Designing a novel type of multifunctional bamboo surface based on an RGO/Ag coating

Jin Wang; Zhe Wang; Haixia Yu; Wangjun Wu; Jian Zhang; Jingpeng Li

doi:10.1016/j.arabjc.2022.104332

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Original article

12 2022

:15;

104332

doi:

10.1016/j.arabjc.2022.104332

Designing a novel type of multifunctional bamboo surface based on an RGO/Ag coating

Jin Wang^{^a}, Zhe Wang^{^b}, Haixia Yu^{^a}, Wangjun Wu^{^a,^b}, Jian Zhang^{^a,⁎}, Jingpeng Li^{^c,⁎}

a Key Laboratory of Bamboo Research of Zhejiang Province, Zhejiang Academy of Forestry, Hangzhou 310023, PR China

b College of Chemistry and Materials Engineering, Zhejiang A&F University, Hangzhou 311300, PR China

c Key Laboratory of High Efficient Processing of Bamboo of Zhejiang Province, China National Bamboo Research Center, Hangzhou 310012, PR China

⁎Corresponding authors. zhjianzj@126.com (Jian Zhang), lijp@caf.ac.cn (Jingpeng Li)

Received: 2022-6-22, Accepted: 2022-10-3,

Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Peer review under responsibility of King Saud University.

Abstract

Super-hydrophobic and fire-retardant coatings have been applied to the bamboo surface. However, it remains a significant challenge to develop coating materials that perform multiple functions simultaneously. In this paper, a novel conductive bamboo timber (RGO@AgBT), coated with reduced graphene oxide (RGO) and silver nanoparticles, was fabricated via a hydrothermal process and a silver mirror reaction process. The RGO@AgBT composites had excellent photo-catalytic activity, meanwhile, the removal rate of rhodamine B (RhB), methylene blue (MB), and methyl orange (MO) after 60 min photo-degradation were 77.6 %, 88.8 %, and 78.4 %, respectively, superior to the raw bamboo timber (BT) and the bamboo timber coated with reduced graphene oxide (RGOBT). The RGO@AgBT samples showed excellent antibacterial performance against Escherichia coli and Staphylococcus aureus. Additionally, the RGO@AgBT samples exhibited improved thermal stability and fire-resistant property with a limiting oxygen index of 30.5 %. Finally, the future directions of multifunctional bamboo research and opportunities for electronic industry application are prospected.

Keywords

Bamboo

Graphene

Antibacterial activity

Photocatalytic

Fire-resistance

Conductivity

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1 Introduction

With the promotion of carbon peak and carbon neutral strategy, the bamboo industry, green industry with ecological and economic value, has ushered in new opportunities. As a kind of natural polymer composite material, bamboo is mainly composed of highly lignified fiber cells and parenchyma cells, which is considered a “green” alternative to wood. Bamboo itself has the advantages of the short growth cycle, high strength, high toughness, renewable, easy to process, multi-porosity, and so on, while it also has quite a few shortcomings that limit its application range. For example, bamboo is prone to water absorption deformation, cracking, light discoloration, easy to mold, easy to bacteria, easy to burn, insulation, and so on, resulting in the limitation of its use in many fields. Certainly, the so-called advantages and disadvantages mentioned above are relative since they could be strengths in one application, while being weaknesses in another case. Therefore, it is urgent for us to design more new features to meet the needs of as numerous applications as possible. Recently, a large number of studies have been carried out around the functional improvement of bamboo surface, which endows bamboo with the characteristics of antibacterial, mold-proof, corrosion resistant, photocatalytic, flame retardant, conductive and superhydrophobic, as shown in Table. 1.

Table 1 Different functional coating prepared on the bamboo surface reported in this study and several previous literature.

Coating/Bamboo	Superhydrophobic/dimensional stability	Photocatalytic/catalysis	Flame retardancy	Antimicrobic (mould, decay fungi,bacteria)	Electrical conduction	References
Ag/Bamboo	√			√	√	Jin et al., 2015
CaCO₃/Bamboo						Li et al., 2015
ZnO/Bamboo	√	√		√		Jin et al., 2014; Li et al., 2015
TiO₂/Bamboo	√			√		Li et al., 2016a; Li et al., 2016b
Fe/TiO₂/Bamboo		√	√	√		Li et al., 2017
ZnO/TiO₂/Bamboo			√	√		Ren et al., 2018a; Ren et al., 2018b
Ag/TiO₂/Bamboo		√		√		Li et al., 2019; Li et al., 2021
RGO/TiO₂/Bamboo		√	√			Wang et al., 2017
RGO/ZnO/Bamboo				√		Wang et al., 2018
RGO/SiO₂/Bamboo	√					Wang et al., 2021
RGO/Ag/Bamboo		√	√	√	√	This study

With the rapid development of the economy, the problem of environmental pollution is becoming more and more obvious, which has seriously threatened people's daily life and health. In particular, the wastewater produced by the printing and dyeing industry will not only have a serious impact on the surrounding environment and affect the living water of human beings, but also cause damage to human health through the enrichment of the food chain. At present, the treatment methods for dye wastewater mainly contain the adsorption method (Wang et al., 2005; Feng et al., 2020), the membrane separation method (Zhang et al., 2016), the chemical oxidation method (Aravind et al., 2018), the electrocatalytic method (Ramachandran et al., 2016), and the photocatalytic degradation method (Reddy D.R et al., 2016). Among the above methods, the photocatalytic method has the advantages of complete degradation of pollutants, low energy consumption, and high cleanliness. The synthesis methods of reduced graphene oxide RGO/Fe₃O₄ nanocomposites and reduced graphene oxide/zinc ferrite (RGO/ZnFe₂O₄) nanocomposites were reported for the photo-oxidative degradation of methylene blue (MB) dye (Vinothkannan et al., 2015; Jenita Rani et al., 2017). The photocatalytic degradation method rapidly decomposes organic pollutants into CO₂, H₂O, and other small molecule compounds through photocatalytic materials under light radiation (Ramachandran et al., 2016; Stanly John Xavie et al., 2014). Different catalytic materials for photodegradation have been prepared by biomass supported materials, such as β-cyclodextrin- functionalized silvernanoparticles (Stanly John Xavier et al., 2014), Ag/GCE sensor (Ramachandran et al., 2016), and efficient fluorescence carbon nanodots (Stanly John Xavie et al., 2019). Recently, the synthesis method of a high-efficiency bamboo-inspired catalytic capillary microreactor was reported with continuous-flow catalytic performance that remained 90 % within 11 h over 5 recycles (Li et al., 2022). It is of great significance to explore and study highly-efficient and stable-photocatalytic materials, such as the porous structure of bamboo-supported catalytic coating materials, for the degradation of organic pollutants in dye wastewater.

