Rational Design of Plasmonic Metal Nanostructures for Solar Energy Conversion

Abstract

Open AccessCCS ChemistryMINI REVIEW1 Apr 2022Rational Design of Plasmonic Metal Nanostructures for Solar Energy Conversion Yawen Wang†, Junchang Zhang†, Wenkai Liang†, He Yang, Tianfu Guan, Bo Zhao, Yinghui Sun, Lifeng Chi and Lin Jiang Yawen Wang† Institute of Functional Nano & Soft Materials Laboratory (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials, Soochow University, Suzhou 215123 , Junchang Zhang† School of Physics and Electronic Engineering, Changshu Institute of Technology, Changshu 215500 , Wenkai Liang† Institute of Functional Nano & Soft Materials Laboratory (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials, Soochow University, Suzhou 215123 , He Yang Institute of Functional Nano & Soft Materials Laboratory (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials, Soochow University, Suzhou 215123 , Tianfu Guan Institute of Functional Nano & Soft Materials Laboratory (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials, Soochow University, Suzhou 215123 , Bo Zhao Institute of Functional Nano & Soft Materials Laboratory (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials, Soochow University, Suzhou 215123 , Yinghui Sun *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] College of Energy, Soochow Institute for Energy and Materials Innovations and Key Laboratory of Advanced Carbon Materials and Wearable Energy Technologies of Jiangsu Province, Soochow University, Suzhou 215006 , Lifeng Chi *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Institute of Functional Nano & Soft Materials Laboratory (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials, Soochow University, Suzhou 215123 and Lin Jiang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Institute of Functional Nano & Soft Materials Laboratory (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials, Soochow University, Suzhou 215123 https://doi.org/10.31635/ccschem.021.202000732 SectionsAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Plasmonic metal nanostructures, possessing unique surface plasmon resonance properties, show excellent capabilities for light trapping and coupling. On this basis, various plasmonic metal nanostructures offer extraordinary opportunities to promote the conversion efficiency of solar energy to electric energy, hydrogen energy or thermal energy, and so on. In this review article, we highlight a number of recent research achievements on the rational design of plasmonic metal nanostructures so as to maximize the utilization of the entire solar spectrum. Compared with single metal nanoparticles, multiplex (such as multicompositions, sizes, or shapes) nanoparticle structures emphasize advantages in broadening the absorption range and improving light-utilization efficiency. This review concludes with discussions regarding challenges in this research field and proposals of prospects for future directions. Download figure Download PowerPoint Introduction Metal nanoparticles (NPs), especially Au NPs and Ag NPs, possess unique localized surface plasmon resonance (LSPR) properties. Caused by incident photons with certain frequencies, collective oscillation of free electrons occurs around the surface of metal NPs, resulting in enhanced electromagnetic fields and precise light manipulation.1–4 By engineering the size, shape, composition, and dielectric environment of NPs, LSPR properties can be customized to achieve optimal performance.5–8 For example, when the sizes of Au nanospheres (NSs) increase from 24 to 221 nm, their plasmonic resonant peaks gradually red shift from 521 to 806 nm, exhibiting distinct color contrast from red to purple, then to yellow. For Au NSs with larger sizes, a phenomenon of spectral broadening is also observed due to increasing radiative losses.9 Due to their fantastic optical properties, plasmonic metal nanostructures show excellent capabilities for sunlight harvesting and solar energy conversion. It is well known that the solar spectrum irradiates a very broad range from 200 nm to 2.5 μm at sea level. According to the distribution of sunlight, ultraviolet, visible, and infrared lights account for around 3%, 43%, and 54% of the total solar energy, respectively.10 Benefiting from the rapid development of synthesis and characterization techniques, varieties of metallic NPs and nanostructures have been fabricated, whose plasmonic resonances are likely to span over the entire UV–vis–near infrared (NIR) region and maximize the utilization of solar energy. Nevertheless, sole metal NPs usually respond to a particular wavelength range, which