Studied Localized Surface Plasmon Resonance Effects of Au ... - MDPI

catalysts Article

Studied Localized Surface Plasmon Resonance Effects of Au Nanoparticles on TiO2 by FDTD Simulations Guo-Ying Yao, Qing-Lu Liu and Zong-Yan Zhao * Faculty of Materials Science and Engineering, Kunming University of Science and Technology, Kunming 650093, China; [email protected] (G.-Y.Y.); [email protected] (Q.-L.L.) * Correspondence: [email protected]; Tel.: +86-871-65109952; Fax: +86-871-65107922 Received: 7 April 2018; Accepted: 21 May 2018; Published: 5 June 2018

Abstract: Localized surface plasmon resonance (LSPR) plays a significant role in the fields of photocatalysis and solar cells. It can not only broaden the spectral response range of materials, but also improve the separation probability of photo-generated electron-hole pairs through local field enhancement or hot electron injection. In this article, the LSPR effects of Au/TiO2 composite photocatalyst, with different sizes and shapes, have been simulated by the finite difference time domain (FDTD) method. The variation tendency of the resonance-absorption peaks and the intensity of enhanced local enhanced electric field were systematically compared and emphasized. When the location of Au nanosphere is gradually immersed into the TiO2 substrate, the local enhanced electric field of the boundary is gradually enhanced. When Au nanoshperes are covered by TiO2 at 100 nm depths, the local enhanced electric field intensities reach the maximum value. However, when Au nanorods are loaded on the surface of the TiO2 substrate, the intensity of the corresponding enhanced local enhanced electric field is the maximum. Au nanospheres produce two strong absorption peaks in the visible light region, which are induced by the LSPR effect and interband transitions between Au nanoparticles and the TiO2 substrate. For the LSPR resonance-absorption peaks, the corresponding position is red-shifted by about 100 nm, as the location of Au nanospheres are gradually immersed into the TiO2 substrate. On the other hand, the size change of the Au nanorods do not lead to a similar variation of the LSPR resonance-absorption peaks, except to change the length-diameter ratio. Meanwhile, the LSPR effects are obviously interfered with by the interband transitions between the Au nanorods and TiO2 substrate. At the end of this article, three photo-generated carrier separation mechanisms are proposed. Among them, the existence of direct electron transfer between Au nanoparticles and the TiO2 substrate leads to the enhanced local enhanced electric field at the boundaries, which is favorable for the improvement of photocatalytic performance of TiO2 . These findings could explain the underlying mechanism of some experimental observations in published experimental works, and helpful to design highly efficient composite photocatalysts that contain noble metal co-catalyst nanoparticles. Keywords: photocatalysis; localized surface plasmon resonance; Au nanoparticle; FDTD simulation

1. Introduction TiO2 is a kind of fascinating photocatalyst that possesses the ability of splitting water, which was first observed by Fujishima and Honda [1]. Afterwards, many scientists have also made many contributions in photocatalytic technology, such as clarifying the intrinsic mechanism, improving the photocatalytic performance, and exploring novel efficient photocatalysts. It is well known that TiO2 can only be activated by UV-light irradiation, whose energy accounts for only 5% of the solar spectrum. In order to make efficient use of solar energy, researchers have performed a large number of studies to improve the photocatalytic performance of TiO2 . The most conventional strategy is Catalysts 2018, 8, 236; doi:10.3390/catal8060236

www.mdpi.com/journal/catalysts

Catalysts 2018, 8, 236

2 of 15

doping with metals [2,3] or non-metals [4,5] into the TiO2 lattice. This approach can narrow the band gap of TiO2 to absorb visible light. However, as the impurities are introduced, it inevitably has some intrinsic shortcomings, and the effects of the modification are very limited. Meanwhile, using a noble metal as a co-catalyst loaded onto the surface of TiO2 is another effective modification strategy to choose [6,7]. The co-catalyst is used as an electron trap to suppress the recombination of photo-generated electron-hole pairs, resulting in the improvement of photocatalytic performance. The optical properties of metal nanoparticles are known to be characterized primarily by the collective vibration of their free electrons, the so-called localized surface plasmon resonance (LSPR). Initially, LSPR plays an important role in biosensors, which have a very high sensitivity in the detection of biomolecular-specific interaction analysis. Until 2008, Awazu et al. placed the LSPR effects in the field of photocatalysis and presented the concept of plasmonic photocatalysis [8], which attracted more researchers to focus on this subject. For example, Tanaka et al. prepared spherical gold particles supported on TiO2 , which exhibited excellent performance due to LSPR, and the apparent quantum efficiency reached 7.2% in the visible-light region [9]. LSPR can directly extend the light absorption range of TiO2 to the visible region without reducing the band gap value of TiO2 , which is different with the doping modification method. For example, Feng et al. have found a new strong absorption peak at 560 nm in the case of Cu/TiO2 composite photocatalyst under the visible-light region [10]; Mubeen et al. also observed that the absorption spectrum of Au/TiO2 composite photocatalyst extends to 620 nm [11]. It is known that metal nanoparticles, such as gold (Au) [12–14], silver (Ag) [15–17], copper (Cu) [18–20], and bismuth (Bi) [21], also show strong optical absorption due to LSPR in the visible-light region. Among then, Au has attracted extensive concern, due to its excellent catalytic performance and remarkable LSPR effects. Thus, it has been used as a catalyst/co-catalyst in many composite photocatalysts, such as Au/Cu2 O [12,22], Au/TiO2 /g-C3 N4 [23], and Au/La2 Ti2 O7 [24]. In these cases, the photocatalytic performance is significantly promoted because Au particles not only act as electron traps and active sites, but also induce the LSPR effects. According to the generation mechanism of LSPR, the size, shape, and composition of metallic nanoparticles are the key factors that predominately determined the effects of LSPR. Therefore, many research groups have focused on how to regulate and utilize the LSPR effect through the selection of materials and the design of micro/nano-structures. On the other hand, the optical properties of metallic nanoparticles are also determined by the electron interband transition with the substrate. However, energetic charge carriers created by the Au interband transitions are frequently ignored in the field of photocatalysis. In the present work, Au nanoparticles supported on a TiO2 substrate are chosen as the research subject. Herein, we report the change of the size and shape of Au nanoparticles loaded on the different positions of the TiO2 substrate. Furthermore, based on finite difference time domain (FDTD) simulated results, the electron interband transition between Au nanoparticles and the TiO2 substrate are determined, and the potential influences on LSPR effects are also discussed. 2. Results and Discussions 2.1. Au Nanospheres In the present work, we considered four different locations of Au nanoparticle loading on the TiO2 substrate as shown in Figure 1: (a) the Au nanoparticle just contacts the surface of the TiO2 substrate, which is labeled as “Model A”; (b) the Au nanoparticle is half-embedded into the TiO2 substrate, and the center of the Au nanoparticle coincides with the surface of the TiO2 substrate, which is labeled as “Model B”; (c) the Au nanoparticle is completely embedded in the TiO2 substrate, which is labeled as “Model C”; and (d) the Au nanoparticle is embedded in the TiO2 substrate to a depth of 100 nm beneath the surface, which is labeled as “Model D”. In these models, the configurations of Model C and Model D are similar to the core-shell structure of Au/TiO2 composite photocatalysts in published experimental work, so they can better model this special nano-structure.


