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Harvesting, sensing and regulating light based on photo-thermal effect of [email protected] mesh
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Research paper
Harvesting, sensing and regulating light based on photo-thermal effect of Cu@CuO mesh Xuan Wu a, Jie Xu a, George Y. Chen b,**, Rong Fan c, Xiaokong Liu a, Haolan Xu a,* b
a Future Industries Institute, University of South Australia, Mawson Lakes Campus, SA 5095, Australia Laser Physics and Photonic Devices Laboratories, School of Engineering, University of South Australia, Mawson Lakes Campus, SA 5095, Australia c School of Natural and Built Environments, University of South Australia, Mawson Lakes Campus, SA 5095, Australia
Received 5 January 2017; revised 30 January 2017; accepted 6 February 2017 Available online ▪ ▪ ▪
Abstract A system of light harvesting, sensing and regulating was designed based on the photo-thermal and Seebeck effect of flexible CuO nanostructures. Cu@CuO meshes were prepared via self-oxidation of Cu mesh and utilized as the photo-thermal material. Upon irradiation by visible light, the temperature of the Cu@CuO mesh dramatically increases. The temperature difference between the irradiated and non-irradiated parts of the Cu@CuO mesh produced a measurable voltage output due to the Seebeck effect. The generated voltage was then converted into a digital signal to control a rotary neutral-density disc to filter the received light. This enabled regulation of the intensity of the incident light at a selected region. This system is cost effective and has potential applications in greenhouses, factories and smart buildings to minimize energy consumption and improve wellbeing. © 2017, Institute of Process Engineering, Chinese Academy of Sciences. Publishing services by Elsevier B.V. on behalf of KeAi Communications Co., Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Keywords: Photothermal; Light; CuO; Nanostructure
1. Introduction Solar energy is recognised as one of most reliable clean and sustainable energy sources that can alleviate the global energy crisis and environmental issues [1,2]. Solar light can be converted into three main type of energy: electricity, chemical energy and heat [3e6]. Tremendous efforts have been devoted to synthesising and designing materials and devices which absorb and convert solar light into applicable energy. One of the most successful applications is solar photovoltaic cells which have been widely deployed to supply electricity for domestic and industrial use [5]. In comparison, typical light-to-chemical energy conversion in nature is photosynthesis conducted by plants [4]. Artificial and biomimetic photocatalysis over catalysts is another * Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (G.Y. Chen), haolan.xu@ unisa.edu.au (H. Xu).
effective pathway to utilise solar light for facilitating chemical reactions [7e10]. Recently, the photo-thermal effect became an attractive energy conversion method, and it is gathering interest world-wide. Photo-thermal materials absorb solar light and convert it into heat, producing a highly localized increase in temperature on the surface of materials. The photo-thermal effect has been intensively applied to generate steam from water for desalination [11e20]. Surprisingly, the energy conversion rate in steam generation can reach ~89%, which is much higher than that of the light-to-electricity and light-to-chemical energy conversion. The photo-thermal effect also plays vital roles in other physical and chemical processes to produce electricity and chemical energy. For instance, a photo-thermal coating on a commercial thermo-electric device generates electricity under irradiation by producing a temperature difference between the two surfaces [21e23]. High-temperature catalytic reaction can be conducted at room temperature under irradiation by loading the catalyst on the surface of photo-thermal material [24,25].
http://dx.doi.org/10.1016/j.gee.2017.02.002 2468-0257/© 2017, Institute of Process Engineering, Chinese Academy of Sciences. Publishing services by Elsevier B.V. on behalf of KeAi Communications Co., Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Please cite this article in press as: X. Wu, et al., Harvesting, sensing and regulating light based on photo-thermal effect of Cu@CuO mesh, Green Energy & Environment (2017), http://dx.doi.org/10.1016/j.gee.2017.02.002
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There is no doubt that the photo-thermal effect will find numerous other applications when integrated with other systems. Herein, we develop a new application that is a light-intensity sensor and regulator based on the photo-thermal effect. CuO is one of the most important semiconductors with a wide array of applications in catalysis, gas sensing, lithiumion batteries and so forth [26]. Recently, CuO was found to be an excellent photo-thermal material for photothermal catalysis and optical sensor [24,27]. Compared to the other intensively utilized photo-thermal materials such as gold, graphene and carbon nanotubes, CuO is much more cost effective. To date, many metal and metal oxides have exploited the Seebeck effect that converts temperature gradients into voltages [28]. Inspired