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Role of CoOx cocatalyst on Ta3N5 photocatalysts studied by transient visible to mid-infrared absorption spectroscopy
Accepted Manuscript Title: Role of CoOx Cocatalyst on Ta3 N5 Photocatalysts Studied by Transient Visible to Mid-Infrared Absorption Spectroscopy Authors: Junie Jhon M. Vequizo, Mirabbos Hojamberdiev, Katsuya Teshima, Akira Yamakata PII: DOI: Reference:
S1010-6030(17)30863-8 http://dx.doi.org/10.1016/j.jphotochem.2017.09.005 JPC 10850
To appear in:
Journal of Photochemistry and Photobiology A: Chemistry
Received date: Revised date: Accepted date:
20-6-2017 25-8-2017 3-9-2017
Please cite this article as: Junie Jhon M.Vequizo, Mirabbos Hojamberdiev, Katsuya Teshima, Akira Yamakata, Role of CoOx Cocatalyst on Ta3N5 Photocatalysts Studied by Transient Visible to Mid-Infrared Absorption Spectroscopy, Journal of Photochemistry and Photobiology A: Chemistryhttp://dx.doi.org/10.1016/j.jphotochem.2017.09.005 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Role of CoOx Cocatalyst on Ta3N5 Photocatalysts Studied by Transient Visible to Mid-Infrared Absorption Spectroscopy
Junie Jhon M. Vequizo,a Mirabbos Hojamberdiev,b Katsuya Teshima,b and Akira Yamakataa,*
a
Graduate School of Engineering, Toyota Technological Institute, 2-12-1 Hisakata,
Tempaku, Nagoya 468-8511, Japan b
Department of Environmental Science and Technology, Faculty of Engineering, Shinshu
University, 4-17-1 Wakasato, Nagano 380-8553, Japan
Corresponding Author *Akira Yamakata, E-mail: [email protected]
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Graphical abstract
Highlights
CoOx cocatalyst efficiently captured holes enhancing charge separation.
By CoOx loading on Ta3N5, the recombination centers were significantly reduced.
Decay kinetics of charge carriers were studied by transient absorption spectroscopy.
Most of the electrons in bare Ta3N5 are deeply trapped at the defects.
CoOx is more effective than MeOH in scavenging of holes in metal nitrides/oxynitrides.
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Abstract Tantalum nitride (Ta3N5) photocatalysts attract considerable attention owing to their visible light absorption below 600 nm and suitable band structures for reduction and oxidation of water to generate hydrogen and oxygen gas. Herein, we investigated the effects of CoOx on the dynamics of photogenerated charge carriers in Ta3N5 powder photocatalysts by using microsecond to millisecond transient visible to mid-IR absorption spectroscopy. Transient absorption measurements revealed that most of the electrons in Ta3N5 are deeply trapped at the defects, most likely originating from oxygen vacancies and reduced Ta species. Two important roles of CoOx were elucidated: effective capture of holes to prolong electron lifetime and reduction of defects as recombination centers, which further elongates the lifetime of both electrons and holes.
Key Words: Photocatalysts, Co-catalysts, Charge separation, Time-resolved measurements
1. Introduction Tantalum nitride (Ta3N5) gains great attention owing to its visible light absorption below 600 nm and suitable band structures for the reduction and oxidation of water to generate hydrogen and oxygen gas, respectively. For instance, it is reported that photoelectrodes consisted of Ta3N5 nanorods exhibited high activity for water oxidation under the irradiation of 440-nm light at 1.23 V vs RHE1: an incident photon-to-current
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conversion efficiency (IPCE) reaches to ~41%. However, in the case of photocatalytic water splitting reaction on Ta3N5 nanoparticles, the activity is very low. The low performance of as-synthesized Ta3N5 is usually attributed to the defects formed during the synthesis of the powders: Ta3N5 particles are commonly prepared under high-temperature and highly reductive nitridation of Ta2O5 precursor.2-3 Surface defects such as nitrogen and oxygen vacancies and reduced Ta species can be easily formed. These defects are believed to decrease the photocatalytic performance. On the contrary, theoretical calculation has predicted that defects in Ta3N5 are essential to generate both H2 and O2 gas compared to that of perfect Ta3N5.4 This prediction seems to be contradicting, so understanding the behaviors of photocarriers at the defects on Ta3N5 is indispensable to further improve the overall water splitting reaction using Ta3N5. It is worthy to mention that the activity of Ta3N5 nanoparticulates can be greatly enhanced by modifying their surface with CoO x,5 and CoOx-MgO6. However, how CoOx affects the behavior of electrons and holes in Ta3N5 is still not fully elucidated. A key route to improve the photocatalytic performance of the metal nitride/oxynitride nanoparticulate photocatalysts (LaTiO2N, Ta3N5 among others) is to understand the mechanism involving photogenerated charge carriers upon band gap excitation. Taking LaTiO2N photocatalyst as an example, we have reported that the oxygen vacancies in LaTiO2N deeply trapped the photogenerated electrons7. As a result, the photocatalytic reduction is greatly decreased, i.e. the activity for H2 generation is very low due to the deep electron trapping at the defects. It is widely established that loading of co-catalysts such as Pt,7-10 IrO2,11-12 and CoOx7, 13-16 enhances the photocatalytic activities. However, in the case
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of LaTiO2N, we have reported that loading of Pt is not that effective compared to CoOx loading.7 We have reported that electron-capture by Pt is very slow proceeding in microseconds since most of electrons are deeply trapped at the defects. On the other hand, hole capture by CoOx proceeds within a few ps and hence it is more efficient. Herein, we investigated the effect of CoOx on Ta3N5 photocatalyst by transient visible to mid-IR absorption spectroscopy, which is very effective tool to study the dynamics of photogenerated charge carriers. Transient absorption measurements revealed that aside from the role of CoOx in capturing holes, loading CoOx cocatalyst can reduce the recombination centers that originated from adventitious defects on the Ta3N5 catalysts. The detailed effects of CoOx-loading on the decay kinetics of photogenerated electrons and holes were investigated and discussed.
