Rapid synthesis of broadband Ag@TiO2 core–shell nanoparticles for solar energy conversion

Solar Energy Materials & Solar Cells 166 (2017) 52–60

Contents lists available at ScienceDirect

Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat

Rapid synthesis of broadband Ag@TiO2 core–shell nanoparticles for solar energy conversion

MARK

⁎

Haoran Li, Yurong He , Ziyu Liu, Baocheng Jiang, Yimin Huang School of Energy Science and Engineering, Harbin Institute of Technology, Harbin 150001, China

A R T I C L E I N F O

A BS T RAC T

Keywords: Ag@TiO2 nanoparticles Broadband absorption One-pot synthesis Solar harvesting

Plasmonics are applied to concentrate light at the nanoscale and are integrated into photothermal technologies to improve solar energy absorption. Here, we report a one-step synthetic route of super broadband Ag@TiO2 core–shell nanoparticles under well-defined conditions using silver nitrate (AgNO3) and titanium butoxide (TBT) in N,N-dimethylformamide as raw materials and hexadecyl trimethyl ammonium chloride (CTAC) as both an inducer and a protective agent. Transmission electron microscopy revealed that the prepared nanoparticles have well-defined core–shell structures. The intermediates and final product were further characterized by UV–vis absorption, which showed that the synthesized Ag@TiO2 core–shell nanoparticles exhibit super broadband absorbance in the visible light range, and the surface plasmon resonance (SPR) peak can be systematically tuned between 414 and 499 nm. In addition, X-ray diffraction and energy-dispersive spectroscopy confirmed the compositions of the nanoparticles. The nanoparticles prepared under well-defined conditions, i.e., a TBT:AgNO3 molar ratio of 3:1, CTAC concentration of 0.6 mM, temperature of 80 °C, and reaction time of 70 min, show an SPR peak at 474 nm, which is close to the maximum solar irradiation intensity and has great potential for promoting solar energy conversion.

1. Introduction Direct collection and conversion of solar energy are considered to be essential parts of energy production worldwide to support future human activities [1–3]. To explore an efficient solar utilization system with enhanced photothermal [4], photoelectric [5–7], and photochemical [8,9] conversion ability, great efforts have been made in both solar energy capture and photonics transfer and conversion. However, the efficiency of solar thermal collectors is seriously limited by the weak absorptivity of conventional heat transfer media, which is a critical problem that needs to be solved [10]. To this end, plasmonics are applied to concentrate light at the nanoscale and are integrated into photothermal technologies to improve light absorption using the surface plasmon resonance (SPR) effect [11,12]. Among numerous plasmon-active materials, nanosized Ag is regarded as a technologically important material owing to its excellent conductivity, chemical stability, catalytic activity, and reflectivity [13]. In addition, the nearfield enhancement of Ag is estimated to be more than 10 times greater than that of similar gold nanoparticles. Therefore, it is especially attractive for surface plasmon applications [14]. However, it is well known that metallic Ag is easily oxidized in air, which greatly limits its practical application. One way to prevent oxidization of such a metal is

⁎

Corresponding author. E-mail address: [email protected] (Y. He).

http://dx.doi.org/10.1016/j.solmat.2017.03.005 Received 1 September 2016; Accepted 6 March 2017 0927-0248/ © 2017 Elsevier B.V. All rights reserved.

to coat it with a layer of stable oxides [15]. Among numerous oxides, TiO2 is widely used owing to its high photocatalytic ability, excellent chemical stability, low toxicity, and low cost [16]. To realize the complementary advantages of Ag and TiO2 nanoparticles, as far as we know, Pastoriza-Santos et al. [17] first reported an approach to synthesize Ag@TiO2 core–shell nanoparticles by simultaneous reduction of AgNO3 in N,N-dimethylformamide (DMF) and controlled hydrolysis and condensation of titanium butoxide (TBT). A few years later, to plausibly explain the contribution of the Ag core to the photocatalytic properties of the outer TiO2 shell, Tsutomu and Kamat [18] synthesized Ag@TiO2 core–shell clusters through a similar method. What makes their work unique is that they hydrolyzed titanium tetraisopropoxide rather than TBT. In the same year, Kim et al. [19] also discovered a new synthesis method to form spherical and monodisperse Ag@TiO2 core–shell nanoparticles. Although they successfully prepared core–shell particles with small diameters ( < 15 nm), the dispersion stability was poor, and TiO2 clusters tended to aggregate in media. In response to this unsatisfactory situation, highly dispersed and monodispersed Ag@TiO2 core– shell nanoparticles were soon prepared through reduction of AgNO3 by hydrazine [20]. However, the use of hydrazine monohydrate as the reducing agent made it harmful to laboratory personnel, which limited

