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Dye-sensitized InGaN nanowire arrays for efficient hydrogen production under visible light irradiation
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2015 Nanotechnology 26 285401 (http://iopscience.iop.org/0957-4484/26/28/285401) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 129.82.28.144 This content was downloaded on 04/07/2015 at 05:05
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Nanotechnology Nanotechnology 26 (2015) 285401 (5pp)
doi:10.1088/0957-4484/26/28/285401
Dye-sensitized InGaN nanowire arrays for efficient hydrogen production under visible light irradiation M G Kibria1, F A Chowdhury1, M L Trudeau2, H Guo3 and Z Mi1 1
Department of Electrical and Computer Engineering, McGill University, 3480 University Street, Montreal, Québec H3A 0E9, Canada 2 Science des Matériaux, IREQ, Hydro-Québec, 1800 Boul. Lionel-Boulet, Varennes, Québec J3X 1S1, Canada 3 Centre for the Physics of Materials, Department of Physics, McGill University, 3600 University Street, Montreal, Québec H3A 2T8, Canada E-mail: [email protected] Received 15 March 2015, revised 9 May 2015 Accepted for publication 1 June 2015 Published 29 June 2015 Abstract
Solar water splitting is a key sustainable energy technology for clean, storable and renewable source of energy in the future. Here we report that Merocyanine-540 dye-sensitized and Rh nanoparticle–decorated molecular beam epitaxially grown In0.25Ga0.75N nanowire arrays have produced hydrogen from ethylenediaminetetraacetic acid (EDTA) and acetonitrile mixture solution under green, yellow and orange solar spectra (up to 610 nm) for the first time. An apparent quantum efficiency of 0.3% is demonstrated for wavelengths 525–600 nm, providing a viable approach to harness deep-visible and near-infrared solar energy for efficient and stable water splitting. Keywords: artificial photosynthesis, solar water splitting, hydrogen, III-nitride, nanowire, dyesensitization (Some figures may appear in colour only in the online journal) 1. Introduction
which significantly enhances the reaction surface area [8]. In contrast to their bulk counterpart, 1D nanowires allow efficient photon absorption and enhanced carrier extraction, leading to improved performance of water splitting. Therefore, development of group-III nitride photocatalysts in the form of nanowires is highly desirable. We have recently demonstrated that stable overall neutral (pH ∼ 7.0) water splitting can be achieved under UV and visible light (up to 560 nm) using molecular beam epitaxially (MBE) grown GaN/InGaN nanowire arrays [9]. While significantly improved quantum efficiency is achieved for wavelengths up to 475 nm, the efficiency of water splitting at longer wavelength (>500 nm) has been limited by the bandgap of InGaN [10]. Although the bandgap of InGaN can further be reduced by tuning the In composition, the development of high-quality InGaN with high In content (>50%) has been extremely challenging [11, 12]. This can be attributed to the large lattice mismatch (∼11%) between InN and
The production of hydrogen and other fuels via solar water splitting has been recognized as one of the key sustainable energy technologies to enable carbon-neutral, storable and affordable energy for future generations [1–3]. Although a number of metal-oxide based photocatalysts have been developed for water splitting under ultraviolet (UV) light, the development of visible light–responsive photocatalyst has been very limited [2, 3]. Recently, group-III nitride has attracted considerable attention owing to its near-perfect band-edge potential (the bandgap straddles the redox potentials of water), and tunable bandgap that encompasses nearly the entire solar spectrum [4, 5]. The extreme chemical stability along with high absorption co-efficient and mobility of III-nitrides further demonstrates their use as an alternative photocatalyst [6, 7]. On the other hand, one-dimensional (1D) nanowires provide an extremely high surface-to-volume ratio, 0957-4484/15/285401+05$33.00
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Nanotechnology 26 (2015) 285401
studies [9]. In order to enhance the carrier extraction efficiency from the In0.25Ga0.75N nanowires and to enhance the hydrogen evolution reaction (HER), Rh nanoparticles were photodeposited onto the nanowire surface from liquid precursors [19, 20]. A high-resolution STEM-HAADF image as shown in figure 1(e) demonstrates successful deposition of Rh nanoparticle onto the In0.25Ga0.75N nanowires. The nanoparticles are ∼3–5 nm in diameter and uniformly distributed on the nonpolar lateral surface of the nanowire. For subsequent experiments ∼3 cm2 wafer sample is used, which corresponds to ∼0.4 mg of nanowire catalyst.
