Sacrificial Template Synthesis and Photothermal Conversion

Apr 17, 2013 - several hundred nanometers deep with the pinnacles pointing to the center of the microspheres. It is expected that such cavities may le...
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Sacrificial Template Synthesis and Photothermal Conversion Enhancements of Hierarchical and Hollow CuInS2 Microspheres Daxiong Wu, Jinfeng Duan, Canying Zhang, Kai Guo, and Haitao Zhu* College of Materials Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042, P.R. China. S Supporting Information *

ABSTRACT: Hierarchical and hollow CuInS2 microspheres are synthesized on the basis of sacrificial templates with solid and hollow CuS microspheres as sacrificial templates, respectively. Transformations from CuS solid microspheres to CuInS2 hierarchical microspheres and CuS hollow microspheres to CuInS2 hollow microspheres can be achieved by a solvothermal process at 180 °C for 24 h with N,Ndimethylformamide as both solvent and reductant. The products are well characterized, and the formation mechanisms are proposed. The assynthesized products have strong optical absorption from 1000 to 2000 nm in addition to normal absorption from 400 to 800 nm, which has not be reported in the literature. The as-synthesized products exhibit a significant photothermal conversion effect under an irradiation of a 1064 nm laser. At a laser power of 0.05 W·cm−2, up to 30% and 20% enhancement of photothermal conversion is observed in the aqueous suspension containing 0.1 wt % CuInS2 hollow microspheres and hierarchical microspheres, respectively. The mechanisms for the enhancements of photothermal conversion in the as-synthesized products are also discussed. from occupied states to the unoccupied states.10−12 The enhancement in NIR photothermal conversion of flowerlike CuS superstructures is attributed to their excellent cavity-mirror effect, leading to a great improvement of the reflection and absorption ability for laser.12 Given that hollow structures may have a cavity-mirror effect even stronger than that of flowerlike superstructures, there is such an inspiration from previous reports10−12 that material with many unoccupied states and a hollow structure may exhibit enhanced photothermal conversion properties. CuInS2, whose band gap is about 1.5 eV, possesses unique optical properties.14−18 Furthermore, there are more unoccupied states in the electronic structure of CuInS2 in comparison to that of CuS, and thus, CuInS2 may exhibit better NIR photothermal conversion properties. Given that cavities may greatly enhance the reflection and absorption NIR ray, CuInS2 hollow structures and CuInS2 hierarchical structures with cavities might be promising candidates for photothermal conversion applications. According to the reported data in the literature, CuInS2 specimens including nanodisks,19 nanocrystals,20−23 films,24,25 flower vaselike nanostructure arrays,26 nanorods27,28 and quantum dots29−31 exhibit strong optical absorption within the wavelengths range from 400 to 800 nm. However, the research on the infrared absorption properties of CuInS2 is scarce because CuInS2 is conventionally regarded as an

1. INTRODUCTION In recent years, photothermal conversion materials have become a hot topic for their potential applications in photothermal therapy of cancer,1−3 as well as in controllable release and delivery of gene and drugs,4−6 visualization of biological components on the basis of photothermal interference contrast,7,8 and so on. These materials are so attractive because they can convert near-infrared (NIR, λ = 700−1100 nm) optical energy into thermal energy, and near-infrared ray can penetrate biological tissues with less absorption.9 However, most of these studies are based on plasmonic noble metals, especially Au nanostructures, whereas semiconductors are seldom included. The reason for this is that semiconductors are thought to absorb ultraviolet rays or visible light rather than infrared rays and exhibit a photoelectric effect rather than a photothermal effect. However, it is not necessarily always the case. Very recently, Chen’s and Li’s groups have reported the significant photothermal conversion properties of CuS nanoparticles under an irradiation of an 808 nm laser.10,11 Hu’s group has developed flowerlike CuS superstructures and found that the photothermal conversion efficiency of the superstructures under the irradiation of a 980 nm laser has been improved by a magnitude of 50% compared to that of their building blocks.12 Chen’s group has also reported the enhanced photothermal conversion of CuS nanoparticles by surface plasmon coupling of Au nanoparticles.13 As a CuS nanoparticle is a semiconductor with a band gap of about 1.85 eV (550 nm absorption edge), the NIR photothermal conversion properties of CuS nanoparticles are attributed to interband transitions © 2013 American Chemical Society

Received: January 23, 2013 Revised: March 24, 2013 Published: April 17, 2013 9121

