Visible Light Driven Photoelectrochemical Properties of Ti@TiO2 ...

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Visible Light Driven Photoelectrochemical Properties of Ti@TiO2 Nanowire Electrodes Sensitized with Core−Shell Ag@Ag2S Nanoparticles Zhichao Shan,† Daniel Clayton,† Shanlin Pan,*,† Panikar Sathyaseelan Archana,† and Arunava Gupta*,†,‡ †

Department of Chemistry, The University of Alabama, Tuscaloosa, Alabama 35487-0336, United States Department of Chemical and Biological Engineering, The University of Alabama, Tuscaloosa, Alabama 35487-0336, United States

‡

S Supporting Information *

ABSTRACT: We present a model electrode system comprised of nanostructured Ti electrode sensitized with Ag@Ag2S core−shell nanoparticles (NPs) for visible light driven photoelectrochemistry studies. The nanostructured Ti electrode is coated with Ti@TiO2 nanowires (NW) to provide a high surface area for improved light absorption and efficient charge collection from the Ag@Ag2S NPs. Pronounced photoelectrochemical responses of Ag@Ag2S NPs under visible light were obtained and attributed to collective contributions of visible light sensitivity of Ag2S, the local field enhancement of Ag surface plasmon, enhanced charge collection by Ti@TiO2 NWs, and the high surface area of the nanostructured electrode system. The shell thickness and core size of the Ag@Ag2S core− shell structure can be controlled to achieve optimal photoelectrochemical performance. XPS, XRD, SEM, high resolution TEM, AC impedance, and other electrochemical methods are applied to resolve the structure−function relationship of the nanostructured Ag@Ag2S NP electrode. electromagnetic fields, separating electrons from holes at a semiconductor surface, scattering electromagnetic radiation, and converting incident photons into hot electrons.17−19 Attempts of coupling plasmonic nanostructures to photocatalytic semiconductors have been made to enhance solar light absorption cross sections of catalytic semiconductors.20−26 In order to maximize the surface-enhanced efficiency, there are several technical challenges which need to be addressed: (1) How to maintain the stability of the plasmon nanostructures during the course of the photocatalytic reaction? (2) How to systematically control the thickness of the catalytic layer27 near a plasmonic antenna system for an improved understanding of the distance dependence of the surface enhancement mechanism? (3) How to maximize the light absorption of the photocatalytic layer while maintaining a thickness suitable for optical plasmon enhancement? Core−shell nanostructures have been recently explored to enhance the efficiency of charge collection by shortening the charge carrier transport distances.28−30 Photocatalytic performance of these core− shell NPs can be optimized by their constituent materials31−34 as well as their core size, shell thickness, and core-to-shell ratio.35−37 We recently studied surface-enhanced Raman scattering (SERS) and electrogenerated chemiluminescence (ECL) effects at polydisperse Au nanoparticles electrodeposited onto a

1. INTRODUCTION As a sustainable and clean energy source, solar energy provides great opportunity to meet the world’s future energy challenges. Solar energy can be harvested for direct electricity generation using solar cells or producing chemical energy sources such as hydrogen via solar water splitting. Enormous effort has been invested in the past few decades in identifying and optimizing photoactive materials for solar energy conversion by tailoring their compositions and morphologies1−3 since Fujishima and Honda’s first experiment demonstrating direct water splitting using the TiO2 photoelectrode.4 Such direct water splitting system via solar energy has been sought for more than three decades and is presently being intensely investigated.5 TiO2 remains the most studied photocatalysts, yet its wide bandgap (3.2 eV) limits operation to wavelength of light less than 400 nm, leading to low conversion efficiencies for hydrogen production using solar radiation. Recent results also show vertically aligned transparent TiO2 and many other nanocomposite materials can be used for solar cell and water splitting applications.6−9 Doped TiO2 materials have also been explored for water splitting using visible light.10−13 Recent attempts to identify more suitable semiconductors have focused on non-TiO2 materials, such as new types of oxynitrides [e.g., (Ga 1−xZnx) (N1−xOx].14 Practical applications of these materials are limited by low-energy conversion efficiency because of their limited light absorption, slow charge separation and transport, and/or slow kinetics of water splitting reactions.15,16 Plasmonic metal nanostructures represent a promising class of materials due to their unique capacities of concentrating © 2014 American Chemical Society

