Highly Transparent and Hazy Cellulose Nanopaper Simultaneously

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Highly Transparent and Hazy Cellulose Nanopaper Simultaneously with a Self-Cleaning Superhydrophobic Surface Sheng Chen, Yijia Song, and Feng Xu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04814 • Publication Date (Web): 08 Mar 2018 Downloaded from http://pubs.acs.org on March 12, 2018

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ACS Sustainable Chemistry & Engineering

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Highly Transparent and Hazy Cellulose Nanopaper Simultaneously

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with a Self-Cleaning Superhydrophobic Surface

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Sheng Chen, Yijia Song, Feng Xu*

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Beijing Key Laboratory of Lignocellulosic Chemistry, Beijing Forestry University,

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Beijing 100083, China

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*

Corresponding author: Feng Xu. E-mail: [email protected]. Tel: 86-10-62337993

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E-mail address for all authors: [email protected] (S. Chen)

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[email protected] (Y. Song)

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[email protected] (F. Xu)

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Full mailing address for all authors: No. 35, Tsinghua East Road, Haidian district, Beijing, 100083, China

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ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT: Wood-derived sustainable materials like cellulose fibers have received

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increased attention for replacing non-renewable substrates in emerging high-tech

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applications. Herein, for the first time, we fabricated a superhydrophobic (static

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contact angle = 159.6°, sliding angle = 5.8°), highly transparent (90.2%) and hazy

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(46.5%) nanopaper made of TEMPO-oxidized cellulose nanofibrils (TOCNF) and

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polysiloxanes. The original TOCNF nanopaper endowed excellent optical and

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mechanical properties; the constructed pearl-necklace-like polysiloxanes fibers on the

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nanopaper surface by further silanization significantly improved water-repellency

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(70.7% for static contact angle) and toughness (118.7%) of the TOCNF nanopaper.

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Our proposed novel nanopaper that simultaneously achieved light-management and

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self-cleaning capabilities not only led to an enhancement (10.43%) in the overall

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energy conversion

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most of photovoltaic performance losses due to dust accumulation by self-cleaning

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process, indicating its potential application in solar cells. This study on

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cellulose-based multifunctional substrates provided new insights into the future

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development of sustainable functional devices.

efficiency of the solar cell by simply coating but also recovered

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KEYWORDS: Transparent, Superhydrophobic, Cellulose nanopaper, Self-cleaning,

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Solar cell

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INTRODUCTION

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Substrates play a crucial role as the foundation for electronic and optoelectronic

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devices. The critical properties of these substrates like mechanical strength, optical

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transparency and wettability determine their eligibility for various applications.1 To

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expand the range of applications for regular materials and fulfill the devices’

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requirements, the multifunctional design strategies are always utilized to develop the

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substrates with desirable and diverse properties or effects.2-5 One example is the

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transparent and simultaneously superhydrophobic substrates, which exhibit potential

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uses in self-cleanable solar cell panels, antifogging goggles, and windows for

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electronic devices.6-8 However, these multifunctional substrates are predominantly

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based on non-renewable materials like plastic and glass.

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In recent years, environmentally friendly materials have received increased

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attention due to the arising of global climate change and resource shortage.9 As

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sustainable and earth-abundant resources, wood-derived materials like cellulose paper

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and wood products are ubiquitous in daily life, however, they are usually limited in

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functionalities by their native properties, such as electrical insulation, optical opacity,

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and hydrophilicity. By chemically modification, reconfiguration, or integration with

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other nanomaterials, the wood-derived substrates can be given desirable functions and

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thereby get ready to be potentially applied in emerging technology areas. For instance,

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the transparent haze cellulose paper or wood composites with the single

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function—light management—achieve an efficiency enhancement of 10.1%∼23.9% 3



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by coating on the solar cell surfaces when compared with the bare cells.1,10-12

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However, the accumulation of dust on the surface of solar cell panels outside remains

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a challenge for the energy conversion efficiency due to the decreased light flux into

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the active layers of cells. Another example of functional wood-derived materials is the

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superhydrophobic cellulose paper and wood blocks, which hold the effect of

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self-cleaning but are limited in expanded applications by their poor transparency.13-18

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To broaden the applicability of these aforementioned sustainable and renewable

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substrates in various fields, integrating the superhydrophobic surfaces that have a

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self-cleaning function into the transparent cellulose paper or wood composites is a

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promising strategy.

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It is noted that superhydrophobic surfaces that are extremely difficult to wet

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require water contact angles (CAs) of greater than 150° and sliding angles (SAs) of

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less than 10°.19,20 Several attempts have been made to convert the hydrophilic

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transparent cellulose papers into hydrophobic ones by modifications, such as

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alkylketene dimer treatment, resin penetration, and epoxy coating.9,21-24 Although the

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water-resistant performance of these modified transparent papers were efficiently

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improved, the self-cleanable ability of paper surfaces were still not achieved due to

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their CAs of less than 150°. To the best of our knowledge, the highly transparent,

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hazy,

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light-management and self-cleaning functions have been not reported previously, and

and

superhydrophobic

cellulose

papers

(films)

4


simultaneously

with

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their potential utilizations in the optoelectronic, electronic, or other high-tech devices

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remain further study and development.

