Article pubs.acs.org/JPCC
Designing a Novel Photothermal Material of Hierarchical Microstructured Copper Phosphate for Solar Evaporation Enhancement Zhentao Hua, Bing Li, Leilei Li, Xiaoyu Yin, Kezheng Chen,* and Wei Wang* Lab of Functional and Biomedical Nanomaterials, College of Materials Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042, China S Supporting Information *
ABSTRACT: Hierarchical microstructured copper phosphate (HCuPO), which could accelerate water evaporation was well designed based on d−d transition of 3d electrons in Cu2+ and fabricated via a solvothermal method. A very strong vis−NIR absorption with the maximum at 808 nm was observed for the HCuPO. Upon irradiation of 808 nm NIR laser light, the HCuPO generated heat with a light-to-heat converting efficiency of 41.8%. The reason for this high efficiency was investigated and assigned to a high probability of nonradiative relaxation, which released the energy in form of heat, happened to the excited 3d electrons of Cu2+. The proposed photothermal mechanism was quite different from the surface−plasmon mechanism of other Cu-based photothermal materials. By adding HCuPO into polydimethylsiloxane (PDMS), HCuPO−PDMS composite sheets were fabricated. Due to the intrinsic hydrophobicity of PDMS matrix, the sheets were floatable on water surface and the heat generated by HCuPO was confined within water−air interface region. A much sharper temperature gradient and more rapid increase of surface temperature were observed compared with the HCuPO−water dispersion in which the HCuPO particles were dispersed in water. Porous HCuPO−PDMS sheets were fabricated in order to further accelerate water evaporation. Under 808 nm laser irradiation with power density of 1000−2000 W·m−2, water evaporation rate of salt water (3.5 wt %) was measured to be 1.13− 1.85 kg·m−2·h−1 for porous floating HCuPO−PDMS, which was 2.2−3.6 times of that measured for ordinary salt water without HCuPO. By using a solar simulator as a light source, a very high solar thermal conversion efficiency of 63.6% was obtained with a power density of 1000 W·m−2, indicating that solar evaporation of salt water could be greatly enhanced by the well-designed HCuPO.
1. INTRODUCTION Water evaporation of seawater driven by solar energy is a crucial process for the thirsty Earth. With increased demand of fresh water in arid countries, tremendous efforts have been devoted to develop new technologies to enhance the solar evaporation rate. For this purpose, light-to-heat converting materials, also termed as photothermal materials, are attracting increasing attention of scientists. Light-to-heat behavior is a process in which photon energy of a specific light is absorbed by photothermal materials and then converted into heat. The photothermal materials can be delivered to solar evaporation ponds and produce heat under sunlight, providing enhanced solar evaporation efficiency without doing harm to water quality. Visible (vis) and near-infrared (NIR) light absorbing materials are more desirable for practical applications in solar evaporation because visible and NIR light both compose nearly 40% of the total solar energy. NIR light responsive photothermal materials are attracting more interest since the wavelength of NIR light is larger than most water drops in cloud and mist and hence can avoid the blocking of cloud and © XXXX American Chemical Society
mist much easier than visible light. As a result, solar energy harvesting from NIR light is more effective than visible light in cloudy days. Being the most well-known NIR responsive photothermal agent, gold (Au) nanostructures,1−3 which show intense NIR absorption subjected to surface plasmon mechanism, have been widely investigated. Aqueous solution containing Au nanoparticles4 and plasmonic film based on Au nanoparticles5 that could accelerate water evaporation were reported by previous literatures. In spite of excellent photothermal effect, practical applications of Au nanostructures are greatly limited due to its high price and poor photostability after a long period of laser irradiation.6 To overcome the drawbacks of Au-based photothermal materials, some new types of light-to-heat nanomaterials including Ge nanocrystals,7 copper chalcogenide nanocrystals,5,8−11 Fe3O4 nanoparticles,12 carbon-based nanomaterials,13−16 WS2 nanosheets,17,18 WO3‑x nanostructures,19,20 and conducting polymers21−23 with high Received: September 5, 2016 Revised: December 14, 2016 Published: December 15, 2016 A
DOI: 10.1021/acs.jpcc.6b08975 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C
was greatly enhanced by the membrane, and self-healing ability was accomplished by PPy, which could preserve a large number of hydrophobic fluoroalkylsilane moieties and release them to migrate to membrane surface so as to restore surface hydrophobicity. In this work, we fabricated HCuPO−PDMS composite sheets, which can float on water surface. PDMS is selected as the matrix for four reasons. The first reason, also the most important reason, is the intrinsic hydrophobicity of PDMS. PDMS has a very small surface energy density of about 20 mN· m−1, which is slightly higher than the well-known hydrophobic PTFE (19 mN·m−1). Consequently, PDMS composites usually have excellent water repellence. The second reason is the good weather resistance of PDMS. Benefitted from high energy of Si−O bonding, aging rate caused by sunlight and air is very slow for PDMS. The third reason is the easy fabrication and low cost of PDMS composites, which is highly desirable for practical applications. We found that HCuPO powder was easily dispersed in PDMS precursor and that HCuPO−PDMS composites with high HCuPO weight percentage could be obtained. The fourth reason is the recyclability of PDMS composite. In real applications of solar evaporation, a reusable photothermal system is more preferred. At present stage, real applications of many photothermal materials are limited by fragility of their sheets/films, which makes them impossible to be reused.5 PDMS is a flexible polymer, and we could fabricate HCuPO−PDMS composite sheets easily to be retracted and recycled. These distinguished properties make the obtained HCuPO−PDMS sheets highly feasible in commercial applications.