2. Experimental Section Ta3N5 powders were synthesized by nitriding 1 g of Ta2O5 precursor powders (99.9%, Wako Pure Chemical Industries Ltd) at 850°C for 10 h × 2 times at a heating rate of 10°C min−1 under an NH3 flow (300 mL min-1) in a vertical tubular furnace. CoOx (2 wt% Co) nanoparticles as a cocatalyst for water oxidation were loaded on Ta3N5 particles by impregnation method using Co(NO3)2∙6H2O (99.9%, Kanto Chemicals) aqueous solution, after which the Co-impregnated Ta3N5 powders were then heat treated under an NH3 flow (200 mL min-1) at 500°C for 1 h and subsequently followed by calcination at 200°C for 1 h.14
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For the transient absorption spectroscopic experiments, the as-synthesized Ta3N5 and CoOx-loaded Ta3N5 powders were used without further treatments. Ta3N5 and CoOx-loaded Ta3N5 powders were fixed separately on a circular CaF2 plate with a density of 1.5 mg cm-1 and placed in the closed IR cell. Prior to transient absorption measurements, the cell was evacuated at room temperature.7, 16 For the reactivity measurement, the powder Ta3N5 was exposed to 20-Torr oxygen and methanol. Microsecond time-resolved visible to mid-IR absorption measurements were performed by using the laboratory-built spectrometers as reported in our previous papers.
17-18
Briefly,
in the mid-IR region (6000 ~ 1000 cm-1), the probe light emitted from a MoSi2 coil was focused on the sample and the transmitted light was introduced to the grating spectrometer. The monochromated light was then detected by an MCT detector (Kolmar) and the output electric signal was amplified with an AC-coupled amplifier (Stanford Research Systems, SR560, 1 MHz). In the case of visible to NIR region (20000 ~ 6000 cm-1), halogen lamp (with a power output of 50 W) was used as probe light and Si or InGaAs photodiodes were utilized as detectors, and the experiments were performed in the reflection mode, i.e. the diffuse reflected light from the sample was transmitted to the spectrometer with installed gratings and then detected by photodetectors. In each experiment, a 355 nm-UV pulse from a Nd:YAG laser (Continuum, Surelite I, duration: 6 ns, power: 0.5 mJ, repetition rate: 10~0.01 Hz) was used to excite the band gap of Ta3N5 and CoOx-Ta3N5 photocatalysts. The time resolution of the spectrometers was limited to be 1 ~ 2μs by the bandwidth of the amplifier.
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3. Results and Discussion 3.1 Transient Absorption Spectra of Ta3N5 and CoOx-loaded Ta3N5 Transient absorption spectra of bare Ta3N5 after band gap excitation using 355 nm (3.49 eV) laser pulses are shown in Figure 1A. The TA spectra were measured from 5 μs to 1 millisecond. The spectra exhibit strong absorption peak at 17000 cm-1 (588 nm, 2.11eV) and a very broad absorption spanning from 13000 - 2000 cm−1, suggesting that band gap excitation induces these absorptions. The absorption peak at 17000 cm−1 is assigned to photogenerated holes since the intensity of this absorption band decreases upon exposure to methanol (MeOH) vapor as shown in Figure 2. The decay is accelerated within 0 – 2 μs in the presence of MeOH compared to that in vacuum, suggesting that holes are consumed by MeOH. Furthermore, the intensity slightly increased by the exposure to oxygen, suggesting the electrons are little consumed by O2. Photogenerated holes have been reported to usually give absorption peaks in the UV to visible region. For instance, holes in TiO2 exhibit peaks at 360,17 450, and 520 nm19-21 and that of hematite α-Fe2O3 is at 580 nm.22-23The absorption peak at 17000 cm-1 for Ta3N5 is similar to that observed for LaTiO2N,7, 16 BaTaO2N,15 and BaNbO2N13 photocatalysts, where the absorption peak was also observed in the visible region (17000 – 15000 cm-1). These absorptions of holes have been proposed to be responsible for the electron transition from valence band (VB) to holes trapped at surface defects such as OH species, O2-, and N3-. The transient absorption observed at 13000 – 2000 cm-1 in Figure 1A can be assigned to shallowly trapped and deeply trapped electrons. The intensity of deeply trapped electrons (for instance at 10000 cm-1) is higher than that of shallowly trapped electrons
