Tandem Structure of QD Cosensitized TiO2 Nanorod Arrays for

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The tandem structure of QD co-sensitized TiO nanorod arrays for solar light driven hydrogen generation Li Cheng Kao, Ya Hsuan Sofia Liou, Chung-Li Dong, Ping-Hung Yeh, and Chi Liang Chen ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.5b01010 • Publication Date (Web): 30 Nov 2015 Downloaded from http://pubs.acs.org on November 30, 2015

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ACS Sustainable Chemistry & Engineering

The tandem structure of QD co-sensitized TiO2 nanorod arrays for solar light driven hydrogen generation Li Cheng Kao†, Sofia Ya Hsuan Liou*,†, Chung Li Dong‡, Ping Hung Yeh‡, Chi Liang Chen

§

† §

Department of Geosciences, National Taiwan University, Taipei 106, Taiwan

Department of Physics, Tamkang University, Tamsui Dist., New Taipei City 25137, Taiwan

National Synchrotron Radiation Research Center (NSRRC), Hsinchu 30076, Taiwan

KEYWORDS: Titanium dioxide, nanorod, quantum dot, water splitting, hydrogen generation

ABSTRACT: One-dimensional (1D) TiO2 nanorod arrays as photoelectrode have great potential for solar photoelectrochemical (PEC) hydrogen generation. However, the large band gap and Ti-growth unit preference of rutile TiO2 limit its solar light utilizing and multi-junction nanostructure photoelectrode design. This paper presents a double-sided tandem structure for quantum dot co-sensitized photoelectrodes with excellent solar PEC hydrogen generation. TiO2 nanorod arrays were grown directly on transparent and conductive glass substrates by hydrothermal method and then coated with CdS or CdSe as photosensitizer to successfully extend their photoresponse to visible light. Given the transparent substrate, TiO2 nanorod arrays could be grown on both sides, allowing the formation of the tandem structure of co-sensitized CdS and CdSe with high reactivity under visible light. The double-sided CdS and CdSe co-sensitized 1D TiO2 photoelectrode exhibited the highest solar-to-hydrogen conversion efficiency of 2.78% and pronounced enhancement of simulated photoconversion efficiency. This success in fabricating a double-sided tandem structure 1D TiO2 photoelectrode provides the opportunity for composite material design based on different band gaps, and this photoelectrode could be applied to other PEC applications.

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INTRODUCTION In recent years, finding sustainable alternative energy sources has become necessary because of the increasing cost of fossil fuels and the drastic effects of global climate change. In 1972, Fujishima and Hondal reported that TiO2 as a semiconductor photoanode can split water into hydrogen and oxygen; this reaction is caused by UV light illumination1. Solar light is the most abundant sustainable energy source, but it is not sufficiently utilized. To combine these two abounding resources, using solar light to split water and obtain hydrogen would be an environmentally friendly method to produce sustainable and clean energy. Solar photoelectrochemical (PEC) hydrogen generation from water has been envisioned to have the potential to supplement and ultimately replace fossil fuels. One-dimensional (1D) TiO2 nanostructured arrays have been widely investigated because of their relatively low cost, large band gap, good chemical stability, and superior charge-collection efficiency2-5. Compared with randomly packed TiO2 nanoparticle films, 1D nanostructured arrays perpendicular to substrates can promote faster transport and slower recombination6, 7. In addition, electron transport in nanoparticle films is limited to the particle network and interparticle contact area8. The 1D structure also provides easy access to the surface, which permits light scattering and enhances photocatalytic activity. Therefore, 1D TiO2 nanostructured arrays have extensive applications for photovoltaic and PEC hydrogen generation9, 10. However, as a photoelectrode for photovoltaic or PEC devices, the wide band gap of TiO2 limits its ultraviolet light absorption. Its poor visible light absorption causes TiO2 to have poor photo-to-current ability under solar light illumination. Therefore, considerable efforts have been developed to improve the visible light absorption of TiO2, including doping non-metal11-13, loaded metal14-16, dye or quantum dot (QD) sensitization17-19, and construction of hybrid structures20-23. Although the tandem structure combined semiconductors with different band gap energies can completely utilize solar light, synthesis techniques for 1D TiO2 with double-sided tandem structure are limited. Owing to the Tigrowth unit preference of different faces of rutile TiO2, synthesis at low temperature limits its growth on 2


