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High performance hydrogenated TiO2 nanorod arrays as a photoelectrochemical sensor for organic compounds under visible light
Electrochemistry Communications 40 (2014) 24–27
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Short communication
High performance hydrogenated TiO2 nanorod arrays as a photoelectrochemical sensor for organic compounds under visible light Shengsen Zhang a, Shanqing Zhang b, Biyu Peng a, Hongjuan Wang a, Hao Yu a, Haihui Wang a, Feng Peng a,⁎ a b
School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China Centre for Clean Environment and Energy, Griffith School of Environment, Gold Coast Campus, Griffith University, QLD 4222, Australia
a r t i c l e
i n f o
Article history: Received 28 October 2013 Received in revised form 26 November 2013 Accepted 13 December 2013 Available online 22 December 2013 Keywords: Photoelectrochemical sensor Hydrogenated TiO2 nanorod arrays Visible light Photo-electrode
a b s t r a c t A photo-electrode of hydrogenated TiO 2 nanorod arrays (H-TNRs) was prepared and used as a photoelectrochemical sensor for organic compound detections for the first time. Under visible light, the H-TNR electrode has shown a highly sensitive and steady photocurrent response due to introduction of oxygen vacancy and mid-gap energy levels. The H-TNRs are a promising material for photoelectrochemical detection of organic compounds under visible light. © 2013 Elsevier B.V. All rights reserved.
1. Introduction In recent decades, titanium dioxide (TiO2) nanomaterials have attracted tremendous attention of researchers and have been a dominant photoelectrochemical material [1–3]. TiO2 nanorod arrays (TNRs) are one of the most intensively researched nanomaterials because of their special architecture, unique physical and chemical properties and stability [4–6]. However, similar to other traditional TiO2 nanomaterials, the TNR suffers from a fundamental drawback of a wide band gap (3.2 eV), which allows the utilization of UV light portion of the solar light that only accounts for 4% of the solar light [7]. Enormous efforts have been devoted to extend the light absorption range of TiO2 into the visible region, such as, nonmetal or metal elements doped TiO2 and dye or narrow band-gap semiconductor composited TiO2 [8]. However, the stability of these dye and semiconductors is far from satisfaction. Recently, the discovery of hydrogenated “black” TiO2 nanoparticles with long-wavelength optical absorption capability and significantly improved conductivity has opened a new avenue and triggered much interest [9–12]. Also, hydrogenation is a simple and facile process in that it does not involve the use of catalyst and other chemicals, does not need further treatments (e.g., separation and purification) of the reaction product and can be scaled-up easily for mass production. In this work, a hydrogenated TiO2 nanorod array (H-TNR) was prepared and used as a photoelectrode for degradation organic compounds in aqueous solution under visible light illumination for the first time. It has been a challenging task to obtain robust photoanode to sensitively
⁎ Corresponding author. E-mail address: [email protected] (F. Peng). 1388-2481/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.elecom.2013.12.013
and indiscriminately degrade and ultimately detect various kinds of organic compounds. 2. Experimental 2.1. Preparation of H-TNRs TNRs were prepared on a fluorine-doped tin oxide (FTO) glass slide using a reported hydrothermal method [13]. Briefly, a 20 mL of deionized water was mixed with 20 mL of concentrated hydrochloric acid (36.5% by weight) in a Teflon-lined stainless steel autoclave (100 mL volume). The mixture was stirred at ambient conditions for 5 min before the addition of 0.48 mL of titanium butoxide (97% Aldrich). After stirring for another 5 min, two pieces of well-cleaned FTO substrates (5 × 2 cm2) were placed into the Teflon-liner at an angle against the wall with the conducting side facing down. The hydrothermal synthesis was conducted at 170 °C for 6 h. The resultant sample, i.e., the TNR electrode was taken out, rinsed extensively with deionized water, and allowed to dry in air. The H-TNR electrode was prepared via the hydrogenation process of the TNRs in hydrogen (20 sccm) and argon (80 sccm) atmosphere at 350 °C in a tubular furnace under ambient pressure for 1 h with a heating rate of 5 °C/min. 2.2. Material characterization The morphology of the samples was characterized by a field-emission scanning electron microscope (FESEM, LEO 1530VP). X-ray diffraction (XRD) analysis was carried out with an X-ray diffractometer (D/maxIIIA, Japan) using Cu Kα radiation source. The chemical nature of Ti and O was studied using X-ray photoelectron spectroscopy (XPS) in
S. Zhang et al. / Electrochemistry Communications 40 (2014) 24–27
25
D
R (002)
Intensity (a.u.)