photothermal conversion efficiency, good photostability, and low cost have been developed in the past few years. Some of these materials have been utilized in solar evaporation. Although excellent advances have been provided, most of them are used for photothermal therapy of tumors, rather than solar evaporation. Besides noble metal nanoparticles, only polypyrrole (PPy)24 and carbon-based materials including amorphous carbon,25 exfoliated graphite,26 carbon black nanoparticles,27 and carbon nanotubes28 have been successfully utilized in solar evaporation. There is much room for further improvements in developing new type of photothermal materials as well as their applications in solar evaporation. The first requirement that a photothermal material should satisfy is an efficient absorption of optical radiation. Copper compounds usually show strong vis−NIR absorption due to d− d transition of Cu2+. Huang’s group reported the Cu2(OH)PO4 as the first NIR-activated photocatalyst known to man in 2013.29 Then some other copper compounds were found to be photocatalytically active under NIR irradiation.30 High absorption in NIR light region ascribed to Cu2+ is one of the preconditions for these copper compounds to act as excellent NIR-activated photocatalysts. This phenomenon inspired our interest in searching for novel copper-based vis−NIR responsive photothermal material. Microstructure is another factor that can influence optical absorption of nanoscaled materials. In order to enhance the photothermal conversion efficiency of CuS, Tian’s group synthesized flower-like CuS superstructures and revealed that the photoabsorption can be remarkably enhanced due to multiple reflection of 980 nm laser among the CuS flower petals.9 The role of hierarchical structures as excellent lasercavity mirrors was later confirmed in WO3−x irradiated by 915 nm NIR light.20 In this work, we synthesized hierarchical microstructured Cu3(PO4)2·3H2O (HCuPO) with strong vis−NIR absorption and pronounced photothermal performances. The mechanism of light-to-heat behavior of HCuPO upon NIR light irradiation was investigated in detail. To the best of our knowledge, this is the first report on photothermal properties of copper phosphate. In order to enhance the energy efficiency, localized heating at the air−water interface should be realized since it can generate a sharper temperature gradient with the same level of energy input.24 As a result, the photothermal materials should be able to float on water surface. Hydrophobic surface modification of photothermal materials is a good strategy to make them floatable. Wang and co-workers25 fabricated floating Fe3O4/C magnetic particles by carbonization of poly(furfuryl alcohol) (PFA) incorporated with Fe3O4 nanoparticles, in which amorphous carbon acted as the photothermal material. Due to their hydrophobicity and low density, Fe3O4/C particles were floatable on water and enhanced the water evaporation rate by a factor of 2.3 of salt water. By using a magnet, Fe3O4/C particles were easily recycled. From the view of Wang’s group,24 surface hydrophobicity given by alkyl or fluoroalkyl group modification may deteriorate due to exposure to highly oxidative chemicals in water and air. To overcome this problem, they fabricated a photothermal membrane with hydrophobicity self-healing capability by deposition of PPy on stainless steel (SS) mesh followed by fluoroalkylsilane modification of the PPy coating. Since the surface morphology of the membrane conformed to Wenzel’s model, it could spontaneously stay at the water surface. Water evaporation rate
2. EXPERIMENTAL SECTION 2.1. Materials. Copper(II) sulfate pentahydrate (CuSO4· 5H2O), diammonium hydrogen phosphate [(NH4)2HPO4], glycol, and sodium chloride (NaCl) were obtained from Shanghai Chemical Corporation (Shanghai, China). PDMS precursor was purchased from Dow Corning (SYLGARD 184). The water used in this study was deionized by a milli-Q Plus system, whose electrical resistance is 18.2 MΩ. 2.2. Synthesis of HCuPO and HCuPO−PDMS Composite Sheets. In a typical process, 2 mmol of CuSO4·5H2O dissolved in glycol (25 mL) and 1 mmol of (NH4)2HPO4 dissolved in deionized water (20 mL) were mixed under vigorous stirring to obtain a homogeneous solution. Afterward the mixture was transferred into a 50 mL Teflon-lined autoclave, sealed, and maintained at 120 °C for 3 h, then allowed to cool to the room temperature naturally. The resulting product was collected by centrifugation, washed with distilled water and ethanol for several times, and finally dried in vacuum for 6 h. HCuPO−PDMS composite sheets were fabricated by adding dry HCuPO powder into PDMS precursor, and then cured at 100 °C for 2 h in a mold made by two pieces of sandpaper (600-grit). In order to expel air bubbles, the precursor was vacuumed before curing. The weight percent of HCuPO powder within the composite sheets was varied from 10% to 40%. To fabricate porous HCuPO−PDMS sheets, a medical device called Dermapen with multiple needles was used to penetrate the sandpapers and HCuPO−PDMS precursor throughout the curing process. By adjusting the reaction conditions, sheet-like CuPOs were obtained. Typically, 0.5 mmol of CuSO4·5H2O dissolved in glycol (24 mL) and 0.5 mmol of (NH4)2HPO4 together with 0.5 g of sodium dodecyl sulfate (SDS) dissolved in deionized B