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arbitrary substrates. Based on the idea of green synthesis, photoelectrode fabrication should provide low cost and renewable synthesis procedure. Recently, Liu and Aydil24 successfully grew TiO2 nanorod arrays on transparent FTO substrates, which allowed the design of a double-sided tandem structure model for QD co-sensitization. The QD sensitization of TiO2 photoelectrodes can be achieved through deposition of narrow-gap semiconducting materials. QD solar cells have large QD extinction coefficients and generate multiple electron–hole pairs per photon, which enhances photoconversion efficiency25, 26. The double-sided model serves as an analogue of tandem structure, which offers new opportunities for design and fabrication of PV and PEC devices27, 28. In this study, TiO2 nanorod arrays were grown directly on optically transparent substrates via simple hydrothermal method, followed by QD sensitization through successive ionic layer adsorption and reaction (SILAR) processes. Given the transparent substrates, a double-sided tandem structure could be achieved for photoelectrode design. These two different QD were sequentially deposited on TiO2 to form a co-sensitized tandem structure. The co-sensitized double-sided structure exhibited superior ability compared with single or co-sensitized one-sided photoelectrode. The double-sided CdS and CdSe cosensitized 1D TiO2 photoelectrode for PEC hydrogen generation substantially improved efficiency, and it provides a simple approach to simultaneously enhance visible light absorption and charge transport.

EXPERIMENTAL SECTION QD Sensitization TiO2 Photoelectrode Fabrication The samples were synthesized on FTO substrates (F:SnO2, TCO-17, 10 Ω/□) by hydrothermal method containing 30 mL of deionized water, 30 mL of hydrochloric acid, and 1 mL of titanium butoxide (97% Aldrich), as reported elsewhere24. The double-sided tandem structure of 1D TiO2 was fabricated on the FTO which had the fluorine-doped tin oxide coating on both sides (Figure S1, Supporting Information). 3



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The experiments were performed at 150 °C for 10 h in an electric oven. For SILAR process of CdS, TiO2 nanorod arrays were sequential dipped into an ethanol solution containing 0.5 M Cd(NO3)2 for 5 min, rinsed with ethanol, dried at 110 °C for 10 min, dipped for another 5 min into 0.5 M Na2S ethanol solution, rinsed with ethanol, and dried at 110 °C. The whole procedure is termed as one cycle, and the incorporated amount of CdS can be increased by repeating the cycles. Sodium selenosulphate (Na2SeSO3) was used as Se source for CdSe. The Na2SeSO3 aqueous solution was synthesized by refluxing Se (0.3 M) in an aqueous solution of Na2SO3 (0.6 M) at 80 °C overnight. The CdSe deposition was similar to the procedure of CdS, except a longer time and higher temperature are required for dipping in Na2SeSO3 solution. After several SILAR cycles, the QD sensitized TiO2 nanorod arrays were calcined at 450 °C for 30 min in argon29, 30. Characterization The crystal structure of the as-prepared film was examined by X-ray diffraction (XRD). The XRD patterns were recorded in a Philips Anatylical X’Pert PRO MPD with Cu Kα radiation (λ = 1.5406 Å) from 20° to 70° at a scanning speed of 2.4° min-1. A FEI QUANTA 200F field-emission scanning electron microscope (FESEM) model fitted with an energy dispersive X-ray spectrometer (EDX) was used to characterize the morphologies of the samples, as well as conduct elemental analysis. The absorption spectra of the films were recorded on a UV-3900 double-beam spectrophotometer. The specimen for TEM observation was prepared using focused ion beam and electron beam systems (DB-FIB, FEI Helios 600i). A thin slice was cut out from the cross section of the films and stuck on a Cu grid. Highresolution transmission electron microscopy (HRTEM) images were obtained with a FEI Tecnai G2 T20 HRTEM operating at 200 kV. The X-ray absorption spectroscopy (XAS) at Ti L-edge and O K-edge were conducted at BL20A at the National Synchrotron Radiation Research Center, Taiwan. Photoelectrochemical Measurements

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PEC measurements were performed using a three-electrode configuration with a Pt wire counter electrode and a saturated Ag/AgCl reference electrode (CH Instruments, model CHI6081D). The electrolyte was a mixture of 0.3 M Na2S and 0.3 M Na2SO3 aqueous solution. The active area of the working electrode was restricted to 0.25 cm2. Linear-sweep voltammograms (LSV) curves were measured under illumination of a solar simulator (Newport, Oriel class A, 94042A), equivalent to AM 1.5G at 100 mW/cm2 within the potential range of −1.2 V to −0.2 V vs. RHE. The photocurrent dynamics of the electrode were recorded with the chronoamperometric I−t program, according to the responses to sudden switching on and off one-sun irradiation at a bias voltage of −0.2 V (vs. Ag/AgCl). An Oriel Cornerstone grating monochromatic (Model 74004) was introduced into the light path to select the excitation wavelengths for incident-photon-to-current-conversion efficiency (IPCE) measurements. An optical power meter (Model 70310, Oriel) equipped with a thermopile head (Ophir Optronics 71964) was used to measure light intensity.

