Robust and Low-Cost Flame-Treated Wood for High-Performance

Research Article www.acsami.org

Robust and Low-Cost Flame-Treated Wood for High-Performance Solar Steam Generation Guobin Xue, Kang Liu, Qian Chen, Peihua Yang, Jia Li, Tianpeng Ding, Jiangjiang Duan, Bei Qi, and Jun Zhou* Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan 430074, China S Supporting Information *

ABSTRACT: Solar-enabled steam generation has attracted increasing interest in recent years because of its potential applications in power generation, desalination, and wastewater treatment, among others. Recent studies have reported many strategies for promoting the efficiency of steam generation by employing absorbers based on carbon materials or plasmonic metal nanoparticles with well-defined pores. In this work, we report that natural wood can be utilized as an ideal solar absorber after a simple flame treatment. With ultrahigh solar absorbance (∼99%), low thermal conductivity (0.33 W m−1 K−1), and good hydrophilicity, the flame-treated wood can localize the solar heating at the evaporation surface and enable a solar-thermal efficiency of ∼72% under a solar intensity of 1 kW m−2, and it thus represents a renewable, scalable, low-cost, and robust material for solar steam applications. KEYWORDS: wood, flame treatment, carbon nanoparticle, photothermal conversion, solar steam generation

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INTRODUCTION Solar-enabled steam generation is considered as one of the most promising solar-energy-harvesting technologies, because of its potential applications in power generation, desalination, wastewater treatment, and liquid−liquid phase separation, among others.1−4 Conventional methods of generating steam from solar energy always employ volumetric absorbers to heat the bulk water directly and thus suffer from large optical and systematic heat losses.5−10 Recently, increasing interest has been directed toward a new absorber design with heat localization, in which heat is confined at the evaporation surface and heat loss from the heating spot to the bulk water is minimized.11 Thus, such an absorber can induce high solarthermal efficiency and produce high-temperature vapor under the same solar intensity.12−14 Recent research on heat localization have mainly focused on exploring photothermal materials with high absorption in the solar spectrum. Carbon materials, semiconductor materials, and plasmonic metal nanoparticles have been demonstrated to be able to enhance solar absorption effectively.11,13,15−21 However, most of these reported materials, films, and floating particles are either expensive or technically demanding for large-scale expansion. Low cost, operability, and stability might be critical limitations to the practical application of these absorbers. Wood is one of the most abundant renewable resources on Earth, with high mechanical strength, low cost, and good machinability.22−24 During their natural growth, trees take in water, ions, and other nutrients from the soil through many vertically aligned microchannels. These microchannels give wood a unique anisotropic and porous structure that has drawn © 2017 American Chemical Society

particular interest in regard to some advanced applications, such as the fabrication of transparent building materials and the development as green chemical reactors.25−28 In this work, we first demonstrate that wood can be an ideal absorber after a very simple flame treatment. The absorbance of the flametreated wood (F-wood) can be as high as ∼99% weighted by the standard air mass 1.5 global (AM 1.5 G) solar spectrum. In addition, owing to its self-floating ability, inherent low thermal conductivity, and natural aligned microchannels for transporting water to the evaporation surface, such flame-treated wood is an excellent solar absorber with strong heat localization, achieving a high solar-thermal efficiencies of ∼72% and ∼81% under solar intensities of 1 and 3 kW m−2, respectively. All of these results demonstrate that F-wood is a promising material for easily manufactured, robust, and lowcost absorbers for use in high-performance solar steam generation.

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EXPERIMENTAL SECTION

Material Preparation. Wood blocks were cut into cylindrical pieces with a sweep saw. After that, the wood samples were pretreated over an alcohol flame for ∼1 min and then polished to expose the aligned pores and make the surface clear. Then, F-wood was obtained by treating the wood samples on the alcohol flame for another 2 min and then directly immersing them in cold water at room temperature to achieve rapid quenching. Fully carbonized wood (C-wood) was Received: February 11, 2017 Accepted: April 12, 2017 Published: April 12, 2017 15052