by these properties, we designed a CuO based device to harvest light and regulate its intensity. In this work, flexible Cu@CuO mesh was prepared. The mesh can absorb incident light and generates a high local temperature. The temperature difference between the hotspot and the cooler part of the Cu@CuO mesh yields a detectable voltage that can be converted into a digital readout. The digital signal can then be applied to control a motorized variable neutral-density (ND) filter to continuously adjust the transmitted light intensity to the desired value. This automated system shown in Fig. 1 can be applied to regulate natural sunlight for health (e.g. skin cancer prevention) and manufacturing (e.g. plant factories) benefits. It can be used for a variety of lighting systems to be deployed in smart greenhouses, stadiums and residential buildings to save electrical energy. Compared to the current light sensing materials and regulating technology [29e31], our system is more cost effective and more precise. 2. Experimental section 2.1. Materials (NH4)2S2O8 and NaOH were purchased from SigmaeAldrich and used without further purification. Milli-Q water
with a resistance of 18.2 MU cm1 was used for all experiments. 2.2. Preparation of Cu@CuO mesh The commercial Cu meshes (200 mesh, 2 1 cm) were first ultrasonicated in Milli-Q water and ethanol to remove contaminants before use. Then, the cleaned Cu mesh was immersed into a mixture of aqueous solution containing NaOH (2.5 M) and (NH4)2S2O8 (0.13 M) at room temperature. The reaction time was set to 2 h, 4 h, and 6 h separately. The resulting meshes were collected and rinsed with Milli-Q water and ethanol, then vacuum dried before further characterization. 2.3. Preparation of Cu@Au mesh The cleaned Cu mesh was immersed into HAuCl4 aqueous solution (0.01 wt%). The galvanic reaction took place on the surface of copper mesh, producing Au particles. After 4 h of reaction, the mesh was withdrawn and rinsed with Milli-Q water and ethanol for several times. 2.4. Characterization Field emission electron microscopy (SEM) images were obtained on a Zeiss Merlin SEM. X-ray photoelectron spectroscopy (XPS) analysis was carried out on a Kratos Axis Ultra with a Delay Line Detector photoelectron spectrometer using an Aluminium monochromatic X-ray source. The infrared (IR) photographs providing calibrated temperature readouts were captured by using an IR camera (FLIRE64501). The voltage measurements were acquired using a multimeter with its two terminals attached to different regions of the sample (i.e. hot and cold) via crocodile clips. The samples were suspended above a glass slide with white paper underneath to prevent the table and glass slide from heating up. The temperature and voltage measurement were conducted at the same time. 3. Results and discussion
Fig. 1. Illustration of the light intensity regulator composed of photo-thermal material of Cu@CuO mesh, analogue digital convert and a variable neutral density disc.
By simply immersing the Cu mesh in the alkaline (NH4)2S2O8 solution, a chemical reaction takes place on the surface of Cu mesh. It was observed that colour of the Cu mesh changed from yellow to blue and eventually to black (Fig. 2a). The SEM images depict the evolution of the surface morphology of the Cu mesh along with the reaction time. As shown in Fig. 2b and c, the cleaned Cu mesh is composed of Cu wires with a diameter of ~38 mm. The surface of the Cu wire is smooth with no obvious nanostructure. After 2 h of reaction, a furry structure emerged on the surface of Cu mesh (Fig. 2d). Highly magnified SEM image shows that this furry structure is composed of nanoribbons (Fig. 2e). The width and length of each nanoribbon is ~220 nm and ~11 mm respectively. As 1-dimensional (1D) nanoribbon is a typical morphology of Cu(OH)2 [32], it
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Fig. 2. (a) Digital photograph of the Cu meshes from different reaction times. SEM images of the Cu mesh after (b,c) 0 h, (d,e) 2 h, and (f, g) 4 h reaction in the alkaline solution with (NH4)S2O8.
implies the formation of Cu(OH)2 on the Cu mesh surfaces by oxidation at this stage. Further increasing the reaction time to 4 h leads to a significant surface morphology change. It can be clearly seen that the 1D nanoribbons were converted into 2D nanoplates, indicating the formation of CuO. When the reaction time was further increased to 6 h, no obvious morphology change was observed in Fig. S1, except corrosion of the Cu mesh (arrowed parts in Fig. S1). An XPS analysis was conducted to investigate the chemical evolution on the surface of Cu mesh during the reaction. The XPS spectrum of the 2 h mesh (Fig. 3a) depicts the photoelectron peaks at the binding energy of 934.5 and 933.2 eV, corresponding to the Cu 2P3/2 of Cu(OH)2 and Cu 2P3/2 of CuO. At this stage, only Cu(OH)2 nanowires and no CuO nanoplates were observed (Fig. 2d, e), which indicates that the main product is Cu(OH)2. However, the Cu(OH)2 begins to convert into CuO. It was noticed that after 4 h reaction, only the XPS peaks of Cu 2P3/2 of CuO (binding energy of 933.2 eV) was detected (Fig. 3b), indicating a completed conversion of Cu(OH)2, corresponding to the morphological change from nanoribbon (Fig. 2e) to nanoplate (Fig. 2g).
