TiO2 film supported by vertically aligned gold nanorod superlattice array for enhanced photocatalytic hydrogen evolution

https://doi.org/10.1016/j.cej.2020.127900Get rights and content

Highlights

  • TiO2 film supported by Au nanorod superlattice array (O-AuNRs/TiO2) was fabricated on a glass.

  • Well-ordered and closely-packed AuNR superlattice dramatically boosts the local electromagnetic field.

  • O-AuNRs architecture plays a key role in enhancing the photocatalytic activity of TiO2 film.

  • O-AuNRs/TiO2 exhibits a highly enhanced photocatalytic hydrogen evolution rate, 58 times of unsupported TiO2 film.

Abstract

Photocatalytic hydrogen generation from water as a renewable non-polluting technique for converting solar energy to chemical energy has recently attracted worldwide attention. However, low hydrogen production efficiency of traditional photocatalysis still remains as a challenge. Here, we report a large area TiO2 film supported by vertically ordered Au nanorods superlattice array (O-AuNRs/TiO2), which demonstrates excellent photocatalytic performances of hydrogen evolution from water under solar and visible light irradiation. The O-AuNRs/TiO2 architecture enables the significant localized surface plasmon resonance (LSPR) enhancement, including both local electromagnetic field effect and hot electron transfer effect, which promotes the photocatalytic hydrogen evolution rate of the TiO2 film by 58 times. The photocatalytic efficiency of O-AuNRs/TiO2 exceeds that of the TiO2 film supported by randomly oriented AuNRs by over five times. Finite difference time domain (FDTD) modeling results support that the strong coupling of O-AuNRs enhances the electromagnetic field intensity along the longitudinal axis in the gaps between adjacent AuNRs and the average electric field enhancement factors at the interface between the AuNRs and TiO2 of O-AuNRs/TiO2. This work demonstrates the substantial performance boost of conventional photocatalyst by LSPR enhancement, thus provides a promising tactic to devise highly efficient photocatalytic system for solar energy conversion.

Graphical abstract

A large area TiO2 film supported by vertically ordered Au nanorod superlattice array (O-AuNRs/TiO2) is fabricated on a glass substrate. Compared to both pristine TiO2 film and the TiO2 film supported by randomly-oriented AuNRs (R-AuNRs/TiO2), the O-AuNRs/TiO2 delivers highly enhanced photocatalytic hydrogen evolution rate, benefiting from the significant localized surface plasmon resonance (LSPR) enhancement by O-AuNRs.

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Introduction

Photocatalytic hydrogen generation from water has long been considered as one of the most promising future strategies for solar energy-to-chemical fuels conversion. Among various materials developed so far, nanoscale titanium dioxide (TiO2) is usually regarded as a benchmarking photocatalyst for metal-oxide-based semiconductors and has been substantially investigated for decades [1], [2]. Nevertheless, the relatively large bandgap energy of TiO2 (3.20 eV for anatase and 3.03 eV for rutile TiO2) makes its light absorption limited to the ultraviolet (UV) region of electromagnetic spectrum, which accounts for less than 5% of the whole solar spectrum and thus hampers the utilization of solar energy. Over the past years, substantial efforts have been devoted to boost visible light-absorption of TiO2 [3], [4], [5], [6], [7]. Recently, plasmon-mediated tactic has been demonstrated to augment the visible light utilization efficiency by means of achieving hot electron injection, improved photoexcitation, and the propelled charge separation in the coupled photocatalyst.

To accomplish efficient utilization of visible light, the nanostructured plasmonic metals (Au, Ag, Cu, and Al) have been widely implemented into various photocatalysts [8], [9], [10], [11], [12], [13], [14]. The light response of such composite photocatalysts can be enhanced and tailored by choosing the suitable components or adjusting the size and shape of the metallic nanostructures. This strategy has been also extensively employed to promote the photocatalytic capability of nanostructured TiO2 by engaging the plasmonic metal nanoparticles (NPs) as light-harvesting antennas, which exhibit unique localized surface plasmon resonance (LSPR) characteristics at visible or even infrared wavelengths [15], [16], [17]. The main LSPR enhancement of photocatalytic performance acts through: [18], [19], [20], [21] 1) scattering light to increase the average photon path length between the metal NPs and semiconductors thus boosting light trapping by a photocatalyst; [22], [23] 2) direct hot electron transfer (DHET) [24], [25], [26], [27], [28], in which the plasmon-excited electrons on metal (hot electrons) are injected into the conduction band (CB) of neighboring semiconductor over the Schottky barrier; 3) plasmon-induced resonant energy transfer (PRET), which is regarded as a process of transferring the plasmonic oscillation energy from the plasmonic metal nanocrystals to the nearby semiconductor based on local electromagnetic field (LEMF) effect. The PRET accelerates the generation of additional electron-hole pairs and suppresses the recombination of generated charge carriers in the nearby semiconductor [29], [30], [31], [32], thus boosting the photocatalytic performance [33].