3 of 15

Catalysts 2018, 8, x FOR PEER REVIEW

3 of 15


3 of 15

Figure 1. location The location of Au nanoparticlesloaded loadedon on the the TiO (a)(a) The AuAu nanoparticle just just Figure 1. The of Au nanoparticles TiO22substrate: substrate: The nanoparticle in contact the surface of the TiOsubstrate; 2 substrate; (b) the Au nanoparticle is half-embedded into the in contact withwith the surface of the TiO (b) the Au nanoparticle is half-embedded into the 2 TiO2 substrate, and the center of the Au nanoparticle coincides with the surface of the TiO 2 substrate; 1. The location of Au of nanoparticles loaded on coincides the TiO2 substrate: The Au just TiO2Figure substrate, and the center the Au nanoparticle with the(a) surface ofnanoparticle the TiO2 substrate; (c) the Au nanoparticle is completely embedded in the TiO 2 substrate; and (d) the Au nanoparticle is in contact with the surface of the TiO 2 substrate; Au nanoparticle is half-embedded into the is (c) the Au nanoparticle is completely embedded in(b) thethe TiO and (d) the Au nanoparticle 2 substrate; embedded in the TiO2 substrate to a depth of 100 nm beneath the surface. TiO2 substrate, and2 the center of Au nanoparticle withsurface. the surface of the TiO 2 substrate; embedded in the TiO substrate tothe a depth of 100 nm coincides beneath the

(c) the Au nanoparticle is completely embedded in the TiO 2 substrate; and (d) the Au nanoparticle is Due to the high spherical symmetry of Au nanospheres, the plasma oscillations are isotropic, embedded in the TiO2 substrate to a depth of 100 nm beneath the surface.

and only exhibit single LSPR absorptionofpeak the wavelength range of 300–900 nm, as shown in Due to the highaspherical symmetry Auinnanospheres, the plasma oscillations are isotropic, Figure 2. For isolated Au nanospheres with different radii, their corresponding resonance absorption and onlyDue exhibit a single LSPR absorption the wavelength range of 300–900 nm, as shown in to the high spherical symmetrypeak of Auinnanospheres, the plasma oscillations are isotropic, peaks areisolated centeredAu at different positions in different the extinction spectra: 517 nm (for R = 25 nm), 530 nm (for Figure For nanospheres with their corresponding resonance absorption and2.only exhibit a single LSPR absorption peak in theradii, wavelength range of 300–900 nm, as shown in R = 50 nm), 581 nm (for R = 75 nm), and 610 nm (for R = 100 nm), respectively. Table 1 shows the peaks are centered at different positions in the extinction spectra: 517 nm (for R = 25 nm), 530 Figure 2. For isolated Au nanospheres with different radii, their corresponding resonance absorption nm corresponding electric near-field intensity at these resonance peaks for different radii of Au centered at different the610 extinction spectra: 517nm), nm (for R = 25 nm), 530 nm1(for (for peaks R = 50are nm), 581 nm (for R =positions 75 nm), in and nm (for R = 100 respectively. Table shows nanospheres. It is obviously observed that these peaks are red-shifted with the increase of the radius R = 50 nm), 581 nm (for Rnear-field = 75 nm), intensity and 610 nm (for R =resonance 100 nm), respectively. Table 1 shows the the corresponding electric at these peaks for different radii of Au of the Au nanospheres, which is in accordance with published results [13,25,26]. Furthermore, as the corresponding electric near-field intensity at these resonance peaks for different radii of Au nanospheres. It isnanospheres obviously increases, observed the thatextinction these peaks are red-shifted with of the radius of Au coefficient also increases. In the the increase case of Au nanospheres. is obviously observed these peakswith are red-shifted with the [13,25,26]. increase of the radius radius of the Au It nanospheres, is that in accordance published results Furthermore, nanospheres with radii of 25which nm, the extinction coefficient is the smallest (the black curve in Figure of the Au nanospheres, which is in accordance with published results [13,25,26]. Furthermore, as the as the2),radius nanospheres increases, extinction coefficient also increases. In the the case of which of is Au almost equal to zero, while thethe corresponding electric near-field intensity reaches radius of Au nanospheres increases, the extinction coefficient also increases. In the case of Au maximum with the Au nanospheres with radii of 75 nm. These simulated results suggest that the Au nanospheres with radii of 25 nm, the extinction coefficient is the smallest (the black curve in nanospheres with radii of 25 nm, the extinction coefficient is the smallest (the black curve in Figure of resonance of LSPR can be by varying the radiusnear-field of the Au intensity nanospheres. Figurefeatures 2), which is almostabsorption equal to zero, while thetuned corresponding electric reaches 2), which is almost equal to zero, while the corresponding electric near-field intensity reaches the the maximum with the Au nanospheres with radii of 75 nm. These simulated results suggest that maximum with the Au nanospheres with radii of 75 nm. These simulated results suggest that the the features of resonance absorption ofofLSPR by varying varyingthe theradius radius nanospheres. features of resonance absorption LSPRcan canbe be tuned tuned by ofof thethe AuAu nanospheres.

Figure 2. The extinction spectra of isolated Au nanospheres with different radii. Table 1. The electric near-field intensity of gold nanoshperes.

Figure 2. TheRadius/nm extinction spectra of isolated Peak Au nanospheres radii. 2 The Absorption of LSPR/nmwithEdifferent

Figure 2. The extinction spectra of isolated Au nanospheres with different radii. 25 517 4.400 Table 1.50The electric near-field intensity of gold nanoshperes. 530 5.983

Table 1. The electric near-field intensity of gold nanoshperes. 75 Radius/nm 100 25 Radius/nm 50 25 75 50 100 75 100

581 The Absorption Peak of LSPR/nm 610 The Absorption 517 Peak of LSPR/nm 530 517 581 530 610 581 610

6.412 E2 4.883 4.400E2 5.983 4.400 6.412 5.983 4.883 6.412

4.883


4 of 15

The absorption spectra of Au nanospheres loaded on the TiO2 substrate at different locations are illustrated in Figure 3. The absorption spectrum of anatase TiO2 (the green curve) perfectly reproduces those results of experimental measurements [1], implying the calculation method and setting parameters are reliable. In the case of Au nanospheres with radii of 100 nm, there are four absorption peaks that are respectively centered at ~350, ~420, ~520, and ~610 nm in Figure 3a for “Model A”. The absorption peak centered at ~350 nm is undoubtedly produced by the TiO2 substrate, which is labeled as “a” in Figure 3a. This further shows that there is a strong interaction between Au and TiO2 . This interaction is mainly determined by the interfacial electronic states, resulting in the electron transition from the high occupied states of Au nanoparticles to the valence band of the TiO2 substrate [27]. Thus, the absorption peak centered at ~420 nm is ascribed to the electronic transitions between Au nanospheres and the TiO2 substrate, which is labeled as “b” in Figure 3a. As per published experimental measurements, the absorption peak centered at ~520 nm is induced by the LSPR effects, which is labeled as “c” in Figure 3a. There is another shoulder absorption peak centered at ~610 nm, which is labeled as “d” in Figure 3a. For nanoparticles, the electronic energy levels are not continuous, compared to bulk materials, but discrete due to the quantum size effects of thenanoparticles and this can result in a sharp and an intense absorption features corresponding to interband transitions in nanoparticles [28]. According to Guerrisi’s conclusion and Liu’s experiment the absorption edge in Au particles is located at ~1.94 eV (~640 nm), which is produced by the interband electronic transition [29,30]. Thus, it can be speculated that this absorption peak is ascribed to the interband transition in Au nanospheres. When the radius of Au nanospheres decreased from 100 to 25 nm, these four absorption peaks show different trends: (1) the absorption properties produced by the TiO2 substrate do not present any obvious variation; (2) the absorption peak led by the electronic transition between Au nanospheres and the TiO2 substrate are gradually blue-shifted and, finally, integrated into the absorption of TiO2 substrate, which is possibly led by the quantum size effects of the Au nanospheres; (3) the LSPR absorption peak of the Au nanospheres is gradually blue-shifted and decreasing, which is also possibly led by the quantum size effects; (4) the absorption peaks produced by the interband transition of Au nanospheres is gradually red-shifted and decreasing, indicating that the optical absorption of Au nanosphere is severely affected by the interaction between LSPR and local interband transitions [31]. Additionally, in the case of Au nanospheres with radii of 25 nm, the variation of optical absorption properties induced by Au nanosphere loading are insignificant, implying that small Au nanospheres do not respond to the incident light. The above characteristics are presented in Au nanospheres with different particle radii, but there are some differences in the details. For example, the absorption peaks “c” and “d” will be split into several distinct peaks, and vary with respect to the blue-shifting as the sphere radius decreases. This simulated result indicates that LSPR and interband transition will induce multi-peak absorption characteristics, especially in case of Au nanospheres that are completely embedded into the TiO2 substrate. It has been experimentally demonstrated that the position of plasmon resonance absorption is sensitively dependent on the particle size and progressively shifts to the longer wavelengths with the increase of the particle size [12,32]. When Au nanospheres are gradually embedded into the TiO2 substrate, the optical absorption properties present some obvious or not obvious variations, as shown in Figure 3 (1) the absorption produced by the TiO2 substrate maintains the constant characteristics, which means the optical properties of TiO2 substrate are not impacted by the size and location of Au nanosphere loading; (2) the absorption intensity led by the electronic transition between Au nanospheres and the TiO2 substrate are gradually decreasing. It can be understood that the interaction between Au nanospheres and TiO2 substrate is gradually weakened with the loading location immersing; (3) the resonance absorption peak is firstly decreasing and then increasing, and split into several distinct peaks; and (4) the absorption induced by the interband transition is gradually increasing, split into several distinct peaks. These simulated results clearly demonstrate that the LSPR effects are also sensitively dependent on the location of Au nanospheres in the TiO2 substrate. In addition, it can be clearly observed that the absorption peak of LSPR is red-shifted, as Au nanospheres are gradually immersed in


