AgI–Ag2S heterostructures for photothermal conversion and solar energy harvesting

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AgI–Ag2 S heterostructures for photothermal conversion and solar energy harvesting Wenxia Zeng, Lulu Suo, Canying Zhang, Daxiong Wu∗, Haitao Zhu∗ College of Materials Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042, PR China

a r t i c l e

i n f o

Article history: Received 15 January 2018 Revised 6 July 2018 Accepted 11 July 2018 Available online xxx Keywords: AgI–Ag2 S Heterostructure Photothermal efficiency Photothermal therapy Solar energy harvesting

a b s t r a c t Heterostructures are emerging as efficient optical absorbers for photothermal therapy and solar thermal applications because of their excellent photothermal properties. In the current work, AgI–Ag2 S heterostructures with enhanced and broadened optical absorption properties were prepared as photothermal materials. The synthesized AgI–Ag2 S heterostructures are more efficient than the corresponding AgI and Ag2 S nanopartilces and exhibit stronger absorption within the spectrum from 500 nm to 1100 nm. In comparison to the solar spectrum, the aqueous suspension containing 0.03 wt% AgI–Ag2 S heterostructures delivers a solar weighted absorption (Am ) value of 97.7% at a penetration distance of 1 cm, which means 97.7% solar energy has been absorbed. Under the illumination of an 808 nm laser at the power of 420 mW, the photothermal efficiency of the AgI–Ag2 S heterostructures is determined to be 52%. For the solar simulation experiment, the solar thermal efficiency of the suspension is evaluated to be 97%. The results of the current work provide a solid evident that heterostructures without metal nanoparticles can also have superior photothermal properties than the corresponding single component nanoparticles. It is also evident that AgI–Ag2 S heterostructures have potential applications in photothermal therapy and solar energy harvesting. © 2018 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

1. Introduction Photothermal materials have attracted tremendous research attention for their promising prospect in the applications of photothermal therapy [1–7], photothermal imaging [8–11], and solar energy harvesting [12–14]. Numerous efforts have been made to pursue high-performance photothermal materials including gold nanoparticles [15–20], silver nanoparticles [21–27], copper sulfide nanoparticles [28–31], grapheme [32–34], carbon nanotube [35–37], and so on. Recently, heterostructures (multicomponent structures with heterointerfaces) are emerging as a novel kind of photothermal materials with enhanced properties in comparison to the corresponding monocomponent photothermal materials [2,3,38–41]. Some heterostructures have broad absorption spectra and more importantly significant absorption in the near infrared region [3,42]. Such properties make them promising in the potential applications of photothermal therapy and imaging because near infrared ray can penetrate biological tissue. It has been pointed out that optical transmission through tissue is optimal in the near infrared region from 700 nm to 1100 nm. Therefore, photothermal conversion materials absorbing near infrared

∗

Corresponding authors. E-mail addresses: [email protected] (D. Wu), [email protected] (H. Zhu).

ray are frequently used in optical imaging and photothermal cancer therapy [43,44]. For instance, the absorption of Au@ZnS core@shell structure is obviously enhanced and the spectrum is broader than that of Au nanoparticles. In addition, the maximum absorption wavelength shifts to 809 nm from 500 nm (for Au nanoparticles) and the photothermal efficiency increases to 64% from 18% (for Au nanoparticles) [45]. Similarly, the absorption spectrum of Au–Cu2 −x Se nanocrystals is significantly broader than that of the Cu2− x Se nanoparticles. At 980 nm, the Au–Cu2− x Se nanocrystals deliver a high photothermal efficiency of 33%, in comparison to 28% for the Cu2− x Se nanoparticles [46]. On the other hand, some heterostructures exhibit broad absorption spectra across the ultraviolet-visible-near infrared regions, which cover the main spectrum of solar irradiation. Therefore, these heterostructures have been extensively investigated as photothermal materials for solar energy harvesting. For example, Xuan’s group reported a solar thermal efficiency of 20.9% for aqueous nanofluids containing TiO2 /Ag composite nanoparticles, which is significantly higher than that of the TiO2 nanofluids (16.07%) [47]. He’s group investigated the solar photothermal performance of bimetallic Au–Ag nanoparticles which demonstrated a solar photothermal efficiency of 41.37%, significantly higher than 31.7% for the blended Au–Ag nanoparticles [48]. Lee’s group proposed a direct solar thermal collector using SiO2− Au nanofluids as working fluids and reported an enhanced solar collector efficiency of 70% [49].