GaN, spinodal decomposition, In phase separation, and In surface segregation [11, 12]. These factors lead to a large number of non-radiative recombination centers that limit device performance. Among different approaches, sensitization of a wide bandgap semiconductor with visible light–sensitive dye is an attractive solution to enhance the light absorption and extend the absorption edge of the photocatalyst [13, 14]. In this approach, organic or inorganic sensitizer molecules are adsorbed on the semiconductor surface. Upon visible light excitation, the dye molecules inject electrons to the conduction band of the semiconductor. These electrons are subsequently used to reduce water to generate H2 either on the semiconductor surface or on a catalytic particle [14]. Although a number of dye and semiconductor combinations have been developed for water splitting under visible light, none of them have reported H2 generation using green to orange solar photons [15]. In this study, we have incorporated visible light–sensitive dyes on InGaN nanowires for efficient and stable production of H2 beyond 500 nm solar photons. To our knowledge this is the first attempt to study the photocatalytic activity of dye-sensitized III-nitride nanowires for water splitting under green, yellow and orange solar spectra (up to 610 nm).
3. Results and discussions In this work, we have adopted Merocyanine-540 (MC-540, Sigma-Aldrich) dye to extend the visible light absorption edge of the nanowires. The MC-540 is a water-soluble anionic cyanine dye (figure 2(a) inset shows the molecular structure of MC-540) and is of particular interest owing to its enhanced photostability and spectral properties [21]. For dye adsorption, the Rh nanoparticle–deposited In0.25Ga0.75N nanowires were immersed in 100 mM acetonitrile (AN) and 1 mM MC-540 solution for 12 h in the dark at room temperature under continuous stirring condition [22]. The pH of the solution was adjusted by adding an appropriate amount of HCl or NaOH. The adsorption of MC-540 dye onto the nanowire surface is confirmed by measuring the absorption spectrum of the MC-540 dye containing AN solution before and after 12 h of adsorption. Figure 2(a) shows the UV–Vis absorption spectrum of 1 mM MC-540 dye containing AN solution before and after dye absorption at different pH. Before adsorption, a broad absorption spectrum expanding from 450 to 600 nm is revealed. As can be seen, the dye molecules were significantly adsorbed onto the nanowire surface at pH ∼ 10.0. At near neutral (pH ∼ 6.0) and acidic (pH ∼ 3.0) solution, the dye adsorption was inefficient. To understand the dye adsorption mechanism, Zeta-potential measurement of nanowire-dispersed solution was performed using a NanoBrook ZetaPALS analyzer [23]. The charge state of the nanowire surface was found to be positive (+11.3 mV), indicating the fact that the negatively charged dye molecules can be electrostatically adsorbed onto the nanowire surface at basic condition. The photocatalytic experiments were performed by immersing the dye adsorbed (at pH ∼ 10.0) and Rh nanoparticle–deposited nanowire samples in 10 mM ethylenediaminetetraacetic acid (EDTA) and 100 mM AN mixture (2:1 ratio) solution and irradiating with 300 W xenon lamp with different long-pass filters. The EDTA is used as a sacrificial electron donor to reduce the oxidized dye molecules and regenerate for subsequent photon absorption. The AN is used as an organic solvent [22]. The reaction-evolved gases were analyzed by a gas chromatograph (TCD) and ultra-pure Ar carrier gas. Figure 2(b) illustrates the rate of H2 evolution from MC-540 dye adsorbed In0.25Ga0.75N/Rh nanowire samples with different photoexcitation conditions. In order to avoid exciting the In0.25Ga0.75N nanowire (band edge 507 nm), long-
2. Experimental details The InGaN nanowires are grown on Si (111) substrate using plasma assisted MBE under nitrogen rich condition [16]. A GaN nanowire template is used for the growth of InGaN nanowires, as schematically illustrated in figure 1(a). Instead of growing one continuous InGaN, two intermediate layers of GaN were introduced in between InGaN layers to avoid phase separation and surface segregation. This unique nanowire heterostructure offers a nearly defect-free nanowire with excellent electrical, structural and optical properties [17]. The nanowires are p-type doped with Mg dopants [10]. Figure 1(b) shows a 45° tilted scanning electron microscopy (SEM) image of as-grown InGaN nanowires on Si (111) substrate. The nanowires are well dispersed and vertically aligned to the substrate with their growth direction along the c-axis, and the sidewalls being the nonpolar m-planes. Scanning transmission electron microscopy–high angle annular dark field (STEM-HAADF) imaging of a single nanowire, as shown in figure 1(c), depicts that the nanowires are nearly 400 nm long and 40–100 nm in diameter. Figure 1(c) (inset) shows a high-resolution transmission electron microscopy (HRTEM) lattice image, illustrating the single crystalline structure of the InGaN nanowire. The bandgap and therefore the In content in the nanowire is determined from the room temperature photoluminescence (PL) spectrum from the as-grown nanowire ensemble, as shown in figure 1(d). A single and broad PL emission centered at ∼507 nm is revealed, which indicates a bandgap of ∼2.44 eV, corresponding to an In content of 25% in InGaN [18]. The broad luminescence indicates intra- and internanowire In fluctuations, and is consistent with previous 2
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Nanotechnology 26 (2015) 285401
Figure 1. (a) Schematic of the nanowire heterostructure. The thickness of each InGaN segment is nearly 45 nm; InGaN segments are
separated by ∼10 nm GaN segments. (b) A 45° tilted scanning electron microscopy (SEM) image and (c) scanning transmission electron microscopy–high angle annular dark field (STEM-HAADF) image of as-grown In0.25Ga0.75N nanowires. The inset shows a high-resolution transmission electron microscopy (HRTEM) lattice image of an In0.25Ga0.75N nanowire. (d) Room temperature photoluminescence from asgrown In0.25Ga0.75N nanowire. (e) Rh nanoparticle–decorated In0.25Ga0.75N nanowire.
