Introduction

Nobel nanoparticles have many of the usual physical and optical properties, including surface plasmon resonance.1,2,3,4,5,6 Surface plasmon resonance produces a stronger electromagnetic field on the surface of gold nanoparticles and has been applied to the enhancement of Raman scattering,7 photoluminescence,8 biolight emission devices9 and solar cells.10

Recently, surface plasmon resonance of noble metals has been used to separate charges on TiO2 films to form negative potential changes and anodic currents under visible light radiation,3,11,12,13,14 which will potentially be able to facilitate the promising application of plasmon-sensitized solar cells and plasmonic solar cells in the future. A few studies3,11,12,13,14 have reported results concerning the phenomenon of plasmon-induced charge separation at TiO2 films loaded with gold nanoparticles. In these experiments, photons resulting from incident light couple with a plasmon to serve as an exciton to separate the charge on gold nanoparticles. The electron is then ejected from the gold nanoparticle onto TiO2 films. The photocurrents of TiO2 films loaded with gold nanoparticles were measured under visible light radiation. However, surface plasmon resonance is an electromagnetic standing wave. Few studies have reported how an electromagnetic standing wave, a frequency-dependent physical parameter, is able to produce a frequency-independent photocurrent (for frequencies of visible light ranging from 1014 to 1015 Hz). By understanding the mechanism of surface plasmon resonance and optimizing the fabrication of a photocell, the efficiency of photovoltaic cells is able to be enhanced.

In this paper, we use gold nanoparticles loaded on TiO2 film to form photocurrents, as shown in Figure 1. Surface plasmon resonance is used to separate the charges on TiO2 films and form the photocurrent. We then use a theoretical model to confirm the phenomenon of charge separation and photocurrent formation and the relationship between surface plasmon resonance and photocurrent formation. This result is applicable to improve the intensity of photocurrents in the field of plasmon-sensitized solar cells.

Figure 1
figure 1

Scheme of plasmon-induced charge separation on gold nanoparticles in a solar cell. The SPR-induced dipole acts as an energy level. SPR enhances the pumping rate of the photoelectronic formation. The photoelectric current will rise. The chemical potential of electrolyte will change due to the external electric field of SPR. The open-circuit voltage is the difference between the conductive band of TiO2 and the chemical potential of the electrolyte. The open-circuit voltage will be shifted at the same time. Gold NPs, gold nanoparticles.

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Materials and methods

Twelve grams of P-25 TiO2 (Degussa, Shinjuku-ku, Japan) powder in 15 ml deionized water mixed with 0.2 ml Triton-X 100 (Sigma, St. Louis, MO, USA) and 0.4 ml acetaldehyde (99.5%; Fluka, Heidelberg, Germany) were vibrated ultrasonically for 3 days. Subsequently, 0.5 ml TiO2 solution was spin-coated on a fluorine-doped tin oxide (FTO) glass substrate at 1000 rad s−1 for 20 s and then sintered at 500 °C for 10 h. After calcinations at 500 °C for 10 h, the FTO had a sheet resistance of 10.3 Ω per square, and the P-25 had a higher anatase content (rutile/anatase=1:4).15

Au nanoparticles (NPs) were synthesized by chemical reduction, by adding 30 ml of 3 mM HAuCl4(aq) (AlfaAesar, Lancashire, UK) to 80 ml of 5 mM tetraoctylammonium bromide (Sigma) in hexane. To this solution was added 25 ml of 0.05 M NaBH4(aq) (Alfa Aesar). The color of the solution turned from yellow to black. Au NPs were modified by tetraoctylammonium bromide. The morphology of the Au NPs in hexane under transmission electron microscopy (TEM) observation and on TiO2 FTO substrate is presented in Figure 2.

Figure 2
figure 2

Morphology of Au NP (a) in hexane under TEM observation and (b) on TiO2 FTO substrate. Uniform 6 nm-diameter Au NP modified by tetraoctylammonium bromide results from the balance between solvent and surfactant. Rough surface of TiO2 on FTO substrate (∼270 nm) increases the surface area to adsorb chemicals.