In addition to the nature, shape, and size of the plasmonic metal [33], [34], [35], the intensity of the LEMF is also largely dependent on the local density, the alignment/orientation, and the interparticle gap of metal crystals [36], [37], [38], [39], [40]. Benefiting from the collective plasmonic intensity of the LSPR-excited LEMF, 3-dimensional (3D) superlattice array, an ordered assembly of colloidal nanostructures, especially the anisotropic Au nanorods (AuNRs), has shown the extraordinarily enhanced performances in spectroscopies [41], [42], [43], [44], solar cells [19], and nanoscale light polarizers [45], [46]. Nevertheless, the size of reported 3D superlattices is usually just a few square micrometers, which hinders their application in the fields where a large area is required. In our previous study, we developed a simple and efficient evaporative self-assembly strategy to build millimeter-scale 3D superlattice arrays composed of dense, regular, and vertically aligned AuNRs for surface-enhanced Raman scattering (SERS) [36]. Apparently, the millimeter-scale superlattice arrays are too small to be an effective photocatalytic system for hydrogen evolution reaction (HER). Herein, a centimeter-scale AuNR superlattice/ordered array (O-AuNRs) was fabricated on a glass substrate to study how the LEMF and DHET from the underlying O-AuNR array affect the exciton carrier generation in TiO2. A remarkably enhanced photocatalytic HER rate of the TiO2 film deposited on top of O-AuNRs by magnetron sputtering (O-AuNRs/TiO2) was achieved, which exceeds that of bare TiO2 film by 58 times.

Section snippets

Chemicals

Chloroauric acid (HAuCl4·4H2O), silver nitrate (AgNO3), sodium chloride (NaCl), hydrochloric acid (HCl), sodium borohydride (NaBH4), sodium hydroxide (NaOH), ascorbic acid (AA), hexadecyltrimethylammonium bromide (CTAB), 11-mercaptoundecylhexaethylene glycol (MUHEG), and triethanolamine (TEOA) were purchased from Sigma-Aldrich. All the chemicals were used as received without further purification.

Synthesis of AuNRs

Au nanorods (AuNRs) were prepared in an aqueous solution using a seed-mediated growth method as

Results and discussion

Among various Au nanostructures, much attention has been paid to those with rod-like shapes due to their unique optical properties and wide applications in optical devices since their first discovery in 1991 [47]. Moreover, the surface plasmon propagation of AuNRs mainly takes place along the longitudinal direction, and the LSPR maxima can be easily tuned between 600 and 1200 nm by adjusting the aspect ratio [48]. To enhance the LSPR effect, the AuNRs were modified with specific capping agents

Conclusions

In summary, a large area of vertically ordered AuNRs (O-AuNRs) superlattice array was prepared on a glass substrate by a simple evaporative self-assembly method. A layer of anatase TiO2 film was then deposited on the AuNR superlattice array by magnetron sputtering to form an O-AuNRs/TiO2 heterogeneous nanostructure. The O-AuNRs/TiO2 architecture significantly enhances the photocatalytic H2 evolution rate of the TiO2 film by 58 times, benefiting from the localized surface plasmon resonance

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work is financially supported by the Innovation and Technology Commission of Hong Kong to the State Key Laboratory of Chemical Biology and Drug Discovery, The Hong Kong Polytechnic University, and National Natural Science Foundation of China (21806099). KYW acknowledges the support from the Patrick S.C. Poon Endowed Professorship.

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    L. Hu and Y. Li contributed equally to this work.

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