


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Physical Study of the Primary and Secondary Photothermal Events in

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