TiO2 Inverse Opal Nanostructures with

Feb 7, 2017 - Ramireddy Boppella† , Saji Thomas Kochuveedu†§, Heejun Kim†, Myung Jin Jeong‡, Filipe Marques Mota† , Jong Hyeok Park‡ , an...
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Plasmon-Sensitized Graphene/TiO2 Inverse Opal Nanostructures with Enhanced Charge Collection Efficiency for Water Splitting Ramireddy Boppella,† Saji Thomas Kochuveedu,†,§ Heejun Kim,† Myung Jin Jeong,‡ Filipe Marques Mota,† Jong Hyeok Park,‡ and Dong Ha Kim*,†,⊥ †

Department of Chemistry and Nano Science, Division of Molecular and Life Sciences, College of Natural Sciences, Ewha Womans University, 52, Ewhayeodae-gil, Seodaemun-gu, Seoul 03760, Korea ‡ Department of Chemical and Biomolecular Engineering, Yonsei University, 50, Yonsei-ro, Seodaemun-gu, Seoul 03722, Korea ⊥ Division of Chemical Engineering and Materials Science, College of Engineering, Ewha Womans University, 52, Ewhayeodae-gil, Seodaemun-gu, Seoul 03760, Korea S Supporting Information *

ABSTRACT: In this contribution we have developed TiO2 inverse opal based photoelectrodes for photoelectrochemical (PEC) water splitting devices, in which Au nanoparticles (NPs) and reduced graphene oxide (rGO) have been strategically incorporated (TiO2@ rGO@Au). The periodic hybrid nanostructure showed a photocurrent density of 1.29 mA cm−2 at 1.23 V vs RHE, uncovering a 2-fold enhancement compared to a pristine TiO2 reference. The Au NPs were confirmed to extensively broaden the absorption spectrum of TiO2 into the visible range and to reduce the onset potential of these photoelectrodes. Most importantly, TiO2@rGO@Au hybrid exhibited a 14-fold enhanced PEC efficiency under visible light and a 2.5-fold enrichment in the applied bias photon-to-current efficiency at much lower bias potential compared with pristine TiO2. Incident photon-toelectron conversion efficiency measurements highlighted a synergetic effect between Au plasmon sensitization and rGO-mediated facile charge separation/transportation, which is believed to significantly enhance the PEC activity of these nanostructures under simulated and visible light irradiation. Under the selected operating conditions the incorporation of Au NPs and rGO into TiO2 resulted in a remarkable boost in the H2 evolution rate (17.8 μmol/cm2) compared to a pristine TiO2 photoelectrode reference (7.6 μmol/cm2). In line with these results and by showing excellent stability as a photoelectrode, these materials are herin underlined to be of promising interest in the PEC water splitting reaction. KEYWORDS: TiO2−graphene, surface plasmons, inverse opal, photocatalysis, photoelectrochemical water splitting

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INTRODUCTION The direct conversion of solar energy into storable and transportable chemical fuels remains to date a challenging issue in sustainable energy research fields. Photoelectrochemical (PEC) water splitting, in particular, has emerged as a promising technology that allows the direct conversion of solar light energy into chemical fuels such as hydrogen.1−3 TiO2 has been italicized as a benchmark electrode material for PEC water splitting, considering its excellent stability in electrolytes under illumination, abundance, and environment friendly and costeffective nature.3−5 Regardless, the large over potential for H2 generation, the rapid recombination of photogenerated electrons and holes, and the inability to absorb visible light still represent important bottlenecks to further enhance the photoactivity of TiO2.4,5 Accordingly, attained conversion levels of a pristine TiO2 photoelectrode remain limited even in the UV region. Notwithstanding the number of efforts made to extend the application of TiO2 into the visible (Vis) and © 2017 American Chemical Society

infrared (IR) regions, charge separation and transportation of photogenerated carriers have remained critical challenges to achieve meaningful photoconversion efficiency with TiO2-based photoelectrodes.6,7 Plasmon enhanced energy conversion has been considered an effective approach to overcome the above-mentioned limitations of PEC photoelectrodes.6−11 Under light illumination plasmonic metal nanostructures have been shown to convert incident photon energy to plasmonic energy. The latter is then transferred from the metal to the wide band gap semiconductor via plasmon-induced resonant energy transfer (PIRET) and/or direct electron transfer (DET) mechanisms.12−16 This energy transfer mechanism not only improves the light absorption of the photoelectrode but also induces Received: November 14, 2016 Accepted: February 7, 2017 Published: February 7, 2017 7075

