Distinctly Improved Photocurrent and Stability in TiO2 Nanotube

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Article

Distinctly Improved Photocurrent and Stability in TiO Nanotube Arrays by Ladder Band Structure 2

Xiangyu Liu, Zhuo Chen, Wenxiao Li, and Maosheng Cao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b05941 • Publication Date (Web): 06 Sep 2017 Downloaded from http://pubs.acs.org on September 15, 2017

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The Journal of Physical Chemistry

Distinctly Improved Photocurrent and Stability in TiO2 Nanotube Arrays by Ladder Band Structure Xiangyu Liu1, Zhuo Chen1*, Wenxiao Li2 and Maosheng Cao1 1

Department of Materials Physics and Chemistry, School of Materials

Science and Engineering, Beijing Institute of Technology, Beijing 100081, People’s Republic of China 2

The High School Affiliated to Minzu University of China, Beijing

100081, People’s Republic of China

ABSTRACT Introducing a ternary interlayer into binary heterostructures to construct a ladder band structure provides a promising way for photoelectrochemical water splitting. Here, we design and fabricate a sandwich structure on TiO2 nanotubes using CdSxSe1-x as the interlayer to obtain a matching band alignment. The photoelectrochemical (PEC) properties of composite photoanodes are optimized by the order of sensitization and elements ratio, wherein the TiO2/CdS/CdS0.5Se0.5/CdSe photoanode shows a significantly enhanced photocurrent of 14.78 mA cm-2 at -0.2 V vs. SCE, exhibiting a nearly 15-fold enhancement, over one order of magnitude. The quantum efficiency apparently increases to 40% at range of 400~520 nm, resulting from that the sensitizing layer with a matching band alignment can facilitate the seperation of photogenerated S1

ACS Paragon Plus Environment


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electron-hole pairs and also extend the absorption range to the visible region due to its narrow bandgaps. Furthermore, its stability was distinctly

improved

by

coating

MoS2

on

the

surface

of

TiO2/CdS/CdS0.5Se0.5/CdSe photoanode. Our findings provide a novel route toward developing highly efficient photoelectrode for water splitting.

1. INTRODUCTION Solar-driven water splitting has attracted extensive interest because it can directly convert solar energy into fuels.1-26 Since Fujishima and Honda reported the first semiconductor device for photoelectrochemical water splitting in 1972

27

, many photocatalytic materials for hydrogen

production have been studied

10,28,29-32

. Among them, titanium dioxide

(TiO2) has advantages of suitable band edge positions, low cost, excellent photostability and non-toxicity.5,31,32 In various nanostructures, vertically aligned TiO2 nanotubes furnish larger surface area and shorter diffusion pathways for photogenerated minority carriers, which can facilitate the charge separation and electron transport.15,18,19,33,34 However, its low solar light absorption due to its wide bandgap and relatively fast electron–hole recombination still limit the photoconversion efficiency.35 Hetero-structurization of semiconductors provides one of the potential strategies for the efficient separation of photogenerated S2


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electrons and holes.10,36 In a type-II band alignment, the band bending at the interface on account of the difference of chemical potential between semiconductors induces a built-in field, which drives the photoexcited electrons and holes to move in opposite directions, resulting in the efficient separation of electron hole pairs.37 Quantum dots with tunable bandgap and band edge position by size control38 or element ratio allow an expected matching of energy levels in a heterojunction and a suitable potential for water splitting. To promote the separation of electron-hole pairs and extend the solar absorption, narrow bandgap quantum dots have extensively been coupled with TiO2 for the formation of type Ⅱ heterostructure, such as TiO2/CdS13, TiO2/CdSe30,38, TiO2/CdTe19,29, etc. However, the photoconversion efficiencies of single-component quantum dot sensitized photoanodes are still limited by the relatively mismatching of their energy levels. To obtain more matching band alignment, introducing a ternary interlayer into binary heterostructures to construct a ladder band structure provides a promising way for photoelectrochemical water splitting. To date, just a few multilayer architectures have been developed by using CdSSe quantum dots. 39-43 In this work, we design and fabricate a sandwich structure on TiO2 nanotubes by using CdSxSe1-x as the interlayer to obtain a matching band alignment. The PEC properties of composite photoanodes are optimized by the order of sensitization and elements ratio. Among them, the S3