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(3000 cm-1), suggesting that most of the electrons are deeply trapped at the mid gap states, which originate from the surface defects in Ta3N5 such as oxygen vacancies and reduced Ta species. This very broad absorption of trapped electrons has been also observed in BaTaO2N photocatalysts.15 On the other hand, the negative absorption signal at < 2000 cm-1 (Figure 1A) can be attributed to thermal emission due to rapid recombination at the defects. Rapid recombination is dependent on the excitation energy, i.e. the larger the excitation intensity, the faster the recombination of electrons and holes. However, the characteristic absorption of free electrons in Ta3N5 can be clearly discernable at low excitation intensity (Figure S1). When CoOx was loaded on Ta3N5, the relative intensities of the absorption peak at 17000 cm-1 and the absorption at < 2000 cm-1 were significantly changed: the intensity at 17000 cm-1 decreased and that of absorption at < 2000 cm-1 increased. The decrease in the intensity at 17000 cm-1 indicates holes are captured by CoOx. In the case of the absorption at < 3000 cm-1, the intensity was drastically increased by the CoOx loading. The spectral shape resembles to that of absorption feature of free electrons, suggesting that the number of free electrons increased by CoOx loading on Ta3N5. When holes are captured by CoOx, the number of surviving electrons increases since the recombination of electrons and holes is suppressed, i.e. electrons remain in Ta3N5 and holes are transferred to CoOx. The hole capturing by CoOx is also evident from the appearance of a new peak at ~12500 cm-1 (800 nm), which is assigned to d-d transition of Co3+ species. It has been reported that oxidation of Co2+ to Co3+ notably changes the color of CoOx from yellow to gray.22-24 Therefore, it would also be plausible that Co2+ captures holes and then converted to Co3+. We have
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observed this peak also appear at 12500 cm-1 for CoOx-loaded LaTiO2N.7, 16 Interestingly, in metal nitride/oxynitride photocatalyst, the capability of hole capture using CoO x cocatalyst is more effective than hole-consuming reaction facilitated in the presence of MeOH as shown in Figure 2.
3.2 Decay kinetics of e- and h+ in bare and CoOx-loaded Ta3N5 The decay kinetics of photogenerated electrons and holes in Ta3N5 and CoOx loaded Ta3N5 are examined by observing the changes in the absorption intensity at 17000 cm-1 (trapped holes), 8000 cm-1 (deeply trapped electrons) and 3000 cm-1 (shallowly trapped electrons) as shown in Figure 3. In the case of the absorption intensity at 17000 cm-1 (Figure 3A), the intensity decreases at 0 – 2 μs by CoOx loading. The decrease in the intensity suggests that the holes are captured by CoOx within 2 s or recombination was enhanced by CoOx-loading. However, we cannot precisely determine the principal reason by just observing the behavior of holes. The behavior of electrons should also be examined. On the other hand, the number of surviving shallowly trapped electrons (3000 cm-1) increased at 0 - 2μs (Figure 3C). From this experimental result, we can rule out the possibility of enhanced recombination because the lifetime of electrons becomes longer upon CoOx loading: when the holes are captured by CoOx, the probability of electrons in Ta3N5 to recombine with holes in CoOx is drastically reduced, which results to a spatially separated electrons and holes in Ta3N5 and CoOx, respectively. The prolongation of electron lifetime (increase of electron density) can also be demonstrated in the presence of
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hole scavenging molecules, MeOH, such as on TiO2 and SrTiO3. However, in the case of Ta3N5, MeOH is not as effective as CoOx. Similar observation has been reported for LaTiO2N, in which MeOH is also not efficient whereas the hole capture by CoOx proceeds within few picoseconds.7, 25 Our results confirm us that CoOx works as very effective hole scavenger than MeOH for Ta3N5 photocatalyst. In contrast, in the case of deeply trapped electrons giving the transient absorption at 13000 ~ 6000 cm-1 (Figure 3B), the results are totally different from that of shallowly trapped electrons: the absorption intensity at 8000 cm-1 decreased upon CoOx loading. This result suggests that the number of deeply trapped electrons is reduced, indicating that the structure of the defects was changed by the CoOx-loading. Heating treatment performed during the deposition of CoOx particles and attachment of smaller CoOx particles at the defects are the plausible reasons why the defect-structure changed upon CoOx loading.
