Home
Search
Collections
Journals
About
Contact us
My IOPscience
3D periodic multiscale TiO2 architecture: a platform decorated with graphene quantum dots for enhanced photoelectrochemical water splitting
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2016 Nanotechnology 27 115401 (http://iopscience.iop.org/0957-4484/27/11/115401) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 128.111.121.42 This content was downloaded on 06/03/2016 at 19:38
Please note that terms and conditions apply.
Nanotechnology Nanotechnology 27 (2016) 115401 (11pp)
doi:10.1088/0957-4484/27/11/115401
3D periodic multiscale TiO2 architecture: a platform decorated with graphene quantum dots for enhanced photoelectrochemical water splitting Zhen Xu1,2, Min Yin1, Jing Sun3, Guqiao Ding3, Linfeng Lu1, Paichun Chang4, Xiaoyuan Chen1 and Dongdong Li1 1
Shanghai Advanced Research Institute, Chinese Academy of Sciences, 99 Haike Road, Zhangjiang Hi-Tech Park, Pudong, Shanghai 201210, People’s Republic of China 2 University of Chinese Academy of Sciences, Beijing 100039, People’s Republic of China 3 State Key Laboratory of Functional Materials for Informatics Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050, People’s Republic of China 4 Department of Creative Industry, Kainan University, No. 1, Kainan Road, Luchu, Taoyuan County 338, Taiwan E-mail: [email protected] and [email protected] Received 25 November 2015, revised 19 January 2016 Accepted for publication 19 January 2016 Published 15 February 2016 Abstract
Micropatterned TiO2 nanorods (TiO2NRs) via three-dimensional (3D) geometry engineering in both microscale and nanoscale decorated with graphene quantum dots (GQDs) have been demonstrated successfully. First, micropillar (MP) and microcave (MC) arrays of anatase TiO2 films are obtained through the sol–gel based thermal nanoimprinting method. Then they are employed as seed layers in hydrothermal growth to fabricate the 3D micropillar/microcave arrays of rutile TiO2NRs (NR), which show much-improved photoelectrochemical water-splitting performance than the TiO2NRs grown on flat seed layer. The zero-dimensional GQDs are sequentially deposited onto the surfaces of the microscale patterned nanorods. Owing to the fast charge separation that resulted from the favorable band alignment of the GQDs and rutile TiO2, the MP-NR-GQDs electrode achieves a photocurrent density up to 2.92 mA cm−2 under simulated one-sun illumination. The incident-photon-to-currentconversion efficiency (IPCE) value up to 72% at 370 nm was achieved on the MP-NR-GQDs electrode, which outperforms the flat-NR counterpart by 69%. The IPCE results also imply that the improved photocurrent mainly benefits from the distinctly enhanced ultraviolet response. The work provides a cost-effective and flexible pathway to develop periodic 3D micropatterned photoelectrodes and is promising for the future deployment of high performance optoelectronic devices. S Online supplementary data available from stacks.iop.org/NANO/27/115401/mmedia Keywords: periodic multiscale structure, TiO2 nanorod arrays, thermal nanoimprint, graphene quantum dots, photoelectrochemical water splitting (Some figures may appear in colour only in the online journal) 1. Introduction
attention owing to its important applications in photoelectrochemical (PEC) water splitting [1, 2], photocatalysis [3, 4], perovskite solar cells [5], sensors [6], because of its abundance, nontoxicity and photostability [7]. As for the
Titanium dioxide (TiO2), with a band gap of 3.0–3.2 eV, has been currently attracting extensive academic and industrial 0957-4484/16/115401+11$33.00
1
© 2016 IOP Publishing Ltd Printed in the UK
Nanotechnology 27 (2016) 115401
Z Xu et al
application in PEC water splitting, promising solutions for efficient solar-to-chemical energy conversion of the TiO2 include avoiding charge trapping on semiconductor surfaces [8], increasing the semiconductor-electrolyte interfacial area [9], and enhancing the light harvesting capability [10], etc. Moreover, a one-dimensional (1D) single crystalline rutile TiO2 nanorod (TiO2NR), providing a direct pathway for charge transportation [11], has been demonstrated as a promising photoanode for PEC water splitting. Therefore, threedimensional (3D) networks consisting of single crystalline 1D nanostructure may possess ideal architecture for high-performance PEC electrodes. The 3D architecture can not only offer long optical paths for efficient light absorption and copious active sites for electrochemical reactions, but provides efficient conducting channels for rapid electron–hole separation and charge transport [12–14]. In recent years, researchers have been enthusiastically dedicated to the development of 3D hierarchical architecture [15], in which the backbone of Si nanowire [11, 12], ZnO nanowire [16] or the TiO2 trunk [17, 18] scaffold is obtained by etching [19] or the hydrothermal/solvothermal method [9]. And the branched nanostructures are achieved through a second hydrothermal growth [9] or a modified chemical vapor deposition [17, 19]. These intricate procedures are either timeconsuming or need costly equipment. With the inherent advantages of low-cost, high resolution, large area and highthroughput [20], nanoimprint lithography is a suitable technique to prepare specified patterns in the scale of nano- and micro-meters [21]. Sol–gel based thermal nanoimprinting is a facile and general route to fabricate large-area nanoscale patterns of various metal oxides, such as Fe2O3 [22], TiO2 [23, 24], and ZrO2 [25] etc. It can be used to achieve the 3D periodic backbone with experimental simplicity, flexibility and mass production. It has been predicted that a maximum photoconversion efficiency of 2.25% can be achieved by rutile TiO2 with an optical band gap of 3.0 eV, under 100 mW/cm2 AM 1.5 global illumination [26]. Yet, the reported photoconversion efficiencies and photocurrent densities of TiO2 photoanodes are substantially lower than the theoretical limit mainly due to the insufficient light absorption and severe charge recombination. Graphene quantum dot (GQD), the emerging zerodimensional carbon material, is reported to have the ability to decrease the electrochemical impedance and facilitate the charge transport kinetics [27, 28]. It has attracted tremendous attention [29–33], because of its intriguing optical and electrical properties due to its tunable size and surface nature [34]. In this study, 3D periodic multiscale TiO2 nanorod arrays are first realized by combining the sol–gel-based thermal nanoimprinting and hydrothermal growth process. The GQDs are subsequently deposited onto the surfaces of the hierarchical patterned nanostructures with the aim of enhancing the PEC performances of the electrodes. The possible mechanism of the GQDs’ enhanced PEC performances are proposed after careful characterization and analysis. Owing to the fast charge separation that resulted from the favorable band alignment of the energy levels between the GQDs and the rutile TiO2 as well as the adequate loading of TiO2NRs,
the MP-NR-GQDs represent superior PEC performances in the UV light region.
2. Experiment Multiscale TiO2 hierarchical structure was fabricated by combining sol–gel-based thermal imprint lithography and hydrothermal growth, as shown in figure 1. The detailed preparation procedures were as below. 2.1. Material preparation Preparation of polymeric mold. A silicone (polydimethysiloxane, PDMS) mold (∼2 mm thickness) with micropit arrays (2.6 μm in depth, 2 μm in diameter and 3 μm in pitch) was provided by NanoCarve Ltd. Prior to use, the PDMS micropit mold was exposed to O2 plasma at 50 W for 1 min with an oxygen flow rate of 0.5 l min−1 [35]. Then, the obtained activated mold with dramatically increased surface O/C ratio was further treated by a molecular vapor deposition (MVD) process to achieve a hydrophobic surface. Specifically, the PDMS mold was placed in the sealed MVD chamber with 2 μl of fluorosilane (1H, 1H, 2H, 2Hperfluorooctyltrichlorosilane) loaded [36]. The temperature of the chamber was increased to 100 °C and lasted for 10 min. A self-assembled monolayer of fluorosilane was introduced on the surface of the PDMS mold and would facilitate the separation in the later process. The whole process is similar to the conventional anti-sticking treatment on the Si mold [37]. Subsequently, micropillar (MP) PDMS mold was prepared by pouring premixed PDMS onto the micropit mold, followed by degassing and a curing process at 60 °C for 3 h. Later, the cured PDMS membrane (about 0.5 mm) with MP arrays was obtained by peeling it off with great care. In general, two kinds of mold were used in this work, i.e. PDMS membranes with micropits and MP arrays.
2.1.1.
2.1.2. Preparation of TiO2 sol solution. Tetrabutyl titanate
(TBOT, Aldrich, 99.9%, 4.25 ml) was mixed with a solution of anhydrous ethanol (25 ml) and diethanolamine (DEA, Aldrich, 99.9%, 3.75 ml) for two hours, followed by the dropwise addition of glacial acetic acid (HAc, Aldrich, 99.9%, 5 ml) and deionized (DI) water (5 ml). The solution was left stirring for another 24 h in a fume hood to obtain a final concentrated solution of 15 ml.
2.1.3. Preparation of seed layer on FTO substrates with different morphology. Sol–gel-based thermal nanoimprinting
lithography was used to prepare a microstructure seed layer of TiO2. Specifically, TiO2 polymeric sol was spin-coated onto the FTO at 1000 rpm for 1 min. Then, two different PDMS molds (i.e. with MP and micropit arrays) were placed on the TiO2 sol substrates with conformal contact, and were then placed in a nanoimprinting chamber with a pressure of 0.3 MPa at 250 °C for 2 h with a heating rate of 10 °C/min. After cooling down to 60 °C, the molds were detached from the TiO2 gel microstructures with great care. Microcave (MC) and MP 2
Nanotechnology 27 (2016) 115401
Z Xu et al
Figure 1. Schematics of the fabrication processes of different TiO2NR electrodes. Route 1 for MC-NR. Route 2 for MP-NR. Route 3 for flat-NR.
after the autoclave was cooled down to room temperature naturally. The samples were further annealed in ambient air at 450 °C for 2 h with a heating rate of 2 °C/min to achieve the well-crystallized rutile TiO2.
