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Green fabrication of coloured superhydrophobic paper from native cotton cellulose
Accepted Manuscript Green fabrication of coloured superhydrophobic paper from native cotton cellulose Qiuying Wen, Fei Guo, Fuchao Yang, Zhiguang Guo PII: DOI: Reference:
S0021-9797(17)30281-3 http://dx.doi.org/10.1016/j.jcis.2017.03.036 YJCIS 22133
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
20 January 2017 3 March 2017 5 March 2017
Please cite this article as: Q. Wen, F. Guo, F. Yang, Z. Guo, Green fabrication of coloured superhydrophobic paper from native cotton cellulose, Journal of Colloid and Interface Science (2017), doi: http://dx.doi.org/10.1016/j.jcis. 2017.03.036
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Green fabrication of coloured superhydrophobic paper from native cotton cellulose Qiuying Wena,b, Fei Guoa,b, Fuchao Yanga and Zhiguang Guoa,b,* a
Hubei Collaborative Innovation Centre for Advanced Organic Chemical Materials and Ministry of
Education Key Laboratory for the Green Preparation and Application of Functional Materials, Hubei University, Wuhan 430062, People’s Republic of China. b
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics,
Chinese
Academy of Sciences, Lanzhou 730000, People’s Republic of China. *E-mail: [email protected]; Fax: +86-931-8277088; Tel: +86-931-4968105
Abstract Paper is kind of essential materials in our daily life. However, it can be easily destroyed by water owing to its superhydrophilic surface. Here, we reported a simple and green fabrication of coloured superhydrophobic paper via swelling and approximate dissolution of cotton followed by precipitation of cellulose and doping coloured
stearates.
The
obtained
paper
exhibited
uniform
colour
and
superhydrophobicity, of which the colour was consistent with the doped stearates owing to the adhesion of stearate powders to the tiny floc fiber surface and we proved that the superhydrophobicity could not be damaged after abrasion resulting from the inner and outer superhydrophobicity and the increased surface roughness. This coloured superhydrophobic paper would be avoided from moisture damage and may be useful in different fields. Keywords:
Green
fabrication;
Coloured
superhydrophobic
superhydrophobicity; Abrasion resistance; Self-cleaning
1
paper;
Uniform
1. Introduction As one of the essential materials, paper has been commonly used in our daily life. However, paper has inherent hydroscopic property because its main composition is cellulose containing some hydrophilic groups, which impedes the application in different fields. Furthermore, it is also an intractable problem that raw materials collection, the massive energy consumption and serious pollution during the process of papermaking make enormous destruction to the ecosystem. 1-3 Based on it, water-repellent paper fabricated through green way would expand the application range in humid conditions, which arouses interests of researchers recently. Superhydrophobicity, referring to the surface with high water contact angle (WCA) larger than 150° and low slide angle (SA) less than 10°，has been considered as an important property toward waterproof application. 4,5 After several decades of developments, superhydrophobicity has been achieved on the surfaces of different kind of materials such as metals, fibers and so on.6-13
Naturally, it provides feasible
ideas to get water-repellent paper through fabricating superhydrophobic paper. Therefore, a few methods have been reported to prepare superhydrophobic paper such as spray-deposition of hydrophobic silicon nanoparticles, 14 dip coating,15-17 pigment coating followed by bio-wax post-treatment,18 fluorocarbon plasma treatment to obtain hydrophobic paper19-21 and so forth. Among them, fabrications of superhydrophobic nanoparticles especially modified silica nanoparticles bring great benefits to the development of superhydrophobic paper. Most of them depended on the treatment of the silica nanoparticles with expensive or toxic modifier to get 2
low-surface-energy, for which severely restricted the industrialization process of superhydrophobic paper.22,23 Although the literature reports about superhydrophobic paper are few owing to the intrinsic hydrophilicity of the paper, it
is still meaningful to fabricate
superhydrophobic paper because the superhydrophobicity can be combined with other performance to achieve functional superhydrophobic paper for different applications. Some groups have also reported functional superhydrophobic paper.24-27 For example, Yang et al. combined superhydrophobicity with structural colour, fabricating coloured films which were tested on a paper printed with letters via spray coating. 28 Latter, they also prepared superamphiphobic paper via spray coating fluorinated silica nanoparticles for light oil–water separation.29 Others fabricated superhydrophobic paper for the biomicrofluidic system based on controlling droplet mobility. 30,31 However, most of the above methods just concentrate on the surface treatment of paper, making the obtained paper superhydrophobicity limited on the outer surfaces. In other words, these kinds of paper have weak mechanical stability and the waterproof function may be lost due to wear and tear during repeated use, let alone after abrasion test. Aiming at this problem, it is of great significance to get robust paper with outer and inner uniform superhydrophobicity through simple and economic way. Our group has also reported superhydrophobic paper, 32,33 however the colour of that superhydrophobic paper was only limited on the colour of white. At the same time, coloured superhydrophobic paper also play an important role in potential academic research and industrial applications, but little according work have been 3