greatly limits their solar energy utilization. Therefore, multiplex (composition, size, shape, or array) NP structures are expected to enable integrated (i.e., optical, electrical, and magnetic) properties, which may play significant roles in boosting photoabsorption and maximizing light-trapping efficiency.11–13 Until now, much progress has been made toward plasmon-induced performance improvement, mainly in solar cells, photodetectors, photocatalysis, photoelectrochemical (PEC) water splitting, solar vapor generation, and other promising applications.14–19 Review articles focusing on the achievements of plasmonic metal nanostructures for solar energy conversion have also been published.20–22 They tend to discuss the application fields of plasmonic metal nanomaterials in terms of plasmonic energy transfer mechanisms, primarily the light-trapping mechanism, hot-electron injection (HEI) mechanism, plasmon-induced resonance energy transfer (PIRET) mechanism, and multiple or hybrid enhancement mechanism.23,24 Nevertheless, a small minority of review articles focuses on the vital functions and significant contributions of multiplex plasmonic NPs and nanostructures. Therefore, we have attempted to summarize visualized structure design and emphasize multiplex metal nanostructures for plasmon-promoted solar energy conversion in widespread application fields, which is distinguished from most previous review articles. Specifically, we will discuss the topic from four aspects, including single-particle metal nanostructure, combined structures of metal NP and metal film, multiple-morphology metal nanostructures, and multicomponent metal nanostructures. Finally, we provide general design principles and challenges for integrating plasmonic metal NPs into other materials or devices. Mechanism of Plasmon-Enhanced Solar Energy Conversion Plasmonic metal nanostructures usually function as light absorbers and are incorporated with other semiconductors or carriers to participate in the reactions. Massive efforts have been made to understand the underlying mechanisms of plasmonic energy transfer, especially from a metal to a semiconductor. Notably, researchers have summarized three major energy transfer mechanisms (light trapping/scattering, HEI, and PIRET), which can be utilized to reach the theoretical maximum efficiency of solar energy conversion. Light trapping/scattering As shown in Figure 1a, LSPR extinction of colloidal metal NPs includes the total contribution of light absorption and light scattering, where the former is dominant for a small NP and the latter occupies a dominant position for a metal NP with a diameter larger than 50 nm.23 Upon integration with a semiconductor, large plasmonic NPs can scatter the incident light. The scattered light then penetrates the semiconductor and increases the photon flux in the semiconductor. In addition, the surface plasmon polariton (SPP) can also enhance the light absorption in the semiconductor structure, where SPP represents the electron oscillation that propagates along the surface of a flat metal film or nanostructure. It is worth noting that the energy of the scattering photon must be larger than the bandgap of the semiconductor to realize the light-trapping effect. More importantly, the light-trapping effect can be maximized if the spectral range of the plasmonic NP is broad enough to have a complete or huge overlap with the absorption band of the semiconductor. Figure 1 | Three major plasmonic energy transfer mechanisms: (a) Light scattering, (b) HEI, (c) PIRET. (a–c) Reprinted with permission from ref 23. Copyright 2018 The Royal Society of Chemistry. (d) Control and separation of HEI and PIRET mechanisms in [email protected]2 nanostructures. Reprinted with permission from ref 25. Copyright 2015 American Chemical Society. Download figure Download PowerPoint HEI The hot electron has an energy higher than the Schottky barrier at the metal–semiconductor interface and can be directly injected into the conduction band of the semiconductor.26 Such electrons can still be regarded as hot carriers, of which the energy level is above the conduction band level. Compared with the photogenerated electrons through interband transitions, the plasmon-induced hot electrons enable a more adequate driving force for photocatalytic reactions and a lower charge recombination. The intimate contact between the metal NP and semiconductor is of vital importance for the implementation of the HEI effect (Figure 1b). A single-electron model has been explored to quantitatively predict the generation of hot electrons in plasmonic metal NP and the injection of hot electrons