5 of 15


5 of 15

the TiO2 substrate. These red-shifts arose due to an overall increase in the refractive index of the dielectric environment surrounding thenanospheres Au nanospheres upon TiO2 coating. The core-shell dielectric environment surrounding the Au upon TiO 2 coating. The core-shell Au-TiO2 Au-TiO TiOof a high refractive index on allofsides thecores, gold nanostructures, with TiOwith 2 shells a highofrefractive index (2.5) on (2.5) all sides the of gold 2 nanostructures, 2 shells cores, experienced a larger red-shift, compared with Model AB, and in which was coated on experienced a larger red-shift, compared with Model A and in B, which TiO2TiO was2 coated onlyonly on one one side of the gold, leaving the other side exposed to air, which had a lower refractive index of ≈ 1.4. side of the gold, leaving the other side exposed to air, which had a lower refractive index of ≈1.4.

(a)

(b)

(c)

(d)

Figure 3. The loaded on on the the TiO TiO2 substrate with different radii Figure 3. The absorption absorption spectra spectra of of Au Au nanospheres nanospheres loaded 2 substrate with different radii and locations: (a) Model A; (b) Model B; (c) Model C; and (d) Model D as labeled in Figure 1. 1. and locations: (a) Model A; (b) Model B; (c) Model C; and (d) Model D as labeled in Figure

In FDTD simulations, the plane-wave source is incident on an Au nanosphere, and the induced In FDTD simulations, the plane-wave source is incident on an Au nanosphere, and the induced electric fields at the particular wavelength around Au nanospheres can be further obtained. In order electric fields at the particular wavelength around Au nanospheres can be further obtained. In order to to further demonstrate the roles of the LSPR effects, the electric field distribution of Au/TiO 2 further demonstrate the roles of the LSPR effects, the electric field distribution of Au/TiO2 composite composite systems for the corresponding absorption peaks of LSPR and interband transitions (“c” systems for the corresponding absorption peaks of LSPR and interband transitions (“c” and “d”) and “d”) are respectively illustrated in Figure 4. Subgraph (a) corresponds with the electric field are respectively illustrated in Figure 4. Subgraph (a) corresponds with the electric field distribution distribution induced by LSPR, and subgraph (b) corresponds with that induced by the interband induced by LSPR, and subgraph (b) corresponds with that induced by the interband transitions. transitions. Their orders are arranged by the sphere radius and the location of the Au nanosphere. Their orders are arranged by the sphere radius and the location of the Au nanosphere. These patterns These patterns show the local enhanced electric field is enhanced around Au nanospheres. With the show the local enhanced electric field is enhanced around Au nanospheres. With the increase in the increase in the radius of the Au nanospheres, both enhanced electric fields are gradually increasing. radius of the Au nanospheres, both enhanced electric fields are gradually increasing. The results show The results show that larger Au nanospheres exhibit more obvious effects on the enhanced electric that larger Au nanospheres exhibit more obvious effects on the enhanced electric fields. Therefore, fields. Therefore, in order to avoid the interference of interband transitions with LSPR, it is in order to avoid the interference of interband transitions with LSPR, it is experimentally attempted to experimentally attempted to prepare Au nanospheres with suitable radii. It can be clearly observed prepare Au nanospheres with suitable radii. It can be clearly observed that the electric field intensity that the electric field intensity of the interband transitions is greater than that of LSPR by the electric of the interband transitions is greater than that of LSPR by the electric field diagram of the large-sized field diagram of the large-sized gold nanoparticles. Furthermore, it has been reported that the gold nanoparticles. Furthermore, it has been reported that the interband transitions of Au correspond interband transitions of Au correspond to the contribution of Au 5d electrons to the electronic to the contribution of Au 5d electrons to the electronic structure of Au [33,34]. Under the excitation of structure of Au [33,34]. Under the excitation of light, the 5d band of gold may generate a large number light, the 5d band of gold may generate a large number of electronic transitions and show a strong of electronic transitions and show a strong electric field. Through LSPR related decay through interband transitions, both LSPR and interband transitions are favorable for light absorption and enhanced the local electric field. When the location of the Au nanospheres is changing, the enhanced electric fields present different variations: both enhanced electric fields firstly decrease, and then


6 of 15

electric field. Through LSPR related decay through interband transitions, both LSPR and interband transitions are favorable for light absorption and enhanced the local electric field. When the location of the Au nanospheres is changing, the enhanced electric fields present different variations: both enhanced electric fields firstly decrease, and then increase, as the location of Au nanospheres is Catalysts 2018, 8, x FOR PEER REVIEW 6 of 15 gradually embedded into the TiO2 substrate. In the case of LSPR, when Au nanospheres are completely embedded the TiO2 substrate (i.e., Model the enhanced electric fieldTiO is 2the smallest, increase, into as the location of Au nanospheres is C), gradually embedded into the substrate. In as theshown case in of Figure 4a. Especially in the cases of R = 25 and 50 nm, the enhanced electric field is equal to LSPR, when Au nanospheres are completely embedded into the TiO2 substrate (i.e., Model C), zero; the in enhanced the other words, field is not by the 4a. LSPR effects, in while a similar electric the fieldelectric is the smallest, as enhanced shown in Figure Especially the cases of Rphenomenon = 25 and 50 could thefield Au nanospheres is half embedded into the electric TiO2 substrate (i.e.,enhanced Model B), nm,be theobserved enhancedwhen electric is equal to zero; in the other words, field is not the LSPR effects, as by shown in Figure 4b.while a similar phenomenon could be observed when the Au nanospheres is half embedded intoof thephotocatalysis, TiO2 substrate (i.e., Model B),of as the shown in enhanced Figure 4b. electric field can effectively In the field the presence local In photo-generated the field of photocatalysis, the pairs. presence of the local enhanced electric field can for effectively separate electron-hole Therefore, except the above discussions the local separateelectric photo-generated electron-hole the above discussionsoffor local enhanced fields enhanced by LSPRpairs. or theTherefore, interbandexcept transition, the distribution thethe strongest enhanced electric fields enhanced by LSPR or the interband transition, the distribution of the 5, local enhanced electric field in the whole simulation region of 300–900 nm are provided in Figure strongest local enhanced electric field in the whole simulation region of 300–900 nm are provided in in which the corresponding wavelength is also indicated. It can be determined that Au nanoparticles Figure 5, interaction in which the corresponding wavelength is the alsostrong indicated. It can be determined that Au have strong with the TiO2 substrate through metal-support interaction compared nanoparticles have strong interaction with the TiO 2 substrate through the strong metal-support to the single Au nanoparticle by comparing the electric field intensity of a single Au nanoparticle interaction compared to the single Au nanoparticle by comparing the electric field intensity of a single (Table 1) with the electric field intensity of the Au/TiO2 composite structure (Figure 5). In the case of Au nanoparticle (Table 1) with the electric field intensity of the Au/TiO2 composite structure (Figure Au nanospheres loaded onto the TiO2 substrate (i.e., Model A), as the radius of the Au nanospheres 5). In the case of Au nanospheres loaded onto the TiO2 substrate (i.e., Model A), as the radius of the increases, the local enhanced electric field also gradually increases, and only appears at the interface of Au nanospheres increases, the local enhanced electric field also gradually increases, and only appears the Au nanoshpheres and the TiO2 substrate. In the other cases of Models B–D, the phenomenon is just at the interface of the Au nanoshpheres and the TiO2 substrate. In the other cases of Models B–D, the the opposite: the local enhanced electric field gradually decreases as the radius of the Au nanospheres phenomenon is just the opposite: the local enhanced electric field gradually decreases as the radius increases. These simulated results suggest that the interface between Au nanospheres and the TiO2 of the Au nanospheres increases. These simulated results suggest that the interface between Au substrate is the and key the factor determine the local enhanced electric field [12,19]. From the combined nanospheres TiOto 2 substrate is the key factor to determine the local enhanced electric field above discussions and the corresponding wavelength the strongest wavelength local enhanced electric field, [12,19]. From the combined above discussions and theofcorresponding of the strongest it can be concluded that the strongest local enhanced electric field is caused by the interband transition local enhanced electric field, it can be concluded that the strongest local enhanced electric field is in caused the caseby ofthe Model A, andtransition it is moreinobvious largerA,Au nanospheres, while the strongest interband the caseinofthe Model and it is more obvious in the larger local Au enhanced electric field is caused by LSPR in the cases of Model B, C, and D, and it is more nanospheres, while the strongest local enhanced electric field is caused by LSPR in the cases obvious of Modelin theB,smaller nanospheres. C, and Au D, and it is more obvious in the smaller Au nanospheres.