reported due to the difficulty in balancing superhydrophobicity and added coloured dye. It is still a challenge to fabricate robust coloured superhydrophobic paper. Based on the previous research, in this work we reported the green fabrication of coloured superhydrophobic paper from absorbent cotton through swelling and approximate dissolution of cotton in zinc chloride (ZnCl2) solution followed by adding water to make the tiny floc cotton fiber precipitate and doping stearates (see the schematic illustration in the Fig. 1 taking the yellow paper as an example and detailed procedures in the experimental section). The stearate powders attached to the precipitated fiber, developing uniform mixtures that exhibited similar colour to the doped powders colour. Through using copper stearate (CuSA2), ferric stearate (FeSA3) and zinc stearate (ZnSA2) respectively, three kinds of coloured paper with superhydrophobicity were presented here: blue, yellow and white. The paper possesses uniform superhydrophobicity and colour from inner to outer, whose superhydrophobicity can be higher after abrasion owing to the uniform superhydrophobicity and the surface roughness increase under abrasion. As the most abundant and widespread biopolymer in nature, cellulose has great potential for both basic study and sustainable development. Using cellulose as raw material,
some
workers
have
fabricated
superhydrophobic
surfaces
and
superhydrophobic cellulose microspheres. 34-36 Here, using absorbent cotton cellulose, we prepared coloured superhydrophobic paper via a green method which includes two aspects as followed. In addition, using the same method, we fabricated superhydrophilic paper without adding superhydrophobic powders as a blank sample. 4
The superhydrophobic paper with blue, yellow and white colours is consistent with colours of the doped stearates respectively owing to the adhesion of stearate powders to the tiny floc fiber surface (Fig. S1). 2. Experimental 2.1. Materials Sodium stearate (CP) (NaSA) was obtained from Shanghai TONGSHI Chemical Co., Ltd, China. Stearic acid (CP), ZnCl2 (AR) and zinc acetate (AR) were all purchased from Shantou XILONG Chemical Co., Ltd, China. Cupric acetate (AR) was purchased from Tianjin QILIANXING Chemical Co., Ltd, China. Ferric chloride crystal (AR) was obtained from Tianjin KEMIOU Chemical Co., Ltd, China. In addition, all other chemicals were analytical grade reagents and were used as received. 2.2. Fabrications of coloured superhydrophobic powders. For FeSA3, 1.83 g NaSA was added into 60 mL hot deionized water under constant heating at 80 ℃ with continuous magnetic stirring and then 20 mL solution containing a little excess ferric chloride crystal was dropwise added into the hot NaSA solutions to react for 1 h. For ZnSA2, the processes resemble that of FeSA3 fabrication except for replacing ferric chloride crystal with zinc acetate. As for CuSA2, 0.5 g cupric acetate and 1.4 g stearic acid was dissolved in 30 ml ethanol respectively. Then the cupric acetate solutions were dropwise added into stearic acid solutions under constant stirring to react for 1 h. All the stearates were dried at 60 ℃ after filtration and then grinded to uniform powders used for next fabrication of coloured 5
superhydrophobic paper. 2.3. Swelling and dissolution of cotton ZnCl2 was dissolved in 20 ml deionized water under magnetic stirring to prepare 65 wt% ZnCl2 transparent solutions. Then 0.285 g absorbent cotton was added into the above solutions. They were heated with oil-bath at 80 ℃ under constant magnetic stirring until the mix solutions became semitransparent and uniform, resulting from the swelling and approximate dissolution of cotton. 2.4. Preparation of coloured superhydrophobic paper 3 ml of the above cellulose solutions was placed into 250 ml of the beaker and 70 ml of deionized water was added into it. After several minutes, the micro-nano cellulose would be precipitated. Then, 0.11 g FeSA3 powders were added into the beaker. The powders floated on the solutions surface due to its property of superhydrophobicity. In order to uniformly disperse the mixture, 130 ml ethanol was added into the beaker. The mixture was stirred with a glass rod and treated with ultrasonic process for 1~2 min. The yellow superhydrophobic powders would attach to the surface of micro-nano cellulose, leading to the development of dispersive and uniform mix solutions. The mix solutions were filtrated using a vacuum pump and the product was washed by ethanol several times to remove the residual reagents such as ZnCl2. The obtained wet paper was placed on a horizontal A4 paper for several minutes and sometimes we should prevent the paper from bending owing to the uneven volatilization of the ethanol. Then the paper can be placed in the middle of flat tissues like sandwich biscuits to be planished using tablet press, which convert the product 6
into flat paper. After that, the final superhydrophobic paper was got after drying at 60 ℃. For blue and white superhydrophobic paper, the preparation resemble that of yellow superhydrophobic paper except for replacing 0.11 g FeSA3 powders with 0.09 g CuSA2, and 0.11 g ZnSA2 respectively. In addition, in order to study the effect of the of content of added coloured stearates on the paper hydrophobicity, we changed the content of the added coloured stearates under a certain cellulosic content to prepare coloured paper and it was summarized in the Table S1.
Fig.