RESULTS AND DISCUSSION Morphology and Structure Characterization of QD sensitization TiO2 TiO2 nanorod arrays were first synthesized to cover FTO substrates by hydrothermal method. Figures 1a and b show the typical FESEM images of TiO2 nanorod arrays grown at 150 °C for 10 h on FTO substrates. The top view of the FESEM images (Figure 1a) shows that the entire surface of the FTO substrate was covered uniformly and densely with TiO2 nanorod arrays. The TiO2 nanorod arrays were relatively smooth and nearly perpendicular to the FTO substrate as shown in Figure 1b. The average length and diameter of the TiO2 nanorod were 2.1 µm and 120 nm, respectively. The TiO2 nanorod arrays were then sequentially sensitized with CdS or CdSe by SILAR. Figure 1c shows the side view of the TiO2 nanorod arrays coated with CdS. Nanoparticles with uniform size were observed along with the nanorod

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shown in the inset, indicating that CdS was successfully deposited. The elemental signals of cadmium and sulfide were detected by EDX analysis after CdS sensitization of TiO2 nanorod arrays (Figure 1e). Figure 1d shows the top view of the TiO2 nanorod arrays coated with CdSe; the inset also displayed that the nanorod was covered with tiny and uniform nanoparticles. EDX analysis of the TiO2 coated with CdSe is shown in Figure 1f.

Figure 1. FESEM images of (a) top view and (b) side view of TiO2 nanorod arrays. (c) top view and (d) side view of CdS and CdSe coated with TiO2, respectively. Insets are the corresponding larger magnification images. EDX analysis of (e) CdS_TiO2 and (f) CdSe_TiO2.

XRD and HRTEM were used to investigate the structural properties of the QD- sensitized nanorod arrays. The XRD pattern of pristine TiO2 nanorod (Figure S2) shows that the diffraction peaks of TiO2 growth on the FTO glass agreed well with that of the tetragonal rutile phase (JCPDS file no.88-1175). Figures 2a and c show the TEM images of TiO2 nanorod arrays coated with CdS and CdSe, respectively, with aggregated diameters of about 20nm to 50 nm. Each aggregate consisted of a group of QD with an average diameter of 10 nm to 20 nm, as distinctively shown in the HRTEM images (Figures 2b and d). 6


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The QD were visible as spots around the nanorod surface and directly attached to it. Thus, the interfaces were clean without organic solvent, which could be the barrier to carrier transport. The TiO2 nanorod with lattice spacing of 0.29 nm demonstrated that it was highly oriented with respect to the substrate surface and grew along with the [0 0 1] direction, which was perpendicular to the substrate. CdS deposited on the TiO2 nanorod arrays had d-spacings, which were estimated for 0.36 and 0.31 nm, corresponding to the (0 0 1) and (1 0 1) planes, respectively. The fringe distances measured in Figure 2d were 0.35 and 0.22 nm, which represented the (0 0 2) and (1 0 3) planes for CdSe, respectively. Both the QD could exhibit an intact crystalline structure rather than amorphous morphology, and these high quality crystallization QD may truly act as photosensitized semiconductors.

Figure 2. TEM images of TiO2 nanorod arrays sensitized by QDs. Low-resolution (left column) and high-resolution (right column) TEM images of (a), (b) TiO2 coated with CdS and (c), (d) TiO2 coated with CdSe.

One-sided QD sensitization TiO2 CdS or CdSe can be deposited on TiO2 nanorod by controlling the SILAR cycles, and the incorporated amount of QD increases with the increase in SILAR cycles. Table 1 are parameters obtained from the 7



