Mohammed A. Ibrahem*a,
Bassam G. Rasheed
b,
Rahman I. Mahdic,
Taha M. Khazala,
Maryam M. Omara and
Mary O'Neilld
aLaser Sciences and Technology Branch, Applied Sciences Department, University of Technology, Baghdad, Iraq. E-mail: mohammed.a.ibrahem@uotechnology.edu.iq
bLaser and Optoelectronic Engineering Department, College of Engineering, Al-Nahrain University, Baghdad, Iraq
cNanotechnology and Advanced Materials Research Centre, University of Technology, Baghdad, Iraq
dSchool of Science and Technology, Nottingham Trent University, Clifton Lane, Nottingham NG11 8NS, UK
First published on 10th June 2020
This work shows the enhancement of the visible photocatalytic activity of TiO2 NPs film using the localized surface plasmonic resonance of Au nanostructures. We adopted a simple yet effective surface treatment to tune the size distribution, and plasmonic resonance spectrum of Au nanostructured films on glass substrates, by hot plate annealing in air at low temperatures. A hybrid photocatalytic film of TiO2:Au is utilized to catalyse a selective photodegradation reaction of Methylene Blue in solution. Irradiation at the plasmonic resonance wavelength of the Au nanostructures provides more effective photodegradation compared to broadband artificial sunlight of significantly higher intensity. This improvement is attributed to the active contribution of the plasmonic hot electrons injected into the TiO2. The broadband source initiates competing photoreactions in the photocatalyst, so that carrier transfer from the catalyst surface to the solution is less efficient. The proposed hybrid photocatalyst can be integrated with a variety of device architectures and designs, which makes it highly attractive for low-cost photocatalysis applications.
In the past few years, plasmonic photocatalysis featuring localized surface plasmon resonance has emerged as a promising new technology that brings in some great advantages over the conventional photocatalysis previously mentioned.7 Particularly, the incorporation of metal nanostructures like gold could help broaden the absorption spectrum of the TiO2 to the visible and near-infrared and also help reduce the recombination rate by establishing a Schottky barrier at the interface.8 Additionally, metal nanostructures could also help enhance light absorption by scattering the incident light which increases its path length inside the hybrid film. More importantly, localized surface plasmon-assisted solar-to-fuel energy conversion is well known via hot-electron generation as a result of the plasmon decay process.9,10 Plasmonic hot electron injection is an efficient mechanism to manipulate the photoresponse of wide bandgap semiconductors and enable them to work in the visible region of the spectrum.8,11 The collective oscillation of charge carriers in Au nanostructures, on excitation of the plasmonic resonance, leads to the formation of highly energetic electrons that can be transferred over the Schottky barrier into the neighbouring semiconductor at appropriate interface conditions.12–14 These plasmonic electrons provide a new physical concept in terms of modifying the charge density of wide bandgap semiconductors and enabling them to have a photoresponse below their optical band gap, which ultimately enhances the photocatalysis process.14 The general principle of the photocatalysis reaction in semiconductors has been reported.15–17 Various plasmonic photocatalysis systems, incorporating nanostructures of different metals, have been reported such as Ag/ZnO,18 Au/CdSe19 and Ag/TiO2.20 Gold nanostructures show a robust and efficient plasmonic response compared to other noble metals makes them highly desirable in achieving efficient photocatalysis process especially when combined with TiO2.
Plasmonic photocatalysis efficiency is often linked to complicated plasmonic metal structures and systems, which require state-of-the-art fabrication techniques and long processing time. For instance, Shaik et al. reported Au nano prisms as a plasmonic antenna with CdSe@CdS core–shell quantum dots to enhance the photocatalysis yields and kinetics.21 Zhao et al. reported an enhancement of the plasmonic photocatalytic activity by building a periodic three-dimensional nanocomposite architecture of Ag/TiO2 nanowires utilizing nanoimprint lithography, vertical e-beam evaporation, nano-transfer, and nano welding.22 Zhou et al. have shown a significant enhancement of the plasmonic photocatalysis of Methylene Blue dye (MB) utilizing Au/Ag NRs/TiO2 core–shell composite nanoparticles as a catalyst agent.23
Herein, a TiO2:Au hybrid film has been used as a catalyst to selectively degrade MB under visible light irradiation, taking advantage of the localized surface plasmonic resonance effect (LSPR) of Au nanostructures. Au nanostructured films are fabricated following a simple, fast and cost-effective technique with an annealing temperature of 300 °C in air. This work aims to provide proof of principle of a photocatalyst, which combines cost-effective fabrication and processing of plasmonic nanostructures with high photocatalysis efficiency.