1 Schematic
illustration of the fabrication procedures of coloured
superhydrophobic paper. Photographs of (a) absorbent cotton, (b) swelling and approximate dissolution of cotton in ZnCl2 solution, (c) tiny floc cotton cellulose precipitate in the deionized water, (d) mix solution of cotton cellulose and FeSA3, (e) and (f) the yellow superhydrophobic paper before and after dying respectively. 2.5. Characterization To characterize the morphology of samples, field emission scanning electron microscope (FESEM) images were obtained on JSM-6701F, both with Au-sputtered specimens. As for the component analysis, Fourier transformer infrared spectra (FTIR) were recorded using Thermo Scientific Nicolet iS10. X-ray photoelectron 7
spectroscopy (XPS, Thermo Scientific ESCALAB 250Xi) measurements were made using the Al Kα line as the excitation source. The water contact angles were measured with JC2000D with a 5 μL distilled water droplet at ambient temperature and the sliding angles were also mesured. The average WCA values of each sample were obtained by measuring each sample at five different sites. All photographs were obtained by a Sony camera (DSCHX200). 3. Results and discussion 3.1 Structure and characterization of coloured superhydrophobic paper Here, it is the yellow paper to represent an example of coloured superhydrophobic paper for analysis. As shown in Fig. 2a - d, the SEM images of yellow superhydrophobic paper and blank sample were represented. It can be observed that the paper of blank sample was composed of micro-nano sheets with smooth surface (Fig. 2b). Comparing Fig. 2b and 2d, we can find the differences of surface topography between these two samples. In the high magnification SEM images (Fig. 2d), it can be found that the yellow superhydrophobic paper was covered by nano particles and the nano particles had much smaller size than the micro-nano sheets of the blank sample surface. Namely, the yellow superhydrophobic paper had much smaller sized rough structure than the blank sample. Based on the SEM images, it is concluded that the addition of coloured superhydrophobic powders not only creates nanoscale rough structure with much smaller size
but also introduces low surface
energy materials, leading to the superhydrophobicity of the paper. It is also reasonable that the colours of paper are attributed to the attachment of coloured powders to the 8
micro-nano cellulose. SEM images of the water proof blue and white paper were also presented in the Fig. S2.
Fig. 2 SEM images of (a, b) blank sample and (c, d) yellow superhydrophobic paper with low and high resolution. In order to prove the chemical composition of the superhydrophobic paper, XPS experiment of the superhydrophobic paper was conducted here. From the XPS spectra Fig. 3a, it can be seen that the main composition of the yellow superhydrophobic paper are carbon, oxygen, zinc, chlorine and iron. In the Fig. 3a and Fig. 3b, the peak at 725.57 eV that is very weak is assigned to Fe 2p of compounds. The addition of FeSA3 powders is very few, which could be the reason that the peak of Fe 2p is not obvious. In the Fig. 3a, the peak at 198.52 eV and the peak at 1022.08 eV are attributed to the Cl 2p and Zn 2p of ZnCl2, which result from 65 wt% ZnCl2 solutions in the process of cotton swelling and approximate dissolution. The C 1s peak at 284.79 eV is assigned to C-C and the O 1s peak at 532.05 eV may be attributed to -COO-. The XPS analysis for blue and white superhydrophobic paper is also 9
represented in the Fig. S5.
Fig. 3 XPS spectra of yellow superhydrophobic paper including (a) the
survey
spectrum and (b) Fe 2p spectrum. To further confirm the chemical composition of the coloured superhydrophobic paper, FTIR of the superhydrophobic paper and blank sample were conducted here (Fig. 4 and Fig. S6). Curve black in the Fig. 4 represent the FTIR spectrum of the yellow superhydrophobic paper. From curves black in the Fig. 4 and Fig. S6, it can be discovered that there are no significant differences in the FTIR spectra of the blue, yellow and white superhydrophobic paper. Comparing the curve black with the curve red in the Fig. 4, we can see that three obvious adsorption peaks appear in the spectrum of the yellow superhydrophobic paper while they are absent in the spectrum 10
of the blank sample. The sharp adsorption peaks centered at about 2919 and 2850 cm-1 are related to the symmetric and asymmetric stretching vibrations of –CH2- and –CH3 groups. However, the Fe 2p peak in the XPS spectra Fig. 3 is not obvious. There are some resons to explain it. Firstly, the proportion of the iron to the methylene groups is 1/48 in per mole FeSA3. Further, the addition of FeSA3 powders in the yellow superhydrophobic paper is very few. In addition, the percentage of Fe in all elements containing carbon, oxygen, zinc, chlorine and iron is very small. The absorption band at around 1540 and 1465 cm-1 in the spectrum of yellow paper is assigned to -COO- symmetric and asymmetric stretching vibrations. Combined with the XPS analysis, it further proves the existence of FeSA3 in the yellow superhydrophobic paper.
Fig. 4 FTIR spectra of yellow superhydrophobic paper and blank sample. The paper without adding superhydrophobic powders exhibited superhydrophilicity (Fig. S3), while all the paper with stearates showed superhydrophobicity with large WCA > 150° (Fig. 5a and b). The SA on the coloured paper was also measured, which was less than 10° (see the SA of yellow paper in the movie S1). The summary about 11
superhydrophobicity including WCA and SA was also represented in the Table 1.
Fig. 5 (a) Water contact angles on blue, yellow and white superhydrophobic paper respectively.
