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CdS co-sensitized titania nanotube for hydrogen generation from photocatalytic splitting water
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Fe2O3/CdS co-sensitized titania nanotube for hydrogen generation from photocatalytic splitting water Chin-Hao Chan, Prabu Samikkannu, Hong-Wen Wang* Department of Chemistry, Center for Nanotechnology, Chung-Yuan Christian University, Chungli, 320, Taiwan, ROC
article info
abstract
Article history:
CdS, Fe2O3-sensitized and Fe2O3/CdS co-sensitized TiO2 nanotube arrays (TNA) were
Received 21 May 2016
fabricated by depositing CdS and Fe2O3 nanoparticles onto the surface of TNA via an
Received in revised form
ultrasonic-assisted chemical bath deposition (CBD) method. Their effects on the photo-
28 July 2016
catalytic properties and hydrogen generation rate were evaluated. The results show that
Accepted 4 August 2016
CdS-sensitized and Fe2O3/CdS co-sensitized TNA have good absorption on long wavelength
Available online 27 August 2016
region and exhibit much higher hydrogen generation rates than those of other TNA specimens. The hydrogen generation rates for TNA-Fe2O3/CdS and TNA-CdS/Fe2O3 were 12
Keywords:
and 1.54 ml cm2 h1, respectively, where the TNA-Fe2O3/CdS refers to the deposition
TiO2 nanotube arrays
of Fe2O3 first on TNA and CdS the second, CdS being the out surface materials and con-
CdS
tacting water. TNA-CdS/Fe2O3 is a reversed structure of TNA-Fe2O3/CdS. The rate of
Fe2O3
hydrogen generation for TNA-Fe2O3/CdS was five times higher than that of blank TNA
Hydrogen generation
(2.43 ml,cm2 h1). It is considered that the low efficiency of TNA-CdS/Fe2O3 is due to a disadvantageous energy band structure for electrons and holes transition. TNA-Fe2O3/CdS has a stepwise energy band structure and the transition of excited electrons and holes is straightforward, while those of TNA-CdS/Fe2O3 has a difficult transition process due to a reverse structure. © 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The need for new energy resources, such as solar energy conversion, has evolved in recent decades. Solar energy can be directly converted into electricity through the use of solar cell [1e3]. Alternatively, solar hydrogen may be generated through the use of photoelectrochemical (PEC) water splitting [4e9]. Through the function of proton exchange membrane (PEM) fuel cells [10e24], electricity can be generated from the dissociation of hydrogen and reduction of oxygen. To generate
solar hydrogen efficiently, PEC water splitting must have a highly efficient photoelectrodes. Among the various photoelectrodes, those based on TiO2 have been actively studied. Since Fujishima and Honda discovered the photocatalytic splitting of water on TiO2 electrodes [25], TiO2 photocatalysis has been extensively investigated in the field of environmental protection and solar-cell materials owing to its great advantages in the decomposition of organic pollutants and relatively long charge carrier transport length, good stability, low-cost and low toxicity.
* Corresponding author. E-mail address: [email protected] (H.-W. Wang). http://dx.doi.org/10.1016/j.ijhydene.2016.08.026 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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One-dimensional nanostructures, particularly TiO2 nanotube arrays (TNA), have received intensive attentions due to their superior photocatalytic and photoelectronic performance [26]. TNA materials are of great interests for H2 generation from photocatalytic water splitting for its enhanced charge-collection efficiency by promoting faster transport and slower electron-hole recombination [27]. Although TiO2 is one of the most important materials widely studied for photoelectrodes and solar cells, the wide bandgap (3.2 eV) of TiO2 limits its photocatalytic property in the UV region. To extend the activity of the photoelectrode into the visible light region, various approaches were employed such as purity of Ti foils [28], doping TiO2 with other impurities [29e33], and sensitizing TiO2 with semiconductors [34e39], which absorb light in the visible region. In this study, semiconductor such as Fe2O3 [34] and CdS [35e38] were deposited on TNA electrodes using ultrasonic chemical bath deposition (CBD) [36]. The utilization of CdS or Fe2O3 materials on TNA is a facile way to induce photocatalytic water decomposition under visible light irradiation due to that the CdS and Fe2O3 has smaller band gap energy (2.4 eV and 2.2 eV, respectively) [34e38]. There are many reports on single component sensitized TiO2 nanotubes [34e39], but not many in CdS/Fe2O3 or Fe2O3/CdS co-sensitized TNA. In this study, the characterization and hydrogen generation of CdS-, Fe2O3-sensitized, CdS/Fe2O3 or Fe2O3/CdS cosensitized TNA were carried out and discussed.
Experimental section
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0.1M Fe(NO3)3 solution and 0.1M NaOH solution for 1 min at room temperature, sequentially. Then the materials were annealed in air at 500 C for 2 h for crystallization.
Preparation of TNA-CdS/Fe2O3 and TNA-Fe2O3/CdS electrodes An ultrasonic chemical bath deposition (CBD) is used for both CdS and Fe2O3 deposition. Two materials were sequentially deposited on TNA, using the same solutions described above, forming CdS/Fe2O3 or Fe2O3/CdS co-sensitized TNA photoelectrodes, respectively. TNA-Fe2O3/CdS means deposition of Fe2O3 on TNA first and then CdS is the second, and being the outer surface layer, while TNA-CdS/Fe2O3 is reversed.