R (101)
a:TNRs b:H-TNRs
R:Rutile F:FTO
F
F
F F
b
F
F
F
a 20
30
40
50
60
70
2θ (degree) Fig. 1. FESEM surface morphologies of the TNRs (A), the top-view (B) and the cross-sectional view (C) of the H-TNRs; XRD spectra of the TNR and H-TNR samples (D).
3. Results and discussions Fig. 1A, B and C shows the typical FESEM images of TNRs and H-TNRs. From Fig. 1A and B, we can see that no significant surface morphologies change after hydrogenation treatment. Fig. 1C reveals that the entire surface of the FTO substrate is covered very uniformly with TiO2 nanorods with the average diameter and length of 150 nm and 1.8 μm, respectively. Fig. 1D displays the XRD patterns of the FTO substrate loading with TNRs before and after the hydrogenation treatment. After subtracting the diffraction peaks from FTO glass, two diffraction peaks centered at 2θ angles of 36.5° and 63.2° were observed in every sample. These two sharp peaks are indexed to the characteristic peaks of tetragonal rutile TiO2 (JCPDS No. 88-1175). The peak centered at 63.2° corresponds to the (002) diffraction which is dominant over the (101) peak centered at 36.5°, providing evidence that the TNRs are highly oriented in the (001) direction on the FTO substrate, which is consistent with the observed growth axis of TNRs. Fig. 1D also shows there is no phase change after hydrogenation, although the TiO2 peak intensity decreases.
Intensity (a.u.)
Ti 2p3/2
458.3 eV
a:TNRs b:H-TNRs
458.3 eV
457.6 eV
454
456 458 460 Binding energy (eV)
462
b a 454
456
458
460
462
464
466
468
Binding Energy(eV)
B O 1s
529.9 eV
Intensity (a.u.)
All photoelectrochemical experiments were performed in a photoelectrochemical cell with a quartz window for illumination. The standard three-electrode cell contains the working electrode (the asprepared TNRs or H-TNRs), an Ag/AgCl reference electrode, and a platinum mesh counter electrode. A voltammograph (CV-27, BAS) was used to control potential bias. A 150 W xenon arc lamp was used as the illumination source. A UV-400 optical filter was used to obtain visible light and block UV light and a radiant power meter (Instruments of Beijing Normal University) was used measure the incident light intensity.
Ti 2p
Intensity (a.u.)
2.3. Photoelectrochemical characterization
A
Intensity (a.u.)
Krato Axis Ultra DLD spectrometer with Al Kα X-ray (hv = 1486.6 eV) at 15 kV and 150 W. UV–vis diffuse reflectance spectroscopy (DRS) measurement was conducted by a UV-1601 spectrophotometer (Shimazu, Japan).
531.7 eV
529
530 531 532 533 Binding energy (eV)
534
b
a:TNRs b:H-TNRs
a 529
530
531
532
533
534
535
536
Binding energy (eV) Fig. 2. Overlay of normalized Ti 2p (A) and O 1s (B) core level XPS spectra of TNRs (a) and H-TNRs (b). The insets show the separated peaks of the Ti 2p and O1s XPS spectra of the HTNRs, respectively.
26
S. Zhang et al. / Electrochemistry Communications 40 (2014) 24–27
B
Intensity (a.u.)