(a) Figure 4. Cont.

Catalysts Catalysts 2018, 2018, 8, 8, 236 x FOR PEER REVIEW Catalysts 2018, 8, x FOR PEER REVIEW

77 of of 15 15 7 of 15

(b) (b) Figure The distributionof thelocal localenhanced enhancedelectric electric field field of Au 2 substrate Figure 4. 4. The distribution ofofthe the local enhanced electric field of of Au Au nanospheres nanosphereson onaaaTiO TiO substrate Figure 4. The distribution nanospheres on TiO 22 substrate induced by different wavelengths: (a) LSPR absorption peaks; and (b) interband transition peaks. The induced by different absorption peaks; andand (b) (b) interband transition peaks. The induced differentwavelengths: wavelengths:(a) (a)LSPR LSPR absorption peaks; interband transition peaks. dashed line represents the interface between the Au nanospheres and the TiO2 substrate. dashed line line represents the interface between the Au andand the the TiOTiO 2 substrate. The dashed represents the interface between the nanospheres Au nanospheres substrate. 2

Figure 5. The distribution of the strongest local enhanced electric field of Au nanospheres loaded on the TiO 2 substrate in the whole simulation region of 300–900 nm. The dashed line represents the Figure 5. The electric field of of AuAu nanospheres loaded on Figure 5. The distribution distributionofofthe thestrongest strongestlocal localenhanced enhanced electric field nanospheres loaded interface between the Au nanospheres and the TiO2 substrate. thethe TiO 2 substrate in the whole simulation region of 300–900 nm. The dashed line represents the on TiO substrate in the whole simulation region of 300–900 nm. The dashed line represents the 2 interface between between the the Au Au nanospheres nanospheresand andthe theTiO TiO22 substrate. substrate. interface

2.2. Au Nanorods

2.2. Au AuCompared Nanorods with Au nanospheres, Au nanorods have different polarization degrees along 2.2. Nanorods different directions, due to the structural anisotropy, as shown in Figure 6. If incident light is parallel Comparedwith withAuAu nanospheres, Au nanorods have different polarization degrees along nanospheres, Au nanorods havefield different polarization degrees along different to Compared the axial direction of the Au nanorods, the electric is perpendicular to the axial direction, different directions, due to the structural anisotropy, asin shown in6.Figure 6. If incident is parallel directions, due to the structural anisotropy, as shown Figure If incident light is light parallel to the to the axial direction of the Au nanorods, the electric field is perpendicular to the axial direction,


8 of 15


8 of 15

which induces the lateral resonance (LSPRT). If incident light is perpendicular to the axial direction of the Au nanorods, the electric field is parallel to the axial direction, which induces the longitudinal resonance (LSPR In experiments, is difficult fix Au nanorods along Thus, axial direction ofL).the Au nanorods,itthe electricto field is perpendicular toparticular the axial directions. direction, which Au nanorods are resonance usually randomly on the TiO2 substrate, andaxial produce twoofdistinct induces the lateral (LSPRT ). orientated If incident light is perpendicular to the direction the Au resonance absorption peaks. In the theoretical simulation, it is induces easy to the fix longitudinal Au nanorods along a nanorods, Catalysts the electric is parallel to the axial direction, which 2018, 8, xfield FOR PEER REVIEW 8 of 15resonance particular direction. In many it Au is a nanorods concern that Auparticular nanorodsdirections. are flattened on the (LSPRL ). In experiments, it isexperiments, difficult to fix along Thus, Au which induces the lateral resonance (LSPRT). If incident light is perpendicular to the axial direction substrate Thus, here we orientated also choose toand be parallel thedistinct axial direction of nanorods [35,36]. are usually randomly onthe theTiO TiO2 2substrate substrate, producetotwo resonance of the Au nanorods, the electric field is parallel to the axial direction, which induces the longitudinal the Au nanorods. the Lpresent work, the axialitdirection of nanorods isdirections. parallel to thedirection. surface absorption peaks. In(LSPR theoretical simulation, istoeasy fixthe AuAu nanorods along a particular resonance ). In experiments, it is difficult fix Auto nanorods along particular Thus, of TiOexperiments, 2Au substrate, torandomly studythat the effects of T with respect different nanorods in are usually orientated on LSPR the TiO 2 flattened substrate, and produce two length-diameter distinct In the many itorder is a concern Au nanorods are onto the substrate [35,36]. Thus, resonance absorption thetotheoretical simulation, it shown is easy to Au along a ratios (L/D) of Au nanorods with In diameters of 100 nm. Asaxial infixFigure 7,Au for nanorods. the isolated here we also choose the TiO2peaks. substrate be parallel to the direction of nanorods the In Au the particular direction. In many experiments, it is a concern that Au nanorods are flattened on the nanorods, when L/D varies of from to three, is theparallel intensity of surface resonance peak LSPR T is present work, thethe axial direction the one Au nanorods to the of the TiOof2 substrate, substrate [35,36]. Thus, here we also choose the TiO2 substrate to be parallel to the axial direction of increased, while the is unchanged at ~560 nm),towhich is consistent in order tothe study thepeak effects LSPR with respect to different length-diameter ratios (L/D) of Au Talmost Au nanorods. In position theof present work, the axial direction of(centered the Au nanorods is parallel the surface with published experimental measurements These results suggest that the shape when of the Au of the TiO 2 substrate, order to study the [13,35,37]. effects LSPR T with respect to different length-diameter nanorods with diameters ofin100 nm. As shown inofFigure 7, for the isolated Au nanorods, the ratios nanorods diameters ofeffects. 100 nm. As shown Figureis 7, increased, for the isolated Au the peak nanorods is the (L/D) non-critical factor for the LSPR T L/D varies from oneoftoAuthree, the with intensity of resonance peak of in LSPR while T nanorods, when the L/D varies from one to three, the intensity of resonance peak of LSPR T is position is almost unchanged (centered at ~560 nm), which is consistent with published experimental increased, while the peak position is almost unchanged (centered at ~560 nm), which is consistent measurements [13,35,37]. These results suggest that the shape of suggest the Authat nanorods non-critical with published experimental measurements [13,35,37]. These results the shape is of the the Au nanorods non-critical factor for the LSPRT effects. factor for the LSPRisT the effects.