TiO2 gel substrates were then obtained. The micropatterned substrates were further annealed in ambient air at 450 °C for 2 h with a heating rate of 2 °C/min to achieve the crystallized seed layer on the FTO. Meanwhile, flat-seed substrate was also prepared according to the aforementioned procedures except for the thermal nanoimprinting process. As a result, substrates with three different topographical seed layers i.e. the MC-seed, MPseed and flat-seed were obtained. More detailed information about the parameters used in the thermal nanoimprinting lithography can be found in the supplementary data.
2.1.5. Preparation of GQDs. GQDs were prepared from
graphene powder via a modified Staudenmaier method [31]. Graphene powder (4 g) was put into H2SO4 (150 ml) and HNO3 (80 ml) with stirring at 15 °C, and was kept for 2 h. Then NaClO3 (40 g) was added gradually and the temperature was kept below 5 °C. The mixture was then stirred at 15 °C for 5 h. After that, the reaction was terminated by adding distilled water (80 ml). The pH value was neutralized to 7 by NaOH, before the mixture was filtered out using an alumina inorganic membrane (Shanghai Shangmu Technology Co. Ltd) with 20 nm pores. The obtained light yellow filtrate was dialyzed in a 3500 Da dialysis bag against DI water for a week to remove excess salt. The purified solutions were transferred to a Teflonlined stainless steel autoclave and heated at 200 °C for 5 h to reduce the oxygen-containing groups, and cooled to room temperature naturally. The resultant light yellow solution of GQDs was obtained.
2.1.4. Hydrothermal growth of TiO2NR film. Rutile TiO2NR
films were prepared on the three different substrates by a slightly modified hydrothermal method [38]. Correspondingly, the samples were marked as MC-NR, MP-NR and flat-NR. Typically, 0.833 ml TBOT was dropped into a mixture of 22.5 mlDI water and 27.5 ml concentrated hydrochloric acid (36.0–38.0 wt%), followed by stirring for 20 min to obtain a transparent solution. The solution was poured into a Teflonlined stainless steel autoclave (100 ml capacity). Three pieces of the FTO substrates (1.5 ×4 cm) coated with different patterned seed layers were placed leaning against the wall of the Teflon-lined autoclave with the conducting side facing down. The hydrothermal synthesis was conducted at 150 °C for 12 h in an electric oven. Afterwards, the samples were taken out, rinsed extensively with DI water and dried in ambient air
2.1.6. Electrophoresis deposition of GQDs on micropatterned hierarchical TiO2NR films. In order to increase the negative
charges of the as-prepared GQDs, the pH was tuned to 8 3
Nanotechnology 27 (2016) 115401
Z Xu et al
using 1 M NaOH solution. Different TiO2NR films (i.e. MCNR, MP-NR and flat-NR) acted as the working electrodes and Pt plate as counter electrodes. Under an applied positive potential of 6 V for 1 h, the functional GQDs were spontaneously deposited onto the surface of the nanorods. The deposition duration was optimized by testing the PEC performances of the flat-NR-GQDs electrodes with different deposition durations (see supplementary data for detailed information).
imprinting molds as displayed in figure S2 and table S1, due to the polymerization-induced inward shrinkage of the patterns during the multiple replications [24, 40]. After hydrothermal growth, all the obtained nanorods from the three substrates exhibit uniform dimensions of about 100 nm in diameter and 2–3 μm in length. Both the MC-NR and the MPNR arrays retain the rough terrain of the seed layers with hexagonal close-packed MCs or MPs (figures 2(a2)–(b2). As can be seen in figures 2(b2)–(b4), the nanorods on MP-NR grow radially on the original MPs, forming the periodic hexagonal close-packed microflower arrays. This is the first report of periodic microflower arrays of rutile TiO2NR, to the best of our knowledge. As can be seen in figures 3(a) and (b), the TiO2NR exhibits a relatively smooth side surface with a rectangular cross-section. As shown in figures 3(c)–(d), the surface of the nanorods becomes much rougher, indicating that the GQDs are homogeneously deposited on the entire surface of the nanorods. The results suggest that GQDs with the advantage of high solubility in various solvents by appropriate functionalization [34] have the capability of conformal wrapping on low-dimensional nanostructures as a functional layer by electrophoresis [31, 41]. Figure 4(a) shows an HRTEM image of an individual pristine TiO2 nanorod with a high resolved (110) lattice fringe of rutile TiO2 (d=0.32 nm), demonstrating that the nanorod grows preferentially along [001] direction in accordance with a strong (002) diffraction peak on the XRD patterns (figure S3). The diameter of the nanorod is about 100 nm (inset of figure 4(a)), which is consistent with the SEM result in figure 3(b). Figure 4(b) shows a representative low-magnification transmission electron microscope (TEM) image of the GQDs, which are uniformly distributed on the TEM carbon film without aggregation. The size of the GQDs is around 3–5 nm. The corresponding HRTEM image (figure 4(c)) shows the lattice spacing of 0.21 nm related to the (1100) plane of GQDs. A lattice-resolved TEM image of TiO2NR-GQDs from figure 4(d) reveals clear lattice fringes with interplanar spacings of 0.32 and 0.21 nm, which are consistent with the (110) plane of rutile TiO2 and (1100) plane of GQDs. It indicates that the GQDs attach onto the TiO2 nanorods firmly even after the intensified ultrasonic dispersion during the TEM samples preparation, which ensures the effective charge transfer channels. Elemental mapping data (figure 4(e)) confirms the existence of O, Ti and C. Although some background noise is apparent in the element of C mapping, we can still infer that the GQDs load over the TiO2 nanorod.
2.2. Materials characterization
The morphology and crystalline structure of the composites were characterized by field-emission scanning electron microscope (FESEM, Hitachi S4800), high-resolution transmission electron microscopy (HRTEM, JEM-2010F) and x-ray diffraction (XRD, Bruker D8 Discover diffractometer). Transmission spectra of the composite films were recorded by a standard UV–Vis spectrometer (Cary 5000, Agilent, USA). The topography of the different seed layers on the FTO slides was analyzed by a 3D laser scanning microscope (VK-9700, Keyence, USA). 2.3. Photoelectrochemical measurements
The entire back surface and edges of the electrodes were coated with insulating epoxy resin to obtain an exposed area of 1 cm2. The PEC water-splitting performances of the different electrodes were evaluated by AUTOLAB (PGSTAT302N/FRA2) using a three-electrode setup with the TiO2NR films as working electrodes, the Ag/AgCl (3 M KCl) electrode as the reference electrode and a platinum foil as a counter electrode in accordance with our previous work [8, 39]. The supporting electrolyte was 1 M potassium hydroxide (KOH, pH=14). The photoanodes were illuminated through the FTO glass substrate with simulated sunlight from a 300 W Xe lamp (Newport Corp.) equipped with an AM 1.5G filter. The photocurrent was measured at a potential of 0 V (versus Ag/AgCl) under chopped simulated light irradiation. Besides, the incident-photon-to-current-conversion efficiency (IPCE, DC mode) measurements were conducted at a zero bias versus Ag/AgCl by the AUTOLAB electrochemical station with the assistance of a commercial spectral response system (PV Measurement, QEX10).
3. Results and discussions 3.1. Structural and morphological analysis
3.2. Optical properties
The preparation of a TiO2 seed layer is a crucial step in the fabrication of highly ordered MCs/MPs consisting of randomly oriented TiO2 nanorods. Figures 2(a1)–(c1) show the FESEM images of seed layer on the FTO with different morphologies. The flat-seed directly prepared through spincoating presents some randomly shallow dents, which may result from the inward shrinkage of the sol during gelation. The obtained MC-seed and MP-seed show smaller diameter and height of the imprinted features than the corresponding
The optical properties and the corresponding Tauc plots for calculating the band gap of different TiO2NRs electrodes are shown in figure 5. It can be inferred that the TiO2NRs are much denser on MP-NR and MC-NR than on flat-NR. Thus, the light absorption of micropatterned hierarchical TiO2NRs in UV light is higher than the flat counterpart. The absorption edges are 340, 380 and 385 nm for flat-NR, MC-NR and MPNR electrodes, respectively. It can be estimated from 4
Nanotechnology 27 (2016) 115401
Z Xu et al
Figure 2. (a1)–(c1) Top view of MC-seed, MP-seed and flat-seed (inset of (b1) and (c1) is the cross-section view); (a2)–(c2), (a3)–(c3) top
views and (a4)–(c4) cross-section views of the corresponding MC-NR, MP-NR and flat-NR.
figure 5(b) that the band gap energy of the flat-NR is about 3.2 eV, which is larger than that of rutile TiO2 (3.0 eV) [7] while approaching that of anatase TiO2. The band gap energy of MC-NR and MP-NR is 2.94 and 2.97 eV, respectively. The black plot belonging to flat-NR is mainly contributed by the uniform flat-seed layer with anatase phase. While the patterned electrode enables some very thin residual seed layer during the imprinting process and with more nanorod (rutile phase) loading, the absorption plots of these electrodes are dominated by the rutile phase. More detailed information about Tauc plots analyses can be found in the supplementary data.