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Max r-A diffractometer. The X-ray photoelectron spectroscopy (XPS) experiments were carried out on a Thermao Escalab 250 system using an Al KR X-rays as the excitation source. The scanning electron microscopy (SEM) images and energy dispersive spectrum (EDS) were taken with FESEM-6700 field-emission scanning electron microscope equipped with an energy dispersive X-ray analyzer (Oxford INCA Energy). The transmission electron microscopy (TEM) images were captured on a JEM-2000EX transmission electron microscope. The UV−vis diffuse reflectance spectra (DRS) of the synthesized powder were recorded on a Varian Cary 500 Scan UV−vis−NIR spectrometer from 200 to 2500 nm, with BaSO4 as the background. 2.3. Evaluations of the Photothermal Conversion Properties. The as-synthesized products were dispersed in distilled water under ultrasonic oscillation to get 0.1 wt % suspensions for photothermal conversion property evaluation. Schematic image of the evaluation system is presented as Figure 1. The system is equipped with an AUT-FSL semiconductor/

absorber material for solar cells. Infrared absorption is not expected in that case as it will cause heat generation and consequently bring adverse effect to the operation of solar cells. On the contrary, infrared absorption and heat generation are favorable to photothermal conversion applications. Thus, it would be very interesting to investigate the infrared absorption and photothermal conversion properties of CuInS2 hollow structures and hierarchical structures. Hollow structures are conventionally prepared thought hard template approach in which templates have to be preprepared and removed afterward.32 Such disadvantages are avoided in a sacrificial template strategy where the templates sever as reactants of the forming reaction of shell materials and are subsequently consumed.33 In this paper, we report on the synthesis of CuInS2 hierarchical microspheres (with open cavities) and CuInS2 hollow microspheres (with close cavities) through a sacrificial template strategy. Then synthesis mechanisms are discussed for better understanding of the strategy. We further investigate the optical properties, especially infrared absorption of the synthesized products, as they are critical to photothermal conversion properties. Finally, we evaluate the photothermal conversion properties of the synthesized products by setting up a system to directly monitor the temperature elevations of aqueous droplets containing 0.1 wt % of the synthesized products under laser irradiation. The enhancement of photothermal conversion effect is then figured out through calculations. The results and understanding arising from the present work will be helpful to the design and synthesis of photothermal conversion agents.

2. EXPERIMENTAL SECTION 2.1. Synthesis of the Products. Reagents involved in the synthesis including CuSO4·5H2O, InCl3·4H2O, thioacetamide, polyvinylpyrrolidone (PVP-k90), and N,N-dimethylformamide (DMF) were of analytical grade and used without further purification. The synthesis of CuInS2 hierarchical microspheres proceeded via a solvothermal route based on a sacrificial template strategy. In a typical synthesis procedure, 0.125 g CuSO4·5H2O, 0.5 g PVP, 0.147 g InCl3·4H2O, and 0.1 g thioacetamide were dissolved in 40 mL DMF to form a solution under magnetic stirring at room temperature. The resulting solution was then transferred into a 50 mL Teflon-lined stainless steel autoclave and maintained at 180 °C for 24 h. The obtained black, solid products were collected by centrifugation, and then washed several times with deionized water and absolute ethanol. Finally, the products were dried in a vacuum at room temperature. CuInS2 hollow microspheres were also synthesized based on a sacrificial template strategy with CuS hollow microspheres as templates. The synthesis procedure of the CuS hollow microspheres was presented in our previous work.34 In the present work, 0.048 g CuS hollow microspheres, 0.074 g InCl3·4H2O, and 0.050 g thioacetamide were added into 40 mL DMF under magnetic stirring at room temperature and then transferred into a 50 mL Teflon-lined stainless steel autoclave. The autoclave was then maintained at 180 °C for 24 h. The resulting products were collected and then washed several times with deionized water and absolute ethanol. Finally, the products were dried in a vacuum at room temperature. 2.2. Characterizations and Optical Absorption Properties Analysis. The phase purity of the as-prepared products was determined by X-ray diffraction (XRD) on a Rigaku D/

Figure 1. Schematic image of the evaluation system of photothermal conversion property.

solid state laser (Aunion Tech. Co., Ltd., Shanghai, China) with a fixed wavelength at 1064 nm. A microliter syringe is applied to control the volume of the droplet to be tested. A thermocouple with an accuracy of ±0.1 °C is fixed immediately to the needle of the microliter syringe and connected to the computer through a temperature collector. The microliter syringe and the thermocouple are fixed at such a position that, when there is no droplet hanging at the end of the needle, the laser beam can pass through just below the needle without hitting the needle or the thermocouple. Thus, the laser beam can be received completely by an optical power and energy meter (THORLABS, PM100D), and the power of the laser beam is recorded digitally. In a typical test procedure, the power of the laser beam is adjusted to be 0.05 W·cm−2 (higher power may lead to elevated temperature and subsequent evaporation of the droplet). Then the laser beam is shut out by a light chopper. A droplet of 10 μL is extruded from the microliter syringe and hanged at the end of the needle. The tip of the thermocouple is immersed in the droplet. After the thermocouple shows an equilibrium temperature close to room temperature (26.8 °C in the present work), the laser beam is let go by shifting the light chopper. Now the laser beam is completely blocked by the droplet, and thus, the optical power and energy meter reads zero. The increasing temperature of the droplet is detected and recorded at a time interval of one second. After 60 s of irradiation, the laser beam is again shut out, and the decreasing temperature of the droplet is also detected and recorded. 9122