Solar Energy Materials & Solar Cells 166 (2017) 52–60

H. Li et al.

adoption of this approach. Therefore, it is necessary to identify a new synthesis route using a nontoxic or low-toxicity reagent for the reaction. Two methods can be applied to prepare Ag@TiO2 core–shell nanoparticles. One is to first synthesize high-quality Ag nanoparticles for use as a core metal for subsequent coating with TiO2. The other is one-pot synthesis of core–shell nanoparticles by forming the Ag cores much more rapidly than the TiO2 shells. As the former synthesis method usually requires a long reaction time and is highly sensitive to the reaction conditions [21], it is not suitable for mass production. On the other hand, attention should also be paid to the controllability of the nanoparticles [22], especially at the SPR location. Although the preparation of Ag@TiO2 core–shell nanoparticles has been widely studied [23,24] and these nanoparticles have been developed for photocatalysis [25–27], their SPR is in the range of 420–450 nm, which limits their ability to absorb solar energy in the visible light range. Therefore, it is also necessary to synthesize nanoparticles with an SPR located around 475 nm, which is the position of the maximum solar irradiation intensity. Here, we take advantage of the properties of DMF both as a reductant of Ag+ ions and as a convenient basic solvent to induce TBT hydrolysis and design a simple one-pot strategy for obtaining TiO2-covered silver nanostructures with interesting plasmonic features. This synthesis route successfully addresses the problems mentioned above. The optical properties, including the absorption performance and SPR peak, of this system are investigated as functions of the TBT:AgNO3 ratio, hexadecyl trimethyl ammonium chloride (CTAC) concentration, reaction temperature, and reaction time. The core–shell structures of the nanoparticles are determined using transmission electron microscopy (TEM). In addition, X-ray diffraction (XRD) and energy-dispersive spectroscopy (EDS) tests are conducted to confirm the components of the prepared nanoparticles. The broadband absorbance of the prepared nanoparticles means that they can enhance the conversion efficiency of incident solar radiation in many applications, e.g., photovoltaic/thermal systems and solar thermoelectric cells [28].

Fig. 1. Synthesis process of Ag@TiO2 nanoparticles.

(10−x) mL of AgNO3 solution was mixed with x mL of TBT solution in a 20 mL vial under 10 min of magnetic stirring. Then y mL of DMF was added to the TBT–Ag solution, followed by (5−y) mL of CTAC/DMF solution. We can change the molar ratio of TBT and AgNO3 (2:1–10:1) as well as the CTAC concentration (0–0.7 mM) in the reaction solutions by simply changing the values of x and y. The solution was magnetically stirred for another 10 min at room temperature and then transferred to a thermostatic water bath (70–95 °C) under static conditions. With increasing reaction time, the solution slowly changed from colorless to reddish and eventually became reddish black. After a sufficient reaction time, heating was stopped, and the suspension was cooled to room temperature. Eventually, all the samples were ultrasonicated in an ultrasonic cleaner for 20 min to obtain a homogeneous dispersion. The cluster suspension of Ag@TiO2 was centrifuged with ethanol three times to minimize the DMF and IPA contents of the suspension.

2. Experimental 2.1. Materials

2.3. Characterization TBT [Ti(OC4H9)4, ≥99.0%], DMF (C3H7NO, ≥99.5%), AgNO3 (≥99.8%), CTAC [C16H33(CH3)3NCl, ≥97%], isopropanol alcohol (IPA, C3H8O, ≥99.5%), polyvinylpyrrolidone (PVP, K29-32), and ethylene glycol (EG, ≥98%) were provided by Aladdin Industrial Corporation, China. All of these reagents were of the analytical reagent grade and were used without further purification.

The absorption spectra were recorded at wavelengths of 300– 800 nm using a two-beam UV–vis spectrophotometer (TU1901, Persee Co., Ltd., China) in a 10 mm cuvette using ethanol as a reference. Before testing, a 1 mL stock solution was centrifuged with ethanol three times, and the clean nanoparticles were suspended in 3.5 mL of ethanol by 10 min of ultrasonic dispersion. TEM was performed using a JEOL JEM-2100 electron microscope at an operating voltage of 200 kV and a beam current of 101 μA. The samples were prepared by mounting several drops of solution on a carbon-coated copper grid and drying the grid in a vacuum oven at 60 °C for 1 h. XRD patterns were obtained using an X-ray diffractometer (D8-Advance, Bruker AXS GmbH, Germany) at room temperature with a monochromatic Cu-Kα radiation source in the step-scan mode with a 2θ angle in the range of 20–80 °. EDS was performed using an X-MaxN 80 Silicon Drift Detector and a field-emission scanning electron microscope (JSM-7500F, JEOL, Japan) with an accelerating voltage of 5 kV and an emission beam of 0.5 μA. To eliminate the influence of the silicon wafer substrate, the sample was directly dropped on the conducting resin.

2.2. Synthesis of Ag and Ag@TiO2 core–shell nanoparticles Ag nanoparticles were prepared as follows [29]. AgNO3/EG solution (40 mL, 1.2 mM) was added to a solution consisting of 1.4g of PVP and 15 mL of EG. The solution was magnetically stirred for 10 min at room temperature and then transferred to a 120 °C thermostatic oil bath. The mixture was stirred for 22 h to yield a suspension of Ag particles. To synthesize Ag@TiO2 core–shell clusters, we modified the procedure for reduction of AgNO3 and the decomposition reaction of TBT in DMF [17,18]. The nanoparticle synthesis process is shown in Fig. 1. TBT and AgNO3 solutions with concentrations of 8.3 and 15 mM, respectively, were prepared in IPA. A balanced amount of CTAC was dissolved in DMF to produce a CTAC/DMF solution with a concentration of 4.8 mg/mL. CTAC was used to prevent aggregation and adhesion of Ag particles on the walls of the vial and to induce structure-directed deposition of TiO2 colloids on the surfaces of Ag particles. Here we controlled the volume ratio of IPA and DMF (2:1) according to the literature [18] to prevent nanoparticle aggregation. Specifically, after the solution preparation procedures were completed,

3. Results and discussion 3.1. Structure and phase formation of Ag@TiO2 nanoparticles The optimized synthesis conditions, i.e., a TBT:AgNO3 molar ratio 53