3.3. Photoelectrochemical performances
The PEC characterizations are first performed on three different electrodes. The linear sweep voltammetry (LSV) plots and photocurrent transients in figures 6(a) and (b) show that the flat-NR electrode exhibits a photocurrent density of 0.68 mA cm−2 which is comparable to that of previous literature [9, 10, 42]. The photocurrent on MC-NR and MP-NR electrodes achieved 1.88 and 2.15 mA cm−2 under zero bias potential versus Ag/AgCl, which are improved by 1.74 and 2.16 times over the flat-NR electrode. The micropatterned TiO2NR electrodes with much denser loading of TiO2NRs bring about enhanced light absorption and active reaction 5
Nanotechnology 27 (2016) 115401
Z Xu et al
Figure 3. SEM images of (a), (b) TiO2NR and (c), (d) TiO2NR-GQDs.
sites [15], which contribute to the much-improved surface catalysis. The intrinsic electrical structures of the photoelectrodes are investigated by the Mott–Schottky analysis, as displayed in figure 6(c). The calculated ND values of the flat-NR, MCNR and MP-NR are 7.2×1017, 1.4×1018 and 1.7×1018 cm−3, respectively. Despite the fact that the carrier densities are estimated values derived from the Mott– Schottky plot which is based on a flat-electrode model [43], the patterned substrates indeed increase the carrier density substantially. More detailed information about Mott–Schottky can be found in the supplementary data. The IPCE measurements are carried out to investigate the contribution of each monochromatic light to the photocurrent (figure 6(d)). The IPCE is calculated as a function of wavelength using IPCE=(1240(mW·nm/mA)I/(λJlight)), where λ is the incident light wavelength (nm), I and Jlight are the photocurrent density (mA/cm2) and incident light irradiance (mW/ cm2) at a specific wavelength [8, 44]. Both the modified electrodes with patterned seed layer represent remarkably enhanced UV responses when compared to the flat-NR electrode. The maximum IPCE for the flat-NR is only 42.5% at 360 nm, whereas the MP-NR has the highest IPCE of 66.0% at 370 nm. It is worth noting that the responsive wavelength shift of the maximum IPCE from 360 to 370 nm could have originated from the increased loading amount of rutile TiO2NRs as discussed above. The GQDs are decorated on the TiO2NR with the aim of further enhancing the PEC performances. It turns out that the GQDs loading can substantially improve the photocurrent
density of the electrodes. The MP-NR-GQDs electrode achieves the best PEC performance with a current density up to 2.92 mA cm−2 at 0 V versus Ag/AgCl (see figures 7(a) and (b)). The Mott–Schottky analyses for the three different electrodes are carried out to study the intrinsic electrical structures, as shown in figure 7(c). Compared to the electrodes without GQDs loading, almost one order of magnitude increase can be observed in the ND values of flat-NR-GQDs (6.58×1018 cm−3), MC-NR-GQDs (8.93×1018 cm−3) and MP-NR-GQDs (1.26×1019 cm−3), as summarized in figure S5. This can be attributed to the high carrier mobility of GQDs [34] which decreases the electrochemical impedance of the composite [31]. Besides, the prolonged lifetimes of carriers (100–300 ps) [45] can also facilitate the charge-transport kinetics. Figure 7(d) shows the IPCE plots of TiO2NR-GQDs electrodes at a zero bias versus Ag/AgCl. The results indicate that the enhanced photocurrent is mainly contributed by UV light response. The GQDs loading gives rise to a pronounced enhancement of IPCE in the whole UV region (300–400 nm) with a maximum of 72% at 370 nm for the MP-NR-GQDs electrode, which is improved by 69 and 9% over the bare flatNR and MP-NR electrodes. 3.4. Possible mechanism
To further understand the mechanism of PEC enhancement by the loading of GQDs, we measured the cyclic voltammetry (CV) curve of the GQDs on Pt sheet electrode to determine their lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO). More detailed information about the CV characterization can be found in the 6
Nanotechnology 27 (2016) 115401
Z Xu et al
Figure 4. TEM characterizations of TiO2NR, GQDs and TiO2NR-GQDs. (a) HRTEM of one pristine TiO2 nanorod. Inset is the lowmagnification view. (b) TEM and (c) HRTEM images of the GQDs. (d) HRTEM image of TiO2NR-GQDs. Inset is the low-magnification view. (e) Scanning TEM elemental mapping of the TiO2NR decorated with GQDs.
supplementary data. According to the empirical formula of ELUMO=−e (Ereduction+4.7) and EHOMO=−e (Eoxidation+4.7) [46, 47] as well as the reduction and oxidation potential of GQDs (−1.20 V and 0.76 V, figure 8(a)), the LUMO and HOMO energy levels of GQDs are −3.5 and −5.46 eV. It is worth noting that both the LUMO and HOMO energy levels are higher than the conduction band (−4.5 eV) and valence band (−7.5 eV) of rutile TiO2 [48]. Thus, it is expected that the photoexcited holes on the VB of TiO2 will transfer to the GQDs to further oxidize water (figure 8(b)) while the unpaired electrons will be left on the backbone of theTiO2, after which they migrate to the cathode to participate in the water-splitting reactions with much suppressed recombination. The favorable band alignment of the energy levels between the GQDs and rutile TiO2 leads to the highly spatial separation of the photo-generated electron–holes with
dramatically prolonged carrier life-times, which in turn contributes to the much-improved PEC performance. It should be pointed out that as the optical band gap is about 2.0 eV for the GQDs, so it would be excited by visible light (specifically 620 nm). Nevertheless, we do not observe any visible light response on the TiO2-GQDs composite photoanode (figure 7(d)), which may be attributed to the small loading amount of the GQDs.
4. Conclusions In conclusion, the novel hierarchical rutile TiO2NRs grown on periodic MC and MP arrays have been developed by combining the sol–gel-based thermal nanoimprinting and hydrothermal methods for the application of PEC water 7
Nanotechnology 27 (2016) 115401
Z Xu et al
Figure 5. (a) Transmission of different TiO2NR-FTO electrodes and (b) the corresponding Tauc plots used for calculating the band gaps of different TiO2NRs.
Figure 6. PEC performance of different TiO2NR electrodes. (a) LSV plots under simulated solar light illumination, (b) I–T plots under
chopped solar light illumination, (c) Mott–Schottky plots under a fixed frequency of 1 kHz in the dark, (d) IPCE spectrum of different TiO2NR electrodes. The samples are measured in the range of 300–700 nm at 0 V (versus Ag/AgCl).
splitting. This is the first report of TiO2NRs on periodic hexagonal close-packed micropattern arrays, which can greatly improve the PEC performance when compared with
the TiO2NR electrode on flat TiO2 seed layer. The hierarchical structure can simultaneously offer a large quantity of photocatalytic sites, excellent light-trapping characteristics, as
8
Nanotechnology 27 (2016) 115401
Z Xu et al
Figure 7. PEC performances of different TiO2NR-GQD electrodes. (a) LSV plots under solar light illumination, (b) I–T plots under chopped solar light illumination, (c) Mott–Schottky plots under a fixed frequency of 1 kHz in the dark, (d) IPCE spectrum of different TiO2NR-GQD electrodes. The samples are measured in the range of 300–700 nm at 0 V (versus Ag/AgCl).
Figure 8. (a) CV curve of the GQDs on the Pt electrode and (b) the schematic energy-level diagram for rutile TiO2-GQD composite.
well as a highly conductive pathway for the collection of charge carriers. Besides, zero-dimensional GQDs are also coupled to the patterned-TiO2NR to further enhance the
performance. Owing to fast charge separation resulting from the favorable band alignment of the energy levels between the GQDs and rutile TiO2, the MP-NR-GQDs delivered a 9
Nanotechnology 27 (2016) 115401
Z Xu et al
photocurrent density up to 2.92 mA cm−2 at 0 V versus Ag/ AgCl and a high IPCEvalue of up to 72% at 370 nm, which outperforms the bare flat-NR by three times and 69%, respectively. The facile and cost-effective method with largescale integration capability presented in this work may shed new light on the design and fabrication strategies for other optoelectronic devices.
[11]
[12]
Acknowledgments [13]
This work is financially supported by the National Natural Science Foundation of China (61474128, 21503261, 61504155 and 11174308), Science & Technology Commission of Shanghai Municipality (14JC1492900), the CAS President’s International Fellowship for Visiting Scientists (2015TWMA2), the Youth Innovation Promotion Association, Chinese Academy of Sciences (2013302), and the Youth Innovation Fund for Interdisciplinary Research of SARI (Y426475234).