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PEC measurements using various photoelectrodes, which adjust SILAR cycles to control the QD amount. Repeated times were marked in brackets. The open circuit potential (Voc), fill factor (ff), and short circuit current (Jsc) of these cell systems are listed in Table 1. The list of short circuit current exhibited a certain potential trend. The total short circuit current increased with the increase in SILAR cycles, indicating that a higher incorporated amount of QD could induce a higher photocurrent. However, the photocurrent sharply dropped when the amount of QD reached a certain level. Thus, the short circuit current decreased after five SILAR cycles for both CdS and CdSe. This phenomenon may be attributed to the fact that more SILAR cycles represent greater amounts of QD deposition, which would cause QD aggregation and growth larger crystal nucleus. Excessive amount of deposited particles would limit the advantage of QD, which could offer large extinction coefficients and generate multiple electron–hole pairs. Moreover, the overloaded QD nanoparticles can become a charge recombination center and block the light penetration. For optimal SILAR cycles, the short circuit current of CdS coated with TiO2 nanorod (denoted as CdS_TiO2) increased from 0.08 mA/cm2 to 3.90 mA/cm2, whereas that of CdSe coated with TiO2 nanorod (denoted as CdSe_TiO2) increased from 0.08 mA/cm2 to 1.07 mA/cm2. For comparison, although CdSe_TiO2 converted less photocurrent than CdS_TiO2, the onset potential of CdSe_TiO2 (Vonset potential = −1.19) had more negative shifts than that of CdS_TiO2 (Vonset potential = −0.93). The negative shifting of the onset potentials demonstrated the passivation of the surface states31. In addition, CdSe_TiO2, which can absorb light up to c.a. 750 nm, had a wider absorption spectrum than CdS_TiO2 (Figure 3a). According to empirical equations reported by Yu et al.32, the size of CdS and CdSe nanoparticles can be estimated from the excitonic peaks of the absorption spectra. The average diameters of CdS and CdSe nanoparticles were estimated to be ca. 8.2 and 23.2 nm, respectively. The calculated CdS and CdSe nanoparticle sizes were identical to those observed in HRTEM. The poorer photo-to-current ability of CdSe_TiO2 photoelectrode was clarified by Kongkanand et al.33, who confirmed that CdSe with different sizes affects the photoresponse of PEC cells. Smaller-sized CdSe presented greater charge in8


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jection rates at the excitonic band. In contrast to the smaller-sized ones, larger-sized particles have better absorption in the visible area, but inject few electrons into TiO2 as effectively as smaller-sized CdSe. According to TEM analysis and empirical equation estimation, the average diameters of CdSe nanoparticles were ca. 23.2 nm, in which the nanoparticle sizes were larger than those in other studies, so the larger-sized particles may cause lower photocurrent. Table 1. Parameters obtained from the PEC measurements using various photoelectrodes with different SILAR cycles to control the QD amount. 2

Electrode

Voc (V)

ff

Jsc (mA/cm )

TiO2

-0.87

0.40

0.08

CdS(3)

-1.15

0.40

2.12

CdS(4)

-1.09

0.31

3.52

CdS(5)

-1.03

0.41

3.90

CdS(7)

-1.01

0.53

3.57

CdS(10)

-0.92

0.57

0.50

CdSe(2)

-1.05

0.37

0.69

CdSe(4)

-1.19

0.59

0.78

CdSe(5)

-1.16

0.68

1.07

CdSe(7)

-1.24

0.48

0.96

To utilize the benefits of both CdS and CdSe, a co-sensitized one-sided model was designed to enhance photocurrent. For comparison of the light absorption properties of QD sensitization TiO2 nanorod, the variations in the UV-vis spectra of the photoelectrodes prepared using the same SILAR cycles, but different deposition sequences, are shown in Figure 3. Considering the numbers of QD sensitization models in our research, the SILAR times were unmarked and only the deposition sequence was presented. The band gap energies of the crystalline semiconductor can be estimated by a related curve of (αhv)2 versus photon energy (hv) plotted in Figure 3b, from the intersection of the extrapolated linear portion. αℎν = Aℎν − E 9


(1)


where α, ν, A, and Eg are absorption coefficient, light frequency, proportionality constant, and band gap energy, respectively34,35. Considering its large band gap, TiO2 absorbed limited visible light and utilized solar light of only up to 400 nm. The intersection of the extrapolated linear portion of CdS and CdSe matched their band gap energy as reported, demonstrating an intact crystalline structure. However, after CdSe was coated with CdS_TiO2 (denoted as CdSe_CdS_TiO2), the co-sensitized electrode extended its absorption range only by about 700 nm, which was not close to the absorption edge of CdSe_TiO2. Although different SILAR cycles could affect the absorption spectrum, the absorbance intensity did not shift the absorption area (Figure S3). Besides, the absorption edge of alignment in the reverse sequence (denoted as CdS_CdSe_TiO2) decreased to wavelength of ca. 600 nm, close to the CdS adsorption spectrum. The absorption spectrum of co-sensitized photoelectrode may contribute to the mixture of CdS and CdSe, which resulted in the absorption of the edges located in the region between single QD sensitization TiO2 photoelectrodes.

2 . 1

TiO2 CdS_TiO2 CdSe_TiO2 CdS_CdSe_TiO2 CdSe_CdS_TiO2

0 . 1 8 . 0 6 . 0

e c n a b r o s b A

(b)

2 2 O O i iT T _ _ _ _S e O2 2 S d O ii Td_ C T C _ _ e _ e Sd Sd Sd S O2 d i T C CC C

(a)

4 . 1

2222 ) V e ( 2222 ) v h . D O (

4 . 0 2 . 0 0 . 0

5 . 3

0 . 3

5 . 2

0 . 2

0 0 8

0 5 7

0 0 7

0 5 6

0 0 6

0 5 5

0 0 5

0 5 4

0 0 4

0 5 3

) V e ( v h

) m n ( h t g n e l e v a W

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Figure 3. (a) UV-vis absorption spectra of pristine TiO2 and TiO2 sensitized by QD. (b) The estimation of band gap energies by 2 curve of (αhv) versus photon energy (hv).