Figure 6. 6. Figure Schematic of two twooftypes types of LSPR LSPR ofofmetallic metallic nanorods: (a) LSPR LSPR ; and LSPRL. 6. Schematic two types of LSPRof metallic nanorods: (a) LSPR T; and T (b) LSPR(b) L. Figure Schematic of of nanorods: (a) T ; and (b) LSPRL.

Figure 7. The extinction spectra of isolated Au nanorods with different length-diameter ratios.

The absorption spectra of the Au nanorods with different L/D and locations are plotted in Figure 8 Compared with the clean TiO2 substrate, as shown in Figure 8a, Au nanorod loading produces some

Figure 7. The extinction spectra of isolated Au nanorods with different length-diameter ratios. Figure 7. The extinction spectra of isolated Au nanorods with different length-diameter ratios.

The absorption spectra spectra of ofthe theAu Aunanorods nanorodswith withdifferent differentL/D L/Dand andlocations locationsare areplotted plottedininFigure Figure8 8Compared Comparedwith withthe theclean cleanTiO TiO substrate,as asshown shownin in Figure Figure 8a, 8a, Au Au nanorod loading produces some 2 2substrate,


9 of 15


9 of 15

new increases, the the positions positions of of peaks new absorption absorption bands bands in in the the visible-light visible-light region. region. As As the the ratio ratio of of L/D L/D increases, peaks b, c, and d are red-shifted. On the other hand, the phenomena are very similar with those of Au b, c, and d are red-shifted. On the other hand, the phenomena are very similar with those of Au nanosphere areare consistent with experimental measurements [35]. nanosphere loading. loading.Moreover, Moreover,these theseobservations observations consistent with experimental measurements When the location of the Au nanorods changes,changes, the absorption propertiesproperties present some variation. [35]. When the location of the Au nanorods the absorption present some Among these absorption bands, the intensity of the c peak that is induced by the LSPR effect is variation. Among these absorption bands, the intensity of the c peak that is induced by the LSPR obviously strong in the case of case Au nanorods loadedloaded on theon surface of theofTiO substrate. As the effect is obviously strong in the of Au nanorods the surface the2 TiO 2 substrate. As Au nanorods are gradually embedded into the TiO substrate, the intensity of different absorption 2 2 substrate, the intensity of different absorption the Au nanorods are gradually embedded into the TiO bands and then then increasing, increasing, and and the the position position is is gradually gradually red-shifted. red-shifted. bands is, is, first, first, gradually gradually decreasing, decreasing, and These simulated results indicate that the intensity and position of LSPR effects are impacted by the These simulated results indicate that the intensity and position of LSPR effects are impacted by the shape and location of Au nanorods. shape and location of Au nanorods.

(a)

(b)

(c)

(d)

Figure 8. 8. The on the Figure The absorption absorption spectra spectra of of Au Au nanorods nanorods loaded loaded on the TiO TiO22 substrate substrate with with different different radii radii and and location: (a) Model A, (b) Model B, (c) Model C, and (d) Model D as labeled in Figure 1. location: (a) Model A, (b) Model B, (c) Model C, and (d) Model D as labeled in Figure 1.

The distribution of the strongest local enhanced electric field in the whole simulation region of The distribution of the strongest local enhanced electric field in the whole simulation region of 300–900 nm of Au nanorods with different L/D ratios and different locations is illustrated in Figure 300–900 nm of Au nanorods with different L/D ratios and different locations is illustrated in Figure 9. 9. Similarly, the local enhanced electric fields are presented at the interface between the Au nanorods Similarly, the local enhanced electric fields are presented at the interface between the Au nanorods and the TiO2 substrate. As the L/D ratio is increasing, the local enhanced electric field is gradually and the TiO2 substrate. As the L/D ratio is increasing, the local enhanced electric field is gradually weakening. This phenomenon is very obvious in the case of Au nanorods embedded into the TiO2 weakening. This phenomenon is very obvious in the case of Au nanorods embedded into the TiO2 substrate (i.e., Model D). On the other hand, when Au nanorods are gradually embedded into the substrate (i.e., Model D). On the other hand, when Au nanorods are gradually embedded into the TiO2 TiO2 substrate, the local enhanced electric field is, first, weakening, and then enhancing. This substrate, the local enhanced electric field is, first, weakening, and then enhancing. This phenomenon phenomenon is very obvious in the case of Au nanorods with an L/D = 1. Moreover, the case of Au is very obvious in the case of Au nanorods with an L/D = 1. Moreover, the case of Au nanorods nanorods covered by the TiO2 substrate at 100 nm depths, like the situation of the Au/TiO2 core–shell covered by the TiO2 substrate at 100 nm depths, like the situation of the Au/TiO2 core–shell system, system, can remarkably enhance the local enhanced electric field, and this phenomenon is more can remarkably enhance the local enhanced electric field, and this phenomenon is more obvious with obvious with the increase in the thickness of the TiO2 shell [13,14]. In the photocatalytic activity the increase in the thickness of the TiO2 shell [13,14]. In the photocatalytic activity testing, this effect testing, this effect can significantly promote the photocatalytic performance of TiO 2. can significantly promote the photocatalytic performance of TiO2 .

Catalysts 2018, 8, 236 Catalysts 2018, 8, x FOR PEER REVIEW

10 of 15 10 of 15

Figure Figure9.9.The Thedistribution distributionof ofthe thestrongest strongestlocal localenhanced enhancedelectric electricfield fieldof ofAu Aunanorods nanorodsloaded loadedon onthe the TiO substrate in the whole simulation region of 300–900 nm. The dashed line represents the interface 2 TiO2 substrate in the whole simulation region of 300–900 nm. The dashed line represents the interface between betweenAu Aunanorods nanorodsand andthe theTiO TiO2 2substrate. substrate.

2.3. 2.3. Au AuNanospheres NanospheresVersus VersusAu AuNanorods NanorodsLoaded Loadedon onthe theTiO TiO2 2Substrate Substrate Due different shape and and size, size, the corresponding LSPR of Au nanoparticles present different Duetotothe the different shape the corresponding LSPR of Au nanoparticles present effects. As the radius of Au nanospheres increases, the LSPR peaks are red-shifted, while as the L/D different effects. As the radius of Au nanospheres increases, the LSPR peaks are red-shifted, while as ratio increases, the LSPR in almost the same WhenWhen the location of the T peaks the L/D ratio increases, the LSPRTremain peaks remain in almost the position. same position. the location ofAu the nanoparticles are gradually embedded from thethe surface to to thethe bulk ofof the 2 substrate, Au nanoparticles are gradually embedded from surface bulk theTiO TiO 2 substrate,the theLSPR LSPR exhibit multi-peak absorption characteristics, especially in the case of Au nanoparticles covered exhibit multi-peak absorption characteristics, especially in the case of Au nanoparticles covered by by the (i.e., Model Model D). D).In Inthe thepresent presentwork, work,it itis is found that adjusting location of the TiO TiO22 substrate (i.e., found that adjusting thethe location of the the nanoparticles TiO couldproduce producemore moreobvious obviousand and rich rich LSPR effects 2 substrate Au Au nanoparticles in in thethe TiO 2 substrate could effects than than changing the size of the Au nanoparticles. If the size and location of the Au nanoparticles changing the size of the Au nanoparticles. If the size and location of the Au nanoparticles are are fixed, fixed, the theLSPR LSPReffects effectscould couldappear appear more moresignificant significant on onthe thesurface surface of ofthe theTiO TiO22 substrate. substrate. In In other other words, words, Au nanoparticle loading onto the surface of the TiO photocatalyst may be better to enhance Au nanoparticle loading onto the surface of the TiO22 photocatalyst may be better to enhance the the corresponding anisotropy and complexity of of AuAu nanorods is correspondingperformance. performance.Furthermore, Furthermore,the thestructural structural anisotropy and complexity nanorods the root forfor thethe non-regular properties ofof the LSPR effects. is the root non-regular properties the LSPR effects.For Forexample, example,Wang Wangetetal. al.have havereported reported that thatthe thetruncated truncatednanorods nanorodsexhibit exhibitaablue-shift blue-shiftof ofthe theLSPR LSPReffects effectsin in comparison comparisonwith with the the rectangular rectangular nanorods nanorods [22]. [22]. It is worth worth noting noting that that changing changing the the location location of of the the Au Au nanoparticles nanoparticles in in the the TiO TiO22 substrate can vary the LSPR effects, varying from the visible-light region to the near-infrared-light substrate can vary the LSPR effects, varying from the visible-light region to the near-infrared-light region, region,while whilechanging changingthe thesize sizeof ofthe theAu Aunanoparticles nanoparticlesonly onlyproduces producesabout about100-nm 100-nmred-shifting. red-shifting. The enhanced electric electricfields fieldsonly onlypresent presentatat interface between nanoparticles The local enhanced thethe interface between thethe Au Au nanoparticles and and the TiO substrate. When the Au nanoparticles are completely covered by the TiO substrate the TiO2 substrate. When the Au nanoparticles are completely covered by the TiO2 substrate 2 2 at a 100at a 100-nm depth, local enhanced electric field is the strongest. In of thephotocatalysis, field of photocatalysis, nm depth, the localthe enhanced electric field is the strongest. In the field LSPR can LSPR canthe promote the photocatalytic performance (enhance visible-light promote photocatalytic performance (enhance visible-light harvesting, harvesting, and separateand the separate electronthe electron-hole pairs efficiently) by the following effects: direct electron transfer, LSPR-mediated hole pairs efficiently) by the following effects: direct electron transfer, LSPR-mediated local electromagnetic fields, and plasmon-induced resonance energy transfer [12,19,32]. However, a recent study suggests that the intensity of the electron transfer is not necessarily correlated with an Au LSPR