Solar Energy Materials & Solar Cells 166 (2017) 52–60

H. Li et al.

Fig. 2. Characterization of Ag@TiO2 nanoparticles. (a) TEM image of Ag@TiO2 core-shell nanoparticles. (b) XRD patterns for the prepared nanoparticles (red line) and nanoparticles annealed at 800 °C for 2 h (blue line). (c) EDS spectra of Ag@TiO2 nanoparticles, and the inset is a table for mean mass and atomic percentages of the Ag, Ti and O elements after ten areas testing. (d) EDS elemental color maps overlapped with Ag (red dots), Ti (green dots) and O (blue dots) elements. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

to determine the phase of the optimized sample. Fig. 2(b) shows the XRD pattern. In the nanocomposite, (111), (200), (220), and (311) peaks for single face-centered-cubic crystalline silver are detected at 2θ angles of 38.1, 44.3, 64.5, and 77.4°, respectively (PDF No. 65–2871). There is no peak for TiO2, indicating that the TiO2 shell is amorphous [19,30] in the natural condition [red line in Fig. 2(b)]. To provide definitive evidence, we calcined the nanoparticles at 800 °C for 2 h to crystallize the shell. After heating [blue line in Fig. 2(b)], the shell was crystallized in the rutile phase, as demonstrated by the diffraction peaks detected at 2θ angles of 27.3, 36.0, 41.2, and 56.6°, which correspond to the (110), (101), (111), and (211) planes of rutile TiO2,

of 3:1, CTAC concentration of 0.6 mM, temperature of 80 °C, and reaction time of 70 min, were determined after several batches of experimental investigations of each factor. The Ag core size and TiO2 shell thickness of more than 200 particles were measured using the Nano Measurer 1.2 software (Fudan University, China). The results showed that the average Ag diameter and TiO2 thickness are 19.7 ± 1.4 nm and 2.2 ± 0.7 nm, respectively. TEM, XRD, and EDS patterns were measured to determine the structure and components of the prepared nanoparticles. The core–shell structure of the prepared Ag@TiO2 nanoparticles is confirmed by the TEM image in Fig. 2(a). XRD analysis was conducted 54

Solar Energy Materials & Solar Cells 166 (2017) 52–60

H. Li et al.

spectra were obtained to determine the effect of R on the optical properties of the prepared nanoparticles. The UV–vis absorbance spectra and SPR peaks of the Ag@TiO2 nanoparticles are shown in Fig. 4(a). All the samples exhibit high absorption in the visible light region owing to the SPR characteristic of Ag nanoparticles. The SPR is derived from the collective oscillation of electrons on the surfaces of the nanoparticles, and the absorption peak depends strongly on the size and shape of the nanoparticles as well as the dielectric environment surrounding them [34,35]. The SPR peak of the Ag@TiO2 nanoparticles remains unchanged (~495 nm) for R values of 2:1 to 8:1, whereas it is rapidly blue-shifted to 487 nm when R=9:1 and eventually changes to 470 nm when R=10:1 [inset of Fig. 4(a)], which may result from a change in the thickness of the TiO2 shells and the formation of individual TiO2 particles. At a low concentration of the TiO2 precursor (TBT), hydrolyzed TiO2 can effectively nucleate on the surfaces of silver nanoparticles, leading to the formation of a core–shell morphology with a very thin shell [Fig. 4(b)]. With increasing TBT concentration, more TiO2 can grow around the silver particles, forming core–shell particles with a thicker shell [Fig. 4(c)–(e)]. However, thicker TiO2 shells cannot always be grown by increasing the TBT concentration [17]. When the concentration is too high, the excessive TiO2 precursors cannot find an available silver surface on which to nucleate owing to the absence of static electrical attraction; thus, they form individual and jointed particles between the grown core–shells [Fig. 4(e)–(g)]. A shoulder peak also exists at a wavelength of around 390 nm, which results from the TiO2 shell and is related to the CTAC concentration. This will be discussed in the next section. On the other hand, the absolute value of the absorbance reflects the concentration of nanoparticles [36]. The concentration of Ag@TiO2 nanoparticles increases with decreasing R when R > 3:1. When R=3:1, the nanoparticle concentration is almost the same as that at R=2:1. The ignorable difference between the absorption profiles at R=2:1 and R=3:1 indicates that the quantity of Ag particles available for light absorption is almost the same. With increasing R, the SPR peak intensity fades gradually because the thick TiO2, assembled layer by layer, conceals the Ag absorption [16]. If this threshold value is reached, the distinct plasmon band of Ag will disappear. In practical application, nanoparticles prepared at R=3:1 are optimal in this work because they exhibit the highest absorption and use less of the precious raw material (AgNO3) than those prepared at R=2:1.

Fig. 3. Normalized intensity spectra of Ag@TiO2 and Ag nanoparticles. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)

respectively (PDF No. 65-1119). EDS was performed to identify the elemental composition of the core–shell nanoparticles. The expected elements, i.e., Ag, Ti, and O, were identified, as shown in Fig. 2(c). On the other hand, the lower intensity of Ti confirms that the content of this element in the composite is very low. Note that Fig. 2(c) cannot show the elemental ratios, as only a small subsection was tested. The results for 10 areas show that the average weight percentages of Ag, Ti, and O are 34.91%, 2.28%, and 62.81%, and the average atomic percentages of the three elements are 74.25%, 2.02%, and 23.73%, respectively, as shown in the inset of Fig. 2(c). EDS elemental color maps [Fig. 2(d)] also show the overlapping of Ag (red dots), Ti (green dots), and O (blue dots) elements. 3.2. Super broadband absorption of Ag@TiO2 nanoparticles The normalized absorption spectrum of the Ag@TiO2 nanoparticles at wavelengths of 300–800 nm is shown in Fig. 3. The SPR peak of the Ag@TiO2 particles is located at 474 nm (blue dot in Fig. 3). To further confirm the SPR peak of the TiO2 coating, we synthesized pure Ag nanoparticles with the same size to evaluate their SPR peak. The SPR peak of the small Ag nanoparticles prepared by reduction of AgNO3 in a PVP/EG solution is around 414 nm (red dot in Fig. 3), which is a typical wavelength for absorption resulting from SPR of metallic Ag nanoparticles. This result confirms that the red shift in the plasmon absorption seen in the core–shell particles depends on the TiO2 shell [31]. Note that the high dielectric constant of the TiO2 shell causes a red shift in the plasmon absorption of the Ag core [32]. The high refractive index of the TiO2 shells (~2.5, compared to ~0.15 for Ag) results in a significant red shift and reveals that the Ag cores are completely covered by the TiO2 shells [17,18,33]. Note that the absorption peak of the prepared Ag@TiO2 nanoparticles is very meaningful for solar energy conversion, as the maximum solar irradiation intensity is located at 475 nm.