3.3 Reduction of the recombination center by CoOx-loading To further investigate the reduction of defects by CoOx loading on Ta3N5, we examined the decay behavior of electrons and holes after 100 μs at 3000 and 17000 cm-1, respectively (shown in Figure 3). The absorption intensity of holes at 17000 cm-1 is larger on bare Ta3N5 than CoOx-loaded samples at 0 ~ 50 s, but that after 100 s is opposite: the decay of holes is accelerated after 100 s on bare Ta3N5. The lifetimes of charge carriers were determined by using double-exponential curve fitting function. The time constants for the decay of holes are estimated to be τ1 = 8.2 ± 0.0 µs and τ2 = 87.2 ± 0.0 µs for bare Ta3N5, while τ1 =
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22.7 ± 0.0 µs and τ2 = 487.0 ± 0.1 µs for CoOx-loaded Ta3N5. The results indicate that the lifetime of holes is significantly elongated in CoOx-loaded Ta3N5, which also suggests that the decay rate of holes is decelerated upon CoOx loading. In the case of shallowly trapped electrons at 3000 cm-1, the results are similar where the decay of electrons after 100 μs is much accelerated in bare Ta3N5 compared to that in CoOx-loaded Ta3N5. The time constants for the decay of electrons are approximated to be τ1 = 269.0 ± 0.0 µs and τ2 = 468.0 ± 0.0 µs for bare Ta3N5, while τ1 = 21.5 ± 0.0 µs and τ2 = 701.0 ± 0.1 µs for CoOxloaded Ta3N5, indicating that the lifetime of electrons is prolonged by loading CoO x. The faster decay of electrons and holes in bare Ta3N5 after 100 s would be due to the electronhole recombination at the defects. However, it is notable that this fast decay component is diminished by CoOx loading, suggesting that the recombination centers that is responsible to the decay after 100 s are significantly reduced by CoOx-loading. This result is consistent to the decrease of the deeply trapped electrons observed in Figure 1 and Figure 3B: the intensity at 10000 - 6000 cm-1 decreased by CoOx-loading. In the case of LaTiO2N, the depth of the electron trap becomes shallower from 6000 to 4000 cm-1. However, in the case of Ta3N5, the deeply trapped electron decreased and the free electrons increased, implying that CoOx clearly affects the defect structure in Ta3N5. The decay kinetics of electrons and holes discussed above give us insights how the photocatalytic activity is enhanced by CoOx loading. The photocatalytic activity of Ta3N5 photocatalyst for water oxidation has already been reported in many studies using powders and in photoelectrode systems.1, 5, 26-30 Without co-catalyst, the activity for water oxidation is very low even in the presence of AgNO3 as electron scavenger. However, loading of
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CoOx drastically increased the activity, signifying the important role of CoOx for activity enhancement. There is no doubt that the effective capture of holes by CoOx cocatalyst is responsible for the improvement of water oxidation. However, based on the findings in this work, two important roles of CoOx are unraveled: (1) hole transfer from Ta3N5 to CoOx to enhance charge separation, thereby increasing the number of surviving free electrons, (2) reduction of defects serving as recombination centers. Loading CoOx on the surface of Ta3N5 changes the structure of its surface defects. The defects can be partially filled by CoOx attached to the Ta3N5 surface, and thus, the electronic properties are changed. 7 In effect, the lifetimes of both shallowly trapped/free electrons and holes are prolonged. This effective hole capture by CoOx and the reduction of recombination centers are responsible for the enhancement of the photocatalytic activity.
4. Conclusion In this work, we have elucidated the effects of CoO x on the dynamics of photogenerated electrons and holes in Ta3N5 powder photocatalysts by using microsecond to millisecond transient visible to mid-IR absorption spectroscopy. Visible to mid-IR transient absorption measurements revealed that most of the electrons in Ta 3N5 are deeply trapped at the defects, which are most likely originating from oxygen vacancies and reduced Ta species. Herein, we unraveled the roles of CoOx for effective capture of holes that resulted to the prolongation of electron lifetime > 1 ms. Interestingly, apart from the enhance charge separation via hole transfer from Ta3N5 to CoOx, the reduction of
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recombination centers originated from adventitious defects in Ta 3N5 is proposed to be the other role of CoOx when they are utilized for surface modification of Ta3N5. This effect further prolongs the lifetime of both electrons and holes. These two significant roles of CoOx shoud be responsible for the higher photocatalytic activity of CoO x-loaded Ta3N5.
ACKNOWLEDGMENT This work was supported by the PRESTO/JST program “Chemical Conversion of Light Energy”, the Grant-in-Aid for Basic Research (B) (No. 16H04188) and Scientific Research on Innovative Areas (Soft Molecular Systems: 16H00852 and Mixed Anion: 17H05491), and the Strategic Research Infrastructure Project of MEXT.
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DA b s o r ban c e (x 10-3)
5 s 10 s 20 s 50 s 100 s 1 ms
6 4 2 0 2 0000
DA b s o r ban c e (x 10-3)
(A)
1 5000
1 0000
5000 1000 -1
Wavenumber / cm 6
Co + h Co 2+
+
4 2
3+
5 s 10 s 20 s 50 s 100 s 1 ms
(B)
0
2 0000
1 5000
1 0000
5000 1000 -1
Wavenumber / cm
Figure 1. Transient absorption spectra of (A) bare Ta3N5 and (B) CoOx-loaded Ta3N5 excited by UV laser pulses (355 nm, 6-ns duration, 0.5 mJ per pulse, and 5 Hz) in a vacuum.
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Oxygen
DAbsorbance
1 0-2
Vacuum
Methanol
1 0-3
1 µs
10 µs
100 µs
Figure 2. Transient decay profile of photogenerated holes (17000 cm-1) in bare Ta3N5 irradiated by UV laser pulses (355 nm and 0.5 mJ per pulse). These decay curves were measured in a vacuum and in the presence of 20Torr O2 and methanol.
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DAbsorbance
1 0-2 1 0-3
Ta3N5
CoOx/Ta3N5
1 0-4 1 0-5 -6
(A) 17000 cm-1
10
1 µs
10 µs
100 µs
1 ms
Time Delay
DAbsorbance
1 0-3
Ta3N5
1 0-4
CoOx/Ta3N5
1 0-5
(B) 8000 cm-1 1 0-6 1 µs
100 µs
1 ms
Time Delay
1 0-3
DAbsorbance
10 µs
CoOx-Ta3N5 1 0-4
Ta3N5
(C) 3000 cm-1 1 0-5
1 µs
10 µs
100 µs
1 ms
Time Delay
Figure 3. Decay kinetics of photogenerated (A) trapped holes (17000 cm-1), (B) deeply trapped electrons (8000 cm-1) and (C) free electrons and/or shallowly trapped electrons (3000 cm-1) in bare and CoOx-loaded Ta3N5. The catalysts were excited by 355 nm laser pulses (0.5 mJ/pulse) in a vacuum.