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smooth surfaces and diameters ranging from 0.8 to 1.0 μm. An inserted TEM image in Figure 2b shows that the microspheres are solid particles. The results indicate that the main products at the early stage of the solvothermal process are CuS microspheres. As the solvothermal process continues, CuS microspheres may act as sacrificial templates that will be gradually consumed and transformed into CuInS2 microspheres. The typical XRD pattern of the as-prepared products collected at the final stage (24 h) of the solvothermal process is also shown in Figure 2a. All peaks in the pattern can be indexed to tetragonal CuInS2 (JCPDS No. 85-1575) without detectable impurities such as CuS, Cu2S, or In2S3. The SEM image (Figure 2c) shows that the final products are microspheres with diameters range from 1 to 1.2 μm. The dark center of the microsphere in the TEM image (inserted in Figure 2c) suggests that the microsphere has a solid central part while the pale area along the perimeter indicates that the perimeter of the microsphere is built by very thin flakes. The SEM image (Figure 2c) reveals that the final products possess highly developed hierarchical structures built by many interleaving two-dimensional (2D) flakes, which is consistent with the TEM image. It seems that several flakes adjoin with one another to form a bunch and many bunches align together at different orientation, forming pyramidal open cavities distributed on the entire surface of the microspheres. The pyramidal cavities are several hundred nanometers wide and several hundred nanometers deep with the pinnacles pointing to the center of the microspheres. It is expected that such cavities may lead to cavity-mirror effects. The composition and valence states of the hierarchical microspheres were further investigated by XPS analysis, with the typical survey and high-resolution spectra shown in Figure 2d. The survey spectrum displays three elements including copper, indium, and sulfur. The core level spectrum of Cu2p is split into Cu2p3/2 (931.3 eV) and Cu2p1/2 (951.3 eV), corresponding to the binding energy of Cu+ but quite different from that of Cu 2 + . 3 5 Therefore, the starting Cu 2 + (CuSO4·5H2O) has been reduced to Cu+ in the reactions that will be discussed in the mechanism section. The In3d core level spectrum exhibits a strong peak at 444.2 eV, corresponding to the In3d5/2 binding energy for CuInS2.35 As was pointed out in the previous reports,28−30 two peaks in the S2p core level spectrum, centered at 161.1 and 162.5 eV are related to Cu−S and In−S, respectively. The binding energies of Cu 2p, In3d, and S 2p are consistent with those of CuInS2 reported in the literature.21,28,29,35,36 It could be concluded that the chemical composition of the hierarchical microspheres is CuInS2. 3.2. Characterization of CuInS2 Hollow Microspheres. CuS hollow microspheres, the sacrificial templates for CuInS2 hollow microspheres, can also be synthesized via sacrificial templates approach, which was presented in our previous works.34 The morphology of the CuS hollow microspheres used in this work is presented as Figure 3a and the corresponding XRD pattern is included in Figure 3e. The diffractions in the XRD pattern of CuS hollow microspheres can be indexed to hexagonal covellite-phase CuS (JCPDS No. 06−0464). Average diameter of the CuS hollow microspheres is about 900 nm and the shell thickness is about 200 nm. It is expected that the templates will transform to CuInS2 hollow microspheres after reaction for 24 h under the current experimental conditions. TEM image (Figure 3b) of the as-

3. RESULTS AND DISCUSSIONS 3.1. Characterization of CuInS2 Hierarchical Microspheres. The XRD pattern (Figure 2a) of the as-synthesized products collected at the early stage (1 h) of the solvothermal process can readily be indexed to hexagonal covellite-phase CuS (JCPDS No. 06-0464). The corresponding SEM image (Figure 2b) reveals that the products are microspheres with relatively

Figure 2. (a) XRD patterns of the as-synthesized products at different stages of solvothermal process, (b) SEM and TEM images of CuS microspheres, (c) SEM and TEM images of CuInS2 hierarchical microspheres and (d) XPS spectra of CuInS2 hierarchical microspheres. 9123

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pattern presented in Figure 3e confirms that the resulting hollow microspheres are of CuInS2 phase (JCPDS No. 85− 1575). It should be noted that most hollow microspheres are perfectly shaped without collapse attributed to the low speed of the gradually conversion from CuS phase to CuInS2 phase as will be discussed in the next section. 3.3. Mechanism of the Sacrificial Templates Synthesis of CuInS2 Microspheres. Copper ions exist as Cu2+ in CuSO4·5H2O and CuS, but as Cu+ in CuInS2 correspondingly. Reduction of Cu2+ to Cu+ is required so as to synthesize CuInS2 hierarchical microspheres from CuSO4·5H2O, as well as to synthesize CuInS2 hollow microspheres from CuS hollow microspheres. In the present case, additional reductants are not necessary because DMF works as both solvent and reductant simultaneously.37 In the synthesis of CuInS2 hierarchical microspheres, however, the whole process can occur in two possible ways. (1) Cu2+ in free ions state is reduced directly to Cu+ by DMF and then reacts with S2− and In3+ in the solution to form CuInS2. (2) Cu2+ reacts with S2− to form CuS, and then Cu2+ on the surface of CuS is reduced to Cu+ followed by simultaneous reaction of Cu+ with S2− and In3+ to form CuInS2. It seems that the process occurs in the latter way because the reduction rate of Cu2+ to Cu+ is much slower in comparison to the hydrolysis rate of thioacetamide to produce S2−. As there are mainly Cu2+ (instead of Cu+) and S2− in the solution at the early stage of the process, CuS precipitates for its very low solubility. Thus, the products at the early stage of the solvothermal process are CuS microspheres. These CuS microspheres, as well as the CuS hollow microspheres in the synthesis of CuInS2 hollow microspheres, will serve as sacrificial templates. The formation process of CuInS2, including the reduction of Cu2+ in CuS to Cu+ by DMF, and then the reaction of Cu+ with S2− and In3+ to form CuInS2, requires longer reaction time. The mechanism of the synthesis of CuInS2 microspheres is illustrated as Figure 4. The sacrificial templates, CuS microFigure 3. (a) TEM and SEM (insets) images of CuS hollow microspheres, (b) TEM and SEM (insets) images of CuInS2 hollow microspheres, (c) SAED pattern and (d) EDS spectrum of CuInS2 hollow microspheres, (e) XRD patterns of CuS and CuInS2 hollow microspheres.