[14]
[15] [16]
References [17]
[1] Qin D D, Bi Y P, Feng X J, Wang W, Barber G D, Wang T, Song Y M, Lu X Q and Mallouk T E 2015 Hydrothermal growth and photoelectrochemistry of highly oriented, crystalline anatase TiO2 nanorods on transparent conducting electrodes Chem. Mater. 27 4180–3 [2] Gui Q F, Xu Z, Zhang H F, Cheng C W, Zhu X F, Yin M, Song Y, Lu L F, Chen X Y and Li D D 2014 Enhanced photoelectrochemical water splitting performance of anodic TiO2 nanotube arrays by surface passivation ACS Appl. Mater. Inter. 6 17053–8 [3] Yu J G, Dai G P, Xiang Q J and Jaroniec M 2011 Fabrication and enhanced visible-light photocatalytic activity of carbon self-doped TiO2 sheets with exposed {001} facets J. Mater. Chem. 21 1049–57 [4] Yu D, Zhu X, Xu Z, Zhong X, Gui Q, Song Y, Zhang S, Chen X and Li D 2014 Facile method to enhance the adhesion of TiO2 nanotube arrays to Ti substrate ACS Appl. Mater. Inter. 6 8001–5 [5] Kim H S, Lee J W, Yantara N, Boix P P, Kulkarni S A, Mhaisalkar S, Gratzel M and Park N G 2013 High efficiency solid-state sensitized solar cell-based on submicrometer rutile TiO2 nanorod and CH3NH3PbI3 perovskite sensitizer Nano Lett. 13 2412–7 [6] Hoshian S, Jokinen V, Hjort K, Ras R H A and Franssila S 2015 Amplified and localized photoswitching of TiO2 by micro- and nanostructuring ACS Appl. Mater. Inter. 7 15593–9 [7] Chen X and Mao S S 2007 Titanium dioxide nanomaterials: synthesis, properties, modifications, and applications Chem. Rev. 107 2891–959 [8] Xu Z, Lin Y, Yin M, Zhang H, Cheng C, Lu L, Xue X, Fan H J, Chen X and Li D 2015 Understanding the enhancement mechanisms of surface plasmon-mediated photoelectrochemical electrodes: a case study on Au nanoparticle decorated TiO2 nanotubes Adv. Mater. Interfaces 2 [9] Cho I S, Chen Z B, Forman A J, Kim D R, Rao P M, Jaramillo T F and Zheng X L 2011 Branched TiO2 nanorods for photoelectrochemical hydrogen production Nano Lett. 11 4978–84 [10] Sun B, Shi T L, Peng Z C, Sheng W J, Jiang T and Liao G L 2013 Controlled fabrication of Sn/TiO2 nanorods for
[18] [19]
[20] [21]
[22]
[23]
[24] [25]
[26]
[27]
10
photoelectrochemical water splitting Nanoscale Res. Lett. 8 462–70 Shankar K, Basham J I, Allam N K, Varghese O K, Mor G K, Feng X J, Paulose M, Seabold J A, Choi K S and Grimes C A 2009 Recent advances in the use of TiO2 nanotube and nanowire arrays for oxidative photoelectrochemistry J. Phys. Chem. C 113 6327–59 Pathak P, Gupta S, Grosulak K, Imahori H and Subramanian V 2015 Nature-inspired tree-like TiO2 architecture: a 3D platform for the assembly of CdS and reduced graphene oxide for photoelectrochemical processes J. Phys. Chem. C 119 7543–53 Qiu Y C, Leung S F, Zhang Q P, Hua B, Lin Q F, Wei Z H, Tsui K H, Zhang Y G, Yang S H and Fan Z Y 2014 Efficient photoelectrochemical water splitting with ultrathin films of hematite on three-dimensional nanophotonic structures Nano Lett. 14 2123–9 Zhang X, Liu Y and Kang Z H 2014 3D branched ZnO nanowire arrays decorated with plasmonic Au nanoparticles for high-performance photoelectrochemical water splitting ACS Appl. Mater. Inter. 6 4480–9 Chen H M, Chen C K, Liu R S, Zhang L, Zhang J J and Wilkinson D P 2012 Nano-architecture and material designs for water splitting photoelectrodes Chem. Soc. Rev. 41 5654–71 Cheng H M, Chiu W H, Lee C H, Tsai S Y and Hsieh W F 2008 Formation of branched ZnO nanowires from solvothermal method and dye-sensitized solar cells applications J. Phys. Chem. C 112 16359–64 Yu Y H, Li J Y, Geng D L, Wang J L, Zhang L S, Andrew T L, Arnold M S and Wang X D 2015 Development of lead iodide perovskite solar cells using three-dimensional titanium dioxide nanowire architectures ACS Nano 9 564–72 Sheng X, He D Q, Yang J, Zhu K and Feng X J 2014 Oriented assembled TiO2 hierarchical nanowire arrays with fast electron transport properties Nano Lett. 14 1848–52 Shi J, Hara Y, Sun C L, Anderson M A and Wang X D 2011 Three-dimensional high-density hierarchical nanowire architecture for high-performance photoelectrochemical electrodes Nano Lett. 11 3413–9 Goh C, Coakley K M and McGehee M D 2005 Nanostructuring titania by embossing with polymer molds made from anodic alumina templates Nano Lett. 5 1545–9 Gates B D, Xu Q B, Stewart M, Ryan D, Willson C G and Whitesides G M 2005 New approaches to nanofabrication: Molding, printing, and other techniques Chem. Rev. 105 1171–96 Ganesan R, Lim S H, Saifullah M S M, Hussain H, Kwok J X Q, Tse R L X, Bo H A P and Low H Y 2011 Direct nanoimprinting of metal oxides by in situ thermal copolymerization of their methacrylates J. Mater. Chem. 21 4484–92 Hampton M J, Williams S S, Zhou Z, Nunes J, Ko D H, Templeton J L, Samulski E T and DeSimone J M 2008 The patterning of sub-500 nm inorganic oxide structures Adv. Mater. 20 2667–73 Ganesan R, Dumond J, Saifullah M S M, Lim S H, Hussain H and Low H Y 2012 Direct patterning of TiO2 using step-and-flash imprint lithography ACS Nano 6 1494–502 Park H H, Zhang X, Lee S W, Kim K-D, Choi D G, Choi J H, Lee J, Lee E S, Park H H and Hill R H 2011 Facile nanopatterning of zirconium dioxide films via direct ultravioletassisted nanoimprint lithography J. Mater. Chem. 21 657–62 Murphy A B, Barnes P R F, Randeniya L K, Plumb I C, Grey I E, Horne M D and Glasscock J A 2006 Efficiency of solar water splitting using semiconductor electrodes Int. J. Hydrogen Energy 31 1999–2017 Zhuo S J, Shao M W and Lee S T 2012 Upconversion and downconversion fluorescent graphene quantum dots: ultrasonic preparation and photocatalysis ACS Nano 6 1059–64
Nanotechnology 27 (2016) 115401
Z Xu et al
[38] Liu B and Aydil E S 2009 Growth of oriented single-crystalline rutile TiO2 nanorods on transparent conducting substrates for dye-sensitized solar cells J. Am. Chem. Soc. 131 3985–90 [39] Xu C, Song Y, Lu L, Cheng C, Liu D, Fang X, Chen X, Zhu X and Li D 2013 Electrochemically hydrogenated TiO2 nanotubes with improved photoelectrochemical water splitting performance Nanoscale Res. Lett. 8 1–7 [40] Shavdina O et al 2015 Large area fabrication of periodic TiO2 nanopillars using microsphere photolithography on a photopatternable sol-gel film Langmuir 31 7877–84 [41] Cheng H H, Zhao Y, Fan Y Q, Xie X J, Qu L T and Shi G Q 2012 Graphene-quantum-dot assembled nanotubes: a new platform for efficient Raman enhancement ACS Nano 6 2237–44 [42] Hwang Y J, Hahn C, Liu B and Yang P 2012 Photoelectrochemical properties of TiO2 nanowire arrays: a study of the dependence on length and atomic layer deposition coating ACS Nano 6 5060–9 [43] Wang G M, Wang H Y, Ling Y C, Tang Y C, Yang X Y, Fitzmorris R C, Wang C C, Zhang J Z and Li Y 2011 Hydrogen-treated TiO2 nanowire arrays for photoelectrochemical water splitting Nano Lett. 11 3026–33 [44] Zhang Z H, Zhang L B, Hedhili M N, Zhang H N and Wang P 2013 Plasmonic gold nanocrystals coupled with photonic crystal seamlessly on TiO2 nanotube photoelectrodes for efficient visible light photoelectrochemical water splitting Nano Lett. 13 14–20 [45] Mueller M L, Yan X, Dragnea B and Li L S 2011 Slow hotcarrier relaxation in colloidal graphene quantum dots Nano Lett. 11 56–60 [46] Wen G A et al 2007 Hyperbranched triazine-containing polyfluorenes: efficient blue emitters for polymer lightemitting diodes (PLEDs) Polymer 48 1824–9 [47] Johansson T, Mammo W, Svensson M, Andersson M R and Inganas O 2003 Electrochemical bandgaps of substituted polythiophenes J. Mater. Chem. 13 1316–23 [48] Scanlon D O et al 2013 Band alignment of rutile and anatase TiO2 Nat. Mater. 12 798–801
[28] Pan D Y, Xi C, Li Z, Wang L, Chen Z W, Luc B and Wu M H 2013 Electrophoretic fabrication of highly robust, efficient, and benign heterojunction photoelectrocatalysts based on graphene-quantum-dot sensitized TiO2 nanotube arrays J. Mater. Chem. A 1 3551–5 [29] Son D I, Kwon B W, Park D H, Seo W S, Yi Y, Angadi B, Lee C L and Choi W K 2012 Emissive ZnO-graphene quantum dots for white-light-emitting diodes Nat. Nanotechnol. 7 465–71 [30] Lin J, Zhang C G, Yan Z, Zhu Y, Peng Z W, Hauge R H, Natelson D and Tour J M 2013 3-Dimensional graphene carbon nanotube carpet-based microsupercapacitors with high electrochemical performance Nano Lett. 13 72–8 [31] Zhu C, Chao D, Sun J, Bacho I M, Fan Z, Ng C F, Xia X, Huang H, Zhang H and Shen Z X 2015 Enhanced lithium storage performance of CuO nanowires by coating of graphene quantum dots Adv. Mater. Interfaces 2 [32] Zhou X M, Tian Z M, Li J, Ruan H, Ma Y Y, Yang Z and Qu Y Q 2014 Synergistically enhanced activity of graphene quantum dot/multi-walled carbon nanotube composites as metal-free catalysts for oxygen reduction reaction Nanoscale 6 2603–7 [33] Ananthanarayanan A, Wang X W, Routh P, Sana B, Lim S, Kim D H, Lim K H, Li J and Chen P 2014 Facile synthesis of graphene quantum dots from 3D graphene and their application for Fe3+ sensing Adv. Funct. Mater. 24 3021–6 [34] Zhang Z P, Zhang J, Chen N and Qu L T 2012 Graphene quantum dots: an emerging material for energy-related applications and beyond Energy Environ. Sci. 5 8869–90 [35] Reymond F, Rossier J S and Girault H H 2002 Polymer microchips bonded by O2-plasma activation Electrophoresis 23 782–90 [36] Yabu H and Shimomura M 2005 Single-step fabrication of transparent superhydrophobic porous polymer films Chem. Mater. 17 5231–4 [37] Truffier-Boutry D, Beaurain A, Galand R, Pelissier B, Boussey J and Zelsmann M 2010 XPS study of the degradation mechanism of fluorinated anti-sticking treatments used in UV nanoimprint lithography Microelectron. Eng. 87 122–4
11
Search
Collections
Journals
About
Contact us
My IOPscience
3D periodic multiscale TiO2 architecture: a platform decorated with graphene quantum dots for enhanced photoelectrochemical water splitting
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2016 Nanotechnology 27 115401 (http://iopscience.iop.org/0957-4484/27/11/115401) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 128.111.121.42 This content was downloaded on 06/03/2016 at 19:38
Please note that terms and conditions apply.