3.3.2. Effect of CTAC concentration CTAC in solution plays multiple roles in the formation of Ag@TiO2 nanoparticles. During nanoparticle synthesis, it is necessary to avoid agglomeration and diverse nanoparticle shapes by employing a stabilizer [37]. An electrical double layer was formed by ions adsorbed on the particle surface, and the corresponding counter-ions gave rise to Coulombic repulsion between particles, providing electrostatic stabilization [38]. Further, we found that the use of CTAC can also prevent adhesion of the Ag particles on the walls of vials. The effect of the CTAC concentration (0–0.7 mM) on Ag@TiO2 nanoparticles was studied at an R value of 3:1, a temperature of 90 °C, and a reaction time of 90 min. UV–vis spectra were obtained to determine the effect of the CTAC concentration on the optical properties of the prepared nanoparticles [Fig. 5(a)]. The nanoparticles prepared without added CTAC show an SPR peak of 440 nm. With increasing CTAC concentration, the SPR peak is rapidly red-shifted to 496 nm at a CTAC concentration of 0.3 mM owing to nanoshell growth. Further addition of CTAC to the reaction solutions results in a slight blue shift of the SPR peak as individual particles are formed. When the CTAC concentration exceeds 0.3 mM, the superfluous Cl− is found free in the solution rather than being absorbed on the surfaces of the Ag nanoparticles. Therefore, the TBT quickly interacts with the Cl− and generates individual TiO2 particles, resulting in a thinner shell. TEM images also show that when the CTAC concentration is 0.1 mM [Fig. 5(c)], the shell is thinner than that synthesized at a concentration of 0.4 mM [Fig. 5(d)], whereas

3.3. Optical properties of Ag@TiO2 nanoparticles Understanding the optical properties of Ag@TiO2 nanoparticles is very important for their practical application. For this purpose, the effects of the TBT:AgNO3 molar ratio (2:1–10:1), CTAC concentration (0–0.7 mM), reaction temperature (70–95 °C), and reaction time (10– 90 min) on the optical and morphological properties were systematically studied. 3.3.1. Effect of TBT:AgNO3 molar ratio For convenience, we define R = MTBT / MAgNO as the molar ratio of 3 TBT and AgNO3, where MTBT and MAgNO are the molar quantities of 3 TBT and AgNO3, respectively. The effect of R (2:1–10:1) on the Ag@ TiO2 nanoparticles was first investigated at a CTAC concentration of 0.3 mM, temperature of 90 °C, and reaction time of 90 min. UV–vis 55

Solar Energy Materials & Solar Cells 166 (2017) 52–60

H. Li et al.

Fig. 4. Effect of R on Ag@TiO2 nanoparticle (CTAC concentration=0.3 mM, temperature=90 °C, reaction time=90 min). (a) UV–vis absorption spectra. The inset shows the SPR peak change under different R. TEM images of Ag@TiO2 nanoparticles prepared at different R: (b) R=2:1, (c) R=3:1, (d) R=4:1, (e) R=6:1, (f) R=8:1 and (g) R=10:1.

In addition, CTAC plays a vital role in controlling the size of Ag particles as well as forming the core–shell structure. In the absence of CTAC, only individual aggregates of Ag and TiO2 nanoparticles were formed instead of the core–shell clusters, showing that CTAC was indispensable for the tropic growth of TiO2 on the surface of Ag nanoparticles. Further, the large Ag nanoparticles [Fig. 5(b)] also confirm that CTAC can control the size of nanoparticles effectively. Here, chloride ions are easily produced in the solvent (IPA) and adsorbed on the Ag particles' surfaces, where they support TBT hydrolysis around the Ag particles and yield a layer of the TiO2 shell. No obvious morphological change appears at higher CTAC concentrations [Fig. 5(d)–(e)]. However, a sufficiently high CTAC concentration is necessary to prevent nanoparticle adhesion on the vial and increase

additional CTAC cannot increase the shell thickness any further [Fig. 5(e)]. Furthermore, the concentration of nanoparticles increases with increasing CTAC content in the reaction mixture until a concentration of 0.6 mM, which suggests that CTAC in the reaction mixture could promote the reduction of AgNO3. When Ag@TiO2 nanoparticles are prepared without CTAC, a shoulder peak characteristic of TiO2 also emerges at ~350 nm, which is attributed to the formation of individual TiO2 particles. The TEM image in Fig. 5(b) shows the separation and the large size of Ag and TiO2 particles. This absorption characteristic peak of TiO2 has been widely reported, e.g., in Refs. [39–42]. When CTAC is added to the reaction solution, the shoulder absorption peak is red-shifted to ~390 nm by the formation of Ag@TiO2 core–shell clusters [38].