https://doi.org/10.1016/j.jtice.2018.07.012 1876-1070/© 2018 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

Please cite this article as: W. Zeng et al., AgI–Ag2 S heterostructures for photothermal conversion and solar energy harvesting, Journal of the Taiwan Institute of Chemical Engineers (2018), https://doi.org/10.1016/j.jtice.2018.07.012

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In the previous reports, most photothermal converting heterostructures contain plasmonic metal nanoparticles as crucial components. The superior performance in photothermal conversion has been mainly ascribed to the enhancement of surface plasmon resonance. However, is it indispensable to include metal nanoparticles in such photothermal converting heterostructures? Can the photothermal properties be enhanced in heterostructures without metal nanoparticles? In order to answer these questions, we prepared a metal-free heterostructure contains two components of silver iodide (AgI) and silver sulfide (Ag2 S). The photothermal properties of the prepared hererostructure were then investigated under the illumination of lasers (808 nm and 1064 nm) and a solar simulator. The AgI–Ag2 S heterostructures show enhanced photothermal properties in comparison to the corresponding AgI and Ag2 S nanopartilces. The results provide evidence that hererostructures without plasmonic metal nanoparticles can also achieve excellent photothermal efficiency. 2. Experimental section All chemicals were analytically pure agents and used without further purification. 2.1. Synthesis of the AgI nanoparticles Polyvinylpyrrolidone (PVP-K30, 0.54 g) was dissolved in distilled water (38.5 mL) under continuous stirring. Then 7 mL AgNO3 aqueous solution (0.1 M) was added followed by the addition of 7 mL KI aqueous solution (0.1 M).The mixture was then stirred in the dark for 30 min at room temperature, resulting in a suspension containing AgI nanoparticles [50]. 2.2. Synthesis of the AgI–Ag2 S heterostructures 7 mL thioacetamide (TAA) aqueous solution (0.15 M) was added dropwise to the above AgI suspension under continuous stirring. Then 40 mL of the resulting mixture was transferred into a 50 mL Teflon-lined stainless steel autoclave, which was sealed and maintained at 180 °C for 20 h. After the hydrothermal treatment, the precipitates were collected by centrifugal separation (10,0 0 0 rpm for 30 min) and washed with deionized water and absolute ethanol alternately for three times. The precipitates were then re-dispersed in deionized water to prepare AgI–Ag2 S suspensions. 2.3. Synthesis of the Ag2 S nanoparitals The Ag2 S nanoparticles were prepared at ambient temperature. 42.5 mg PVP-K30 and 42.5 mg AgNO3 were dissolved in deionized water (50 mL) under magnetic stirring. After stirring for 30 min, 7 mL TAA aqueous solution (0.15 M) was added drop by drop. The reaction was stopped after stirring for another 30 min. The resulting mixture was separated by centrifugation (10,0 0 0 rpm for 30 min) and the precipitates were washed with deionized water and absolute ethanol alternately for three times. The precipitates were then re-dispersed in deionized water to prepare Ag2 S suspensions.