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0.2 350
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D 2500
Photocurrent density (μΑ cm-2)
C Photocurrent density (μΑ cm-2)
H-TNRs
300
I ( μ A cm-2 )
1.0
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A
2
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800
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1600
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Fig. 3. UV–vis DR spectra of the TNR and H-TNR samples (A); The voltammograms of TNR and H-TNR photoanodes obtained at a scan rate of 5 mV/s under visible light (B) and UV light (C) irradiation; The stability of H-TNR photoanode(D). Inset shows the photocurrent responses for the TNR and H-TNR photoanodes.
This can be attributed to the increase of defect density in TiO2 structure, which has also been observed in the recent studies of hydrogenated TiO2 [9,10]. Fig. 2A clearly shows the characteristic peaks of Ti 2p XPS have a slight shift to low binding energy after hydrogenation treatment (Fig. 2A). As shown in inset of Fig. 2A, the Ti 2p3/2 XPS spectrum of HTNRs can be separated to two peaks at 458.3 and 457.6 eV, which can be ascribed to Ti4 + and Ti3 + [14]. The result suggests that oxygen vacancies (Ti3 +) were created in H-TNRs during hydrogenation. In Fig. 2B, the peak of 529.9 eV corresponds to the characteristic peak of Ti–O–Ti [9,15]. Additional separated peak centered at 531.7 eV of HTNRs in inset of Fig. 2B is attributed to Ti–OH [15], indicating the TNR surface was functionalized by hydroxyl groups after hydrogenation. Fig. 3A shows the H-TNR film exhibits a slight enhancement in the UV light region (ca. 4%), and strong absorbance in the visible light region (ca. 50%) than TNRs film due to the presence of oxygen vacancies, leading to mid-gap energy levels corresponding to the excitation from the valence band to the impurity band [10]. This result indicates that the hydrogenation treatment extended the absorption of the TNRs into the visible light region. The voltammetry measurement of TNRs and H-TNRs was carried out in 0.1 M NaNO3 electrolyte, and the results were presented in Fig. 3B and C. Under visible light, the negligible anodic photocurrent (Iph) observed for TNRs from −0.3 V to 0.6 V (vs Ag/AgCl), whereas the Iph of H-TNRs first increased monotonically with the applied potential, and then leveling off and reaching a saturation photocurrent (Isph). The inset of Fig. 3B shows no significant photocurrent for both the samples in the dark. Under visible light illumination, the Isph of HTNRs and TNRs is ca. 300.0 μA/cm2 and 3.0 μA/cm2 at the potential of 0.4 V, respectively. These results indicate that the H-TNRs possessed much better photoelectrochemical properties than TNRs under visible light, which can be attributed to as follows. In a typical hydrogenation
process, the sample is treated at high temperature in H2 atmosphere, which allows H2 to penetrate into the TiO2 crystal lattice where hydrogen could react with some oxygen, creating oxygen vacancy and Ti3+ sites, and subsequently lead to shallow donor states between the conduction band and valence band, namely mid-gap energy levels that realize the absorption of longer wavelength of light (N400 nm) [9,10,16]. Also hydrogen atoms can stay in the lattice as shallow donors then enhance the charge transport capability (electrical conductivity) [10,17]. In this work, we used the photoelectrochemical method to evidence the improvement of electrical conductivity. As shown in Fig. 3C, under UV light illumination, the Isph of H-TNRs and TNRs is ca. 2500.6 μA/cm2 and 38.7 μA/cm2 at a potential of 1.1 V, respectively. This is ca. 65 times increase. As both H-TNRs and TNRs possess strong absorption in UV region as shown in Fig. 3A, the dramatical increase of the photocurrent was mainly due to the significant improvement of electronic conductivity that resulted from the creation of mid-energy levels [10], rather than the slight UV absorption enhancement after the hydrogenation process. It is well-established that the improvement of electrical conductivity that is beneficial to the separation of electron– hole pair and electron collection from the oxidation of water and organic compounds [18]. In contrast, the enhancement of the photocurrent of the H-TNRs under visible light was even more dramatical, reaching ca.100 times compared with the TNRs. Again, the improved electronic conductivity of the H-TNRs samples plays a significant role here besides the enhanced visible light absorption. Additionally, the stability of H-TNR photoanode was evaluated by measuring the photocurrents for 2400 s under a visible light illumination of 100 mW/cm2. As shown in Fig. 3C, the plot reveals the excellent stability of H-TNR photoanode, which proves that the influences of the gaseous atmosphere, humidity and thermal effect could be effectively avoided. This result suggests H-TNR film is suitable for sensing applications under the visible light illumination.