DOI: 10.1021/acs.jpcc.6b08975 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C
Perfect Light Co., China) was employed to simulate real solar irradiation. A radiation meter (LM8, Coherent, USA) was used to measure the light intensity of 808 laser and solar simulator. The temperature was recorded by an online type thermocouple thermometer (UT325, Uni-Trend Technology Co., Ltd., China). An analytical balance (BSA124S, Sartorius, German) was used to monitor the weight loss of water during evaporation tests. Digital IR thermal images were captured by a thermal imaging camera (Seek Thermal XR, USA).
water (8 mL) were mixed under vigorous stirring to obtain a homogeneous solution. Afterward the mixture was transferred into a 50 mL Teflon-lined autoclave, sealed, and maintained at 120 °C for 4 h, then was cooled to the room temperature. The resulting product was collected in the way similar to that for HCuPO. 2.3. Measurement of Temperature Rise and Photothermal Conversion Efficiency (η1) of HCuPO. For the temperature rise of HCuPO solution, 808 nm NIR laser was delivered through a 4 mL quartz cuvette containing 0.3 mL of HCuPO dispersion with different concentrations with a power density output of 0.36, 0.90, and 1.44 W. The NIR laser was 20 cm away left aside from quartz cuvette with a spot size of 0.72 cm2, and the power density was calculated to be 0.50, 1.25, and 2.0 W·cm−2, respectively. A thermocouple with an accuracy of ±0.1 °C was inserted into the dispersion perpendicular to the laser path. In order to avoid direct irradiation of the laser on the probe, the laser spot was focused on the left corner of the quartz cuvette, whereas the probe was located on the right corner with a distance of about 0.9 cm to the laser spot. The temperature was recorded per 10 s for 10 min. For the photothermal conversion efficiency (η1) of HCuPO, 1.5 mL of HCuPO−water dispersion (7.5 mg·mL−1, 1.52 g) was suspended in a quartz cuvette and irradiated by 808 nm NIR laser at an output of 1.44 W for 14 min (laser on), followed by naturally cooling to room temperature without NIR laser irradiation for 20 min (laser off). The NIR laser was 20 cm away left aside from quartz cuvette with a spot size of 0.72 cm2. The temperature was recorded one time per 30 s using a thermocouple thermometer with an accuracy of ±0.1 °C perpendicular to the path of the laser. 2.4. Water Evaporation Measurement of HCuPO− PDMS Composite Sheets. Water evaporation experiments were carried out with 4.5 mL of salt water (3.5% NaCl) in a 1.0 cm × 1.0 cm quartz cuvette with 4.5 cm in height at constant ambient temperature of 22 °C. Each HCuPO−PDMS sheet was cut into a square with edge length of 0.95 cm to cover the salt water surface. A solar simulator or 808 nm NIR laser was set on top of the water surface. Light radiation intensity was tuned from 1000−2000 W·m−2 with the assistance of a radiation meter. A block of polystyrene foam with low thermal conductivity was put between the balance tray and quartz cuvette in order to minimize heat loss from water caused by high thermal conductivity of the metallic tray. Weight of water loss was monitored by an analytical balance. 2.5. Characterization. XRD studies were conducted on a Rigaku D/max-2500 X-ray powder diffractometer using Cu Kα radiation (λ = 1.5406). The morphological investigations were carried out with field-emission scanning electron microscopy (FESEM, JEOL JSM-6700F). UV−vis−NIR spectrophotometer (Cary500, Varian) was utilized to collect the optical absorption spectra of HCuPO. Photoluminescence spectra were recorded by a fluorescence spectrometer (F4600, Hitachi Ltd., Japan) using a solid-state 808 nm laser as the excitation source at room temperature. Photothermal conversion efficiency experiments were carried out using a continuouswave diode NIR laser (T808D3W, Xi’an Minghui Optoelectronic Technology, China) with a center wavelength of 808 ± 10 nm. The laser beam diameter at aperture was 3 mm × 3 mm. Water evaporation experiments were performed by another 808 nm NIR laser (LSR808, Ningbo Yuanming Laser Technology, China) with a much larger light spot in order to cover the water surface of 1 cm × 1 cm. A solar simulator (PLS-SXE300C,
3. RESULTS AND DISCUSSION 3.1. Composition and Morphology Characterization. Figure S1 (Supporting Information) shows the XRD patterns of the products. All the diffraction peaks can be ascribed to the copper phosphate hydroxide hydrate Cu3 (PO 4 ) 2 ·3H 2 O (JCPDS No. 22−0548). SEM images in Figure 1 demonstrate
Figure 1. SEM images of HCuPO.