absorption properties were investigated on a UV-vis-NIR spectrometer (Lambda 750S, transmission mode, suspension in quartz cell with 1 cm thickness). Hemispherical transmittance was measured on a spectrophotometer equipped with an integration sphere [51]. 2.5. Evaluation of the photothermal properties The photothermal properties under the illumination of infrared lasers and solar simulator were studied on a lab-made evaluation system with the schematic presented as Fig. S1. An 808 nm laser (Aunion Tech. LDP-808-1.5, 420 mW, 3 mm spot on sample surface), a 1064 nm laser (Aunion Tech. AUT-FCL-1064-10 0 0T, 420 mW, 3 mm spot on sample surface), and a solar simulator (Education Au-light CEL-PE300L-3A, 100 mW/cm−2 ) are equipped. The light source is a xenon lamp (20 0–120 0 nm) with a light filter (CELAM 1.5) to tune the spectrum in order to match the solar spectrum. In the typical testing procedure, suspension (1 mL for laser illumination, 2 mL for solar simulator illumination) was filled in a quartz cuvette (4 × 1 × 1 cm) and a thermocouple (±0.1 °C) was inserted in the suspension. The thermocouple was kept away from the direct illumination of light. The cuvette was surrounded by insulation to minimize the heat dissipation. The insulation was cut open to allow the cuvette to be exposed to light illumination (section area of 1.5 cm2 for solar simulator and 1.0 cm2 for lasers). The suspension was stirred continuously to achieve uniform temperature distribution. After the temperature in the suspension reached equilibrium with room temperature, the light source (lasers or solar simulator) was turned on. The temperature in the suspension increased gradually because of the photothermal effect. After a period of time, the temperature reached a maximum value and became stable. At this maximum temperature, the rate of heat generation from photothermal effect equaled to the rate of heat dissipation to the environment. Then the light source was turned off and the suspension was gradually cooled down to room temperature. The temperature in the suspension was detected by the thermocouple in real time and recorded for further analysis. As previously reported in the literatures [12,52–56], the light utilization efficiency (ηu ) of the suspensions can be calculated according to the below equation (see supporting information for more details),

ηu =

Bmw cw  Tmax Pin

(1)

where B is the heat dissipation constant, mw and cw are the mass and specific heat capacity of water in the suspension, Tmax is the maximum temperature rise in the suspension, and Pin is the incident power of illumination. Likewise, the photothermal conversion efficiency of the nanoparticles (ηc ) can be defined by the below equation,

ηc =

Bmw cw  Tmax − Q0



Pin 1 − 10−Aλ



(2)

In Eq. 2, Aλ is the absorbance of the nanoparticles and Q0 is the heat generation by the same mass of water under identical conditions. Other parameters have the same definition as those in Eq. 1.

2.4. Characterization of the samples 3. Results and discussion The phase composition of the as-prepared products was determined by X-ray diffraction (XRD) on a Rigaku D/MAX-2500/PC diffractometer. The morphology and size of the as-prepared products were studied on a JEM-20 0 0EX transmission electron microscope (TEM). An energy dispersive X-ray spectroscopy (EDS) analyzer attached to a FEI Magellan 400 SEM was used to analyze the elemental composition of the AgI–Ag2 S heterostructure. The optical

At the first step of the synthesis, AgNO3 reacts with KI to form AgI in the presence of PVP solution. Transmission electron microscopy (TEM) observation reveals that the AgI colloids are spherical particles of around 35 nm (Fig. 1a). The corresponding X ray diffraction (XRD) pattern (Fig. S2) shows the cubic phase of AgI. After TAA was added to the resulting AgI suspension, the mixture

Please cite this article as: W. Zeng et al., AgI–Ag2 S heterostructures for photothermal conversion and solar energy harvesting, Journal of the Taiwan Institute of Chemical Engineers (2018), https://doi.org/10.1016/j.jtice.2018.07.012

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Fig. 1. (a) TEM image of the AgI nanoparticles, (b) XRD patterns of AgI–Ag2 S heterostructure, (c) TEM image of the AgI–Ag2 S heterostructures, (d) HRTEM image of the AgI–Ag2 S heterostructure.