DOI: 10.1021/acsami.6b14618 ACS Appl. Mater. Interfaces 2017, 9, 7075−7083

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ACS Applied Materials & Interfaces charge separation in the PEC device.12−15 However, the average lifetime of hot electrons ( 0.32 mA cm−2 at 1.23 V vs RHE) (Figure 6a). The result reflected a synergism between TiO2 and the thin layer, corroborated by the photocurrent density generated under light illumination in the sole presence of the TiO2 film (Figure S6). The integration

Figure 5. Diffused reflectance UV−vis spectra of (a) TiO2@Au at different Au deposition times and (b) all prepared photoelectrodes. 7078

DOI: 10.1021/acsami.6b14618 ACS Appl. Mater. Interfaces 2017, 9, 7075−7083

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ACS Applied Materials & Interfaces

maximum photocurrent, thus resulting in an improved PEC H2 generation. Upon contact with TiO2, the Schottky barrier formed at the semiconductor interface alters the band alignment,46,47 resulting in a change in the flat band potential (Efb). The Efb directly influences the onset potential due to the shift in the Fermi level toward more negative potentials. The Efb values of the as-prepared photelectrodes were further deduced from the Mott−Schottky plots measured under dark condition (Figure 6c). Compared to the TiO2 reference, the Efb values were negatively shifted for Au@TiO2 and Au@rGO/TiO2 with values of −0.018 and −0.031 V vs RHE, respectively. The change in the Efb shifts the Fermi level toward the conduction band, thus reducing the onset potential.48 The aforementioned cathodic shift represents an efficient charge separation and transportation in the TiO2@Au compared to that of pristine TiO2. No shift in the Efb could be observed upon addition of rGO, corroborating previous reports showing that graphene does not alter the flat band potential.48 PEC water-splitting experiments were carried out using a three-electrode system described in the Experimental Section. The amounts of H 2 and O 2 were analyzed by gas chromatography equipped with a thermal conductivity detector (TCD). H2 and O2 dissolution in the electrolyte solution and gas leakage were considered minor. Results have been summarized in Figure 7. The resulting ratio of H2 and O2

of this TiO2 layer further induced a 0.145 V cathodic shift of the photocurrent onset potential. When using Na2SO3, herein selected as a suitable hole scavenger agent as suggested elsewhere,38 the presence of the TiO2 layer was confirmed to lead to an enhanced charge transfer efficiency (η) (Figure S7). The induced shift was mainly attributed to a blocked back electron injection from FTO to the photoelectrode, in particular at lower bias potential.39,40 The deposition of Au NPs was adjusted to maximize the resulting photocurrent of the prepared samples (Figure 6b). A photocurrent decline with a continuous increase of the Au content was attributed to a decrease of both the photon flux reaching the TiO2 surface and the exposed metal oxide surface area.7 Conversely, a continuous augment of the rGO content was observed to lead to a decrease of the onset potential and the corresponding current density of the prepared materials (Figure S8). Similarly, this was attributed to a reduction of the TiO2/electrolyte interfaces where the photochemical reaction takes place. The photocurrent density of both TiO2@Au (for 6 h deposition time) and TiO2@rGO (coating with 0.5 g/L GO suspension) electrodes at optimized condition was 0.8 and 0.76 mA cm−2, unveiling a boost of ca. 70% and 60% compared to the TiO2 reference, respectively (Figure 6a). The simultaneous integration of rGO and Au NPs resulted in the highest photocurrent density achieved among the evaluated samples (1.29 mA cm−2 at 1.23 V vs RHE). The value, representing a 2fold increase compared to the photocurrent density attained with pristine TiO2 at the same potential value, was believed to be the result of the plasmonic effect of Au NPs and the incorporation of rGO. The plasmonic effect of Au NPs was evaluated in detail in the following section. The excellent conductivity and electron-accepting nature of rGO were herein hinted to effectively enhance the charge carriers’ transport from TiO2, thereby increasing the photocurrent density of TiO2. The assumption was corroborated by collected electrochemical impedance spectroscopy (EIS) data summarized in Figure 6d.41,42 The small charge transfer resistance of TiO2@Au@rGO is ascribed to the superior charge transport properties of rGO.43 The enhanced photocurrent was further attributed to an increase of the carrier density (Nd), determined from the slope of the Mott−Schottky plot using eq 1, upon integration of rGO and Au NPs35 Nd = (2/e0εε0)[d(1/C 2)/dv]−1

Figure 7. Evaluation rates of H2 and O2 under simulated sunlight illumination at 1.23 V vs RHE with the four prepared photoelectrodes.