Nanotechnology Nanotechnology 27 (2016) 115401 (11pp)
doi:10.1088/0957-4484/27/11/115401
3D periodic multiscale TiO2 architecture: a platform decorated with graphene quantum dots for enhanced photoelectrochemical water splitting Zhen Xu1,2, Min Yin1, Jing Sun3, Guqiao Ding3, Linfeng Lu1, Paichun Chang4, Xiaoyuan Chen1 and Dongdong Li1 1
Shanghai Advanced Research Institute, Chinese Academy of Sciences, 99 Haike Road, Zhangjiang Hi-Tech Park, Pudong, Shanghai 201210, People’s Republic of China 2 University of Chinese Academy of Sciences, Beijing 100039, People’s Republic of China 3 State Key Laboratory of Functional Materials for Informatics Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050, People’s Republic of China 4 Department of Creative Industry, Kainan University, No. 1, Kainan Road, Luchu, Taoyuan County 338, Taiwan E-mail: [email protected] and [email protected] Received 25 November 2015, revised 19 January 2016 Accepted for publication 19 January 2016 Published 15 February 2016 Abstract
Micropatterned TiO2 nanorods (TiO2NRs) via three-dimensional (3D) geometry engineering in both microscale and nanoscale decorated with graphene quantum dots (GQDs) have been demonstrated successfully. First, micropillar (MP) and microcave (MC) arrays of anatase TiO2 films are obtained through the sol–gel based thermal nanoimprinting method. Then they are employed as seed layers in hydrothermal growth to fabricate the 3D micropillar/microcave arrays of rutile TiO2NRs (NR), which show much-improved photoelectrochemical water-splitting performance than the TiO2NRs grown on flat seed layer. The zero-dimensional GQDs are sequentially deposited onto the surfaces of the microscale patterned nanorods. Owing to the fast charge separation that resulted from the favorable band alignment of the GQDs and rutile TiO2, the MP-NR-GQDs electrode achieves a photocurrent density up to 2.92 mA cm−2 under simulated one-sun illumination. The incident-photon-to-currentconversion efficiency (IPCE) value up to 72% at 370 nm was achieved on the MP-NR-GQDs electrode, which outperforms the flat-NR counterpart by 69%. The IPCE results also imply that the improved photocurrent mainly benefits from the distinctly enhanced ultraviolet response. The work provides a cost-effective and flexible pathway to develop periodic 3D micropatterned photoelectrodes and is promising for the future deployment of high performance optoelectronic devices. S Online supplementary data available from stacks.iop.org/NANO/27/115401/mmedia Keywords: periodic multiscale structure, TiO2 nanorod arrays, thermal nanoimprint, graphene quantum dots, photoelectrochemical water splitting (Some figures may appear in colour only in the online journal) 1. Introduction
attention owing to its important applications in photoelectrochemical (PEC) water splitting [1, 2], photocatalysis [3, 4], perovskite solar cells [5], sensors [6], because of its abundance, nontoxicity and photostability [7]. As for the
Titanium dioxide (TiO2), with a band gap of 3.0–3.2 eV, has been currently attracting extensive academic and industrial 0957-4484/16/115401+11$33.00
1
© 2016 IOP Publishing Ltd Printed in the UK
Nanotechnology 27 (2016) 115401
Z Xu et al
application in PEC water splitting, promising solutions for efficient solar-to-chemical energy conversion of the TiO2 include avoiding charge trapping on semiconductor surfaces [8], increasing the semiconductor-electrolyte interfacial area [9], and enhancing the light harvesting capability [10], etc. Moreover, a one-dimensional (1D) single crystalline rutile TiO2 nanorod (TiO2NR), providing a direct pathway for charge transportation [11], has been demonstrated as a promising photoanode for PEC water splitting. Therefore, threedimensional (3D) networks consisting of single crystalline 1D nanostructure may possess ideal architecture for high-performance PEC electrodes. The 3D architecture can not only offer long optical paths for efficient light absorption and copious active sites for electrochemical reactions, but provides efficient conducting channels for rapid electron–hole separation and charge transport [12–14]. In recent years, researchers have been enthusiastically dedicated to the development of 3D hierarchical architecture [15], in which the backbone of Si nanowire [11, 12], ZnO nanowire [16] or the TiO2 trunk [17, 18] scaffold is obtained by etching [19] or the hydrothermal/solvothermal method [9]. And the branched nanostructures are achieved through a second hydrothermal growth [9] or a modified chemical vapor deposition [17, 19]. These intricate procedures are either timeconsuming or need costly equipment. With the inherent advantages of low-cost, high resolution, large area and highthroughput [20], nanoimprint lithography is a suitable technique to prepare specified patterns in the scale of nano- and micro-meters [21]. Sol–gel based thermal nanoimprinting is a facile and general route to fabricate large-area nanoscale patterns of various metal oxides, such as Fe2O3 [22], TiO2 [23, 24], and ZrO2 [25] etc. It can be used to achieve the 3D periodic backbone with experimental simplicity, flexibility and mass production. It has been predicted that a maximum photoconversion efficiency of 2.25% can be achieved by rutile TiO2 with an optical band gap of 3.0 eV, under 100 mW/cm2 AM 1.5 global illumination [26]. Yet, the reported photoconversion efficiencies and photocurrent densities of TiO2 photoanodes are substantially lower than the theoretical limit mainly due to the insufficient light absorption and severe charge recombination. Graphene quantum dot (GQD), the emerging zerodimensional carbon material, is reported to have the ability to decrease the electrochemical impedance and facilitate the charge transport kinetics [27, 28]. It has attracted tremendous attention [29–33], because of its intriguing optical and electrical properties due to its tunable size and surface nature [34]. In this study, 3D periodic multiscale TiO2 nanorod arrays are first realized by combining the sol–gel-based thermal nanoimprinting and hydrothermal growth process. The GQDs are subsequently deposited onto the surfaces of the hierarchical patterned nanostructures with the aim of enhancing the PEC performances of the electrodes. The possible mechanism of the GQDs’ enhanced PEC performances are proposed after careful characterization and analysis. Owing to the fast charge separation that resulted from the favorable band alignment of the energy levels between the GQDs and the rutile TiO2 as well as the adequate loading of TiO2NRs,
the MP-NR-GQDs represent superior PEC performances in the UV light region.
2. Experiment Multiscale TiO2 hierarchical structure was fabricated by combining sol–gel-based thermal imprint lithography and hydrothermal growth, as shown in figure 1. The detailed preparation procedures were as below. 2.1. Material preparation Preparation of polymeric mold. A silicone (polydimethysiloxane, PDMS) mold (∼2 mm thickness) with micropit arrays (2.6 μm in depth, 2 μm in diameter and 3 μm in pitch) was provided by NanoCarve Ltd. Prior to use, the PDMS micropit mold was exposed to O2 plasma at 50 W for 1 min with an oxygen flow rate of 0.5 l min−1 [35]. Then, the obtained activated mold with dramatically increased surface O/C ratio was further treated by a molecular vapor deposition (MVD) process to achieve a hydrophobic surface. Specifically, the PDMS mold was placed in the sealed MVD chamber with 2 μl of fluorosilane (1H, 1H, 2H, 2Hperfluorooctyltrichlorosilane) loaded [36]. The temperature of the chamber was increased to 100 °C and lasted for 10 min. A self-assembled monolayer of fluorosilane was introduced on the surface of the PDMS mold and would facilitate the separation in the later process. The whole process is similar to the conventional anti-sticking treatment on the Si mold [37]. Subsequently, micropillar (MP) PDMS mold was prepared by pouring premixed PDMS onto the micropit mold, followed by degassing and a curing process at 60 °C for 3 h. Later, the cured PDMS membrane (about 0.5 mm) with MP arrays was obtained by peeling it off with great care. In general, two kinds of mold were used in this work, i.e. PDMS membranes with micropits and MP arrays.
2.1.1.
2.1.2. Preparation of TiO2 sol solution. Tetrabutyl titanate
(TBOT, Aldrich, 99.9%, 4.25 ml) was mixed with a solution of anhydrous ethanol (25 ml) and diethanolamine (DEA, Aldrich, 99.9%, 3.75 ml) for two hours, followed by the dropwise addition of glacial acetic acid (HAc, Aldrich, 99.9%, 5 ml) and deionized (DI) water (5 ml). The solution was left stirring for another 24 h in a fume hood to obtain a final concentrated solution of 15 ml.
2.1.3. Preparation of seed layer on FTO substrates with different morphology. Sol–gel-based thermal nanoimprinting
lithography was used to prepare a microstructure seed layer of TiO2. Specifically, TiO2 polymeric sol was spin-coated onto the FTO at 1000 rpm for 1 min. Then, two different PDMS molds (i.e. with MP and micropit arrays) were placed on the TiO2 sol substrates with conformal contact, and were then placed in a nanoimprinting chamber with a pressure of 0.3 MPa at 250 °C for 2 h with a heating rate of 10 °C/min. After cooling down to 60 °C, the molds were detached from the TiO2 gel microstructures with great care. Microcave (MC) and MP 2
Nanotechnology 27 (2016) 115401
Z Xu et al
Figure 1. Schematics of the fabrication processes of different TiO2NR electrodes. Route 1 for MC-NR. Route 2 for MP-NR. Route 3 for flat-NR.
after the autoclave was cooled down to room temperature naturally. The samples were further annealed in ambient air at 450 °C for 2 h with a heating rate of 2 °C/min to achieve the well-crystallized rutile TiO2.