Nowadays, bamboo products are adopted as raw materials in the electronic industry, such as bamboo keyboards, bamboo mice, and bamboo cell phone shells. As electronic products develop rapidly in recent years, various electronic products emerge in an endless stream and are constantly updated, of which the common characteristics are short-life cycles for the fast updates. What is worse, most people have the experience of discarding electronic products, which will cause pollution (Berrin Tansel, 2016). Therefore, it is beneficial for environment-friendly electronic products made of biodegradable biomass-based materials. According to the electrical resistivity, materials are classified into conductors, insulators, and semiconductors, whose electrical resistivity is less than 10^-5 Ω/m, more than 10⁸ Ω/m, and 10^-5 ∼ 10⁸ Ω/m, respectively. When the conductivity of the material is less than 10^-7S/m, it is regarded as insulator material that is basically non-conductive (N. Chand. et al., 2006). Bamboo or wood-based materials with equilibrium moisture content of 15 % are generally adopted as insulating materials (Du. et al., 2002). Hence, this study is aimed to prepare bamboo samples with conductivity.

Graphene and its derivatives provide a fundamental platform for emerging novel electronic and optoelectronic devices (Sadasivuni et al., 2015, Kafy et al., 2015, Yan et al., 2014 ). Through the simple process of the one-pot method, the composites with better properties can be obtained by compounding different bamboo components under the condition of maintaining certain microstructure. The special physical structure of bamboo cellulose can be adopted as the substrate of supercapacitor electrode materials, whose water absorption and swelling effects are conducive to the absorption of electrolyte, and the internal mesoporous structure can provide a channel for ion diffusion to electrochemical energy storage materials (Gui. et al., 2013). With the development of conductive nanomaterials (copper nanowires), conductive polymer (PEDOT: PSS) and conductive hydrogels, there are more choices for electrode materials of ACEL devices (Hong et al., 2014, Schrage. et al., 2009, Alonso. et al.,2016 ). Silver nanowires are a typical one-dimensional metal nanomaterial with excellent mechanical flexibility, good transparency, and high conductivity (MacDonald. et al., 2004, Hsu. et al., 2014). Silver nanowires with a high aspect ratio can form a network structure with low area density, which is a good choice for flexible ACEL devices (Zeng et al., 2010). The self-generating sensing nanomaterials are prepared by retaining the cellulose in wood and compounding it with copper after hot pressing, which can be applied to big data statistics and promote big data analysis of the intelligent sports industry (Luo et al., 2019).

As a kind of green and natural conductive material, the wood-based conductive composites can be developed into an organic semiconductor, which has huge application space in the field of organic sensors, organic lasers, organic memories, and so on (Zhu et al., 2020).

Meanwhile, silver nanoparticles have a variety of antibacterial mechanisms, which is broad-spectrum antibiotics worthy of promotion in nanomaterials, and the resistance of bacteria to silver ions is extremely rare (Simon, 2003). In the face of the emergence of drug-resistant bacteria, silver nanomaterials have the potential to become new antibacterial agents, which can be widely used in the field of antibacterial for the benefit of mankind. Compared with carbon nanotubes, graphene-based nanomaterials can provide greater surface area and better dispersion in most solvents (Bossy et al., 2013). After a series of redox reactions, graphene oxide (GO) is obtained from the lamellar graphene, which has a large number of oxygen-containing groups (hydroxyl, carboxyl, and epoxy groups) on its surface, forming hydrogen bonds with water, increasing its compatibility in water, greatly facilitating the modification of other chemicals on its surface as well as expanding its application scope.

In our previous studies, we have prepared bamboo with superhydrophobic surfaces or catalytic functions, or antibacterial properties, the bamboo-based materials will have a wider foreground and application space. While in this study, the graphene-based nano-silver coating has been fabricated on the bamboo surface, which has multiple functions such as antibacterial, conductive, flame retardant, and photo-catalysis.

2 Materials and methods

2.1

2.1 Materials

Graphite powder (less than20 μm) was provided by the Shanghai Boyles Chemical Co., ltd. Sulfuric acid (wt.95 ∼ 98 %), while nitric acid (wt.68 %) were provided by the Lanxi Liudongshan Chemical Co., ltd. The following reagents including methylene blue, methyl orange, rhodamine B, glucose, potassium permanganate, hydrogen peroxide, and anhydrous ethanol were purchased from the Chengdu Kelon Chemical Co., ltd. Silver nitrate, hydrochloric acid (36 ∼ 38 %), and ammonia water were provided by Hangzhou Huipu Chemical Instrument Co., ltd. The nutrient AGAR medium and the microbiology races including E. coli & S. aureus were provided by the Beijing Beina Chuanglian Biotechnology Research Institute. BT samples were processed into blocks, and the size was set as 50 mm × 20 mm × 5 mm and 20 mm × 20 mm × 5 mm (longitudinal × tangential × radial). The BT samples were pretreated for each 30 min by water washing and alcohol washing and then dried under vacuum at 50 °C for 24 h.

2.2

2.2 Synthesis of RGO on the surface of bamboo

According to the modified Hummer's method (Marcano. et al., 2010), the GO sample was prepared and dissolved in deionized water. According to the mass volume ratio, 2 mg/L GO solution was obtained and sonicated to achieve a uniform GO dispersion. BT samples and the GO dispersion were mixed in a volume ratio of 2:1 and transferred into a Teflon-lined stainless-steel autoclave for a hydrothermal reaction at 140 ℃ for 3 h. Then, the treated bamboo samples were removed from the mixed solution. The treated BT samples with an RGO coating (RGOBT) were ultrasonically rinsed with deionized water for 3 min and dried at 50℃ for 24 h in a vacuum.