(1)

with e0 being the electron charge, ε the dielectric constant of anatase TiO2 (55 for anatase),44 ε0 the permittivity of vacuum, Nd the carrier density, and V the applied bias at the electrode. The calculated carrier densities followed the order TiO2@ rGO@Au (8.3 × 1019) > TiO2@Au (3.3 × 1019) > TiO2@rGO (2.93 × 1019) > TiO2 (1.01 × 1019). Whereas a higher carrier density is expected upon integration of rGO, the rather high value with the loading of Au NPs was however attributed to an electron density increase at the Au/TiO2 interface following a facile extraction of electrons from the conduction band of TiO2.45 During their PEC performance, samples were further compared according to their corresponding onset potential. In particular, TiO2@Au unveiled a clear cathodic shift compared with a pristine TiO2 reference, the same tendency being observed with TiO2@rGO@Au. In both cases the result was ascribed to the plasmonic effect of the Au NPs (Figure S9). Achieving a lower photocurrent onset potential is of prime importance to reduce the applied bias required to attain the

evolution rates was close to the stoichiometric value of 2.0 for all photoelectrodes. The hydrogen evolution rates followed the order TiO2@rGO@Au (17.8 μmol/cm2) > TiO2@Au (13.6 μmol/cm2) > TiO2@rGO (11.1 μmol/cm2) > TiO2 (7.6 μmol/ cm2). Our novel TiO2@rGO@Au material further showed a slightly improved Faradaic efficiency (93.9%) compared with the reference TiO2 sample (91.1%). The applied bias photon-to-current efficiency (ABPE) of each photoelectrode was calculated based on eq 249,50 ABPE =

Jph (1.23 − V ) plight

(2)

where Jph is the photocurrent density measured in a twoelectrode configuration, V the applied bias between working electrode and counter electrode, and Jlight the incident light intensity (100 mW cm−2; AM 1.5G) as measured by a calibrated Si cell. A similar trend in the J−V curve was observed in a two-electrode configuration as shown in Figure S10. Figure 7079

DOI: 10.1021/acsami.6b14618 ACS Appl. Mater. Interfaces 2017, 9, 7075−7083

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indexed to the prolonged lifetime of photogenerated charge carriers. To shed additional light on the obtained results, the prepared nanostructures were selectively evaluated under visible light irradiation, i.e., ≥420 nm (Figure 8c). The low photocurrent density generated with TiO2 (0.01 mA cm−2) was ascribed to a light-trapping phenomenon of the inverse opal structure from multiple light scattering. Most importantly, despite the significant absorbance increase noted in Figure 5b, the hybridization of TiO2 with rGO was herein confirmed to solely result in an increase of photocurrent density. Conversely, a more prominent increase under visible light was observed upon decoration of TiO2 with Au NPs. The resulting material revealed a 10-fold enhancement in photocurrent density under the same operating conditions, which clearly suggest a contribution of the SPR of Au NPs to the PEC activity in visible light. This effect corroborates the remarkable ICPE enhancement observed in Figure 8b. Regarding the TiO2@ rGO@Au photoelectrode, results evidenced in Figure 8b and 8c suggest a synergism between the Au NPs, rGO, and TiO2. Indeed, the obtained IPCE value obtained does not seem to only rely on an additional effect between the incorporation of rGO and Au NPs. To assess the instant photoresponse of the prepared electrodes, current density versus potential (J−V) measurements under on−off light illumination switching were obtained (Figure S11). A steep increase of transient photocurrent was observed upon light illumination, with an immediate decrease of the current density in the dark. In addition, to verify the practical applicability of the evaluated photoelectrodes, corresponding stability levels were assessed through chronoamperomertic i−t curves at 1.23 V versus RHE (Figure 8d). With photocurrent density values constant over 9000 s and consistent with the values obtained from the linear sweep voltammagrams, we believe these materials evidence excellent stability. Insights of the Mechanism. Previous results have suggested that the observed enhancement in IPCE in the visible region is attributed to the SPR absorbance of Au NPs, whereas a further increase in the UV−vis wide band region was attributed to the improved charge transfer efficiency by the presence of rGO. Our experimental results further confirm the aforementioned observation. The assumed synergism between the Au NPs, rGO, and TiO2 and the possible charge transfer mechanism in the TiO2@rGO@Au system have been elucidated in Figure 9. Enhanced light absorption and charge generation are based on the direct hot electron transfer from the metal to the semiconductor (DET)13,15,52,53 and/or excitation of electron− hole pairs in the semiconductor by transferring plasmon