56

Solar Energy Materials & Solar Cells 166 (2017) 52–60

H. Li et al.

Fig. 5. Effect of CTAC concentration on Ag@TiO2 nanoparticle (R=3:1, temperature=90 °C, reaction time=90 min). (a) UV–vis absorption spectra. The inset shows the SPR peak change under different CTAC concentrations. (b) TEM image of Ag@TiO2 nanoparticles prepared without CTAC addition. TEM images of typical particles prepared at different CTAC concentrations: (c) 0.1 mM, (d) 0.4 mM and (e) 0.7 mM.

the SPR peak. Actually, as long as the lacunae between particles match the wavelength of the incident light, these collective plasmon modes will be observed. Here, a higher reaction temperature leads to a larger Ag@TiO2 nanoparticle radius and a narrower interparticle space, resulting in the red shift of the SPR peak. In addition, the higher temperature can also cause the shell thickness to increase. Fig. 6(c) confirms that the shell thickness is very thin (less than 1 nm) for the nanoparticles prepared at 70 °C. Here, one may also observe that the absorption peak bifurcations for nanoparticles prepared at lower temperatures (70 and 75 °C) are not well-defined. High temperature promotes the reduction of AgNO3 to Ag and hydrolysis of TBT on Ag atomic clusters. Considering that the reduction process is faster than the hydrolysis process, a higher temperature will lead to a higher atomic ratio of Ag to TiO2 in the Ag@TiO2 atomic clusters. After coating, the nucleated Ag core will no longer grow, and subsequent TBT hydrolysis will result in thick TiO2 shells, and the thickness even exceeds 10 nm in some locations [Fig. 6(d)–(e)]. On the other hand, a higher reaction temperature also results in rapid coalescence of the Ag@TiO2 atomic clusters [Fig. 6(d)–(e)], which shortens the duration of the reduction and hydrolysis processes. That is, immediately before the coalescence process, the Ag@TiO2 atomic clusters have a high atomic ratio of Ag to TiO2, and the subsequent hydrolysis will produce random coverage of the shells. Therefore, a higher temperature will result in thicker and more irregular TiO2 shells.

the absorption intensity of the particles [Fig. 5(a)]. 3.3.3. Effect of temperature The reaction temperature is also an important factor in the preparation of nanoparticles. We synthesized Ag@TiO2 nanoparticles at different temperatures at an R value of 3:1, a CTAC concentration of 0.6 mM, and a reaction time of 90 min. The UV–vis spectra of the nanoparticles prepared at different temperatures are shown in Fig. 6(a). The intensity of the spectrum increases as the reaction temperature increases from 70 to 95 °C. This behavior indicates improvement of the nanoparticle concentration and increased particle generation by ripening. The spectrum of the nanoparticles reacted at 70 °C exhibits the weakest SPR peak, which appears at 444 nm, whereas that of the nanoparticles reacted at 95 °C has the strongest SPR peak, which appears at 499 nm. The red shift of the SPR peak reveals almost linear temperature-tunable plasmon behavior of the nanoparticles [inset of Fig. 6(a)] that can be attributed to nanoparticle aggregation as well as increasing shell thickness. With increasing temperature, both reduction of AgNO3 and hydrolysis of TBT are accelerated. Further, the distance between the Ag@TiO2 nanoparticles is decreased by the increase in the free energy as well as the decrease in the stability of the formed aggregate [43]. Therefore, the agglomerations in the samples [44] and the coupling of surface plasmons between neighboring and contiguous particles [45] contribute to the red shift of 57

Solar Energy Materials & Solar Cells 166 (2017) 52–60

H. Li et al.

Fig. 6. Effect of temperature on Ag@TiO2 nanoparticle (R=3:1, CTAC concentration=0.6 mM, reaction time=90 min). (a) UV–vis absorbance, and SPR peak change at different temperatures (inset). TEM images of typical nanoparticles prepared at (b) 70 °C, (c) 80 °C, (d) 85 °C and (e) 95 °C.

show the spectra of nanoparticles at reaction times of less than 10 min, as the resulting nanoparticles are very small and the quantity is rather limited, so the samples are hard to prepare. At a reaction time of 10 min, the SPR peak of the nanoparticles appears at 413 nm, which is a typical absorption wavelength associated with SPR of metallic Ag nanoparticles [46–48], indicating that little TiO2 has formed at this point. This is also confirmed by the TEM image in Fig. 7(b). After 15 min of reaction, the SPR peak is rapidly red-shifted to 444 nm. The SPR peak of the nanoparticles also becomes concentrated around 474 nm after 70 min of reaction; a TEM image is shown in Fig. 7(d). This absorption peak is very meaningful for solar energy conversion, as the maximum solar irradiation intensity is located at 475 nm. Therefore, these nanoparticles can effectively absorb solar energy to the greatest extent. When the reaction time increases further from 70 to 90 min, the absorbance peak does not change within the accuracy range of the test instrument.

These results show that a low reaction temperature is beneficial for monodisperse Ag@TiO2 nanoparticle synthesis, as it is necessary to maintain the formation of a uniform and thinner oxide shell in the reaction solution, which guarantees the desired nanoparticle structure, i.e., Ag cores in thinner TiO2 shells rather than Ag cores embedded in matrix-type TiO2 shells. On the other hand, a low temperature leads to less intermixing of core and shell atoms between interfaces. Although the absorbance increases with increasing temperature, this deficiency of the nanoparticles prepared at a low temperature can be remedied as long as the reaction time is sufficient. Therefore, taking all these factors into consideration, the reduction and hydrolysis processes are promoted even at a higher temperature, whereas a low temperature is beneficial for obtaining a uniform shell coating. Thus, the following study was conducted at a temperature of 80 °C.