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S1010-6030(17)30863-8 http://dx.doi.org/10.1016/j.jphotochem.2017.09.005 JPC 10850
To appear in:
Journal of Photochemistry and Photobiology A: Chemistry
Received date: Revised date: Accepted date:
20-6-2017 25-8-2017 3-9-2017
Please cite this article as: Junie Jhon M.Vequizo, Mirabbos Hojamberdiev, Katsuya Teshima, Akira Yamakata, Role of CoOx Cocatalyst on Ta3N5 Photocatalysts Studied by Transient Visible to Mid-Infrared Absorption Spectroscopy, Journal of Photochemistry and Photobiology A: Chemistryhttp://dx.doi.org/10.1016/j.jphotochem.2017.09.005 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Role of CoOx Cocatalyst on Ta3N5 Photocatalysts Studied by Transient Visible to Mid-Infrared Absorption Spectroscopy
Junie Jhon M. Vequizo,a Mirabbos Hojamberdiev,b Katsuya Teshima,b and Akira Yamakataa,*
a
Graduate School of Engineering, Toyota Technological Institute, 2-12-1 Hisakata,
Tempaku, Nagoya 468-8511, Japan b
Department of Environmental Science and Technology, Faculty of Engineering, Shinshu
University, 4-17-1 Wakasato, Nagano 380-8553, Japan
Corresponding Author *Akira Yamakata, E-mail: [email protected]
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Graphical abstract
Highlights
CoOx cocatalyst efficiently captured holes enhancing charge separation.
By CoOx loading on Ta3N5, the recombination centers were significantly reduced.
Decay kinetics of charge carriers were studied by transient absorption spectroscopy.
Most of the electrons in bare Ta3N5 are deeply trapped at the defects.
CoOx is more effective than MeOH in scavenging of holes in metal nitrides/oxynitrides.
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Abstract Tantalum nitride (Ta3N5) photocatalysts attract considerable attention owing to their visible light absorption below 600 nm and suitable band structures for reduction and oxidation of water to generate hydrogen and oxygen gas. Herein, we investigated the effects of CoOx on the dynamics of photogenerated charge carriers in Ta3N5 powder photocatalysts by using microsecond to millisecond transient visible to mid-IR absorption spectroscopy. Transient absorption measurements revealed that most of the electrons in Ta3N5 are deeply trapped at the defects, most likely originating from oxygen vacancies and reduced Ta species. Two important roles of CoOx were elucidated: effective capture of holes to prolong electron lifetime and reduction of defects as recombination centers, which further elongates the lifetime of both electrons and holes.
Key Words: Photocatalysts, Co-catalysts, Charge separation, Time-resolved measurements
1. Introduction Tantalum nitride (Ta3N5) gains great attention owing to its visible light absorption below 600 nm and suitable band structures for the reduction and oxidation of water to generate hydrogen and oxygen gas, respectively. For instance, it is reported that photoelectrodes consisted of Ta3N5 nanorods exhibited high activity for water oxidation under the irradiation of 440-nm light at 1.23 V vs RHE1: an incident photon-to-current
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conversion efficiency (IPCE) reaches to ~41%. However, in the case of photocatalytic water splitting reaction on Ta3N5 nanoparticles, the activity is very low. The low performance of as-synthesized Ta3N5 is usually attributed to the defects formed during the synthesis of the powders: Ta3N5 particles are commonly prepared under high-temperature and highly reductive nitridation of Ta2O5 precursor.2-3 Surface defects such as nitrogen and oxygen vacancies and reduced Ta species can be easily formed. These defects are believed to decrease the photocatalytic performance. On the contrary, theoretical calculation has predicted that defects in Ta3N5 are essential to generate both H2 and O2 gas compared to that of perfect Ta3N5.4 This prediction seems to be contradicting, so understanding the behaviors of photocarriers at the defects on Ta3N5 is indispensable to further improve the overall water splitting reaction using Ta3N5. It is worthy to mention that the activity of Ta3N5 nanoparticulates can be greatly enhanced by modifying their surface with CoO x,5 and CoOx-MgO6. However, how CoOx affects the behavior of electrons and holes in Ta3N5 is still not fully elucidated. A key route to improve the photocatalytic performance of the metal nitride/oxynitride nanoparticulate photocatalysts (LaTiO2N, Ta3N5 among others) is to understand the mechanism involving photogenerated charge carriers upon band gap excitation. Taking LaTiO2N photocatalyst as an example, we have reported that the oxygen vacancies in LaTiO2N deeply trapped the photogenerated electrons7. As a result, the photocatalytic reduction is greatly decreased, i.e. the activity for H2 generation is very low due to the deep electron trapping at the defects. It is widely established that loading of co-catalysts such as Pt,7-10 IrO2,11-12 and CoOx7, 13-16 enhances the photocatalytic activities. However, in the case
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of LaTiO2N, we have reported that loading of Pt is not that effective compared to CoOx loading.7 We have reported that electron-capture by Pt is very slow proceeding in microseconds since most of electrons are deeply trapped at the defects. On the other hand, hole capture by CoOx proceeds within a few ps and hence it is more efficient. Herein, we investigated the effect of CoOx on Ta3N5 photocatalyst by transient visible to mid-IR absorption spectroscopy, which is very effective tool to study the dynamics of photogenerated charge carriers. Transient absorption measurements revealed that aside from the role of CoOx in capturing holes, loading CoOx cocatalyst can reduce the recombination centers that originated from adventitious defects on the Ta3N5 catalysts. The detailed effects of CoOx-loading on the decay kinetics of photogenerated electrons and holes were investigated and discussed.