that the products have flower-like hierarchical structure with a diameter of 2−5 μm. Each flower is assembled by nanosheets with thickness of several nanometers. As mentioned above, hierarchical structures constructed by plenty of nanosheets may favor reflection of light beam and enhance photoabsorption of HCuPO. By adjusting the reaction conditions, sheet-like CuPOs that do not have hierarchical structures (Figure S2A, Supporting Information) were obtained for the purpose to investigate the effect of morphology on photoabsorption. XPS spectra (Figure 2) indicate that Cu 2p3/2, O 1s, and P 2p1/2 have a main peak at 936.0, 531.6, and 133.8 eV, respectively, which are in good agreement with Cu3(PO4)2 reported elsewhere.32 The peak at 284.8 eV is indexed to C 1s, and the presence of C element in the HCuPO sample may be attributed to residual organic groups produced from glycol solvent used in the preparation process. 3.2. Photothermal Properties of HCuPO. The photothermal properties of the prepared HCuPO were assessed by the value of photothermal conversion efficiency (η1). Usually, η1 was measured under a light source with a specific wavelength rather than a broad wavelength band. In order to compare the η1 value of our HCuPO with other reported materials, we employed a laser light source with good monochromaticity to carry out the experiments. The wavelength was selected as 808 nm, which exactly matched the absorption maximum (λmax) of HCuPO (Figure 3A). Before measuring, HCuPO powder was dispersed in ultrapure water to obtain HCuPO−water dispersion samples. As shown in Figure 3B, compared to pure water a significantly greater temperature rise was exhibited by HCuPO−water samples under 808 nm laser irradiation. With gradient HCuPO concentration of 1.25−7.5 mg·mL−1 and laser power density of 0.50−2.0 W·cm−2, remarkable temperature increases of 8.7− 12.9, 14.9−29.2, and 20.1−45.1 °C were observed after 10 min laser irradiation, whereas a tiny increase of only 1.0−4.0 °C was C
DOI: 10.1021/acs.jpcc.6b08975 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Figure 2. XPS survey scan, and Cu 2p, O 1s, P 2p, and C 1s spectra of HCuPO.