Hydrogen generation The hydrogen generations from the CdS, Fe2O3-sensitized, CdS/Fe2O3 and Fe2O3/CdS co-sensitized TNA as well as a blank TNA film were recorded under a 400 W Hg lamp's illumination. The specimens were cut into a 2.5 cm2 area and sputtered a Pt layer on the TNA backside. The sputtering condition for Pt layer was 40 mA for 300 s. TNA specimens were placed in a quartz container with DI water and sealed. The hydrogen gas was collected using water replacement method for 8 h.
Preparation of TiO2 nanotube arrays
Characterization
Titanium foil (0.125 mm thick, 99.9% purity) was anodized at 60 V in an electrolyte containing 94.5 wt.% Ethylene glycol (EG), 0.5 wt.% NH4F and 5 wt.% deionized (DI) water at room temperature for 5 h. Ti foil was cleaned by ultra-sonication in a mixture of acetone and isopropanol for 30 min, followed by subsequent rinsing in DI water and drying. Electrochemical anodization of the Ti foils was performed using a Cu sheet as the counter electrode. After the first anodization, the Ti foil was ultrasonicated in ethanol to remove the electrolytes, followed by annealing at 450 C for 30 min to crystallize it. A secondary anodization was carried out on the as-annealed film in the same electrolyte solution for 15 min with an applied potential of 60 V.
The morphology and length of the anodic TNA after calcinations were observed by a field emission scanning electron microscope (JEOL JSM-7600F) equipped with an energy dispersive spectroscope (EDS, Oxford, 80 nm2). The crystalline phase and structure of TNA were analyzed using a PANalytical PW3040/60 X'Pert Pro X-ray diffractometer with a Cu target and Ni filter at a scanning rate of 4 /min from 2q ¼ 10 e80 . The chemical status of elements on the surface 1e10 nm of TNA were analyzed using X-ray Photoelectron Spectroscope (XPS, Thermo VG-Scientific Sigma Probe U.K.) using the energy source Al Ka. Photoluminescence (PL) data were obtained using a HITACHI F-7000 and the specimens were illuminated using a 246 nm light source. UVevis spectra of TNA and modified TNA were obtained using a SHIMADZU UV-2550 spectrophotometer, scanning from 190 to 800 nm. The photocurrent densityevoltage (IeV) curves were obtained u sing an electrochemical analyzer (CH Instruments 6273B, CH Instruments Co., USA) in a three-electrode configuration with a Pt wire net as the counter electrode and a saturated Ag/AgCl electrode as the reference electrode. A voltage sweep from 1.5 V to 0.5 V (versus Ag/AgCl) with a sweep rate of 10 mV s1 was employed for the electrochemical tests. A fixed area of 1 cm2 TNA was placed in a quartz container which contained 0.35 M Na2SO3 þ 0.25 M Na2S solution. The working electrode (TNA) was illuminated with a standard AM 1.5 solar simulator (SAN-EI Electric Co., Ltd XES40S1) from a distance of 10 cm.
Preparation of TNA-CdS electrodes CdS sensitizer was deposited on the TNA electrodes by immersing TNA into an aqueous solution of 0.1M Cd (ClO4)2 solution and 0.1M Na2S solution for 1 min at room temperature, sequentially. The process was known as a chemical bath deposition (CBD). An ultrasonic bath was employed in order to improve the uniformity of deposition. Comparison was made between CBD with and without ultrasonic assistance.
Preparation of TNA-Fe2O3 electrodes Fe2O3 sensitizer was deposited ultrasonically on the TNA electrodes by immersing TNA into an aqueous solution of
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Results and discussion Surface morphologies As shown in Fig. 1(a) and (b), FE-SEM images of the CdSsensitized TNA with and without ultrasonic in CBD process, respectively. It was clear that the deposition of CdS nanoparticles was uniform when CBD was done with ultrasonic process. On the other hand, the CdS was in a disorder manner on TNA when CBD was done without ultrasonic process (Fig. 1(b)). This disorder can probably lead to a decrease of the photocatalytic property due to the recombination of photoinduced charge carriers. Therefore, the co-sensitizers were deposited on TNA using ultrasonic CBD. The ultrasonic power in a chemical bath deposition provides an additional kinetic energy to overcome the surface tension of the solution into the tube, thereby reducing possible agglomeration of deposited nanoparticles to achieve uniform distribution. The surface tension of the solution was broken down due to the so-called cavitation bubbles collapsing. The cavitation bubbles are created instantaneously when the negative pressure caused by an ultrasonic wave crossing the liquid is large enough. The liquid breaks down and voids are created. These bubbles will collapse violently due to compression and each collapsing bubble can be considered as a micro-reactor in which temperatures and pressures are much higher than atmosphere. In this way, deposition materials such as CdS and Fe2O3 were considered to be uniformly coated on TNA, as evidences shown in Fig. 1(c) and (d). Fig. 1 (c) and (d) are the higher magnification of FESEM images of the CdS- and Fe2O3sensitized TNA, respectively. Although Fe2O3 is not as obvious
as CdS nanoparticles, the ultrasonic chemical bath does result in a uniform Fe2O3 layer over TNA. Surface elemental composition of TNA has been analyzed using EDS, which proves that the Fe element does exist on TNA.