synthesized products reveals the hollow structure of the resulting microspheres. The average diameter of the resulting hollow microspheres is about 1 μm, which is slightly larger than that of the CuS hollow microspheres, the templates. The shell thickness of the resulting hollow microspheres increases to 250 nm accordingly. The significant difference between the resulting hollow microspheres and the templates lies in the surface feature. The resulting hollow microspheres possess hierarchical architecture while the templates hollow microspheres exhibit relatively smooth surface. The morphology evolution tendency is analogous to that of the CuInS2 hierarchical microspheres. The selected area electron diffraction (SAED) pattern of the resulting hollow microspheres is shown as Figure 3c. The SAED rings can be indexed to (112), (200)/ (204) and (312)/(116) of CuInS2 phase. EDS analysis of the resulting hollow microspheres shown in Figure 3d demonstrates that the chemical components only consist of Cu, In, and S, and their molar ratio is 1:0.9:2.1, which is close to the stoichiometry of CuInS2. The signal of carbon is attributed to the binder fixing the sample to the holder. Finally, the XRD

Figure 4. Suggested formation mechanism for the hierarchical and hollow CuInS2 microspheres.

spheres and CuS hollow microspheres, possess relatively smooth surfaces and smaller diameters in comparison to the final products. As the solubility of CuS is extremely low, the concentration of Cu2+ is so low that Cu2+ can only exist on the surface of the templates, without diffusing into the body of the solution. Under the present solvothermal circumstance, Cu2+ on the surface of the templates can be reduced to Cu+ by DMF, at a very low speed. The resulted Cu+ then reacts with S2− and In3+ to form CuInS2. The newly formed CuInS2 exists as small flakes on the surfaces of the templates, and thus, the surfaces 9124

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become rougher. It should be emphasized that the Cu+ also cannot leave the surfaces and diffuse into the body of the solution for their extremely low concentration. Thus, CuInS2 can only be formed gradually on the position where Cu+ exists. It is the so-called in situ transformation mechanism by which the shape and structure of the sacrificial templates will be inherited by the final products. As the solvothermal process continues, in situ transformation of CuS toward CuInS2 extends from the surface toward the center of the sacrificial templates. The newly formed CuInS2 flakes gradually grow, interleave, connect, and develop hierarchical structures. Finally, the sacrificial templates, CuS microspheres and CuS hollow microspheres, completely transform to hierarchical and hollow CuInS2 microspheres. The diameters of the final products are slightly larger than that of the sacrificial templates due to the highly developed hierarchical structures. 3.4. Optical Absorption Properties of the Products. The band gap of CuInS2 is reported to be about 1.5 eV; thus, an absorption edge of over 800 nm can be expected. Both the as-synthesized CuInS2 hierarchical microspheres and CuInS2 hollow microspheres are black powders. The UV−vis diffuse reflectance spectra (DRS) of the synthesized products (Figure 5) reveal strong absorption in the ultraviolet and visible region