2.3

2.3 Synthesis of silver coating on the surface of RGOBT

The ammonia solution (wt.28 %) was added to 50 ml of 0.5 M AgNO₃ solution in a beaker, and stirred constantly until the transparent colorless [Ag(NH₃)₂]⁺ solution was formed. The RGOBT samples were placed into [Ag(NH₃)₂]⁺ solution and soaked for 1 h, then the bamboo blocks were transferred into 50 ml of 0.2 M glucose solution for 5 min, with the remaining [Ag(NH₃)₂]⁺ solution poured into the glucose solution for 30 min. Finally, the treated bamboo samples were removed, ultrasonically rinsed with deionized water for 3 min, and then dried at 50 °C for 24 h in a vacuum. The RGOBT coated with silver nanoparticles after the hydrothermal step was abbreviated as RGO@AgBT.

2.4

2.4 Characterization

The surface morphology was characterized by scanning electron microscopy (SEM, Quanta 200, FEI, USA). The surface chemical compositions were determined via energy to disperse spectroscopy (EDX, attached to the SEM). The crystalline structures were identified by X-ray diffraction (XRD, D/MAX 2200, Rigaku, Japan) using Cu $K_{α}$ radiation ( $λ = 1.5418 \bar{A}$ ) at a $2 θ$ scan rate of 4° $\cdot {m i n}^{- 1}$ , 40 kV, 40 mA, ranging from 5° to 80°. The presence of functional groups in the samples was confirmed through Fourier transform infrared (FTIR) spectroscopy (Spectrum One, Perkin Elmer, USA). The surface elemental composition analysis was conducted based on the X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific-K-Alpha 1063, UK) with an Al Ka monochromatic X-ray source, in which all the binding energies were calibrated with reference to the C₁^s peak (284.8 eV). The BT, RGOBT and RGO@AgBT samples were characterized by SEM, FTIR, EDX, XRD, and XPS.

2.5

2.5 Photo-catalytic activity test

The photocatalytic degradation of the BT, RGOBT, and RGO@AgBT samples under UV irradiation, which were mixed in the three kinds of mixed solutions including RhB, MB, and MO, was tested. The above three dye solutions were disposed of with the same concentration of 10 mg/L. The bamboo samples with dimensions of 20 mm $\times$ 20 mm $\times$ 5 mm were dipped into 30 ml of the dye solution and stirred in the dark for 60 min. Then, the dispersion was centrifuged to measure UV–visible absorption of the MB, MO and RhB solutions. We set the concentration of each dye solution after dark treatment as the initial concentration $C_{0}$ . The bamboo-dye mixed solutions were placed 30 cm directly below the 1000 W high-pressure Hg lamp with the main wave crest at 465 nm. Under normal temperature and stirring conditions, the temperature of the light degradation reaction was kept at 20℃ and controlled by recirculating cooling water system. The pH value of the bamboo-dye mixed solutions was measured and adjusted with a pH value of 6.35, 5.79 and 5.06, for MO, MB, and RhB, respectively. The bamboo-dye mixed solutions were irradiated by UV for 60 min. The dye solution was analyzed every 10 min by using a visible spectrophotometer (TU-1901, Beijing Purkinje, China). The absorbance of MO at 464 nm wavelength, MB at 668 nm wavelength, and RhB at 554 nm wavelength were measured. The efficiency was calculated by the following formula: $Y = 100 \times \frac{C - C_{0}}{C_{0}}$ where $C_{0}$ indicates the initial concentration after dark treatment, and C refers to the concentration at the end time under UV irradiation every 10 min.

2.6

2.6 Electrical conductivity test

The electrical resistance of bamboo materials (BT and RGO@AgBT) was measured by using a resistance tester. The thickness of the coating belonged to the micro-nanometer level, which was difficult to measure. A closed loop circuit was designed to determine whether the bamboo samples can conduct electricity. The length of the bamboo coating in the above circuit was 50 mm. The electrical resistance was calculated by the average value of 5 replicates.

2.7

2.7 Antibacterial test

Antibacterial tests were conducted according to the bacterial inhibition ring method (agar plate diffusion test /CEN/TC 248 WG 13) and the reduction of bacterial growth test method (Xue. et al., 2012). E. coli (BNCC269342) and S. aureus (BNCC186335) were applied in the antibacterial test. The freeze-dried bacteria seeds were activated in nutrient broth at 37 °C for 24 h. The agar medium was then cast into the petri dishes and cooled in laminar airflow. Approximately 105 colony-forming units of E. coli were inoculated on the dishes. The BT, RGOBT and RGO@AgBT samples, with a diameter of 5 mm and thickness of about 2 mm, were processed and then planted onto the agar plates. After incubation at 37 °C for 24 h, the diameter of the inhibition ring was measured. The diameter of the inhibition ring was calculated by the average value of three replicates.

2.8

2.8 Fire resistance test

In order to test the combustion performance of the bamboo samples (BT, RGOBT, RGO@AgBT), the three kinds of samples were ignited at different oxygen concentrations. The limiting oxygen index (LOI) was measured by using a JF-5oxygenindex tester (Nanjing Jiangning Co., ltd., China). The three kinds of bamboo samples underwent the same combustion process on the alcohol lamp, which was repeated three times, and the photos of their combustion state were recorded by the camera.