X-ray photoelectron spectroscopy (XPS) The surface elements of CdS and Fe2O3-sensitized TNA were analyzed by XPS as shown in Fig. 2 and Fig. 3. For the CdSsensitized TNA, the Ti, O, C, Cd and S elements were observed and the corresponding photoelectron peaks appeared at binding energies 459.8 (Ti 2p), 531.2 (O1s), 285.3 (C 1s), 405.9 (Cd 3d) and 161.7 eV (S 2p), respectively. The photoelectron peak of C element is due to the residual carbon from the precursor solution and adventitious hydrocarbon from XPS instrument itself. The atomic ratio of Cd:S is ca. 1, confirming the formation of CdS. Fig. 2 (b) presents the high-resolution XPS spectrum of Cd 3d and S 2p for CdS-sensitized TNA. The binding energies corresponding to Cd 3d5/2 and Cd 3d3/2 are 405.9 and 412.7 eV, respectively, indicating the Cd2þ in CdS. The binding energies corresponding to S 2p is 161.7 eV, indicating the presence of S2 in CdS [37]. For the Fe2O3-sensitized TNA, the Ti, O, C and Fe elements are observed and the corresponding photoelectron peaks appear at binding energies 459.1 (Ti 2p), 530.5 (O 1s), 285.0 (C 1s) and 712.1 eV (Fe 2p), respectively. High-resolution XPS of the Fe element is shown in Fig. 3(b). The peaks located at 712.1 and 725.9 eV can be assigned to the Fe 2p3/2 and the Fe 2p1/2, respectively, in agreement with those for Fe2O3 [40e45]. Moreover, the appearance of the shake-up satellite line at 720.0 eV [41e45], a characteristic for the presence of Fe3þ state,
Fig. 1 e FE-SEM images of (a), (b) CdS-sensitized TNA with and without ultrasonic process during CBD, and (c) (d) high magnifications of CdS- and Fe2O3-sensitized TNA, respectively.
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Cd 3d
(a)
(a)
O 1s
Intensity (a.u.)
O 1s
Intensity (a.u.)
Cd 3p Ti 2p
C 1s S 2s
600
500
400 300 200 Binding Energy (eV)
Intensity (a.u.)
100
Ti 3p
0
700
600
500 400 300 Binding Energy (eV)
170
168 166 164 162 Binding Energy (eV)
160
158
156
Fig. 2 e (a) XPS survey spectra, and (b) high-resolution XPS spectra of CdS-sensitized TNA.
is clearly observed at Fig. 3(b). The peak position of Fe 2p3/2 has been intensively investigated and the values of between 710.6 and 711.3 eV have been reported [41e45]. The Fe 2p3/2 peak has associated satellite peaks. The satellite peak of Fe 2p3/2 for Fe2O3 is located approximately 8 eV higher than the main Fe 2p3/2 peak [41e45]. The satellite peak obtained at 720.0 eV is clearly distinguishable and does not overlap either the Fe 2p3/2 or Fe 2p1/2 peaks.
200
100
0
Fe 2p3/2
(b)
S 2p 2(SO4 )
172
800
S 2p
(b)
174
C 1s
Ti 3p
Fe 2p1/2
Intensity (a.u.)
700
S 2p
Ti 2p Fe 2p
satellite
735
730
725
720
715
710
705
700
Binding Energy (eV) Fig. 3 e (a) XPS survey spectra, and (b) high-resolution XPS spectra of Fe2O3-sensitized TNA. fluorescence intensity in Fig. 4 shows that blank TNA has the highest PL intensity. Therefore, blank TNA exhibits the highest recombination phenomena. All sensitized TNA showed a relatively low PL intensity. The drastic quenching of PL intensity suggests that CdS, Fe2O3 and their co-sensitized counterparts all remarkably enhanced the charge separation of photo-generated carriers.
Ultravioletevisible spectroscopy (UV) Photoluminescence (PL) PL spectra for blank TNA and sensitized TNA can be seen in Fig. 4. There was a clear decline in the intensity, indicating that after the deposition of compound Fe2O3 or CdS, the sensitized layer can increase the separation and transfer efficiency of electrons generated by the light, thereby preventing electrons and holes recombine to make electronic effective delivery and enhance its photocatalytic activity. It is well known that the PL signals of semiconductor materials result from the recombination of photo-induced charge carriers. In general, the lower the PL intensity, the lower the recombination rate of photo-induced electronehole pairs, and the higher the photocatalytic activity of semiconductor photocatalysts. The CdS, Fe2O3 and co-sensitized TNA emit visible light at room temperature with excitation wavelength 246 nm. The
The UVevisible absorption spectra of CdS, Fe2O3-sensitized and co-sensitized TNA as well as a blank TNA film are shown in Fig. 5. The absorption edges of the TNA-CdS and TNA-Fe2O3 electrodes appear at about 550 and 650 nm, respectively. For the co-sensitized electrodes (TNA-CdS/Fe2O3 and TNA-Fe2O3/ CdS), the absorption edge resembles that of Fe2O3-modified electrode. Apparently, the co-sensitized electrodes have complementary effects in the light harvest that it can improve the light absorption shortage of long wavelength legion of TNA-CdS. These results indicate that the narrowing of band gap of TNA using CdS and Fe2O3 nanoparticles. The band gap energy of TNAs can be estimated from the following equation: Eg ¼
1239:8 l
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where Eg is the band gap (eV) and l (nm) is the wavelength of the absorption edge in the spectrum.