It can be suggested from Fig. 2f and g that the complex nanostructure on the Cu@CuO mesh can enhance light absorption and thus light-to-heat conversion [24,33]. In the following experiment, the photo-thermal effect of the Cu@CuO mesh is investigated. A solar simulator with adjustable power was used as the light source to irradiate a Cu@CuO mesh (Fig. 4a). An IR camera was used to record and analyse the surface temperature and temperature gradient of the Cu@CuO mesh under irradiation (Fig. 4bef). The Cu@CuO mesh obtained after 4 h of oxidation (denoted as Cu@CuO-4h) shows an excellent photo-thermal response. As shown in Fig. 4bef, when the incident light intensity is 0.70, 1.38, 1.88, 2.50 and 3.18 W/cm2, the maximum surface temperature of Cu@CuO-4h mesh reaches 51.9, 79.8, 101.8, 130.7 and 150.0 C respectively. In comparison, under the same irradiation the surface temperature of a Cu mesh (Fig. S2 and black square in Fig. 4g) is much lower than that of the Cu@CuO-4h mesh (red dot in Fig. 4g). The maximum surface temperature of Cu mesh is only 51.8 C, which indicates that Cu is not a good photo-thermal material. The Cu@CuO mesh obtained after 6 h of oxidation (denoted as Cu@CuO-6h) can reach a maximum surface temperature of 110.0 C (Fig. S3
Fig. 3. XPS spectra of the Cu mesh after (a) 2 h and (b) 4 h of reaction in the alkaline solution with (NH4)2S2O8. Please cite this article in press as: X. Wu, et al., Harvesting, sensing and regulating light based on photo-thermal effect of Cu@CuO mesh, Green Energy & Environment (2017), http://dx.doi.org/10.1016/j.gee.2017.02.002
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Fig. 4. (a) Photograph of the Cu@CuO-4h mesh undergoing temperature and electrical measurements; (bef) IR camera images of the Cu@CuO-4h mesh under irradiation with light intensities of 0.70, 1.38, 1.88, 2.50 and 3.18 W/cm2 respectively; (g) maximum surface temperature as a function of light intensity for the Cu mesh (black square), Cu@CuO-4h mesh (red dot), Cu@CuO-6h mesh (blue triangle) and Cu@Au mesh (pink triangle); (h) generated voltage over the Cu mesh (black square), Cu@CuO-4h mesh (red dot), Cu@CuO-6h mesh (blue triangle) and Cu@Au mesh (pink triangle) via the Seebeck effect.
and blue triangle in Fig. 4g), which is higher than that of the Cu mesh but lower than that of the Cu@CuO-4h mesh. These results confirm that CuO is an excellent photo-thermal material. The Cu@CuO mesh obtained after 4 h of oxidation of Cu mesh possesses superior photo-thermal performance. As one of the well-known photo-thermal materials, plasmonic Au particles, have been intensively investigated [34,35]. For comparison, we also prepared the Cu@Au mesh to investigate its photo-thermal effect. The Cu@Au mesh was synthesized by simply immersing the Cu mesh in the HAuCl4 solution. A galvanic reaction took place on the surface of Cu mesh which led to the formation of Au particles. As shown in Fig. S4a and b, after 4 h of galvanic reaction, the surface of the
Cu mesh was densely covered by particles. The SEM images with high magnification show that the particles are of polyhedral shape. The average particle size is ~0.8 mm. The XPS spectrum of the mesh verifies the binding energy of Au 4f7/2 and Au 4f5/2 (Fig. S4c), confirming the formation of Au particles on the Cu mesh surface. The photo-thermal effect of the Cu@Au mesh was also investigated. The surface temperature increased with the increment of incident light intensity (Fig. S5 and Fig. 4g). However, the maximum temperature is 97.5 C, which is much lower than that of Cu@CuO-4h mesh. Therefore, compared to the Cu@Au mesh, the Cu@CuO mesh has superior photo-thermal performance, while the cost of CuO is much lower than that of Au.