Thus, the strong absorption of as-synthesized CuInS 2 hierarchical microspheres and CuInS2 hollow microspheres from 1000 to 2000 nm is novel. The detailed mechanism of the novel optical absorption properties of the as-synthesized products is still under investigation. Briefly, it can be attributed to the cavity-mirror effect and interband transitions of the assynthesized products. The CuInS2 hierarchical microspheres possess open pyramid cavities on the whole surface. The CuInS2 hollow microspheres have close spherical cavities in the center. All these cavities are submicrometer in size, which is close to the wavelength of the absorbed light. The cavity-mirror effect leads to a great improvement of the reflection and absorption ability for optical irradiation. As a reference, CuS hollow microspheres are dark powders and exhibit reasonable absorption from 400 to 800 nm, but comparable absorption can also be observed from 1000 to 2000 nm (Figure 5). It can also be attributed to the cavity-mirror effect as the CuS hollow microspheres possess cavities similar to that of CuInS2 hollow microspheres. However, it should be noted that the absorbance of CuInS2 hierarchical microspheres and CuInS2 hollow microspheres is much higher than that of the CuS hollow microspheres. The difference in absorbance might come from the difference in the band structures. For example, the band gap of CuInS2 is about 1.5 eV, while that of CuS is about 1.85 eV. Thus, CuInS2 has stronger absorption of visible light. Furthermore, there are more unoccupied states in the band structures of CuInS2 in comparison to that of CuS, providing more opportunities for interband transitions of electrons and subsequently more opportunities for the absorption of photons. In other words, cavity-mirror effect and interband transitions together lead to the novel optical absorption of the CuInS2 hierarchical microspheres and hollow microspheres. As reference, the UV−vis−NIR spectra of water and 0.01 M phosphate buffered saline (PBS) were also recorded. The results are presented as Figure S1 (Supporting Information). Both spectra show strong absorption over 1300 nm, which can be roughly attributed to the vibrational transitions of O−H in H2O molecules. At 1064 nm, which is the wavelength of the laser used in the present work, both spectra exhibit a noticeable but relatively weak absorption which may lead to a certain degree of photothermal conversion effect. In contrast, CuInS2 microspheres show strong absorptions at 1064 nm. It should also be noted that the two spectra have little difference in the range from 200 to 2500 nm, because the absorption bands of the inorganic ions in PBS are far over 2500 nm. 3.5. Photothermal Conversion Enhancements of the Products. The photothermal conversion properties of three aqueous suspensions (containing 0.1 wt % CuInS2 hierarchical microspheres, CuInS2 hollow microspheres, and CuS hollow microspheres, respectively) are evaluated according to the typical test procedure, with distilled water and 0.01 M PBS as references. The results are presented as Figure 6. When the laser beam is blocked by the light chopper, the temperature of the droplet remains at a value close to the room temperature (26.8 °C). Then (at 0 s) the light chopper shifts, and the droplet is exposed to the irradiation of the laser beam. The temperature of the droplet increases rapidly due to heat generation from photothermal conversion. When the temperature of the droplet increases to be significantly higher than the room temperature, exothermic process begins with an increasing rate of heat release (Φr). After a short time (about 20 s) of irradiation, the temperature of the droplet reaches an equilibrium value at which the rate of heat release (Φr) equates

Figure 5. UV−vis diffuse reflectance spectra (DRS) of CuInS2 hollow microspheres (red), CuInS2 hierarchical microspheres (blue), and CuS hollow microspheres (purple).

with absorption edge stretch to more than 800 nm, which is consistent with reported data. The fluctuations of the curves at about 900 nm in Figure 5 are attributed to the shift of the light source. Surprisingly, the products exhibit even stronger absorption in the range from 1000 to 2000 nm. As CuInS2 has the potential to attain high conversion efficiency of a solar cell, attention is drawn to its optical absorption properties in the UV and visible region. Absorption in the IR region is not expected as it does not contribute to the conversion efficiency of solar cell but does cause heat generation. Heat generation should be avoided in solar cell devices. However, such limitation does not exist for photothermal conversion agents. Heat generation is regarded as an advantage in this case. Thus, the as-synthesized CuInS2 hierarchical microspheres and CuInS2 hollow microspheres can be considered as candidates for photothermal conversion agents. The reported data of different CuInS2 samples in the literature, including nanodisks,19 nanocrystals,20−23 films,24,25 flower vaselike nanostructure arrays,26 nanorods,27,28 and quantum dots,29−31 did not include absorptions over 800 nm. 9125

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hierarchical microspheres, and CuS hollow microspheres, respectively. Among these suspensions, the one containing CuInS2 hollow microspheres exhibit the highest enhancement (more than 30%) of photothermal conversion, followed by the suspension containing CuInS2 hierarchical microspheres which exhibit more than 20% enhancement of photothermal conversion. Although the photothermal conversion enhancement of the suspension containing CuS hollow microspheres is the lowest, more than 10% enhancement can still be observed. Given that the photothermal conversion effects are observed at very low laser power (0.05 W·cm−2), such enhancements are significant. The tendency of the photothermal conversion properties within the three products is consistent with that of the optical absorption properties as illustrated in Figure 5. However, it should be noted that the temperature elevations in the present work are less than that in Hu’s report, probably due to the differences in testing procedures, such as wavelength and power of laser irradiation, volume, and heat release of tested liquid, and so on. Unlike Au nanostructures, whose photothermal conversion is derived from surface plasmon resonance of free electrons, CuInS2 microspheres do not have free electrons under irradiation of 1064 nm laser. Just like that of CuS, the photothermal conversion of CuInS2 microspheres can be attributed to band−band transitions10 which is further enhanced by cavity-mirror effect,12 as demonstrated in Figure 7. When the laser beams reach the hollow microspheres, some

Figure 6. Temperature elevation of water and the suspensions as a function of irradiation time.