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In this work, for the first time, we fabricated a superhydrophobic, highly

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transparent and hazy nanopaper composed of earth-abundant wood fibers and 3D

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nanostructured polysiloxanes via a facile process: vacuum filtration and in situ

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siloxanes growth. As an example, the application of this multifunctional substrate in

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solar cells was demonstrated; the obtained nanopaper here could not only lead to a

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significant enhancement in short circuit density and conversion efficiency of a solar

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cell by light management but also recover most of photovoltaic performance losses

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due to dust accumulation by self-cleaning process. Moreover, the superhydrophobic

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and transparent nanopaper was able to potentially be applied beyond just solar cells to

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other sustainable, biodegradable and “green” electronics, optoelectronics, and

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functional devices.

89 90

EXPERIMENTAL SECTION

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Materials. A commercial dissolving wood pulp (Southern Yellow Pine) from

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Tianjin Haojia Cellulose Co., Ltd. (Tianjin, China) was used as native cellulose fibers.

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Sulfuric acid (H2SO4), 2,2,6,6-tetramethylpiperidinyl-1-oxyl (TEMPO), sodium

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bromide (NaBr), sodium hypochlorite (NaClO), hydrochloric acid (HCl), sodium

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hydroxide (NaOH), methyltrichlorosilane (MTCS), toluene, ethanol, and polyvinyl

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alcohol (PVA) were all purchased from Aladdin Chemistry Co., Ltd. (Shanghai, 5



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China). All chemicals were of analytical grade and used as received without further

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modification.

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Preparation of Nanofibrillated Cellulose Fibers. Cellulose nanocrystals (CNCs)

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were obtained from wood pulp fibers by well-established acid hydrolysis method with

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slight modifications.25 In brief, the pulp (5 g) after milling to pass through a 60-mesh

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screen was hydrolyzed with a 60 wt% H2SO4 solution (200 mL) at 55 °C for 2.5 h

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under stirring. The obtained suspension was then washed with water until pH

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neutrality and further dispersed by ultrasonic treatment to produce a homogeneous

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colloidal solution. TEMPO-oxidized cellulose was synthesized according to the

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procedure reported elsewhere.26 The dissolving pulp was oxidized by TEMPO

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mediated treatment with NaBr and NaClO for 12h at room temperature. The pH of the

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reaction system was kept constant at 10 by adding HCl or NaOH using a pH stat. The

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native and the oxidized cellulose fibers after thorough wash with water were then

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mechanically disintegrated by a high-pressure homogenizer (APV-2000, 15 passes at

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60 MPa) to produce cellulose nanofibrils (CNFs) and TEMPO-oxidized cellulose

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nanofibrils (TOCNFs) suspensions, respectively.

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Fabrication of Nanopapers. Nanopapers were fabricated by vacuum filtering the

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cellulose/water dispersion. In detail, the CNC, CNF, and TOCNF dispersions (0.1

115

wt%, 50 g) after stirring for 1 h at 800 rpm were vacuum filtrated with a glass suction

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filtration kit (T-50, Jinteng Co. Ltd., Tianjin, China) using a mixed cellulose ester

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membrane filter (pore size: 0.22 µm). Then, the wet sheets were carefully placed 6


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between filter papers and dried under pressure (0.01 MPa) at 105 °C for 10 min. The

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thickness of nanopapers was measured by a micrometer.

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Superhydrophobic Modification. The TOCNF-P sample was further modified

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by immersing this nanopaper into the 0.5

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room temperature for 10, 20, 30, and 40 min, respectively, as reported previously.27,28

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Vessels were closed to the air but exposed to the chamber environment during the

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solution and sample introductions. After the reaction, the samples were then removed

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and rinsed with various solvents in the following order: toluene, ethanol,

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ethanol:deionized water (1:1), and deionized water. Finally, the samples were dried in

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an oven at 55 °C for 4 h.

M

MTCS solution of anhydrous toluene at

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Attachment of Nanopaper to Solar Cell. The superhydrophobic transparent

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nanopaper (H40-TOCNF-P) was cut into a rectangle with an area of 1.5 × 1.2 cm2 for

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attachment to the solar cell. A drop of PVA (5 wt%, ~ 400 µL) was deposited on the

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surface of the existing polysilicon solar cell. Then the trimmed nanopaper was

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carefully placed on top of the cell to form intimate contact and to assure that the

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nanopaper covered the entire active area. The laminated sample was then dried at

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room temperature.