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When incident light was focused on one part of the Cu@CuO mesh shown in Fig. 4bef, the local surface temperature dramatically increased, while the temperature of the none-exposed part is much lower. This gives rise to a temperature gradient along the Cu@CuO mesh (Fig. 4bef). The electrons flow from the hotter region to the cooler region, inducing a voltage and a current, which is described by the Seebeck effect. The temperature gradient along the Cu@CuO mesh increased with increasing light intensity (Fig. 4bef). The maximum temperature-difference on the Cu@CuO-4h mesh reached ~120 C (Fig. 4f), which induced a voltage of ~74.5 mV shown in Fig. 4h. With the same incident light intensity, the Cu and Cu@Au mesh generated lower voltages of 52.5 and 69.0 mV respectively. The Cu@CuO-6h generated a maximum voltage of 74.0 mV, which is close to that of the Cu@CuO-4h mesh. Therefore, considering the cost of materials and manufacturing, the photo-thermal effect and the generated voltage, the Cu@CuO-4h mesh is the most promising candidate. Acting as a light sensor, the Cu@CuO-4h mesh generates a voltage upon irradiation that can be amplified and converted into a digital signal via an analogue-to-digital converter. This enables the feedback system demonstrated in Fig. 5 to control a motorized neutral-density (ND) disc for controlling the transmitted light intensity. The ND disc has a gradient of filtering ability, meaning the attenuation of light is a function of its rotation angle. As a result, the incident light intensity on a selected region behind the ND disc can be fine-tuned for specific applications. The algorithm and software behind the control was implemented by Labview, where the digital input signal was compared with a reference value. The difference signal was scaled then sent out as an analogue signal to drive the motorized ND disc. The video (Supporting information) demonstrates the process of light sensing and regulating. For practical applications where the area of daylighting is large,
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the ND disc can be replaced by a roll of graded fabric that shifts the correct patch of filter to the opening using a rotor. Supplementary video related to this article can be found at http://dx.doi.org/10.1016/j.gee.2017.02.002. 4. Conclusions In summary, we developed a light harvesting, sensing and regulating system based on the photo-thermal effect and Seebeck effect of a flexible Cu@CuO mesh. The mesh was synthesized by simple self-oxidation of Cu mesh in an alkaline solution. The Cu@CuO mesh showed superior photo-thermal response to the Cu@Au mesh. Under irradiation, the surface temperature of the Cu@CuO mesh reached 150.0 C. The large temperature discrepancy between the hotspot and the cool region of the Cu@CuO mesh induced a measurable voltage based on the Seebeck effect. The generated voltage was converted into digital signal which in turn controlled the motorized ND to adjust the intensity of incident light to the desired value. This system is cost effective and easily reproducible. It has great potential to be employed as light intensity controllers in greenhouses, factories and smart buildings to save energy and bring health benefits. Conflict of interest The authors declare no conflict of interests. Acknowledgements This work was supported by the Future Industries Institute, University of South Australia (Foundation Fellow).
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.gee.2017.02.002. References
Fig. 5. Photograph of the Cu@CuO-4h based light sensor and control system. Inset: enlarged view of the neutral-density disc and sensor.
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[12] Y. Liu, J.W. Lou, M.T. Ni, C.Y. Song, J.B. Wu, N.P. Dasgupta, P. Tao, W. Shang, T. Deng, ACS Appl. Mater. Interfaces 8 (2016) 772e779. [13] N.S. Lewis, Science 351 (2016) 353. [14] S. Ishii, R.P. Sugavaneshwar, T. Nagao, J. Phys. Chem. C 120 (2016) 2343e2348. [15] D.W. Zhao, H.Z. Duan, S.T. Yu, Y. Zhang, J.Q. He, X.J. Quan, P. Tao, W. Shang, J.B. Wu, C.Y. Song, T. Deng, Sci. Rep. 5 (2015) 17276. [16] L.B. Zhang, B. Tang, J.B. Wu, R.Y. Li, P. Wang, Adv. Mater. 27 (2015) 4889e4894. [17] S.T. Yu, Y. Zhang, H.Z. Duan, Y.M. Liu, X.J. Quan, P. Tao, W. Shang, J.B. Wu, C.Y. Song, T. Deng, Sci. Rep. 5 (2015) 13600. [18] O. Neumann, A.D. Neumann, E. Silva, C. Ayala-Orozco, S. Tian, P. Nordlander, N.J. Halas, Nano Lett. 15 (2015) 7880e7885. [19] Y. Ito, Y. Tanabe, J.H. Han, T. Fujita, K. Tanigaki, M.W. Chen, Adv. Mater. 27 (2015) 4302e4307. [20] K. Bae, G. Kang, S.K. Cho, W. Park, K. Kim, W.J. Padilla, Nat. Commun. 6 (2015) 10103. [21] S.M. Sun, W.Z. Wang, D. Jiang, L. Zhang, J. Zhou, Appl. Catal. B Environ. 158 (2014) 136e139. [22] L.B. Zhang, R.Y. Li, B. Tang, P. Wang, Nanoscale 8 (2016) 14600e14607. [23] E. Miyako, H. Nagata, R. Funahashi, K. Hirano, T. Hirotsu, Chemsuschem 2 (2009) 740e742. [24] J. Xu, X. Li, X. Wu, W. Wang, R. Fan, X. Liu, H. Xu, J. Phys. Chem. C 120 (2016) 12666e12672.