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TiO2/CdS/CdS0.5Se0.5/CdSe photoanode shows an enhanced photocurrent of 14.78 mA cm-2 at -0.2 V vs. SCE which is nearly 15-fold enhancement compared with unsensitized TiO2 nanotubes and reaches a higher IPCE value (41.1% at 470nm). The improved performance mainly results from the suitable potential gradient for charge separation and transportation, a higher charge carrier density (1.08×1019 cm-3) and a longer photoelectron lifetime. Moreover, we fabricated MoS2 as a protection layer on the surface of TiO2/CdS/CdS0.5Se0.5/CdSe electrode to protect the cadmium chalcogenide from the photocorrosion with the solution and promote charge separation and transportation, further to improve its stability. This work could illuminate designing and exploiting highly efficient photoelectrode for water splitting. Our present findings are significant not only for the enhancement in PEC water splitting but also for how to design highly efficient photoelectrode.

2. EXPERIMENTAL SECTION 2.1. Preparation of TiO2 nanotube arrays. The highly ordered TiO2 nanotube arrays were prepared by an electrochemical anodic oxidation technique. Prior to anodization, metal titanium foils were cleaned in an ultrasonic bath of trichloromethane, acetone and ethanol for 15 min subsequently. The anodization was carried out using a two-electrode system connected to a DC power supply with the titanium S4


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foil as the working electrode and a graphite electrode as the counter electrode, respectively. The electrolyte consisted of 0.3 wt% NH4F in ethylene glycol solution and 2 vol% water. First, the titanium foil was anodized at 60 V for 1.5 h, and then the anodized titanium foil was ultrasonically removed in the mixed solution of DI water and ethanol. Subsequently, a second anodization of the treated titanium foil was anodized at 60 V for 4 h. The anodization processes were carried out at 5 ºC. After the two-step anodization, the prepared TiO2 nanotube arrays electrode was rinsed in acetone. All samples were annealed at 450 ºC for 1h. 2.2. Preparation of quantum dots sensitized TiO2 nanotube arrays. The highly ordered TiO2 nanotube arrays were sensitized with quantum dots by successive ionic layer adsorption and reaction (SILAR). In order to investigate the impacts of the sensitization order and interlayer elements

ratio

on

band

alignment

TiO2/CdS/CdS0.5Se0.5/CdSe,

respectively,

we

fabricated



TiO2/CdSe/CdS0.5Se0.5/CdS,

TiO2/CdSe/CdS0.67Se0.33/CdS and TiO2/CdSe/CdS0.33Se0.67/CdS electrodes. Methanol solution containing 0.1 M Cd(NO3)2 was employed as Cd2+ source. The mixed solution of DI water and methanol (volume ratio is 3:7) containing 0.1 M Na2S·9H2O was employed as S2- source. Aqueous solution containing 0.15 M Na2SO3 and 0.1 M selenium powder which S5



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was heated at 80℃ for 12h in nitrogen atmosphere was employed as Se2source. The prepared TiO2 nanotube film was dipped into the Cd2+ source solution for 1 min to adsorb Cd2+ ion onto the film. The film was then rinsed with methanol and immersed into the S2- or Se2- source solution for 1 min to adsorb S2- or Se2-. The adsorbed S2- or Se2- reacted with Cd2+ to form CdS or CdSe compound which was anchored onto the TiO2 film. The film was further washed with methanol to remove the excessive ions to complete one cycle. The CdSxSe1-x compounds were uploaded on the TiO2 nanotubes by the same way in which the volume ratio of S2- to Se2were adjusted to 1:2, 1:1 and 2:1, respectively. 2.3. Characterization. X-ray diffraction (XRD) patterns of samples were obtained by a PANalytical X-pert diffractometer (PANalytical, Netherlands) with Cu Kα radiation at 40 kV and 40 mA at room temperature in the 2θ range from 10° to 80°. Scanning electron microscope (SEM) images were collected by a Hitachi S-4800 microscope. Transmission electron microscopy (TEM) samples were prepared by immerging a 300 mesh copper grid with an ultrathin carbon film into a solution containing various samples. Transmission electron microscopy (FEI Tecnai G2 F20, 200 kV) was used to observe the morphology of the samples. The chemical states of Ti, Cd, Se and S in the composite