11 of 15


11 of 15

local electromagnetic fields, and plasmon-induced resonance energy transfer [12,19,32]. However, band [38]. Govorov and co-workers have reported that photo-generated holes in the interband a recent study suggests that the intensity of the electron transfer is not necessarily correlated with transition are created far from the Fermi surface (2.4 eV relative to EF), while holes in the plasmonic an Au LSPR band [38]. Govorov and co-workers have reported that photo-generated holes in the wave are distributed near the Fermi surface [39]. This means that the 5d band of gold generates a interband transition are created far from the Fermi surface (2.4 eV relative to EF ), while holes in the large amount of electrons, and these electrons transition to the conduction band of Au and then to plasmonic wave are distributed near the Fermi surface [39]. This means that the 5d band of gold the conduction band of TiO2. This promotes efficient separation of electron-hole pairs. In the present generates a large amount of electrons, and these electrons transition to the conduction band of Au work, it can be found that the local enhanced electric fields of the LSPR effect are easily impacted by and then to the conduction band of TiO2 . This promotes efficient separation of electron-hole pairs. the shape, size, and location of the Au nanoparticles, compared to the LSPR absorption. Furthermore, In the present work, it can be found that the local enhanced electric fields of the LSPR effect are easily there is the complex interaction between the interband transition and LSPR absorption. Thus, how to impacted by the shape, size, and location of the Au nanoparticles, compared to the LSPR absorption. effectively utilize LSPR effect needs careful consideration to reasonably choose these determined Furthermore, there is the complex interaction between the interband transition and LSPR absorption. factors according to the enhanced mechanism for the photocatalytic performance. Thus, how to effectively utilize LSPR effect needs careful consideration to reasonably choose these determined factorsMethods according to the enhanced mechanism for the photocatalytic performance. 3. Computational 3. Computational Methods In recent years, with the aid of computers and various numerical processing methods to solve Maxwell’s equations, great achievements have been made in the field of LSPR, such as the FDTD In recent years, with the aid of computers and various numerical processing methods to solve method, discrete dipole approximation (DDA), T-matrix method, discrete source method (DSM), and Maxwell’s equations, great achievements have been made in the field of LSPR, such as the FDTD so on. Among these methods, the FDTD method can directly simulate the near and far field method, discrete dipole approximation (DDA), T-matrix method, discrete source method (DSM), and so distribution, and can use a non-uniform meshing grid for the irregular models, resulting in relatively on. Among these methods, the FDTD method can directly simulate the near and far field distribution, high simulation accuracy. The FDTD method is based on the calculation of the Maxwell equations and can use a non-uniform meshing grid for the irregular models, resulting in relatively high simulation using the Yee cellular discretization shown in Figure 10. It can be seen from Figure 9 that each accuracy. The FDTD method is based on the calculation of the Maxwell equations using the Yee electric/magnetic field component is surrounded by four magnetic/electric field components. The cellular discretization shown in Figure 10. It can be seen from Figure 10 that each electric/magnetic electric field and the magnetic field are alternately sampled in time. The sampling time interval is field component is surrounded by four magnetic/electric field components. The electric field and half a time step, and solved iteratively in time. Combined with the initial conditions and the boundary the magnetic field are alternately sampled in time. The sampling time interval is half a time step, conditions, the electromagnetic field distribution in space can be obtained. Through the obtained and solved iteratively in time. Combined with the initial conditions and the boundary conditions, electric field, the reflectivity, transmissivity, and absorption of incident electromagnetic waves can the electromagnetic field distribution in space can be obtained. Through the obtained electric field, also be obtained. the reflectivity, transmissivity, and absorption of incident electromagnetic waves can also be obtained.

Figure Figure10. 10.The TheYee Yee cells cells of of electromagnetic electromagnetic field field in in FDTD FDTD simulations. simulations.

Usually, we used used the the scattering scattering cross-section cross-section (Q (Qscat),), the absorption cross-section (Qabs), and the Usually, we scat the absorption cross-section (Qabs ), and the extinction cross-section(Q(Qexti) )toto characterize LSPR effects Au nanoparticles on the TiO 2 extinction cross-section characterize the the LSPR effects of Auof nanoparticles on the TiO exti 2 substrate. substrate. The scattering cross-section characterizes the effective radiation enhancement range of the The scattering cross-section characterizes the effective radiation enhancement range of the LSPR, LSPR, the absorption cross-section is the effective absorption energy of theelectrons surface and theand absorption cross-section is the effective absorption energy space of space the surface electrons for forming the LSPR. While the extinction cross-section is the combined effect of the above for forming the LSPR. While the extinction cross-section is the combined effect of the above two two phenomena, which can be used to characterize the integrated effect of the surrounding medium phenomena, which can be used to characterize the integrated effect of the surrounding medium and metal nanoparticles on the incident electromagnetic field, and observe the effective resonance wavelength between the LSPR and the incident light field. The peak wavelength corresponds to the


12 of 15

and metal nanoparticles on the incident electromagnetic field, and observe the effective resonance wavelength between the LSPR and the incident light field. The peak wavelength corresponds to the LSPR wavelength of Au particles. The above physical quantities are related to the size and shape of the gold particles and the surrounding media environment and are defined as follows: εm − ε0 } ε m − 2ε 0

(1)

8 4 6 ε m − ε 0 2 k4 2 = α k πa | | ε m + 2ε 6π 3 0

(2)

Qexti = Qabs + Qscat

(3)

Qabs = kIm{α} = 4ka3 Im{

Qscat =

where k is the wave vector of the incident light, α is the polarizability of the Au nanoparticles, a is the radius of the Au nanoparticles, εm and ε0 are the permittivity of the Au nanoparticles and the environmental medium, respectively, and Im is the imaginary part. In order to simplify the published experimental work [40,41], only one Au nanoparticle in the periodic boundary is considered for the numerical simulation of free electrons’ collective oscillation bringing about the LSPR phenomenon in the present work. In the present work, the dielectric constant of Au can be described using Johnson and Christy’s report [42], and the dielectric constant of TiO2 (anatase) can be introduced using our previous density functional theory calculated results. Other FDTD parameters, descriptions, and settings to extract the optical properties in the present work are listed in Table 2. Table 2. FDTD parameter descriptions and settings to investigate the optical properties of the proposed Au-based nanoparticles. Parameter

Isolated Au Nanoparticle

Au/TiO2 Composites

Spatial cell sizes(dx = dy = dz) Number of time steps Source Wavelength Background index Boundary conditions Number of PML layers