was subjected to the hydrothermal process after which a dark precipitate was collected. The XRD pattern (Fig. 1b) shows that the precipitates mainly contain monoclinic Ag2 S (JCPDS No. 14-0072) and cubic AgI (JCPDS No. 09-0399). Quantitative phase composition analysis based on the XRD pattern shows that the phase content of AgI is 38.5 wt%, and the rest is Ag2 S (Fig. S3). Moreover, the TEM image (Fig. 1c) reveals that the product consists of near-spherical particles with average size round 30 nm. There are two distinct regions with different contrasts in a single particle. More than 100 particles were analyzed and most of them are heterostructures with conjoint regions. In the high-resolution (HR) TEM image (Fig. 1e), the lattice spacing of 0.36 nm is coincident with (110) plane of monoclinic Ag2 S, while the lattice spacing of 0.23 nm is coincident with (200) plane of cubic AgI. Energy dispersive X-ray spectroscopy (EDS) analysis further confirms the co-existence of silver, sulfur and iodine, with the atomic ratio of 55:26:19. Accordingly, the content of AgI is determined to be 40 wt% which is close to the result from XRD analysis (38.5 wt%). Based on the above discussion, it is evident that AgI–Ag2 S heterostructures have been synthesized. In order to compare the properties of the synthesized AgI–Ag2 S heterostructures to that of the corresponding monocomponent nanoparticles, Ag2 S nanoparticles were also prepared. The TEM image and XRD pattern of the Ag2 S nanoparticles are shown in Fig. S4. The Ag2 S nanoparticles are of monoclinic phase and spherical shape with monodispersed size of 35 nm. The optical absorption spectra of the nanoparticles were presented as Fig. 2a. The spectrum for AgI nanopartilces exhibits the short-wavelength absorption edged at ∼420 nm and the absorbance is close to zero at the wavelength from 420 to 1100 nm. In contrast, the absorption of the Ag2 S nanoparticles is strong in ultraviolet region but decreases rapidly as wavelength increases across the visible region. At the wavelength from 800 to 1100 nm, the absorption of the Ag2 S nanoparticles becomes negligible. For the AgI–Ag2 S heterostructures, however, the absorption is significant in the whole detected spectrum. Importantly, the absorption is stronger at the wavelength from 500 to 1100 nm com-

pared to that of the AgI and Ag2 S nanoparticles. The comparison of the normal transmission spectrum and the hemispherical transmission spectrum of the suspension containing 0.01 wt% AgI–Ag2 S heterostructures is presented in Fig. S5. The results show minor difference between the hemispherical transmittance and normal transmittance of the suspension. By assuming that the difference between hemispherical transmittance and normal transmittance can represent the degree of scattering, we came to the conclusion that the scattering at low concentration (0.01 wt%) is not significant. As it is mentioned previously, photothermal materials for photothermal therapy are required to have significant absorption in the near infrared region, while broad absorption across the visible and near infrared regions favors the application in solar thermal conversion. Therefore, the AgI–Ag2 S heterostructures can be expected to show enhanced photothermal properties in comparison to the corresponding monocomponent nanoparticles. To evaluate the potential application of the AgI–Ag2 S heterostructures in photothermal therapy, a lab-made device (Fig. S1) equipped with near infrared lasers has been developed to investigate the photothermal properties of the aqueous suspensions containing AgI–Ag2 S heterostructures (0.01 wt%). The photothermal properties of aqueous suspensions containing AgI and Ag2 S nanoparticles (0.01 wt%) were also tested for comparison. The results (under the illumination of an 808 nm laser at 420 mW) are presented in Fig. 2b with water as reference. As previously mentioned, the temperature increases gradually when the suspension is exposed to the illumination of the laser. Then it reaches an equilibrium temperature at which the heat generation from photothermal conversion equals to the heat dissipation to the environment. According to the heat transfer theory, the heat dissipation is directly proportional to the temperature difference between the suspension and the environment which equals to the maximum temperature rise presented in Fig. 2b. Therefore, the maximum temperature rise can be applied to compare the heat generation from photothermal conversion. For the AgI suspension, the maximum temperature rise is 6.5 °C under the irradiation of the

Please cite this article as: W. Zeng et al., AgI–Ag2 S heterostructures for photothermal conversion and solar energy harvesting, Journal of the Taiwan Institute of Chemical Engineers (2018), https://doi.org/10.1016/j.jtice.2018.07.012