2. Experimental Section Ta3N5 powders were synthesized by nitriding 1 g of Ta2O5 precursor powders (99.9%, Wako Pure Chemical Industries Ltd) at 850°C for 10 h × 2 times at a heating rate of 10°C min−1 under an NH3 flow (300 mL min-1) in a vertical tubular furnace. CoOx (2 wt% Co) nanoparticles as a cocatalyst for water oxidation were loaded on Ta3N5 particles by impregnation method using Co(NO3)2∙6H2O (99.9%, Kanto Chemicals) aqueous solution, after which the Co-impregnated Ta3N5 powders were then heat treated under an NH3 flow (200 mL min-1) at 500°C for 1 h and subsequently followed by calcination at 200°C for 1 h.14
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For the transient absorption spectroscopic experiments, the as-synthesized Ta3N5 and CoOx-loaded Ta3N5 powders were used without further treatments. Ta3N5 and CoOx-loaded Ta3N5 powders were fixed separately on a circular CaF2 plate with a density of 1.5 mg cm-1 and placed in the closed IR cell. Prior to transient absorption measurements, the cell was evacuated at room temperature.7, 16 For the reactivity measurement, the powder Ta3N5 was exposed to 20-Torr oxygen and methanol. Microsecond time-resolved visible to mid-IR absorption measurements were performed by using the laboratory-built spectrometers as reported in our previous papers.
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Briefly,
in the mid-IR region (6000 ~ 1000 cm-1), the probe light emitted from a MoSi2 coil was focused on the sample and the transmitted light was introduced to the grating spectrometer. The monochromated light was then detected by an MCT detector (Kolmar) and the output electric signal was amplified with an AC-coupled amplifier (Stanford Research Systems, SR560, 1 MHz). In the case of visible to NIR region (20000 ~ 6000 cm-1), halogen lamp (with a power output of 50 W) was used as probe light and Si or InGaAs photodiodes were utilized as detectors, and the experiments were performed in the reflection mode, i.e. the diffuse reflected light from the sample was transmitted to the spectrometer with installed gratings and then detected by photodetectors. In each experiment, a 355 nm-UV pulse from a Nd:YAG laser (Continuum, Surelite I, duration: 6 ns, power: 0.5 mJ, repetition rate: 10~0.01 Hz) was used to excite the band gap of Ta3N5 and CoOx-Ta3N5 photocatalysts. The time resolution of the spectrometers was limited to be 1 ~ 2μs by the bandwidth of the amplifier.
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3. Results and Discussion 3.1 Transient Absorption Spectra of Ta3N5 and CoOx-loaded Ta3N5 Transient absorption spectra of bare Ta3N5 after band gap excitation using 355 nm (3.49 eV) laser pulses are shown in Figure 1A. The TA spectra were measured from 5 μs to 1 millisecond. The spectra exhibit strong absorption peak at 17000 cm-1 (588 nm, 2.11eV) and a very broad absorption spanning from 13000 - 2000 cm−1, suggesting that band gap excitation induces these absorptions. The absorption peak at 17000 cm−1 is assigned to photogenerated holes since the intensity of this absorption band decreases upon exposure to methanol (MeOH) vapor as shown in Figure 2. The decay is accelerated within 0 – 2 μs in the presence of MeOH compared to that in vacuum, suggesting that holes are consumed by MeOH. Furthermore, the intensity slightly increased by the exposure to oxygen, suggesting the electrons are little consumed by O2. Photogenerated holes have been reported to usually give absorption peaks in the UV to visible region. For instance, holes in TiO2 exhibit peaks at 360,17 450, and 520 nm19-21 and that of hematite α-Fe2O3 is at 580 nm.22-23The absorption peak at 17000 cm-1 for Ta3N5 is similar to that observed for LaTiO2N,7, 16 BaTaO2N,15 and BaNbO2N13 photocatalysts, where the absorption peak was also observed in the visible region (17000 – 15000 cm-1). These absorptions of holes have been proposed to be responsible for the electron transition from valence band (VB) to holes trapped at surface defects such as OH species, O2-, and N3-. The transient absorption observed at 13000 – 2000 cm-1 in Figure 1A can be assigned to shallowly trapped and deeply trapped electrons. The intensity of deeply trapped electrons (for instance at 10000 cm-1) is higher than that of shallowly trapped electrons
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(3000 cm-1), suggesting that most of the electrons are deeply trapped at the mid gap states, which originate from the surface defects in Ta3N5 such as oxygen vacancies and reduced Ta species. This very broad absorption of trapped electrons has been also observed in BaTaO2N photocatalysts.15 On the other hand, the negative absorption signal at < 2000 cm-1 (Figure 1A) can be attributed to thermal emission due to rapid recombination at the defects. Rapid recombination is dependent on the excitation energy, i.e. the larger the excitation intensity, the faster the recombination of electrons and holes. However, the characteristic absorption of free electrons in Ta3N5 can be clearly discernable at low excitation intensity (Figure S1). When CoOx was loaded on Ta3N5, the relative intensities of the absorption peak at 17000 cm-1 and the absorption at < 2000 cm-1 were significantly changed: the intensity at 17000 cm-1 decreased and that of absorption at < 2000 cm-1 increased. The decrease in the intensity at 17000 cm-1 indicates holes are captured by CoOx. In the case of the absorption at < 3000 cm-1, the intensity was drastically increased by the CoOx loading. The spectral shape resembles to that of absorption feature of free electrons, suggesting that the number of free electrons increased by CoOx loading on Ta3N5. When holes are captured by CoOx, the number of surviving electrons increases since the recombination of electrons and holes is suppressed, i.e. electrons remain in Ta3N5 and holes are transferred to CoOx. The hole capturing by CoOx is also evident from the appearance of a new peak at ~12500 cm-1 (800 nm), which is assigned to d-d transition of Co3+ species. It has been reported that oxidation of Co2+ to Co3+ notably changes the color of CoOx from yellow to gray.22-24 Therefore, it would also be plausible that Co2+ captures holes and then converted to Co3+. We have
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observed this peak also appear at 12500 cm-1 for CoOx-loaded LaTiO2N.7, 16 Interestingly, in metal nitride/oxynitride photocatalyst, the capability of hole capture using CoO x cocatalyst is more effective than hole-consuming reaction facilitated in the presence of MeOH as shown in Figure 2.