pass filters with cut-off wavelengths between 500 and 610 nm were adopted. The H2 evolution rate was estimated from 2 h of reaction. The photocatalytic activity decreased with increasing wavelength because of a decrease in the number of absorbed photons. This can be well correlated with the absorption spectrum of the dye as shown in figure 2(a). The photocatalytic activity at longer wavelength (>610 nm) is limited by the photoabsorption of the dye. Our control experiment shows that in the absence of either dye molecules or sacrificial electron donors, no photocatalytic activity is observed for sub-bandgap excitation (>500 nm). This confirms that the observed photocatalytic activity is initiated via photon absorption by the adsorbed dye molecules, and proceeds by subsequent electron injection events followed by the oxidation of the dyes by the sacrificial reagent. The photocatalytic activity observed here is nearly 15 times higher than recently reported dye-sensitized potassium hexaniobate nanoscrolls (NS-K4Nb6O17) with subbandgap excitation [24]. The apparent quantum efficiency is further estimated to be 0.3% for wavelengths between 525 and 600 nm, following the procedure discussed in our previous studies [10, 25, 26]. Figure 3 shows a plot of H2 evolution with irradiation time from the mixture of EDTA and AN with 525 nm long-pass filter. Steady evolution of H2 is observed for over 2 h. The saturation of H2 evolution after 2 h of photocatalytic reaction can be attributed to the leaching of the MC540 dye under photoexcitation and photochemical reaction
condition [24]. The saturation in H2 evolution is partly due to the degradation of the MC-540 dye under the photoexcitation condition and the resulting increase in the temperature of the reaction solution. The photocatalytic activity can be recovered when the reaction is repeated after adding more MC-540 dye in the reaction solution. Figure 4 illustrates the photocatalytic reaction mechanism of dye-sensitized In0.25Ga0.75N nanowires under visible light irradiation (>500 nm). The lowest-occupied molecular orbital (LUMO) of MC-540 is reported to be located at −1.2 V versus NHE at pH ∼ 7.0 and the conduction band minimum (CBM) of In0.25Ga0.75N is located at −1.0 V versus NHE at pH ∼ 7.0 [5, 27]. Therefore efficient electron injection is possible from the LUMO of MC-540 to CBM of In0.25Ga0.75N. When GaN is used as the host semiconductor for the dyes, no photocatalytic activity is observed, indicating the fact that the CBM of GaN is positioned at more negative potential than the LUMO level of the dye. Therefore, electron injection from the dye to the semiconductor is suppressed for GaN. The injected electrons in the CBM of In0.25Ga0.75N are subsequently transferred to the Rh nanoparticles deposited on the nanowire surface, as illustrated in figure 4. Note that, in the case of p-doped nitrides (as in the present study), a downward band bending at the nanowire surface is commonly reported [10, 26]. Therefore, efficient electron extraction from the nanowire to the Rh nanoparticle was observed in our previous study [10]. In contrast, when an 3
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Nanotechnology 26 (2015) 285401
Figure 4. Reaction mechanism of MC-540 dye-sensitized
In0.25Ga0.75N/Rh nanowires.
and intrinsic nanowire surfaces [9, 20]. Since the Fermi level of Rh (−4.9 eV) is more negative than the electron affinity of In0.25Ga0.75N:Mg (−3.95 eV), the photogenerated electrons in the conduction band can easily migrate from the nanowire to Rh nanoparticles. Furthermore, our control experiment reveals that in the absence of an Rh nanoparticle, negligible photocatalytic activity is observed, indicating the significant influence of a suitable reaction site for proton reduction. The EDTA finally reduces the oxidized dye molecules and regenerates for subsequent photon absorption. The reaction mechanism discussed above can be summarized according to the following steps [24, 28, 29]: MC540 + hƲ (λ > 500 nm) → *MC540 *MC540 → In 0.25Ga 0.75N ( e−) + (MC540)+ Figure 2. (a) UV–Vis absorption spectrum of 100 mM AN and 1 mM MC-540 dye before and after 12 h of adsorption. (b) H2 evolution from 10 mM EDTA and 100 mM AN mixture under 300 W xenon lamp with different long-pass filters.