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Fig. 2. (a) UV–Vis–NIR spectra of AgI, Ag2 S and AgI–Ag2 S heterostructure nanoparticles, (b) Temperature rise for water and aqueous suspensions containing 0.01 wt% AgI, Ag2 S, and AgI–Ag2 S nanoparticles under the illumination of an 808 nm laser, (c) The maximum temperature rise for the aqueous suspensions (0.01 wt%) containing AgI and Ag2 S nanoparticles with different ratio under the illumination of an 808 nm laser, (d) The temperature rise for the AgI–Ag2 S heterostructure aqueous suspensions with different concentrations.

Table 1 The typical data of light utilization efficiency (ηu ) for suspensions containing 0.01 wt% nanopartilces. Materials

ηu

ηu

ηu

H2 O AgI Ag2 S AgI–Ag2 S

808 nm (%) – 4.9 ± 0.52 16.1 ± 0.59 33.4 ± 0.5

1064 nm (%) – 15.5 ± 0.55 18.5 ± 0.65 28.9 ± 0.58

Solar (%) 19.2 ± 0.55 25.4 ± 0.45 35.5 ± 0.57 50.6 ± 0.62

808 nm laser. The corresponding maximum temperature rise for the Ag2 S suspension is 12.9 °C. By comparison, the suspension containing AgI–Ag2 S heterostructure gives a maximum temperature rise of 26.7 °C, which is 141.1% and 120.7% as high as that of the AgI and Ag2 S suspensions. The light utilization efficiencies (ηu ) of the suspensions are presented in Table 1. The ηu for the AgI, Ag2 S, and AgI–Ag2 S suspensions are 4.9%, 16.1%, and 33.4%, respectively. The results recorded under the illumination of a 1064 nm laser (420 mW) show similar tendency. The maximum temperature rises for the AgI–Ag2 S, AgI, and Ag2 S suspensions are 27.8 °C, 15.8 °C and 17.8 °C, respectively (Fig. S6a). Likewise, the AgI–Ag2 S suspension exhibits much higher maximum temperature rise than the other two suspensions. The ηu for the AgI–Ag2 S suspension is calculated to be 28.9%, compared to 15.5% and 18.5% for the AgI and Ag2 S suspensions (Table 1). The results indicate that, compared to the corresponding monocomponent nanoparticles, the AgI–Ag2 S heterostructures show enhanced photothermal property in the near infrared region and therefore have potential application in photothermal therapy. To further demonstrate the enhanced photothermal property of the AgI–Ag2 S heterostructures, suspensions containing both AgI and Ag2 S nanoparticles with different mass ratios (1:9 to 9:1,

total concentration = 0.01 wt%) were prepared and tested under the illumination of the 808 nm laser (420 mW). The maximum temperature rises in the suspensions vary from 10.1 6.6 °C, as presented in Fig. 2c. It should be emphasized that all these maximum temperature rises are much lower than that of the suspensions containing AgI–Ag2 S heterostructure (26.7 °C) with the same total concentration. The results indicate that the enhanced photothermal property of the AgI–Ag2 S heterostructures cannot be achieved by simply mixing AgI and Ag2 S nanoparticles. Therefore, the enhanced photothermal property can be better attributed to the novel structure: the presence of heterointerfaces usually induces the quenching of radiative de-excitation process and therefore enhances the non-radiative de-excitation process and delivers more heat [57]. It is possible that the de-excitation of the excited electrons can easily turn into phonons or the vibration of atoms because of the loose packing of atoms at the heterointerfaces or boundaries. In this way, more energy goes towards heat generation instead of radiation. Thus the enhanced photothermal property is attributed to the formation of AgI–Ag2 S heterogeneous structures. The stability of the AgI–Ag2 S heterostructures is also an important factor to be considered. As illustrated in Fig. S7, the maximum temperature rises of the suspension containing the AgI–Ag2 S heterostructures show no obvious change after 4 cycles of photothermal heating and cooling within a time span of 550 min. The results indicate that the AgI–Ag2 S heterostructures are stable during the photothermal heating and cooling cycles. The concentrations of the AgI–Ag2 S suspensions can also influence the maximum temperature rise, as illustrated in Fig. 2e (808 nm laser, 420 mW). The maximum temperature rise increases from 9.5 °C to 16.5 °C, 26.7 °C, 38 °C and 42.5 °C as the concentration increases from 0.002 wt% to 0.005 wt%, 0.01 wt%, 0.02 wt% and 0.03 wt%. Therefore, it is possible to achieve different temperature levels by