Currentevoltage (IeV) curve Fig. 6 shows the photocurrent density versus measured potential (IeV) curves for the CdS, Fe2O3-sensitized and cosensitized TNA. These measurements were performed under illumination of AM1.5 at 100 mW/cm2. For TNA-blank, TNACdS, TNA-Fe2O3, TNA-Fe2O3/CdS, and TNA-CdS/Fe2O3, the observed photocurrent densities at 0.0 V are 0.188 mA/cm2, 1.618 mA/cm2, 0.400 mA/cm2, 0.904 mA/cm2 and 0.600 mA/ cm2, respectively. TNA-CdS exhibits the highest photocurrent density and TNA-Fe2O3/CdS is the second. The performance of TNA-Fe2O3/CdS was better than that of TNA-CdS/Fe2O3. It is thought that this dependents on their cascade structure and their stepwise energy levels for electrons (e) and holes (hþ) transition become drastically different, as shown in Schemes 1 and 2 [46]. As shown in Scheme 1 for TNA-Fe2O3/CdS, the
Fig. 6 e Current density versus potential for various electrodes measured under illumination of AM 1.5 light at 100 mW/cm2. Electrolyte: 0.35 M Na2SO3 þ 0.25 M Na2S.
TNA-blank TNA-CdS TNA-Fe2O3 Intensity (a.u.)
TNA-CdS/Fe2O3 TNA-Fe2O3/CdS
300
400 Wavelength (nm)
500
600
Fig. 4 e PL spectra of the CdS, Fe2O3-sensitized and cosensitized TNA.
Scheme 1 e Energy band diagram of TNA-Fe2O3/CdS.
Fig. 5 e UVevis absorption spectra of the CdS, Fe2O3sensitized and co-sensitized TNA.
electron generated from CdS is favorably moving to higher potential (downward) to Fe2O3, and then to TiO2. Although the electrons may not jump from Fe2O3 to TiO2 due to more negative potential (upward), the conduction band of TiO2 is still lower than that of CdS and the transition of electrons from CdS to TiO2 is possible. The generated holes from TiO2 will also easily move to lower potential (upward) to CdS, a perfect stepwise structure. However, in Scheme 2, conduction band of TiO2 and CdS is more negative than that of Fe2O3, making Fe2O3-generated electrons less likely to be transmitted to the titanium dioxide. In the meantime, the holes from TiO2 to CdS and then to Fe2O3 also encounter difficulties of transition due to higher potential of Fe2O3.
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The relative band edge levels of CdS and Fe2O3 were considered. After contact, Fermi levels of two semiconductors became stabilized. For TNA-CdS, which has the highest hydrogen generation rate in this study, the stepwise energy level is straight forward for electron downward and hole upward transition. For TNA-Fe2O3/CdS co-sensitized electrode (CdS is out surface), the conduction band of CdS lies above that of Fe2O3, and the valence band of Fe2O3 lies below that of CdS. When this electrode is irradiated by light with energy higher than its band gap, electron-hole pairs are created and the structure is advantageous to the transport of electrons and holes. On the other hand, for the inverse structure, TNA-CdS/ Fe2O3 electrode, where Fe2O3 is out surface, the conduction band of Fe2O3 lies below that of CdS, and the valence band of CdS lies above that of Fe2O3. The generated electrons and holes have difficulties to transition and therefore more opportunities to recombine. Therefore, TNA-Fe2O3/CdS electrode has higher hydrogen generation rate while TNA-CdS/Fe2O3 electrode has considerable lower hydrogen generation rate.
Conclusion Scheme 2 e Energy band diagram of TNA-CdS/Fe2O3.
Hydrogen generation The hydrogen generation from the CdS, Fe2O3-sensitized and co-sensitized TNA as well as a blank TNA film in water under 400 W Hg lamp's illumination are shown in Fig. 7. The hydrogen generation rate in 8 h for TNA-blank, TNA-CdS, TNA-Fe2O3, TNA-Fe2O3/CdS, and TNA-CdS/Fe2O3 are 2.43, 13.3, 3.75, 12.0, 1.54 ml/cm2h, respectively. It is clear that TNA-CdS and TNA-Fe2O3/CdS co-sensitized structure were much higher than other conditions. When CdS and Fe2O3 were inversely deposited on TNA as a TNA-CdS/Fe2O3 cascade structure, a much lower hydrogen generation rate was obtained compared to other electrodes. So the sequence of sensitizers is considered to play an important role on the hydrogen generation.
Hydrogen generation (ml)
Acknowledgment
TNA-blank TNA-CdS TNA-Fe2O3
400
The authors are grateful for the support from Ministry of Science and Technology, Taiwan, R.O.C. (MOST 102-2113-M033-008-MY3).
TNA-Fe2O3/CdS
300
TNA-CdS/Fe2O3
references
200
100
0
According to SEM and XPS, the CdS and Fe2O3 were successfully deposited on TNA. From PL and UVevis absorption spectra, the CdS- and Fe2O3/CdS-sensitized TNA have redshift absorption and less electron-hole recombination. From IeV curves, CdS and Fe2O3/CdS sensitized TNA electrodes exhibited higher photocurrent density than the blank TiO2 and other conditions. The hydrogen production rate of TNAFe2O3/CdS (12 ml/cm2 h1) is approximately 5 times higher than that of TNA-blank (2.43 ml cm2 h1). The efficiency of TNA-CdS/Fe2O3 is even lower than the TNA-blank. By depicting the energy level of these materials, as shown in Schemes 1 and 2, the structure of TNA-CdS/Fe2O3 leads a difficult transition of electrons and holes. The band edge of TNA-Fe2O3/CdS after contacting is considered to be advantageous for chargetransfer and causes a higher hydrogen generation rate.
0
2
4 Time (hr)
6
8
Fig. 7 e The hydrogen generation for various electrodes measured under 400 W Hg lamp illumination. Electrolyte: pure water.