2.5 nm 6000 Total-field scattered-field 300–1500 nm 1 PML 8

2.5 nm 6000 Plane wave 300–1500 nm 1 PML 12

4. Conclusions In the present work, the effects of LSPR of Au nanoparticle loading onto a TiO2 substrate were simulated by the FDTD method. Especially, the variation tendency of the resonance-absorption peaks and the intensity of local enhanced electric field were systematically compared and emphasized. The features of the resonance absorption of LSPR can be tuned by varying the size, shape, and location of Au nanoparticles. When Au nanoparticles are completely embedded into the TiO2 substrate up to 100-nm, LSPR and interband transition will induce multi-peak absorption characteristics. In these determined factors, the LSPR effects are sensitively dependent on the location of Au nanoparticles in the TiO2 substrate. When the location of Au nanospheres are gradually immersed into the TiO2 substrate, the local enhanced electric field of the boundary is gradually enhanced. When it is covered by TiO2 into 100 nm depths, the local enhanced electric field intensities reach the maximum value. However, when Au nanorods are loaded onto the surface of the TiO2 substrate, the intensity of the corresponding local enhanced electric field is the maximum. Au nanospheres produce two strong absorption peaks in the visible light region, which are induced by the LSPR effect and interband transitions between Au nanoparticles and the TiO2 substrate. For the LSPR resonance-absorption peaks, the corresponding position is red-shifted by about 100 nm, as the location of the Au nanospheres is gradually immersed into the TiO2 substrate. On the other hand, the size change of the Au nanorods does not lead to a


13 of 15

similar variation of the LSPR resonance-absorption peaks, except to change the length-diameter ratio. Meanwhile, the LSPR effects are obviously interfered with by the interband transitions between the Au nanorods and the TiO2 substrate. In the Au/TiO2 composite structure, there is a strong interaction between the Au nanoparticles and the TiO2 substrate, which is compared to the electric field intensity of the Au single-nanoparticle and the composite structure. Furthermore, although interband transitions will interfere with the LSPR, both the interband transitions and the LSPR of Au will have an electric field enhancement effect. Finally, hot electrons are likely to be generated in the 5d band of Au instead of LSPR. The local enhanced electric fields of the LSPR effect are easily impacted by the shape, size, and location of Au nanoparticles, compared to the LSPR absorption. Furthermore, there is the complex interaction between the interband transition and LSPR absorption. Thus, how to effectively utilize the LSPR effect needs careful consideration in order to reasonably choose these determined factors according to the enhanced mechanism of the photocatalytic performance. Author Contributions: G.-Y.Y. carried out the FDTD calculations, analyzed the calculated results, and prepared the manuscript; Q.-L.L. helped analyze the calculated results and revised the manuscript. Z.-Y.Z. designed and planed the research work, guided G.-Y.Y. to complete the present work, and revised the manuscript. Acknowledgments: The authors would like to acknowledge the financial support from the National Natural Science Foundation of China (grant no. 21473082), and the 18th Yunnan Province Young Academic and Technical Leaders Reserve Talent Project (grant no. 2015HB015). Conflicts of Interest: The authors declare no conflict of interest.

References 1. 2. 3.

4. 5. 6. 7.

8.

9.

10.

11.

Fujishima, A.; Honda, K. Electrochemical photocatalysis of water at semiconductor electrode. Nature 1972, 238. [CrossRef] Xu, M.; Da, P.; Wu, H.; Zhao, D.; Zheng, G. Controlled sn-doping in TiO2 nanowire photoanodes with enhanced photoelectrochemical conversion. Nano Lett. 2012, 12, 1503. [CrossRef] [PubMed] Scuderi, V.; Impellizzeri, G.; Romano, L.; Scuderi, M.; Brundo, M.V.; Bergum, K.; Zimbone, M.; Sanz, R.; Buccheri, M.A.; Simone, F.; et al. An enhanced photocatalytic response of nanometric TiO2 wrapping of Au nanoparticles for eco-friendly water applications. Nanoscale 2014, 6, 11189–11195. [CrossRef] [PubMed] Cao, J.; Zhang, Y.; Tong, H.; Li, P.; Kako, T.; Ye, J. Selective local nitrogen doping in a TiO2 electrode for enhancing photoelectrochemical water splitting. Chem. Commun. 2012, 48, 8649–8651. [CrossRef] [PubMed] Long, R.; English, N.J. First-principles calculation of synergistic (n, p)-codoping effects on the visible-light photocatalytic activity of anatase TiO2 . J. Phys. Chem. C 2010, 114, 11984–11990. [CrossRef] Foo, W.J.; Zhang, C.; Ho, G.W. Non-noble metal Cu-loaded TiO2 for enhanced photocatalytic H2 production. Nanoscale 2013, 5, 759. [CrossRef] [PubMed] Liu, L.; Ji, Z.; Zou, W.; Gu, X.; Deng, Y.; Gao, F.; Tang, C.; Dong, L. In situ loading transition metal oxide clusters on TiO2 nanosheets as co-catalysts for exceptional high photoactivity. ACS Catal. 2013, 3, 2052–2061. [CrossRef] Awazu, K.; Fujimaki, M.; Rockstuhl, C.; Tominaga, J.; Murakami, H.; Ohki, Y.; Yoshida, N.; Watanabe, T. A plasmonic photocatalyst consisting of silver nanoparticles embedded in titanium dioxide. J. Am. Chem. Soc. 2008, 130, 1676–1680. [CrossRef] [PubMed] Tanaka, A.; Hashimoto, K.; Kominami, H. A very simple method for the preparation of Au/TiO2 plasmonic photocatalysts working under irradiation of visible light in the range of 600–700 nm. Chem. Commun. 2017, 53, 4759–4762. [CrossRef] [PubMed] Zhang, S.; Peng, B.; Yang, S.; Wang, H.; Yu, H.; Fang, Y.; Peng, F. Non-noble metal copper nanoparticles-decorated TiO2 nanotube arrays with plasmon-enhanced photocatalytic hydrogen evolution under visible light. Int. J. Hydrog. Energy 2015, 40, 303–310. [CrossRef] Mubeen, S.; Hernandez-Sosa, G.; Moses, D.; Lee, J.; Moskovits, M. Plasmonic photosensitization of a wide band gap semiconductor: Converting plasmons to charge carriers. Nano Lett. 2011, 11, 5548–5552. [CrossRef] [PubMed]


12.

13.

14.

15.

16.

17. 18.

19.

20.

21. 22.

23.

24.

25.

26.

27. 28. 29. 30.