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Fig. 3. (a) Transmittance spectra of water and suspensions containing 0.01 wt% AgI, Ag2 S and AgI–Ag2 S nanoparticles, (b) Solar spectral irradiance and the term I(λ)(1 − e − α (λ)l ) of water and the suspensions, (c) Solar weighted absorption fraction (%) spectrum for water and the suspensions at different penetration distances, (d) Temperature rises and the solar thermal efficiencies for water and the suspensions under the illumination of the solar simulator.

Table 2 The typical data of the light utilization efficiency (ηu ) of the suspensions and the photothermal conversion efficiency (ηc ) of the AgI–Ag2 S heterostructures. Concentration ηu H2 O 0.002% 0.005% 0.01% 0.02% 0.03% 0.05%

808 nm (%) – 11.9 ± 0.75 20.6 ± 0.68 33.4 ± 0.5 47.5 ± 0.58 53.1 ± 0.57 51.4 ± 0.7

ηc

ηu

ηc

ηu

808 nm (%) – 51.6 ± 0.66 50.1 ± 0.63 49.6 ± 0.56 51.5 ± 0.70 55.3 ± 0.58 53.2 ± 0.52

1064 nm (%) – 12.4 ± 0.53 20.8 ± 0.57 28.9 ± 0.61 35.8 ± 0.55 37.4 ± 0.67 41.5 ± 0.69

1064 nm (%) – 42.4 ± 0.56 40.1 ± 0.63 41.3 ± 0.65 39.4 ± 0.59 42.0 ± 0.52 43.7 ± 0.67

Solar (%) 19.2 ± 0.55 36.7 ± 0.65 41.9 ± 0.58 50.6 ± 0.62 85.3 ± 0.68 97.0 ± 0.57 96.1 ± 0.66

adjusting the concentration of the suspension in order to fulfill the requirement of different applications. The corresponding ηu for different concentrations are also calculated and presented in Table 2. The efficiency increases from 11.9% to 53.1% when the concentration increases from 0.002 wt% to 0.03 wt% (Table 2). With the light source changed to the 1064 nm laser, the experiments deliver similar results. The maximum temperature rise is 39.9 °C at the concentration of 0.05 wt%, and the corresponding ηu is calculated to be 41.5%. The photothermal conversion efficiencies (ηc ) of the AgI–Ag2 S heterostructures are also calculated and presented in Table 2. The data vary slightly because of the measurement error and the fluctuation. The typical efficien-

Table 3 The typical data concerning efficiency (under laser irradiation) and the corresponding concentration in the current work and the literatures. Materials

Efficiency (%)

Wavelength of Concentration Ref laser (nm) (Dose)

AgI–Ag2 S AgI–Ag2 S Au-Ag2 S hybrid Au-ZnS hybrid Au-Cu7 S4 hybrid Au-SiO2 hybrid Au-Au2 S hybrid Au-Cu2− x Se hybrid Fe3 O4 @CuS