photoanode

were

evaluated

by

X-ray

photoelectron

spectroscopy (XPS) (PerkinElmer Physics PHI 5300). Absorption spectra S6


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were measured by a Hitachi U4100 spectrometer at room temperature. The photoelectrochemical measurement was performed in a three-electrode configuration. The mixed aqueous solution containing 0.1 M Na2S and 0.1 M Na2SO3 was used as the electrolyte. TiO2 nanotube arrays with or without quantum dots were used as the working electrodes (photoanodes), saturated calomel electrode as the reference electrode, and a platinum foil as a counter electrode. All TiO2/quantum dots samples were fabricated into photoanodes with a well-defined area of 1.13 cm2. The photocurrent measurements were carried out by applying an external bias to the cell using an electrochemical station (Zahner IM6e, Germany) with a scan rate of 10 mV s-1 and using a 150 W Xe solar simulator (Newport 94021A) with AM 1.5G filter as light source. Electrochemical impedance spectroscopic (EIS) were collected by the same instrument. Incident-photon-to-current-conversion efficiency (IPCE) spectra were measured on a QE/IPCE Measurement Kit (Crowntech QTest Station 1000AD) with a tungsten halogen lamp (CT-TH-150), a calibrated silicon diode and a monochromator (Crowntech QEM24-S 1/4 m).

3. RESULTS AND DISCUSSION Highly ordered TiO2 nanotubes were prepared by anodization and subsequently sensitized with CdS, CdSe and CdS1-xSex quantum dots by successive

ionic

layer

adsorption

and

reaction

S7


(SILAR).

The


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morphologies of the pure TiO2 nanotubes and quantum dot sensitized TiO2 nanotubes were shown in Figure 1. Figure 1a reveals that aligned TiO2 nanotube arrays were vertically grown on the Ti foil and these nanotubes exhibit a uniform diameter distribution (around 77 nm). The average thickness of nanotube wall and the length were around 20 nm and 18.9 µm (as shown in Figure S1), respectively. We can see in Figure 1b that cadmium chalcogenide nanoparticles were indeed decorated onto TiO2 nanotube surface uniformly which are also confirmed by TEM. Figure 1c shows a TEM image of the pure TiO2 nanotubes, in which the diameters of the nanotubes are consistent with that in the SEM image. The lattice fringe spacing of 0.352 nm corresponds to the interplanar distance of the (101) planes in anatase TiO2 (inset of Figure 1c). The HRTEM image of quantum dot sensitized TiO2 nanotubes in Figure 1d shows obviously different lattice fringes from anatase TiO2 and the lattice fringe spacing of 0.346 nm, 0.225nm and 0.298nm corresponds to (111), (220) and (200) planes in cubic structure of CdSe, revealing that cadmium chalcogenide nanoparticles with diameter ~ 4nm indeed were attached onto TiO2 nanotube surface. Energy-dispersive X-ray (EDX) mappings of quantum dots sensitized TiO2 nanotubes (Figure 1e) further confirmed the existence of CdS and CdSe on TiO2 nanotubes.

S8


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Figure 1. (a) SEM images of pure TiO2 nanotubes, inset is the cross-section of nanotubes. (b) SEM images of quantum dots sensitized TiO2 nanotubes. (c) TEM images of pure TiO2 nanotubes, inset is the High-resolution transmission electron micrograph. (d) TEM images of quantum dots sensitized TiO2 nanotubes. (e) EDX elemental mapping of Ti, S, Se, O and Cd in quantum dots sensitized TiO2 nanotubes.

In addition, the X-ray diffraction (XRD) patterns of pure TiO2 nanotubes on titanium substrates correspond to diffraction peaks of Ti (JCPDS No. 44-1294) and anatase TiO2 (JCPDS No. 21-1272) (as shown in Figure S2). We also noted that no obvious CdS or CdSe peaks were observed in XRD patterns of quantum dots sensitized TiO2 nanotubes probably because the diffraction peaks of small-size quantum dots were too weak. To further determine the existence of CdS, CdSe and CdSxSe1-x quantum dots, the X-ray photoelectron spectroscopy (XPS) was employed, as shown in Figure S3. The appearance of Ti2p1/2, Ti2p3/2, Cd3d3/2, Cd3d5/2, S2p, Se3p and Se3d confirms the coexistence of Ti, Cd, S9