3.2 Decay kinetics of e- and h+ in bare and CoOx-loaded Ta3N5 The decay kinetics of photogenerated electrons and holes in Ta3N5 and CoOx loaded Ta3N5 are examined by observing the changes in the absorption intensity at 17000 cm-1 (trapped holes), 8000 cm-1 (deeply trapped electrons) and 3000 cm-1 (shallowly trapped electrons) as shown in Figure 3. In the case of the absorption intensity at 17000 cm-1 (Figure 3A), the intensity decreases at 0 – 2 μs by CoOx loading. The decrease in the intensity suggests that the holes are captured by CoOx within 2 s or recombination was enhanced by CoOx-loading. However, we cannot precisely determine the principal reason by just observing the behavior of holes. The behavior of electrons should also be examined. On the other hand, the number of surviving shallowly trapped electrons (3000 cm-1) increased at 0 - 2μs (Figure 3C). From this experimental result, we can rule out the possibility of enhanced recombination because the lifetime of electrons becomes longer upon CoOx loading: when the holes are captured by CoOx, the probability of electrons in Ta3N5 to recombine with holes in CoOx is drastically reduced, which results to a spatially separated electrons and holes in Ta3N5 and CoOx, respectively. The prolongation of electron lifetime (increase of electron density) can also be demonstrated in the presence of
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hole scavenging molecules, MeOH, such as on TiO2 and SrTiO3. However, in the case of Ta3N5, MeOH is not as effective as CoOx. Similar observation has been reported for LaTiO2N, in which MeOH is also not efficient whereas the hole capture by CoOx proceeds within few picoseconds.7, 25 Our results confirm us that CoOx works as very effective hole scavenger than MeOH for Ta3N5 photocatalyst. In contrast, in the case of deeply trapped electrons giving the transient absorption at 13000 ~ 6000 cm-1 (Figure 3B), the results are totally different from that of shallowly trapped electrons: the absorption intensity at 8000 cm-1 decreased upon CoOx loading. This result suggests that the number of deeply trapped electrons is reduced, indicating that the structure of the defects was changed by the CoOx-loading. Heating treatment performed during the deposition of CoOx particles and attachment of smaller CoOx particles at the defects are the plausible reasons why the defect-structure changed upon CoOx loading.
3.3 Reduction of the recombination center by CoOx-loading To further investigate the reduction of defects by CoOx loading on Ta3N5, we examined the decay behavior of electrons and holes after 100 μs at 3000 and 17000 cm-1, respectively (shown in Figure 3). The absorption intensity of holes at 17000 cm-1 is larger on bare Ta3N5 than CoOx-loaded samples at 0 ~ 50 s, but that after 100 s is opposite: the decay of holes is accelerated after 100 s on bare Ta3N5. The lifetimes of charge carriers were determined by using double-exponential curve fitting function. The time constants for the decay of holes are estimated to be τ1 = 8.2 ± 0.0 µs and τ2 = 87.2 ± 0.0 µs for bare Ta3N5, while τ1 =
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22.7 ± 0.0 µs and τ2 = 487.0 ± 0.1 µs for CoOx-loaded Ta3N5. The results indicate that the lifetime of holes is significantly elongated in CoOx-loaded Ta3N5, which also suggests that the decay rate of holes is decelerated upon CoOx loading. In the case of shallowly trapped electrons at 3000 cm-1, the results are similar where the decay of electrons after 100 μs is much accelerated in bare Ta3N5 compared to that in CoOx-loaded Ta3N5. The time constants for the decay of electrons are approximated to be τ1 = 269.0 ± 0.0 µs and τ2 = 468.0 ± 0.0 µs for bare Ta3N5, while τ1 = 21.5 ± 0.0 µs and τ2 = 701.0 ± 0.1 µs for CoOxloaded Ta3N5, indicating that the lifetime of electrons is prolonged by loading CoO x. The faster decay of electrons and holes in bare Ta3N5 after 100 s would be due to the electronhole recombination at the defects. However, it is notable that this fast decay component is diminished by CoOx loading, suggesting that the recombination centers that is responsible to the decay after 100 s are significantly reduced by CoOx-loading. This result is consistent to the decrease of the deeply trapped electrons observed in Figure 1 and Figure 3B: the intensity at 10000 - 6000 cm-1 decreased by CoOx-loading. In the case of LaTiO2N, the depth of the electron trap becomes shallower from 6000 to 4000 cm-1. However, in the case of Ta3N5, the deeply trapped electron decreased and the free electrons increased, implying that CoOx clearly affects the defect structure in Ta3N5. The decay kinetics of electrons and holes discussed above give us insights how the photocatalytic activity is enhanced by CoOx loading. The photocatalytic activity of Ta3N5 photocatalyst for water oxidation has already been reported in many studies using powders and in photoelectrode systems.1, 5, 26-30 Without co-catalyst, the activity for water oxidation is very low even in the presence of AgNO3 as electron scavenger. However, loading of