8a shows the calculated ABPE as a function of the external potential (V vs Pt). The pristine TiO2 sample exhibited an

Figure 8. (a) Applied bias photon-to-current efficiency (ABPE, %) as a function of applied potential employing two electrodes. (b) Incident photon-to-electron conversion efficiency (IPCE) as a function of wavelength at 1.23 V vs RHE. (c) Amperometric I−t curves of photoelectrodes measured at 1.23 V vs RHE with repeated on−off cycles under visible light irradiation. (d) Chronoamperometry measurements at 1.23 V vs RHE revealed the superior stability of the resulting photoelectrodes over 9000 s.

optimal ABPE efficiency of 0.15% at 0.77 V vs Pt. A remarkable boost in the ABPE efficiency could be observed with both TiO2@rGO and TiO2@Au samples, attaining 0.26% and 0.24% values at 0.69 and 0.77 V vs Pt, respectively. Most remarkably, the TiO2@rGO@Au sample achieved a maximum conversion efficiency of 0.38% at 0.68 V vs Pt, representing a striking 2.5fold enhancement compared to the TiO2 reference. Incident photon-to-electron conversion efficiency (IPCE) determined as a function of the wavelength at 1.23 V vs RHE unveiled an overall increase compared with the pristine TiO2 sample. IPCE, calculated according to eq 3, is a suitable parameter to characterize the efficiency of different photoelectrodes, as it does not depend on selected light sources IPCE = (1240I )/(λJlight )

(3)

with I being the measured photocurrent density at a specific wavelength, λ the wavelength of incident light, and Jlight the measured irradiance at a specific wavelength. The sequential loading of rGO and Au NPs was confirmed to substantially enhance IPCE over the UV region (Figure 8b). Upon coupling of a plasmonic metal with a semiconductor, the resulting energy transfer not only improves the light absorption of the photoelectrode but also induces charge separation in the TiO2@Au-based PEC device. In addition, the Au NPs show a noticeable electric field in the wavelength range between 300 and 450 nm, which is overlapped with the absorption edge of TiO2.51 Most notably, the PEC activity could also be extended to the visible wavelength region for samples incorporating Au NPs. Interestingly, however, despite the remarkable absorption increase upon introduction of rGO in the entire visible region (Figure 5b), the IPCE spectrum was not significantly enhanced with TiO2@rGO in the corresponding wavelength range. In this case, the enhanced IPCE in the UV region was thus

Figure 9. Schematic representation of the proposed mechanism illustrating the charge separation and transportation in the TiO2@ rGO@Au photoelectrode under light illumination. 7080