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Se and S in the composite photoanode. To evaluate the effect of quantum dots on the PEC properties of TiO2 nanotubes, the PEC measurements were performed in a three-electrode cell under simulated sunlight illumination at 100 mW cm-2. Figure 2 shows the photocurrent density versus applied bias curves for the pure TiO2 electrode and the TiO2/quantum dot electrodes with different order of sensitization and different elements ratios of the CdS1-xSex interlayer. From Figure 2a, the dark current densities are essentially negligible even at high potentials of -0.2 V vs. SCE and the photocurrent of the pure TiO2 electrode is 0.94 mA cm-2 at -0.2 V vs. SCE. In comparison with the pure TiO2 electrode, the photocurrent densities of all these TiO2/quantum dots electrodes increase significantly, indicating that the main contribution to photocurrents comes from the quantum dots, which absorb more photons because of its band gap. TiO2/CdSe/CdS0.5Se0.5/CdS electrode reaches a higher photocurrent of 7.32 mA cm-2 at -0.2 V vs. SCE than TiO2/CdS and TiO2/CdSe electrode, which reaches 5.98 mA cm-2 and 6.60 mA cm-2 at -0.2 V vs. SCE respectively. The higher PEC performance may result from the constructed band structure of CdSe/CdS0.5Se0.5/CdS sandwich sensitizing layer. In order to optimize the energy levels between quantum dots and TiO2, we regulated sensitization order and interlayer elements ratio of quantum dot-sensitized TiO2 nanotubes and summarized the photoelectrochemical data in the table S1. The values of photocurrent S10


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density vary according to the order of sensitization as shown in Figure 2b. TiO2/CdS/CdSxSe1-x/CdSe electrodes reach a higher photocurrent density range between 10.2 and 14.8 mA cm-2 than TiO2/CdSe/CdSxSe1-x/CdS electrodes

(7.3

~

9.5

mA

cm-2).

The

advantage

of

TiO2/CdS/CdSxSe1-x/CdSe electrodes mainly results from the more matching band alignments, where the conductive band of CdSe is higher than that of CdS, which are consistent with Pan’s results46. The band alignment in this order of sensitization could induce a potential gradient for consecutive electron transfer. Among the samples with different interlayer elements ratios, the TiO2/CdS/CdS0.5Se0.5/CdSe electrode achieves the highest photocurrent density 14.78 mA cm-2 at -0.2 V vs. SCE and TiO2/CdS/CdS0.33Se0.67/CdSe, TiO2/CdS/CdS0.67Se0.33/CdSe electrodes reach 12.34 mA cm-2 and 10.24 mA cm-2 at -0.2 V vs. SCE respectively. The interlayer band gap decreases and the conduction band level lifts with the increase of Se component, which affect the potential gradient for electron transfer.

S11



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Figure 2. (a) Photocurrent density vs. applied potential curves of the pure TiO2 nanotubes, TiO2/CdS and TiO2/CdSe/CdS0.5Se0.5/CdS. (b) Photocurrent density vs. applied potential curves of the pure TiO2 nanotubes, TiO2/CdS/CdSxSe1-x/CdSe and TiO2/CdSe/CdSxSe1-x/CdS under AM 1.5G solar illumination. (c) Calculated photoconversion efficiencies for the pure TiO2 nanotubes and quantum dots (CdS, CdSxSe1-x and CdSe) sensitized TiO2 nanotubes. (d) Optical absorption spectra of the pure TiO2 nanotubes and quantum dots (CdS, CdSxSe1-x and CdSe) sensitized TiO2 nanotubes on Titanium substrates.

We have calculated the photoconversion efficiency according to the following equation47, η =I (1.23 -V)/Jlight, where V is the applied voltage versus reversible hydrogen electrode (RHE), I is the photocurrent density at the measured potential, and Jlight is the irradiance intensity of 100 mW cm-2 (AM 1.5G). As seen in Figure 2c, the TiO2/CdS and TiO2/CdSe electrode reaches 3.36% at -0.4 V vs. SCE and 3.45% at -0.3 V vs. SCE respectively, showing an increase compared with the pure TiO2 electrode (0.54% at -0.3 V vs. SCE). And sensitization with CdSxSe1-x quantum S12


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dots as the interlayer between the CdS layer and the CdSe layer can further

improve

the

photoconversion

efficiencies.