⎛ T − Tsurr ⎞ t = −τs ln θ = −τs ln⎜ ⎟ ⎝ Tmax − Tsurr ⎠
obtained for pure water. Figure S3 (Supporting Information) shows digital photographs of HCuPO powder on papers before and after irradiation by 808 nm laser for a few seconds. It is clear to see that the paper under the HCuPO powder was burned out by the heat generated from HCuPO. In comparison, no trace of burn could be found on the bare paper with other conditions being equal. These data unambiguously proved the excellent photothermal properties of HCuPO. The energy from NIR light photons could be absorbed and then converted to heat highly efficiently by HCuPO. The value of conversion efficiency, η1, was calculated using the following eq 1:33,34 η1 =
where Tmax is the equilibrium temperature, Tsurr is ambient temperature of the surroundings, and (Tmax − Tsurr) was 22.5 °C according to Figure 3C. τs was calculated to be 331.99 s. Qdis represents heat dissipated from light absorbed by the quartz sample cell itself and was calculated independently to be 15.324 mW using a quartz sample cell containing pure water without HCuPO. I is incident laser power (1.44 W), and A808 is absorbance intensity of the sample at 808 nm, which was measured to be 2.878 for HCuPO−water dispersion (7.5 mg· mL−1) from its UV−vis−NIR absorption spectrum (Figure 3A). Thus, the 808 nm laser heat conversion efficiency of the prepared HCuPO was calculated to be 41.8%, which is lower than poly(oligo(ethylene glycol) methacrylate)-stabilized polypyrrole (PPy) nanoparticles (47%, 808 nm)35 and Cu7.2S4 nanocrystals (56.7%, 980 nm),6 but higher than most of the other reported photothermal agents including PPy−Au nanoparticles (24.0%, 808 nm),36 Cu9S5 nanocrystals (25.7%, 980 nm), 34 Cu 2−xSe nanocrystals (22%, 980 nm),11 WO 3−x hierarchical nanostructures (28%, 980 nm), 20 and WS 2 nanosheets (32.83%, 808 nm).18 For comparison, light-to-heat behavior of sheet-like CuPOs was also investigated by the same manner. As shown in Figure S2B, smaller temperature increase was observed for sheet-like CuPOs with other conditions being equal. This phenomenon
hS(Tmax − Tsurr) − Q dis I(1 − 10−A808)
(3)
(1)
where h is heat transfer coefficient, S is the surface area of the container, and the value of hS was determined by measuring the rate of temperature drop after removing the light source according to eq 2: cm hS = τs (2) where c and m are heat capacity (4.2 J·g−1·K−1) and mass (1.52 g) of the sample, respectively, and τs is the time constant determined by applying the linear time data from the cooling period versus negative natural logarithm of driving force temperature (Figure 3D) according to eq 3: D
DOI: 10.1021/acs.jpcc.6b08975 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Figure 3. (A) UV−vis−NIR absorption spectrum of HCuPO−water dispersion (7.5 mg·mL−1, 1.5 mL). (B) Temperature rise (ΔT) of HCuPO− water dispersion (0.3 mL) with gradient concentrations under 808 nm laser irradiation. (C) Photothermal effect of the HCuPO−water dispersion (7.5 mg·mL−1, 1.5 mL) with the NIR laser (808 nm, 1.44 W) in which the irradiation lasted for 690 s, and then the laser was shut off. (D) Time constant for heat transfer from the system is determined to be τs = 331.991 s by applying the linear time data from the cooling period (after 960 s) versus negative natural logarithm of driving force temperature, which is obtained from the cooling stage of panel C.
change was observed either in the UV−vis−NIR spectra or the morphology of HCuPO before and after 808 nm laser (2.0 W· cm−2) irradiation for 30 min (data not shown). Figure 5 shows the photothermal cycling behavior of 2.5 mg·mL−1 HCuPO dispersed in water for an on−off laser irradiation (808 nm, 2.0 W·cm−2) procedure for six cycles. Almost invariable temperature rise of 25 °C was obtained after repetitive process for six cycles, further indicating the photostability of the HCuPO, which favors the use of long-term practical applications.
coincided with the weaker absorption of sheet-like CuPOs observed at 808 nm in its UV−vis−NIR diffuse reflection spectrum (Figure 4). These results indicated that the
Figure 4. UV−vis−NIR diffuse reflection spectra of copper phosphate powder with different morphology.
photothermal properties of CuPOs are influenced by its morphology. Hierarchical structures favor multiple reflection of 808 nm laser among the CuPOs flower petals and enhance the photoabsorption of 808 nm light, which supports the conclusion made by Hu’s group for their CuS and WO3−x superstructures.9,20 Photostability was also investigated for our HCuPO. We carried out SEM characterization and UV−vis−NIR diffuse reflection measurement after laser illumination. Almost no
Figure 5. Photothermal cycling of HCuPO−water dispersion (2.5 mg· mL−1, 0.3 mL) for an on−off laser irradiation procedure (2.0 W·cm−2). E
DOI: 10.1021/acs.jpcc.6b08975 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Figure 6. Contact angles of HCuPO−PDMS sheets cured in (A) an ordinary steel mold and (B) a sandpaper box. (C−F) Weight loss profiles of 3.5% salt water with and without HCuPO−PDMS sheets under 808 nm laser or solar simulator irradiation with varied power density.