TiO2 gel substrates were then obtained. The micropatterned substrates were further annealed in ambient air at 450 °C for 2 h with a heating rate of 2 °C/min to achieve the crystallized seed layer on the FTO. Meanwhile, flat-seed substrate was also prepared according to the aforementioned procedures except for the thermal nanoimprinting process. As a result, substrates with three different topographical seed layers i.e. the MC-seed, MPseed and flat-seed were obtained. More detailed information about the parameters used in the thermal nanoimprinting lithography can be found in the supplementary data.
2.1.5. Preparation of GQDs. GQDs were prepared from
graphene powder via a modified Staudenmaier method [31]. Graphene powder (4 g) was put into H2SO4 (150 ml) and HNO3 (80 ml) with stirring at 15 °C, and was kept for 2 h. Then NaClO3 (40 g) was added gradually and the temperature was kept below 5 °C. The mixture was then stirred at 15 °C for 5 h. After that, the reaction was terminated by adding distilled water (80 ml). The pH value was neutralized to 7 by NaOH, before the mixture was filtered out using an alumina inorganic membrane (Shanghai Shangmu Technology Co. Ltd) with 20 nm pores. The obtained light yellow filtrate was dialyzed in a 3500 Da dialysis bag against DI water for a week to remove excess salt. The purified solutions were transferred to a Teflonlined stainless steel autoclave and heated at 200 °C for 5 h to reduce the oxygen-containing groups, and cooled to room temperature naturally. The resultant light yellow solution of GQDs was obtained.
2.1.4. Hydrothermal growth of TiO2NR film. Rutile TiO2NR
films were prepared on the three different substrates by a slightly modified hydrothermal method [38]. Correspondingly, the samples were marked as MC-NR, MP-NR and flat-NR. Typically, 0.833 ml TBOT was dropped into a mixture of 22.5 mlDI water and 27.5 ml concentrated hydrochloric acid (36.0–38.0 wt%), followed by stirring for 20 min to obtain a transparent solution. The solution was poured into a Teflonlined stainless steel autoclave (100 ml capacity). Three pieces of the FTO substrates (1.5 ×4 cm) coated with different patterned seed layers were placed leaning against the wall of the Teflon-lined autoclave with the conducting side facing down. The hydrothermal synthesis was conducted at 150 °C for 12 h in an electric oven. Afterwards, the samples were taken out, rinsed extensively with DI water and dried in ambient air
2.1.6. Electrophoresis deposition of GQDs on micropatterned hierarchical TiO2NR films. In order to increase the negative
charges of the as-prepared GQDs, the pH was tuned to 8 3
Nanotechnology 27 (2016) 115401
Z Xu et al
using 1 M NaOH solution. Different TiO2NR films (i.e. MCNR, MP-NR and flat-NR) acted as the working electrodes and Pt plate as counter electrodes. Under an applied positive potential of 6 V for 1 h, the functional GQDs were spontaneously deposited onto the surface of the nanorods. The deposition duration was optimized by testing the PEC performances of the flat-NR-GQDs electrodes with different deposition durations (see supplementary data for detailed information).
imprinting molds as displayed in figure S2 and table S1, due to the polymerization-induced inward shrinkage of the patterns during the multiple replications [24, 40]. After hydrothermal growth, all the obtained nanorods from the three substrates exhibit uniform dimensions of about 100 nm in diameter and 2–3 μm in length. Both the MC-NR and the MPNR arrays retain the rough terrain of the seed layers with hexagonal close-packed MCs or MPs (figures 2(a2)–(b2). As can be seen in figures 2(b2)–(b4), the nanorods on MP-NR grow radially on the original MPs, forming the periodic hexagonal close-packed microflower arrays. This is the first report of periodic microflower arrays of rutile TiO2NR, to the best of our knowledge. As can be seen in figures 3(a) and (b), the TiO2NR exhibits a relatively smooth side surface with a rectangular cross-section. As shown in figures 3(c)–(d), the surface of the nanorods becomes much rougher, indicating that the GQDs are homogeneously deposited on the entire surface of the nanorods. The results suggest that GQDs with the advantage of high solubility in various solvents by appropriate functionalization [34] have the capability of conformal wrapping on low-dimensional nanostructures as a functional layer by electrophoresis [31, 41]. Figure 4(a) shows an HRTEM image of an individual pristine TiO2 nanorod with a high resolved (110) lattice fringe of rutile TiO2 (d=0.32 nm), demonstrating that the nanorod grows preferentially along [001] direction in accordance with a strong (002) diffraction peak on the XRD patterns (figure S3). The diameter of the nanorod is about 100 nm (inset of figure 4(a)), which is consistent with the SEM result in figure 3(b). Figure 4(b) shows a representative low-magnification transmission electron microscope (TEM) image of the GQDs, which are uniformly distributed on the TEM carbon film without aggregation. The size of the GQDs is around 3–5 nm. The corresponding HRTEM image (figure 4(c)) shows the lattice spacing of 0.21 nm related to the (1100) plane of GQDs. A lattice-resolved TEM image of TiO2NR-GQDs from figure 4(d) reveals clear lattice fringes with interplanar spacings of 0.32 and 0.21 nm, which are consistent with the (110) plane of rutile TiO2 and (1100) plane of GQDs. It indicates that the GQDs attach onto the TiO2 nanorods firmly even after the intensified ultrasonic dispersion during the TEM samples preparation, which ensures the effective charge transfer channels. Elemental mapping data (figure 4(e)) confirms the existence of O, Ti and C. Although some background noise is apparent in the element of C mapping, we can still infer that the GQDs load over the TiO2 nanorod.
2.2. Materials characterization
The morphology and crystalline structure of the composites were characterized by field-emission scanning electron microscope (FESEM, Hitachi S4800), high-resolution transmission electron microscopy (HRTEM, JEM-2010F) and x-ray diffraction (XRD, Bruker D8 Discover diffractometer). Transmission spectra of the composite films were recorded by a standard UV–Vis spectrometer (Cary 5000, Agilent, USA). The topography of the different seed layers on the FTO slides was analyzed by a 3D laser scanning microscope (VK-9700, Keyence, USA). 2.3. Photoelectrochemical measurements
The entire back surface and edges of the electrodes were coated with insulating epoxy resin to obtain an exposed area of 1 cm2. The PEC water-splitting performances of the different electrodes were evaluated by AUTOLAB (PGSTAT302N/FRA2) using a three-electrode setup with the TiO2NR films as working electrodes, the Ag/AgCl (3 M KCl) electrode as the reference electrode and a platinum foil as a counter electrode in accordance with our previous work [8, 39]. The supporting electrolyte was 1 M potassium hydroxide (KOH, pH=14). The photoanodes were illuminated through the FTO glass substrate with simulated sunlight from a 300 W Xe lamp (Newport Corp.) equipped with an AM 1.5G filter. The photocurrent was measured at a potential of 0 V (versus Ag/AgCl) under chopped simulated light irradiation. Besides, the incident-photon-to-current-conversion efficiency (IPCE, DC mode) measurements were conducted at a zero bias versus Ag/AgCl by the AUTOLAB electrochemical station with the assistance of a commercial spectral response system (PV Measurement, QEX10).
3. Results and discussions 3.1. Structural and morphological analysis
3.2. Optical properties
The preparation of a TiO2 seed layer is a crucial step in the fabrication of highly ordered MCs/MPs consisting of randomly oriented TiO2 nanorods. Figures 2(a1)–(c1) show the FESEM images of seed layer on the FTO with different morphologies. The flat-seed directly prepared through spincoating presents some randomly shallow dents, which may result from the inward shrinkage of the sol during gelation. The obtained MC-seed and MP-seed show smaller diameter and height of the imprinted features than the corresponding
The optical properties and the corresponding Tauc plots for calculating the band gap of different TiO2NRs electrodes are shown in figure 5. It can be inferred that the TiO2NRs are much denser on MP-NR and MC-NR than on flat-NR. Thus, the light absorption of micropatterned hierarchical TiO2NRs in UV light is higher than the flat counterpart. The absorption edges are 340, 380 and 385 nm for flat-NR, MC-NR and MPNR electrodes, respectively. It can be estimated from 4
Nanotechnology 27 (2016) 115401
Z Xu et al
Figure 2. (a1)–(c1) Top view of MC-seed, MP-seed and flat-seed (inset of (b1) and (c1) is the cross-section view); (a2)–(c2), (a3)–(c3) top
views and (a4)–(c4) cross-section views of the corresponding MC-NR, MP-NR and flat-NR.
figure 5(b) that the band gap energy of the flat-NR is about 3.2 eV, which is larger than that of rutile TiO2 (3.0 eV) [7] while approaching that of anatase TiO2. The band gap energy of MC-NR and MP-NR is 2.94 and 2.97 eV, respectively. The black plot belonging to flat-NR is mainly contributed by the uniform flat-seed layer with anatase phase. While the patterned electrode enables some very thin residual seed layer during the imprinting process and with more nanorod (rutile phase) loading, the absorption plots of these electrodes are dominated by the rutile phase. More detailed information about Tauc plots analyses can be found in the supplementary data.