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Fe2O3/CdS co-sensitized titania nanotube for hydrogen generation from photocatalytic splitting water Chin-Hao Chan, Prabu Samikkannu, Hong-Wen Wang* Department of Chemistry, Center for Nanotechnology, Chung-Yuan Christian University, Chungli, 320, Taiwan, ROC
article info
abstract
Article history:
CdS, Fe2O3-sensitized and Fe2O3/CdS co-sensitized TiO2 nanotube arrays (TNA) were
Received 21 May 2016
fabricated by depositing CdS and Fe2O3 nanoparticles onto the surface of TNA via an
Received in revised form
ultrasonic-assisted chemical bath deposition (CBD) method. Their effects on the photo-
28 July 2016
catalytic properties and hydrogen generation rate were evaluated. The results show that
Accepted 4 August 2016
CdS-sensitized and Fe2O3/CdS co-sensitized TNA have good absorption on long wavelength
Available online 27 August 2016
region and exhibit much higher hydrogen generation rates than those of other TNA specimens. The hydrogen generation rates for TNA-Fe2O3/CdS and TNA-CdS/Fe2O3 were 12
Keywords:
and 1.54 ml cm2 h1, respectively, where the TNA-Fe2O3/CdS refers to the deposition
TiO2 nanotube arrays
of Fe2O3 first on TNA and CdS the second, CdS being the out surface materials and con-
CdS
tacting water. TNA-CdS/Fe2O3 is a reversed structure of TNA-Fe2O3/CdS. The rate of
Fe2O3
hydrogen generation for TNA-Fe2O3/CdS was five times higher than that of blank TNA
Hydrogen generation
(2.43 ml,cm2 h1). It is considered that the low efficiency of TNA-CdS/Fe2O3 is due to a disadvantageous energy band structure for electrons and holes transition. TNA-Fe2O3/CdS has a stepwise energy band structure and the transition of excited electrons and holes is straightforward, while those of TNA-CdS/Fe2O3 has a difficult transition process due to a reverse structure. © 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The need for new energy resources, such as solar energy conversion, has evolved in recent decades. Solar energy can be directly converted into electricity through the use of solar cell [1e3]. Alternatively, solar hydrogen may be generated through the use of photoelectrochemical (PEC) water splitting [4e9]. Through the function of proton exchange membrane (PEM) fuel cells [10e24], electricity can be generated from the dissociation of hydrogen and reduction of oxygen. To generate
solar hydrogen efficiently, PEC water splitting must have a highly efficient photoelectrodes. Among the various photoelectrodes, those based on TiO2 have been actively studied. Since Fujishima and Honda discovered the photocatalytic splitting of water on TiO2 electrodes [25], TiO2 photocatalysis has been extensively investigated in the field of environmental protection and solar-cell materials owing to its great advantages in the decomposition of organic pollutants and relatively long charge carrier transport length, good stability, low-cost and low toxicity.
* Corresponding author. E-mail address: [email protected] (H.-W. Wang). http://dx.doi.org/10.1016/j.ijhydene.2016.08.026 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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One-dimensional nanostructures, particularly TiO2 nanotube arrays (TNA), have received intensive attentions due to their superior photocatalytic and photoelectronic performance [26]. TNA materials are of great interests for H2 generation from photocatalytic water splitting for its enhanced charge-collection efficiency by promoting faster transport and slower electron-hole recombination [27]. Although TiO2 is one of the most important materials widely studied for photoelectrodes and solar cells, the wide bandgap (3.2 eV) of TiO2 limits its photocatalytic property in the UV region. To extend the activity of the photoelectrode into the visible light region, various approaches were employed such as purity of Ti foils [28], doping TiO2 with other impurities [29e33], and sensitizing TiO2 with semiconductors [34e39], which absorb light in the visible region. In this study, semiconductor such as Fe2O3 [34] and CdS [35e38] were deposited on TNA electrodes using ultrasonic chemical bath deposition (CBD) [36]. The utilization of CdS or Fe2O3 materials on TNA is a facile way to induce photocatalytic water decomposition under visible light irradiation due to that the CdS and Fe2O3 has smaller band gap energy (2.4 eV and 2.2 eV, respectively) [34e38]. There are many reports on single component sensitized TiO2 nanotubes [34e39], but not many in CdS/Fe2O3 or Fe2O3/CdS co-sensitized TNA. In this study, the characterization and hydrogen generation of CdS-, Fe2O3-sensitized, CdS/Fe2O3 or Fe2O3/CdS cosensitized TNA were carried out and discussed.
Experimental section
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0.1M Fe(NO3)3 solution and 0.1M NaOH solution for 1 min at room temperature, sequentially. Then the materials were annealed in air at 500 C for 2 h for crystallization.
Preparation of TNA-CdS/Fe2O3 and TNA-Fe2O3/CdS electrodes An ultrasonic chemical bath deposition (CBD) is used for both CdS and Fe2O3 deposition. Two materials were sequentially deposited on TNA, using the same solutions described above, forming CdS/Fe2O3 or Fe2O3/CdS co-sensitized TNA photoelectrodes, respectively. TNA-Fe2O3/CdS means deposition of Fe2O3 on TNA first and then CdS is the second, and being the outer surface layer, while TNA-CdS/Fe2O3 is reversed.