S. Zhang et al. / Electrochemistry Communications 40 (2014) 24–27
Photocurrent density (μΑ cm-2)
A
480
k k: 2.50 mM j: 2.08 mM i: 1.67 mM h: 1.25 mM g: 0.83 mM f: 0.42 mM e: 0.21 mM d: 0.14 mM c: 0.08 mM b: 0.04 mM a: 0.0 mM
400 320 240 160
a
80 0 -0.2
-0.1
0.0
0.1
0.2
0.3
0.4
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0.6
Potential (V)
B
200
a: Malonic acid
I net (μΑ cm-2)
b: Glucose
c
150
c: KHP
100
b 50
a
0 0.0
0.5
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1.5
Acknowledgment
200
We acknowledge the financial support from the NSFC (No. 20873044; 21328301). y=8.03x+1.05 2 R =0.9973
160
I net (μΑ cm-2)
similar voltammograms to those shown in Fig. 4A (not shown here). It is well established that Isph in the presence of organics can be classified into two parts, one from the oxidation of water (I0sph) and the other from the oxidation of the organics (Inet) [18]. The Isph and I0sph data can be obtained from the Isph–E curves at 0.4 V in Fig. 4A. The Inet could be calculated by subtracting I0sph from Isph and plotted against the organics concentration (C) in Fig. 4B. The result shows good linear relationships between Inet and molar concentrations of all the investigated organic compounds, demonstrating that H-NTRs can sensitively and steadily detect the concentrations of organic compounds. More interestingly, the equivalent concentrations (Ceq) can be calculated by molar concentration (CM) with its corresponding electron transfer numbers (n), i.e., Ceq = nCM. The n for the malonic acid, glucose and KHP is 8, 24 and 30, respectively [19]. Subsequently, Inet of malonic acid, glucose and KHP were normalized and plotted against Ceq, in Fig. 4C. As expected, the data (in 0–20 meq) were linear fitted with a R2 value of 0.9973. The excellent linear relationship suggests that the H-TNR photoanode is able to indiscriminately oxidize organic compounds in the same extent (to H2O and CO2 end products) regardless of its identity and adsorptivity to the TiO2 surface. This characteristic allows H-TNRs to work as a universal visible light photoelectrochemical material with high sensitivity and wide detection linear range for the sensing of organic compounds. Furthermore, it can achieve selective detection of organic compounds if it is incorporated with a separation means such as high performance liquid chromatography (HPLC). In fact, as a universal detector, a TiO2 photoelectrochemical sensor was incorporated with HPLC and had achieved the selective detection of various sugars in water [20]. In conclusion, the as-prepared H-TNRs electrode exhibited highly sensitive and steady photocurrent to the investigated organic compounds under visible light, indicating that it could be adopted as a promising photoelectrochemical sensor for organic compounds in water solution.
2.0
C (mM)
C
27
References
120
80
Malonic acid Glucose KHP
40
0 0
4
8
12
16
20
24
Ceq (meq) Fig. 4. (A) Voltammograms of H-TNR film photoanode from 0.1 M NaNO3 solution with various concentrations of malonic acid; (B, C) Relationships between Inet and concentrations at the H-TNR electrode.