Special Issue: Spectroscopy of Nano- and Biomaterials Symposium Received: May 3, 2014 Revised: July 7, 2014 Published: July 10, 2014 14037

dx.doi.org/10.1021/jp504346k | J. Phys. Chem. B 2014, 118, 14037−14046

The Journal of Physical Chemistry B

Article

nanostructured Ti@TiO2 NW electrode.38 Such polydisperse plasmonic nanostructures support a broad wavelength region for enhanced optical spectroscopy studies and large surface areas for electrochemical applications. Here, we present a nanostructured electrode system comprised of Ag@Ag2S core− shell NPs attached to a Ti@TiO2 NW photoelectrode substrate. This integrated NW and core−shell NP structures will greatly help address the challenging issues outlined above regarding surface-enhanced photocatalytic systems. The Ag2S shell thickness and Ag core size can be controlled by simply controlling the electroless deposition time of the respective constituents. Pronounced visible light responses are obtained and attributed to collective contribution of visible light sensitive Ag2S, enhanced charge collection by the Ti NWs, large surface area of the nanostructured electrode, and plasmon local field enhancement.

Figure 1. Schematic Ti@TiO2 NW electrodes modified with core− shell Ag@Ag2S NPs for surface-enhanced photoelectrochemical studies under visible light.

tometer. XRD patterns were obtained using a Bruker D2 phaser diffractometer. 2.4. Photoelectrochemistry Measurements. Action spectra and I−V curves were obtained using a home-built photocurrent testing system comprised of a glass cell, a Keithley 2400 source meter, and a xenon arc lamp (Newport 66902) coupled with an Oriel 1.5 air mass (AM) spectral filter. An additional 400 nm long pass filter was used when only visible light was needed. The photocurrent response to the incident light wavelength (or action spectrum) was measured in a two electrode system with a spectrometer (MD 1000, Optical Building Blocks) coupled to the solar simulator. A Pt counter electrode was used. Electrochemical impedance spectra (EIS) of samples were measured in a three-electrode cell using a CHI 760C potentiostat at DC potential of 0 V versus Ag/AgCl and an AC potential frequency range of 10000−0.1 Hz with an amplitude of 10 mV in 0.1 M NaOH electrolyte. A commercially free software (ZsimpWin) was used for fitting the experimental EIS data. Mott−Schottky plots were obtained with an AC potential frequency 1000 Hz under light and dark conditions. The amplitude of AC potential of 10 mV was used for the potential scan.

2. EXPERIMENTAL SECTION 2.1. Synthesis of Ti@TiO2 NWs Substrate. The Ti@TiO2 NWs were fabricated using a hydrothermal reaction process as described in our recent work.38 Ti plates of 99% purity with dimensions of 15 × 15 mm and thickness of 0.50 mm were first cleaned with acetone in an ultrasonic bath for 20 min and then rinsed in excess DI water. The cleaned Ti plates were heated at 190 °C for 12 h in a 23 mL Teflon-lined stainless steel autoclave containing 10 mL 0.6 M HCl solution. After cooling to room temperature, the as-prepared Ti NW substrates were thoroughly washed with distilled water and dried in air. Subsequently, the samples were annealed at 450 °C for 2 h (in air) and allowed to cool to room temperature to form the desired Ti@TiO2 NW substrate. The presence of the TiO2 layer was confirmed by XRD and its photocatalytic capability under ultraviolet (UV) light. 2.2. Synthesis of Ag@Ag2S Core−Shell NPs on Ti@ TiO2 NW Substrates. Ag NPs were synthesized by the Tollens reaction.39 Typically, 0.2 mL 0.1 M NaOH was added to 10 mL of 0.01 M AgNO3 solution prior to dropwise addition of 1.0 M ammonia solution until a clear solution containing a silver− ammonia complex [Ag(NH3)2]+ was obtained. 10 mL of 0.05 M dextrose solution was then added into the above fresh [Ag(NH3)2]+ solution then freshly prepared Ti@TiO2 NW substrates were quickly dipped into the above solution under stirring. The color of the solution changed to a dark brown, indicating the formation of Ag NPs in the solution. The amount of the Ag NPs attached to the Ti@TiO2 NW substrates was varied by altering the substrate dipping time. The Ag NPs/Ti@ TiO2 NWs were taken out and washed in excess DI water. Ag@ Ag2S core−shell NPs were obtained by sulfurizing the Ag NPs in 20 mL of 8 mM sulfur/DMSO solution at 100 °C to convert the surface of Ag NPs to Ag2S shells. The shell thickness was controlled by the reaction time duration. The as-prepared sample was finally washed with CS2 (caution: highly flammable, toxic, and irritant) and acetone and dried, sequentially, at room temperature to obtain Ag@Ag2S core−shell NPs on Ti@TiO2 substrates as shown in Figure 1. 2.3. Structural Characterization. Nanostructure morphologies and high-resolution images were characterized with a JEOL 7600F field emission scanning electron microscope (SEM) and a FEI Tecnai F-20 transmission electron microscope (TEM). X-ray photoelectron spectroscopy was performed using a Kratos XIS 165 system. Absorbance spectra were measured using a Varian Cary 50 UV−vis spectropho-