to the rate of heat generation (Φg). Then the temperature of the droplet fluctuates around the equilibrium value. After 60 s of illumination, the laser beam is shut out, and the temperature of the droplet decreases to room temperature. At the equilibrium state, the rate of heat generation (Φg) can be represented by the rate of heat release (Φr) which is expressed as Newton’s low of cooling: Φr = h·ΔT·A, where h is the heat transfer coefficient, A is the surface area of the droplet, and ΔT is the temperature difference between the droplet and the atmosphere. As the volumes of the droplets are set to be exactly 10 μL, the surface area of the droplets can be expected to be identical. Assuming that 0.1 wt % solid particles would not reduce the heat transfer coefficient significantly, h·A in the formula can be regarded as constant. Thus, the rate of heat release (Φr) and, further, the rate of heat generation (Φg) can be represented by ΔT. Now it is possible to evaluate the enhancements of photothermal conversion effects by comparing the ΔT of different droplets. There are two ways to determine ΔT. (1) Choose the maximum temperature (Tmax) of the whole process to represent the equilibrium temperature; thus, ΔT = Tmax − T0, where T0 is the room temperature. (2) Choose the average temperature (Tave) from the 20th second to the 60th second to represent the equilibrium temperature; thus, ΔT = Tave − T0. By choosing water or 0.01 M PBS as reference, the enhancement of photothermal conversion effect can be represented by the difference of ΔTs (sample) over ΔTR (reference), as presented in Table 1. As there is little deviation between the curves of water and 0.01 M PBS (Figure 6), Tmax and Tave of water and 0.01 M PBS are identical. The results suggest significant enhancement of photothermal conversion in suspensions containing 0.1 wt % CuInS2 hollow microspheres, CuInS2

Figure 7. Schematic demonstration of photothermal conversion enhancement based on cavity-mirror effect and interband transitions.

photons are absorbed by the shell, while the rest pass through the shell and enter the inner cavities of the hollow microspheres. Photons inside the cavities have little chance to escape as they will be reflected many times until most of them are finally absorbed. Thus, the absorbance of laser is significantly enhanced, which is so-called cavity-mirror effect. Cavity-mirror effect also functions in the case of hierarchical microspheres as there are plenty of open cavities all over the surfaces of the hierarchical microspheres. Photons are reflected in these open cavities and the absorption is enhanced as well. When the materials absorb photons, electron transitions from occupied states (blue line in Figure 7) to unoccupied states

Table 1. Enhancement of Photothermal Conversion Effects samplesa

Tmax (°C)

ΔT =Tmax − T0 (°C)

enhancement (%)

Tave (°C)

ΔT = Tave − T0 (°C)

enhancement (%)

water 0.01 M PBS suspension 1 suspension 2 suspension 3

32.5 32.5 34.4 33.8 33.1

5.7 5.7 7.6 7.0 6.3

0.0 0.0 33.3 22.8 10.5

32.1 32.1 34.0 33.4 32.8

5.3 5.3 7.2 6.6 6.0

0.0 0.0 35.8 24.5 13.2

a

Suspension 1: water + 0.1 wt % CuInS2 hollow microspheres; suspension 2: water + 0.1 wt % CuInS2 hierarchical microspheres; suspension 3: water + 0.1 wt % CuS hollow microspheres. 9126

