Metallic nanoparticle-on-mirror: Multiple-band light harvesting and efficient photocurrent generation under visible light irradiation
Graphical Abstract
A photoanode concept for photoelectrochemical water splitting based on metal nanoparticles (Au, Ag, Cu) deposited on a Ti mirror to photosensitize an intermediate TiO2 layer is developed to induce the harvesting of light with sub-bandgap photon energies. The photoelectrochemical enhancement is mainly affected by a favorable interplay between thin-film interference cavity modes with both intraband and interband excitations.
Introduction
Harnessing the solar energy for photoelectrochemical (PEC) water splitting through catalysis is considered a promising route for clean energy generation to cope up with the current energy crisis. Since the pioneering work of Fujishima and Honda in 1972 [1], PEC systems have been widely studied as green energy alternatives to conventional fuel technology [2], [3]. Among available applications, artificial photosynthesis [4], PEC water splitting to generate H2 gas [2], [3], [5], and solar cells [6], [7] are of particular importance. However, despite an intensive study during the last 50 years, it is still challenging to develop processes to obtain efficient renewable sources and, in particular, enhanced photoactivity in the visible spectral range [8], [9].
According to the fundamental mechanism of photo(electro)catalysis, it is notable that light absorption for the generation of energetic charge carriers is the governing factor of the process [10]. To improve photo-activity, enhancing solar light absorption of the photocatalytic systems is essential. However, the traditional semiconducting materials used in the photocatalytic applications have a wide bandgap meaning that they absorb light in the UV region, which is only 3–5% of the solar spectrum [11], [12], while the other, i.e., ~46% of visible and ~49% of NIR light are only barely utilized. Extending the light absorption range of the photoelectrochemical materials to visible-NIR regions is a practical solution for more efficient utilization of solar energy to improve the overall solar conversion efficiency [13]. However, it is difficult to harvest the long-wavelength light efficiently for solar-to-chemical energy conversion due to its low photonic energy. Such low energy nature is unable to excite the typical large bandgap semiconductors being incompetent to photo-induce charge carriers separation [14].
Among available semiconducting materials, titanium dioxide (TiO2) is one of the promising materials for photocatalytic applications due to its outstanding physicochemical properties such as chemical stability[15], relatively low cost, as well as large availability [16]. Furthermore, the position of TiO2 valence (VB) and conduction (CB) bands fits well with the splitting of water into O2 and H2 gases [17], [18]. However, the wide bandgap of TiO2, which is 3.0 – 3.2 eV depending on the crystallographic phase (anatase or rutile), restricts its efficiency setting a limit to light absorption to UV photons [16], [19]. Numerous approaches such as bandgap engineering via doping by non-metal elements (N, C, H, P, B) [9], [20], metals (Fe, Cu, Cr, V) [21], as well as sensitization of TiO2 surface with dye molecules or quantum dots [22], [23], among others, have been applied to shift the bandgap of the TiO2 to the visible spectral range.
Another feasible alternative is to modify the surface of TiO2 with metallic nanostructures that enable localized surface plasmon resonance (LSPR) phenomena [24]. The interaction of light with nanostructured metals results in the excitation of collective charge‐density oscillations and the appearance of a strong LSPR extinction band(s) in the vis/NIR range [25]. It is well known that the absorption of light by LSPR of metal nanostructures is tunable and depends strongly on the size, shape, aspect ratio of the nanostructures, the dielectric properties of both metal, and the surrounding environment [25], [26].
When deposited on a semiconductor support, plasmonic nanostructures can generate charge carriers, so-called “hot” electrons, which can be injected into the semiconductor substrate [27]. Upon visible light illumination, hot electrons/holes are generated in Au nanoparticles (NPs), e.g. deposited on TiO2 such “hot” carriers can be utilized in a PEC configuration for water splitting. Several plasmon-induced Au-TiO2 photoelectrode configurations have been proposed to enhance the PEC activity, varying the TiO2 layer geometry from planar dense to mesoporous films, or arrays of one-dimensional nanostructures such as nanotubes, nanowires, or nanorods [16]. At the same time, plasmonic Au nanoparticles (NPs) can be synthesized either by wet chemical synthesis from Au precursors using reducing chemical agents or photo-deposition [28], [29], or by physical vapor deposition techniques such as resistive or e-beam evaporation [30], [31], and magnetron or Ar-plasma sputtering [32].
An alternative way to increase the PEC efficiency is to place the plasmonic NPs on a thin transparent semiconducting layer supported by a metallic mirror substrate (Nano Particles-on-Mirror, NPoM) [33]. The NPoM configuration exhibits multiple resonance peaks due to hybridization between localized surface plasmons in the nanoparticles and Fabry−Perót cavity mode reflections within the transparent semiconducting spacer caused by the highly reflective mirror substrate [34]. The NPoM configuration has shown promising characteristics for surface-enhanced spectroscopies used for bio- and chemical sensing [35], [36], [37]. In all these cases, however, the spacer is a dielectric material pre-deposited or formed in situ by oxidation of the metallic mirror [31], [35], [37]. While reports on plasmon-enhanced photocurrent in TiO2–noble metal NP structures date back to 1996 [38], the NPoM configuration has not been studied extensively for the PEC activity. Käll et al. demonstrated at the first time the possibility of application of NPoM systems for PEC water splitting in the visible spectral range using Au as both reflecting mirror and back contact (a continuous Au film) and absorbing material (Au NPs) in the wavelength region of 550–800 nm [34]. Furthermore, all studies published so far utilize exclusively Au as reflecting mirror and absorbing material [39], [40], [41], which limits the NPoM systems in real-life applications due to its high cost.