To further understand the QD interaction in different deposition sequences, PEC studies were carried out to obtain the connection between deposition sequences and the ability of photo-to-current. Figure 4a 10


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presents LSV under solar light illumination, within the potential range of −1.2 V to −0.2 V vs. RHE. Clearly, all QD sensitization TiO2 photoelectrodes exhibited photocurrent enhancement because QD effectively absorbed the visible light. These two co-sensitized one-sided electrodes performed the photocurrent density similarly, but were less efficient than the single QD sensitized TiO2. Under the same SILAR cycles, the excess amount of QD deposition, which would block light penetration, could be excluded as a factor in decreasing photocurrent density. Compared with the photocurrent density of pristine TiO2 electrode, QD co-sensitized TiO2 electrode improved the photocurrent density; however, the maximum photocurrent at −0.2 V was quite low, with c.a. 0.5 mA/cm2. Based on the UV-vis spectra and LSV results, it is reasonable to have an interpretation on the low photocurrent density of QD cosensitized TiO2 electrode. The QD were fabricated by sequential dipping TiO2 nanorod into different solutions, which combined the ions together. However, both CdS and CdSe were synthesized by dipping into ethanol solution containing 0.5 M Cd(NO3)2, allowing Cd2+ on the TiO2 nanorod to bond to sulfide or selenide for the next dipping sequence. The co-sensitized alignment would allow TiO2 nanoparticle films to deposit QD layer by layer. Previous studies have proven that the stepwise band structure exhibits the Fermi–Dirac distribution and causes electrons to flow36. For comparison, the 1D TiO2 structure allowed QD to be deposited in a dispersive manner, and the cadmium ion may become the center to bond sulfide and selenide. These dispersive QD could not form an aligned Fermi level; thus, the coating sequence was not related to efficiency. The lower photocurrent density of the co-sensitized one-sided model may be attributed to the random dispersion of QD deposition; these adjacent QD could become the recombination center for each other because they have no appropriate alignment in accordance with the Fermi–Dirac distribution (Figure S4). The photocurrent dynamics of the PEC cells were recorded according to the response to sudden switching on and off one-sun irradiation at a bias voltage of -0.2 V (vs. Ag/AgCl). The comparison between the single QD sensitized photoelectrode and co-sensitized onesided photoelectrode is depicted in Figure S5. The two competitive processes of electron generation and 11



recombination in the different deposited models were induced from Figure S4 according to the following equations:

D = exp − 2)

and D is defined as D =

(3)

where t is the time and τ is the transient time constant that is the time at ln D = -137-39. Figure 4b illustrates ln D as a function of time for the photocurrent transient response of t. The determined transient time constant of CdS_TiO2 was higher than that of the co-sensitized one-sided photoelectrode, indicating that the single QD model was much more efficient for electron transport than the co-sensitized onesided model. The lower τ of the co-sensitized one-sided photoelectrode was attributed to the recombination center formation of co-sensitization QD, thereby trapping the photo-excited electron.

5 . 3

O O iT i T _ _ e S O2 S d i O2 i C TT Ce _d _ _ e _ S S S S O2 d d d d i T C C C C

(a)

0 . 4

(b)

0.0

-0.5

2 2

5 . 2

-1.0

5 . 1

ln (D)

0 . 2

-1.5

-2.0

CdS_TiO2

0 . 1

) 2222 m c / A m ( y t i s n e D t n e r r u C

0 . 3

-2.5

5 . 0 0 . 0

CdSe_CdS_TiO2 CdS_CdSe_TiO2

-3.0

2 . 1 -

0 . 1 -

8 . 0 -

6 . 0 -

4 . 0 -

2 . 0 -

) l C g A / g A s v ( e g a t l o V l a i t n e t o P

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Time (sec)

Figure 4. (a) PEC measurements of current density–voltage curves at a scan rate of 10 mV/s, under a light intensity of 100 2 mW/cm . (b) Plot of ln D vs. t from photocurrent dynamics of the PEC cells for the comparison between the single QD sensitized photoelectrodes and co-sensitized one-sided photoelectrodes.