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Fig. 1 Shows XRD of TiO2 recorded in the powder form. The particles show a typical diffraction pattern of the anatase crystal structure. |
Laser light with a wavelength of 532 nm and power of 10 mW was utilized as a light source to photoexcite the hybrid photocatalytic film immersed in 2 ml of 5 ppm solution of MB in a glass container put on a stainless steel bench which act as a heat sink to help stabilize the reaction temperature. The light beam was expanded using a beam expander to cover an area of 100 mm2 of the plasmonic photocatalyst, resulting in irradiance of 2.3 mW cm−2.
Artificial sunlight (150 W Xe lamp with Air Mass 1.5 Global Filter) was used as a second light source to study the photodegradation of MB. In this case, an irradiance of 94 mW cm−2 was used. A UV filter has been used with the solar simulator source to block the UV light from going through and cause unwanted photoreaction process. The temperature of aqueous solution during the reaction at both light sources was in the range of 23 ∓ 2 °C. The liquid temperature is monitored using contactless IR thermometer (Fluke 62 MAX+). The photodegradation process was monitored by measuring the absorbance spectrum of MB using the UV-Vis spectrophotometer for different irradiation times (5, 10, 20, 30, 40, 50 and 60 minutes) of the laser and the artificial sunlight source. Two control experiments were also carried out only for irradiation with the laser. For the first, MB absorbance was monitored without a photocatalyst and, for the second, a TiO2 film on glass without Au nanoislands was used as a photocatalyst.
Annealing at 200 °C leads to a drastic variation in the shape and size of the Au structures forming complex melting patterns with large diameters as shown in Fig. 2-C. The variation of the full width at half maximum (FWHM), calculated from the size distribution histograms, indicating a gold recrystallization process towards more uniform Au shapes and sizes. The surface morphology of the Au film at 300 °C is more uniform with distinct columnar structures separated by gaps. The FWHM is narrowed to 37 nm. The modifications of the Au nanoislands shape with temperature is an adaptation to reduce their surface energy. A narrowing of the size distribution is linked to the drastic change of the nanostructure colour from dark blue to reddish (can be observed by the naked eye as shown later in the inset of Fig. 4). Such a colour is attributed to a plasmonic resonance resulting from more uniform shapes of Au nanoparticles.28 Fig. 3 summarises changes to the average diameter and surface roughness of the Au nanostructures with annealing temperature. The diameter increases from about 90 nm to about 107 nm with temperature from 100–200 °C with an increment of 14%. While further increasing the annealing temperature to 300 °C shows a slower increase of only 4%, suggesting that growth is saturating.
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Fig. 3 The average diameter of Au nanoislands in addition to surface roughness with different annealing temperatures. |
It also shows a significant change in the FWHM of the absorption peak with annealing temperature. Optical absorption of the as-deposited Au film shows a broad peak with the absorption resonance at around 660 nm. Annealing at 100 °C for 1 hour in the air blueshifts the plasmonic resonance to 610 nm and narrow it to an FWHM of 219 nm. Annealing at 200 °C and 300 °C results in further narrowing of the FWHM, to 86 nm to 53 nm respectively, and a blue shift of the absorption resonance, to 555 nm and 521 nm respectively. The inset in Fig. 4 shows optical images of the Au films before (as deposited) and after annealing at 300 °C for 1 hour. The colour of the film is clearly changed from dark blue to red after annealing which supports the morphological transformation from random shaped Au islands to uniform-like Au particles shown earlier in the AFM images (Fig. 2-A and D). Moreover, our results show that annealing of 10 nm Au film at 300 °C for 60 min in the air is sufficient to obtain the almost full morphological transformation from random islands to uniform particles in much less time than previously reported.30,31 It is found that there is no significant difference in the resonance absorption FWHM and peak position of Au film annealed at 300 °C for 60 min and 120 min following the same annealing conditions. Fig. 5 shows the absorption spectra of two TiO2 thin films, one with and one without an underlying Au nanostructure layer annealed at 300 °C for 1 hour in the air prior to TiO2 deposition. Both spectra show a characteristic absorption peak in the UV due to the electronic interband transition in TiO2.