In 0.25Ga 0.75N ( e−) → In 0.25Ga 0.75N + Rh ( e−) Rh ( 2e−) + 2H + → H 2 (MC540)+ + EDTA ( e−) → MC540 + EDTA(oxidized)
4. Conclusion In summary, we have demonstrated that the dye-sensitization of the molecular beam epitaxially grown In0.25Ga0.75N nanowires can be a viable approach for efficient H2 production in the presence of a sacrificial reagent under green, yellow and orange solar spectrum (up to 610 nm). Although, at present, the quantum efficiency is low, the enhancement of efficiency is in progress by optimizing nanowire structure, dye-adsorption, and co-catalyst deposition. This work will pave the way towards harnessing of deep-visible and nearinfrared solar photons using suitable and stable dye molecules for efficient and stable water splitting. Figure 3. Repeated cycles of H2 evolution from EDTA and AN mixture under 300 W xenon lamp with 525 nm long-pass filter.
Acknowledgments n-type or intrinsic In0.25Ga0.75N nanowire photocatalyst is used, the photocatalytic activity is substantially low. This can be caused by the potential barrier resulting from the presence of an upward band bending of the conduction band on n-type
This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) and the Climate Change and Emissions Management (CCEMC) 4
M G Kibria et al
Nanotechnology 26 (2015) 285401
Corporation, Canada. Part of the work was performed in the Micro-fabrication Facility at McGill University. Electron microscopy images and analysis were carried out at IREQ of Hydro-Québec and the Facility for Electron Microscopy Research (FEMR), McGill University.
[15] Chen X, Shen S, Guo L and Mao S S 2010 Semiconductorbased photocatalytic hydrogen generation Chem. Rev. 110 6503 [16] Fernández-Garrido S, Kaganer V M, Sabelfeld K K, Gotschke T, Grandal J, Calleja E, Geelhaar L and Brandt O 2013 Self-regulated radius of spontaneously formed GaN nanowires in molecular beam epitaxy Nano Lett. 13 3274 [17] Nguyen H P T, Zhang S, Cui K, Han X, Fathololoumi S, Couillard M, Botton G A and Mi Z 2011 p-Type modulation doped InGaN/GaN dot-in-a-wire white-light-emitting diodes monolithically grown on Si (111) Nano Lett. 11 1919 [18] Moses P G and Van de Walle C G 2010 Band bowing and band alignment in InGaN alloys Appl. Phys. Lett. 96 021908 [19] Maeda K, Teramura K, Lu D, Takata T, Saito N, Inoue Y and Domen K 2006 Photocatalyst releasing hydrogen from water Nature 440 295 [20] Wang D, Pierre A, Kibria M G, Cui K, Han X G, Bevan K H, Guo H, Paradis S, Hakima A R and Mi Z 2011 Wafer-level photocatalytic water splitting on GaN nanowire arrays grown by molecular beam epitaxy Nano Lett. 11 2353–7 [21] Khazraji A C, Hotchandani S, Das S and Kamat P V 1999 Controlling dye (Merocyanine-540) aggregation on nanostructured TiO2 films an organized assembly approach for enhancing the efficiency of photosensitization J. Phys. Chem. B 103 4693 [22] Abe R, Sayama K and Arakawa H 2004 Dye-sensitized photocatalysts for efficient hydrogen production from aqueous I− solution under visible light irradiation J. Photochem. Photobiol., B 166 115 [23] Li S, Zhang J, Kibria M G, Mi Z, Chaker M, Ma D, Nechache R and Rosei F 2013 Remarkably enhanced photocatalytic activity of laser ablated Au nanoparticle decorated BiFeO3 nanowires under visible-light Chem. Commun. 49 5856 [24] Maeda K, Eguchi M, Youngblood W J and Mallouk T E 2008 Niobium oxide nanoscrolls as building blocks for dyesensitized hydrogen production from water under visible light irradiation Chem. Mater. 