Without knowing the fundamental electronic structures of the QD-sensitized photoelectrodes and how it changes under operando condition, it is difficult to better understand and thus better engineer for more 12


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practical use of these photoelectrodes. Therefore, in order to investigate the electronic structure of these TiO2 nanorod coated with QD and how the light irradiation affects the electronic structures, in situ XAS was performed to reveal the possible mechanism of hydrogen generation. Figure S6 shows the O K- and Ti L-edges recorded with and without illumination by simulated solar light. O K-edge probes the O 2pTi 3d hybridized unoccupied electronic states and Ti L-edge probes directly the 3d states. It is clear that no substantial spectral differences of O K- and Ti L-edges can be found in both CdSe_CdS_TiO2 and CdS_CdSe_TiO2 in the dark and under illuminated condition which suggest there are no change of O 2p and Ti 3d states and these systems are not light active, which is consistent with the obtained results from photocurrent density in which CdSe_CdS_TiO2 and CdS_CdSe_TiO2 show nearly the same photocurrent density, as shown in Fig. 4(a). In Fig. 5, significant enhancements of photocurrent densities were found in the CdSe_TiO2 and CdS_TiO2. The photocurrent density is correlated to the electronic structure in the conduction band. In order to reveal how the role of the electronic structure affects the photocurrent density, the in situ XAS at Ti L- and O K- edges of these photoelectrodes were further compared in Fig. 5. In contrast to the Fig. S5, a remarkable spectral difference is revealed in the CdSe_TiO2 and CdS_TiO2 under light illumination. Upon the light irradiation, the raising of the intensity of peak t2g in Ti L-edge in Fig. 5(a) indicate the increase of Ti 3d(t2g) states. The peak A and B in the O K-edge (lower part of Fig. 6(a)), respectively, correspond to the electron excitation to O 2p-Ti 3d (t2g) and O 2p-3d (eg) hybridized states. Notably, the peak A (peak B) is increased (decreased) in the intensity in irradiated condition, implying there are more unoccupied t2g states induced, which is consistent with enhancement of Ti 3d (t2g) state in Ti Ledge (upper part of Fig. 5(a)). The results above suggest the charge transfer from TiO2 to CdS may occur. For CdSe_TiO2, the t2g peak in Ti L-edge is decreased as irradiated with light, indicating it gains some charge. From view point of O K-edge, the hybridized O 2p-Ti 3d states is increased in intensity, implying the electron transfer may probably occur between Ti and O sites. The interfacial charge trans13



fer phenomena obtained from XAS thus suggest that in CdS_TiO2 there is more efficient charge migration from TiO2 to CdS, and less efficient charge migration from TiO2 to CdSe since some charge transfer occurs between Ti and O. The different charge transfer behavior in CdSe_TiO2 and CdS_TiO2 is due to the oxygen in TiO2 has different oxygen environment when TiO2 is attached to CdSe and CdS, as revealed by comparing the O K-edge of CdSe_TiO2 and CdS_TiO2 in the dark condition. The different charge migration path gives rise to different PEC catalytic ability. Thus, the charge transfer observed herein maybe the consequence of the higher current density of CdS_TiO2 than that of CdSe_TiO2.

Photon Energy (eV)

Photon Energy (eV)

456

456

459

459

(b) CdSe_TiO2

(a) CdS_TiO2

Ti L-edge

Ti L-edge

t2g t2g

Intenstiy (arb. units)

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Dark Illumination

Dark Illumination A

B

B O K-edge

O K-edge

A

Dark Illumination

Dark Illumination 524

528

532

Photon Energy (eV)

536

524

528

532

536

Photon Energy (eV)

Figure 5. In situ X-ray absorption spectra at Ti L- and O K- edges of (a) CdS_TiO2, and (b) CdSe_TiO2.

Double-sided tandem structure of QD co-sensitization TiO2 The double-sided tandem structure was designed for solar hydrogen generation with excellent solar light utilization to achieve the benefits from both QD and avoid the dilemma occurring in co-sensitized onesided photoelectrode. Only few studies about growth of the double-sided tandem structure of 1D TiO2 electrode by hydrothermal method have been reported because of the difficulty in synthesizing 1D TiO2 perpendicular to the arbitrary substrates caused by the crystal growth preference. However, according to the research by Liu and Aydil24, it is practicable to sputter the fluorine-doped tin oxide coating on both 14


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sides of glass which allow 1D TiO2 to form the tandem structure and then sensitize CdS and CdSe on each side. The CdS sensitized side faced toward the incident light in all PEC measurements of the double-sided tandem structure model. For optimal ability of solar light driven hydrogen generation, Figure 6(a) shows the solar-to-hydrogen conversion efficiency (η) of the double-sided tandem structure photoelectrodes with different QD deposition times. The solar-to-hydrogen conversion efficiency was calculated according to η(%) = jp[(Erev-Eapp)/Io] × 100, where E rev = 1.23 V and Eapp =Emeas − Eocp. Emeas is the electrode potential (vs. Ag/AgCl) of the working electrode, jp is the photocurrent density, and Io is the intensity of the incident light40,