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Measurements and Characterizations. The morphology of native wood pulp

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fibers was viewed under a polarizing microscope (PM, BX43, Olympus). The finely

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dispersed nanocrystals and nanofibrils were scanned using an atomic force

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microscope (AFM, Multimode-8, Bruker). The content of carboxyl groups in 7



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TOCNFs was determined by conductimetric titrations with a digital conductivity

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meter (DDS-11A, Shanghai Hongyi), and the degree of oxidation (DO) of the

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TOCNFs was calculated based on the following equation:29,30

‫= ܱܦ‬

162 × ‫ܸ( × ܥ‬ଶ − ܸଵ ) ‫ ݓ‬− 36 × ‫ܸ( × ܥ‬ଶ − ܸଵ )

(1)

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where C is the NaOH concentration (mol l-1); V1 and V2 are the amount of consumed

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NaOH (ml) in the titration; w is the weight of the sample (g). The morphology of

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regular paper and nanopapers was characterized by a scanning electron microscope

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(SEM, Hitachi SU-8010) equipped with energy dispersive X-ray spectroscopy (EDS).

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Fourier transform infrared (FTIR) spectrometer (Nicolet iN10-MX, Thermo

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Scientific) equipped with an attenuated total reflectance (ATR) accessory and X-ray

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diffraction (XRD, D8 Advance, Bruker) were utilized to study the chemical structure

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and crystalline structure of nanopapers, respectively. The crystallinity index (CrI) was

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calculated with the following equation:31

‫= ܫݎܥ‬

‫ܫ‬ଶ଴଴ − ‫ܫ‬௔௠ ‫ܫ‬ଶ଴଴

(2)

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where I200 is the intensity of 200 peak at about 2θ = 22.6°, and Iam is the peak intensity

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of the amorphous portion at about 2θ = 18.0°. The content of siloxanes grafted on the

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nanopaper surface was estimated via the weight increase after superhydrophobic

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modification. The optical properties of the nanopapers were tested by using a UV-vis

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spectrometer (UV-2550, Shimadzu) equipped with an integrating sphere accessory.

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The tensile tests were conducted using a Zwick testing machine equipped with a 100

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N loading cell. The wettability of nanopapers was evaluated by average water CA and 8


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SA which were measured at five different positions on each sample using a contact

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angle meter (SL200KS, Kino) equipped with a high-speed video camera. The thermal

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stability of the nanopaper was characterized by the thermogravimetric analysis (TGA)

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using a thermogravimetric analyzer (TG/DTA6300, Seiko) at a heating rate of 10 °C

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min-1 under a N2 atmosphere. The photovoltaic characteristics of solar cells were

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measured using a voltage-current source meter (2400 Keithley) under illumination of

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Orel Solar Simulator (AM 1.5G, 100 mW cm-2).

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RESULTS AND DISCUSSION

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From Wood to Superhydrophobic Cellulose Nanopaper. Trees have a

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hierarchical structure, as shown in Figure 1a. Wood fibers with the diameter of dozens

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of micrometers are built from fibrils to fibrils bundles. By chemical processes, i.e.,

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acid hydrolysis and TEMPO oxidization, and mechanical treatment namely

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homogenization, we obtained three types of nanofibrillated cellulose fibers: CNCs,

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CNFs, and TOCNFs (Figure 1b). CNC is the intact rod-like cellulose crystalline core

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after dissolving amorphous domains using H2SO4. Moreover, the introduction of

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sulfate groups produces a negative charge on its surface, which is contributed to the

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generation of homogenous and stable CNC suspensions.32,33 CNF is the nano-scale

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elements of cellulose fibers after mechanical fibrillation, typically with a diameter of

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3-50 nm and a length of a few micrometers.34 TOCNF, highly carboxylated CNF, is

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also produced by fibrillation of cellulose fibers, but a chemical pretreatment by 9



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TEMPO was carried out prior to the mechanical treatment. As a result, its hydroxyl

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group at C-6 was selectively oxidized into carboxyl group, which can reduce

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mechanical energy consumption.35 Then, by vacuum filtering the dispersions made of

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these cellulose fibers and water, three corresponding nanopapers with ~40 µm

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thickness were fabricated and coded as CNC-P, CNF-P, and TOCNF-P, respectively.

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Finally, the TOCNF-P was further modified via MTCS to construct the

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superhydrophobic surface (Figure 1c). The obtained hydrophobic TOCNF-Ps after

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different modification time (10, 20, 30, 40 min) were coded as H10-TOCNF-P,

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H20-TOCNF-P, H30-TOCNF-P, and H40-TOCNF-P, respectively.

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10


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Figure 1. (a) Hierarchical structure of a tree. (b) Schematic illustration of the

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preparation of CNC, CNF, and TOCNF. Bottom middle and right, molecular structure

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of CNF and TOCNF. (c) Schematic illustration of the fabrication of cellulose

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nanopapers and superhydrophobic modification.