3.3. Water Evaporation Accelerated by HCuPO−PDMS Sheets. Encouraged by its desirable light-to-heat converting properties, we investigated the performance of the HCuPO as a photothermal material in water evaporation. In order to enhance evaporation rate, the photothermal material used for water evaporation should be floatable on water surface to generate localized and concentrated heat at water−air interface. Although HCuPO quickly sank to the bottom of water, the HCuPO−PDMS sheets could easily float on water surface (Figure S4, Supporting Information), indicating our success of fabricating floatable HCuPO. The contact angle was measured to be 97.6° as shown in Figure 6A. For enhanced hydrophobicity, we further fabricated HCuPO−PDMS sheets with nanostructured rough surface. It has been known that surface morphology greatly influences the wetting/antiwetting behavior of materials. The excellent water repellence of lotus leaf is believed to be the result of their unique nanostructured surface. Plenty of superhydrophobic surfaces with water contact angles more than 120° have been fabricated by mimicking the
nanostructured rough surface of lotus leaf. For this purpose, we employed sandpaper boxes as molds to make HCuPO−PDMS sheets. Before curing, HCuPO−PDMS prepolymer was poured into sandpaper boxes and then vacuumed to expel air bubbles. After being cured, the rough surface morphology of sandpapers could be well replicated by HCuPO−PDMS sheets. As shown, water contact angle was measured to be 124.0° (Figure 6B), which was 26.4° larger than the HCuPO−PDMS sheet cured in an ordinary steel mold (Figure 6A). So the excellent floating ability of HCuPO−PDMS sheet was ascribed to not only the intrinsic hydrophobicity of PDMS matrix but also the surface morphology of HCuPO−PDMS sheets. Figure 6C−E shows the weight loss profiles of salt water with and without HCuPO−PDMS sheets under 808 nm laser light irradiation. When the light power density was set to be 1000 W· m−2, the evaporation rate was measured to be 0.57−0.76 kg· m−2·h−1 for nonporous HCuPO−PDMS sheets floating on the water surface with HCuPO weight content of 10−40%, whereas a much slower rate of 0.50 kg·m−2·h−1 was measured for salt F
DOI: 10.1021/acs.jpcc.6b08975 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C water without HCuPO. As anticipated, the floating sheet with highest HCuPO percentage (40%) exhibited best performances. For example, the evaporation rate measured at 2000 W· m−2 increased from 0.94 to 1.53 kg·m−2·h−1 as the HCuPO percentage increased from 10% to 40%. IR images of HCuPO− PDMS sheet samples (0.95 cm × 0.95 cm) with varied HCuPO percentage (10%, 25%, 40%) demonstrated that under illumination of 2000 W·m−2 for 5 min the temperature of the 40% sample rose to 130 °C, while the 10% sample could only rise to 115 °C (Figure 7).
HCuPO−PDMS sheet, the upper surface temperature reached 56 °C as recorded in the top view image (Figure 8A). In the side view image (Figure 8B), a localized hot zone was observed in the top portion of water. The highest temperature was detected at the surface as 38 °C, but the temperature at the bottom was only 26 °C. The results indicated that the heat was well confined within the air−water interface region by floating HCuPO due to interface heating effect. Bulk water that could not participate in evaporating did not consume the heat generated by HCuPO. In contrast, for the dispersed HCuPO, the highest temperature was detected at the bottom of water (Figure 8D) as the result of precipitation of dispersed HCuPO to the bottom (Figure S4B, Supporting Information). The temperature of the water surface was measured to be 32 °C (Figure 8C), which was slightly lower than the bottom temperature, indicating that the heat was invalidly consumed by the bulk water rather than the surface water. As a result, the evaporation rate accomplished by floating HCuPO was much higher than that by dispersed HCuPO (1.53 vs 1.03 kg·m−2· h−1). As shown in Figure S4A, bubbles were observed under the HCuPO−PDMS sheet during the heating process. This phenomenon suggested to us that the PDMS sheet impeded the escape of water steam, and a porous sheet could resolve this problem. The experiment results identify that faster evaporation could be accomplished by floating HCuPO−PDMS sheets with porous structure. With equivalent HCuPO content of 40% and power density of 2000 W·m−2, the evaporation rate of porous HCuPO−PDMS sheet was measured to be 1.85 kg·m−2·h−1, while 1.53 kg·m−2·h−1 for nonporous HCuPO−PDMS. According to the data obtained from water evaporation experiments, the light energy to heat of water evaporation conversion efficiency (η2) of the photothermal system can be calculated by eq 4:24
Figure 7. IR images of nonporous HCuPO−PDMS composite sheets with varied HCuPO percentage of (A) 10%, (B) 25%, and (C) 40% under 808 nm laser illumination of 2000 W·m−2 for 5 min.