41

. Based on the experience of single QD sensitization, excessive

amounts of deposited QD would block light penetration and cause QD aggregation; hence, each side of the SILAR cycles was limited to five times. The double-sided tandem structure photoelectrode with appropriate QD combination exhibited higher solar-to-hydrogen conversion efficiency than single QD sensitization photoelectrode while deposited the same SILAR cycles. Among these QD co-sensitization photoelectrodes, the photoelectrode with three SILAR cycles for each side (denoted as CdS_TiO2_CdSe) performed maximum solar-to-hydrogen conversion efficiency, and the plots of the solar-to-hydrogen conversion efficiency vs. applied bias potentials of CdS_TiO2_CdSe are presented in Figure 6b. The CdS_TiO2_CdSe achieved the highest efficiency of c.a. 2.78% at a rather low bias of −0.63 V vs. Ag/AgCl. By contrast, the pristine TiO2 photoelectrode yielded only 0.02% under solar light illumination. In addition, this two QD co-sensitization double-sided tandem structure photoelectrode underwent negative shifting on the onset potential (Vonset potential = −0.97), thereby improving interfacial charge transportation in the heterostructures. To eliminate the discrepancy between the irradiance of the simulation light source used in the laboratory and solar light, the corresponding IPCE spectra were measured (Figure S7) and integrated with a standard AM1.5G solar spectrum (ASTM G-173-03) using the following equation:

η% =

λ1.23 − V'()* IPCEλEλ dλ

15


(4)


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where E(λ) is solar irradiance at a special wavelength (λ), Vbias is applied bias vs. RHE, and IPCE is the obtained photoresponse profiles of the photoelectrodes at specific wavelength (λ) at −0.2 V vs. RHE42,43. Figure 7a shows the simulated photoconversion efficiencies as a function of wavelength ranging from 400 nm to 700 nm. CdS_TiO2, CdSe_TiO2, and CdS_TiO2_CdSe achieved simulated photoconversion efficiency of 0.68%, 0.72%, and 5.25%, respectively. Although CdSe_TiO2 showed low photocurrent in LSV, its simulated photoconversion efficiency was comparable with that of CdS_TiO2. The standard AM 1.5G solar spectrum between 550 and 700 nm could be better utilized by CdSe, thereby enhancing the simulated photoconversion efficiency. CdS_TiO2_CdSe revealed a pronounced IPCE of ca. 30% to 45% within the wavelength range of 460 nm to 700 nm. Even at 700 nm, the double-sided tandem structure photoanode still showed prominent IPCE improvement. Compared with the one-sided sensitized model, the co-sensitized double-sided tandem model showed substantially enhanced simulated photoconversion efficiency in the visible region. This enhancement was attributed to three possible reasons. First, the double-sided tandem structure photoelectorde extended its absorption spectrum, thereby more efficiently utilizing solar light. When the light source illuminated the photoanode, wavelengths up to 510 nm were absorbed by CdS. The transparent substrates allowed the other part of light that could not be absorbed by CdS to penetrate into another side and be absorbed by CdSe. CdSe has a narrower band gap than CdS, so it can extend its absorption spectrum to ca. 750 nm. The larger particle size of CdSe may have led to low photocurrent, but resulted in better absorption in the visible area, which is advantageous in the present research. The CdSe sensitized side of the double-sided model faced toward the light illumination (denoted as CdSe_TiO2_CdS) shown in Figure 7b might prove this inference. The IPCE of the same double-sided QD sensitized photoanode, but with reverse aspect light illumination, showed lower conversion efficiency because CdSe absorbed most parts of visible light. Less light penetrated through the substrate utilized by CdS, making it harder for the photo-excited electron to transfer. Second, the Fermi levels of these three semiconductors rearranged at equilibrium in the electrolyte solution system. 16


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The enhanced IPCE was attributed to the Fermi level alignment among CdS, CdSe, and TiO2. According to previous research27, 36, the conduction band edge of CdSe was elevated when its Fermi level was aligned with CdS in a stepwise structure. Therefore, the conduction band edges of CdS and CdSe were both higher than that of TiO2, allowing efficient transport of photo-excited electrons from QD to TiO2. Alignment of the conduction bands could accelerate charge separation and decelerate hole recombination. Third, the simulated photoconversion efficiency of CdS_TiO2_CdSe in the wavelength range of 400 nm to 500 nm increased more than the sum of single CdS and CdSe sensitized. This high simulated photoconversion efficiency was expected, and both CdS and CdSe layers showed substantial absorption in this region. The CdS sensitized side, in contrast to large-sized CdSe ones, revealed higher photocurrent density, indicating better quantum effect and enabling rapid overall photo-excited charge transport (Figure S8).