53.1 41.5 64 86 63 35 60 32 20.7

808 1064 809 809 980 815 815 980 808

0.03 wt% 0.05 wt% 0.24 nM 0.24 nM 2.0 g/L 3 pM 300 pM 200 μg/mL 1 mg/mL

This work This work [45] [45] [42] [58] [58] [46] [59]

cies are 52% (808 nm) and 41% (1064 nm), and do not depend on concentration. The photothermal properties of the AgI–Ag2 S heterostructures are compared with the typical data reported previously, and the results are shown in Table 3. It can be seen that the photothermal properties of the AgI–Ag2 S heterostructures are comparable to most of the heterostructures containing gold component. It seems that, without the surface plasmon resonance of metal component, heterostructures can still have significant photothermal conversion effect. The presence of heterointerfaces in the

Please cite this article as: W. Zeng et al., AgI–Ag2 S heterostructures for photothermal conversion and solar energy harvesting, Journal of the Taiwan Institute of Chemical Engineers (2018), https://doi.org/10.1016/j.jtice.2018.07.012

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Fig. 4. (a) Transmittance spectra of water and suspensions containing various concentrations of AgI–Ag2 S nanoparticles, (b) Solar spectral irradiance and the term I(λ)(1 − e − α (λ)l ) for the AgI–Ag2 S suspensions with different concentrations, (c) Solar weighted absorption fractions for AgI–Ag2 S suspensions with different concentrations, and (d) Temperature rises for the AgI–Ag2 S suspensions with different concentrations under the illumination of the solar simulator.

heterostructures should be regarded as the main reason for the enhanced photothermal properties. As mentioned previously, the AgI–Ag2 S heterostructures show significant absorption in both visible and near infrared regions and therefore may have potential applications in solar energy harvesting. The transmission spectra of the aqueous suspensions containing AgI–Ag2 S heterostructures, AgI nanoparticles and Ag2 S nanoparticles are presented in Fig. 3a (penetration distance = 1 cm). The AgI suspension and the Ag2 S suspension show low transmittance below 400 nm but high transmittance over 500 nm. In contrast, the AgI–Ag2 S suspension shows low transmittance across the ultraviolet-visible-near infrared bands which cover the main spectrum of the solar irradiation. Such property enables the application in solar energy harvesting. The absorbing ability of the suspensions can be described as the solar weighted absorption fraction Am which is defined by the following equation, [60]



Am =



∫λλmax I (λ ) 1 − e−α (λ)l dλ min

∫λλmax I (λ )dλ

(3)

min

where I(λ) is the spectral solar irradiance, α (λ) is the absorption coefficient and l is the penetration distance. By assuming that scattering is negligible for small particles (less than 40 nm), one can calculate α (λ) from the transmittance according to the Beer–Lambert Law [61,62]. Then the term I(λ)(1 − e − α (λ)l ) can be calculated at a fixed penetration distance of 1 cm. The results are

presented in Fig. 3b together with I(λ). The area under the curve of I(λ) represents the total energy of solar irradiation from 300 nm to 2500 nm, whereas the area under the curve of I(λ)(1 − e − α (λ)l ) represents the energy absorbed by the suspension. As can be seen from Fig. 3b, the curve of I(λ)(1 − e − α (λ)l ) for water covers only a small area in the near infrared region, which stands for less than 20% of the total energy of solar irradiation. For the AgI–Ag2 S suspension, however, the term I(λ)(1 − e − α (λ)l ) increases significantly in comparison to that of the AgI suspension and Ag2 S suspension, especially at the wavelengths below 1100 nm. To precisely compare the absorbing ability of the suspensions, the Am value was calculated from Eq. 2 and the results are presented in Fig. 3c. In general, the Am value increases as the penetration distance increases. At a penetration distance of 1 cm, the Am value for water is lower than 20% while AgI suspension and Ag2 S suspension deliver Am values of 28.7% and 43.5%, respectively. In contrast, the Am value for the AgI–Ag2 S heterostructure suspension is determined to be 65.5%. The results indicate that the AgI–Ag2 S heterostructure suspension (0.01 wt%) can absorb 65.5% solar energy at a penetration distance of 1 cm. The solar thermal property of the AgI–Ag2 S heterostructures was evaluated by exposing the suspensions (0.01 wt%) to the illumination of a solar simulator. The temperature profiles are presented in Fig. 3d. The AgI–Ag2 S heterostructure suspension gives a maximum temperature rises of 13.7 °C, compared to 7.9 °C for water, 10.4 °C for the AgI suspension and 11.1 °C for Ag2 S suspension. Determined from Eq. S10, the corresponding solar