The

TiO2/CdS/CdS0.5Se0.5/CdSe electrode yields the highest photoconversion efficiency of 9.19% at -0.6 V vs. SCE, the TiO2/CdS/CdS0.33Se0.67/CdSe, TiO2/CdS/CdS0.67Se0.33/CdSe,

TiO2/CdSe/CdS0.5Se0.5/CdS,

TiO2/CdSe/CdS0.67Se0.33/CdS and TiO2/CdSe/CdS0.33Se0.67/CdS electrode reaches 7.49% at -0.6 V vs. SCE, 6.07% at -0.5 V vs. SCE, 4.83% at -0.5 V vs. SCE, 6.18% at -0.6 V vs. SCE and 5.42% at -0.5 V vs. SCE, respectively, showing an increase compared with the pure TiO2 electrode (0.54% at -0.3 V vs. SCE). The results reveal that different order of sensitization and different elements ratios of the CdS1-xSex interlayer in TiO2/quantum dots electrodes greatly influence their PEC performance. The optical absorption of quantum dots sensitized TiO2 nanotubes was measured to further analyze the key factor for their PEC performance. The diffuse reflectance absorbance spectra of the photoanodes are illustrated in Figure 2d. The pure TiO2 nanotube film shows little photo absorption in the visible range above 500 nm due to its large band gap (anatase TiO2 ~3.2eV). Compared with the pure TiO2 nanotubes, the TiO2/CdS and TiO2/CdSe photoanodes exhibit an obviously red shift of broad absorption edge to 525 nm and 580 nm respectively. These result indicate that the deposition of CdS and CdSe quantum dots on TiO2 nanotube film enlarges the light absorption range to visible region due to S13



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the narrower band gap of cadmium chalcogenide. Upon sensitization with CdSxSe1-x quantum dots as the interlayer between the CdS layer and the CdSe layer, the absorption range of the photoanodes can be further extended to 650 nm, indicating the more effective photo absorption properties in the visible region. Among all photoanodes, the TiO2/CdS/CdS0.5Se0.5/CdSe photoanode exhibits a broader and stronger absorption with an absorption edge at about 675 nm. To explore the better PEC performance in TiO2/CdS/CdSxSe1-x/CdSe electrodes, we have further optimized elements ratios of the CdS1-xSex interlayer. We performed IPCE measurements of the quantum dots sensitized TiO2 nanotubes at -0.5 V vs. SCE, as shown in Figure 3a. IPCE can be expressed by the following equation, where I is the measured photocurrent density at a specific wavelength, λ is the wavelength of incident light, and Jlight is the measured irradiance at a specific wavelength. IPCE (%) = [1240×I (mA cm-2)]/[λ (nm) ×Jlight (W cm-2)]×100 The IPCE curve of pure TiO2 nanotubes has a narrow peak with an edge at about 400 nm. The IPCE values of the TiO2/CdS/CdS0.5Se0.5/CdSe photoanode increase from zero at 680 nm to 41.1% at 470nm, and then gradually

decrease

to

TiO2/CdS/CdS0.33Se0.67/CdSe

almost and

zero

at

300

nm.

The


photoanodes show the same variation trend, but they have lower S14


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maximum values of 34.9% at 460nm and 28.14% at 480nm. The IPCE results of the samples are consistent with their photocurrent data. The TiO2/quantum dots photoanodes significantly improve the photoresponse in the visible light region.

Figure 3. (a) IPCE spectra of the pure TiO2 nanotubes and TiO2/CdS/CdSxSe1-x/CdSe photoanodes (b) Mott–Schottky plots collected in the dark for the pure TiO2 nanotubes and TiO2/CdS/CdSxSe1-x/CdSe photoanodes. (c) The decay of open-circuit photovoltage with time of the pure TiO2 nanotubes and TiO2/CdS/CdSxSe1-x/CdSe photoanodes. (d) The estimated photoelectron lifetime of the pure TiO2 nanotubes and TiO2/CdS/CdSxSe1-x/CdSe photoanodes.