Gold (Au) is the most widely investigated noble metal in the plasmonic field due to its superior chemical and physical stability. A variety of plasmonic nanostructures has been produced by various chemo-physical methods [42], [43]. However, the work function of Au is 5.1 eV, which forms a Schottky barrier of ~1 eV with TiO2 and by that limiting the hot carriers mostly to the intraband excitation [44]. Furthermore, the high cost of Au limits its wide application in the PEC water splitting cells, then Ag and Cu may be considered as alternative plasmonic metals. The main advantage of these metals is that their work functions are smaller than that of Au (4.64–4.75 eV for Ag and 4.65 eV for Cu) [44], [45]; therefore, they form a lower Schottky barrier with the TiO2, which were determined as 0.83 eV for Ag/TiO2 [46], and 0.63 eV for Cu/TiO2 interfaces [44]. On the other hand, the interband transition energy levels of these plasmonic metals vary substantially from 2.15 eV for Cu to 2.4 eV for Au, and 4 eV for Ag [44]. Therefore, hot carriers in Ag can only be excited by intraband transitions in the visible range, while in Au and Cu it is possible to generate hot electrons with relatively low energy also by interband excitations [44], [47].
In this work, we study a metallic NPs/TiO2/Ti (NPoM) architecture and demonstrate the possibility to photosensitize a TiO2 layer sandwiched between metal nanoparticles (either Au, Ag, or Cu) and an underlying reflective metallic Ti mirror. The latter also acts as back contact and electron collector. The TiO2 layer thicknesses are varied from 20 to 500 nm. We show that such NPoM systems generate multiple resonance peaks within the visible spectral range, i.e., λ = 380–800 nm, due to multiple reflections within the TiO2 layer resulting in measurable photocurrent. Furthermore, experimental internal quantum efficiency calculations demonstrate at the first time that in Au- and Cu-based NPoMs the hot carriers can be efficiently excited by both intraband and interband electron transitions. At the same time, the Ag-based NPoMs display an asymmetric photocurrent peak at the LSPR band broadened by the constructive interference indicating that the utilization of intraband-exited electrons is amplified in the NPoM configuration even at wavelengths much beyond its plasmonic band. Our results reveal that the mirror component can be considered as an additional degree of freedom in the parameters that influence the efficiency of PEC systems, demonstrating a supplementary route toward the development and optimization of novel and cost-efficient solar energy light-harvesting devices.
Section snippets
Physicochemical characterization of the NPoM systems
The schematic representation of the NanoParticles-on-Mirror (NPoM) photoelectrochemical system is shown in Fig. 1a. It consists of an e-beam deposited 100-nm-thick reflective metallic Ti layer on a 100-nm-thick SiO2 on Si substrate effectively preventing light transmission, while the same NPoM systems were also deposited on quartz and glass substrates. These substrates were subsequently covered by a magnetron sputtered TiO2 layer of various thicknesses. The 3-nm-thick Au layer was then
Conclusion
Herein we developed photoelectrochemical NanoParticles-on-Mirror (NPoM) systems, which consist of a reflective Ti mirror and metallic NPs separated by semiconducting TiO2 layers with thicknesses varying from 20 to 500 nm. The optical spectra of such samples follow the typical Fabry-P é rot (FP) interference patterns and vary according to their thickness, i.e. the number of FP interference fringes increases from one to four in the visible spectral range (380–800 nm) with increasing the TiO2
Deposition of Ti films
Before Ti deposition, the Si/SiO2 substrates (SiO2/Si; 3 in., µChemicals, Si(100) p-type + 100 nm SiO2) were cleaned ultrasonically in acetone, ethanol, and deionized water (10 min for each step), then dried under an N2 stream. The Ti metal coating was e-beam evaporated from a Ti target (Hauner HMW, 99.999%) operated in a direct current mode, with a deposition power of 150 W, at room temperature. During the deposition, the working pressure was held constant at 1 × 10−3 mbar and the Ti metal
CRediT authorship contribution statement
A.B.T., M.A., and S.H. synthesized the photoanodes. A.B.T, T.S., R.M., and N.V. performed the numerical optical simulations. The photoanodes were characterized and analyzed by A.B.T. and M.A. for photoelectrochemistry, HR-SEM, XPS and XRD. A.B.T. wrote the draft with the help of M.A. and P.S. This work was initiated by A.B.T. and P.S. P.S. supervised the project.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors acknowledge the DFG and DFG Cluster of Excellence Engineering of Advanced Materials (EAM) for financial support. A.B.T would like to acknowledge the Emerging Talents Initiative (ETI) of Friedrich-Alexander University, Germany, grant number 5500102, and the DFG (grant number 442826449; SCHM 1597/38-1 and FA 336/13-1) for financial support. We thank Dr. A.V. Solomonov for discussion and design of graphical table of content.
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