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Mercapto-propyl-tri-methoxy-silane (95%; Alfa Aesar) was used as a molecular linker connecting the substrate with the Au NPs through self-assembled monolayers. The TiO2/FTO was immersed in the solution (NH4OH/H2O2/H2O=5:5:1 in volume) for 3 h in order to get modified OH− on the surface of the substrate. The TiO2/FTO was then immersed in 1% 3-mercaptopropyltrimethoxysilane hexane for 1 day, and then in a Au NPs colloid for 1 day. Au NPs were modified on the substrate. Molecular linkers, including 2 wt-% 2-mercaptoethanol (98%; Alfa Aesar) and 2 wt-% 1,3-propanedithiol (97%; Alfa Aesar) in hexane, were then added to modify the surface of the Au NPs. The Au NPs were immersed with the molecular linkers for 1 day. The sample was immersed in the Au NPs colloid again for 1 day. By repeating the above process five times, layer-by-layer Au NPs were fabricated on the substrate as an anode electrode. Thickness and fraction of layer-by-layer Au NPs on the TiO2/FTO substance are presented in Figure 3. Pt was sputtered on the indium-doped tin oxide glass as the cathode electrode.

Figure 3
figure 3

(a) Thickness and (b) fraction of layer-by-layer Au NPs on TiO2/FTO substance. According to the effective medium theory, the relationship between thickness and coverage for Au NPs on TiO2/FTO substance is fitted by Fresnel equation via ellipsometric spectroscopy. The thickness of one layer of Au NPs on TiO2/FTO substance is ∼6 nm, corresponding to the TEM observation. As the number of layer-by-layer Au NPs increases, the coverage also increases. Finally, the coverage of layer-by-layer Au NPs will grow slowly due to the saturation of the reactive surface.

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The electrolyte was a water-based solution (35% ethanol and 65% water), including 0.1 M Ce(NO3)3/0.05 M Ce(NO3)4 (99%; ICN Biomedicals, Inc., Aurora, OH, USA).

The morphologies of the Au NPs were observed by TEM. The as-grown product was observed by a Hitachi model HF-2000 transmission electron microscope operating at 200 kV. The absorption of Au NPs was observed by UV-visible spectrometry (Hitachi U-2001 spectrophotometer). The sample was directly injected into a quartz tube for UV-visible light analysis. The optical properties and thickness were determined by ellipsometry (h-VASE; Woollam, Lincoln, NE, USA) in the visible light range. The electric properties, current and voltages, were measured by a high voltage Source-Measure unit (Keithley 237). The power of the simulated solar energy (AM1.5) was 100 mW cm−2. The J–V curve under solar simulator radiation was recorded.

Results and discussion

Energy level of surface plasmon resonance

A metal cluster with a diameter greater than 3 nm has bulk-like physical properties and energy levels that are not discrete.16 In our case, the diameter of gold NPs is ∼6 nm as shown in Figure 2a. The Fermi-level of bulk-like gold NPs is the work function. Surface plasmon resonance induces excitons to separate the positive and negative charges. The density of states of the surface plasmon resonance range from 510 to 580 nm in the visible light region (see also Figure S1 in Supplementary Information). In a solar cell, the Fermi-level of bulk-like gold NPs contacts that of the semiconductor due to the balance of the electronic density on the surface. Energy level, E, referred by the expectation value of the surface plasmon resonance’s induced dipole, is under the work function of gold NPs. The difference in energy between E and the work function is Eg, as shown in Figure 1. The SPR of gold NPs induces a dipole, which acts as an energy level to support the transition of photoelectrons. Photoelectrons then transfer from the SPR level of the gold NP to the conduction band of the semiconductor.17 Photoelectrons are injected into the anode electrode and are inhibited from returning due to the Schottky barrier.18 The charged gold NP is renewed by the donor from the electrolyte, as shown in Figure 1.

SPR enhances photoelectric voltage

Open-circuit voltage (Voc) is determined by the difference between the chemical potential of the electrolyte and the conductive band (CB) of a semiconductor (). For a regular solution case, the Gibbs free energy of an electrolyte is presented as the following:

where μ is the Gibbs free energy of electrolyte, is the ratio of Ce4+, is the ratio of Ce3+, is the Gibbs free energy of Ce4+, is the Gibbs free energy of Ce3+, and Ω is the interaction relationship between and (Ω is constant for a regular solution.). When an external electrical field is applied to the system, the free energy is represented as G′=G−qV, where q is the charge of the system, V is the potential of the external electrical field and G is the Gibbs free energy. For the Ce4+ system, the free energy is due to the four electrons of the system. On the other hand, for the Ce3+ system, the free energy is , because of the three electrons of the system.

The open-circuit voltage (Voc) is represented as the following:

The open-circuit voltage (Voc) is proportional to the external electrical field.