into a nearby semiconductor. Notably, it shows that generation and injection are strongly sensitive to the size and shape of the NPs, as well as the presence of hot spots in the plasmonic nanostructures. PIRET The PIRET mechanism is dependent on the overlap between the resonant spectrum of the NP and the absorption band of the semiconductor, the distance between the two materials, and the orientation of the respective dipole moments.24 The plasmon acts as a donor in the photocatalysis, which is able to concentrate and amplify the intensity of the electromagnetic field. During the PIRET process, the energy in the plasmonic material can be nonradiatively transferred to the semiconductor through an electromagnetic field or a dipole–dipole interaction (Figure 1c). It has been demonstrated that the generation rate of electron–hole pairs in the semiconductor is proportional to the square of the local intensity of the electric field. Hence, the formation of electron–hole pairs close to the photocatalyst surface can be enhanced with plasmon-induced strong near-field electromagnetic resonance. On the other hand, the LSPR of plasmonic metal can reduce the thickness that is required in the semiconductor to completely absorb the incident light. On this basis, efficient separation and avoidable recombination of charge carriers can be realized because of the short distance for them to migrate to the reaction sites.27 On the basis of understanding the plasmonic energy transfer mechanisms, it is also important to control them. As shown in Figure 1d, it has been reported that the occurence of HEI or PIRET in a typical [email protected]2 system can be manipulated by regulating the physical contact and the spectral overlap between the metal and the semiconductor.25 Researchers proposed four different nanostructures to experimentally investigate the two key influencing factors. In a simple [email protected]2 core–shell NP system, the HEI process happens, but a lack of spectral overlap prevents PIRET from occurring. In [email protected]2@TiO2, the introduction of SiO2 layer with a thickness of about 10 nm hinders hot-electron transfer and thus neither HEI nor PIRET can take place. In [email protected]2@TiO2, the PIRET process can happen owing to the spectral overlap between Ag’s LSPR band and TiO2’s absorption band edge. In [email protected]2, both HEI and PIRET can happen to promote plasmon-enhanced photoconversion in TiO2. Theoretical simulations have developed models revealing the maximum efficiency of solar energy conversion in photovoltaics and PEC cells based on plasmon energy, semiconductor energy, and plasmon dephasing.28 For a single NP, scattering has become the most efficient mechanism at dephasing times close to the bulk metal (20–30 fs, radius∼50 nm), when the plasmon energy was above the semiconductor’s band gap. HEI plays the dominant role at dephasing times (3–10 fs, radius∼15 nm) and at a plasmon energy below the band gap of the semoconductor. PIRET resulted in optimal enhancement when the dephasing (5–10 fs, radius∼15 nm) was similar to the semiconductor and the spectral overlap happened between plasmonic metal and semiconductor. Generally, PIRET is expected to be the most efficient avenue to enhance photovoltaic and PEC conversion. In summary, the understanding of primary mechanisms provides design principles of plasmonic metal nanostructures and lays foundations to realize optimized sunlight utilization. Single-Particle Metal Nanostructure Au NPs have been widely brought into plasmon-based solar cells to enhance photovoltaic performance, mainly because of their good scattering capability, rich species diversity, and tunable optical properties. Energy conversion assisted by the plasmonic effect of Au NPs has been considered as a universal strategy to effectively convert solar energy directly into electricity. Sunlight is strongly confined at the surface of metal nanostructures with near-field enhancement, far-field scattering, or hot-carrier generation.29,30 Generally, plasmonic NPs have been employed either between interfaces or into active layers in varieties of solar cells, such as perovskite solar cells (PSCs), organic solar cells (OSCs), quantum dot solar cells (QDSCs), and dye-sensitized solar cells (DSSCs), to increase light absorption and maximize light coupling.31–34 Over the past two decades, fundamental research in both experiment and theory has rapidly developed and obtained significant progress.35 Remarkably, works reported by Yu et al.36 in 2006 have showed that the presence of Au NPs could improve the energy conversion efficiency in the amorphous silicon solar cells, due to