193 194

Morphology and Chemical Structure. The morphology of the cellulose fibers

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and the nanopapers before and after silanization treatment plays a critical role in the

196

properties of these substrates. As illustrated by the PM image in Figure 2a, the

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original cellulose fibers in wood pulp had a width of ∼30 µm and a length of several

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millimeters, and the bright and relatively dark regions indicated the existence of both

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crystalline and amorphous domains. The regular paper built up by these fibers had a

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significantly rough surface and porous structure (leftmost in Figure 2b), where the

201

many cavities caused light scattering and thereby limited optical transparency.

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Figure 2. (a) PM image (leftmost) of original wood pulp and AFM images (right

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three) of nanofibrillated cellulose fibers. (b) The surface morphology of regular paper

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and nanopapers before modification under SEM. (c) The surface morphology of

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H40-TOCNF-P under SEM with different magnifications (left three) and elemental

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mapping of Si by EDS (rightmost).

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Compared with the original cellulose fibers, significant decrease in dimensions

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during chemical or/and mechanical treatments would occurred due to the extensive

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forces for the nanofibrillated cellulose fibers. Preliminarily, the nanoscale dimensions

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of these fibers were evaluated by the Tyndall effect36 seen through light scattering

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when their suspensions were illuminated by a laser beam (Figure S1). As can be seen,

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the cellulose/water suspensions with the same solid content (0.1 wt%) exhibited

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different intensities of Tyndall scattering seen by the bright “pathway”, which was

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dependent on the dimensions of cellulose fibers. The slight scattering observed in

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CNC suspension indicated the minimum dimension of CNCs and the stable colloidal

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dispersion. The significant Tyndall scattering and translucent appearance of CNF

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suspension were resulted from the largest dimension of CNF. For TOCNF suspension,

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an in-between intensity of scattering was observed, demonstrating that its dimension

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was between the CNC and CNF. After TEMPO oxidization, the TOCNFs had the

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carboxyl groups of 0.927 mmol g-1 and the DO of 0.155 mol mol-1, which was

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consistent with the reported literature.37,38 12


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The exact dimensions of nanofibrillated cellulose fibers and the surface

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morphology of corresponding nanopapers are further illustrated in the AFM images

226

(Figure 2a) and SEM images (Figure 2b), respectively. The three types of

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nanofibrillated cellulose fibers had similar width ranging from 40 nm to 60 nm but

228

different length, thus leading to various degrees of roughness for obtained nanopapers.

229

In detail, CNCs were rod-like nanoparticles with a length of ~300 nm; the nanopaper

230

built up by CNCs (CNC-P) had a significantly flattening and smooth surface. CNFs

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had the maximum length reaching ~3 µm and the rough and uneven surface was

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observed in the CNF-P. The length of TOCNF and the surface roughness of

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TOCNF-P were in between the above two types of nanofibrillated cellulose fibers and

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corresponding nanopapers, respectively. Besides, the cross-sectional SEM images of

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the regular and nanostructured paper are shown in Figure S2. The belt-like original

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wood fibers arranged sparsely with gaps therebetween in the regular paper; in contrast,

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the CNC-P, CNF-P, and TOCNF-P possessed very densely laminated structures.

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The surface morphology of the TOCNF-P changed significantly after MTCS

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modification for different duration time, as shown in Figure S3 and Figure 2c. For

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H10-TOCNF-P, a large number of spherical-shaped particles with different diameters

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were clearly observed on the surface, shown in Figure S3a. After prolonging the

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MTCS reaction time from 20 to 40 min, the siloxanes fibers formed on the surface of

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TOCNF-P transformed from discrete to relatively dense 3D-network configuration.

244

For H40-TOCNF-P, the siloxanes fibers with a rough surface were constructed 13



245

(leftmost in Figure 2c); we infer that these pearl-necklace-like fibers were composed

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of interconnected ‘ball-like’ siloxanes particles. The content of these grafted siloxanes

247

on nanopaper surface was estimated via the weight increase after modification. As

248

shown in Figure S4, the weight of the siloxanes on TOCNF-P increased from 0.134 g

249

g-1 to 0.312 g g-1, prolonging the reaction time from 10 min to 40 min. In the

250

high-resolution SEM images of H40-TOCNF-P surface, we observed that the fibrous

251

siloxanes network, which was similar with those reported in the literatures,27,28,39 was

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relatively uniform in its entirety, though the siloxanes fibers were irregularly bent and

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randomly cross-linked with each other.

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Figure 3. Schematic illustration of (a) reaction between MTCS and TOCNF, (b)

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molecular structure of polysiloxanes, (c) pearl-necklace-like polysiloxanes fibers and

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3D network, (d) water-repellence of the rough surface for modified cellulose

258

nanopaper, and (e) light distribution through modified cellulose nanopaper incident on

259

a solar cell. 14


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260 261

The schematic illustration of MTCS coating on the surface of TOCNF-P is shown

262

in Figure 3. A possible mechanism of polysiloxanes formation was proposed based on

263

the literatures.27,28,40,41 The trace water in the toluene solution led to the hydrolysis of

264

MTCS, then the obtained silanols would react with the isolated hydroxyl groups on

265

the surface of TOCNF-P or other silanols (Figure 3a, b). During this reaction, the

266

Si-O-Si linkages were established on the substrates. Because the silanols with both

267

hydrophilic groups (-OH) and hydrophobic groups (-CH3) have the propensity to

268

self-assemble,27 the nanospheres or nanofibers of siloxanes continued to grow in three

269

dimensions and react with excessive Si-OH groups. Therefore, the 3D fibrous

270

siloxanes network that consisted of rough pearl-necklace-like fibers, as illustrated in

271

Figure 3c, was formed on the nanopaper surface, which is also shown by the SEM

272

images in Figure 2c.