Figure 6. (a) Solar-to-hydrogen conversion efficiency (η) of the double-sided tandem structure photoelectrodes with different QD deposition times. (b) The solar-to-hydrogen conversion efficiency vs. applied bias potentials of CdS_TiO2_CdSe.

17



4.5 1.4 4.0 1.2 1.0

2.5

0 0 7

700

0 5 6

650

0 0

600

6) m n ( h t 0g 5 n 5e l e v a W

550

0 0 5

500

0 5 4

450

0 0 4

0.0

-1

400

0

0.5 CdSe_TiO2

0.0

0 1

1.0

CdS_TiO2

0.2

0 2

1.5 0.4

0 3

2.0

0.6

0 4

3.0 CdS_TiO2 _CdSe

0.8

) % ( y c n e i c i f f E

0 5

3.5

(b)

0 6

-2

5.0

ed S S d C C _ _ O2 i O2 i T T _ _ e S S d d C C

5.5

0 7

1.6

0 8

1.8

Solar irradiance (Wm nm )

0 9

(a)

Simulated photoconversion efficiency %

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Wavelength (nm)

Figure 7. (a) The simulated photoconversion efficiencies of CdS_TiO2, CdSe_TiO2, and CdS_TiO2_CdSe as a function of wavelength by integrating their IPCE spectra. (b) The IPCE comparison for CdS_TiO2_CdSe and CdSe_TiO2_CdS.

In summary, we successfully fabricated the double-sided tandem structure of CdS and CdSe cosensitized TiO2 nanorod array photoelectrode for PEC hydrogen generation. This double-sided photoelectrode exhibited strong light absorption within almost the entire visible region using only the sun as light source. The band alignment among CdS, CdSe, and TiO2 improved the electron transport, thereby enhancing the solar-to-hydrogen conversion efficiency and simulated photoconversion efficiency. The QD deposition in co-sensitized one-sided photoanode with a 1D model could hardly improve the phototo-current ability. This work demonstrated the potential advantages of a double-sided tandem structure photoelectrode for PEC cells and its possible other applications, thereby providing important insights into the design and fabrication of composite nanostructures. ASSOCIATED CONTENT Supporting Information. Figure S1. The comparison between single-sided FTO and double-sided FTO. The scratch (red circles) aided understanding another side of FTO. Figure S2. XRD patterns of pristine TiO2, CdS coated with TiO2, and CdSe coated with TiO2. Figure S3. The absorption spectrum of CdSe with different SILAR cycles. Figure S4. Schematic diagram illustrating the random dispersion of QD deposition on co18


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sensitized one-sided model. Figure S5. Photocurrent dynamics in response to on-off irradiation. Figure S6.O K- and Ti L-edges of CdSe_CdS_TiO2 and CdS_CdSe_TiO2 under solar light illumination. Figure S7. Measured IPCE of QD sensitized TiO2 collected at the incident wavelength range from 400 to 700 nm. Figure S8. Schematic diagram illustrating charge-transfer processes in the model of double-sided tandem structure of CdS and CdSe co-sensitized TiO2.This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]; Fax: +886 2 23636095; Tel: +886 2 33669861 Author Contributions All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT The authors would like to thank the Ministry of Science and Technology of the Republic of China for financially supporting this research. ABBREVIATIONS FTO, fluorine-doped tin oxide. REFERENCES (1) Akira, F.; Kenichi, H. Electrochemical photolysis of water at a semiconductor electrode. Nature 1972, 238, 37-38. (2) Hou, Y.; Li, X.; Zou, X.; Quan, X.; Chen, G. Photoeletrocatalytic activity of a Cu2O-loaded selforganized highly oriented TiO2 nanotube array electrode for 4-Chlorophenol degradation. Environ. Sci. Technol. 2009, 43, 858-863. (3) Liang, K.; Tay, B. K.; Kupreeva, O. V.; Orekhovskaya, T. I.; Lazarouk, S. K.; Borisenko, V. E. Fabrication of double-walled titania nanotubes and their photocatalytic activity. ACS Sustainable Chem. Eng. 2014, 2, 991–995. (4) Kao, L. C.; Lin, C. J.; Dong, C. L.; Chen, C. L.; Liou, S. Y. H. Transparent free-standing film of 119



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Table of Contents artwork The tandem structure of QD co-sensitized TiO2 nanorod arrays for solar light driven hydrogen generation Li Cheng Kao, Sofia Ya Hsuan Liou*, Chung Li Dong, Ping Hung Yeh, Chi Liang Chen

Synopsis: The model of double-sided tandem structure of CdS and CdSe co-sensitized TiO2.


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Tandem Structure of QD Cosensitized TiO2 Nanorod Arrays for

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