[25] Y. Zheng, W. Wang, D. Jiang, L. Zhang, X. Li, Z. Wang, J. Mater. Chem. A 4 (2016) 105e112. [26] Q. Zhang, K. Zhang, D. Xu, G. Yang, H. Huang, F. Nie, C. Liu, S. Yang, Prog. Mater. Sci. 60 (2014) 208e337. [27] G.Y. Chen, X. Wu, X. Liu, D.G. Lancaster, T.M. Monro, H. Xu, Sci. Rep. 7 (2017) 41895. [28] F.J. DiSalvo, Science 285 (1999) 703e706. [29] D. Men, F. Zhou, L. Hang, X. Li, G. Duan, W. Cai, Y. Li, J. Mater. Chem. C 4 (2016) 2117e2122. [30] D. Liu, F. Zhou, C. Li, T. Zhang, H. Zhang, W. Cai, Y. Li, Angew. Chem. Int. Ed. 54 (2015) 9596e9600. [31] M. Konstantin, GCP08B - Dynamic Daylighting & Sun Control Systems Dynamic Daylighting & Sun Control Systems, http://www. cpidaylighting.com/save-file.php?knowledge¼1&n¼Dynamicþ Daylightingþ%26þSunþControlþSystems&f¼%2FcmsAdmin% 2Fuploads%2FDynamic-Daylighting-AIA.pdf, (Accessed 30 January 2017). [32] H.L. Xu, W.Z. Wang, W. Zhu, L. Zhou, M.L. Ruan, Cryst. Growth Des. 7 (2007) 2720e2724. [33] Q. Tian, M. Tang, Y. Sun, R. Zou, Z. Chen, M. Zhu, S. Yang, J. Wang, J. Wang, J. Hu, Adv. Mater. 23 (2011) 3542e3547. [34] L. Au, D. Zheng, F. Zhou, Z.-Y. Li, X. Li, Y. Xia, ACS Nano 2 (2008) 1645e1652. [35] D.-K. Lim, A. Barhoumi, R.G. Wylie, G. Reznor, R.S. Langer, D.S. Kohane, Nano Lett. 13 (2013) 4075e4079.
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ScienceDirect Green Energy & Environment xx (2017) 1e6 www.keaipublishing.com/gee
Research paper
Harvesting, sensing and regulating light based on photo-thermal effect of Cu@CuO mesh Xuan Wu a, Jie Xu a, George Y. Chen b,**, Rong Fan c, Xiaokong Liu a, Haolan Xu a,* b
a Future Industries Institute, University of South Australia, Mawson Lakes Campus, SA 5095, Australia Laser Physics and Photonic Devices Laboratories, School of Engineering, University of South Australia, Mawson Lakes Campus, SA 5095, Australia c School of Natural and Built Environments, University of South Australia, Mawson Lakes Campus, SA 5095, Australia
Received 5 January 2017; revised 30 January 2017; accepted 6 February 2017 Available online ▪ ▪ ▪
Abstract A system of light harvesting, sensing and regulating was designed based on the photo-thermal and Seebeck effect of flexible CuO nanostructures. Cu@CuO meshes were prepared via self-oxidation of Cu mesh and utilized as the photo-thermal material. Upon irradiation by visible light, the temperature of the Cu@CuO mesh dramatically increases. The temperature difference between the irradiated and non-irradiated parts of the Cu@CuO mesh produced a measurable voltage output due to the Seebeck effect. The generated voltage was then converted into a digital signal to control a rotary neutral-density disc to filter the received light. This enabled regulation of the intensity of the incident light at a selected region. This system is cost effective and has potential applications in greenhouses, factories and smart buildings to minimize energy consumption and improve wellbeing. © 2017, Institute of Process Engineering, Chinese Academy of Sciences. Publishing services by Elsevier B.V. on behalf of KeAi Communications Co., Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Keywords: Photothermal; Light; CuO; Nanostructure
1. Introduction Solar energy is recognised as one of most reliable clean and sustainable energy sources that can alleviate the global energy crisis and environmental issues [1,2]. Solar light can be converted into three main type of energy: electricity, chemical energy and heat [3e6]. Tremendous efforts have been devoted to synthesising and designing materials and devices which absorb and convert solar light into applicable energy. One of the most successful applications is solar photovoltaic cells which have been widely deployed to supply electricity for domestic and industrial use [5]. In comparison, typical light-to-chemical energy conversion in nature is photosynthesis conducted by plants [4]. Artificial and biomimetic photocatalysis over catalysts is another * Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (G.Y. Chen), haolan.xu@ unisa.edu.au (H. Xu).