Hydrogen generation The hydrogen generations from the CdS, Fe2O3-sensitized, CdS/Fe2O3 and Fe2O3/CdS co-sensitized TNA as well as a blank TNA film were recorded under a 400 W Hg lamp's illumination. The specimens were cut into a 2.5 cm2 area and sputtered a Pt layer on the TNA backside. The sputtering condition for Pt layer was 40 mA for 300 s. TNA specimens were placed in a quartz container with DI water and sealed. The hydrogen gas was collected using water replacement method for 8 h.
Preparation of TiO2 nanotube arrays
Characterization
Titanium foil (0.125 mm thick, 99.9% purity) was anodized at 60 V in an electrolyte containing 94.5 wt.% Ethylene glycol (EG), 0.5 wt.% NH4F and 5 wt.% deionized (DI) water at room temperature for 5 h. Ti foil was cleaned by ultra-sonication in a mixture of acetone and isopropanol for 30 min, followed by subsequent rinsing in DI water and drying. Electrochemical anodization of the Ti foils was performed using a Cu sheet as the counter electrode. After the first anodization, the Ti foil was ultrasonicated in ethanol to remove the electrolytes, followed by annealing at 450 C for 30 min to crystallize it. A secondary anodization was carried out on the as-annealed film in the same electrolyte solution for 15 min with an applied potential of 60 V.
The morphology and length of the anodic TNA after calcinations were observed by a field emission scanning electron microscope (JEOL JSM-7600F) equipped with an energy dispersive spectroscope (EDS, Oxford, 80 nm2). The crystalline phase and structure of TNA were analyzed using a PANalytical PW3040/60 X'Pert Pro X-ray diffractometer with a Cu target and Ni filter at a scanning rate of 4 /min from 2q ¼ 10 e80 . The chemical status of elements on the surface 1e10 nm of TNA were analyzed using X-ray Photoelectron Spectroscope (XPS, Thermo VG-Scientific Sigma Probe U.K.) using the energy source Al Ka. Photoluminescence (PL) data were obtained using a HITACHI F-7000 and the specimens were illuminated using a 246 nm light source. UVevis spectra of TNA and modified TNA were obtained using a SHIMADZU UV-2550 spectrophotometer, scanning from 190 to 800 nm. The photocurrent densityevoltage (IeV) curves were obtained u sing an electrochemical analyzer (CH Instruments 6273B, CH Instruments Co., USA) in a three-electrode configuration with a Pt wire net as the counter electrode and a saturated Ag/AgCl electrode as the reference electrode. A voltage sweep from 1.5 V to 0.5 V (versus Ag/AgCl) with a sweep rate of 10 mV s1 was employed for the electrochemical tests. A fixed area of 1 cm2 TNA was placed in a quartz container which contained 0.35 M Na2SO3 þ 0.25 M Na2S solution. The working electrode (TNA) was illuminated with a standard AM 1.5 solar simulator (SAN-EI Electric Co., Ltd XES40S1) from a distance of 10 cm.
Preparation of TNA-CdS electrodes CdS sensitizer was deposited on the TNA electrodes by immersing TNA into an aqueous solution of 0.1M Cd (ClO4)2 solution and 0.1M Na2S solution for 1 min at room temperature, sequentially. The process was known as a chemical bath deposition (CBD). An ultrasonic bath was employed in order to improve the uniformity of deposition. Comparison was made between CBD with and without ultrasonic assistance.
Preparation of TNA-Fe2O3 electrodes Fe2O3 sensitizer was deposited ultrasonically on the TNA electrodes by immersing TNA into an aqueous solution of
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Results and discussion Surface morphologies As shown in Fig. 1(a) and (b), FE-SEM images of the CdSsensitized TNA with and without ultrasonic in CBD process, respectively. It was clear that the deposition of CdS nanoparticles was uniform when CBD was done with ultrasonic process. On the other hand, the CdS was in a disorder manner on TNA when CBD was done without ultrasonic process (Fig. 1(b)). This disorder can probably lead to a decrease of the photocatalytic property due to the recombination of photoinduced charge carriers. Therefore, the co-sensitizers were deposited on TNA using ultrasonic CBD. The ultrasonic power in a chemical bath deposition provides an additional kinetic energy to overcome the surface tension of the solution into the tube, thereby reducing possible agglomeration of deposited nanoparticles to achieve uniform distribution. The surface tension of the solution was broken down due to the so-called cavitation bubbles collapsing. The cavitation bubbles are created instantaneously when the negative pressure caused by an ultrasonic wave crossing the liquid is large enough. The liquid breaks down and voids are created. These bubbles will collapse violently due to compression and each collapsing bubble can be considered as a micro-reactor in which temperatures and pressures are much higher than atmosphere. In this way, deposition materials such as CdS and Fe2O3 were considered to be uniformly coated on TNA, as evidences shown in Fig. 1(c) and (d). Fig. 1 (c) and (d) are the higher magnification of FESEM images of the CdS- and Fe2O3sensitized TNA, respectively. Although Fe2O3 is not as obvious
as CdS nanoparticles, the ultrasonic chemical bath does result in a uniform Fe2O3 layer over TNA. Surface elemental composition of TNA has been analyzed using EDS, which proves that the Fe element does exist on TNA.