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Femtosecond Laser Pulse at Their Surface Plasmon Resonance. J. Am. Chem. Soc. 2006, 128, 2115−2120. (2) Dickerson, E. B.; Dreaden, E. C.; Huang, X. H.; El-Sayed, I. H.; Chu, H. H.; Pushpanketh, S.; McDonald, J. F.; El-Sayed, M. A. Gold Nanorod Assisted Near-Infrared Plasmonic Photothermal Therapy (PPTT) of Squamous Cell Carcinoma in Mice. Cancer Lett. 2008, 269, 57−66. (3) Maltzahn, G.; von Centrone, A.; Park, J.-H.; Ramanathan, R.; Sailor, M. J.; Hatton, T. A.; Bhatia, S. N. SERRS-Coded Gold Nanorods as a Multifunctional Platform for Densely-Multiplexed Near-Infrared Imaging and Photothermal Heating. Adv. Mater. 2009, 21, 3175−3180. (4) Lee, S. E.; Liu, G. L.; Kim, F.; Lee, L. P. Remote Optical Switch for Localized and Selective Control of Gene Interference. Nano Lett. 2009, 9, 562−570. (5) Yavuz, M. S.; Cheng, Y. Y.; Chen, J. Y.; Cobley, C. M.; Zhang, Q.; Rycenga, M.; Xie, J. W.; Kim, C.; Song, K. H.; Schwartz, A. G.; et al. Gold Nanocages Covered by Smart Polymers for Controlled Release with Near-Infrared Light. Nat. Mater. 2009, 8, 935−939. (6) Chen, W. Nanoparticle Self-Lighting Photodynamic Therapy for Cancer Treatment. J. Biomed. Nanotechnol. 2008, 4, 369−376. (7) Boyer, D.; Tamarat, P.; Maali, A.; Lounis, B.; Orrit, M. Photothermal Imaging of Nanometer-Sized Metal Particles Among Scatterers. Science 2002, 297, 1160−1163. (8) Berciaud, S.; Cognet, L.; Blab, G. A.; Lounis, B. Photothermal Heterodyne Imaging of Individual Nonfluorescent Nanoclusters and Nanocrystals. Phys. Rev. Lett. 2004, 93, 257402. (9) Chen, W. R.; Adams, R. L.; Carubelli, R.; Nordquist, R. E. LaserPhotosensitizer Assisted Immunotherapy: a Novel Modality for Cancer Treatment. Cancer Lett. 1997, 115, 25−30. (10) Li, Y. B.; Lu, W.; Huang, Q. A.; Huang, M. A.; Li, C.; Chen, W. Copper Sulfide Nanoparticles for Photothermal Ablation of Tumor Cells. Nanomedicine 2010, 5, 1161−1171. (11) Zhou, M.; Zhang, R.; Huang, M. A.; Lu, W.; Song, S. L.; Melancon, M. P.; Tian, M.; Liang, D.; Li, C. A Chelator-Free Multifunctional [64Cu]CuS Nanoparticle Platform for Simultaneous Micro-PET/CT Imaging and Photothermal Ablation Therapy. J. Am. Chem. Soc. 2010, 132, 15351−15358. (12) Tian, Q.; Tang, M.; Sun, Y.; Zou, R.; Chen, Z.; Zhu, M.; Yang, S.; Wang, J.; Wang, J.; Hu, J. Hydrophilic Flower-Like CuS Superstructures as an Efficient 980 nm Laser-Driven Photothermal Agent for Ablation of Cancer Cells. Adv. Mater. 2011, 23, 3542−3547. (13) Lakshmanan, S. B.; Zou, X.; Hossu, M.; Ma, L.; Yang, C.; Chen, W. Local Field Enhanced Au/CuS Nanocomposites as Efficient Photothermal Transducer Agents for Cancer Treatment. J. Biomed. Nanotechnol. 2012, 8, 883−890. (14) Nairn, J. J.; Shapiro, P. J.; Twamley, B.; Pounds, T.; Wandruszka, R. V.; Fletcher, T. R.; Williams, M.; Wang, C. M.; Norton, M. G. Preparation of Ultrafine Chalcopyrite Nanoparticles via the Photochemical Decomposition of Molecular Single-Source Precursors. Nano Lett. 2006, 6, 1218−1223. (15) Allen, P. M.; Bawendi, M. G. Ternary I-III-VI Quantum Dots Luminescent in the Red to Near-Infrared. J. Am. Chem. Soc. 2008, 130, 9240−9241. (16) Nanu, M.; Schoonman, J.; Goossens, A. Nanocomposite ThreeDimensional Solar Cells Obtained by Chemical Spray Deposition. Nano Lett. 2005, 5, 1716−1719. (17) Scheer, R.; Walter, T.; Schock, H. W.; Fearheiley, M. L.; Lewerenz, H. J. CuInS2 Based Thin Film Solar Cell with 10.2% Efficiency. Appl. Phys. Lett. 1993, 63, 3294. (18) Li, L.; Pandey, A.; Werder, D. J.; Khanal, B. P.; Pietryga, J. M.; Klimov, V. I. Efficient Synthesis of Highly Luminescent Copper Indium Sulfide-Based Core/Shell Nanocrystals with Surprisingly Long-Lived Emission. J. Am. Chem. Soc. 2011, 133, 1176−1179. (19) Koo, B.; Patel, R. N.; Korgel, B. A. Wurtzite−Chalcopyrite Polytypism in CuInS2 Nanodisks. Chem. Mater. 2009, 21, 1962−1966. (20) Zhong, H.; Lo, S. S.; Mirkovic, T.; Li, Y.; Ding, Y.; Li, Y.; Scholes, G. D. Noninjection Gram-Scale Synthesis of Monodisperse

(green lines in Figure 7) are triggered. The probability of electron transition is closely related to the band structures of the materials. Increase in the number of unoccupied states may result in higher probability of electron transition, which is in fact the case of CuInS2. When the electrons return from the excited states, their paths may change where there are many unoccupied states. They may shift many times from one unoccupied state to another before finally going back to the ground state. The frequent transitions of electrons between the unoccupied states might shake the system to a certain degree, which favors heat generation and hence leads to the enhancement of photothermal conversion. As there are more unoccupied states in CuInS2 microspheres in comparison to CuS microspheres, CuInS2 microspheres exhibit higher enhancement of photothermal conversion.

4. CONCLUSIONS In summary, CuInS2 hierarchical microspheres and CuInS2 hollow microspheres can be synthesized through a solvothermal procedure based on a sacrificial template mechanism, with CuS microspheres and CuS hollow microspheres as sacrificial templates, respectively. The as-synthesized microspheres inherit shapes and structures from the sacrificial templates with additional highly developed hierarchy on the surfaces. Close cavities and open cavities are the characteristic features of the structures. The as-synthesized hierarchical and hollow CuInS2 microspheres exhibit strong absorption of infrared rays from 1000 to 2000 nm, in addition to the normal absorption from 400 to 800 nm. Significant enhancements of photothermal conversion are observed in aqueous suspensions containing hierarchical CuInS2 microspheres (up to 20% enhancement) and hollow CuInS2 microspheres (up to 30% enhancement). The photothermal conversion enhancement of the aqueous suspension containing hollow CuInS2 microspheres is found to be significantly higher than that of the aqueous suspension containing hollow CuS microspheres of similar size. The assynthesized products are supposed to have potential applications as photothermal conversion agents.

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ASSOCIATED CONTENT

* Supporting Information S

UV−vis−NIR spectra of water and 0.01 M phosphate buffered saline (BPS). This material is available free of charge via the Internet at http://pubs.acs.org .