The latter also shows a broad hump in the visible due to the delocalization of electrons in the Au as a result of the plasmonic resonance absorption. The figure also shows a slight increase in the UV absorption of TiO2 incorporating Au nanostructures which could be attributed to the localized plasmonic field and/or to light scattering by Au inside TiO2 film which increases the overall absorption.32 TiO2 has a very limited photoresponse in the visible due to the presence of surface defects. However, when decorated with Au nanostructures, its photoresponse is tailored and enhanced significantly in the visible as a result of the plasmonic resonance absorption as indicated by the absorption spectrum (blue line) in Fig. 5. The inset shows the plasmonic absorption contribution calculated by subtracting the absorption of TiO2 from the overall absorption spectrum of TiO2:Au. The resonant absorption peak is broadened and red-shifted when combined with TiO2 film compared to its spectrum discussed earlier (the green line in Fig. 4). This is probably due to the difference in the refractive index of TiO2 compared to air which has a significant impact on the optical properties of Au nanostructures.
Fig. 6-C shows the temporal variation of the absorption spectrum of MB examined by measuring the UV-Vis absorption of the dye periodically during irradiation in the presence of the hybrid TiO2:Au film as a catalyst. Additionally, a low initial concentration of MB was used and the hybrid film was kept in MB solution for 60 min prior to irradiation to reach the adsorption–desorption equilibrium following Azeez et al.36 The figure inset shows an image of the MB solution before and after irradiation with the laser for 60 min. Fig. 6-D plots the MB absorbance (A), at a wavelength of 664 nm, normalised with respect to the starting absorbance (Ao), with irradiation time up to 60 min, when TiO2:Au (blue line) and TiO2 films (red line) are used as photocatalysts. The photodegradation profile shows a significant reduction in MB concentration, in which about 60% of the dye is degraded with time when the TiO2:Au hybrid photocatalyst is used as a photocatalyst compared to a minor reduction in MB concentration with the TiO2 film. The observed reduction in MB concentration when a TiO2 film is used as a catalyst is observed elsewhere37–39 and attributed to the surface defects which can be photoexcited with photon energy less than the semiconductor bandgap.
Most proposed plasmonic photodegradation reactions involve suspension of the plasmonic mixture in MB which is considered to be time-consuming and can only be used for one reaction at the time.21,40 In contrary, our method adopt a clean, fast and simple method to initiate the photoreaction by making it as a film possibly coating the wall of the container. This method does not need to separate the hybrid mixture after reaction and could potentially be reused in another reaction. Despite our simple photocatalytic configuration, our results show equivalent photodegradation performance to more sophisticated photocatalytic structures.22,41 The green line in Fig. 6-D shows very little degradation when the MB solution is irradiated with the laser without any photocatalyst present. This could be attributed to photobleaching.42 A control experiment using Au nanostructures only was not made due to the extremely fast process of plasmonic hot electrons stimulated in Au nanostructures (around 50 fs)43 accompanied with the small catalytic area of Au compared to the hybrid photocatalyst film.
Plasmonic excitation using a light source with a resonant wavelength is expected to enable an efficient and sustainable photocatalysis reaction compared to sunlight irradiation due to a selective initiation of the plasmonic photodegradation reaction. To better confirm this, the hybrid film was irradiated with artificial sunlight (94 mW cm−2) and the absorption of MB was monitored for the same irradiation period as shown in Fig. 6-D. Data show that the MB absorption is slightly reduced with sunlight irradiation and this reduction has two regions. Initially, the MB absorption reduces almost linearly with irradiation time up to 40 min when it saturates at a relatively high absorbance level. This saturation is unexpected, given the relatively high intensity of the sunlight source, which has a small but significant component of the spectrum overlapping the plasmonic resonance.
To interpret these results, we discuss the differences between the effects of the two light sources on the electronic transitions of the photocatalyst. By using the green laser, we only activate the plasmonic effect. On the other hand, only the green portion of the artificial sunlight spectrum (UV light is discarded) is resonant with the Au nanostructures. Irradiation with artificial sunlight excites a range of sub-bandgap defects states within the TiO2 bandgap. Some of these do not have sufficient energy to reach the conduction band and nonradiatively relax back to lower energy level, heating the lattice of the semiconductor.44 This photothermal effect could negatively impact the plasmonic hot electrons leading to charge collisions and energy dissipation. Other photochemical transitions may result in the generation of new traps reducing the lifetime of the surface free carriers.
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