3.3.4. Effect of reaction time In this section, the influence of the reaction time is considered in order to form nanoparticles with a tunable SPR peak at an R value of 3:1, a CTAC concentration of 0.6 mM, and a temperature of 80 °C. The UV–vis spectra and SPR peak change during evolution of the nanoparticles are shown in Fig. 7(a). The absorbance increases with the reaction time, indicating enhancement of the nanoparticle concentration. After 40 min of reaction, the enhancement ratio gradually decreases because the predecessors fade. The inset of Fig. 7(a) shows that the SPR peak is red-shifted during the reaction. Here we do not

3.4. Synthesis mechanism for Ag@TiO2 nanoparticles The synthesis of Ag@TiO2 nanoparticles involved two steps: reduction of AgNO3 to Ag nanoparticles (Eq. (1)) and coating of hydrolyzed TiO2 colloid on the Ag surface (Eq. (2)).

58

HCONMe2 + 2Ag+ +H2 O → 2Ag + Me2NH + CO2 + 2H+

(1)

Ti(C3H7O)4 + 2H2O → TiO2 + 4C3H8O

(2)

Solar Energy Materials & Solar Cells 166 (2017) 52–60

H. Li et al.

Fig. 7. Effect of reaction time on Ag@TiO2 nanoparticle (R=3:1, CTAC concentration=0.6 mM, temperature=80 °C). (a) UV–vis absorbance spectra and SPR peak change at different times (inset). TEM images of the samples taken from a reaction solution at (c) 10 min and (d) 70 min, respectively.

Fig. 8. Schematic illustration for the formation of Ag@TiO2 core-shell nanoparticles.

TiO2 core–shell nanoparticles can be successfully prepared using a simple one-pot synthesis method. The SPR peak of the core–shell cluster is tunable in the wavelength range of 414–499 nm by adjusting the TBT:AgNO3 molar ratio, CTAC concentration, temperature, and reaction time. This broadband absorption has never been reported elsewhere as far as we know. The use of CTAC was indispensable for the tropic growth of TiO2 on the surfaces of Ag nanoparticles. In addition, it can also control the size of the nanoparticles effectively and prevent nanoparticle adhesion on the vial. What also make this work attractive is that the nanoparticles can be synthesized using a lower temperature and shorter reaction time. The nanoparticles prepared in this work are expected to be useful as a solar energy harvesting material and a photocatalyst because of the multifunctionality obtained by combining Ag and TiO2. Related work is currently underway and will be reported in the near future.

The formation process of the core–shell cluster is illustrated in Fig. 8. The reductant was chosen because it can reduce the Ag+ ions first, and the hydrolysis of TBT in DMF is slow [16]; these behaviors are beneficial for the formation of an oxidation shell around the metal core. In the inception phase of the reaction, Ag+ ions are reduced to Ag0. Nucleation of Ag nanoparticles is initially rapid because DMF in the solution acts as a strong reducing agent. On the other hand, excessive reductant in the solution could promote secondary nucleation more effectively. Hence, the quantity of Ag nanoparticles resulting from secondary nucleation would be increased. As a result, complete Ag nanoparticles would be obtained because abundant Ostwald ripening would occur, supporting further growth. In addition, chloride ions are adsorbed on the metal nanoparticles, and then TBT quickly interacts with the chloride ions. The adsorption of Cl− on the nucleated particles prevents anisotropic growth of Ag nanoparticles and is responsible for their sphericity. The condensation polymerization of TBT progresses slowly on the surface of the Ag particles to form a layer of the TiO2 shell. The experimental findings obtained in this work suggest that Ag@

4. Conclusions In this work, super broadband Ag@TiO2 core–shell nanoparticles 59

Solar Energy Materials & Solar Cells 166 (2017) 52–60

H. Li et al.

were synthesized under well-defined conditions using AgNO3 and TBT in DMF as raw materials and CTAC as an inducer and protective agent. All the samples exhibited strong absorption in the visible light region owing to SPR of Ag nanoparticles. The SPR peak of the core–shell cluster can be tuned in a wavelength range of 414–499 nm by adjusting the synthesis parameters, i.e., the TBT:AgNO3 molar ratio, CTAC concentration, temperature, and reaction time. This is the result of agglomeration and changes in the thickness of the TiO2 shells as well as formation of individual TiO2 particles. In the absence of CTAC, only individual aggregates of Ag and TiO2 nanoparticles were formed instead of the core–shell clusters, which showed that CTAC was indispensable for the tropic growth of TiO2 on the surfaces of the Ag nanoparticles. Further, the appearance of much larger Ag nanoparticles in the absence of CTAC also confirmed that CTAC can control the size of nanoparticles effectively. A low reaction temperature was also found to be beneficial for Ag@TiO2 nanoparticle synthesis, as it is necessary to maintain formation of a uniform and thinner oxide shell in the reaction solution. The SPR peak of nanoparticles obtained under the optimum conditions was concentrated around 474 nm, which is beneficial for solar energy conversion, as the maximum solar irradiation intensity is located at 475 nm.

[18]

[19] [20]

[21] [22]

[23] [24] [25]

[26]

[27]

[28]

Acknowledgments

[29]

This work is financially supported by the National Natural Science Foundation of China (Grant No. 51322601), the Natural Science Founds of Heilongjiang Province for Distinguished Young Scholars (Grant No. JC2016009), and the Fundamental Research Funds for the Central Universities (Grant No. HIT. BRETIV. 201315).