3 Results and discussion

3.1

3.1 SEM analysis

Fig. 1 shows the typical low- and high- magnification SEM images of BT (a, b), RGOBT (d, e) and RGO@AgBT (g, h). It could be seen from Fig. 1a and Fig. 1b that the raw BT showed a clean and smooth surface. The internode cells of bamboo plants are arranged vertically without horizontally arranged cells. Among them, the parenchymal cells accounted for more than 52 % of the total tissue. Similar to the stop-edge structure, there are microporous structures with different sizes on the pit membrane of bamboo, which is one of the key factors for the penetration and diffusion of fluid in bamboo. Through the hydrothermal treatment of GO dispersion, GO nanosheets were loaded on the surface of RGOBT (Fig. 1 d & e), which are stacked on the surface of bamboo with a disordered distribution. The low-magnification image of RGO@AgBT in Fig. 1g showed that Ag nanoparticles were deposited on the bamboo surface, making the surface of BT rough. In addition, the high-magnification image of RGO@AgBT in Fig. 1h showed that the silver nanoparticles were attached to the structure of RGO. According to the size scale of silver nanoparticles, the agglomeration of various silver nanoparticles covered the whole surface of RGO@AgBT. It is intuitive from the digital photos (Fig. 1 c & f & i) of the cross-sections of the BT, RGOBT and RGO@AgBT samples that the penetration depth of GO into the bamboo after treatment was small, while the penetration depth of the surface after loading nano silver was large. The thickness of the RGO/Ag coating on RGO@AgBT was greater than that of RGO coating on RGOBT.

Typical low- and high- magnification SEM images of BT (a, b), RGOBT (d, e), and RGO@AgBT (g, h). Digital photograph of the radial section of BT(c), RGOBT (f) and RGO@AgBT (i).

3.2

3.2 EDS analysis

Fig. 2(a, b, c) showed the chemical elements and the element content of BT, RGOBT, and RGO@AgBT. The Au element detected was produced by the gold plating process (Yu. et al., 2012). The carbon signals and oxygen signals are believed to be originated from the bamboo substrate and RGO coating materials (Shen. et al., 2015). The element content of carbon and oxygen showed that both the mass ratio and atomic ratio of carbon obviously increased after the hydrothermal process of RGO coating formation, mainly attributed to the content of carbon element of RGO coating, which was higher than BT sample. Fig. 2c indicated that the silver elements could be detected from the RGO@AgBT samples. The silver nanoparticles were grown on the RGO of the bamboo surface after the silver mirror reaction process. The C, O, and Ag element mapping images of RGO@AgBT were shown in Fig. 2(d, e, h). It was found that the silver element area is densely distributed and Ag NPs were loaded on the surface of RGO@AgBT uniformly, where the element areas of carbon and oxygen are relatively low. As can be seen from Fig. 1.h, the Ag element was distributed among the area of C elements, but the contrast (d, f) showed that the C element was not significant, indicating that the Ag NPs were evenly distributed among the layers of RGO.

EDX spectra of BT (a), RGOBT (b), and RGO@AgBT (c). The inset tables present the element contents. The C(d), O(e) and Ag(f) element mapping images of RGO@AgBT.

3.3

3.3 XRD analysis

The XRD patterns of BT, RGOBT, and RGO@AgBT were shown in Fig. 3. The raw bamboo samples showed the typical peaks at 2θ of 16.5° and 22.5°, corresponding to the cellulose (Li. et al., 2011). The RGO@AgBT samples showed the typical peaks at 2θ of 38.02°, 44.12°, 64.58°, and 77.13°, respectively, corresponding to the (1 1 1), (2 0 0), (2 2 0), (1 1 0), and (3 3 1) crystal plane of Ag (JCPDS No. 04–0783) (Tsujiy. et al., 2004, Deng. et al., 2012), which proves that the silver coating was successfully formed on the RGOBT surface during the reaction.

XRD pattern on BT (a), RGOBT (b) and RGO@AgBT (c).

3.4

3.4 FTIR analysis

Evidently, the peaks in the three FTIR spectra of BT, RGOBT and RGO@AgBT in Fig. 4 were similar to each other. In the high-frequency region, the three samples showed absorption peaks at 2919 cm⁻¹ and 2852 cm⁻¹, which were caused by symmetric telescopic vibration and asymmetric telescopic vibration of CH₃ and CH₂. The absorption peak at 1641 cm⁻¹ was caused by C⚌C vibration (Hu. et al., 2015). The absorption peak at 1730 cm⁻¹ corresponded to the carboxyl group (–COOH) on the surface of GO, which indicates that GO was not fully reduced to RGO. The RGO and the residual GO were coated on the bamboo surface. In addition, a strong interaction was formed between GO and the hydroxyl group of the bamboo. In the process of the silver mirror reaction, the silver nanoparticles were dispersed on the RGO network. The peak intensity of the oxygen group decreased due to the increased temperature in the hydrothermal process, meanwhile, the oxygen-containing functional groups were reduced.

FTIR spectra of BT (a), RGOBT (b), and RGO@AgBT (c).

3.5

3.5 XPS analysis

Here, we employed XPS to characterize the chemical composition of the BT, RGOBT, and RGO@AgBT. As shown in Fig. 5, the RGO@AgBT mainly contained C, O, N, and Ag elements. Fig. 5 (c) showed that the reduction of the C—O—C epoxy bond in GO resulted in the formation of a C—O bond at 285.4ev. The C⚌O carbonyl signal peak at 288.1ev in GO formed a C⚌O carbonyl signal peak of 287.7ev after the interruption of bond recombination during the reduction process. Fig. 5 (d) showed the narrow spectrum of Ag3d in RGO@AgBT. It can be seen that spin splitting occurs in the Ag3d orbit and the spin–orbit components were well separated. In Fig. 5 (d), the binding energies of Ag3d_5/2 and Ag3d_3/2 shown were around 366 ev and 373 ev, respectively. The silver element from the RGO@AgBT surface could be proved in the spectrometer with the Ag3d dominant peak (366.5 eV) and the companion peak (372.5 eV) in the Ag standard spectra. The two peaks at the binding energies 367.2 and 373.2 belonged to Ag₂O and could be attributed to Ag(NH₃)₂⁺ ions (Li. et al., 2021), which proved that silver ions were completely reduced in the process of the silver mirror reaction. Besides, the Ag element corresponded to the two peaks at 573.6 eV (Ag 3p³) and 604.05 eV (Ag 3p¹). Therefore, a Ag/Ag₂O coating was formed on the surface of the RGOBT samples.

XPS full spectra (a) of BT, RGOBT, and RGO@AgBT, (b) O1s region of RGO@AgBT sample, (c) C1s region of RGO@AgBT sample, (d) Ag 3d region of RGO@AgBT sample.