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CoOx drastically increased the activity, signifying the important role of CoOx for activity enhancement. There is no doubt that the effective capture of holes by CoOx cocatalyst is responsible for the improvement of water oxidation. However, based on the findings in this work, two important roles of CoOx are unraveled: (1) hole transfer from Ta3N5 to CoOx to enhance charge separation, thereby increasing the number of surviving free electrons, (2) reduction of defects serving as recombination centers. Loading CoOx on the surface of Ta3N5 changes the structure of its surface defects. The defects can be partially filled by CoOx attached to the Ta3N5 surface, and thus, the electronic properties are changed. 7 In effect, the lifetimes of both shallowly trapped/free electrons and holes are prolonged. This effective hole capture by CoOx and the reduction of recombination centers are responsible for the enhancement of the photocatalytic activity.
4. Conclusion In this work, we have elucidated the effects of CoO x on the dynamics of photogenerated electrons and holes in Ta3N5 powder photocatalysts by using microsecond to millisecond transient visible to mid-IR absorption spectroscopy. Visible to mid-IR transient absorption measurements revealed that most of the electrons in Ta 3N5 are deeply trapped at the defects, which are most likely originating from oxygen vacancies and reduced Ta species. Herein, we unraveled the roles of CoOx for effective capture of holes that resulted to the prolongation of electron lifetime > 1 ms. Interestingly, apart from the enhance charge separation via hole transfer from Ta3N5 to CoOx, the reduction of
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recombination centers originated from adventitious defects in Ta 3N5 is proposed to be the other role of CoOx when they are utilized for surface modification of Ta3N5. This effect further prolongs the lifetime of both electrons and holes. These two significant roles of CoOx shoud be responsible for the higher photocatalytic activity of CoO x-loaded Ta3N5.
ACKNOWLEDGMENT This work was supported by the PRESTO/JST program “Chemical Conversion of Light Energy”, the Grant-in-Aid for Basic Research (B) (No. 16H04188) and Scientific Research on Innovative Areas (Soft Molecular Systems: 16H00852 and Mixed Anion: 17H05491), and the Strategic Research Infrastructure Project of MEXT.
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DA b s o r ban c e (x 10-3)
5 s 10 s 20 s 50 s 100 s 1 ms
6 4 2 0 2 0000
DA b s o r ban c e (x 10-3)
(A)
1 5000
1 0000
5000 1000 -1
Wavenumber / cm 6
Co + h Co 2+
+
4 2
3+
5 s 10 s 20 s 50 s 100 s 1 ms
(B)
0
2 0000
1 5000
1 0000
5000 1000 -1
Wavenumber / cm
Figure 1. Transient absorption spectra of (A) bare Ta3N5 and (B) CoOx-loaded Ta3N5 excited by UV laser pulses (355 nm, 6-ns duration, 0.5 mJ per pulse, and 5 Hz) in a vacuum.
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Oxygen
DAbsorbance
1 0-2
Vacuum
Methanol
1 0-3
1 µs
10 µs
100 µs
Figure 2. Transient decay profile of photogenerated holes (17000 cm-1) in bare Ta3N5 irradiated by UV laser pulses (355 nm and 0.5 mJ per pulse). These decay curves were measured in a vacuum and in the presence of 20Torr O2 and methanol.
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DAbsorbance
1 0-2 1 0-3
Ta3N5
CoOx/Ta3N5
1 0-4 1 0-5 -6
(A) 17000 cm-1
10
1 µs
10 µs
100 µs
1 ms
Time Delay
DAbsorbance
1 0-3
Ta3N5
1 0-4
CoOx/Ta3N5
1 0-5
(B) 8000 cm-1 1 0-6 1 µs
100 µs
1 ms
Time Delay
1 0-3
DAbsorbance
10 µs
CoOx-Ta3N5 1 0-4
Ta3N5
(C) 3000 cm-1 1 0-5
1 µs
10 µs
100 µs
1 ms
Time Delay
Figure 3. Decay kinetics of photogenerated (A) trapped holes (17000 cm-1), (B) deeply trapped electrons (8000 cm-1) and (C) free electrons and/or shallowly trapped electrons (3000 cm-1) in bare and CoOx-loaded Ta3N5. The catalysts were excited by 355 nm laser pulses (0.5 mJ/pulse) in a vacuum.
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