20 6770 [25] Kibria M G, Chowdhury F A, Zhao S, Trudeau M L, Guo H and Mi Z 2015 Defect-engineered GaN:Mg nanowire arrays for overall water splitting under violet light Appl. Phys. Lett. 106 113105 [26] Kibria M G, Zhao S, Chowdhury F A, Wang Q, Nguyen H P T, Trudeau M L, Guo H and Mi Z 2014 Tuning the surface Fermi level on p-type gallium nitride nanowires for efficient overall water splitting Nat. Commun. 5 3825 [27] Abe R, Sayama K and Arakawa H 2002 Efficient hydrogen evolution from aqueous mixture of I− and acetonitrile using a merocyanine dye-sensitized Pt/TiO2 photocatalyst under visible light irradiation Chem. Phys. Lett. 362 441 [28] Abe R, Shinmei K, Hara K and Ohtani B 2009 Robust dyesensitized overall water splitting system with two-step photoexcitation of coumarin dyes and metal oxide semiconductors Chem. Commun. 24 3577 [29] Abe R, Sayama K and Sugihara H 2004 Effect of water/ acetonitrile ratio on dye-sensitized photocatalytic H2 evolution under visible light irradiation J. Sol. Energy Eng. 127 413
References [1] Tachibana Y, Vayssieres L and Durrant J R 2012 Artificial photosynthesis for solar water-splitting Nat. Photonics 6 511 [2] Kudo A and Miseki Y 2009 Heterogeneous photocatalyst materials for water splitting Chem. Soc. Rev. 38 253 [3] Li X, Yu J, Low J, Fang Y, Xiao J and Chen X 2015 Engineering heterogeneous semiconductors for solar water splitting J. Mater. Chem. A 3 2485–534 [4] Wu J 2009 When group-III nitrides go infrared: new properties and perspectives J. Appl. Phys. 106 011101 [5] Van de Walle C G and Neugebauer J 2003 universal alignment of hydrogen levels in semiconductors, insulators and solutions Nature 423 626 [6] AlOtaibi B, Nguyen H P T, Zhao S, Kibria M G, Fan S and Mi Z 2013 Highly stable photoelectrochemical water splitting and hydrogen generation using a double-band InGaN/GaN core/shell nanowire photoanode Nano Lett. 13 4356–61 [7] Luo W, Liu B, Li Z, Xie Z, Chen D, Zou Z and Zhang R 2008 Stable response to visible light of InGaN photoelectrodes Appl. Phys. Lett. 92 262110 [8] Tong H, Ouyang S, Bi Y, Umezawa N, Oshikiri M and Ye J 2012 Nano‐photocatalytic materials: possibilities and challenges Adv. Mater. 24 229 [9] Kibria M G, Nguyen H P T, Cui K, Zhao S, Liu D, Guo H, Trudeau M L, Paradis S, Hakima A-R and Mi Z 2013 Onestep overall water splitting under visible light using multiband InGaN/GaN nanowire heterostructures ACS Nano 7 7886 [10] Kibria M G, Chowdhury F A, Zhao S, AlOtaibi B, Trudeau M L, Guo H and Mi Z 2015 Visible light driven efficient overall water splitting using p-type metal-nitride nanowire arrays Nat. Commun. 6 6797 [11] Hoffbauer M A, Williamson T L, Williams J J, Fordham J L, Yu K M, Walukiewicz W and Reichertz L A 2013 In-rich InGaN thin films: progress on growth, compositional uniformity, and doping for device applications J. Vac. Sci. Technol. B 31 03C114 [12] Yam F and Hassan Z 2008 InGaN: an overview of the growth kinetics, physical properties and emission mechanisms Superlattices Microstruct. 43 1 [13] Swierk J R and Mallouk T E 2013 Design and development of photoanodes for water-splitting dye-sensitized photoelectrochemical cells Chem. Soc. Rev. 42 2357 [14] Youngblood W J, Lee S H A, Maeda K and Mallouk T E 2009 Design and development of photoanodes for water-splitting dye-sensitized photoelectrochemical cells Acc. Chem. Res. 42 1966
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Dye-sensitized InGaN nanowire arrays for efficient hydrogen production under visible light irradiation
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2015 Nanotechnology 26 285401 (http://iopscience.iop.org/0957-4484/26/28/285401) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 129.82.28.144 This content was downloaded on 04/07/2015 at 05:05
Please note that terms and conditions apply.