3.6

3.6 Synthesis mechanism

According to the above analysis, a schematic illustration of the coating process and a possible synthetic mechanism for the RGO@AgBT composite are shown in Fig. 6. The GO solution was first absorbed by the raw BT and reduced to an RGO coating with a graphene network on the bamboo surface by the hydrothermal process (Tissera. et al., 2015). Then, the multiple layers of the RGO sheets were assembled onto the bamboo surface via hydrogen bonding and physical adsorption. In the second step of the silver-mirror reaction, the RGOBT was firstly immersed in the mixture solution of silver nitrate and ammonia solution, and then the glucose solution was added. Under the water bath conditions for a period of time, the silver layer was formed on the surface of RGOBT. The process of reaction process can be described as (Zhou. et al., 2009):

（1）

AgNO₃ + NH₃^-H₂O → AgOH↓+NH₄⁺+NO₃^–

（2）

AgOH + 2NH₃^-H₂O → [Ag(NH₃)₂]OH + 2H₂O

（3）

[Ag(NH₃)₂]OH → [Ag(NH₃)₂]⁺+OH^–

（4）

C₅H₁₁O₅-COH + 2[Ag(NH₃)₂]OH → C₅H₁₁O₅-CO₂NH₄ + 2Ag↓+3NH₃ + H₂O

Schematic illustration of the preparation process and synthetic mechanism of RGO@Ag BT by two steps.

3.7

3.7 Evaluation of the photo-catalytic activity

Considering that bamboo was a porous material, in order to compare the photo-catalytic performance of different bamboo samples, we intended to eliminate the influence of bamboo on the adsorption of dyes. The porous structure and pore size distribution were analyzed by a mercury injection test. The porous structure test was performed by using an automated mercury porosimeter (AutoPore V 9600, Norcross, GA, USA) to force mercury into the pores of bamboo samples. The pressure range was 0.5–33000 psi with the pore diameter range of 350 μm-5 nm. Test results were shown in Table. 2.

Table 2 MIP test results of BT and RGOBT@Ag samples.

Samples	Total Intrusion Volume(ml/g)	Median Pore Diameter(nm)	Total Pore Area(m²/g)	Bulk Density (g/ml)	Porosity (%)
BT	0.9261	33.19	99.079	0.5971	55.2958
RGOBT@Ag	1.0918	38.17	101.285	0.5468	59.7035

According to the classification standard of the International Union of Pure and Applied Chemistry (IUPAC), the pore size of porous materials was divided into three categories: micropores (∼less than 2 nm), mesopores (2 nm-50 nm), and macrospores (∼greater than50 nm) (Fengel, 1969). After the comparison of the pore characteristics of BT and RGO@AgBT, it was found that the pore structure and pore size distribution of the two samples had no remarkable difference. The porosity of BT and RGO@AgBT reached 55.3 % and 59.7 %, and the total invasion volume of the BT and RGO@AgBT were 0.92 ml/g and 1.09 ml/g, respectively. Therefore, while there is no obvious difference in pore ratio and pore distribution between BT and RGO@AgBT, we can still prove that RGO@AgBT has a better photocatalytic effect on dyes than BT by comparing their removal rates of dyes (MO, MB and RhB).

The light degradation of MO under UV light treated by the BT, RGOBT and RGO@AgBT samples were shown in Fig. 7(A). It was found that the MO concentration decreased when the above three samples were immersed in the MO solutions for 60 min and the efficacy of RGO@AgBT reached the highest. The reason for the decrease in MO concentration was that bamboo materials could absorb MO molecules in solution via the space between the cell walls and the cell lumens (Lian et al., 2014). As a result, the concentration of MO solution dropped. The RGO nanosheets coated on the BT surface formed the graphene network on RGOBT and provided more active sites to adsorb MO molecules. By the photo-catalysis reaction, the color of the MO solution gradually faded by the treatment of the RGO@AgBT samples. The RGO@AgBT could not only absorb the MO molecules but also degrade the dye molecules via catalytic reaction. After 60 min light degradation under UV to the RGO@AgBT sample, the decline rate of MO concentration was 78.4 %. The light degradation of RhB under UV light treated by BT, RGOBT and RGO@AgBT was shown in Fig. 7(B). The concentration of RhB solution was dropped significantly by the RGO@AgBT treatment, leading to the light degradation of nano silver particles and dye composition. Another factor resulting in a drop in concentration was the adsorption effect of the porous structure of the bamboo and graphene materials. After the 60 min light degradation under UV light, the decline rate of RhB concentration was 77.6 %.

Photodegradation of MO(A), RhB(B) and MB(C) under UV light over BT (a), RGOBT (b) and RGO@AgBT (c). The corresponding dye solutions are shown as inset photos.

The light degradation of MB under UV light treated by BT, RGOBT, and RGO@AgBT were shown in Fig. 7(C). The mechanism of chemical action was similar to that of MO and RhB. After the 60 min light degradation under UV light treated by the RGO@AgBT sample, the decline rate of MB concentration was 88.8 %.

After comparing the decline rate of the three types of organic pollutants under UV light treated by the RGO@AgBT sample, the light degradation efficiency of MO, MB, and RhB was 78.4 %, 88.8 % and 77.6 %, respectively. The light degradation efficiency of MB was the highest. The RGO@AgBT samples showed high photocatalytic activity, especially for the light degradation of MB.

In our study, all the three samples (BT, RGOBT and RGO@AgBT) could adsorb the dye to reduce its concentration.

However, due to the small difference in the influence of the porous, the influence of the adsorption of the three samples is ignored in our comparative analysis. In the process of photodegradation of organic dyes by xenon lamps, RGO@AgBT, as a photocatalyst, can greatly improve the degradation efficiency of dyes, of which the catalytic process was described as below. This catalytic process utilized the excellent adsorption performance of RGO to effectively physically adsorb organic dyes (Cheng et al., 2015), and then Ag NPs obtained energy under xenon lamp irradiation to undergo electron transition and capture electrons from the solution. The generated electrons were transferred to the dye through RGO. After accepting electrons, dyes are degraded through a series of free radical reactions to form a series of small molecule compounds (Yen. et al., 2015, Ramachandran. et al., 2016). Due to the metal support interaction, the electron transfer at the RGO@AgBT catalyst interface was induced (Cheng. et al., 2016), and the conductive grid structure formed by RGO increased the electron transfer capacity, greatly improving the catalytic efficiency of Ag NPs, as well as the photodegradation efficiency of organic dyes.