3. RESULTS AND DISCUSSION 3.1. Morphology and Structural Characteristics of Ti@ TiO2 NWs, Ag NP/Ti@TiO2 NWs, and Ag@Ag2S NPs/Ti@ TiO2 NWs. Ti@TiO2 NWs with diameters varying from 50 to 100 nm and lengths of up to 500 nm, such as those in Figure 2A, have been synthesized. The estimated surface coverage of Ti@TiO2 NWs from SEM is about 6 × 109/cm2. The estimated oxide layer from high-resolution TEM is around 2 nm with some variation from wire-to-wire. The detailed hydrothermal reaction mechanism for the formation of NWs with a rectangular shape is hypothesized to be dependent on the selective chemical etching of Ti under high temperature and pressure in HCl. The dimension and coverage of the NWs are found be to very sensitive to the purity of the Ti substrate and its surface roughness. For example, an electropolished Ti substrate produces densely packed and short NWs, while the as-received Ti plate yields a standing NW structure. The detailed reaction mechanism for such NW formation is yet to be fully understood. Figure 2B shows a top-view SEM image of Ti@TiO2 NWs coated with Ag@Ag2S NPs obtained by dipping the Ag NP-coated NW electrode in S/DMSO solution for 10 s at 100 °C. Small Ag@Ag2S NPs with sizes around 25 nm (in diameter) are densely coated onto the Ti@TiO2 NWs with their density and sizes determined by the electroless deposition time of silver. SEM studies show no significant morphological 14038



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Figure 3. Anodic polarization curves of freshly prepared Ag/Ti@TiO2 NW electrodes in 0.1 M NaOH before and after treatment with S/ DMSO solution for 1, 3, 5, and 10 s. Scan rate: 0.2 V/s.

direct observation of possible surface-enhanced photoelectrochemical studies because a background current as low as possible is needed from the Ag NP electrode while benefiting from its plasmon enhancement under visible light irradiation. We also ensured the Ti@TiO2 NWs are coated with a thin layer of TiO2 to decrease the dark current density as we scan the potential in positive direction for following photoelectrochemistry studies. Prior to measuring photoelectrochemical performance of the nanostructured electrode system, XPS, XRD, and high-resolution TEM have been used to investigate the surface bonding energies of Ag/Ag2S, its crystallinity, and lattice spacing, respectively. The XPS data for the binding energy of Ag and Ag@Ag2S is depicted in Figure 4.

Figure 2. Typical SEM images of bare Ti@TiO2 NWs prepared on Ti substrate using (A) hydrothermal reaction and (B) Ti@TiO2 NWs coated with Ag@Ag2S NPs using the electroless deposition method. Figure 4. XPS spectra of Ag/Ti@TiO2 NWs and Ag@Ag2S/Ti@TiO2 NWs showing shift in binding energy of Ag 3d peaks.

difference before and after converting Ag NPs to Ag2S NPs. The presence of Ag@Ag2S NPs is confirmed by scanning transmission electron microscopy (STEM) as shown in Figure S1 of the Supporting Information. The Ag2S layer serves as a photocatalytic substrate under visible light and an insulating protection layer for the Ag core. To confirm how well the Ag2S top layer would prevent corrosion of the Ag core, the obtained Ag@Ag2S NPs have been tested using a dynamic potential control method (linear or cyclic voltammetry) in 0.1 M NaOH. An anodic peak is expected for partially coated or bare Ag NPs in NaOH solution because of the formation of Ag2O as shown in Figure 3. Longer dipping times for Ag NPs in S/DMSO solution yield lower anodic current; no Ag anodic current is obtained after dipping the Ag NPs for 5 s, indicating the Ag NP cores are fully protected by a Ag2S shell. The anodic current turn-on potential Eon, which is defined by the potential where the anodic current starts rising, increases when the substrate dipping time increases from 1 to 5 s and is accompanied by a decrease in the anodic current, indicating the coated Ag NPs are resistive to corrosion in NaOH. This is essential for the