(b)
Photographs of water
droplets sitting on the coloured
superhydrophobic paper. Table 1 Water contact angles and slide angles of the coloured superhydrophobic paper and blank sample surfaces colour
θCA
θSA
yellow
153.3°
~5°
blue
154.3°
~8°
white
151.7°
~6°
blank sample
0°
-
θCA: water contact angle; θSA: slide angle 3.2. Mechanical property of coloured superhydrophobic paper It can be understood that the superhydrophobicity of most of superhydrophobic surfaces would be damaged under abrasion impact owing to the superhydrophobicity only limited on the outer surface. In order to improve the mechanical stability of superhydrophobic surface, some works have been reported. 37,38 Here, the coloured paper can maintain good superhydrophobicity after abrasion. In order to clarify the 12
hydrophobicity of the yellow paper under abrasion, the abrasion test was performed on the yellow paper here. The paper was placed on the sandpaper and loaded with a 200 g weight, which was dragged for the length of 10 cm along the ruler (see in the movie S4 and Fig. 6b). After twice, the frictional place of the yellow paper still exhibited excellent superhydrophobicity as seen in the Fig. 6d, indicating a good mechanical stability of the superhydrophobic paper. The WCA of the paper after the abrasion test was higher than 153° (Fig. 6d), which would have great potential in future paper industry for inner and outer superhydrophobicity. In order to further reveal it, the SEM image of the yellow paper after abrasion was also obtained (Fig. 6c). Comparing the SEM images of the yellow paper before and after abrasion (Fig. 6c and Fig. 2d), it can be discovered that the micro-nano particles covered on the cellulose reduced obviously after abrasion while the roughness of micro-nano structure increased. This may be the reason that the coloured paper still maintained excellent superhydrophobicity after abrasion. In addition, flexibility of the yellow superhydrophobic paper was also proved here as seen in the Fig. 6a. It can be bended by fingers to some extent. The above tests showed that the prepared coloured superhydrophobic paper have good mechanical property.
13
Fig. 6 (a) Photograph of yellow superhydrophobic paper bended by fingers. (b) Optical image of abrasion test on yellow superhydrophobic paper. (c) SEM images of yellow superhydrophobic paper after abrasion. (d) Photographs of water droplets sitting on the yellow superhydrophobic paper after abrasion and photograph of the water contact angle of ~ 153° on it in the upper-right corner.
3.3. water-proof function and application in our daily life Water-proof function is significant for paper in applications. In order to explore resistance to water of the superhydrophobic paper, we chose the yellow paper to represent a sample of coloured paper for water proof function test. As shown in the Fig. 7a-b and the movie S2, a silver mirror phenomenon can be seen at a certain angle of view when the yellow paper was immersed in the water, which was a signature of the trapped air between the interface of liquid and solid. In the movie S2, the silver mirror phenomenon can also be seen after the letters “superhydrophobic” were written on the yellow paper surface. This shows that air can be naturally trapped while the water was spreading on the writeable superhydrophobic paper surface. Owing to the property of air trapping in the grooves of superhydrophobic paper to insulate from liquid phase, the substrates exhibit great water proof function.
14
Fig. 7 (a-b) Silver mirror phenomenon of yellow paper immersed in the water before and after writing letters on it. (c) Self-cleaning test of yellow paper on the slightly tilted glass sheet and the dust particles as contaminants. (d-e) Photographs of typing paper before and after immersion in the coffee for 30s. (f-g) Photographs of yellow paper before and after immersion in the coffee for 60s. It is generally known that lotus leaf exhibits self-cleaning property owing to the high WCA and low SA. When the water slides off the lotus leaf, the dust on the surfaces can be taken away along with the water. Here, the self-cleaning function of the yellow paper was also proved. As seen in the Fig. 7c and the movie S3, the yellow paper with some dust particles was placed on a slightly tilted glass sheet. The water was sprinkled on the yellow paper surface through using a wash bottle. We can see that the dust particles were dropped away from the paper by the water. After a while, the yellow paper became cleaning with no dust particles remained on the paper surface, which indicated that our work would be potential for self-cleaning applications. To further clarify the potential application of superhydrophobic paper in our daily life, an immersing test was represented here. The above yellow paper after abrasion 15
test and the common used typing paper were immersed into the coffee as comparision (Fig. 7d-g). Fig. 4e and g respectively shows the photographs of the yellow paper and the typing paper after immersion, we can see that the typing paper were wetted obviously and exhibited the colour of the coffee in half a minute while the yellow paper still maintained nearly intact as before immersion even after a much longer immersion (see in the movie S5, movie S6 and Fig. S4). 4. Conclusions In summary, we have fabricated superhydrophobic coloured paper from absorbent cotton and coloured stearate powders via a green method. The process contains no toxic modifier. In addition, the obtained paper not only exhibit consistent colour with the doped stearates owing to the adhesion of stearate powders to the tiny floc fiber surface but also possess uniform superhydrophobicity from inner to outer. After abrasion, the superhydrophobicity would not be damaged owing to the inner and outer superhydrophobicity
and
the
surface
roughness
increase.
The
coloured
superhydrophobic paper prepared by a green fabrication method in our work may have great potential applications in different fields, such as microfluidic system and paper industry, and the menthod used here can also satisfy the requirements of large-scale preparation. Acknowledgement This study is supported by the National Nature Science Foundation of China (No. 51522510 and 51675513), and the National 973 Project (2013CB632300) for financial support. 16
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21
Graphical abstract
The green coloured superhydrophobic paper was fabricated from native cotton cellulose via swelling and approximate dissolution of cotton followed by precipitation of cellulose and doping coloured stearates. The process contains no toxic modifier and the obtained paper exhibits satisfactory inner and outer uniform superhydropfobicity and colour.