effective pathway to utilise solar light for facilitating chemical reactions [7e10]. Recently, the photo-thermal effect became an attractive energy conversion method, and it is gathering interest world-wide. Photo-thermal materials absorb solar light and convert it into heat, producing a highly localized increase in temperature on the surface of materials. The photo-thermal effect has been intensively applied to generate steam from water for desalination [11e20]. Surprisingly, the energy conversion rate in steam generation can reach ~89%, which is much higher than that of the light-to-electricity and light-to-chemical energy conversion. The photo-thermal effect also plays vital roles in other physical and chemical processes to produce electricity and chemical energy. For instance, a photo-thermal coating on a commercial thermo-electric device generates electricity under irradiation by producing a temperature difference between the two surfaces [21e23]. High-temperature catalytic reaction can be conducted at room temperature under irradiation by loading the catalyst on the surface of photo-thermal material [24,25].
http://dx.doi.org/10.1016/j.gee.2017.02.002 2468-0257/© 2017, Institute of Process Engineering, Chinese Academy of Sciences. Publishing services by Elsevier B.V. on behalf of KeAi Communications Co., Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Please cite this article in press as: X. Wu, et al., Harvesting, sensing and regulating light based on photo-thermal effect of Cu@CuO mesh, Green Energy & Environment (2017), http://dx.doi.org/10.1016/j.gee.2017.02.002
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There is no doubt that the photo-thermal effect will find numerous other applications when integrated with other systems. Herein, we develop a new application that is a light-intensity sensor and regulator based on the photo-thermal effect. CuO is one of the most important semiconductors with a wide array of applications in catalysis, gas sensing, lithiumion batteries and so forth [26]. Recently, CuO was found to be an excellent photo-thermal material for photothermal catalysis and optical sensor [24,27]. Compared to the other intensively utilized photo-thermal materials such as gold, graphene and carbon nanotubes, CuO is much more cost effective. To date, many metal and metal oxides have exploited the Seebeck effect that converts temperature gradients into voltages [28]. Inspired by these properties, we designed a CuO based device to harvest light and regulate its intensity. In this work, flexible Cu@CuO mesh was prepared. The mesh can absorb incident light and generates a high local temperature. The temperature difference between the hotspot and the cooler part of the Cu@CuO mesh yields a detectable voltage that can be converted into a digital readout. The digital signal can then be applied to control a motorized variable neutral-density (ND) filter to continuously adjust the transmitted light intensity to the desired value. This automated system shown in Fig. 1 can be applied to regulate natural sunlight for health (e.g. skin cancer prevention) and manufacturing (e.g. plant factories) benefits. It can be used for a variety of lighting systems to be deployed in smart greenhouses, stadiums and residential buildings to save electrical energy. Compared to the current light sensing materials and regulating technology [29e31], our system is more cost effective and more precise. 2. Experimental section 2.1. Materials (NH4)2S2O8 and NaOH were purchased from SigmaeAldrich and used without further purification. Milli-Q water
with a resistance of 18.2 MU cm1 was used for all experiments. 2.2. Preparation of Cu@CuO mesh The commercial Cu meshes (200 mesh, 2 1 cm) were first ultrasonicated in Milli-Q water and ethanol to remove contaminants before use. Then, the cleaned Cu mesh was immersed into a mixture of aqueous solution containing NaOH (2.5 M) and (NH4)2S2O8 (0.13 M) at room temperature. The reaction time was set to 2 h, 4 h, and 6 h separately. The resulting meshes were collected and rinsed with Milli-Q water and ethanol, then vacuum dried before further characterization. 2.3. Preparation of Cu@Au mesh The cleaned Cu mesh was immersed into HAuCl4 aqueous solution (0.01 wt%). The galvanic reaction took place on the surface of copper mesh, producing Au particles. After 4 h of reaction, the mesh was withdrawn and rinsed with Milli-Q water and ethanol for several times. 2.4. Characterization Field emission electron microscopy (SEM) images were obtained on a Zeiss Merlin SEM. X-ray photoelectron spectroscopy (XPS) analysis was carried out on a Kratos Axis Ultra with a Delay Line Detector photoelectron spectrometer using an Aluminium monochromatic X-ray source. The infrared (IR) photographs providing calibrated temperature readouts were captured by using an IR camera (FLIRE64501). The voltage measurements were acquired using a multimeter with its two terminals attached to different regions of the sample (i.e. hot and cold) via crocodile clips. The samples were suspended above a glass slide with white paper underneath to prevent the table and glass slide from heating up. The temperature and voltage measurement were conducted at the same time. 3. Results and discussion
Fig. 1. Illustration of the light intensity regulator composed of photo-thermal material of Cu@CuO mesh, analogue digital convert and a variable neutral density disc.