It was reported that the organics in the aqueous could be quantified electrochemically by measuring the photocurrents originated from the photoelectrocatalytic degradation [19]. The voltammograms of H-TNR film photoanode with various concentrations of malonic acid under the same conditions as the above were shown in Fig. 4A. The Iph increased dramatically as the applied potential below 0.4 V, and then arrived Isph. Other two organic compounds (glucose and potassium hydrogen phthalate (KHP)) solutions were investigated and exhibited
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Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Short communication
High performance hydrogenated TiO2 nanorod arrays as a photoelectrochemical sensor for organic compounds under visible light Shengsen Zhang a, Shanqing Zhang b, Biyu Peng a, Hongjuan Wang a, Hao Yu a, Haihui Wang a, Feng Peng a,⁎ a b
School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China Centre for Clean Environment and Energy, Griffith School of Environment, Gold Coast Campus, Griffith University, QLD 4222, Australia
a r t i c l e
i n f o
Article history: Received 28 October 2013 Received in revised form 26 November 2013 Accepted 13 December 2013 Available online 22 December 2013 Keywords: Photoelectrochemical sensor Hydrogenated TiO2 nanorod arrays Visible light Photo-electrode
a b s t r a c t A photo-electrode of hydrogenated TiO 2 nanorod arrays (H-TNRs) was prepared and used as a photoelectrochemical sensor for organic compound detections for the first time. Under visible light, the H-TNR electrode has shown a highly sensitive and steady photocurrent response due to introduction of oxygen vacancy and mid-gap energy levels. The H-TNRs are a promising material for photoelectrochemical detection of organic compounds under visible light. © 2013 Elsevier B.V. All rights reserved.
1. Introduction In recent decades, titanium dioxide (TiO2) nanomaterials have attracted tremendous attention of researchers and have been a dominant photoelectrochemical material [1–3]. TiO2 nanorod arrays (TNRs) are one of the most intensively researched nanomaterials because of their special architecture, unique physical and chemical properties and stability [4–6]. However, similar to other traditional TiO2 nanomaterials, the TNR suffers from a fundamental drawback of a wide band gap (3.2 eV), which allows the utilization of UV light portion of the solar light that only accounts for 4% of the solar light [7]. Enormous efforts have been devoted to extend the light absorption range of TiO2 into the visible region, such as, nonmetal or metal elements doped TiO2 and dye or narrow band-gap semiconductor composited TiO2 [8]. However, the stability of these dye and semiconductors is far from satisfaction. Recently, the discovery of hydrogenated “black” TiO2 nanoparticles with long-wavelength optical absorption capability and significantly improved conductivity has opened a new avenue and triggered much interest [9–12]. Also, hydrogenation is a simple and facile process in that it does not involve the use of catalyst and other chemicals, does not need further treatments (e.g., separation and purification) of the reaction product and can be scaled-up easily for mass production. In this work, a hydrogenated TiO2 nanorod array (H-TNR) was prepared and used as a photoelectrode for degradation organic compounds in aqueous solution under visible light illumination for the first time. It has been a challenging task to obtain robust photoanode to sensitively
⁎ Corresponding author. E-mail address: [email protected] (F. Peng). 1388-2481/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.elecom.2013.12.013
and indiscriminately degrade and ultimately detect various kinds of organic compounds. 2. Experimental 2.1. Preparation of H-TNRs TNRs were prepared on a fluorine-doped tin oxide (FTO) glass slide using a reported hydrothermal method [13]. Briefly, a 20 mL of deionized water was mixed with 20 mL of concentrated hydrochloric acid (36.5% by weight) in a Teflon-lined stainless steel autoclave (100 mL volume). The mixture was stirred at ambient conditions for 5 min before the addition of 0.48 mL of titanium butoxide (97% Aldrich). After stirring for another 5 min, two pieces of well-cleaned FTO substrates (5 × 2 cm2) were placed into the Teflon-liner at an angle against the wall with the conducting side facing down. The hydrothermal synthesis was conducted at 170 °C for 6 h. The resultant sample, i.e., the TNR electrode was taken out, rinsed extensively with deionized water, and allowed to dry in air. The H-TNR electrode was prepared via the hydrogenation process of the TNRs in hydrogen (20 sccm) and argon (80 sccm) atmosphere at 350 °C in a tubular furnace under ambient pressure for 1 h with a heating rate of 5 °C/min. 2.2. Material characterization The morphology of the samples was characterized by a field-emission scanning electron microscope (FESEM, LEO 1530VP). X-ray diffraction (XRD) analysis was carried out with an X-ray diffractometer (D/maxIIIA, Japan) using Cu Kα radiation source. The chemical nature of Ti and O was studied using X-ray photoelectron spectroscopy (XPS) in
S. Zhang et al. / Electrochemistry Communications 40 (2014) 24–27
25
D
R (002)
Intensity (a.u.)