15



273 274

Figure 4. (a) FTIR spectra and (b) XRD patterns of unmodified nanopapers and

275

H40-TOCNF-P.

276 277

The elemental mapping of Si by EDS on H40-TOCNF-P (rightmost in Figure 2c)

278

shows that the siloxanes were homogeneously distributed on the nanopaper surface.

279

The successful silanization of the nanopaper was also further confirmed by the FTIR

280

spectra (Figure 4a). Compared with all the unmodified nanopapers, two new peaks

281

were observed in the H40-TOCNF-P sample: the adsorption bands at 1273 cm-1 and

282

781 cm-1, which are assigned to the asymmetric stretching vibrations of Si-CH3 and

283

the characteristic vibrations of Si-O-Si, respectively.39,42 These groups and linkages 16


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284

were consistent with the predicted chemical structures of polysiloxanes we proposed

285

in Figure 3. Besides, XRD patterns of unmodified nanopapers and H40-TOCNF-P are

286

shown in Figure 4b. The characteristic XRD peaks at 2θ = 15.1°, 16.5°, 22.6°, and

287

34.5°, corresponding to the crystalline planes with Miller indices of 1-10, 110, 200,

288

and 004, respectively,43,44 were observed in all the unmodified nanopapers and the

289

H40-TOCNF-P. This indicated that the cellulose Iβ crystalline structure was

290

preserved during the silanization reaction. However, these peaks exhibited different

291

intensities for various samples, implying the different crystallinities of substrates. As

292

expected, CNC-P had the highest CrI of 48.3% due to the almost removal of

293

amorphous domains from cellulose during the preparation of CNCs. The CNF-P and

294

TOCNF-P had the similar CrI (32.3% and 30.8%, respectively) but lower than that of

295

CNC-P. For H40-TOCNF-P with the CrI of 25.2%, the peaks intensity of the cellulose

296

Iβ crystalline structure was weakened after modification when compared with that of

297

TOCNF-P, as a result of the formation of polysiloxanes on the nanopaper surface.

17



298 299

Figure 5. (a) Photos of unmodified nanopapers and H40-TOCNF-P placed close to

300

and away from the school emblem underneath. (b, c) The total transmittance and

301

transmittance haze of unmodified nanopapers and H40-TOCNF-P.

302 303

Optical Properties. The optical properties of nanopapers, including optical

304

transmittance and haze, which are critical for substrates toward wide applications in

305

optoelectronic devices, are shown in Figure 5. The high optical transparency was

306

demonstrated by the clearly identified school emblem of Beijing Forestry University

307

when placing the nanopapers close to the images underneath (top in Figure 5a); the

308

significant light scattering effect was demonstrated by the fuzzy school emblem when 18


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309

placing the nanopapers about 5 cm away from the images underneath (bottom in

310

Figure 5a). In addition, the wavelength versus transmittance and transmission haze are

311

shown in Figure 5b and c, respectively. The haze value is defined as the ratio of

312

diffuse transmittance to total transmittance, which was determined according to the

313

literature45 in this work. As can be seen, CNC-P exhibited the highest transmittance of

314

91.3% but the lowest haze of only 20.1%, at 550 nm, which may be attributed to that

315

the rod-like CNCs with small length constructed densely, allowing more light to

316

propagate through and suppressing light scatter behavior. Conversely, CNF-P

317

exhibited the lowest transmittance (69.7% at 550 nm) but the highest haze (61.4% at

318

550 nm), which may be due to their rough surface caused by the large dimensions of

319

CNFs. Interestingly, TOCNF-P not only had a high optical transmittance (90.4% at a

320

wavelength of 550 nm), with was close to that of CNC-P, but it also exhibited a

321

higher transmission haze (49.3% at 550 nm) than that of CNC-P. A possible

322

explanation for this could be that each individual TOCNF led to small forward

323

scattering rather than significant back scattering due to its nanoscale diameter and

324

appropriate length, and thus the obtained densely laminated TOCNF-P allowed most

325

of light to propagate through and retained an appropriate light-scattering effect, as

326

reported in the literature.46

327

Compared with the cellulose-based or wood-based substrates with high light

328

transmittance

(~90%)

and

haze

(60%-90%)

reported

329

recently,1,11,12,45 our nanopaper made of TOCNF exhibited a similar light 19


in the

publications


330

transmittance but a relatively lower haze. However, the maximized haze in

331

transparent substrates is not always preferred when integrated into different

332

optoelectronic devices, which require different levels of light scattering.1 Additionally,

333

the surprising and interesting result in the present work is that the obtained

334

H40-TOCNF-P after modification still retained a high transmittance (90.2% at 550

335

nm) and high haze (46.5% at 550 nm). To the best of our knowledge, this is the first

336

time to construct a superhydrophobic surface (CA > 150°) that holds self-cleanable

337

ability on the transparent and hazy cellulose paper without suppressing its optical

338

properties.