3.8

3.8 Analysis of the electrical conductivity

The electrical conductivity of the BT, RGOBT, and RGO@AgBT samples were shown in Fig. 8. The lamp, connected to the BT or RGOBT samples circuit, was off because the bamboo samples had no conductivity (Fig. 8a and Fig. 8b). However, the RGO@AgBT samples could make the lamp turn on. The prepared RGO@AgBT sample was found to possess excellent conductivity (Fig. 8c). The two as-prepared RGO@AgBT samples were randomly attached to make the lamp turn on in the connecting circuit (Fig. 8d). Thus, the RGO@AgBT surface was fully loaded with conductive materials, allowing the bamboo to transfer to the conductor. The electrical resistance of bamboo samples with 50 mm length was 992 Ω. The novel type of RGO@AgBT would be adopted as the biomass-based conductive material to expand the application field of bamboo-based materials.

Test photos of electrical conductivity for BT (a), RGOBT (b) and RGO@AgBT (c, d).

3.9

3.9 Analysis of the antibacterial activity

Fig. 9 showed the antibacterial test results of BT, RGOBT and RGO@AgBT against E. coli and S. aureus under the condition of incubation for 24 h at 37 °C. Compared with the antibacterial activity of BT, RGOBT and RGO@AgBT against E. coli (Fig. 9a), it was found that BT and RGOBT showed no significant antibacterial activity against E. coli. The RGO/Ag coated bamboo could inhibit the growth of E. coli around the sample and distinct zones of inhibition could be seen clearly. The inhibition zone of raw BT and RGOBT samples were nearly 5 mm as shown in Fig. 9 a. The average inhibition zone of RGO@AgBT samples was 10 mm as shown in Fig. 9a. It was proved that RGO@AgBT had better antibacterial activity against E. coli than BT or RGOBT. Meanwhile, the antibacterial activity of BT, RGOBT and RGO@AgBT against S. aureus (Fig. 9b) was compared. Compared with the antibacterial activity of BT, RGOBT and RGO@AgBT against S. aureus (Fig. 9b), it was found that BT and RGOBT showed no significant antibacterial activity against S. aureus. The inhibition zone of raw bamboo samples and RGOBT samples were 5 mm as shown in Fig. 9b. The average inhibition zone of RGO@AgBT samples was 8 mm as shown in Fig. 9b. It was proved that RGO@AgBT had better antibacterial activity against S. aureus than BT or RGOBT. According to the results of the diameter of the inhibition zone, the antibacterial activity of RGO@AgBT against E. coli was better than S. aureus.

Antibacterial digital photos（a: E. coli, b: S. aureus; A: BT, B: RGOBT, C: RGO@AgBT）.

Silver-based materials had good antibacterial properties and there were different opinions on the antibacterial mechanism of nano-silver, mainly including changes in cell membrane permeability, DNA damage, dehydrogenase inactivation, and oxidative damage of reactive oxygen radicals (Xu. et al., 2011). Graphene-supported nano silver could control the physical properties of nano-silver very efficiently, and better play the antibacterial role of nano-silver (Ma. et al., 2011). So in our method of making RGO@AgBT, we set the RGO/Ag as bamboo surface coating materials, based on the special lamellar structure of graphene.which had antibacterial activity and a large specific surface area, the silver nanoparticles were loaded on the water-soluble functional fossil graphene as a carrier, so RGO@AgBT had a synergistic antibacterial effect. The RGO@AgBT not only enabled the silver nanoparticles to be firmly supported on the graphene sheet, maintaining the stability of the silver nanoparticles, but also endowed the silver nanoparticles with water solubility. More importantly, due to the synergistic antibacterial effect of graphene and silver nanoparticles, the performance of this coating material was better than the superposition of single component performance. The excellent antibacterial properties of Ag/RGO complexes on bamboo were attributed to the synergistic effect of the adsorption properties of GO or RGO and the antibacterial properties of silver, i.e., the process of capture kill.

3.10

3.10 Flammability analysis

As shown in Fig. 10, the LOI value of BT was 28 % while the LOI value of RGOBT was only 23.2 %. The LOI value of RGO@AgBT increased to 30.5 % after the silver mirror reaction. Thus, the RGOBT samples coated with silver nanoparticles had better flame resistance than BT and RGOBT. Graphene material is composed of carbon and is flammable, when BT was coated with an RGO sheet, the graphene-based materials would make the bamboo-based composite more flammable (Bao et al., 2012). Because of the silver nanoparticles coated on the bamboo surface, the RGO@AgBT was harder to burn, which displayed significant improvement in flame resistance. The combustion processes of the BT, RGOBT and RGO@AgBT samples were shown in Fig. 11. The BT sample could be easily ignited with a strong flame during combustion. The whole piece of bamboo was burned out in 64 s, but BT was burned away, with ash remaining on the surface. As shown in Fig. 11b, the RGOBT burned intensely, with a higher flame height than BT, which was burned out in 44 s. In Fig. 11c, the RGO@AgBT could be heated and burned in an air atmosphere, which could extinguish itself when the flame is left unattended. The results showed that the RGO@AgBT samples had improved flame retardancy compared with the BT or the RGOBT samples.

Limiting oxygen index of BT, RGOBT and RGO@AgBT.

Digital photos of combustion processes of BT (a), RGOBT (b), and RGO@AgBT (c).