R (101)
a:TNRs b:H-TNRs
R:Rutile F:FTO
F
F
F F
b
F
F
F
a 20
30
40
50
60
70
2θ (degree) Fig. 1. FESEM surface morphologies of the TNRs (A), the top-view (B) and the cross-sectional view (C) of the H-TNRs; XRD spectra of the TNR and H-TNR samples (D).
3. Results and discussions Fig. 1A, B and C shows the typical FESEM images of TNRs and H-TNRs. From Fig. 1A and B, we can see that no significant surface morphologies change after hydrogenation treatment. Fig. 1C reveals that the entire surface of the FTO substrate is covered very uniformly with TiO2 nanorods with the average diameter and length of 150 nm and 1.8 μm, respectively. Fig. 1D displays the XRD patterns of the FTO substrate loading with TNRs before and after the hydrogenation treatment. After subtracting the diffraction peaks from FTO glass, two diffraction peaks centered at 2θ angles of 36.5° and 63.2° were observed in every sample. These two sharp peaks are indexed to the characteristic peaks of tetragonal rutile TiO2 (JCPDS No. 88-1175). The peak centered at 63.2° corresponds to the (002) diffraction which is dominant over the (101) peak centered at 36.5°, providing evidence that the TNRs are highly oriented in the (001) direction on the FTO substrate, which is consistent with the observed growth axis of TNRs. Fig. 1D also shows there is no phase change after hydrogenation, although the TiO2 peak intensity decreases.
Intensity (a.u.)
Ti 2p3/2
458.3 eV
a:TNRs b:H-TNRs
458.3 eV
457.6 eV
454
456 458 460 Binding energy (eV)
462
b a 454
456
458
460
462
464
466
468
Binding Energy(eV)
B O 1s
529.9 eV
Intensity (a.u.)
All photoelectrochemical experiments were performed in a photoelectrochemical cell with a quartz window for illumination. The standard three-electrode cell contains the working electrode (the asprepared TNRs or H-TNRs), an Ag/AgCl reference electrode, and a platinum mesh counter electrode. A voltammograph (CV-27, BAS) was used to control potential bias. A 150 W xenon arc lamp was used as the illumination source. A UV-400 optical filter was used to obtain visible light and block UV light and a radiant power meter (Instruments of Beijing Normal University) was used measure the incident light intensity.
Ti 2p
Intensity (a.u.)
2.3. Photoelectrochemical characterization
A
Intensity (a.u.)
Krato Axis Ultra DLD spectrometer with Al Kα X-ray (hv = 1486.6 eV) at 15 kV and 150 W. UV–vis diffuse reflectance spectroscopy (DRS) measurement was conducted by a UV-1601 spectrophotometer (Shimazu, Japan).
531.7 eV
529
530 531 532 533 Binding energy (eV)
534
b
a:TNRs b:H-TNRs
a 529
530
531
532
533
534
535
536
Binding energy (eV) Fig. 2. Overlay of normalized Ti 2p (A) and O 1s (B) core level XPS spectra of TNRs (a) and H-TNRs (b). The insets show the separated peaks of the Ti 2p and O1s XPS spectra of the HTNRs, respectively.
26
S. Zhang et al. / Electrochemistry Communications 40 (2014) 24–27
B
Intensity (a.u.)