Nanotechnology Nanotechnology 26 (2015) 285401 (5pp)
doi:10.1088/0957-4484/26/28/285401
Dye-sensitized InGaN nanowire arrays for efficient hydrogen production under visible light irradiation M G Kibria1, F A Chowdhury1, M L Trudeau2, H Guo3 and Z Mi1 1
Department of Electrical and Computer Engineering, McGill University, 3480 University Street, Montreal, Québec H3A 0E9, Canada 2 Science des Matériaux, IREQ, Hydro-Québec, 1800 Boul. Lionel-Boulet, Varennes, Québec J3X 1S1, Canada 3 Centre for the Physics of Materials, Department of Physics, McGill University, 3600 University Street, Montreal, Québec H3A 2T8, Canada E-mail: [email protected] Received 15 March 2015, revised 9 May 2015 Accepted for publication 1 June 2015 Published 29 June 2015 Abstract
Solar water splitting is a key sustainable energy technology for clean, storable and renewable source of energy in the future. Here we report that Merocyanine-540 dye-sensitized and Rh nanoparticle–decorated molecular beam epitaxially grown In0.25Ga0.75N nanowire arrays have produced hydrogen from ethylenediaminetetraacetic acid (EDTA) and acetonitrile mixture solution under green, yellow and orange solar spectra (up to 610 nm) for the first time. An apparent quantum efficiency of 0.3% is demonstrated for wavelengths 525–600 nm, providing a viable approach to harness deep-visible and near-infrared solar energy for efficient and stable water splitting. Keywords: artificial photosynthesis, solar water splitting, hydrogen, III-nitride, nanowire, dyesensitization (Some figures may appear in colour only in the online journal) 1. Introduction
which significantly enhances the reaction surface area [8]. In contrast to their bulk counterpart, 1D nanowires allow efficient photon absorption and enhanced carrier extraction, leading to improved performance of water splitting. Therefore, development of group-III nitride photocatalysts in the form of nanowires is highly desirable. We have recently demonstrated that stable overall neutral (pH ∼ 7.0) water splitting can be achieved under UV and visible light (up to 560 nm) using molecular beam epitaxially (MBE) grown GaN/InGaN nanowire arrays [9]. While significantly improved quantum efficiency is achieved for wavelengths up to 475 nm, the efficiency of water splitting at longer wavelength (>500 nm) has been limited by the bandgap of InGaN [10]. Although the bandgap of InGaN can further be reduced by tuning the In composition, the development of high-quality InGaN with high In content (>50%) has been extremely challenging [11, 12]. This can be attributed to the large lattice mismatch (∼11%) between InN and
The production of hydrogen and other fuels via solar water splitting has been recognized as one of the key sustainable energy technologies to enable carbon-neutral, storable and affordable energy for future generations [1–3]. Although a number of metal-oxide based photocatalysts have been developed for water splitting under ultraviolet (UV) light, the development of visible light–responsive photocatalyst has been very limited [2, 3]. Recently, group-III nitride has attracted considerable attention owing to its near-perfect band-edge potential (the bandgap straddles the redox potentials of water), and tunable bandgap that encompasses nearly the entire solar spectrum [4, 5]. The extreme chemical stability along with high absorption co-efficient and mobility of III-nitrides further demonstrates their use as an alternative photocatalyst [6, 7]. On the other hand, one-dimensional (1D) nanowires provide an extremely high surface-to-volume ratio, 0957-4484/15/285401+05$33.00
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© 2015 IOP Publishing Ltd Printed in the UK
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Nanotechnology 26 (2015) 285401
studies [9]. In order to enhance the carrier extraction efficiency from the In0.25Ga0.75N nanowires and to enhance the hydrogen evolution reaction (HER), Rh nanoparticles were photodeposited onto the nanowire surface from liquid precursors [19, 20]. A high-resolution STEM-HAADF image as shown in figure 1(e) demonstrates successful deposition of Rh nanoparticle onto the In0.25Ga0.75N nanowires. The nanoparticles are ∼3–5 nm in diameter and uniformly distributed on the nonpolar lateral surface of the nanowire. For subsequent experiments ∼3 cm2 wafer sample is used, which corresponds to ∼0.4 mg of nanowire catalyst.
GaN, spinodal decomposition, In phase separation, and In surface segregation [11, 12]. These factors lead to a large number of non-radiative recombination centers that limit device performance. Among different approaches, sensitization of a wide bandgap semiconductor with visible light–sensitive dye is an attractive solution to enhance the light absorption and extend the absorption edge of the photocatalyst [13, 14]. In this approach, organic or inorganic sensitizer molecules are adsorbed on the semiconductor surface. Upon visible light excitation, the dye molecules inject electrons to the conduction band of the semiconductor. These electrons are subsequently used to reduce water to generate H2 either on the semiconductor surface or on a catalytic particle [14]. Although a number of dye and semiconductor combinations have been developed for water splitting under visible light, none of them have reported H2 generation using green to orange solar photons [15]. In this study, we have incorporated visible light–sensitive dyes on InGaN nanowires for efficient and stable production of H2 beyond 500 nm solar photons. To our knowledge this is the first attempt to study the photocatalytic activity of dye-sensitized III-nitride nanowires for water splitting under green, yellow and orange solar spectra (up to 610 nm).