3.3. Photoelectrochemical performances
The PEC characterizations are first performed on three different electrodes. The linear sweep voltammetry (LSV) plots and photocurrent transients in figures 6(a) and (b) show that the flat-NR electrode exhibits a photocurrent density of 0.68 mA cm−2 which is comparable to that of previous literature [9, 10, 42]. The photocurrent on MC-NR and MP-NR electrodes achieved 1.88 and 2.15 mA cm−2 under zero bias potential versus Ag/AgCl, which are improved by 1.74 and 2.16 times over the flat-NR electrode. The micropatterned TiO2NR electrodes with much denser loading of TiO2NRs bring about enhanced light absorption and active reaction 5
Nanotechnology 27 (2016) 115401
Z Xu et al
Figure 3. SEM images of (a), (b) TiO2NR and (c), (d) TiO2NR-GQDs.
sites [15], which contribute to the much-improved surface catalysis. The intrinsic electrical structures of the photoelectrodes are investigated by the Mott–Schottky analysis, as displayed in figure 6(c). The calculated ND values of the flat-NR, MCNR and MP-NR are 7.2×1017, 1.4×1018 and 1.7×1018 cm−3, respectively. Despite the fact that the carrier densities are estimated values derived from the Mott– Schottky plot which is based on a flat-electrode model [43], the patterned substrates indeed increase the carrier density substantially. More detailed information about Mott–Schottky can be found in the supplementary data. The IPCE measurements are carried out to investigate the contribution of each monochromatic light to the photocurrent (figure 6(d)). The IPCE is calculated as a function of wavelength using IPCE=(1240(mW·nm/mA)I/(λJlight)), where λ is the incident light wavelength (nm), I and Jlight are the photocurrent density (mA/cm2) and incident light irradiance (mW/ cm2) at a specific wavelength [8, 44]. Both the modified electrodes with patterned seed layer represent remarkably enhanced UV responses when compared to the flat-NR electrode. The maximum IPCE for the flat-NR is only 42.5% at 360 nm, whereas the MP-NR has the highest IPCE of 66.0% at 370 nm. It is worth noting that the responsive wavelength shift of the maximum IPCE from 360 to 370 nm could have originated from the increased loading amount of rutile TiO2NRs as discussed above. The GQDs are decorated on the TiO2NR with the aim of further enhancing the PEC performances. It turns out that the GQDs loading can substantially improve the photocurrent
density of the electrodes. The MP-NR-GQDs electrode achieves the best PEC performance with a current density up to 2.92 mA cm−2 at 0 V versus Ag/AgCl (see figures 7(a) and (b)). The Mott–Schottky analyses for the three different electrodes are carried out to study the intrinsic electrical structures, as shown in figure 7(c). Compared to the electrodes without GQDs loading, almost one order of magnitude increase can be observed in the ND values of flat-NR-GQDs (6.58×1018 cm−3), MC-NR-GQDs (8.93×1018 cm−3) and MP-NR-GQDs (1.26×1019 cm−3), as summarized in figure S5. This can be attributed to the high carrier mobility of GQDs [34] which decreases the electrochemical impedance of the composite [31]. Besides, the prolonged lifetimes of carriers (100–300 ps) [45] can also facilitate the charge-transport kinetics. Figure 7(d) shows the IPCE plots of TiO2NR-GQDs electrodes at a zero bias versus Ag/AgCl. The results indicate that the enhanced photocurrent is mainly contributed by UV light response. The GQDs loading gives rise to a pronounced enhancement of IPCE in the whole UV region (300–400 nm) with a maximum of 72% at 370 nm for the MP-NR-GQDs electrode, which is improved by 69 and 9% over the bare flatNR and MP-NR electrodes. 3.4. Possible mechanism
To further understand the mechanism of PEC enhancement by the loading of GQDs, we measured the cyclic voltammetry (CV) curve of the GQDs on Pt sheet electrode to determine their lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO). More detailed information about the CV characterization can be found in the 6
Nanotechnology 27 (2016) 115401
Z Xu et al
Figure 4. TEM characterizations of TiO2NR, GQDs and TiO2NR-GQDs. (a) HRTEM of one pristine TiO2 nanorod. Inset is the lowmagnification view. (b) TEM and (c) HRTEM images of the GQDs. (d) HRTEM image of TiO2NR-GQDs. Inset is the low-magnification view. (e) Scanning TEM elemental mapping of the TiO2NR decorated with GQDs.
supplementary data. According to the empirical formula of ELUMO=−e (Ereduction+4.7) and EHOMO=−e (Eoxidation+4.7) [46, 47] as well as the reduction and oxidation potential of GQDs (−1.20 V and 0.76 V, figure 8(a)), the LUMO and HOMO energy levels of GQDs are −3.5 and −5.46 eV. It is worth noting that both the LUMO and HOMO energy levels are higher than the conduction band (−4.5 eV) and valence band (−7.5 eV) of rutile TiO2 [48]. Thus, it is expected that the photoexcited holes on the VB of TiO2 will transfer to the GQDs to further oxidize water (figure 8(b)) while the unpaired electrons will be left on the backbone of theTiO2, after which they migrate to the cathode to participate in the water-splitting reactions with much suppressed recombination. The favorable band alignment of the energy levels between the GQDs and rutile TiO2 leads to the highly spatial separation of the photo-generated electron–holes with
dramatically prolonged carrier life-times, which in turn contributes to the much-improved PEC performance. It should be pointed out that as the optical band gap is about 2.0 eV for the GQDs, so it would be excited by visible light (specifically 620 nm). Nevertheless, we do not observe any visible light response on the TiO2-GQDs composite photoanode (figure 7(d)), which may be attributed to the small loading amount of the GQDs.
4. Conclusions In conclusion, the novel hierarchical rutile TiO2NRs grown on periodic MC and MP arrays have been developed by combining the sol–gel-based thermal nanoimprinting and hydrothermal methods for the application of PEC water 7
Nanotechnology 27 (2016) 115401
Z Xu et al
Figure 5. (a) Transmission of different TiO2NR-FTO electrodes and (b) the corresponding Tauc plots used for calculating the band gaps of different TiO2NRs.
Figure 6. PEC performance of different TiO2NR electrodes. (a) LSV plots under simulated solar light illumination, (b) I–T plots under
chopped solar light illumination, (c) Mott–Schottky plots under a fixed frequency of 1 kHz in the dark, (d) IPCE spectrum of different TiO2NR electrodes. The samples are measured in the range of 300–700 nm at 0 V (versus Ag/AgCl).
splitting. This is the first report of TiO2NRs on periodic hexagonal close-packed micropattern arrays, which can greatly improve the PEC performance when compared with
the TiO2NR electrode on flat TiO2 seed layer. The hierarchical structure can simultaneously offer a large quantity of photocatalytic sites, excellent light-trapping characteristics, as
8
Nanotechnology 27 (2016) 115401
Z Xu et al
Figure 7. PEC performances of different TiO2NR-GQD electrodes. (a) LSV plots under solar light illumination, (b) I–T plots under chopped solar light illumination, (c) Mott–Schottky plots under a fixed frequency of 1 kHz in the dark, (d) IPCE spectrum of different TiO2NR-GQD electrodes. The samples are measured in the range of 300–700 nm at 0 V (versus Ag/AgCl).
Figure 8. (a) CV curve of the GQDs on the Pt electrode and (b) the schematic energy-level diagram for rutile TiO2-GQD composite.
well as a highly conductive pathway for the collection of charge carriers. Besides, zero-dimensional GQDs are also coupled to the patterned-TiO2NR to further enhance the
performance. Owing to fast charge separation resulting from the favorable band alignment of the energy levels between the GQDs and rutile TiO2, the MP-NR-GQDs delivered a 9
Nanotechnology 27 (2016) 115401
Z Xu et al
photocurrent density up to 2.92 mA cm−2 at 0 V versus Ag/ AgCl and a high IPCEvalue of up to 72% at 370 nm, which outperforms the bare flat-NR by three times and 69%, respectively. The facile and cost-effective method with largescale integration capability presented in this work may shed new light on the design and fabrication strategies for other optoelectronic devices.
[11]
[12]
Acknowledgments [13]
This work is financially supported by the National Natural Science Foundation of China (61474128, 21503261, 61504155 and 11174308), Science & Technology Commission of Shanghai Municipality (14JC1492900), the CAS President’s International Fellowship for Visiting Scientists (2015TWMA2), the Youth Innovation Promotion Association, Chinese Academy of Sciences (2013302), and the Youth Innovation Fund for Interdisciplinary Research of SARI (Y426475234).