The peak positions at 368.5 and 374.5 eV are attributed to the Ag 3d5/2 and 3d3/2 energy levels. These two peaks shift to 368.1 and 374.1 eV, respectively, after the Ag NPs are converted to Ag2S. This result is consistent with the previously reported data for Ag2S.40,41 Figure 5 shows the XRD patterns of bare Ti@TiO2 NWs, Ag NP/Ti@TiO2 NWs, and Ag@Ag2S NPs/Ti@TiO2 NWs. XRD of the bare Ti@TiO2 NWs show diffraction peaks of both the Ti substrate and the two phases of TiO2, anatase and rutile, coexisting in the NW structure. After coating with Ag NPs, two Ag diffraction peaks, ⟨200⟩ and ⟨220⟩, are observed at 44° and 64°, respectively. After converting the Ag NPs to Ag@Ag2S NPs, four Ag2S diffraction peaks are observed at 28.7°, 31.2°, 33.3°, and 34.1°, corresponding to Ag2S ⟨111⟩, ⟨112⟩, ⟨120⟩, and ⟨121⟩, respectively. Other peaks from Ag, Ti, and TiO2 remain after the Ag2S coating, further confirming that Ag and Ag2S coexist on the Ti@TiO2 NW electrode. To resolve the core−shell structure adopted by the Ag2S−Ag NPs, high-resolution TEM 14039



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the interior of the Ti@TiO2 NWs and Ag NPs. To further probe the concentration spatial distribution at the nanometer scale for S and Ag in single core−shell unit, combined STEM and EDS methods are used to probe the element spatial distribution with electron beam scan across a selected Ag@ Ag2S NP. As shown in Figure 6D, the black and red patterns show concentration profiles of S and Ag across a selected Ag@ Ag2S particle with size of 23 nm in diameter indicated by the black arrow in Figure 6C. For the 23 nm particle, the shell thickness is about 8 nm thick, and the core size is about 15 nm. On the basis of the TEM images and EDS line profile data, Ag@Ag2S core−shell structures prepared with a 10 s dip time in DMSO/S solution produces NPs with a top shell thickness around 8 nm. 3.2. Absorption Spectra and Average Ag2S Shell Thickness Measurement. To eliminate the strong light scattering effect of Ti@TiO2 NWs, diffuse reflectance absorption spectra of Ag and Ag2S are obtained on glass slides to characterize the light absorption characteristics of the nanostructured Ag2S and bare Ag electrodes. As shown in Figure 7, the glass substrate treated with Tollen’s reaction has a

Figure 5. XRD patterns of Ti@TiO2 NWs, Ag/Ti@TiO2 NWs, and Ag@Ag2S/Ti@TiO2 NWs.

Figure 7. UV−vis spectra of Ag NPs before (black) and after (red) treatment with S/DMSO solution to form Ag2S NPs. The spectra were taken for Ag NPs prepared on bare cover glass rather than the Ti@ TiO2 NW substrate in order to eliminate strong light scattering and background signal.

Figure 6. High-resolution TEM images of (A) a selected single Ag and (B) Ag@Ag2S NPs detached from a Ti@TiO2 NW electrode with lattice structure and spacing labeled in the figures. STEM image of a selected Ag@Ag2S NP attached to a Ti@TiO2 NW and element concentration profiles of S and Ag are shown in panels C and D, respectively.

major plasmon absorbance peak of Ag NPs near 450 nm. The Ag2S coating shows a dramatic decrease in the plasmon peak accompanied by an obvious absorption peak broadening in both the UV and visible light region above 600 nm. The extrapolated band gap Ebg of Ag2S is estimated to be around 2 eV. This is near the band gap of nanostructure Ag2S in the literature.42,43 Control of the shell thickness for the Ag@Ag2S NPs is obtained by varying the dipping time of Ag NPs in S/DMSO solution. Yet determining the average shell thickness by highresolution TEM for polydisperse NPs would be timeprohibitive. To determine the average shell thickness, the total charge for reduction of Ag2S to Ag is measured (cf. Figure 8) and used to calculate the thickness of Ag2S on top of Ag by following the Faraday’s law. The average sizes of the core−shell nanostructure were obtained from previous TEM study for the shell thickness calculation. The collected total charge of Ag2S reduction reaction increases with the dipping time; the reduction peak position shifts toward a more negative potential because of the increase in the film resistivity with the increase

was used to determine the lattice spacing and spatial distribution of bare Ag and Ag@Ag2S NPs. Analysis of all NPs using this technique would be time-prohibitive. Therefore, high-resolution TEM images are presented in Figure 6 for more detailed structural information. The bare Ag NP with a size of 22 nm in diameter shows a lattice spacing of 2.3 Å, corresponding to the ⟨111⟩ plane of Ag. After treating with S/DMSO solution for 10 s, a single Ag NP coated with Ag2S is imaged and found to have a size of 30 nm and lattice spacing of 2.8 Å attributed to the ⟨122⟩ plane of Ag2S. The Ag core located near the side of a Ti@TiO2 NW has a lattice spacing of around 2.04 Å, corresponding to the ⟨200⟩ plane of Ag. The asymmetric spatial distribution of Ag2S shell thickness with the thickest portion pointing far away from the surface of the Ti@ TiO2 NW is attributed to the limited mass transfer of sulfur to 14040