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D 2500
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C Photocurrent density (μΑ cm-2)
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Time(s)
Fig. 3. UV–vis DR spectra of the TNR and H-TNR samples (A); The voltammograms of TNR and H-TNR photoanodes obtained at a scan rate of 5 mV/s under visible light (B) and UV light (C) irradiation; The stability of H-TNR photoanode(D). Inset shows the photocurrent responses for the TNR and H-TNR photoanodes.
This can be attributed to the increase of defect density in TiO2 structure, which has also been observed in the recent studies of hydrogenated TiO2 [9,10]. Fig. 2A clearly shows the characteristic peaks of Ti 2p XPS have a slight shift to low binding energy after hydrogenation treatment (Fig. 2A). As shown in inset of Fig. 2A, the Ti 2p3/2 XPS spectrum of HTNRs can be separated to two peaks at 458.3 and 457.6 eV, which can be ascribed to Ti4 + and Ti3 + [14]. The result suggests that oxygen vacancies (Ti3 +) were created in H-TNRs during hydrogenation. In Fig. 2B, the peak of 529.9 eV corresponds to the characteristic peak of Ti–O–Ti [9,15]. Additional separated peak centered at 531.7 eV of HTNRs in inset of Fig. 2B is attributed to Ti–OH [15], indicating the TNR surface was functionalized by hydroxyl groups after hydrogenation. Fig. 3A shows the H-TNR film exhibits a slight enhancement in the UV light region (ca. 4%), and strong absorbance in the visible light region (ca. 50%) than TNRs film due to the presence of oxygen vacancies, leading to mid-gap energy levels corresponding to the excitation from the valence band to the impurity band [10]. This result indicates that the hydrogenation treatment extended the absorption of the TNRs into the visible light region. The voltammetry measurement of TNRs and H-TNRs was carried out in 0.1 M NaNO3 electrolyte, and the results were presented in Fig. 3B and C. Under visible light, the negligible anodic photocurrent (Iph) observed for TNRs from −0.3 V to 0.6 V (vs Ag/AgCl), whereas the Iph of H-TNRs first increased monotonically with the applied potential, and then leveling off and reaching a saturation photocurrent (Isph). The inset of Fig. 3B shows no significant photocurrent for both the samples in the dark. Under visible light illumination, the Isph of HTNRs and TNRs is ca. 300.0 μA/cm2 and 3.0 μA/cm2 at the potential of 0.4 V, respectively. These results indicate that the H-TNRs possessed much better photoelectrochemical properties than TNRs under visible light, which can be attributed to as follows. In a typical hydrogenation
process, the sample is treated at high temperature in H2 atmosphere, which allows H2 to penetrate into the TiO2 crystal lattice where hydrogen could react with some oxygen, creating oxygen vacancy and Ti3+ sites, and subsequently lead to shallow donor states between the conduction band and valence band, namely mid-gap energy levels that realize the absorption of longer wavelength of light (N400 nm) [9,10,16]. Also hydrogen atoms can stay in the lattice as shallow donors then enhance the charge transport capability (electrical conductivity) [10,17]. In this work, we used the photoelectrochemical method to evidence the improvement of electrical conductivity. As shown in Fig. 3C, under UV light illumination, the Isph of H-TNRs and TNRs is ca. 2500.6 μA/cm2 and 38.7 μA/cm2 at a potential of 1.1 V, respectively. This is ca. 65 times increase. As both H-TNRs and TNRs possess strong absorption in UV region as shown in Fig. 3A, the dramatical increase of the photocurrent was mainly due to the significant improvement of electronic conductivity that resulted from the creation of mid-energy levels [10], rather than the slight UV absorption enhancement after the hydrogenation process. It is well-established that the improvement of electrical conductivity that is beneficial to the separation of electron– hole pair and electron collection from the oxidation of water and organic compounds [18]. In contrast, the enhancement of the photocurrent of the H-TNRs under visible light was even more dramatical, reaching ca.100 times compared with the TNRs. Again, the improved electronic conductivity of the H-TNRs samples plays a significant role here besides the enhanced visible light absorption. Additionally, the stability of H-TNR photoanode was evaluated by measuring the photocurrents for 2400 s under a visible light illumination of 100 mW/cm2. As shown in Fig. 3C, the plot reveals the excellent stability of H-TNR photoanode, which proves that the influences of the gaseous atmosphere, humidity and thermal effect could be effectively avoided. This result suggests H-TNR film is suitable for sensing applications under the visible light illumination.