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thermal efficiencies for water, the AgI suspension, the Ag2 S suspension, and the AgI–Ag2 S heterostructure suspension are 19.2%, 25.4%, 35.5%, and 50.6%, respectively (Table 1). The results indicate that the AgI–Ag2 S heterostructures have solar thermal properties superior to that of the AgI and Ag2 S nanoparticles. The concentrations of the suspensions can significantly impact the solar thermal efficiency. The transmittance spectra of the suspensions with various concentrations of the AgI–Ag2 S nanoparticles are presented in Fig. 4a. The transmittance drops as concentration increases. Fig. 4b shows that I(λ)(1 − e − α (λ)l ) approaches to the solar spectral irradiance I(λ) when the concentrations of the AgI–Ag2 S suspensions increase. At the concentration of 0.02 wt%, the curve of I(λ)(1 − e − α (λ)l ) comes very close to the curve of solar spectral irradiance I(λ). As shown in Fig. 4c, Am for the AgI–Ag2 S suspension increases rapidly from 65.5% to 97.7% when the concentration increases from 0.01 wt% to 0.03 wt% at a penetration distance of 1 cm. The maximum temperature rises for the AgI–Ag2 S suspension under the illumination of the solar simulator also increase as the concentrations increase (Fig. 4d). The maximum temperature rise increases from 9.9 °C to 20.7 °C as the concentration increases from 0.002 wt% to 0.03 wt%, and the corresponding solar thermal efficiency increases from 36.7% to 97% at a penetration distance of 1 cm (Table 2). Further increase in concentration would not enhance the solar conversion efficiency because most incident energy has been absorbed at the concentration of 0.03 wt%. The efficiency should be stable at higher concentrations. The efficiency for 0.05 wt% is slightly lower than that for 0.03 wt% probably because of experimental fluctuation and the slightly increased scattering of light at an increased concentration. 4. Conclusion In this study, AgI–Ag2 S heterostructures were prepared through a wet chemical method with AgI nanoparticles as templates and TAA as sulfur source. The AgI–Ag2 S heterostructures have significant absorption within a broad spectrum from 350 nm to 1350 nm. In comparison to the corresponding AgI nanoparticles and Ag2 S nanoparticles, the AgI–Ag2 S heterostructures show enhanced photothermal properties under the illumination of both infrared lasers and solar simulator. Such enhanced photothermal properties cannot be achieved by simply mixing AgI nanoparticles and Ag2 S nanoparticles, but should be attributed to the presence of heterointerfaces in the heterostructures. The current work provides evidence that heterostructures can exhibit enhanced photothermal effect even if plasmonic metal components are not included in the heterostructures. Acknowledgments This study was financially supported by the National Natural Science Foundation of China (51741206 and 51472134) and the Natural Science Foundation of Shandong Province (ZR2017MEM004). Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi: 10.1016/j.jtice.2018.07.012. References [1] Jaque D, Martınez Maestro L, del Rosal B, Haro-Gonzalez P, Benayas A, Plaza JL, Martın Rodrıguez E, Garcıa Sole J. Nanoparticles for photothermal therapies. Nanoscale 2014;6(16):9494–530. doi:10.1039/c4nr00708e. [2] Lu F, Wang J, Yang L, Zhu J. Facile one-pot synthesis of colloidal stable, monodispersed, highly PEGylated CuS@mSiO2 nanocomposites for the combination of photothermal and chemotherapy. ChemComm 2015;51(46):9447–50. doi:10.1039/C5CC01725D.

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Please cite this article as: W. Zeng et al., AgI–Ag2 S heterostructures for photothermal conversion and solar energy harvesting, Journal of the Taiwan Institute of Chemical Engineers (2018), https://doi.org/10.1016/j.jtice.2018.07.012