To further analyze the advantage of TiO2/CdS/CdSxSe1-x/CdSe electrodes in their PEC performance, electrochemical impedance spectroscopy measurements were conducted at a frequency of 100 Hz in the dark. The Mott–Schottky plots of the pure TiO2 nanotubes and S15



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TiO2/CdS/CdSxSe1-x/CdSe photoanodes are shown in Figure 3b. All quantum dot sensitized TiO2 nanotubes exhibit a positive slope in the Mott-Schottky plots, as expected for n-type semiconductor. Significantly, the TiO2/CdS/CdSxSe1-x/CdSe electrode shows a clearly smaller slop of Mott-Schottky plot compared to the pure TiO2 nanotubes, revealing an increase of donor densities, which is based on the Mott–Schottky equation, 1/c2 = ( E - EFB - kT/e)/Ndeε0ε, where c is the space charge capacitance in the semiconductor, E is the applied potential, EFB is the flat band potential, T is the temperature, k is the Boltzmann constant, e is the electron charge, Nd is the doner density, and ε0 and ε are the vacuum permittivity and the relative permittivity, respectively. The charge carrier densities (Nd) of these samples were calculated from the Mott-Schottky plots using the following equation: = 2

. The

calculated charge carrier densities of TiO2/CdS/CdS0.5Se0.5/CdSe, TiO2/CdS/CdS0.33Se0.67/CdSe, nanotubes were cm-3,

TiO2/CdS/CdS0.67Se0.33/CdSe and TiO2

1.08 × 1019, 6.13 × 1018, 4.57 × 1018 and 4.81 × 1018

respectively. The increased charge carrier density of the

TiO2/CdS/CdS0.5Se0.5/CdSe electrode results from the upward shift to the conduction band edge of the Fermi level, which facilitates the charge separation at the electrode/electrolyte interface by increasing the degree of band bending at the TiO2 surface.31 The enhanced charge separation and transportation in the matching band alignments are inferred to be one S16


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of

the

reasons

for

the

enhanced

PEC

performance

of

the

TiO2/CdS/CdS0.5Se0.5/CdSe and TiO2/CdS/CdS0.33Se0.67/CdSe electrode. However,

the

inadequately


matching

which

band

probably

alignment introduces

of more

recombination centers may be responsible for the decrease in charge carrier density compared to that of the pure TiO2 nanotubes. To further analyze the enhanced PEC performance, the open circuit photovoltage decays (OCPD) of TiO2/CdS/CdSxSe1-x/CdSe electrodes were measured to characterize their inherent electronic properties. The OCPD measurement consists of turning off illumination at a steady state and supervising the succeeding decay of photovoltage () with time, as shown in Figure 3c. When the illumination is interrupted, the excess electrons are removed due to recombination, thus photoelectron lifetime can be estimated to evaluate the recombination kinetics of the photoelectrons and holes with the photovoltage decay rate directly related to the photoelectron lifetime by the following expression: =

!"

, where is the potential dependent photoelectron lifetime,

#$ is Boltzmann’s constant, % is the temperature, is the electron charge, and dVoc/dt is the open circuit photovoltage transient. The calculated photoelectron

lifetime

of

TiO2/CdS/CdSxSe1-x/CdSe electrodes is

illustrated in Figure 3d. Compared with the pure TiO2 nanotubes, TiO2/CdS/CdSxSe1-x/CdSe

electrodes

show

S17


relatively

longer


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photoelectron

lifetimes,

which

contribute

to

Page 18 of 31

their

better

PEC

performances. Significantly, the TiO2/CdS/CdS0.5Se0.5/CdSe electrode achieves the longest photoelectron lifetime due to its more matching band alignments where the serial sensitization of CdS, CdS0.5Se0.5 and CdSe layers is favorable to form into suitable potential gradient for electron transfer. According to the results above, we deduced that the enhanced PEC performance of the TiO2/CdS/CdS0.5Se0.5/CdSe electrode mainly results from the matching band alignment. Figure 4 illustrates the possible band alignment. As the Se component increases, the interlayer band gap decreases and the conduction band level lifts. Consequently, with the regulation of Se component, the electrode should realize a ladder energy band structure with powerful driving force to facilitate the seperation of photogenerated electron-hole pairs and promote consecutive electron transfer along the gradient. The multilayer structure also extends the absorption range.