enhanced electromagnetic radiation arising from scattering effect in Au NPs. Panigrahi et al.37 employed 30 nm-sized Au NPs into the perovskite layer of the PSCs, where the plasmonic energy in Au NPs could be transferred into the perovskite system in the form of hot electrons (Figure 2a). The measured absorbance for the perovskite layer after Au NPs treatment was higher than that for bare perovskite layer, leading to efficient light trapping (Figure 2b).37 Consequently, with the incorporation of Au NPs, the current density was increased from 15.69 to 19.35 mA cm−2, and the obtained power conversion efficiency (PCE) was improved from 10.8% to 13.5% (Figure 2c). Nevertheless, bare metal NPs often serve as recombination centers, possibly causing exciton quenching losses at the surface of metal NPs.39 To address this issue, Ye et al.40 coated Au NPs with SiO2 to fabricate [email protected]2 core–shell NPs which were then introduced between the TiO2 layer and perovskite layer. As a result, the PCE of PSCs was increased from 15.8% to 20.3% with enhanced light trapping and mobilities of charge carriers. Findings like these reveal that the incorporation of plasmonic metal NPs could be a hopeful method for PSCs to reach their theoretical PCE limits. Figure 2 | (a) The illustration of device structure. (b) UV–vis spectra of perovskite absorbers with and without Au NPs. (c) J–V curves of PSCs with (red) and without (black) Au NPs under AM 1.5 simulated sunlight. (a–c) Reprinted with permission from ref 37. Copyright 2019 The Royal Society of Chemistry. (d) High-resolution transmission electron microscopy (HRTEM) characterization of [email protected]2@Pa NPs. Insert is Pa’s chemical structure. (e) Normalized UV–vis absorption spectra of Ag NPs, [email protected]2 NPs, and [email protected]2@Pa NPs. (f) J–V reverse (filled) and forward (open) scan curves of MAPbI3 PSCs with (circles) or without (squares) [email protected]2@Pa NPs. Inset illustrates the device structure. (d–f) Reprinted with permission from ref 38. Copyright 2019 American Chemical Society. Download figure Download PowerPoint Compared with Au NPs, Ag NPs are relatively cheap plasmonic nanomaterials and more importantly, Ag NPs provide particular advantages in generating stronger electromagnetic field enhancement and supporting surface plasmons within the short wavelength regions.41 In earlier studies, small sized (10–20 nm) Ag NPs fabricated by electrodeposition or thermal evaporation methods have been applied to solar cells to overcome the weak absorbance of devices.42,43 Later, with improved scientific research, device configuration and performance have been continuously optimized. Baek et al.44 demonstrated that the addition of 40–50 nm-sized Ag NPs directly influenced the carrier collection process and facilitated the mobility of carrier in OSCs, thus maximizing the internal quantum efficiency (IQE) to nearly 100% and achieving a PCE of 10.1%. In addition, Yao et al.38 proposed a titania/benzoic-acid-fullerene bishell-decorated Ag core nanostructure ([email protected]2@Pa) (Figure 2d), which was then applied to both OSCs and PSCs, where TiO2 enabled the removal of exciton quenching and fullerene shell ensured uniform particle dispersion within active layer. Compared with the Ag NP and [email protected]2 nanostructure, the optical absorption peak of [email protected]2@Pa nanostructure was red-shifted to 424 nm and the absorption intensity below 400 nm was enhanced (Figure 2e). Arising from the activation of exciton–plasmon coupling, the existence of plasmonic NPs could enhance free charge carrier generation and optimize the carrier transport properties. As illustrated in Figure 2f, the PCE of MAPbI3 devices was greatly improved from 18.37% to 20.24% with the presence of [email protected]2@Pa. Apart from Au and Ag nanostructures, Al nanostructures are also attractive candidates for performance improvement in photovoltaic devices, due to their tunable spectral response from ultraviolet to NIR regions, abundant reserve, and lower cost.32 Liang et al.16 employed well-dispersed Al nanodisk arrays into OPVs and realized remarkably increased PCEs, since Al nanodisk arrays enhanced the light absorption of the active layer and caused better alignment of energy level. More importantly, an additional procedure of insulating layer modification could be omitted, attributable to the naturally formed oxide layer on the Al surface when exposed to air. Additionally, plasmon-enhanced light harvesting has significant applications in solar steam generation, seawater desalination, and water purification.45–48 It has been reported that an ideal solar