[14]
[15] [16]
References [17]
[1] Qin D D, Bi Y P, Feng X J, Wang W, Barber G D, Wang T, Song Y M, Lu X Q and Mallouk T E 2015 Hydrothermal growth and photoelectrochemistry of highly oriented, crystalline anatase TiO2 nanorods on transparent conducting electrodes Chem. Mater. 27 4180–3 [2] Gui Q F, Xu Z, Zhang H F, Cheng C W, Zhu X F, Yin M, Song Y, Lu L F, Chen X Y and Li D D 2014 Enhanced photoelectrochemical water splitting performance of anodic TiO2 nanotube arrays by surface passivation ACS Appl. Mater. Inter. 6 17053–8 [3] Yu J G, Dai G P, Xiang Q J and Jaroniec M 2011 Fabrication and enhanced visible-light photocatalytic activity of carbon self-doped TiO2 sheets with exposed {001} facets J. Mater. Chem. 21 1049–57 [4] Yu D, Zhu X, Xu Z, Zhong X, Gui Q, Song Y, Zhang S, Chen X and Li D 2014 Facile method to enhance the adhesion of TiO2 nanotube arrays to Ti substrate ACS Appl. Mater. Inter. 6 8001–5 [5] Kim H S, Lee J W, Yantara N, Boix P P, Kulkarni S A, Mhaisalkar S, Gratzel M and Park N G 2013 High efficiency solid-state sensitized solar cell-based on submicrometer rutile TiO2 nanorod and CH3NH3PbI3 perovskite sensitizer Nano Lett. 13 2412–7 [6] Hoshian S, Jokinen V, Hjort K, Ras R H A and Franssila S 2015 Amplified and localized photoswitching of TiO2 by micro- and nanostructuring ACS Appl. Mater. Inter. 7 15593–9 [7] Chen X and Mao S S 2007 Titanium dioxide nanomaterials: synthesis, properties, modifications, and applications Chem. Rev. 107 2891–959 [8] Xu Z, Lin Y, Yin M, Zhang H, Cheng C, Lu L, Xue X, Fan H J, Chen X and Li D 2015 Understanding the enhancement mechanisms of surface plasmon-mediated photoelectrochemical electrodes: a case study on Au nanoparticle decorated TiO2 nanotubes Adv. Mater. Interfaces 2 [9] Cho I S, Chen Z B, Forman A J, Kim D R, Rao P M, Jaramillo T F and Zheng X L 2011 Branched TiO2 nanorods for photoelectrochemical hydrogen production Nano Lett. 11 4978–84 [10] Sun B, Shi T L, Peng Z C, Sheng W J, Jiang T and Liao G L 2013 Controlled fabrication of Sn/TiO2 nanorods for
[18] [19]
[20] [21]
[22]
[23]
[24] [25]
[26]
[27]
10
photoelectrochemical water splitting Nanoscale Res. Lett. 8 462–70 Shankar K, Basham J I, Allam N K, Varghese O K, Mor G K, Feng X J, Paulose M, Seabold J A, Choi K S and Grimes C A 2009 Recent advances in the use of TiO2 nanotube and nanowire arrays for oxidative photoelectrochemistry J. Phys. Chem. C 113 6327–59 Pathak P, Gupta S, Grosulak K, Imahori H and Subramanian V 2015 Nature-inspired tree-like TiO2 architecture: a 3D platform for the assembly of CdS and reduced graphene oxide for photoelectrochemical processes J. Phys. Chem. C 119 7543–53 Qiu Y C, Leung S F, Zhang Q P, Hua B, Lin Q F, Wei Z H, Tsui K H, Zhang Y G, Yang S H and Fan Z Y 2014 Efficient photoelectrochemical water splitting with ultrathin films of hematite on three-dimensional nanophotonic structures Nano Lett. 14 2123–9 Zhang X, Liu Y and Kang Z H 2014 3D branched ZnO nanowire arrays decorated with plasmonic Au nanoparticles for high-performance photoelectrochemical water splitting ACS Appl. Mater. Inter. 6 4480–9 Chen H M, Chen C K, Liu R S, Zhang L, Zhang J J and Wilkinson D P 2012 Nano-architecture and material designs for water splitting photoelectrodes Chem. Soc. Rev. 41 5654–71 Cheng H M, Chiu W H, Lee C H, Tsai S Y and Hsieh W F 2008 Formation of branched ZnO nanowires from solvothermal method and dye-sensitized solar cells applications J. Phys. Chem. C 112 16359–64 Yu Y H, Li J Y, Geng D L, Wang J L, Zhang L S, Andrew T L, Arnold M S and Wang X D 2015 Development of lead iodide perovskite solar cells using three-dimensional titanium dioxide nanowire architectures ACS Nano 9 564–72 Sheng X, He D Q, Yang J, Zhu K and Feng X J 2014 Oriented assembled TiO2 hierarchical nanowire arrays with fast electron transport properties Nano Lett. 14 1848–52 Shi J, Hara Y, Sun C L, Anderson M A and Wang X D 2011 Three-dimensional high-density hierarchical nanowire architecture for high-performance photoelectrochemical electrodes Nano Lett. 11 3413–9 Goh C, Coakley K M and McGehee M D 2005 Nanostructuring titania by embossing with polymer molds made from anodic alumina templates Nano Lett. 5 1545–9 Gates B D, Xu Q B, Stewart M, Ryan D, Willson C G and Whitesides G M 2005 New approaches to nanofabrication: Molding, printing, and other techniques Chem. Rev. 105 1171–96 Ganesan R, Lim S H, Saifullah M S M, Hussain H, Kwok J X Q, Tse R L X, Bo H A P and Low H Y 2011 Direct nanoimprinting of metal oxides by in situ thermal copolymerization of their methacrylates J. Mater. Chem. 21 4484–92 Hampton M J, Williams S S, Zhou Z, Nunes J, Ko D H, Templeton J L, Samulski E T and DeSimone J M 2008 The patterning of sub-500 nm inorganic oxide structures Adv. Mater. 20 2667–73 Ganesan R, Dumond J, Saifullah M S M, Lim S H, Hussain H and Low H Y 2012 Direct patterning of TiO2 using step-and-flash imprint lithography ACS Nano 6 1494–502 Park H H, Zhang X, Lee S W, Kim K-D, Choi D G, Choi J H, Lee J, Lee E S, Park H H and Hill R H 2011 Facile nanopatterning of zirconium dioxide films via direct ultravioletassisted nanoimprint lithography J. Mater. Chem. 21 657–62 Murphy A B, Barnes P R F, Randeniya L K, Plumb I C, Grey I E, Horne M D and Glasscock J A 2006 Efficiency of solar water splitting using semiconductor electrodes Int. J. Hydrogen Energy 31 1999–2017 Zhuo S J, Shao M W and Lee S T 2012 Upconversion and downconversion fluorescent graphene quantum dots: ultrasonic preparation and photocatalysis ACS Nano 6 1059–64
Nanotechnology 27 (2016) 115401
Z Xu et al
[38] Liu B and Aydil E S 2009 Growth of oriented single-crystalline rutile TiO2 nanorods on transparent conducting substrates for dye-sensitized solar cells J. Am. Chem. Soc. 131 3985–90 [39] Xu C, Song Y, Lu L, Cheng C, Liu D, Fang X, Chen X, Zhu X and Li D 2013 Electrochemically hydrogenated TiO2 nanotubes with improved photoelectrochemical water splitting performance Nanoscale Res. Lett. 8 1–7 [40] Shavdina O et al 2015 Large area fabrication of periodic TiO2 nanopillars using microsphere photolithography on a photopatternable sol-gel film Langmuir 31 7877–84 [41] Cheng H H, Zhao Y, Fan Y Q, Xie X J, Qu L T and Shi G Q 2012 Graphene-quantum-dot assembled nanotubes: a new platform for efficient Raman enhancement ACS Nano 6 2237–44 [42] Hwang Y J, Hahn C, Liu B and Yang P 2012 Photoelectrochemical properties of TiO2 nanowire arrays: a study of the dependence on length and atomic layer deposition coating ACS Nano 6 5060–9 [43] Wang G M, Wang H Y, Ling Y C, Tang Y C, Yang X Y, Fitzmorris R C, Wang C C, Zhang J Z and Li Y 2011 Hydrogen-treated TiO2 nanowire arrays for photoelectrochemical water splitting Nano Lett. 11 3026–33 [44] Zhang Z H, Zhang L B, Hedhili M N, Zhang H N and Wang P 2013 Plasmonic gold nanocrystals coupled with photonic crystal seamlessly on TiO2 nanotube photoelectrodes for efficient visible light photoelectrochemical water splitting Nano Lett. 13 14–20 [45] Mueller M L, Yan X, Dragnea B and Li L S 2011 Slow hotcarrier relaxation in colloidal graphene quantum dots Nano Lett. 11 56–60 [46] Wen G A et al 2007 Hyperbranched triazine-containing polyfluorenes: efficient blue emitters for polymer lightemitting diodes (PLEDs) Polymer 48 1824–9 [47] Johansson T, Mammo W, Svensson M, Andersson M R and Inganas O 2003 Electrochemical bandgaps of substituted polythiophenes J. Mater. Chem. 13 1316–23 [48] Scanlon D O et al 2013 Band alignment of rutile and anatase TiO2 Nat. Mater. 12 798–801
[28] Pan D Y, Xi C, Li Z, Wang L, Chen Z W, Luc B and Wu M H 2013 Electrophoretic fabrication of highly robust, efficient, and benign heterojunction photoelectrocatalysts based on graphene-quantum-dot sensitized TiO2 nanotube arrays J. Mater. Chem. A 1 3551–5 [29] Son D I, Kwon B W, Park D H, Seo W S, Yi Y, Angadi B, Lee C L and Choi W K 2012 Emissive ZnO-graphene quantum dots for white-light-emitting diodes Nat. Nanotechnol. 7 465–71 [30] Lin J, Zhang C G, Yan Z, Zhu Y, Peng Z W, Hauge R H, Natelson D and Tour J M 2013 3-Dimensional graphene carbon nanotube carpet-based microsupercapacitors with high electrochemical performance Nano Lett. 13 72–8 [31] Zhu C, Chao D, Sun J, Bacho I M, Fan Z, Ng C F, Xia X, Huang H, Zhang H and Shen Z X 2015 Enhanced lithium storage performance of CuO nanowires by coating of graphene quantum dots Adv. Mater. Interfaces 2 [32] Zhou X M, Tian Z M, Li J, Ruan H, Ma Y Y, Yang Z and Qu Y Q 2014 Synergistically enhanced activity of graphene quantum dot/multi-walled carbon nanotube composites as metal-free catalysts for oxygen reduction reaction Nanoscale 6 2603–7 [33] Ananthanarayanan A, Wang X W, Routh P, Sana B, Lim S, Kim D H, Lim K H, Li J and Chen P 2014 Facile synthesis of graphene quantum dots from 3D graphene and their application for Fe3+ sensing Adv. Funct. Mater. 24 3021–6 [34] Zhang Z P, Zhang J, Chen N and Qu L T 2012 Graphene quantum dots: an emerging material for energy-related applications and beyond Energy Environ. Sci. 5 8869–90 [35] Reymond F, Rossier J S and Girault H H 2002 Polymer microchips bonded by O2-plasma activation Electrophoresis 23 782–90 [36] Yabu H and Shimomura M 2005 Single-step fabrication of transparent superhydrophobic porous polymer films Chem. Mater. 17 5231–4 [37] Truffier-Boutry D, Beaurain A, Galand R, Pelissier B, Boussey J and Zelsmann M 2010 XPS study of the degradation mechanism of fluorinated anti-sticking treatments used in UV nanoimprint lithography Microelectron. Eng. 87 122–4
11