339 340

Figure 6. Tensile stress-strain curves of regular paper, unmodified nanopapers, and

341

H40-TOCNF-P. The value of toughness for different papers is also shown.

342 343

Mechanical Properties. The mechanical properties like strength and toughness of

344

nanopapers are important for a wide range of applications in practice. To this end, we

345

performed tensile tests for regular paper, unmodified nanopapers, and H40-TOCNF-P;

346

their stress-strain curves are shown in Figure 6. As can be seen, the regular paper and 20


Page 20 of 40

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347

CNC-P had limited mechanical performances. Regular paper had a very low strength

348

(~49.3 MPa), which may be attributed that native wood fibers with large dimensions

349

are unable to bind tightly and densely, but a relatively high toughness (0.85 J M-3),

350

which may be due to the original good toughness of native wood fibers. As CNCs are

351

rod-like and rigid crystalline structures, leading to the limited contact area between

352

crystals, we believe that their ability to conjoin tightly is significantly hindered and

353

thereby the obtained CNC-P exhibited the lowest strength of ~48.8 MPa and the

354

lowest toughness of ~0.18 J M-3. However, CNF-P, TOCNF-P, and H40-TOCNF-P

355

were much stronger (with the tensile strength of 92.8-103.7 MPa) and much tougher

356

(with the toughness of 1.12-2.45 J M-3) than the regular paper and CNC-P. This

357

improvement of mechanical properties may be related to the enhanced cohesion, such

358

as dispersion force and hydrogen bonds, between CNFs or TOCNFs.47 As shown in

359

Figure 2a, CNFs and TOCNFs had nanoscale width (40-60 nm) and retained

360

sufficient length (2-3 µm), which caused increased contact area and more overlapping

361

between these building blocks of nanopaper. Interestingly, H40-TOCNF-P exhibited

362

the highest toughness of 2.45 J M-3 better than that of unmodified TOCNF-P and even

363

CNF-P. This was probably due to the formation of 3D polysiloxanes networks, in

364

which the stable crosslinks between pearl-necklace-like fibers, as illustrated in Figure

365

2c and Figure 3e, increased the ability of H40-TOCNF-P to absorb energy or sustain

366

deformation without breaking. Additionally, the superhydrophobic modification

367

caused only a slight decline in tensile strength (7.33%), which may be ascribed to the 21



368

decreased dispersion force or/and hydrogen bonding47 between TOCNFs as some

369

hydroxyl groups interacted with silanols during siloxanes reaction.

370 371

Figure 7. (a) Water contact angle of regular paper, TOCNF-P, and H40-TOCNF-P.

372

(b) Photos of regular paper, TOCNF-P, and H40-TOCNF-P with the water droplet on

373

the surface.

374 375

Self-Cleaning Effect and Thermal Analysis. In view of the superior

376

performances of TOCNF-P in optical property (high transmittance and high haze),

377

which is crucial to the light-management function, this nanopaper was further

378

modified using MTCS to construct superhydrophobic surfaces. The wettability of

379

regular paper, unmodified TOCNF-P, and modified TOCNF-P with different reaction

380

time was characterized by the static CAs of the substrates (Figure 7). The regular

381

paper exhibited a completely hydrophilic property, indicated by the zero CA and the 22


Page 22 of 40

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382

wetted paper by water droplet, which was due to the original hydrophilicity of regular

383

wood pulp48,49 and a large number of cavities in this sample. This hydrophilicity of

384

regular paper was further confirmed by the sequential images in Figure S5, where the

385

water droplet was absorbed immediately by the regular paper when contacting the

386

substrate surface and disappeared within 51 ms. However, the TOCNF-P exhibited a

387

certain hydrophobicity with a CA of about 93.5°, which may be attributed the

388

decreased hydrophilicity of TOCNF and the tight binding and lamination of building

389

blocks in TOCNF-P.

390

As expected, the modified TOCNF-P experienced a sharp increase in CA when

391

compared with the unmodified TOCNF-P, as shown in Figure S6 and Figure 7. The

392

obtained H10-TOCNF-P after siloxanes reaction for 10 min had a CA of up to 120.4°.