DOI: 10.1021/acsami.7b01992 ACS Appl. Mater. Interfaces 2017, 9, 15052−15057

Research Article

ACS Applied Materials & Interfaces directly obtained by annealing wood samples in a muffle furnace at a temperature of 350 °C for 1 h. Material Characterization. The morphologies of the samples were characterized by field-emission scanning electron microscopy (FE-SEM, JSM-6330F). Optical transmittance and reflectance spectra were measured in the range of 300−2500 nm with a Shimadzu UV3600 spectrophotometer. Fourier transform infrared (FTIR) spectroscopy was performed on a VERTEX 70 spectrometer (Bruker, Karlsruhe, Germany). Raman scattering measurements were performed on a Renishaw-inVia Raman spectrometer at room temperature using the 514.5-nm line of an Ar+ laser. Thermal conductivities were measured using a steady-state method with homemade test equipment (Figure S1).29 Steam Generation Tests. Steam generation was measured by floating F-wood in a quartz beaker containing deionized (DI) water. The solar light was supplied by a solar simulator (Newport). Different solar intensities were obtained using a condensing lens. Changes in the mass of water were measured with a high-accuracy balance (MettlerToledo, ME204E). Infrared photographs were recorded with a FLUKE TiX520 thermal imager. The temperature of water was measured by thermocouples and recorded by a data logger (TC-08).

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Figure 2. SEM images of F-wood. (a) Top and (b) cross-sectional views of F-wood, showing the long, well-defined channels. (c) Enlarged SEM image of the inner pores. Apparent deposited carbon nanoparticles can be observed on the surface and inner wall of the channel. (d) Morphology of the deposited carbon nanoparticles.

RESULTS AND DISCUSSION F-wood samples were fabricated in three steps. First, a basswood block was cut into cylindrical pieces perpendicular to the growth direction. Then, the wood pieces were heated directly on the flame of an alcohol lamp for 2 min. After that, the hot wood was directly immersed in cold water at room temperature to achieve quenching. Then, the pale yellow wood became very dark, as shown in Figure 1a, which intuitively

surface of F-wood had pores with two different sizes: fiber-cell pores with diameters of several micrometers and vessel pores with diameters of 50−100 μm.28 The long channels of vessel pores are used to pump water and inorganic salt for trees during their growth process, and in this case, they could be efficient for transporting water for evaporation.32 Moreover, both the disordered fiber-cell pores and the vessel pores can help to capture the incident light.17,18 Here, a very interesting phenomenon should be noted, namely, that the flame treatment also deposited a layer of carbon particles on the surface and inside wall of the microchannels, as shown in Figure 2c,d and Figure S4. The carbon nanoparticles had diameters of about 30 nm, which should be efficient in enhancing the absorbance of the surface.33 To quantitatively characterize the performance of the absorber, we measured absorption spectra in the solar spectrum range (wavelengths from 300 to 2500 nm). As shown in Figure 3a, F-wood was found to have ultrahigh absorbance (over 99%) in the ultraviolet and visible regions (300−780 nm), as well as ∼97% absorbance in the near-infrared region (780−2500 nm). Weighted by the standard air mass 1.5 global (AM 1.5 G) solar spectrum, the overall light absorbance of F-wood for sunlight can achieve a high value of ∼99% (Figure S5).34 To explore the reasons for the high absorbance of F-wood, pristine wood and fully carbonized wood (C-wood) (Figure S6) were tested for comparison. The average absorbance of pristine wood was found to be only ∼60% in the ultraviolet and visible regions and ∼50% in the near-infrared region (black line). The absorbance of C-wood was found to be close to that of F-wood in the ultraviolet and visible regions but slightly lower than that of Fwood in the near-infrared region (blue line). These results indicate that the high absorbance of F-wood is induced by the carbonized surface with pores, as well as the deposited carbon nanoparticles.17,18 The thermal conductivity of the absorber is another critical parameter for a heat localization system.33 Here, F-wood has an inherent low thermal conductivity of 0.33 W m−1 K−1 (Figure 3b), which is favorable for thermal insulation. Another