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Ag NP are sensitive to the particle size and their coverage on Ti@TiO2 NWs as shown in Figure S3 of the Supporting Information. Peak broadening and smaller amplitude photocurrent for extended silver deposition time are attributed to plasmon coupling and increase in light scattering by silver nanoparticles. After coating the Ag NPs with Ag2S, enhanced and broad photocurrent response to visible light was obtained in contrast to the pristine Ag NPs/Ti@TiO2 photoelectrode. In the UV region, increase in the amount of Ag2S leads to growth in the photocurrent intensity due to the UV absorbance of Ag2S. In the visible region, the photocurrent increases with increasing coating of Ag2S shell and then decreases if the Ag NPs are overdeveloped in the DMSO/S solution. Figure 10 shows the I−V curves of the photoelectrodes under visible light irradiation from a solar simulator with radiation in the UV wavelength range blocked by a broad-band filter (>400 nm). Photocurrent density of Ag NPs/Ti@TiO2 is 39 μA/cm2 at 0.2 V, which is slightly higher than 32 μA/cm2 of the bare Ti@TiO2 NW electrode at the same bias. After being sulfurized, Ag NPs show improved photocurrent and the amplitude of enhancement depends on the sulfurization time which was used to control the Ag2S shell thickness. A 2-fold increase in photocurrent is obtained for Ag@Ag2S NP electrodes with the Ag2S shell thickness of 5 and 8 nm. Further increase of shell thickness yields smaller photocurrent increase as shown by photocurrent density of film with 14 and 20 nm Ag2S coating. Another notable feature of the action spectra is that only a single peak in the visible region is obtained for Ag2S after an extended period of sulfurization reaction, and the peak position shifts as we increase the silver particle coverage and sizes as shown in Figure S3 of the Supporting Information. Table 1 summarizes photocurrent measurable peak positions and their relative photocurrent intensities from samples obtained under various conditions of silver deposition and sulfurization time. The enhancement photocurrent with the thinner shell thickness from entries 1 to 5 is partially due to the surface-enhanced light absorption of Ag2S in the presence of plasmon active Ag core. This is confirmed by the presence of the first photocurrent peak λ1 near 460 nm, corresponding to the plasmonic absorption of the silver core. Direct charge injection from the silver core is unlikely because of the complete coating of the sulfide layer as evidenced by our previous TEM and electrochemical reduction of Ag2S shell studies. Shell thickness of 8 nm is a short distance to produce improved photoexcitation because of the local field effect of the silver core architecture. On the other hand, the Ag cores also have excellent electrical contact to the Ti@TiO2 NWs as shown by the anodic polarization curve in Figure 3, therefore, electron−hole pairs can separate more effectively at the interface of Ag2S and Ag prior to being collected by the Ti@ TiO2 NWs. Moreover, the NWs provide a high surface area which allows the anchoring of a large number of Ag NPs. This structure yields a high optical density for effective light absorption by increasing the average effective optical path length, causing an increase in the population of the excited states in the ultrathin Ag2S shell layer. Further growth of the Ag2S shell thickness results in a small Ag core size, which is unlikely to provide any field enhancement. Sulfurization of bare Ag NPs always produces more photocurrent than bare Ag NPs (entries 6−9 of Table 1) because of the semiconductor characteristic of Ag2S itself. Large Ag2S particle size and higher coverage produce red-shift in photocurrent peak from 470 to

Figure 8. Cathodic reduction behavior of Ag2S shell of Ag@Ag2S/Ti@ TiO2 NW electrode for estimating the average thickness of the shell thicknesses obtained by controlling the dipping time in the DMSO/S solution. The total reduction charge is calculated and converted to Ag2S shell thickness as shown in the inset.

of the shell thickness. Shell thickness reaches 25 nm when the bare Ag NPs are overdeveloped in the DMSO/S solution. Ag2S shell thickness increases from 5 to 25 nm when the dip coating time increases from 5 to 100 s, as shown in the inset of Figure 8. 3.3. Photoelectrochemistry Results and Enhancement Mechanism. Figure 9 shows the effects of Ag NPs and Ag@

Figure 9. Action spectra of photoanodes comprised of Ti@TiO2 NWs and Ag/Ti@TiO2 NWs and Ag@Ag2S/Ti@TiO2 NWs with various shell thicknesses. Inset is the zoom-in figure showing only the visible light region of the action spectra. Corresponding IPCE is shown in Figure S2 of the Supporting Information.