In order to confirm the interface heating effect, we also measured the evaporation rate of 4.5 mL of salt water in which HCuPO powder was dispersed. The total mass of HCuPO was determined to be 58.5 mg, equal to that of HCuPO contained in 0.146 g of HCuPO−PDMS composite sheet (0.95 cm × 0.95 cm, 1.25 mm in thickness) with HCuPO percentage of 40%. Figure 8A−D shows IR images of 4.5 mL of salt water filled in a quartz cuvette with floating HCuPO−PDMS and dispersed HCuPO, respectively, under equal power density of 2000 W· m−2 and equal illumination time of 30 min. For the floating
η2 =
Qe Qs
(4)
where Qs is the incidence light power density of 1000 W·m−2 and Qe is the power of evaporation of the water, which can be calculated by eq 5: Qe =
dmHe = vHe dt
(5)
where m is the evaporation mass, t is the evaporation time, and v is the evaporation rate. He is the heat of water evaporation with a value of 2260 kJ·kg−1. In this work, v was measured to be 1.13, 0.76, and 0.67 kg· m−2·h−1 for porous floating HCuPO−PDMS, nonporous floating HCuPO−PDMS, and dispersed HCuPO (Figure 6C−E and Figure S5, Supporting Information), respectively, with equal HCuPO amount of 58.5 mg under the power density of 1000 W·m−2. Then η2 was calculated to be as high as 70.9% for the porous floating HCuPO−PDMS, whereas only 47.7% and 42.0% for nonporous floating HCuPO−PDMS and dispersed HCuPO, respectively. An even lower η2 value of 31.6% was obtained for the control group, i.e., salt water without HCuPO. HCuPO−PDMS composite sheets with different thickness were prepared to investigate the influence of sheet thickness on the photothermal effect. As shown in Figure S6A (Supporting Information), v was measured to increase from 0.50 to 1.13 kg· m−2·h−1 for porous HCuPO−PDMS as the thickness increased
Figure 8. IR images of salt water filled in a quartz cuvette with (A,B) nonporous floating HCuPO−PDMS and (C,D) dispersed HCuPO, which were irradiated by 808 nm laser (2000 W·m−2, 30 min). G
DOI: 10.1021/acs.jpcc.6b08975 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C
Figure 9. Schematic illustration of light-to-heat converting mechanism of HCuPO under 808 nm laser light irradiation.
3.4. Photothermal Mechanism of HCuPO. The performances of a photothermal material are determined by how well it absorbs light and also how well it converts the light into energy. The blue curve in Figure 4 shows the UV−vis−NIR diffuse reflectance spectrum of HCuPO. The peak in the UV region is attributed to ligand-to-metal charge transfer (LMCT) from ligand orbitals to Cu2+ orbitals.30 A very strong vis−NIR absorption band from 460 to 1200 nm with the maximum at 808 nm was observed, suggesting that our HCuPO meets the first requirement of an excellent vis−NIR photothermal material. This broad strong band is characteristic of Cu(II) compounds and attributed to d−d transitions of Cu2+ with a 3d9 electron configuration.29,30 It indicates to us that under the irradiation of vis−NIR light 3d electrons of Cu2+ can be excited from ground state to high level states within d bands. According to theoretical calculations by Huang et al., who reported Cu2(OH)PO4 as the first NIR-activated photocatalyst,29−31 in Cu2(OH)PO4 there are unoccupied d bands located at 0.79−1.89 eV above the Fermi energy in the bandgap. Due to the small energy gap between occupied d bands and unoccupied d bands, 3d electrons of Cu2+ can be excited by NIR photons and leave holes in valence bands. This feature is a precondition for Cu2(OH)PO4 to be photocatalytically active under NIR excitation. As a result, although the energy of NIR photons is much lower than the band gap (Eg) of Cu2(OH)PO4 (2.86 eV),29 electron−hole pairs still can be generated by NIR irradiation. A later study on other copper compounds further confirmed their conclusion.30,31 This is very different from traditional photocatalysts that require light photons with energy larger than the band gap to generate electron−hole pairs. With the UV−vis−NIR diffuse reflectance data, the Eg value of the prepared HCuPO was calculated to be 2.88 eV (Figure S7, Supporting Information), which is much higher than 808 nm laser photons (1.53 eV). Since our HCuPO shares many similarities in both chemical composition and NIR absorption characteristics with the aforementioned copper based NIR photocatalysts, we believe that electron−hole pairs can also generate in HCuPO under 808 nm laser irradiation. So we can assume that the prepared HCuPO is a semiconductor similar to Cu2(OH)PO4 in which electrons can easily jump from occupied d bands to unoccupied d bands upon 808 nm laser excitation. A large number of laser photon-excited charge carriers (denoted as e− in Figure 9) generated and left holes (denoted as h+ in Figure 9) in occupied d bands. Each carrier gained energy approximately equal to the energy gap between the occupied d bands and unoccupied d bands of the HCuPO, as happened in Cu2(OH)PO4. However, different from