X-ray photoelectron spectroscopy (XPS) The surface elements of CdS and Fe2O3-sensitized TNA were analyzed by XPS as shown in Fig. 2 and Fig. 3. For the CdSsensitized TNA, the Ti, O, C, Cd and S elements were observed and the corresponding photoelectron peaks appeared at binding energies 459.8 (Ti 2p), 531.2 (O1s), 285.3 (C 1s), 405.9 (Cd 3d) and 161.7 eV (S 2p), respectively. The photoelectron peak of C element is due to the residual carbon from the precursor solution and adventitious hydrocarbon from XPS instrument itself. The atomic ratio of Cd:S is ca. 1, confirming the formation of CdS. Fig. 2 (b) presents the high-resolution XPS spectrum of Cd 3d and S 2p for CdS-sensitized TNA. The binding energies corresponding to Cd 3d5/2 and Cd 3d3/2 are 405.9 and 412.7 eV, respectively, indicating the Cd2þ in CdS. The binding energies corresponding to S 2p is 161.7 eV, indicating the presence of S2 in CdS [37]. For the Fe2O3-sensitized TNA, the Ti, O, C and Fe elements are observed and the corresponding photoelectron peaks appear at binding energies 459.1 (Ti 2p), 530.5 (O 1s), 285.0 (C 1s) and 712.1 eV (Fe 2p), respectively. High-resolution XPS of the Fe element is shown in Fig. 3(b). The peaks located at 712.1 and 725.9 eV can be assigned to the Fe 2p3/2 and the Fe 2p1/2, respectively, in agreement with those for Fe2O3 [40e45]. Moreover, the appearance of the shake-up satellite line at 720.0 eV [41e45], a characteristic for the presence of Fe3þ state,
Fig. 1 e FE-SEM images of (a), (b) CdS-sensitized TNA with and without ultrasonic process during CBD, and (c) (d) high magnifications of CdS- and Fe2O3-sensitized TNA, respectively.
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Cd 3d
(a)
(a)
O 1s
Intensity (a.u.)
O 1s
Intensity (a.u.)
Cd 3p Ti 2p
C 1s S 2s
600
500
400 300 200 Binding Energy (eV)
Intensity (a.u.)
100
Ti 3p
0
700
600
500 400 300 Binding Energy (eV)
170
168 166 164 162 Binding Energy (eV)
160
158
156
Fig. 2 e (a) XPS survey spectra, and (b) high-resolution XPS spectra of CdS-sensitized TNA.
is clearly observed at Fig. 3(b). The peak position of Fe 2p3/2 has been intensively investigated and the values of between 710.6 and 711.3 eV have been reported [41e45]. The Fe 2p3/2 peak has associated satellite peaks. The satellite peak of Fe 2p3/2 for Fe2O3 is located approximately 8 eV higher than the main Fe 2p3/2 peak [41e45]. The satellite peak obtained at 720.0 eV is clearly distinguishable and does not overlap either the Fe 2p3/2 or Fe 2p1/2 peaks.
200
100
0
Fe 2p3/2
(b)
S 2p 2(SO4 )
172
800
S 2p
(b)
174
C 1s
Ti 3p
Fe 2p1/2
Intensity (a.u.)
700
S 2p
Ti 2p Fe 2p
satellite
735
730
725
720
715
710
705
700
Binding Energy (eV) Fig. 3 e (a) XPS survey spectra, and (b) high-resolution XPS spectra of Fe2O3-sensitized TNA. fluorescence intensity in Fig. 4 shows that blank TNA has the highest PL intensity. Therefore, blank TNA exhibits the highest recombination phenomena. All sensitized TNA showed a relatively low PL intensity. The drastic quenching of PL intensity suggests that CdS, Fe2O3 and their co-sensitized counterparts all remarkably enhanced the charge separation of photo-generated carriers.
Ultravioletevisible spectroscopy (UV) Photoluminescence (PL) PL spectra for blank TNA and sensitized TNA can be seen in Fig. 4. There was a clear decline in the intensity, indicating that after the deposition of compound Fe2O3 or CdS, the sensitized layer can increase the separation and transfer efficiency of electrons generated by the light, thereby preventing electrons and holes recombine to make electronic effective delivery and enhance its photocatalytic activity. It is well known that the PL signals of semiconductor materials result from the recombination of photo-induced charge carriers. In general, the lower the PL intensity, the lower the recombination rate of photo-induced electronehole pairs, and the higher the photocatalytic activity of semiconductor photocatalysts. The CdS, Fe2O3 and co-sensitized TNA emit visible light at room temperature with excitation wavelength 246 nm. The
The UVevisible absorption spectra of CdS, Fe2O3-sensitized and co-sensitized TNA as well as a blank TNA film are shown in Fig. 5. The absorption edges of the TNA-CdS and TNA-Fe2O3 electrodes appear at about 550 and 650 nm, respectively. For the co-sensitized electrodes (TNA-CdS/Fe2O3 and TNA-Fe2O3/ CdS), the absorption edge resembles that of Fe2O3-modified electrode. Apparently, the co-sensitized electrodes have complementary effects in the light harvest that it can improve the light absorption shortage of long wavelength legion of TNA-CdS. These results indicate that the narrowing of band gap of TNA using CdS and Fe2O3 nanoparticles. The band gap energy of TNAs can be estimated from the following equation: Eg ¼
1239:8 l
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where Eg is the band gap (eV) and l (nm) is the wavelength of the absorption edge in the spectrum.