393

After prolonging the reaction time to 30 and 40 min, the superhydrophobic surfaces

394

with CA of 154.5° and 159.6°, respectively, were prepared. As illustrated by the

395

rightmost photo in Figure 7b, the water droplet that settled on the superhydrophobic

396

surface of H40-TOCNF-P was almost a sphere, demonstrating the significant

397

water-repellency of this nanopaper. Additionally, the anti-wetting property of

398

H40-TOCNF-P was also indicated by the dynamic impact behavior of the water

399

droplet impinging on the prepared surface of the nanopaper, which was illustrated by

400

the sequential images extracted from the recorded video of CA measurement every 3

401

ms (Figure S7). We observed that the water droplet was deformed into a spheroidic

402

shape when impinging on the H40-TOCNF-P surface and then bounced back 23



403

completely. After bouncing for several times, the water droplet came to rest by fully

404

consuming its initial potential energy, which is also demonstrated in Movie S1.

405

Moreover, the SA for the surface of H40-TOCNF-P was only 5.8°, suggesting a low

406

adhesion force between water droplet and the substrate surface, which is crucial to

407

self-cleaning applications.50 Illustrated by Figure 3d, the high CA and low SA of

408

modified nanopaper indicated the suspension but not the penetration between the

409

water droplet and the micro/nanopores of paper surface as the Cassie-Baxter state

410

described.51 These results of surface wettability, which is ruled by the surface energy

411

and roughness,52,53 clearly indicated the superhydrophobicity of the modified

412

nanopaper. The decreased surface energy and the increased surface roughness caused

413

by the formation of 3D polysiloxanes networks during the silanization reaction

414

imparted an excellent water-repellency to the TOCNF-P.

415

Self-cleaning is an important characteristic of superhydrophobic surfaces, which

416

thereby have many potential applications in interdisciplinary technological fields.6,54

417

Herein, some dust particles were randomly put onto the surface of H40-TOCNF-P to

418

test the self-cleaning effect of this modified nanopaper. As shown in Figure S8, when

419

dripped onto the surface of the transparent nanopaper, the water droplet formed a

420

sphere and rolled away along the slope. The dust particles were attached to the surface

421

of the water droplet (as shown by the arrow) and carried away, leaving a very clean

422

roll trace (as shown by the double dashed lines). This self-cleaning effect was also

423

clearly demonstrated in Movie S2. The dust particles on the surface of H40-TOCNF-P 24


Page 24 of 40

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424

were collected and taken away by rolling water, and then a clean surface was

425

achieved.

426

Good shape stability and thermal stability are required for the practical

427

applications of the obtained multifunctional cellulose nanopaper. As shown in Figure

428

S9, the H40-TOCNF-P exhibited enhanced shape stability after water rinse. This was

429

attributed to that the hydrophilic cellulose paper was converted into the

430

superhydrophobic paper during modification; therefore, its wet resistance was

431

extremely improved. Figure S10 presents the TGA curve and the corresponding first

432

derivative weight loss curve of the H40-TOCNF-P. The thermal decomposition of this

433

cellulose nanopaper processed rapidly at about 201–354 °C; below the temperature of

434

201 °C, the H40-TOCNF-P exhibited a high thermal stability. As observed in the

435

differential TGA curve, the main decomposition process had two peak temperatures

436

of 249 °C and 312 °C, which were probably related to the decomposition of the

437

TOCNF and the grafted siloxanes, respectively.22

438

25



Page 26 of 40

439

Figure 8. Current density-voltage (J-V) curves of the bare solar cell and the solar cell

440

with different nanopapers.

441 442

Table 1. Photovoltaic characteristics of the bare solar cell and the solar cell with

443

different nanopapers.

Voc

Jsc

FF

Efficiency

(V)

(mA cm-2)

(%)

(%)

Bare Cell

2.494

5.282

76.47

10.07

Cell + TOCNF-P

2.508

5.915

77.37

11.48

Cell + H40-TOCNF-P

2.531

5.692

77.21

11.12

Cell + Contaminated H40-TOCNF-P

2.469

3.493

60.95

6.00

Cell + Self-cleaned H40-TOCNF-P

2.546

5.396

78.35

10.76

444 445

Enhancement of Solar Cell Efficiency. Light management and self-cleaning

446

properties play a crucial role in improving the overall conversion efficiency of solar

447

cells. The superhydrophobic, highly transparent and highly hazy nanopaper we

448

prepared in this work exhibited a potential application in optoelectronic devices like

449

solar panels outside. To indicate this fact, the effect of light management and

450

self-cleaning was evaluated by characterizing the photovoltaic performance of a 26


Page 27 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


451

polysilicon solar cell without and with TOCNF-P, H40-TOCNF-P, dust-contaminated

452

and self-cleaned H40-TOCNF-P. The obtained current density-voltage (J-V) curves

453

are given in Figure 8. The electrical properties of the solar cell, including open circuit

454

voltage (Voc), short circuit density (Jsc), fill factor (FF, the maximum output power

455

divided the product of Voc and Jsc), and the overall conversion efficiency were

456

extracted from the J-V curves and shown in Table 1.