S. Zhang et al. / Electrochemistry Communications 40 (2014) 24–27
Photocurrent density (μΑ cm-2)
A
480
k k: 2.50 mM j: 2.08 mM i: 1.67 mM h: 1.25 mM g: 0.83 mM f: 0.42 mM e: 0.21 mM d: 0.14 mM c: 0.08 mM b: 0.04 mM a: 0.0 mM
400 320 240 160
a
80 0 -0.2
-0.1
0.0
0.1
0.2
0.3
0.4
0.5
0.6
Potential (V)
B
200
a: Malonic acid
I net (μΑ cm-2)
b: Glucose
c
150
c: KHP
100
b 50
a
0 0.0
0.5
1.0
1.5
Acknowledgment
200
We acknowledge the financial support from the NSFC (No. 20873044; 21328301). y=8.03x+1.05 2 R =0.9973
160
I net (μΑ cm-2)
similar voltammograms to those shown in Fig. 4A (not shown here). It is well established that Isph in the presence of organics can be classified into two parts, one from the oxidation of water (I0sph) and the other from the oxidation of the organics (Inet) [18]. The Isph and I0sph data can be obtained from the Isph–E curves at 0.4 V in Fig. 4A. The Inet could be calculated by subtracting I0sph from Isph and plotted against the organics concentration (C) in Fig. 4B. The result shows good linear relationships between Inet and molar concentrations of all the investigated organic compounds, demonstrating that H-NTRs can sensitively and steadily detect the concentrations of organic compounds. More interestingly, the equivalent concentrations (Ceq) can be calculated by molar concentration (CM) with its corresponding electron transfer numbers (n), i.e., Ceq = nCM. The n for the malonic acid, glucose and KHP is 8, 24 and 30, respectively [19]. Subsequently, Inet of malonic acid, glucose and KHP were normalized and plotted against Ceq, in Fig. 4C. As expected, the data (in 0–20 meq) were linear fitted with a R2 value of 0.9973. The excellent linear relationship suggests that the H-TNR photoanode is able to indiscriminately oxidize organic compounds in the same extent (to H2O and CO2 end products) regardless of its identity and adsorptivity to the TiO2 surface. This characteristic allows H-TNRs to work as a universal visible light photoelectrochemical material with high sensitivity and wide detection linear range for the sensing of organic compounds. Furthermore, it can achieve selective detection of organic compounds if it is incorporated with a separation means such as high performance liquid chromatography (HPLC). In fact, as a universal detector, a TiO2 photoelectrochemical sensor was incorporated with HPLC and had achieved the selective detection of various sugars in water [20]. In conclusion, the as-prepared H-TNRs electrode exhibited highly sensitive and steady photocurrent to the investigated organic compounds under visible light, indicating that it could be adopted as a promising photoelectrochemical sensor for organic compounds in water solution.
2.0
C (mM)
C
27
References
120
80
Malonic acid Glucose KHP
40
0 0
4
8
12
16
20
24
Ceq (meq) Fig. 4. (A) Voltammograms of H-TNR film photoanode from 0.1 M NaNO3 solution with various concentrations of malonic acid; (B, C) Relationships between Inet and concentrations at the H-TNR electrode.
It was reported that the organics in the aqueous could be quantified electrochemically by measuring the photocurrents originated from the photoelectrocatalytic degradation [19]. The voltammograms of H-TNR film photoanode with various concentrations of malonic acid under the same conditions as the above were shown in Fig. 4A. The Iph increased dramatically as the applied potential below 0.4 V, and then arrived Isph. Other two organic compounds (glucose and potassium hydrogen phthalate (KHP)) solutions were investigated and exhibited
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