S18


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Figure 4. Illustration of the possible band alignment of TiO2, CdS, CdSxSe1-x and CdSe.

To improve the stability of the TiO2/CdS/CdS0.5Se0.5/CdSe electrode, MoS2 was fabricated on the surface of quantum dots sensitized TiO2 nanotubes due to its excellent physical and chemical properties, such as higher conductivity and photostability.48-51 We immersed the sample into ammonium thiomolybdate solution and annealing at 425℃ subsequently. As shown in Figure S6 in the supporting information, the XPS results indicate that the appearance of Mo3d5/2 at 299.1eV, Mo3d3/2 at 232.3 eV and S2s at 266.3 eV confirms the existence of MoS2 coating. The stability measurements were tested at -0.2 V vs. SCE under continuous solar illumination

for

8000

seconds.

As

seen

in

Figure

5,

the

TiO2/CdS/CdS0.5Se0.5/CdSe electrode with MoS2 coating achieves a more stable PEC conversion, suggesting that the MoS2 coating depresses photocorrosion

of

the

cadmium

chalcogenide.

Importantly,

the

TiO2/CdS/CdS0.5Se0.5/CdSe electrode with MoS2 coating still owns a better PEC performance with the improved stability. The current density is 13.26 mA cm-2 at -0.2 V vs. SCE and the IPCE value doesn’t fall at 300-700nm (see Figure S9 in the supporting information). We deduced that

MoS2

as

a

protection

layer

on

the

surface

of

TiO2/CdS/CdS0.5Se0.5/CdSe electrode further optimizing performance mainly results from two aspects. On the one hand, MoS2 can be more S19



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stable at outermost layer in contact with the solution and also protect the cadmium chalcogenide from the photocorrosion with the solution under illumination. On the other hand, the valence band level of MoS2 is above CdSe52, which promoted the photogenerated holes accumulated in CdSe layer to further transport to MoS2 layer by potential gradient.

Figure 5. Current density decay percentage of the quantum dots sensitized TiO2 nanotubes with and without MoS2 coating measured at -0.2 V vs. SCE under 100 mW cm-2 solar illumination.

4. CONCLUSION Using CdSxSe1-x as the interlayer between the CdS layer and the CdSe layer may form a ladder structure of the energy band to obtain a matching band alignment. The effects of CdS/CdSxSe1-x/CdSe as the sensitizing layer on the PEC performance of the TiO2 nanotubes were investigated. The


photoanode

shows

the

highest

photocurrent of 14.78 mA cm-2 at -0.2 V vs. SCE, which is 15 times higher than that of pure TiO2 nanotubes. Incident photon to current S20


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conversion efficiency (41.1% at 470nm) measurement reveals that the TiO2/CdS/CdS0.5Se0.5/CdSe composite photoanode significantly improves the photo response in the visible light region. The enhanced efficiency results from broadening light absorption range and the matching band alignment to promote charge separation and transfer. Furthermore, MoS2 coating on the surface of TiO2/CdS/CdS0.5Se0.5/CdSe electrode can not only improve its stability, but also maintain its PEC performance. The present study may provide a ladder structure using a ternary compound as the interlayer of the sensitizing layer to obtain a matching band alignment and significantly improve the PEC performance.

Supporting Information. SEM of TiO2 nanotubes; XRD patterns of quantum dots sensitized TiO2 nanotubes; XPS of the TiO2/CdS/CdS0.5Se0.5/CdSe photoanode and TiO2/CdSxSe1-x photoanodes; Table of PEC data; Bode phase plots of TiO2/CdS/CdSxSe1-x/CdSe photoanodes; XPS of the MoS2 coated TiO2/CdS/CdS0.5Se0.5/CdSe electrode; Current density decay of the TiO2/CdS/CdS0.5Se0.5/CdSe electrode with and without MoS2 coating; Mott–Schottky plots,

Photocurrent density vs. applied potential curve

and IPCE spectra of the TiO2/CdS/CdS0.5Se0.5/CdSe electrode with MoS2 coating.

S21



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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Tel.: +861068913469.

ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grant No. 51472031).

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TOC Graphic

S31


Distinctly Improved Photocurrent and Stability in TiO2 Nanotube

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