[30]

[31] [32] [33]

References [34] [1] Y. Xiong, R. Long, D. Liu, X. Zhong, C. Wang, Z.Y. Li, Y. Xie, Solar energy conversion with tunable plasmonic nanostructures for thermoelectric devices, Nanoscale 4 (2012) 4416–4420. [2] N. Kalfagiannis, P.G. Karagiannidis, C. Pitsalidis, N.T. Panagiotopoulos, C. Gravalidis, S. Kassavetis, P. Patsalas, S. Logothetidis, Plasmonic silver nanoparticles for improved organic solar cells, Sol. Energy Mater. Sol. C 104 (2012) 165–174. [3] T. Khatib, A. Mohamed, K. Sopian, A review of solar energy modeling techniques, Renew. Sust. Energy Rev. 16 (2012) 2864–2869. [4] H. Zhang, H.J. Chen, X. Du, D. Wen, Photothermal conversion characteristics of gold nanoparticle dispersions, Sol. Energy 100 (2014) 141–147. [5] M. Chalh, S. Vedraine, B. Lucas, B. Ratier, Plasmonic Ag nanowire network embedded in zinc oxide nanoparticles for inverted organic solar cells electrode, Sol. Energy Mat. Sol. C 152 (2016) 34–41. [6] S. Chandra, J. Doran, S.J. McCormack, M. Kennedy, A.J. Chatten, Enhanced quantum dot emission for luminescent solar concentrators using plasmonic interaction, Sol. Energy Mater. Sol. C 98 (2012) 385–390. [7] K.M. Lee, C.W. Hu, H.W. Chen, K.C. Ho, Incorporating carbon nanotube in a lowtemperature fabrication process for dye-sensitized TiO2 solar cells, Sol. Energy Mater. Sol. C 92 (2008) 1628–1633. [8] S. Linic, S.U. Aslam, C. Boerigter, M. Morabito, Photochemical transformations on plasmonic metal nanoparticles, Nat. Mater. 14 (2015) 567–576. [9] A. Ramchiary, S.K. Samdarshi, T. Shripathi, Hydrogenated mixed phase Ag/TiO2 nanoparticle–a super-active photocatalyst under visible radiation with multi-cyclic stability, Sol. Energy Mater. Sol. C 155 (2016) 117–127. [10] T.P. Otanicar, P.E. Phelan, J.S. Golden, Optical properties of liquids for direct absorption solar thermal energy systems, Sol. Energy 83 (2009) 969–977. [11] H. Duan, Y. Xuan, Synthesis and optical absorption of Ag/CdS core/shell plasmonic nanostructure, Sol. Energy Mat. Sol. C, 121(4), pp. 8–13. [12] N.J. Hogan, A.S. Urban, C. Ayala-Orozco, A. Pimpinelli, P. Nordlander, N.J. Halas, Nanoparticles heat through light localization, Nano Lett. 14 (2014) 4640–4645. [13] A. Frattini, N. Pellegri, D. Nicastro, O.D. Sanctis, Effect of amine groups in the synthesis of ag nanoparticles using aminosilanes, Mater. Chem. Phys. 94 (2005) 148–152. [14] C.M. Cobley, S.E. Skrabalak, D.J. Campbell, Y. Xia, Shape-controlled synthesis of silver nanoparticles for plasmonic and sensing applications, Plasmonics 4 (2009) 171–179. [15] K. Awazu, M. Fujimaki, C. Rockstuhl, J. Tominaga, H. Murakami, Y. Ohki, N. Yoshida, T. Watanabe, A plasmonic photocatalyst consisting of silver nanoparticles embedded in titanium dioxide, J. Am. Chem. Soc. 130 (2008) 1676–1680. [16] M.M. Khan, S.A. Ansari, M.I. Amal, J. Lee, M.H. Cho, Highly visible light active Ag@TiO2 nanocomposites synthesized using an electrochemically active biofilm: a novel biogenic approach, Nanoscale 5 (2013) 4427–4435. [17] I. Pastoriza-Santos, D.S. Koktysh, A.A. Mamedov, M. Giersig, N.A. Kotov, L.M. Liz-

[35]

[36]

[37] [38]

[39]

[40]

[41]

[42]

[43] [44]

[45]

[46]

[47]

[48]