from 0.30 to 1.25 mm. These results indicated that although PDMS is not a good heat conductor, water beneath the composite sheet still could be heated. The reasons may lie in two aspects. On one hand, block of heat-transfer in the sheet caused by air gaps was avoided as evidenced from the SEM image captured for the cross-section of HCuPO−PDMS sheet (Figure S6B, Supporting Information), which demonstrated that HCuPO contacted PDMS matrix without air gaps. On the other hand, PDMS is intrinsically a transparent polymer and does not greatly absorb laser light in NIR band. From the UV− vis−NIR absorption spectra in Figure S6C (Supporting Information), one can see that HCuPO−PDMS sheet exhibited a much stronger absorption band in the range of 460−1200 nm than pure PDMS sheets. With the same output of 314 mW (Figure S6D, Supporting Information), laser power detected by the radiation meter, which was put below the pure PDMS sheet with a thickness of 1.25 mm, was 301 mW (Figure S6E, Supporting Information), whereas 0 mW was detected in the case of HCuPO−PDMS sheet (1.25 mm, 40% HCuPO, Figure S6F, Supporting Information). In other words, NIR laser could transmit pure PDMS sheets but could not transmit HCuPO− PDMS sheet with equal thickness of 1.25 mm. As a result though the laser illuminated upper surface of HCuPO−PDMS sheet, NIR photons could interact with HCuPO on the bottom surface, where the heat generated could compensate heat loss resulting from the poor heat conductivity of PDMS. A solar simulator was further employed as a light source to simulate real solar irradiation. As shown in Figure 6F, the evaporation rate of 3.5% NaCl solution under a power density of 1000 W·m−2 was enhanced to 1.01 kg·m−2·h−1, which was 1.71 times of control group (0.59 kg·m−2·h−1), by porous floating HCuPO−PDMS sheet (40% HCuPO). η2 was calculated to be 63.6%, 56.7%, 40.2%, and 37.0% for porous floating HCuPO−PDMS, nonporous floating HCuPO−PDMS, dispersed HCuPO, and control group, respectively. In order to further analyze the contribution of vis−NIR light to evaporation, a filter with a cutoff of 420 nm was equipped in front of the Xe light so that UV light (1 μm) Imaging and Photothermal Cancer Therapy with Carbon Nanotubes. Nano Res. 2010, 3, 779−793. (14) Burke, A.; Ding, X. F.; Singh, R.; Kraft, R. A.; Levi-Polyachenko, N.; Rylander, M. N.; Szot, C.; Buchanan, C.; Whitney, J.; Fisher, J.; et al. Long-Term Survival Following a Single Treatment of Kidney Tumors with Multiwalled Carbon Nanotubes and Near-Infrared Radiation. Proc. Natl. Acad. Sci. U. S. A. 2009, 106, 12897−12902. (15) Yang, K.; Feng, L. Z.; Shi, X. Z.; Liu, Z. Nano-Graphene in Biomedicine: Theranostic Applications. Chem. Soc. Rev. 2013, 42, 530−547. (16) Feng, L. Z.; Yang, X. Z.; Tan, X. F.; Peng, R.; Wang, J.; Liu, Z. Polyethylene Glycol and Polyethylenimine Dual-Functionalized NanoGraphene Oxide for Photothermally Enhanced Gene Delivery. Small 2013, 9, 1989−1997. (17) Cheng, L.; Liu, J. J.; Gu, X.; Gong, H.; Shi, X. Z.; Liu, T.; Wang, C.; Wang, X. Y.; Liu, G.; Xing, H. Y.; et al. PEGylated WS2 Nanosheets as a Multifunctional Theranostic Agent for In Vivo Dual-modal CT/ photoacoustic Imaging Guided Photothermal Therapy. Adv. Mater. 2014, 26, 1886−1893. (18) Yong, Y.; Zhou, L. J.; Gu, Z. J.; Yan, L.; Tian, G.; Zheng, X. P.; Liu, X. D.; Zhang, X.; Shi, J. X.; Cong, W. S.; et al. WS2 Nanosheet as a New Photosensitizer Carrier for Combined Photodynamic and Photothermal Therapy of Cancer Cells. Nanoscale 2014, 6, 10394− 10403. (19) Liu, J. H.; Han, J. G.; Kang, Z. C.; Golamaully, R.; Xu, N. N.; Li, H. P.; Han, X. L. In Vivo Near-infrared Photothermal Therapy and Computed Tomography Imaging of Cancer Cells Using Novel Tungsten-based Theranostic Probe. Nanoscale 2014, 6, 5770−5776. (20) Li, B.; Zhang, Y. X.; Zou, R. J.; Wang, Q.; Zhang, B. J.; An, L.; Yin, F.; Hua, Y. Q.; Hu, J. Q. Self-assembled WO3‑x Hierarchical J
DOI: 10.1021/acs.jpcc.6b08975 J. Phys. Chem. C XXXX, XXX, XXX−XXX