3. Results and discussions In this work, we have adopted Merocyanine-540 (MC-540, Sigma-Aldrich) dye to extend the visible light absorption edge of the nanowires. The MC-540 is a water-soluble anionic cyanine dye (figure 2(a) inset shows the molecular structure of MC-540) and is of particular interest owing to its enhanced photostability and spectral properties [21]. For dye adsorption, the Rh nanoparticle–deposited In0.25Ga0.75N nanowires were immersed in 100 mM acetonitrile (AN) and 1 mM MC-540 solution for 12 h in the dark at room temperature under continuous stirring condition [22]. The pH of the solution was adjusted by adding an appropriate amount of HCl or NaOH. The adsorption of MC-540 dye onto the nanowire surface is confirmed by measuring the absorption spectrum of the MC-540 dye containing AN solution before and after 12 h of adsorption. Figure 2(a) shows the UV–Vis absorption spectrum of 1 mM MC-540 dye containing AN solution before and after dye absorption at different pH. Before adsorption, a broad absorption spectrum expanding from 450 to 600 nm is revealed. As can be seen, the dye molecules were significantly adsorbed onto the nanowire surface at pH ∼ 10.0. At near neutral (pH ∼ 6.0) and acidic (pH ∼ 3.0) solution, the dye adsorption was inefficient. To understand the dye adsorption mechanism, Zeta-potential measurement of nanowire-dispersed solution was performed using a NanoBrook ZetaPALS analyzer [23]. The charge state of the nanowire surface was found to be positive (+11.3 mV), indicating the fact that the negatively charged dye molecules can be electrostatically adsorbed onto the nanowire surface at basic condition. The photocatalytic experiments were performed by immersing the dye adsorbed (at pH ∼ 10.0) and Rh nanoparticle–deposited nanowire samples in 10 mM ethylenediaminetetraacetic acid (EDTA) and 100 mM AN mixture (2:1 ratio) solution and irradiating with 300 W xenon lamp with different long-pass filters. The EDTA is used as a sacrificial electron donor to reduce the oxidized dye molecules and regenerate for subsequent photon absorption. The AN is used as an organic solvent [22]. The reaction-evolved gases were analyzed by a gas chromatograph (TCD) and ultra-pure Ar carrier gas. Figure 2(b) illustrates the rate of H2 evolution from MC-540 dye adsorbed In0.25Ga0.75N/Rh nanowire samples with different photoexcitation conditions. In order to avoid exciting the In0.25Ga0.75N nanowire (band edge 507 nm), long-
2. Experimental details The InGaN nanowires are grown on Si (111) substrate using plasma assisted MBE under nitrogen rich condition [16]. A GaN nanowire template is used for the growth of InGaN nanowires, as schematically illustrated in figure 1(a). Instead of growing one continuous InGaN, two intermediate layers of GaN were introduced in between InGaN layers to avoid phase separation and surface segregation. This unique nanowire heterostructure offers a nearly defect-free nanowire with excellent electrical, structural and optical properties [17]. The nanowires are p-type doped with Mg dopants [10]. Figure 1(b) shows a 45° tilted scanning electron microscopy (SEM) image of as-grown InGaN nanowires on Si (111) substrate. The nanowires are well dispersed and vertically aligned to the substrate with their growth direction along the c-axis, and the sidewalls being the nonpolar m-planes. Scanning transmission electron microscopy–high angle annular dark field (STEM-HAADF) imaging of a single nanowire, as shown in figure 1(c), depicts that the nanowires are nearly 400 nm long and 40–100 nm in diameter. Figure 1(c) (inset) shows a high-resolution transmission electron microscopy (HRTEM) lattice image, illustrating the single crystalline structure of the InGaN nanowire. The bandgap and therefore the In content in the nanowire is determined from the room temperature photoluminescence (PL) spectrum from the as-grown nanowire ensemble, as shown in figure 1(d). A single and broad PL emission centered at ∼507 nm is revealed, which indicates a bandgap of ∼2.44 eV, corresponding to an In content of 25% in InGaN [18]. The broad luminescence indicates intra- and internanowire In fluctuations, and is consistent with previous 2
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Nanotechnology 26 (2015) 285401
Figure 1. (a) Schematic of the nanowire heterostructure. The thickness of each InGaN segment is nearly 45 nm; InGaN segments are
separated by ∼10 nm GaN segments. (b) A 45° tilted scanning electron microscopy (SEM) image and (c) scanning transmission electron microscopy–high angle annular dark field (STEM-HAADF) image of as-grown In0.25Ga0.75N nanowires. The inset shows a high-resolution transmission electron microscopy (HRTEM) lattice image of an In0.25Ga0.75N nanowire. (d) Room temperature photoluminescence from asgrown In0.25Ga0.75N nanowire. (e) Rh nanoparticle–decorated In0.25Ga0.75N nanowire.