60

Marzán, One-pot synthesis of Ag@TiO2 core-shell nanoparticles and their layer-bylayer assembly, Langmuir 16 (2000) 2731–2735. H. Tsutomu, P.V. Kamat, Charge separation and catalytic activity of Ag@TiO2 coreshell composite clusters under UV-irradiation, J. Am. Chem. Soc. 127 (2005) 3928–3934. Y.H. Kim, Y.S. Kang, B.G. Jo, Preparation and Characterization of Ag-TiO2 CoreShell-Type Nanoparticles, J. Ind. Eng. Chem. 10 (2004) 739–744. H. Sakai, T. Kanda, H. Shibata, T. Ohkubo, M. Abe, Preparation of highly dispersed core/shell-type titania nanocapsules containing a single Ag nanoparticle, J. Am. Chem. Soc. 128 (2006) 4944–4945. S.E. Skrabalak, L. Au, X. Li, Y. Xia, Facile synthesis of Ag nanocubes and Au nanocages, Nat. Protoc. 2 (2007) 2182–2190. D. Barreca, A. Gasparotto, E. Tondello, Metal/oxide interfaces in inorganic nanosystems: what's going on and what's next*, J. Mater. Chem. 21 (2011) 1648–1654. W. Ping, D. Wang, T. Xie, H. Li, Y. Min, W. Xiao, Preparation of monodisperse Ag/ anatase TiO2 core–shell nanoparticles, Mater. Chem. Phys. 109 (2008) 181–183. S. Angkaew, P. Limsuwan, Preparation of silver-titanium dioxide core-shell (Ag@ TiO2) nanoparticles: effect of Ti-Ag mole ratio, Proc. Eng., 32, 2012, pp. 649–655.. X.H. Yang, H.T. Fu, K. Wong, X.C. Jiang, A.B. Yu, Hybrid Ag@TiO2 core-shell nanostructures with highly enhanced photocatalytic performance, Nanotechnology 24 (2013) 4977–4984. Y. An, L. Yang, J. Hou, Z. Liu, B. Peng, Synthesis and characterization of carbon nanotubes-treated Ag@TiO2 core–shell nanocomposites with highly enhanced photocatalytic performance, Opt. Mater. 36 (2014) 1390–1395. A. Khanna, K.V. Shetty, Solar light-driven photocatalytic degradation of Anthraquinone dye-contaminated water by engineered Ag@TiO2 core–shell nanoparticles, Desalin. Water Treat. 54 (2015) 744–757. H.W. Chen, C.Y. Hong, C.W. Kung, C.Y. Mou, K.C.W. Wu, H.C. Ho, A gold surface plasmon enhanced mesoporous titanium dioxide photoelectrode for the plasticbased flexible dye-sensitized solar cells, J. Power Sources 288 (2015) 221–228. P.Y. Silvert, R. Herrera-Urbina, K. Tekaia-Elhsissen, Preparation of colloidal silver dispersions by the polyol process, J. Mater. Chem. 7 (1997) 293–299. H.Y. Chuang, D.H. Chen, Fabrication and photocatalytic activities in visible and UV light regions of Ag@TiO2 and NiAg@TiO2 nanoparticles, Nanotechnol 20 (2009) 4227–4228. P. Ramasamy, D.M. Seo, S.H. Kim, J. Kim, Effects of TiO2 shells on optical and thermal properties of silver nanowires, J. Mater. Chem. 22 (2012) 11651–11657. L.M, P. Mulvaney, The assembly of coated nanocrystals, J. Phys. Chem. B 107 (2003) 7312–7326. H.Y. Chuang, D.H. Chen, Fabrication and photocatalytic activities in visible and UV light regions of Ag@TiO2 and NiAg@TiO2 nanoparticles, Nanotechnol 20 (2009) 4227–4228. S. Pal, Y.K. Tak, J.M. Song, Does the antibacterial activity of silver nanoparticles depend on the shape of the nanoparticle* A study of the gram-negative bacterium Escherichia coli, Appl. Environ. Microbiol. 73 (2007) 1712–1720. K.L. Kelly, E. Coronado, L.L. Zhao, G.C. Schatz, The optical properties of metal nanoparticles: the influence of size, shape, and dielectric environment, J. Phys. Chem. B 107 (2003) 668–677. W. Haiss, N.T. Thanh, J. Aveyard, D.G. Fernig, Determination of size and concentration of gold nanoparticles from UV–vis spectra, Anal. Chem. 79 (2007) 4215–4221. R. Richards, Surface and Nanomolecular Catalysis, CRC Press, Florida, U.S., 2006. M.R.H. Nezhad, M. Alimohammadi, J. Tashkhourian, S.M. Razavian, Optical detection of phenolic compounds based on the surface plasmon resonance band of Au nanoparticles, Spectrochim. Acta A 71 (2008) 199–203. A.H. Yuwono, J. Xue, J. Wang, H.I. Elim, W. Ji, Y. Li, T.J. White, Transparent nanohybrids of nanocrystalline TiO2 in PMMA with unique nonlinear optical behavior, J. Mater. Chem. 13 (2003) 1475–1479. N. Serpone, D. Lawless, R. Khairutdinov, Size effects on the photophysical properties of colloidal anatase tio2 particles: size quantization versus direct transitions in this indirect semiconductor*, J. Phys. Chem. 99 (1995) 16646–16654. Y.H. Chang, C.M. Liu, H.E. Cheng, C. Chen, Effect of geometric nanostructures on the absorption edges of 1-d and 2-d tio2 fabricated by atomic layer deposition, Acs Appl. Mater. Inter 5 (2013) 3549–3555. A. Al-Haddad, Z. Wang, R. Xu, H. Qi, R. Vellacheri, U. Kaiser, Y. Lei, Dimensional dependence of the optical absorption band edge of TiO2 nanotube arrays beyond the quantum effect, J. Phys. Chem. C 119 (2015) 16331–16337. N.T.K. Thanh, Z. Rosenzweig, Development of an aggregation-based immunoassay for anti-protein A using gold nanoparticles, Anal. Chem. 74 (2002) 1624–1628. A.K. Samal, L. Polavarapu, S. Rodal-Cedeira, L.M. Liz-Marzán, J. Pérez-Juste, I. Pastoriza-Santos, Size tunable Au@Ag core–shell nanoparticles: synthesis and surface-enhanced raman scattering properties, Langmuir 29 (2013) 15076–15082. A.C. Manikas, A. Aliberti, F. Causa, E. Battista, P.A. Netti, Thermoresponsive PNIPAAm hydrogel scaffolds with encapsulated AuNPs show high analyte-trapping ability and tailored plasmonic properties for high sensing efficiency, J. Mater. Chem. B 3 (2015) 53–58. P. Magudapathy, P. Gangopadhyay, B.K. Panigrahi, K.G.M. Nair, S. Dhara, Electrical transport studies of Ag nanoclusters embedded in glass matrix, Physica B 299 (2001) 142–146. A. Podlipensky, A. Abdolvand, G. Seifert, H. Graener, Femtosecond laser assisted production of dichroitic 3D structures in composite glass containing Ag nanoparticles, Appl. Phys. A 80 (2005) 1647–1652. M.B. Ahmad, J.J. Lim, K. Shameli, N.A. Ibrahim, M.Y. Tay, B.W. Chieng, Antibacterial activity of silver bionanocomposites synthesized by chemical reduction route, Chem. Cent. J. 6 (2012) 101.