Figure 1. Schematic of F-wood for solar steam generation. (a) Photographs of blocks of pristine wood and F-wood. The wood blocks were ∼3.2 cm in diameter and 1 cm in thickness. (b) Schematics of the inner structures of the (I) pristine wood and (II) F-wood. (c) Schematic illustration of F-wood for solar steam generation. F-wood floats on the water, pumps water spontaneously through its inner channels to the heated surface, and generates high-temperature steam for fresh water.

indicates a high absorbance for visible light. Because the flame treatment lasted for a very short time, only the surface of the wood was carbonized (Figure S2). In addition to carbonization, carbon nanoparticles were also deposited on the wood from the incomplete combustion of alcohol (Figure 1b).30 However, the entire piece of F-wood retained the properties of wood, including good mechanical properties and self-floating ability. These properties will benefit the stable applications of F-wood in harsh environments, such as stand-alone desalination systems in the ocean (Figure 1c). The Raman spectrum showed that the surface of the flametreated wood had a structure analogous to that of graphite with some degree of disordering (Figure S3).30,31 Scanning electron microscopy (SEM) images (Figure 2a,b) revealed the inherent disordered and well-defined inner pores in the wood. The 15053


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Figure 3. Characterizations of F-wood. (a) Absorption spectra of F-wood, C-wood, and pristine wood in the range from 300 to 2500 nm. (b) Thermal conductivity of the dry F-wood (black line) and a sample with the channels filled with water (red line). (c) FTIR spectrum of F-wood. The inset shows the contact angle image after a water droplet (4 μL) has been dropped on the surface of F-wood. (d,e) High-resolution SEM images of the F-wood surface (d) after soaking for one month and (e) after ultrasonic treatment for 1 h at 100 W after the soaking. The insets show photographs of F-wood after the treatments, with no obvious changes after the limit test.

With its ultrahigh sunlight absorbance, low thermal conductivity, good hydrophilicity, and robust structure, Fwood can be used directly for efficient solar steam generation. As shown in Figure 4a-I, under a solar intensity of 1 kW m−2, the surface temperature of the floating F-wood was 43 °C, which was much higher than that of the bulk water (31.5 °C, Figure S8). The surface temperature increased with the intensity of the solar irradiation. Under solar illumination of 3 kW m−2, the surface temperature reached ∼63 °C, and vapor could be visibly observed, as shown in Figure 4a-III and 4a-IV. The temperature differences (ΔTw) between the heated surface and the water at a position 1 cm under the surface were also measured. As shown in Figure 4b, ΔTw in a pure water system was only 0.5 °C at 1 kW m−2, whereas for the F-wood system, ΔTw was about 3.3 °C, which increased to 16.4 °C under solar illumination of 3 kW m−2. This demonstrates that F-wood can effectively localize the solar heating at the surface and minimize the heat lost in heating the bulk water. The steam generation rates of F-wood under different light densities were also tested, as shown in Figure 4c. All evaporation rates were measured for 60 min at steady state (after illumination for 1 h). Under a solar intensity of 1 kW m−2, the water loss with F-wood was as high as 1.05 kg m−2 h−1, about 2.3 times higher than that of pure water. For comparison, the steam generation rate of wood was only 0.7 kg m−2 h−1, which indicates the important role of

interesting point that should be noted is that the thermal conductivity of F-wood filled with water is 0.54 W m−1 K−1, even lower than the thermal conductivity of pure water (∼0.6 W m−1 K−1). Further, it is known that the thermal conductivity of a liquid measured by a steady-state method should be larger than the actual thermal conductivity, because of heat convection. However, in our system, the convection was greatly weakened in the narrow microchannels inside the wood, which would reduce the heat loss during solar-thermal operation. The third important characteristic of F-wood for heat localization is its hydrophilicity, which ensures water flow to the heated surface.35,36 Because the wood was flame-treated in air and subjected to a quenching treatment, plenty of oxygencontaining functional groups were formed (Figure 3c),37−39 making the F-wood surface hydrophilic (Figure S7). As shown in the inset of Figure 3c, a water droplet of 4 μL dropping on the surface of F-wood will leak into F-wood in several seconds. During the solar steam generation process, the wood will float on the water and keep the top surface wet by pumping water from the bottom spontaneously. In addition, the microstructure and carbon nanoparticles were found to be very robust. As shown in Figure 3d,e, carbon nanoparticles remained adhered to the porous carbonized wood even after it had been soaked in water for one month and then subjected to ultrasonic treatment for 1 h at 100 W. 15054


