Photothermal conversion enhancement of triangular nanosheets for solar energy harvest

Highlights

• Photo-thermal conversion enhancement of triangular nanosheets is investigated.

• Shape-induced tripolar resonance mode by triangular nanosheets benefits photo-thermal conversion.

• Blended nanofluids with silver triangular nanosheets and gold nanorods are proposed and tested.

• Blended nanofluids with 0.0001 vol% achieve a solar absorption efficiency of 76.9% experimentally.

Abstract

Metallic plasmonic nanofluids have been adopted to enhance the photothermal conversion in direct absorption solar thermal collectors, however, their absorption band is not broad enough to realize a sufficient adsorption. Driven by the Local Surface Plasmon Resonance (LSPR) effect, this work explores the enhancement of triangular nanosheets in photothermal conversion, to facilitate the optimization of blended nanofluids. Firstly, the size effects of triangular nanoparticles on photothermal conversion are examined using the Discrete Dipole Approximation method. Then, the blended nanofluids based on silver triangular nanosheets and gold nanorods, are proposed and a high efficiency of 76.9% is achieved experimentally with a very low volume concentration (0.0001%). The enhancement mechanism is revealed furtherly by numerically investigating the excited electric field. We believe that this work can shed light on the design of blended nanofluids based on triangular nanosheets for solar energy harvest.

Introduction

With a strong desire for renewable and clean energy [1], [2], the utilization of solar energy has drawn extensive attention [3], [4], [5]. Photothermal conversion of solar energy is a direct method that possesses a high achievable conversion efficiency, according to the review work of Ho et al. [6], [7]. It has been investigated and applied widely in many industries, such as electricity generation, steam sterilization and fuel production [8]. Most conventional solar collectors belong to surface collectors, where only absorption surface is adopted to harvest solar radiation, leading to a high temperature of absorption surface and a considerable heat loss via radiation. Direct absorption solar collectors (DASCs), which utilise the high radiation absorption property of nanoparticles suspended in a working fluid, are proposed to enhance the conversion efficiency [9]. One of the working fluids is a kind of plasmonic nanofluids, which has been investigated widely and will be focused in the present work.

Generally, photothermal conversion performance is closely dependent on the extinction efficiency [10], [11], [12], which is defined as the ratio of the attenuation energy to the incident radiation energy. As an important parameter in electromagnetic scattering, it is decided by both the absorption efficiency and scattering efficiency. For pure water, as the scattering effect can be neglected, its extinction efficiency can be evaluated by the absorption effect. Hence, extinction coefficient ($k_{\mathrm{e},\mathrm{p}\mathrm{w}}$) can be calculated as absorption coefficient ($k_{\mathrm{a},\mathrm{p}\mathrm{w}}$), furtherly expressed as [13]

$$k_{\mathrm{e},\mathrm{p}\mathrm{w}}\approx k_{\mathrm{a},\mathrm{p}\mathrm{w}}=\frac{4\pi k}{\lambda },$$

where k represents the absorption index of water (referred by Hale and Querry [14]) and λ denotes the wavelength of incident radiation. The dependence of $k_{\mathrm{a},\mathrm{p}\mathrm{w}}$ on wavelength, obtained from Eq. (1), is presented in Fig. 1, accompanied by solar spectral radiant intensity [15]. Whilst pure water demonstrates a higher absorption coefficient at the near-infrared and infrared wavelength range (1300–2500 nm), it fails to exploit those between 250 and 1300 nm, 80% of the whole radiation. Subsequently, DASCs based on nanofluids [16], [17], [18] are proposed to broaden the solar absorption band and enhance photothermal conversion performance.

The photothermal conversion in nanofluids is as follows: with photons approaching nanofluids, some of them are absorbed by water and nanoparticles; some are scattered (with their propagation direction changed) while the rest of them will leave the nanofluids without any conversion. Normally, a certain type of nanoparticles can improve the conversion performance for the incident photons, but this enhancement is limited to a narrow range of wavelength, referred to as absorption wavelength band. This means an insufficiency to cover the whole range between 250 and 1300 nm. Subsequently, to utilize solar energy within this range, various blended nanofluids were proposed [19], [20], [21]. For example, Zeng and Xuan [19] enhanced absorption efficiency using a blended nanofluids based on SiO₂/Ag plasmonic nanoparticles and multi-wall carbon nanotubes. Chen et al. [20] investigated the complementary effect in CuO-ATO based nanofluids, by building a broad absorption band. However, the nanofluids above may require a high concentration, which could improve the instability in some practical circumstances [21].

Especially, nanofluids based on metallic nanoparticles start to draw extensive attention in photothermal conversion due to a phenomenon, called Local Surface Plasmon Resonance (LSPR) [22], [23], [24], [25]. This effect (LSPR) can be excited, as the incident frequency is consistent with the oscillation frequency of electrons in metal nanomaterials. The generated resonance can enhance the electromagnetic field excited and then improve the absorption efficiency. In this effect, the incident photons, colliding with metallic nanoparticles, can cause free electrons to corporately oscillate under proper conditions [26]. The corresponding incident wavelength is to characterize the location of the LSPR, known as resonance wavelength (λ_LSPR). Actually, the performance of nanofluids, based on a certain kind of metallic particle, can be improved within a narrow absorption band. To widen the absorption band, different blended nanofluids have been proposed based on metal nanoparticles of different shapes, including nanorods [27], [28], nanoellipsoids [28], [29], nanospheres [30] and rectangular nanosheets [28]. Du and Tang [28], as our previous work, numerically examined the nanofluids based on nanorods and nanoellipsoids, and they found that LSPR can be optimized by the shape of nanoparticles. In addition, the rectangular nanosheets-induced LSPR as well, can furtherly enhance the absorption by optimizing the morphological parameters of nanosheets. Nevertheless, the absorption band is not yet wide enough, especially for the wavelength between 650 and 1300 nm.

With the findings of the strong extinction spectra of the LSPR in noble metal nanoparticles, more and more advances have been obtained. Lee et al. [31] investigated the absorption efficiency of silver nanoparticles with sharp edges. They found that sharp edges can generate multiple absorption peaks, but more edges are not necessary for broader absorption. Cheng et al. [32] investigated the effect of sharp corner on absorption peak, and they found that the sharp corner can enhance the absorption efficiency because of a point effect. Meanwhile, in the research field of optical communication, Fletcher et al. [33] reported a tripolar resonance mode in triangular nanosheets and its successful application in nano-antenna. They claimed that the unique resonance mode can arouse a strong electric field at the tips of triangular surface. Promisingly, their work inspired us a potential application of triangular nanosheets in solar-thermal conversion. Therefore, this work is aimed to explore the role of triangular nanosheets in photothermal conversion, for some ideas to design blended nanofluids with complementary coupling interaction.

In this paper, a theoretical model of extinction efficient is introduced using the Discrete Dipole Approximation (DDA) method in Section 2 and the results are discussed in Section 3. Section 4 presents our experimental validation, followed by a further discussion of enhancement mechanism, based on both numerical and experimental analysis. Finally, a conclusion is given in Section 5.