457

Compared with the performance of the bare solar cell, an enhancement of 11.98%

458

and 7.76% in Jsc and a corresponding 14.00% and 10.43% boost in conversion

459

efficiency were observed in the solar cell with TOCNF-P and H40-TOCNF-P,

460

respectively. This was mainly attributed to the two nanopapers’ excellent optical

461

properties: high transmittance, which allowed most light to propagate through the

462

nanopaper and reach the active layer of solar cell with less losses, and high haze,

463

which could lead the normal incident light to become extremely diffusive and

464

therefore increase the travelling path length of photos in the solar cell, thus improving

465

the possibility of a photon being captured within the active region of solar

466

cell,10,12,55,56 as illustrated in Figure 3e. On the other hand, our nanopapers may be

467

functioning as the anti-reflection coating by decreasing refractive index mismatch

468

between solar cell and air to suppress light reflection. After being randomly

469

contaminated with some dust particles, the solar cell with H40-TOCNF-P exhibited a

470

severe decrease in Jsc (38.63%) and conversion efficiency (46.04%). This decline of

471

photovoltaic performance of solar cell was mainly due to the dust particles blocking 27



Page 28 of 40

472

the incident light significantly, indicated by the slight variation of Voc and FF.7 After

473

cleaning process, the increase of photovoltaic performance of the solar cell with

474

self-cleaned H40-TOCNF-P was clearly observed. Compared with that of the solar

475

cell with native H40-TOCNF-P, the Jsc and the corresponding efficiency had

476

recovered 94.80% and 96.76%, respectively. This photovoltaic performance was still

477

slightly better than that of the bare solar cell.

478

The enhancement of Jsc and conversion efficiency by our nanopapers was

479

relatively lower than that by transparent wood materials reported in literatures,11,12

480

which may be mainly due to the relatively lower transmittance and haze of our

481

nanopapers or the differential of various types of solar cells. However, our modified

482

nanopaper, with significant functions of both light management and self-cleaning, has

483

been demonstrated to be able to simultaneously mitigate the adverse effect of light

484

reflection and dust accumulation on solar cells. Additionally, this sustainable and

485

multifunctional cellulose-based material had the potential practical utility in other

486

optoelectronic devices or common objects such as building windows, vehicle

487

windshields, packages, and goggles.

488 489 490

CONCLUSION In summary, we developed a novel multifunctional nanopaper that simultaneously

491

achieves

high

transmittance

(90.2%),

high

optical

492

superhydrophobicity (CA = 159.6°, SA = 5.8°) for the first time. The renewable 28


haze

(46.5%),

and

Page 29 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


493

TOCNF-P from earth-abundant wood fibers endowed this multifunctional nanopaper

494

excellent optical and mechanical properties, while further silanization modification

495

imparts the TOCNF-P with superb water-repellency. With light-management and

496

self-cleaning capabilities, our superhydrophobic, transparent and hazy cellulose

497

nanopaper could not only enhance the energy conversion efficiency of solar cells but

498

also recover most of photovoltaic performance losses due to dust accumulation on

499

surface. Therefore, the proposed nanopaper here was potentially feasible to be utilized

500

in solar cells and even a wide range of applications in sustainable and biodegradable

501

“green” electronics, optoelectronics, or other functional devices.

502 503

ASSOCIATED CONTENT

504

Supporting Information

505

The Supporting Information is available free of charge.

506

Photos of Tyndall effect for three types of nanofibrillated cellulose fibers

507

suspension; SEM images of cross sections for regular paper and nanopapers;

508

SEM images of surface morphology and static CA for H10-TOCNF-P,

509

H20-TOCNF-P, and H30-TOCNF-P; The weight of siloxane grafted on the

510

TOCNF-P; Sequential images of dynamic impact behavior of the water droplet

511

impinging on the surface of regular paper and H40-TOCNF-P; Photo of

512

self-cleaning

513

H40-TOCNF-P. (PDF)

effect

for

H40-TOCNF-P.

Thermal

29


analysis

of

the


514

Movie S1 shows that water droplets quickly roll away and bounce on the

515

surface of H40-TOCNF. (AVI)

516

Movie S2 shows the self-cleaning property of H40-TOCNF-P to remove dust

517

particles. (AVI)

518 519

AUTHOR INFORMATION

520

Corresponding Author

521

*E-mail: [email protected]

522

ORCID

523

Sheng Chen: 0000-0001-6094-3084

524

Feng Xu: 0000-0003-2184-1872

525

Notes

526

The authors declare no competing financial interest.

527 528

ACKONWLEDGMENTS

529

The authors gratefully acknowledge the financial support from the National Key R&D

530

Program of China (2017YFD0600204).

531 532

REFERENCES

30


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Bongiovanni, R.; Turri, S., Multifunctional Luminescent Down ‐ Shifting

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A multifunctional nanopaper based on sustainable and renewable cellulose fibers was developed and exhibited potential applications in solar cells.

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Highly Transparent and Hazy Cellulose Nanopaper Simultaneously

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