22
S0021-9797(17)30281-3 http://dx.doi.org/10.1016/j.jcis.2017.03.036 YJCIS 22133
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
20 January 2017 3 March 2017 5 March 2017
Please cite this article as: Q. Wen, F. Guo, F. Yang, Z. Guo, Green fabrication of coloured superhydrophobic paper from native cotton cellulose, Journal of Colloid and Interface Science (2017), doi: http://dx.doi.org/10.1016/j.jcis. 2017.03.036
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Green fabrication of coloured superhydrophobic paper from native cotton cellulose Qiuying Wena,b, Fei Guoa,b, Fuchao Yanga and Zhiguang Guoa,b,* a
Hubei Collaborative Innovation Centre for Advanced Organic Chemical Materials and Ministry of
Education Key Laboratory for the Green Preparation and Application of Functional Materials, Hubei University, Wuhan 430062, People’s Republic of China. b
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics,
Chinese
Academy of Sciences, Lanzhou 730000, People’s Republic of China. *E-mail: [email protected]; Fax: +86-931-8277088; Tel: +86-931-4968105
Abstract Paper is kind of essential materials in our daily life. However, it can be easily destroyed by water owing to its superhydrophilic surface. Here, we reported a simple and green fabrication of coloured superhydrophobic paper via swelling and approximate dissolution of cotton followed by precipitation of cellulose and doping coloured
stearates.
The
obtained
paper
exhibited
uniform
colour
and
superhydrophobicity, of which the colour was consistent with the doped stearates owing to the adhesion of stearate powders to the tiny floc fiber surface and we proved that the superhydrophobicity could not be damaged after abrasion resulting from the inner and outer superhydrophobicity and the increased surface roughness. This coloured superhydrophobic paper would be avoided from moisture damage and may be useful in different fields. Keywords:
Green
fabrication;
Coloured
superhydrophobic
superhydrophobicity; Abrasion resistance; Self-cleaning
1
paper;
Uniform
1. Introduction As one of the essential materials, paper has been commonly used in our daily life. However, paper has inherent hydroscopic property because its main composition is cellulose containing some hydrophilic groups, which impedes the application in different fields. Furthermore, it is also an intractable problem that raw materials collection, the massive energy consumption and serious pollution during the process of papermaking make enormous destruction to the ecosystem. 1-3 Based on it, water-repellent paper fabricated through green way would expand the application range in humid conditions, which arouses interests of researchers recently. Superhydrophobicity, referring to the surface with high water contact angle (WCA) larger than 150° and low slide angle (SA) less than 10°，has been considered as an important property toward waterproof application. 4,5 After several decades of developments, superhydrophobicity has been achieved on the surfaces of different kind of materials such as metals, fibers and so on.6-13
Naturally, it provides feasible
ideas to get water-repellent paper through fabricating superhydrophobic paper. Therefore, a few methods have been reported to prepare superhydrophobic paper such as spray-deposition of hydrophobic silicon nanoparticles, 14 dip coating,15-17 pigment coating followed by bio-wax post-treatment,18 fluorocarbon plasma treatment to obtain hydrophobic paper19-21 and so forth. Among them, fabrications of superhydrophobic nanoparticles especially modified silica nanoparticles bring great benefits to the development of superhydrophobic paper. Most of them depended on the treatment of the silica nanoparticles with expensive or toxic modifier to get 2
low-surface-energy, for which severely restricted the industrialization process of superhydrophobic paper.22,23 Although the literature reports about superhydrophobic paper are few owing to the intrinsic hydrophilicity of the paper, it
is still meaningful to fabricate
superhydrophobic paper because the superhydrophobicity can be combined with other performance to achieve functional superhydrophobic paper for different applications. Some groups have also reported functional superhydrophobic paper.24-27 For example, Yang et al. combined superhydrophobicity with structural colour, fabricating coloured films which were tested on a paper printed with letters via spray coating. 28 Latter, they also prepared superamphiphobic paper via spray coating fluorinated silica nanoparticles for light oil–water separation.29 Others fabricated superhydrophobic paper for the biomicrofluidic system based on controlling droplet mobility. 30,31 However, most of the above methods just concentrate on the surface treatment of paper, making the obtained paper superhydrophobicity limited on the outer surfaces. In other words, these kinds of paper have weak mechanical stability and the waterproof function may be lost due to wear and tear during repeated use, let alone after abrasion test. Aiming at this problem, it is of great significance to get robust paper with outer and inner uniform superhydrophobicity through simple and economic way. Our group has also reported superhydrophobic paper, 32,33 however the colour of that superhydrophobic paper was only limited on the colour of white. At the same time, coloured superhydrophobic paper also play an important role in potential academic research and industrial applications, but little according work have been 3