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Figure 4. Steam generation performance of F-wood. (a) Infrared radiation thermal images of F-wood under different solar intensities and photograph of visible steam generation at only 3 kW m−2. The surface temperature of F-wood reached (I) 43, (II) 54, and (III) 63 °C under solar illumination of 1, 2, and 3 kW m−2, respectively. (b) Temperature difference between the heated surface and the water at a position 1 cm under the surface under different solar irradiations. (c) Vapor-evaporation-induced mass changes of water as a function of time under different solar irradiations. (d) Steam generation efficiency (black squares) and evaporation rate (blue circles) under different solar irradiations. All of the experiments were conducted at an ambient temperature of 26 °C and a relative humidity of ∼40%.

∼81% under a solar intensity of 3 kW m−2. This efficiency is comparable to the best values in the previously reported literatures11,17,18 Given its low cost and robust structure, therefore, we believe that F-wood is a very promising material for solar steam generation application, as well as power generation, desalination, and water purification, among other applications.

enhancing the absorbance with the surface treatment (Figure S9). With increasing solar intensity, the steam generation rate of F-wood increased, reaching 3.46 kg m−2 h−1 (Figure 4d) under solar illumination of 3 kW m−2. Here, the solar converting property of the ultrasonic-treated wood was also tested and was found to be similar to that of a newly fabricated sample, further indicating the good stability of F-wood (Figure S10). To assess the performance of F-wood, the thermal efficiency (ηth) was calculated as11 ηth = Q e/(ACoptQ s)

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CONCLUSIONS In summary, we have demonstrated that wood in nature can be used for high-performance solar steam generation. After a very simple flame treatment, the wood exhibits ultrahigh absorbance, low thermal conductivity, and good hydrophilicity and shows a high solar-thermal efficiency of ∼72% at only 1 kW m−2. This result, in combination with a low cost and robust structure, means that F-wood holds great potential as a cost-effective and scalable absorber for converting sunlight into thermal energy for practical applications.

(1)

where A is the cross-sectional area of F-wood, Copt denotes the optical concentration, and Qs is the normal direct solar irradiation (1 kW m−2) . Qe denotes the power consumed for vapor generation, which can be estimated as Q e = mλ + mC ΔT

(2)

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where m denotes the mass flux, λ is the latent heat of the phase change, C is the specific heat capacity of water (4.2 J g−1 K−1), and ΔT denotes the temperature increase of the water. As shown in Figure 4d, pure water was found to have an efficiency of only 30% under an illumination of 1 kW m−2, because of its poor light absorbance and bulk water heating strategy. By comparison, F-wood had a much higher efficiency of ∼72% under the same solar intensity. With increasing solar intensity, the efficiency of F-wood increased, achieving a high value of

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b01992. Schematic of the experimental setup for measuring thermal conductivity. SEM images of pristine wood, Fwood, and C-wood. Photograph showing F-wood selffloating on the surface of water. Raman spectrum of F15055


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wood. Absorption spectrum of F-wood and the standard AM 1.5 G solar spectrum. Temperature of surface water and water at a position 1 cm under the surface. Mass changes due to vapor evaporation of water as a function of time for natural wood and ultrasonic-treated F-wood (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jun Zhou: 0000-0003-4799-8165 Author Contributions

G.X. and K.L. contributed equally. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (51672097, 51606082, 51322210), the National Program for Support of Top-Notch Young Professionals, the China Postdoctoral Science Foundation (2015M570639), the Fundamental Research Funds for the Central Universities (HUST: 2015MS004), and the Director Fund of WNLO. The authors acknowledge the facility support of the Center for Nanoscale Characterization & Devices, WNLO-HUST, and the Analysis and Testing Center of Huazhong University of Science and Technology.

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Robust and Low-Cost Flame-Treated Wood for High-Performance

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