Ag2S NPs on the photoelectrochemical properties of the Ti@ TiO2 NW electrodes. The Ti@TiO2 NW electrode only shows photocurrent response in the UV region. In contrast, the Ag NP/Ti@TiO2 NW electrode exhibit pronounced photocurrent under visible light illumination above 400 nm. The visible light response near the surface plasmon resonance region of Ag NPs in NaOH electrolyte is attributed to the charge injection directly from Ag NPs to Ti@TiO2 when their surface plasmons are excited.44 The photocurrent intensity decrease in the UV region upon Ag NP coating can be explained by the decreased UV light absorption by the underlying Ti@TiO2 NWs in the presence of Ag NPs, which partially blocks the UV light irradiation by the Ti@TiO2 NWs. Visible light response of bare 14041



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Figure 10. Dark and light I−V curves of photoanodes comprised of (A) Ti@TiO2 NWs and (B) Ag/Ti@TiO2 NWs, Ag@Ag2S/Ti@TiO2 NWs with (C) 5, (D) 8, (E) 14, and (F) 20 nm. Photocurrent was taken with an AM1.5 solar simulation with its UV light modified with a > 400 nm long pass filter to remove UV excitation. All I−V curves were taken in a two-electrode system with a Pt counter electrode.

Table 1. Estimated Photocurrent Peak Positions (λ1 and λ2) as Indicated in Figure 9 and Corresponding Peak Current without Background Current Subtraction, and Dependence on Growth Times of Silver Film and Sulfurization sample entry no.

t (min) silver mirror reaction time

t (sec) S/ DMSO

λ1 (nm)

iλ1 (μA)

λ2 (nm)

iλ2 (μA)

1 2 3 4 5 6 7 8 9 10 11 12 13 14

1.5 1.5 1.5 1.5 1.5 1.5 3 5 7 1.5 3 5 7 14

0 5 10 20 40 0 0 0 0 180 180 180 300 300

465 465 465 465 460 465 465 465 465 − − − − −

0.47 0.66 0.81 0.55 0.52 0.14 0.09 0.08 0.05

− 540 525 − − − − −

− 0.59 0.73 − − − − − − 0.15 0.12 0.1 0.06 0.03

470 535 570 585 620

Figure 11. Action spectra cycles of Ag@Ag2S/Ti@TiO2 NWs (10 nm shell thickness) in (A) 0.1 M NaOH, (B) 0.1 M sodium citrate (Na2C6H5O7), (C) 0.1 M Na2SO4 + 0.05 M Na2S, and (D) the photocurrent at 465 nm with various cycles; bias potential is 0.05 V.

620 nm and photocurrent decrease from 0.15 μA to 0.03 μA, as shown by entries 10−14 of Table 1. 3.4. Ag2S Photoanode Stability Testing. To test if Ag@ Ag2S NPs are suitable for long-term photocatalytic applications, we tested their temporal electrochemical stability properties in various electrolytes and at various electrode potentials. Stability testing results of Ag@Ag2S/Ti@TiO2 photoanode in NaOH, sodium citrate, and a mixture of sulfate and sulfide electrolytes are shown in Figure 11 by repeating the action spectra scanning under constant potential. Visible light-induced photocurrent in the region of 450 nm decreases over time in NaOH and sodium citrate as shown in Figure 11, panels A and B. This is attributed to the oxidation of silver and silver sulfide via the half reaction 2Ag + 2OH− − 2e− → Ag2O + H2O and/or Ag2S + 2OH− − 2e− → Ag2O + S + H2O to form the oxide layer. In contrast,

the photocurrent of Ag@Ag2S/Ti@TiO2 NWs in a mixed sulfate and sulfide electrolyte is very stable, as shown in Figure 11C because of the low molar solubility of Ag2S in concentrated Na2S which can fill the holes in Ag2S upon photoexcitation. Figure 11D shows the variation of Ag@Ag2S photocurrents at 465 nm over the action scanning cycles in the three different electrolytes. It clearly shows that photocurrent decreases by 37% and 18% in NaOH and sodium citrate, respectively, and no obvious photocurrent decrease is observed in the sulfide and sulfate electrolyte. To better illustrate the function of the Ag2S shell of a Ag@ Ag2S/Ti@TiO2 electrode, we tested photoelectrochemical stability of bare Ag NPs. Figure 12 shows stability results of Ag/Ti@TiO2 in three different electrolytes. Similar to Ag@ 14042