reported due to the difficulty in balancing superhydrophobicity and added coloured dye. It is still a challenge to fabricate robust coloured superhydrophobic paper. Based on the previous research, in this work we reported the green fabrication of coloured superhydrophobic paper from absorbent cotton through swelling and approximate dissolution of cotton in zinc chloride (ZnCl2) solution followed by adding water to make the tiny floc cotton fiber precipitate and doping stearates (see the schematic illustration in the Fig. 1 taking the yellow paper as an example and detailed procedures in the experimental section). The stearate powders attached to the precipitated fiber, developing uniform mixtures that exhibited similar colour to the doped powders colour. Through using copper stearate (CuSA2), ferric stearate (FeSA3) and zinc stearate (ZnSA2) respectively, three kinds of coloured paper with superhydrophobicity were presented here: blue, yellow and white. The paper possesses uniform superhydrophobicity and colour from inner to outer, whose superhydrophobicity can be higher after abrasion owing to the uniform superhydrophobicity and the surface roughness increase under abrasion. As the most abundant and widespread biopolymer in nature, cellulose has great potential for both basic study and sustainable development. Using cellulose as raw material,
some
workers
have
fabricated
superhydrophobic
surfaces
and
superhydrophobic cellulose microspheres. 34-36 Here, using absorbent cotton cellulose, we prepared coloured superhydrophobic paper via a green method which includes two aspects as followed. In addition, using the same method, we fabricated superhydrophilic paper without adding superhydrophobic powders as a blank sample. 4
The superhydrophobic paper with blue, yellow and white colours is consistent with colours of the doped stearates respectively owing to the adhesion of stearate powders to the tiny floc fiber surface (Fig. S1). 2. Experimental 2.1. Materials Sodium stearate (CP) (NaSA) was obtained from Shanghai TONGSHI Chemical Co., Ltd, China. Stearic acid (CP), ZnCl2 (AR) and zinc acetate (AR) were all purchased from Shantou XILONG Chemical Co., Ltd, China. Cupric acetate (AR) was purchased from Tianjin QILIANXING Chemical Co., Ltd, China. Ferric chloride crystal (AR) was obtained from Tianjin KEMIOU Chemical Co., Ltd, China. In addition, all other chemicals were analytical grade reagents and were used as received. 2.2. Fabrications of coloured superhydrophobic powders. For FeSA3, 1.83 g NaSA was added into 60 mL hot deionized water under constant heating at 80 ℃ with continuous magnetic stirring and then 20 mL solution containing a little excess ferric chloride crystal was dropwise added into the hot NaSA solutions to react for 1 h. For ZnSA2, the processes resemble that of FeSA3 fabrication except for replacing ferric chloride crystal with zinc acetate. As for CuSA2, 0.5 g cupric acetate and 1.4 g stearic acid was dissolved in 30 ml ethanol respectively. Then the cupric acetate solutions were dropwise added into stearic acid solutions under constant stirring to react for 1 h. All the stearates were dried at 60 ℃ after filtration and then grinded to uniform powders used for next fabrication of coloured 5
superhydrophobic paper. 2.3. Swelling and dissolution of cotton ZnCl2 was dissolved in 20 ml deionized water under magnetic stirring to prepare 65 wt% ZnCl2 transparent solutions. Then 0.285 g absorbent cotton was added into the above solutions. They were heated with oil-bath at 80 ℃ under constant magnetic stirring until the mix solutions became semitransparent and uniform, resulting from the swelling and approximate dissolution of cotton. 2.4. Preparation of coloured superhydrophobic paper 3 ml of the above cellulose solutions was placed into 250 ml of the beaker and 70 ml of deionized water was added into it. After several minutes, the micro-nano cellulose would be precipitated. Then, 0.11 g FeSA3 powders were added into the beaker. The powders floated on the solutions surface due to its property of superhydrophobicity. In order to uniformly disperse the mixture, 130 ml ethanol was added into the beaker. The mixture was stirred with a glass rod and treated with ultrasonic process for 1~2 min. The yellow superhydrophobic powders would attach to the surface of micro-nano cellulose, leading to the development of dispersive and uniform mix solutions. The mix solutions were filtrated using a vacuum pump and the product was washed by ethanol several times to remove the residual reagents such as ZnCl2. The obtained wet paper was placed on a horizontal A4 paper for several minutes and sometimes we should prevent the paper from bending owing to the uneven volatilization of the ethanol. Then the paper can be placed in the middle of flat tissues like sandwich biscuits to be planished using tablet press, which convert the product 6
into flat paper. After that, the final superhydrophobic paper was got after drying at 60 ℃. For blue and white superhydrophobic paper, the preparation resemble that of yellow superhydrophobic paper except for replacing 0.11 g FeSA3 powders with 0.09 g CuSA2, and 0.11 g ZnSA2 respectively. In addition, in order to study the effect of the of content of added coloured stearates on the paper hydrophobicity, we changed the content of the added coloured stearates under a certain cellulosic content to prepare coloured paper and it was summarized in the Table S1.
Fig.