DOI: 10.1021/acsami.6b14618 ACS Appl. Mater. Interfaces 2017, 9, 7075−7083

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ACS Applied Materials & Interfaces resonance energy (PRET) mechanism.15,52,53 The latter has been reported to occur when the SPR absorption region of the plasmonic nanostructure and the absorption band of the semiconductor overlap.9 Herein, given that no spectral overlap occurs between Au and TiO2, the plasmonic energy transfer is assumed to be governed by a DET mechanism. The observed IPCE for TiO2@Au photoelectrode at ca. 522 nm was regardless weak. The weak DET effect in TiO2@Au is due to the Schottky barrier present at the Au/TiO2 interface hindering a charge transfer from the plasmon to the semiconductor. Upon contact of the Au NPs with TiO2, the ∼1.0 eV Schottky barrier formed at the Au/TiO2 interface is expected to hinder the transfer of electrons to TiO2.45,54 Under light irradiation, however, the energy of the hot electrons generated in the SPR excitation process is above 1.0 eV with respect to the Au Fermi level.45 The energetic electrons are believed to overcome the Schottky barrier and be transferred to the CB of TiO2, leading to an effective charge separation.51,55 The hot electrons in TiO2 are then consecutively transferred to the rGO sheets, in agreement with the lower redox potential of graphene compared to the CB of TiO2.54−56 A superior charge transfer efficiency was accordingly evidenced in the presence of rGO (Figure S13). Due to the high electron-accepting nature of rGO, upon light illumination the latter may also directly accept hot electrons from Au in the excited state, as the Fermi level of graphene is shifted below the Dirac point.19,57,58 However, the hot electron transfer and corresponding transfer efficiency from Au to rGO is relatively low compared with the transfer efficiency from Au to TiO2.57 Regardless, this process remains of prime importance to reduce the recombination of energetic species that are not rapidly transferred from Au to TiO2. The charge separation is therefore assumed to occur at both Au/ rGO and Au/TiO2 interfaces. The charge separation and transportation in TiO2@rGO@Au under light irradiation can be understood, as illustrated in Figure 9. The excellent charge transport properties with superior conductivity of rGO could capture the photogenerated electrons and accelerate the separation and transportation process to the respective electrode.54,59,60 In addition, this process is assumed to hinder charge recombination between electrons in the semiconductor material and holes in the Au NPs. The mechanism herein discussed highlights the key role of both rGO and Au NPs in improving the PEC activity of TiO2. As shown, we believe that the TiO2@rGO@Au sample unveils a synergism of prime importance leading to an enhanced PEC activity compared to the TiO2 reference and both TiO2@rGO and TiO2@Au samples.

excellent electron transport properties of rGO further increased the charge carrier lifetime, leading to an increase in both the charge extraction and the transport efficiency. As a result of the synergism found between rGO and the Au NPs a notable enhancement in the IPCE was attained. TiO2@rGO@Au photoelectrode was further shown to achieve a H2 evolution rate of 17.8 μmol/cm2, reflecting a nearly 2.5-fold boost compared to the pristine TiO2 sample. These results and the excellent PEC activity of TiO2@rGO@Au photoelectrode are believed to highlight this material as a promising candidate for practical PEC hydrogen generation.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b14618. Synthesis of polystyrene beads, graphene oxide, and gold colloidal solution, characterization tools, additional SEM images of hybrid structures, calculation of charge transfer efficiency and its related graphs (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; Fax: +82-2-3277-4546; Tel.: +822-3277-4517. ORCID

Ramireddy Boppella: 0000-0002-0370-5490 Filipe Marques Mota: 0000-0002-0928-3583 Jong Hyeok Park: 0000-0002-6629-3147 Dong Ha Kim: 0000-0003-0444-0479 Present Address §

S.T.K.: Institute of Applied Physics, Johannes Kepler University Linz, Altenberger Strasse 69, 4040 Linz, Austria.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Research Foundation of Korea Grant funded by the Korean Government (2014R1A2A1A09005656; 2011-0030255).

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REFERENCES

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CONCLUSIONS In this work we designed a hybrid photoelectrode composite for photoelectrochemical (PEC) water splitting in which Au nanoparticles (NPs) and reduced graphene oxide (rGO) were incorporated into 3D TiO2 inverse opal structures. Incorporation of a TiO2 thin layer spin coated on the FTO substrate was confirmed to significantly reduce back recombination. Compared to a pristine TiO2 reference the resulting material (TiO2@rGO@Au) revealed a remarkable improvement in PEC activity and a 2.5-fold applied bias photon-to-current efficiency increase due to its improved light-harvesting ability and faster charge transfer. The surface plasmon resonance effect of Au NPs was confirmed to increase the spectral response of TiO2 to the visible range, to hinder charge recombination due to Fermi level equilibration, and to reduce the onset potential. The 7081

DOI: 10.1021/acsami.6b14618 ACS Appl. Mater. Interfaces 2017, 9, 7075−7083

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DOI: 10.1021/acsami.6b14618 ACS Appl. Mater. Interfaces 2017, 9, 7075−7083