The SPR of gold NPs applies a strong external electric field in the near field. As the number of gold NPs increases, the intensity of SPR rises, as shown in Figure 4. When the array of gold NPs is random, as in Figure 2b, no special peak of SPR appears due to the geometric effect. The open-circuit voltage (Voc) is proportional to the external electric field for our experimental results, as shown in Figure 5c, corresponding to the above result. SPR produces a stronger electric field that enhances the open-circuit voltage of a photovoltaic cell.

Figure 4
figure 4

(a) Absorption and (b) relationship between SPR intensity and volume of layer-by-layer Au NPs on TiO2/FTO substrate. SPR peak of layer-by-layer Au NPs on TiO2/FTO substance is at 520 nm from UV-visible spectroscopic observation. SPR intensity is recorded at 520 nm. The count of background contributed by the TiO2/FTO substance is 24.6. Volume of layer-by-layer Au NPs is calculated by ellipsometric data, which is multiplied by coverage fraction. The relationship between SPR intensity and volume is a linear one (correlative coefficient is +0.97). As the volume of Au NPs increase, the intensity of SPR also increases. The extinction intensity is referred to as the SPR intensity.

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Figure 5
figure 5

(a) Photocurrent and the relationship between SPR intensity and (b) short circuit current (Jsc); (c) open circuit current (Voc) and (d) efficiency of optic–electric conversion for layer-by-layer Au NPs on TiO2/FTO substance under the solar simulator (AM1.5) illumination. Relationships between SPR intensity and short circuit current (Jsc) and open circuit current (Voc) are linear. The relationship between efficiency of optic-electric conversion and SPR intensity is a second-order relationship. The correlative coefficient between SPR intensity and short circuit current (Jsc), open circuit current (Voc), and efficiency are +0.94, +0.96 and +0.95, respectively. The background of SPR intensity is removed.

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SPR enhances photoelectric current

The electron excitation process19,20 in a photovoltaic cell is shown as follows:

where g is the generation, τ is the lifetime, φn is the energy level for electrons and n is the particle number. When processing for a short time interval, g, φn and n are represented as follows:

Then this can be applied in the electron excitation process:

And and . Then

In the above equation, the chemical effective capacity is , the physical effective current is iph=eg, and the reaction effective resistance is . In the above equation, the term, eg, is the special solution for the effective current: .

SPR can produce a strong electric field that functions as the hot spot to enhance the pumping rate of the electrons between bands. During this process, the generation, g, will increase. As the generation increases, the effective current in a photovoltaic cell will rise. The short-circuit current will rise at the same time. In our experiment, the short-circuit current is proportional to the SPR intensity, as is shown in Figure 5b, which presents the relation: .

SPR enhances photoelectric properties

In a photovoltaic cell, the current–voltage curve of a solar cell yields important operational parameters,21 among which are the short-circuit current Jsc, the open-circuit voltage Voc, the current Jmp and voltage Vmp at the maximum power point Pmax. The fill factor (FF) is defined as

The solar conversion efficiency (η) is given by

where Ps is the input solar irradiance (in mW cm−2).

In a p–n junction solar cell model,22 the Vmp, Jmp and FF can be determined as:

where Js is the saturated current in a photovoltaic cell, and

When SPR is applied in a solar cell, and are put into the above equations, then Vmp′, Jmp′ and FF′ are represented in the SPR solar cell case as:

where and . (b−1)+aV/kBT and −ln(b+aV/kBT) act as the antagonism for the FF′, which makes the relationship between SPR intensity and the fill factor irregular, as shown in Table 1. The solar conversion efficiency (η′) is given by

Table 1 Photoelectric characteristic of layer-by-layer Au NPs
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The solar conversion efficiency (η′) is a second-order equation of the external electric SPR field. In our experiment, the solar conversion efficiency (η′) is a second-order equation of SPR intensity, as shown in Figure 5d.

By modeling the phenomenon of photoelectric conversion efficiency induced by SPR, the mechanism of solar conversion efficiency can be better understood, which could be used to enhance the efficiency of plasmon-sensitized solar cells in the future.

Conclusion

Gold nanoaprticles separate charges under visible light radiation. We fabricated gold nanoparticles using the layer-by-layer method on a TiO2 film to form multilayers in order to increase the intensity of the photocurrent. The efficiency of photoelectric conversion increased as the thickness increased. After optimization, nobel metal nanoparticles are possible candidates to replace dyes in solar cells and to act as plasmon-sensitized solar cells and plasmonic solar cells in the future.