1 Schematic
illustration of the fabrication procedures of coloured
superhydrophobic paper. Photographs of (a) absorbent cotton, (b) swelling and approximate dissolution of cotton in ZnCl2 solution, (c) tiny floc cotton cellulose precipitate in the deionized water, (d) mix solution of cotton cellulose and FeSA3, (e) and (f) the yellow superhydrophobic paper before and after dying respectively. 2.5. Characterization To characterize the morphology of samples, field emission scanning electron microscope (FESEM) images were obtained on JSM-6701F, both with Au-sputtered specimens. As for the component analysis, Fourier transformer infrared spectra (FTIR) were recorded using Thermo Scientific Nicolet iS10. X-ray photoelectron 7
spectroscopy (XPS, Thermo Scientific ESCALAB 250Xi) measurements were made using the Al Kα line as the excitation source. The water contact angles were measured with JC2000D with a 5 μL distilled water droplet at ambient temperature and the sliding angles were also mesured. The average WCA values of each sample were obtained by measuring each sample at five different sites. All photographs were obtained by a Sony camera (DSCHX200). 3. Results and discussion 3.1 Structure and characterization of coloured superhydrophobic paper Here, it is the yellow paper to represent an example of coloured superhydrophobic paper for analysis. As shown in Fig. 2a - d, the SEM images of yellow superhydrophobic paper and blank sample were represented. It can be observed that the paper of blank sample was composed of micro-nano sheets with smooth surface (Fig. 2b). Comparing Fig. 2b and 2d, we can find the differences of surface topography between these two samples. In the high magnification SEM images (Fig. 2d), it can be found that the yellow superhydrophobic paper was covered by nano particles and the nano particles had much smaller size than the micro-nano sheets of the blank sample surface. Namely, the yellow superhydrophobic paper had much smaller sized rough structure than the blank sample. Based on the SEM images, it is concluded that the addition of coloured superhydrophobic powders not only creates nanoscale rough structure with much smaller size
but also introduces low surface
energy materials, leading to the superhydrophobicity of the paper. It is also reasonable that the colours of paper are attributed to the attachment of coloured powders to the 8
micro-nano cellulose. SEM images of the water proof blue and white paper were also presented in the Fig. S2.
Fig. 2 SEM images of (a, b) blank sample and (c, d) yellow superhydrophobic paper with low and high resolution. In order to prove the chemical composition of the superhydrophobic paper, XPS experiment of the superhydrophobic paper was conducted here. From the XPS spectra Fig. 3a, it can be seen that the main composition of the yellow superhydrophobic paper are carbon, oxygen, zinc, chlorine and iron. In the Fig. 3a and Fig. 3b, the peak at 725.57 eV that is very weak is assigned to Fe 2p of compounds. The addition of FeSA3 powders is very few, which could be the reason that the peak of Fe 2p is not obvious. In the Fig. 3a, the peak at 198.52 eV and the peak at 1022.08 eV are attributed to the Cl 2p and Zn 2p of ZnCl2, which result from 65 wt% ZnCl2 solutions in the process of cotton swelling and approximate dissolution. The C 1s peak at 284.79 eV is assigned to C-C and the O 1s peak at 532.05 eV may be attributed to -COO-. The XPS analysis for blue and white superhydrophobic paper is also 9
represented in the Fig. S5.
Fig. 3 XPS spectra of yellow superhydrophobic paper including (a) the
survey
spectrum and (b) Fe 2p spectrum. To further confirm the chemical composition of the coloured superhydrophobic paper, FTIR of the superhydrophobic paper and blank sample were conducted here (Fig. 4 and Fig. S6). Curve black in the Fig. 4 represent the FTIR spectrum of the yellow superhydrophobic paper. From curves black in the Fig. 4 and Fig. S6, it can be discovered that there are no significant differences in the FTIR spectra of the blue, yellow and white superhydrophobic paper. Comparing the curve black with the curve red in the Fig. 4, we can see that three obvious adsorption peaks appear in the spectrum of the yellow superhydrophobic paper while they are absent in the spectrum 10
of the blank sample. The sharp adsorption peaks centered at about 2919 and 2850 cm-1 are related to the symmetric and asymmetric stretching vibrations of –CH2- and –CH3 groups. However, the Fe 2p peak in the XPS spectra Fig. 3 is not obvious. There are some resons to explain it. Firstly, the proportion of the iron to the methylene groups is 1/48 in per mole FeSA3. Further, the addition of FeSA3 powders in the yellow superhydrophobic paper is very few. In addition, the percentage of Fe in all elements containing carbon, oxygen, zinc, chlorine and iron is very small. The absorption band at around 1540 and 1465 cm-1 in the spectrum of yellow paper is assigned to -COO- symmetric and asymmetric stretching vibrations. Combined with the XPS analysis, it further proves the existence of FeSA3 in the yellow superhydrophobic paper.
Fig. 4 FTIR spectra of yellow superhydrophobic paper and blank sample. The paper without adding superhydrophobic powders exhibited superhydrophilicity (Fig. S3), while all the paper with stearates showed superhydrophobicity with large WCA > 150° (Fig. 5a and b). The SA on the coloured paper was also measured, which was less than 10° (see the SA of yellow paper in the movie S1). The summary about 11
superhydrophobicity including WCA and SA was also represented in the Table 1.
Fig. 5 (a) Water contact angles on blue, yellow and white superhydrophobic paper respectively.