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silver(II) oxide around 0.9 V versus NHE. However, there is no Ag oxidation peak observed at a potential 1.0 V due to the oxidation of S2− to S. All the observed redox potentials of bare silver, silver sulfide are listed in an energy diagram in Figure S4 of the Supporting Information in order to better understand the photoelectrochemical performance of Ti@TiO2 NWs sensitized with Ag@Ag2S NPs. We tested the possible water splitting capability of the Ag@Ag2S/Ti@TiO2 electrode, but no appreciable amount of oxygen was produced, indicating that the Ag@Ag2S electrode material is not capable of splitting water directly under sunlight. Its photoelectrochemical properties can be used to direct solar water splitting if the cell is arranged in a tandem configuration in the presence of a sacrificial material and/or external bias from a solar cell. 3.5. AC Impedance and Mott−Schottky Plot. To elucidate the photoinduced charge transfer processes and to obtain more quantitative information about the photoanode, electrochemical AC impedance spectroscopy (EIS) has been performed. Figure 14 shows the typical Nyquist plots of the Ag@Ag2S NPs coated Ti@TiO2

Figure 12. Action spectrum cycles of Ag/Ti@TiO2 NWs in (A) 0.1 M NaOH, (B) 0.1 M sodium citrate (Na3C6H5O7), (C) 0.1 M Na2SO4 + 0.05 M Na2S, and (D) the photocurrent at 465 nm with different cycle, the bias potential is 0.05 V.

Ag2S/Ti@TiO2 electrode, bare Ag NP electrodes show a similar decrease in their photocurrent intensity in the visible light region in NaOH and sodium citrate because of the silver oxide formation upon silver oxidation in basic aqueous solution, although the photocurrent density is less than the Ag@Ag2S/ Ti@TiO2 electrode. The bare Ag NP electrode showed very large background current in mixed electrolyte of sulfide and sulfate in the dark because of the production of silver ions and enhanced dissolution of silver ions in the presence of sulfide, as shown in Figure 12C. A stable wavelength-dependent photocurrent response was obtained after 16 cycles of the action spectra because of the formation of Ag2S. As summarized in Figure 12D, photocurrent of bare Ag NPs showed a 35% decrease in its visible light photocurrent in NaOH and a decrease of 11% in sodium citrate. It should also be noted that Ag2O does not generate appreciable photocurrent at a bias of 0.2 V unless a high potential is applied under light excitation to produce AgO.45,46 For the dark current, it is well-known that silver particles are readily oxidized during anodic polarization. As shown in Figure 13, there are two main current peaks generally observed in the dark scanning voltammetry curve of Ag. The first current peak at 0.6 V versus NHE is due to the oxidation of silver to silver(I) oxide, and the second peak is attributed to the formation of

Figure 14. Nyquist plots of Ag@Ag2S/Ti@TiO2 NW (8 nm) photoanode in dark, under visible irradiation (>400 nm) and 1 sun solar (AM 1.5) irradiation, respectively. Inset is the equivalent circuit used to fit the experimental data. Fitting results are shown in Table 2.

NW electrode is in the dark, under irradiation with visible light (>400 nm) and a full simulated solar spectrum. Impedance spectra of other control electrodes, such as bare Ti@TiO2 NWs, Ag@Ag2S/Ti@TiO2 NW electrodes, are shown in Figure S5 of Supporting Information. All impedance spectra are fitted using an equivalent RC circuit model, shown in Figure 14, comprised of a resistor (Rs) representing the resistivity of the electrolyte between the working and reference electrode, a charge transfer resistance (Rct) representing the photoinduced charge transfer resistivity, and a capacitance (C) in parallel with the (Rct) analogous to the double layer charging capacity of the solid−liquid junction. All fitting results are summarized in Table 2. No major differences in Rs and C are found for all three electrodes. Rct is found to be smaller under visible irradiation than in the dark because of the photoinduced charge transfer events which provide photocurrent from the photocatalytic anode to the Ti substrate. Rct is further decreased under full solar irradiation due to the greater UV sensitivity of the Ti@TiO2 NW electrode. For the Ag@Ag2S/Ti@TiO2 (8 nm) photoanode, Rct under visible light irradiation is reduced