4 Conclusion

In summary, we have demonstrated a two-step protocol method of fabricating silver nanoparticles in a graphene network on the bamboo surface. A possible synthetic mechanism was analyzed as follows. The raw BT firstly absorbed GO, and then formed a graphene structure on its surface, followed by the formation of a silver protective layer on the graphene network through a silver mirror reaction. The RGO@AgBT possessed excellent light catalytic activity, and the light degradation efficiency of MO, MB, and RhB was 78.4 %, 88.8 %, and 77.6 %, respectively. Moreover, the RGO@AgBT possessed superior electrical conductivity with an electrical resistance of 992 Ω between the two test points of 50 mm distance. The antibacterial test showed that the diameter of the bacteriostatic ring of RGO@AgBT was 10 mm and 8 mm, respectively, against E. coli and S. aureus. The antibacterial activity of RGO@AgBT was significantly improved, much higher than that of BT or RGOBT. Additionally, the RGO@AgBT exhibited improved thermal stability and fire-resistant properties, with an LOI value of 30.5 %.

Acknowledgments

The work was financially supported by the Cooperation Project of Zhejiang Province Project of Scientifific Research Institutes (Grant NO. 2021F1065-3), Cooperation Project of Zhejiang Province & China Forestry Academy (Grant NO. 2014SY13) and National Natural Science Foundation of China (Grant NO. 31470586).

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Show Sections

[1] Alonso E.T., Karkera G., Jones G.F., Craciun M.F., Russo S., . Homo- geneously bright, flexible, and foldable lighting devices with functionalized graphene electrodes. ACS Appl. Mater. Inter.. 2016;8:16541-16545.
[Google Scholar]

[2] Aravind P., Selvaraj H., Ferro S., Neelavannan G.M., Sundaram M., . A one-pot approach: Oxychloride radicals enhanced electrochemical oxidation for the treatment of textile dye wastewater trailed by mixed salts recycling. J. Clean. Prod.. 2018;182:246-258.
[Google Scholar]

[3] Bao C., Song L., Wilkie C.A., Yuan B., Guo Y., Hu Y., Gong X., . Graphite oxide, graphene, and metal-loaded graphene for fire safety applications of polystyrene. J. Mater. Chem.. 2012;22:16399-16406.
[Google Scholar]

[4] Bossy C., Ali-Boucetta H., Kostarelos K., . Safety considerations for graphene: lessons learnt from carbon nanotubes. Accounts Chem. Res.. 2013;46:692-701.
[Google Scholar]

[5] Chand N., Jain D., Nigrawal A., . Investigations on gradient a.c. conductivity characteristics of bamboo (Dendrocalamus strictus) B Mater. Sci.. 2006;29:193-196.
[Google Scholar]

[6] Cheng Z., Liao J., He B., Zhang F., Zhang F., Huang X., Zhou L., . One-step fabrication of graphene oxide enhanced magnetic composite gel for highly efficient dye adsorption and catalysis. ACS Sustain. Chem. Eng.. 2015;3:1677-1685.
[Google Scholar]

[7] Cheng Y., Zhao Q., Li Y., Peng W., Zhang G., Zhang F., Fan X., . Gold nanoparticles supported on layered TiO₂–RGO hybrid as an enhanced and recyclable catalyst for microwave-assisted hydration reaction. RSC Adv.. 2016;6:76151-76157.
[Google Scholar]

[8] Deng Z., Zhu H., Peng B., Chen H., Sun Y., Gang X., Jin P., Wang J., . Synthesis of PS/Ag nanocomposite spheres with catalytic and antibacterial activities. Acs Appl. Mater. Inter.. 2012;4:5625-5632.
[Google Scholar]

[9] Du H., Li R., Xiu H., Sun J., Chen Y., Zhao Y., . Relations among resistance percentage, temperature and moisture of oak in the seasoning process. J. Beihua Univ. (Nat. Sci.). 2002;3:349-351.
[Google Scholar]

[10] Feng C., Ren P., Li Z., Tan W., Zhang H., Jin Y., Ren F., . Graphene/ waste-newspaper cellulose composite aerogels with selective adsorption of organic dyes: preparation, characterization and adsorption mechanism. New J. Chem.. 2020;44:2256-2267.
[Google Scholar]

[11] Fengel D., . The ultrastructure of cellulose from wood. Wood Sci. Technol.. 1969;3:203-217.
[Google Scholar]

[12] Gui Z., Zhu H., Gillette E., Han X., Rubloff G.W., Hu L., Lee S.B., . Natural cellulose fiber as substrate for supercapacito. ACS Nano. 2013;7:6037-6046.
[Google Scholar]

[13] Hong C.M., Lodge T.P., Frisbie C.D., . Solution-processable electrochem iluminescent ion gels for flexible, low-voltage, emissive displays on plastic. J. Am. Chem. Soc.. 2014;136:3705-3712.
[Google Scholar]

[14] Hsu P.C., Kong D., Wang S., Wang H., Welch A.J., Wu H., Cui Y., . Electro- lessly deposited electrospunmetal nanowire transparent electrodes. J. Am. Chem. Soc.. 2014;136:10593-10596.
[Google Scholar]

[15] Hu X., Tian M., Qu L., Zhu S., Han G., . Multifunctional cotton fabrics with graphene/polyurethane coatings with far-infrared emission, electrical conductivity, and ultraviolet-blocking properties. Carbon. 2015;95:625-633.
[Google Scholar]

[16] Jenita Rani, M.A. Jothi Rajan, G. Gnana kumar. 2017. Reduced graphene oxide/ZnFe2O4 nanocomposite as an efficient catalyst for the photocatalytic degradation of methylene blue dye. Res Chem Intermediat. 43, 2669-2690.

[17] Jin C., Li J., Wang J., Han S., Wang Z., Sun Q., . Cross-linked ZnO nano- walls immobilized onto bamboo surface and their use as recyclable photocatalysts. J. Nanomater. 2014:1-7.
[Google Scholar]

[18] Jin C., Li J., Han S., Wang J., Yao Q., Sun Q., . Silver mirror reaction as an approach to construct a durable, robust superhydrophobic surface of bamboo timber with high conductivity. J. Alloys Compd.. 2015;635:300-306.
[Google Scholar]

[19] Kafy A., Sadasivuni K.K., Kim H.C., Akther A., Kim J., . Designing flexible energy and memory storage materials using cellulose modified graphene oxide nanocomposites. Phys. Chem. Chem. Phys.. 2015;17:5923-5931.
[Google Scholar]

[20] Li J., Yu Hui., Wu Z., Wang j., He S., Ji J., Li N., Bao Y., Huang C., Chen Z., Chen Y., Jin C. 2016. Room temperature synthesis of crystalline anatase TiO2 on bamboo timber surface and their short-term antifungal capability under natural weather conditions. Colloids Surfaces A. 508, 117-123.

