Self-Assembly of Soft and Conformable Broadband Absorbing Nanocellulose-Gold Nanoparticle Composites

Abstract

Broadband light-absorbing materials are of large interest for numerous applications ranging from solar harvesting and photocatalysis to low reflection coatings. Fabrication of these materials is often complex and typically utilizes coating techniques optimized for flat and hard materials. Here, we show a self-assembly based strategy for generating robust but mechanically flexible broadband light-absorbing soft materials that can conform to curved surfaces and surface irregularities. The materials were fabricated by adsorbing large quantities of gold nanoparticles (AuNPs) on the nanofibrils of hydrated bacterial cellulose (BC) membranes by tailoring the interaction potential between the cellulose nanofibrils and the AuNPs. The highly efficient self-assembly process resulted in very dense multilayers of AuNPs on the nanofibrils, causing extensive broadening of the localized surface plasmon resonance band and a striking black appearance of the BC membranes. The nanocomposite materials showed an absorptance >96% in both the visible and the near-infrared wavelength range. The AuNP-functionalized BC membranes demonstrated excellent conformability to curved and structured surfaces and could adopt the shape of highly irregular surface structures without any obvious changes in their optical properties. The proposed self-assembly based strategy enables the fabrication of soft and conformable broadband light-absorbing nanocomposites with unique optical and mechanical properties using sustainable cellulose-based materials.

Keywords: absorptance; gold nanoparticles; nanocellulose; nanocomposite; plasmonic materials.

Figure 1

Schematic description of the self-assembly process for generating high densities of adsorbed AuNPs to bacterial nanocellulose membranes and corresponding photographs of (i) native BC and (ii) hydrated BC-AuNP membranes, ø 2 cm.

Figure 2

(a) Photographs of hydrated BC (i), freeze–dried BC (ii), and air-dried BC (iii). The diameter of the membranes is 2 cm. (b) Reflectance (DHR) spectra of hydrated BC, freeze-dried BC, and air-dried BC in the visible wavelength range. (c) SEM micrograph of freeze-dried BC, scale bar: 500 nm. (d) SEM micrograph of air-dried BC, scale bar: 500 nm. (e) Reflectance (DHR) spectra of hydrated BC, freeze–dried BC, and air-dried BC in the near-infrared wavelength range (visible wavelength range indicated with gray).

Figure 3

(a) Photographs of (i) air-dried, (ii) freeze–dried, and (iii) hydrated BC-AuNP. The diameter of the membranes is 2 cm. (b) Reflectance spectra of BC-AuNP in the visible wavelength range. (c) SEM micrograph of air-dried BC-AuNP, scale bar: 500 nm. Inset scale bar: 100 nm. (d) SEM micrograph of freeze-dried BC-AuNP, scale bar: 500 nm. (e) Reflectance spectra of BC-AuNP between 250–2500 nm (visible wavelength range indicated in gray).

Figure 4

(a,b) Absorptance spectra of air-dried, freeze–dried, and hydrated BC-AuNP. (c) Photographs of hydrated BC-AuNP on a curved surfaces showing the conformability of the BC-AuNP composites on screws with a diameter of (i,ii) 5 mm, (iii) 3 mm, and (iv) 8 mm as substrates, and on a finger (v). The pitch of the threads in (iv) is 1.25 mm. Scale bars: (i–iii) 5 mm, (iv) 8 mm, and (v) 1 cm.

Figure 5

(a) Thickness of hydrated BC and BC-AuNP composites prior to drying. Significance was tested using unpaired t-test with Welch’s correction, P < 0.05, n = 6. (b) Water absorption of BC and BC-AuNP composites after freeze–drying and air-drying, n = 3. Statistical analysis was performed with an ordinary one-way ANOVA test complemented with Tukey’s multiple comparison test (*P < 0.1 and ****P < 0.0001). (c) Cycles of compression and rewetting of BC and (d) BC AuNP composites at two compression levels, 15% and 50%, and subsequent rehydration, normalized at hydrated pre-compression height, n = 3. Error bars show standard deviations.

Figure 6

Normalized stress relaxation curves of hydrated (a) BC and (b) BC-AuNP composites subjected to 0.1–2 N axial compression. Average values are displayed (n = 3). Storage (G′) and loss modulus (G″) of hydrated (c) BC and (d) BC-AuNPs composites recorded during the last 5 min of the relaxation step. Data displayed as mean and standard deviation, n = 3.