14 of 15

Lu, B.; Liu, A.; Wu, H.; Shen, Q.; Zhao, T.; Wang, J. Hollow Au–Cu2 O core–shell nanoparticles with geometry-dependent optical properties as efficient plasmonic photocatalysts under visible light. Langmuir 2016, 32, 3085–3094. [CrossRef] [PubMed] Dai, L.; Song, L.; Huang, Y.; Zhang, L.; Lu, X.; Zhang, J.; Chen, T. Bimetallic Au/Ag core–shell superstructures with tunable surface plasmon resonance in the near-infrared region and high performance surface-enhanced raman scattering. Langmuir 2017, 33, 5378–5384. [CrossRef] [PubMed] Li, Y.; DiStefano, J.G.; Murthy, A.A.; Cain, J.D.; Hanson, E.D.; Li, Q.; Castro, F.C.; Chen, X.; Dravid, V.P. Superior plasmonic photodetectors based on Au@MoS2 core–shell heterostructures. ACS Nano 2017, 11, 10321–10329. [CrossRef] [PubMed] Ong, W.-J.; Putri, L.K.; Tan, L.-L.; Chai, S.-P.; Yong, S.-T. Heterostructured AgX/g–C3 N4 (X = Cl and Br) nanocomposites via a sonication-assisted deposition-precipitation approach: Emerging role of halide ions in the synergistic photocatalytic reduction of carbon dioxide. Appl. Catal. B Environ. 2016, 180, 530–543. [CrossRef] Chen, L.; Sun, H.; Zhao, Y.; Zhang, Y.; Wang, Y.; Liu, Y.; Zhang, X.; Jiang, Y.; Hua, Z.; Yang, J. Plasmonic-induced sers enhancement of shell-dependent Ag@Cu2 O core–shell nanoparticles. RSC Adv. 2017, 7, 16553–16560. [CrossRef] Liu, Y.; Zhou, F.; Zhan, S.; Yang, Y. Preparation of Ag/AgBr–Bi2 MoO6 plasmonic photocatalyst films with highly enhanced photocatalytic activity. J. Inorg. Organ. Polym. Mater. 2017, 27, 1365–1375. [CrossRef] Clarizia, L.; Vitiello, G.; Luciani, G.; Somma, I.D.; Andreozzi, R.; Marotta, R. In situ photodeposited nanoCu on TiO2 as a catalyst for hydrogen production under UV/visible radiation. Appl. Catal. A Gener. 2016, 518, 142–149. [CrossRef] Sugawa, K.; Tsunenari, N.; Takeda, H.; Fujiwara, S.; Akiyama, T.; Honda, J.; Igari, S.; Inoue, W.; Tokuda, K.; Takeshima, N.; et al. Development of plasmonic Cu2 O/Cu composite arrays as visible- and near-infrared-light-driven plasmonic photocatalysts. Langmuir 2017, 33, 5685–5695. [CrossRef] [PubMed] Liu, P.-H.; Wen, M.; Tan, C.-S.; Navlani-García, M.; Kuwahara, Y.; Mori, K.; Yamashita, H.; Chen, L.-J. Surface plasmon resonance enhancement of production of H2 from ammonia borane solution with tunable Cu2−x S nanowires decorated by Pd nanoparticles. Nano Energy 2017, 31, 57–63. [CrossRef] Sun, Y.; Zhao, Z.; Zhang, W.; Gao, C.; Zhang, Y.; Dong, F. Plasmonic bi metal as cocatalyst and photocatalyst: The case of Bi/(BiO)2 CO3 and Bi particles. J. Colloid Interface Sci. 2017, 485, 1–10. [CrossRef] [PubMed] Wang, H.J.; Yang, K.H.; Hsu, S.C.; Huang, M.H. Photothermal effects from Au–Cu2 O core-shell nanocubes, octahedra, and nanobars with broad near-infrared absorption tunability. Nanoscale 2016, 8, 965. [CrossRef] [PubMed] Zada, A.; Qu, Y.; Ali, S.; Sun, N.; Lu, H.; Yan, R.; Zhang, X.; Jing, L. Improved visible-light activities for degrading pollutants on TiO2 /g–C3 N4 nanocomposites by decorating SPR Au nanoparticles and 2,4-dichlorophenol decomposition path. J. Hazard. Mater. 2017, 342, 715–723. [CrossRef] [PubMed] Zhu, M.; Cai, X.; Fujitsuka, M.; Zhang, J.; Majima, T. Au/La2 Ti2 O7 nanostructures sensitized with black phosphorus for plasmon-enhanced photocatalytic hydrogen production in visible and near-infrared light. Angew. Chem. Int. Ed. 2017, 129, 2096–2100. [CrossRef] Yoo, S.M.; Rawal, S.B.; Lee, J.E.; Kim, J.; Ryu, H.-Y.; Park, D.-W.; Lee, W.I. Size-dependence of plasmonic Au nanoparticles in photocatalytic behavior of Au/TiO2 and Au@SiO2 /TiO2 . Appl. Catal. A Gener. 2015, 499, 47–54. [CrossRef] Zhu, Y.; Yang, S.; Cai, J.; Meng, M.; Li, X. A facile synthesis of Agx Au1−x /TiO2 photocatalysts with tunable surface plasmon resonance (SPR) frequency used for rhb photodegradation. Mater. Lett. 2015, 154, 163–166. [CrossRef] Jiang, Z.Y.; Zhao, Z.Y. Density functional theory study on the metal−support interaction between au9 cluster and anatase TiO2 (001) surface. Phys. Chem. Chem. Phys. 2017, 118, 3514–3522. [CrossRef] [PubMed] Rao, C.N.R.; Kulkarni, G.U.; Thomas, P.J.; Edwards, P.P. Size-dependent chemistry: Properties of nanocrystals. Chem. A Eur. J. 2002, 8, 28–35. [CrossRef] Winsemius, P.; Guerrisi, M.; Rosei, R. Splitting of the interband absorption edge in Au: Temperature dependence. Phys. Rev. B 1975, 12, 4570–4572. [CrossRef] Liu, L.; Li, P.; Adisak, B.; Ouyang, S.; Umezawa, N.; Ye, J.; Kodiyath, R.; Tanabe, T.; Ramesh, G.V.; Ueda, S. Gold photosensitized SrTiO3 for visible-light water oxidation induced by Au interband transitions. J. Mater. Chem. A 2014, 2, 9875–9882. [CrossRef]


31. 32. 33. 34. 35.

36. 37. 38.

39.

40.

41. 42.

15 of 15

Pakizeh, T. Optical absorption of plasmonic nanoparticles in presence of a local interband transition. J. Phys. Chem. C 2011, 115, 21826–21831. [CrossRef] Li, J.; Cushing, S.K.; Bright, J.; Meng, F.; Senty, T.R.; Zheng, P.; Bristow, A.D.; Wu, N. Ag@Cu2 O core-shell nanoparticles as visible-light plasmonic photocatalysts. ACS Catal. 2014, 3, 47–51. [CrossRef] Sönnichsen, C.; Franzl, T.; Wilk, T.; von Plessen, G.; Feldmann, J.; Wilson, O.; Mulvaney, P. Drastic reduction of plasmon damping in gold nanorods. Phys. Rev. Lett. 2002, 88, 077402. [CrossRef] [PubMed] Jin, R. Quantum sized, thiolate-protected gold nanoclusters. Nanoscale 2010, 2, 343–362. [CrossRef] [PubMed] Hu, C.; Rong, J.; Cui, J.; Yang, Y.; Yang, L.; Wang, Y.; Liu, Y. Fabrication of a graphene oxide–gold nanorod hybrid material by electrostatic self-assembly for surface-enhanced raman scattering. Carbon 2013, 51, 255–264. [CrossRef] Wang, Z.; Cao, M.; Yang, L.; Liu, D.; Wei, D. Hemin/Cu nanorods/self-doped TiO2 nanowires as a novel photoelectrochemical bioanalysis platform. Analyst 2017, 142, 2805–2811. [CrossRef] [PubMed] Oyelere, A.K.; Chen, P.C.; Huang, X.; El-Sayed, I.H.; El-Sayed, M.A. Peptide-conjugated gold nanorods for nuclear targeting. Bioconjug. Chem. 2007, 18, 1490. [CrossRef] [PubMed] Priebe, J.B.; Michael, K.; Henrik, J.; Matthias, B.; Dirk, H.; Angelika, B. Water reduction with visible light: Synergy between optical transitions and electron transfer in Au–TiO2 catalysts visualized by in situ EPR spectroscopy. Angew. Chem. Int. Ed. 2013, 52, 11420–11424. [CrossRef] [PubMed] Govorov, A.O.; Zhang, H.; Gun’ko, Y.K. Theory of photoinjection of hot plasmonic carriers from metal nanostructures into semiconductors and surface molecules. J. Phys. Chem. C 2013, 117, 16616–16631. [CrossRef] Liu, S.; Chen, G.; Prasad, P.N.; Swihart, M.T. Synthesis of monodisperse Au, Ag, and Au–Ag alloy nanoparticles with tunable size and surface plasmon resonance frequency. Chem. Mater. 2011, 23, 4098–4101. [CrossRef] Kimura, K.; Naya, S.I.; Jin-Nouchi, Y.; Tada, H. TiO2 crystal form-dependence of the au/TiO2 plasmon photocatalyst’s activity. J. Phys. Chem. C 2012, 116, 7111–7117. [CrossRef] Johnson, P.B.; Christy, R.W. Optical constants of the noble metals. Phys. Rev. B 1972, 6, 4370–4379. [CrossRef] © 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).