pass filters with cut-off wavelengths between 500 and 610 nm were adopted. The H2 evolution rate was estimated from 2 h of reaction. The photocatalytic activity decreased with increasing wavelength because of a decrease in the number of absorbed photons. This can be well correlated with the absorption spectrum of the dye as shown in figure 2(a). The photocatalytic activity at longer wavelength (>610 nm) is limited by the photoabsorption of the dye. Our control experiment shows that in the absence of either dye molecules or sacrificial electron donors, no photocatalytic activity is observed for sub-bandgap excitation (>500 nm). This confirms that the observed photocatalytic activity is initiated via photon absorption by the adsorbed dye molecules, and proceeds by subsequent electron injection events followed by the oxidation of the dyes by the sacrificial reagent. The photocatalytic activity observed here is nearly 15 times higher than recently reported dye-sensitized potassium hexaniobate nanoscrolls (NS-K4Nb6O17) with subbandgap excitation [24]. The apparent quantum efficiency is further estimated to be 0.3% for wavelengths between 525 and 600 nm, following the procedure discussed in our previous studies [10, 25, 26]. Figure 3 shows a plot of H2 evolution with irradiation time from the mixture of EDTA and AN with 525 nm long-pass filter. Steady evolution of H2 is observed for over 2 h. The saturation of H2 evolution after 2 h of photocatalytic reaction can be attributed to the leaching of the MC540 dye under photoexcitation and photochemical reaction
condition [24]. The saturation in H2 evolution is partly due to the degradation of the MC-540 dye under the photoexcitation condition and the resulting increase in the temperature of the reaction solution. The photocatalytic activity can be recovered when the reaction is repeated after adding more MC-540 dye in the reaction solution. Figure 4 illustrates the photocatalytic reaction mechanism of dye-sensitized In0.25Ga0.75N nanowires under visible light irradiation (>500 nm). The lowest-occupied molecular orbital (LUMO) of MC-540 is reported to be located at −1.2 V versus NHE at pH ∼ 7.0 and the conduction band minimum (CBM) of In0.25Ga0.75N is located at −1.0 V versus NHE at pH ∼ 7.0 [5, 27]. Therefore efficient electron injection is possible from the LUMO of MC-540 to CBM of In0.25Ga0.75N. When GaN is used as the host semiconductor for the dyes, no photocatalytic activity is observed, indicating the fact that the CBM of GaN is positioned at more negative potential than the LUMO level of the dye. Therefore, electron injection from the dye to the semiconductor is suppressed for GaN. The injected electrons in the CBM of In0.25Ga0.75N are subsequently transferred to the Rh nanoparticles deposited on the nanowire surface, as illustrated in figure 4. Note that, in the case of p-doped nitrides (as in the present study), a downward band bending at the nanowire surface is commonly reported [10, 26]. Therefore, efficient electron extraction from the nanowire to the Rh nanoparticle was observed in our previous study [10]. In contrast, when an 3
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Nanotechnology 26 (2015) 285401
Figure 4. Reaction mechanism of MC-540 dye-sensitized
In0.25Ga0.75N/Rh nanowires.
and intrinsic nanowire surfaces [9, 20]. Since the Fermi level of Rh (−4.9 eV) is more negative than the electron affinity of In0.25Ga0.75N:Mg (−3.95 eV), the photogenerated electrons in the conduction band can easily migrate from the nanowire to Rh nanoparticles. Furthermore, our control experiment reveals that in the absence of an Rh nanoparticle, negligible photocatalytic activity is observed, indicating the significant influence of a suitable reaction site for proton reduction. The EDTA finally reduces the oxidized dye molecules and regenerates for subsequent photon absorption. The reaction mechanism discussed above can be summarized according to the following steps [24, 28, 29]: MC540 + hƲ (λ > 500 nm) → *MC540 *MC540 → In 0.25Ga 0.75N ( e−) + (MC540)+ Figure 2. (a) UV–Vis absorption spectrum of 100 mM AN and 1 mM MC-540 dye before and after 12 h of adsorption. (b) H2 evolution from 10 mM EDTA and 100 mM AN mixture under 300 W xenon lamp with different long-pass filters.
In 0.25Ga 0.75N ( e−) → In 0.25Ga 0.75N + Rh ( e−) Rh ( 2e−) + 2H + → H 2 (MC540)+ + EDTA ( e−) → MC540 + EDTA(oxidized)
4. Conclusion In summary, we have demonstrated that the dye-sensitization of the molecular beam epitaxially grown In0.25Ga0.75N nanowires can be a viable approach for efficient H2 production in the presence of a sacrificial reagent under green, yellow and orange solar spectrum (up to 610 nm). Although, at present, the quantum efficiency is low, the enhancement of efficiency is in progress by optimizing nanowire structure, dye-adsorption, and co-catalyst deposition. This work will pave the way towards harnessing of deep-visible and nearinfrared solar photons using suitable and stable dye molecules for efficient and stable water splitting. Figure 3. Repeated cycles of H2 evolution from EDTA and AN mixture under 300 W xenon lamp with 525 nm long-pass filter.
Acknowledgments n-type or intrinsic In0.25Ga0.75N nanowire photocatalyst is used, the photocatalytic activity is substantially low. This can be caused by the potential barrier resulting from the presence of an upward band bending of the conduction band on n-type
This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) and the Climate Change and Emissions Management (CCEMC) 4
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Nanotechnology 26 (2015) 285401
Corporation, Canada. Part of the work was performed in the Micro-fabrication Facility at McGill University. Electron microscopy images and analysis were carried out at IREQ of Hydro-Québec and the Facility for Electron Microscopy Research (FEMR), McGill University.
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