(b)
Photographs of water
droplets sitting on the coloured
superhydrophobic paper. Table 1 Water contact angles and slide angles of the coloured superhydrophobic paper and blank sample surfaces colour
θCA
θSA
yellow
153.3°
~5°
blue
154.3°
~8°
white
151.7°
~6°
blank sample
0°
-
θCA: water contact angle; θSA: slide angle 3.2. Mechanical property of coloured superhydrophobic paper It can be understood that the superhydrophobicity of most of superhydrophobic surfaces would be damaged under abrasion impact owing to the superhydrophobicity only limited on the outer surface. In order to improve the mechanical stability of superhydrophobic surface, some works have been reported. 37,38 Here, the coloured paper can maintain good superhydrophobicity after abrasion. In order to clarify the 12
hydrophobicity of the yellow paper under abrasion, the abrasion test was performed on the yellow paper here. The paper was placed on the sandpaper and loaded with a 200 g weight, which was dragged for the length of 10 cm along the ruler (see in the movie S4 and Fig. 6b). After twice, the frictional place of the yellow paper still exhibited excellent superhydrophobicity as seen in the Fig. 6d, indicating a good mechanical stability of the superhydrophobic paper. The WCA of the paper after the abrasion test was higher than 153° (Fig. 6d), which would have great potential in future paper industry for inner and outer superhydrophobicity. In order to further reveal it, the SEM image of the yellow paper after abrasion was also obtained (Fig. 6c). Comparing the SEM images of the yellow paper before and after abrasion (Fig. 6c and Fig. 2d), it can be discovered that the micro-nano particles covered on the cellulose reduced obviously after abrasion while the roughness of micro-nano structure increased. This may be the reason that the coloured paper still maintained excellent superhydrophobicity after abrasion. In addition, flexibility of the yellow superhydrophobic paper was also proved here as seen in the Fig. 6a. It can be bended by fingers to some extent. The above tests showed that the prepared coloured superhydrophobic paper have good mechanical property.
13
Fig. 6 (a) Photograph of yellow superhydrophobic paper bended by fingers. (b) Optical image of abrasion test on yellow superhydrophobic paper. (c) SEM images of yellow superhydrophobic paper after abrasion. (d) Photographs of water droplets sitting on the yellow superhydrophobic paper after abrasion and photograph of the water contact angle of ~ 153° on it in the upper-right corner.
3.3. water-proof function and application in our daily life Water-proof function is significant for paper in applications. In order to explore resistance to water of the superhydrophobic paper, we chose the yellow paper to represent a sample of coloured paper for water proof function test. As shown in the Fig. 7a-b and the movie S2, a silver mirror phenomenon can be seen at a certain angle of view when the yellow paper was immersed in the water, which was a signature of the trapped air between the interface of liquid and solid. In the movie S2, the silver mirror phenomenon can also be seen after the letters “superhydrophobic” were written on the yellow paper surface. This shows that air can be naturally trapped while the water was spreading on the writeable superhydrophobic paper surface. Owing to the property of air trapping in the grooves of superhydrophobic paper to insulate from liquid phase, the substrates exhibit great water proof function.
14
Fig. 7 (a-b) Silver mirror phenomenon of yellow paper immersed in the water before and after writing letters on it. (c) Self-cleaning test of yellow paper on the slightly tilted glass sheet and the dust particles as contaminants. (d-e) Photographs of typing paper before and after immersion in the coffee for 30s. (f-g) Photographs of yellow paper before and after immersion in the coffee for 60s. It is generally known that lotus leaf exhibits self-cleaning property owing to the high WCA and low SA. When the water slides off the lotus leaf, the dust on the surfaces can be taken away along with the water. Here, the self-cleaning function of the yellow paper was also proved. As seen in the Fig. 7c and the movie S3, the yellow paper with some dust particles was placed on a slightly tilted glass sheet. The water was sprinkled on the yellow paper surface through using a wash bottle. We can see that the dust particles were dropped away from the paper by the water. After a while, the yellow paper became cleaning with no dust particles remained on the paper surface, which indicated that our work would be potential for self-cleaning applications. To further clarify the potential application of superhydrophobic paper in our daily life, an immersing test was represented here. The above yellow paper after abrasion 15
test and the common used typing paper were immersed into the coffee as comparision (Fig. 7d-g). Fig. 4e and g respectively shows the photographs of the yellow paper and the typing paper after immersion, we can see that the typing paper were wetted obviously and exhibited the colour of the coffee in half a minute while the yellow paper still maintained nearly intact as before immersion even after a much longer immersion (see in the movie S5, movie S6 and Fig. S4). 4. Conclusions In summary, we have fabricated superhydrophobic coloured paper from absorbent cotton and coloured stearate powders via a green method. The process contains no toxic modifier. In addition, the obtained paper not only exhibit consistent colour with the doped stearates owing to the adhesion of stearate powders to the tiny floc fiber surface but also possess uniform superhydrophobicity from inner to outer. After abrasion, the superhydrophobicity would not be damaged owing to the inner and outer superhydrophobicity
and
the
surface
roughness
increase.
The
coloured
superhydrophobic paper prepared by a green fabrication method in our work may have great potential applications in different fields, such as microfluidic system and paper industry, and the menthod used here can also satisfy the requirements of large-scale preparation. Acknowledgement This study is supported by the National Nature Science Foundation of China (No. 51522510 and 51675513), and the National 973 Project (2013CB632300) for financial support. 16
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Graphical abstract
The green coloured superhydrophobic paper was fabricated from native cotton cellulose via swelling and approximate dissolution of cotton followed by precipitation of cellulose and doping coloured stearates. The process contains no toxic modifier and the obtained paper exhibits satisfactory inner and outer uniform superhydropfobicity and colour.
22