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86 532 84022676. Fax: +86 532 84022787. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51172117, 50872061), the Natural Science Foundation of Shandong Province, and the Foundation of Qingdao Science and Technology.

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REFERENCES

(1) Huang, X. H.; El-Sayed, I. H.; Qian, W.; El-Sayed, M. A. Gold Nanoparticles Propulsion from Surface Fueled by Absorption of 9127

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The Journal of Physical Chemistry C

Article

Pyramidal CuInS2 Nanocrystals and Their Size-Dependent Properties. ACS Nano 2010, 4, 5253−5262. (21) Zhong, H.; Zhou, Y.; Ye, M.; He, Y.; Ye, J.; He, C.; Yang, C.; Li, Y. Controlled Synthesis and Optical Properties of Colloidal Ternary Chalcogenide CuInS2 Nanocrystals. Chem. Mater. 2008, 20, 6434− 6343. (22) Nose, K.; Soma, Y.; Omata, T.; Matsuo, S. O. Y. Synthesis of Ternary CuInS2 Nanocrystals; Phase Determination by Complex Ligand Species. Chem. Mater. 2009, 21, 2607−2613. (23) Kruszynska, M.; Borchert, H.; Parisi, J.; Olesiak, J. K. Synthesis and Shape Control of CuInS2 Nanoparticles. J. Am. Chem. Soc. 2010, 132, 15976−15986. (24) Todorov, T.; Cordoncillo, E.; Sánchez-Royo, J. F.; Carda, J.; Escribano, P. CuInS2 Films for Photovoltaic Applications Deposited by a Low-Cost Method. Chem. Mater. 2006, 18, 3145−3150. (25) Li, L.; Coates, N.; Moses, D. Solution-Processed Inorganic Solar Cell Based on in Situ Synthesis and Film Deposition of CuInS2 Nanocrystals. J. Am. Chem. Soc. 2010, 132, 22−23. (26) Das, K.; Datta, A.; Chaudhuri, S. CuInS2 Flower Vaselike Nanostructure Arrays on a Cu Tape Substrate by the Copper Indium Sulfide on Cu-Tape (CISCuT) Method: Growth and Characterization. Cryst. Growth Des. 2007, 7, 1547−1552. (27) Connor, S. T.; Hsu, C. M.; Weil, B. D.; Aloni, S.; Cui, Y. Phase Transformation of Biphasic Cu2S−CuInS2 to Monophasic CuInS2 Nanorods. J. Am. Chem. Soc. 2009, 131, 4962−4966. (28) Xiao, J.; Xie, Y.; Tang, R.; Qian, Y. Synthesis and Characterization of Ternary CuInS2 Nanorods via a Hydrothermal Route. J. Solid State Chem. 2001, 161, 179−183. (29) Yue, W.; Han, S.; Peng, R.; Shen, W.; Geng, H.; Wu, F.; Tao, S.; Wang, M. CuInS2 Quantum Dots Synthesized by a Solvothermal Route and Their Application as Effective Electron Acceptors for Hybrid Solar Cells. J. Mater. Chem. 2010, 20, 7570−7578. (30) Li, T. L.; Teng, H. Solution Synthesis of High-Quality CuInS2 Quantum Dots as Sensitizers for TiO2 Photoelectrodes. J. Mater. Chem. 2010, 20, 3656−3664. (31) Li, T. L.; Lee, Y. L.; Teng, H. CuInS2 Quantum Dots Coated with CdS as High-Performance Sensitizers for TiO2 Electrodes in Photoelectrochemical Cells. J. Mater. Chem. 2011, 21, 5089−5098. (32) Lou, X. W.; Archer, L. A.; Yang, Z. Hollow Micro-/ Nanostructures: Synthesis and Applications. Adv. Mater. 2008, 20, 3987−4019. (33) Yin, Y. D.; Rioux, R. M.; Erdonmez, C. K.; Hughes, S.; Somorjai, G. A.; Alivisatos, A. P. Formation of Hollow Nanocrystals Through the Nanoscale Kirkendall Effect. Science 2004, 304, 711−714. (34) Zhu, H.; Wang, J.; Wu, D. Fast Synthesis, Formation Mechanism, and Control of Shell Thickness of CuS Hollow Spheres. Inorg. Chem. 2009, 48, 7099−7104. (35) Wagner, C. D.; Riggs, W. M.; Davis, L. E.; Moulder, J. F.; Muilenberg, G. E. Handbook of X-Ray Photoelectron Spectroscopy; Perkin-Elmer Corp.: Eden Prairie, U.S.A., 1978. (36) Courtel, F. M.; Paynter, R. W.; Marsan, B.; Morin, M. Synthesis of n-Type CuInS2 Particles Using N-Methylimidazole, Characterization and Growth Mechanism. Chem. Mater. 2009, 21, 3752−3761. (37) Qi, Y.; Tang, K.; Zeng, S.; Zhou, W. Template-Free One-Step Fabrication of Porous CuInS2 Hollow Microspheres. Microporous Mesoporous Mater. 2008, 114, 395−400.

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