[21] Li J., Lu Y., Yang D., Sun Q., Liu Y., Zhao H., . Lignocellulose aerogel from wood-ionic liquid solution (1-allyl-3-methylimidazoliumchloride) under freezing and thawing conditions. Biomacromolecules. 2011;12:1860-1867.
[Google Scholar]

[22] Li J., Sun Q., Fan B., Zheng H., Yan C., Jin C., . Fabrication of biomimetic superhydrophobic plate-like CaCO₃ coating on the surface of bamboo timber inspired from the biomineralization of nacre in seawater. Nano Rep.. 2015;1:9-14.
[Google Scholar]

[23] Li J., Sun Q., Han S., Wang J., Wang Z., Jin C., . Reversibly light-switchable wettability between superhydrophobicity and superhy- drophilicity of hybrid ZnO/bamboo surfaces via alternation of UV irradiation and dark storage. Prog. Org. Coat.. 2015;87:155-160.
[Google Scholar]

[24] Li J., Lu Y., Wu Z., Bao Y., Xiao R., Yu H., Chen Y., . Durable, self-cleaning and superhydrophobic bamboo timber surfaces based on TiO₂ films combined with fluoroalkylsilane. Ceram. Int.. 2016;42:9621-9629.
[Google Scholar]

[25] Li J., Ren D., Wu Z., Huang C., Yang H., Chen Y., Yu H., . Visible- light-mediated antifungal bamboo based on Fe-doped TiO₂ thin films. RSC Adv.. 2017;7:55131-55140.
[Google Scholar]

[26] Li J., Su M., Wang A., Wu Z., Chen Y., Qin D., Jiang Z., . In situ formation of Ag nanoparticles in mesoporous TiO₂ films decorated on bamboo via self-sacrifificing reduction to synthesize nanocomposites with efficient antifungal activity. Int. J. Mol. Sci.. 2019;20:5497.
[Google Scholar]

[27] Li J., Ma R., Wu Z., He S., Chen Y., Bai R., Wang J., . Visible-light-driven Ag-modifified TiO₂ thin films anchored on bamboo material with antifungal memory activity against Aspergillus Niger. J. Fungi. 2021;7:592.
[Google Scholar]

[28] Li J., Ma R., Lu Y., Wu Z., Liu R., Su M., Jin X., Zhang R., Bao Y., Chen Y., Qin D., Yang D., Jiang Z., . Bamboo-inspired design of a stable and high- efficiency catalytic capillary microreactor for nitroaromatics reduction. Appl. Catal. B-Environ.. 2022;310:121297
[Google Scholar]

[29] Lian X., Li Y., An D., Zhou Y., Wang Q., Zhang N., . Synthesis of porous ZnO nanostructures using bamboo fibers as templates. Mater. Sci.-Poland. 2014;32:514-520.
[Google Scholar]

[30] Luo J., Wang Z., Xu L., Wang A.C., Han K., Jiang T., Lai Q., Bai Y., Tang W., Fan F.R., . Flexible and durable wood-based triboelectric nanogenerators for self-powered sensing in athletic big data analytics. Nat. Commun.. 2019;10:1-9.
[Google Scholar]

[31] Ma J., Zhang J., Xiong Z., Yong Y., Zhao X., . Preparation, characterization and antibacterial properties of silver-modified graphene oxide. J. Mater. Chem.. 2011;21:3350-3352.
[Google Scholar]

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[Google Scholar]

[33] Marcano D.C., Kosynkin D.V., Berlin J.M., Sinitskii A., Sun Z., Slesarev A., Alemany L.B., Lu W., Tour J.M., . Improved synthesis of graphene oxide. ACS Nano. 2010;4:4806-4814.
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[34] K.Ramachandran, D.Kalpana, Y.Sathishkumar, YangSooLee, K. Ravichandran, G. Gnana kumar. 2016. A facile green synthesis of silver nanoparticles using Piper betle biomass and its catalytic activity toward sensitive and selective nitrite detection. J Ind Eng Chem. 35, 29-35.

[35] Reddy D.R., Dinesh G.K., Anandan S., Sivasankar T., . Sonophotocatalytic treatment of Naphthol Blue Black dye and real textile wastewater using synthesized Fe doped TiO₂. Chem. Eng. Process.. 2016;99:10-18.
[Google Scholar]

[36] Ren D., Li J., Bao Y., Wu Z., He S., Wang A., Guo F., Chen Y., . Low-temperature synthesis of flower-like ZnO microstructures supported on TiO₂ thin films as efficient antifungal coatings for bamboo protection under dark conditions. Colloid Surf. A. 2018;555:381-388.
[Google Scholar]

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Designing a novel type of multifunctional bamboo surface based on an RGO/Ag coating

Abstract

Keywords

Bamboo

Graphene

Antibacterial activity

Photocatalytic

Fire-resistance

Conductivity

1 Introduction

2 Materials and methods

2.1 Materials

2.2 Synthesis of RGO on the surface of bamboo

2.3 Synthesis of silver coating on the surface of RGOBT

2.4 Characterization

2.5 Photo-catalytic activity test

2.6 Electrical conductivity test

2.7 Antibacterial test

2.8 Fire resistance test

3 Results and discussion

3.1 SEM analysis

3.2 EDS analysis

3.3 XRD analysis

3.4 FTIR analysis

3.5 XPS analysis

3.6 Synthesis mechanism

3.7 Evaluation of the photo-catalytic activity

3.8 Analysis of the electrical conductivity

3.9 Analysis of the antibacterial activity

3.10 Flammability analysis

4 Conclusion

Acknowledgments

Declaration of Competing Interest

References

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