absorber should have broadband absorption over the solar spectrum, excellent photothermal conversion efficiency, and good thermal insulation ability.49,50 In 2013, Halas et al.51 recorded plasmon-induced steam formation at individual 98 nm-sized Au NPs surface and investigated several influence factors during the process, including the size of nanobubble around the Au NPs, the intensity of incident light, and the surface temperature of NPs. Since then, many researchers have focused their interest in plasmonic solar-vapor conversion. Fang et al.52 deposited Ag NPs on diatomite, and such structures showed outstanding solar-vapor evaporation efficiency (92.2%) under one-sun illumination, because of the increased light harvesting and localized heating effect. Gao et al.53 fabricated the first ever solar absorber gel using new-typed Au nanoflowers, which had positive influence on plasmonic solar vaporization, mainly attributed to broadened light absorption, high photothermal conversion, and effective macro–nano heat insulation (Figures 3a and 3b). Upon such design, the evaporation efficiency could achieve 85% under one-sun illumination (Figure 3c). Huang et al.54 loaded trepang-shaped Au nanostructures into polymeric aerogels, which were then applied to practical seawater steam generation (Figure 3d). Attributable to rational manipulation of the structural anisotropy, a Au nanotrepang showed strong and widened absorption over 300 to 1100 nm (Figure 3e). Under one-sun irradiation, the solar steam generation rate of samples modified with Au nanotrepang reached 2.7 kg m−2 h−1, which was 16.1 times higher than those without the Au nanotrepang. During six on–off cycles, it was summarized that the absorber network exhibited good reusability and stability under different light intensities (Figure 3f). To this end, through ingenious design of the morphology of individual metal NPs, rather than relying on particle aggregation, the LSPR peaks could also be broadened to achieve promoted conversion performance. Figure 3 | (a) Nanoscopic view of the solar absorber gel comprised of Au nanoflowers. (b) UV–vis spectral comparison of Au NP and Au nanoflower silica gels and their solutions. (c) The temperature differences and evaporation efficiencies for contrast systems under one sun. (a–c) Reproduced with permission from ref 53. Copyright 2018 WILEY-VCH Verlag GmbH & Co. KGaA. (d) Schematics of photothermal steam generation. (e) Change of UV–vis absorption spectra from Au NR to Au nanotrepang. (f) Stability test of the sample under different solar irradiation intensities. (d–f) Reprinted with permission from ref 54. Copyright 2020 The Royal Society of Chemistry. Download figure Download PowerPoint Until now, varieties of carbon-based materials (such as graphene and carbon quantum dots) have showed wonderful light absorption and light-to-heat conversion effect due to the energy transfer from the excited electrons to the atomic lattice vibration.55–58 In comparison, plasmonic nanomaterials can form subwavelength localized hot spots through the light interactions and also have advantages in the and other physical and chemical properties. on a of plasmonic nanomaterials and carbon-based materials are expected to be an strategy to enhance light trapping and coupling. et a plasmonic NPs, and carbon to achieve extraordinary performance. Au showed an absorption peak at nm, absorption peak at As for a Au [email nanostructure, in of it showed outstanding absorbance the The introduction of Au and facilitated the separation of photogenerated electron–hole the photothermal properties of Consequently, with Au [email and NPs, Au [email better for charge transfer, resulting in a higher water evaporation rate of kg m−2 and a photothermal conversion efficiency of under one a single of plasmonic metal NPs or their structures have shown improved solar energy conversion efficiency. it in the optical absorption range and increasing absorption Therefore, the rational design of multiplex metal nanostructures is important and such as combined structures of metal NPs and metal film, metal nanostructures, and multicomponent metal nanostructures. Metal Nanostructures structures of metal NPs and metal film single-particle metal nanostructures have been considered as to the increasing because of their physical and chemical properties. For example, single-particle metal nanostructure usually has plasmonic resonant On the other hand, to spectral more nanostructures are to be which on current synthesis and it is to investigate multiplex metal nanostructures with optical and properties. 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