By simply immersing the Cu mesh in the alkaline (NH4)2S2O8 solution, a chemical reaction takes place on the surface of Cu mesh. It was observed that colour of the Cu mesh changed from yellow to blue and eventually to black (Fig. 2a). The SEM images depict the evolution of the surface morphology of the Cu mesh along with the reaction time. As shown in Fig. 2b and c, the cleaned Cu mesh is composed of Cu wires with a diameter of ~38 mm. The surface of the Cu wire is smooth with no obvious nanostructure. After 2 h of reaction, a furry structure emerged on the surface of Cu mesh (Fig. 2d). Highly magnified SEM image shows that this furry structure is composed of nanoribbons (Fig. 2e). The width and length of each nanoribbon is ~220 nm and ~11 mm respectively. As 1-dimensional (1D) nanoribbon is a typical morphology of Cu(OH)2 [32], it
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Fig. 2. (a) Digital photograph of the Cu meshes from different reaction times. SEM images of the Cu mesh after (b,c) 0 h, (d,e) 2 h, and (f, g) 4 h reaction in the alkaline solution with (NH4)S2O8.
implies the formation of Cu(OH)2 on the Cu mesh surfaces by oxidation at this stage. Further increasing the reaction time to 4 h leads to a significant surface morphology change. It can be clearly seen that the 1D nanoribbons were converted into 2D nanoplates, indicating the formation of CuO. When the reaction time was further increased to 6 h, no obvious morphology change was observed in Fig. S1, except corrosion of the Cu mesh (arrowed parts in Fig. S1). An XPS analysis was conducted to investigate the chemical evolution on the surface of Cu mesh during the reaction. The XPS spectrum of the 2 h mesh (Fig. 3a) depicts the photoelectron peaks at the binding energy of 934.5 and 933.2 eV, corresponding to the Cu 2P3/2 of Cu(OH)2 and Cu 2P3/2 of CuO. At this stage, only Cu(OH)2 nanowires and no CuO nanoplates were observed (Fig. 2d, e), which indicates that the main product is Cu(OH)2. However, the Cu(OH)2 begins to convert into CuO. It was noticed that after 4 h reaction, only the XPS peaks of Cu 2P3/2 of CuO (binding energy of 933.2 eV) was detected (Fig. 3b), indicating a completed conversion of Cu(OH)2, corresponding to the morphological change from nanoribbon (Fig. 2e) to nanoplate (Fig. 2g).
It can be suggested from Fig. 2f and g that the complex nanostructure on the Cu@CuO mesh can enhance light absorption and thus light-to-heat conversion [24,33]. In the following experiment, the photo-thermal effect of the Cu@CuO mesh is investigated. A solar simulator with adjustable power was used as the light source to irradiate a Cu@CuO mesh (Fig. 4a). An IR camera was used to record and analyse the surface temperature and temperature gradient of the Cu@CuO mesh under irradiation (Fig. 4bef). The Cu@CuO mesh obtained after 4 h of oxidation (denoted as Cu@CuO-4h) shows an excellent photo-thermal response. As shown in Fig. 4bef, when the incident light intensity is 0.70, 1.38, 1.88, 2.50 and 3.18 W/cm2, the maximum surface temperature of Cu@CuO-4h mesh reaches 51.9, 79.8, 101.8, 130.7 and 150.0 C respectively. In comparison, under the same irradiation the surface temperature of a Cu mesh (Fig. S2 and black square in Fig. 4g) is much lower than that of the Cu@CuO-4h mesh (red dot in Fig. 4g). The maximum surface temperature of Cu mesh is only 51.8 C, which indicates that Cu is not a good photo-thermal material. The Cu@CuO mesh obtained after 6 h of oxidation (denoted as Cu@CuO-6h) can reach a maximum surface temperature of 110.0 C (Fig. S3
Fig. 3. XPS spectra of the Cu mesh after (a) 2 h and (b) 4 h of reaction in the alkaline solution with (NH4)2S2O8. Please cite this article in press as: X. Wu, et al., Harvesting, sensing and regulating light based on photo-thermal effect of Cu@CuO mesh, Green Energy & Environment (2017), http://dx.doi.org/10.1016/j.gee.2017.02.002
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Fig. 4. (a) Photograph of the Cu@CuO-4h mesh undergoing temperature and electrical measurements; (bef) IR camera images of the Cu@CuO-4h mesh under irradiation with light intensities of 0.70, 1.38, 1.88, 2.50 and 3.18 W/cm2 respectively; (g) maximum surface temperature as a function of light intensity for the Cu mesh (black square), Cu@CuO-4h mesh (red dot), Cu@CuO-6h mesh (blue triangle) and Cu@Au mesh (pink triangle); (h) generated voltage over the Cu mesh (black square), Cu@CuO-4h mesh (red dot), Cu@CuO-6h mesh (blue triangle) and Cu@Au mesh (pink triangle) via the Seebeck effect.