Figure 13. Linear scanning voltammetry of Ag/Ti@TiO2 NWs and Ag@Ag2S/Ti@TiO2 NWs (10 nm shell thickness) in 0.1 M NaOH. Scan rate: 50 mV/s. 14043



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Table 2. AC Impedance Results Obtained by Fitting the Experimental Data Fitting in Figure 14 and Figure S5 of the Supporting Information and Carrier Density Results Calculated from Figure 15 R (Ω) and C (μF) dark

visible > 400 nm

AM 1.5 UV + vis

carrier density

Rs R C Rs R C Rs R C

Ag@Ag2S (8 nm)

error %

Ag@Ag2S (20 nm)

error %

TiO2

error %

24 9.0 × 104 8 22 4.0 × 104 8 21 1.8 × 104 8 7.14 × 1020 /cm−3

4 8 2 6 7 3 5 5 3

28 1.2 × 105 10 30 6.0 × 104 9 28 2.4 × 104 9 2.78 × 1020 /cm−3

5 11 3 4 7 3 4 6 3

22 1.4 × 105 7 23 8.0 × 104 8 18 9585 8 1.24 × 1019 /cm−3

3 8 2 5 8 3 4 4 3

order to confirm the core−shell structure of Ag@Ag2S formed by a robust and simple dipping method. An optimal Ag2S shell thickness of 8 nm is used to demonstrate the concept of a surface-enhanced photoelectrochemical reaction at the nanostructured electrode system. Electrochemical polarization curves show that the Ag2S shell serves not only as a photoactive layer but also as an ideal insulating layer to prevent corrosion of the Ag core structure. The enhanced visible light response of the core−shell structure is attributed to (1) visible light sensitive Ag2S, (2) light absorption enhancement by plasmon active Ag, (3) a large interface area of Ti NW electrode, and (4) effective charge collection of Ti@TiO2 NWs. Further efforts are needed to quantitatively analyze these contribution factors.

about 50% in contrast to bare the Ti@TiO2 photoanode, and decreases one-third compared to the Ag@Ag2S/Ti@TiO2 (20 nm), indicating a more effective photoelectrochemical reaction occurring due to the localized electromagnetic field enhancement of the Ag core. Rs of Ti@TiO2 after removing the 400 nm long-pass filter decreases to 9585 Ω, which is smaller than Ag@ Ag2S/Ti@TiO2, because of the UV sensitivity of TiO2. Mott−Schottky plots (Figure 15) at 1000 HZ are used to determine the flat band potential (Efb) and carrier density (N)

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ASSOCIATED CONTENT

* Supporting Information S

STEM image of Ag@Ag2S particles, IPCE, action spectra of Ag/Ti@TiO2 and Ag2S/Ti@TiO2, the band potential of TiO2 and Ag2S and the oxidation potential of Ag2O/Ag, AgO/Ag2O, and O2/H2O in basic solution, and Nyquist and Mott− Schottky plots of nanostructured electrodes comprised of Ti@ TiO2 NWs and electrodes Ag@Ag2S/Ti@TiO2. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 15. Mott−Schottky plots obtained at 1000 HZ in dark from a bare Ti@TiO2 NW electrode in contrast to Ag@Ag2S/Ti@TiO2 NWs.

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of the modified Ti@TiO2 NW electrodes in order to estimate the conduction band position and the conductivity of the photoanode, respectively.47 Results at other frequencies are shown in Figure S6 of the Supporting Information. The estimated Efb values are −0.53, −0.5, and −0.52 V (vs NHE) for bare Ti@TiO2 NW, Ag@Ag2S/Ti@TiO2 (8 nm), and Ag@ Ag2S/Ti@TiO2 (20 nm), respectively, indicating that the Efb is still determined by the thick Ti@TiO2 NW electrode, whose conduction band is aligned with the conduction band of Ag@ Ag2S NPs for effective charge injection upon light irradiation. The estimated N values obtained from the slopes of the Mott− Schottky plots are also found to be close to each other for these three electrodes, as summarized in Table 2. The increase of N for Ag@Ag2S/TiO2 is from the Ag core to enhance the photocatalytic activity of the shell because of the higher number of carriers available for effective charge transport via Ag core and Ti@TiO2 NWs.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Science Foundation under Award CHE-1153120. We are very grateful to the Central Analytical Facility (CAF) of The University of Alabama for surface analysis instrument support.

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4. CONCLUSIONS In summary, a model system of nanostructured electrode of Ti@TiO2 NW sensitized with Ag@Ag2S NPs is reported for presenting a surface-enhanced photocatalytic process. Structural and detailed electrochemical studies are performed in 14044



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