Currentevoltage (IeV) curve Fig. 6 shows the photocurrent density versus measured potential (IeV) curves for the CdS, Fe2O3-sensitized and cosensitized TNA. These measurements were performed under illumination of AM1.5 at 100 mW/cm2. For TNA-blank, TNACdS, TNA-Fe2O3, TNA-Fe2O3/CdS, and TNA-CdS/Fe2O3, the observed photocurrent densities at 0.0 V are 0.188 mA/cm2, 1.618 mA/cm2, 0.400 mA/cm2, 0.904 mA/cm2 and 0.600 mA/ cm2, respectively. TNA-CdS exhibits the highest photocurrent density and TNA-Fe2O3/CdS is the second. The performance of TNA-Fe2O3/CdS was better than that of TNA-CdS/Fe2O3. It is thought that this dependents on their cascade structure and their stepwise energy levels for electrons (e) and holes (hþ) transition become drastically different, as shown in Schemes 1 and 2 [46]. As shown in Scheme 1 for TNA-Fe2O3/CdS, the
Fig. 6 e Current density versus potential for various electrodes measured under illumination of AM 1.5 light at 100 mW/cm2. Electrolyte: 0.35 M Na2SO3 þ 0.25 M Na2S.
TNA-blank TNA-CdS TNA-Fe2O3 Intensity (a.u.)
TNA-CdS/Fe2O3 TNA-Fe2O3/CdS
300
400 Wavelength (nm)
500
600
Fig. 4 e PL spectra of the CdS, Fe2O3-sensitized and cosensitized TNA.
Scheme 1 e Energy band diagram of TNA-Fe2O3/CdS.
Fig. 5 e UVevis absorption spectra of the CdS, Fe2O3sensitized and co-sensitized TNA.
electron generated from CdS is favorably moving to higher potential (downward) to Fe2O3, and then to TiO2. Although the electrons may not jump from Fe2O3 to TiO2 due to more negative potential (upward), the conduction band of TiO2 is still lower than that of CdS and the transition of electrons from CdS to TiO2 is possible. The generated holes from TiO2 will also easily move to lower potential (upward) to CdS, a perfect stepwise structure. However, in Scheme 2, conduction band of TiO2 and CdS is more negative than that of Fe2O3, making Fe2O3-generated electrons less likely to be transmitted to the titanium dioxide. In the meantime, the holes from TiO2 to CdS and then to Fe2O3 also encounter difficulties of transition due to higher potential of Fe2O3.
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The relative band edge levels of CdS and Fe2O3 were considered. After contact, Fermi levels of two semiconductors became stabilized. For TNA-CdS, which has the highest hydrogen generation rate in this study, the stepwise energy level is straight forward for electron downward and hole upward transition. For TNA-Fe2O3/CdS co-sensitized electrode (CdS is out surface), the conduction band of CdS lies above that of Fe2O3, and the valence band of Fe2O3 lies below that of CdS. When this electrode is irradiated by light with energy higher than its band gap, electron-hole pairs are created and the structure is advantageous to the transport of electrons and holes. On the other hand, for the inverse structure, TNA-CdS/ Fe2O3 electrode, where Fe2O3 is out surface, the conduction band of Fe2O3 lies below that of CdS, and the valence band of CdS lies above that of Fe2O3. The generated electrons and holes have difficulties to transition and therefore more opportunities to recombine. Therefore, TNA-Fe2O3/CdS electrode has higher hydrogen generation rate while TNA-CdS/Fe2O3 electrode has considerable lower hydrogen generation rate.
Conclusion Scheme 2 e Energy band diagram of TNA-CdS/Fe2O3.
Hydrogen generation The hydrogen generation from the CdS, Fe2O3-sensitized and co-sensitized TNA as well as a blank TNA film in water under 400 W Hg lamp's illumination are shown in Fig. 7. The hydrogen generation rate in 8 h for TNA-blank, TNA-CdS, TNA-Fe2O3, TNA-Fe2O3/CdS, and TNA-CdS/Fe2O3 are 2.43, 13.3, 3.75, 12.0, 1.54 ml/cm2h, respectively. It is clear that TNA-CdS and TNA-Fe2O3/CdS co-sensitized structure were much higher than other conditions. When CdS and Fe2O3 were inversely deposited on TNA as a TNA-CdS/Fe2O3 cascade structure, a much lower hydrogen generation rate was obtained compared to other electrodes. So the sequence of sensitizers is considered to play an important role on the hydrogen generation.
Hydrogen generation (ml)
Acknowledgment
TNA-blank TNA-CdS TNA-Fe2O3
400
The authors are grateful for the support from Ministry of Science and Technology, Taiwan, R.O.C. (MOST 102-2113-M033-008-MY3).
TNA-Fe2O3/CdS
300
TNA-CdS/Fe2O3
references
200
100
0
According to SEM and XPS, the CdS and Fe2O3 were successfully deposited on TNA. From PL and UVevis absorption spectra, the CdS- and Fe2O3/CdS-sensitized TNA have redshift absorption and less electron-hole recombination. From IeV curves, CdS and Fe2O3/CdS sensitized TNA electrodes exhibited higher photocurrent density than the blank TiO2 and other conditions. The hydrogen production rate of TNAFe2O3/CdS (12 ml/cm2 h1) is approximately 5 times higher than that of TNA-blank (2.43 ml cm2 h1). The efficiency of TNA-CdS/Fe2O3 is even lower than the TNA-blank. By depicting the energy level of these materials, as shown in Schemes 1 and 2, the structure of TNA-CdS/Fe2O3 leads a difficult transition of electrons and holes. The band edge of TNA-Fe2O3/CdS after contacting is considered to be advantageous for chargetransfer and causes a higher hydrogen generation rate.
0
2
4 Time (hr)
6
8
Fig. 7 e The hydrogen generation for various electrodes measured under 400 W Hg lamp illumination. Electrolyte: pure water.
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