and blue triangle in Fig. 4g), which is higher than that of the Cu mesh but lower than that of the Cu@CuO-4h mesh. These results confirm that CuO is an excellent photo-thermal material. The Cu@CuO mesh obtained after 4 h of oxidation of Cu mesh possesses superior photo-thermal performance. As one of the well-known photo-thermal materials, plasmonic Au particles, have been intensively investigated [34,35]. For comparison, we also prepared the Cu@Au mesh to investigate its photo-thermal effect. The Cu@Au mesh was synthesized by simply immersing the Cu mesh in the HAuCl4 solution. A galvanic reaction took place on the surface of Cu mesh which led to the formation of Au particles. As shown in Fig. S4a and b, after 4 h of galvanic reaction, the surface of the
Cu mesh was densely covered by particles. The SEM images with high magnification show that the particles are of polyhedral shape. The average particle size is ~0.8 mm. The XPS spectrum of the mesh verifies the binding energy of Au 4f7/2 and Au 4f5/2 (Fig. S4c), confirming the formation of Au particles on the Cu mesh surface. The photo-thermal effect of the Cu@Au mesh was also investigated. The surface temperature increased with the increment of incident light intensity (Fig. S5 and Fig. 4g). However, the maximum temperature is 97.5 C, which is much lower than that of Cu@CuO-4h mesh. Therefore, compared to the Cu@Au mesh, the Cu@CuO mesh has superior photo-thermal performance, while the cost of CuO is much lower than that of Au.
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When incident light was focused on one part of the Cu@CuO mesh shown in Fig. 4bef, the local surface temperature dramatically increased, while the temperature of the none-exposed part is much lower. This gives rise to a temperature gradient along the Cu@CuO mesh (Fig. 4bef). The electrons flow from the hotter region to the cooler region, inducing a voltage and a current, which is described by the Seebeck effect. The temperature gradient along the Cu@CuO mesh increased with increasing light intensity (Fig. 4bef). The maximum temperature-difference on the Cu@CuO-4h mesh reached ~120 C (Fig. 4f), which induced a voltage of ~74.5 mV shown in Fig. 4h. With the same incident light intensity, the Cu and Cu@Au mesh generated lower voltages of 52.5 and 69.0 mV respectively. The Cu@CuO-6h generated a maximum voltage of 74.0 mV, which is close to that of the Cu@CuO-4h mesh. Therefore, considering the cost of materials and manufacturing, the photo-thermal effect and the generated voltage, the Cu@CuO-4h mesh is the most promising candidate. Acting as a light sensor, the Cu@CuO-4h mesh generates a voltage upon irradiation that can be amplified and converted into a digital signal via an analogue-to-digital converter. This enables the feedback system demonstrated in Fig. 5 to control a motorized neutral-density (ND) disc for controlling the transmitted light intensity. The ND disc has a gradient of filtering ability, meaning the attenuation of light is a function of its rotation angle. As a result, the incident light intensity on a selected region behind the ND disc can be fine-tuned for specific applications. The algorithm and software behind the control was implemented by Labview, where the digital input signal was compared with a reference value. The difference signal was scaled then sent out as an analogue signal to drive the motorized ND disc. The video (Supporting information) demonstrates the process of light sensing and regulating. For practical applications where the area of daylighting is large,
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the ND disc can be replaced by a roll of graded fabric that shifts the correct patch of filter to the opening using a rotor. Supplementary video related to this article can be found at http://dx.doi.org/10.1016/j.gee.2017.02.002. 4. Conclusions In summary, we developed a light harvesting, sensing and regulating system based on the photo-thermal effect and Seebeck effect of a flexible Cu@CuO mesh. The mesh was synthesized by simple self-oxidation of Cu mesh in an alkaline solution. The Cu@CuO mesh showed superior photo-thermal response to the Cu@Au mesh. Under irradiation, the surface temperature of the Cu@CuO mesh reached 150.0 C. The large temperature discrepancy between the hotspot and the cool region of the Cu@CuO mesh induced a measurable voltage based on the Seebeck effect. The generated voltage was converted into digital signal which in turn controlled the motorized ND to adjust the intensity of incident light to the desired value. This system is cost effective and easily reproducible. It has great potential to be employed as light intensity controllers in greenhouses, factories and smart buildings to save energy and bring health benefits. Conflict of interest The authors declare no conflict of interests. Acknowledgements This work was supported by the Future Industries Institute, University of South Australia (Foundation Fellow).
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.gee.2017.02.002. References
Fig. 5. Photograph of the Cu@CuO-4h based light sensor and control system. Inset: enlarged view of the neutral-density disc and sensor.
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Please cite this article in press as: X. Wu, et al., Harvesting, sensing and regulating light based on photo-thermal effect of Cu@CuO mesh, Green Energy & Environment (2017), http://dx.doi.org/10.1016/j.gee.2017.02.002