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Enhanced photoelectrochemical performance of CdO-TiO2 nanotubes prepared by direct impregnation
Applied Surface Science 476 (2019) 136–143
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Full Length Article
Enhanced photoelectrochemical performance of CdO-TiO2 nanotubes prepared by direct impregnation
T
Xueqin Wanga,1, Kai Chenga,1, Shuna Douc, Qihui Chena, Junlei Wanga, Zhenyuan Songa, ⁎ Jiaojing Zhanga, Hua Songa,b, a
College of Chemistry & Chemical Engineering, Northeast Petroleum University, Daqing 163318, China Provincial Key Laboratory of Oil & Gas Chemical Technology, Northeast Petroleum University, Daqing 163318, China c Strategic Planning and Development Division, Zhonghua Petrochemical Co. LTD, Qingzhou 262513, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: TiO2 nanotubes Cadmium oxide Impregnation-calcination Photoelectrochemical
A direct impregnation technique was adopted to prepare a series of CdO-TiO2 nanotubes. Self-organized TiO2 nanotubes were prepared using an optimized two-step anodization process. The morphology, crystallinity, elemental composition, and photoelectrochemical properties of the CdO-TiO2 nanotubes were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV–Vis diffuse reflection spectra (UV–Vis DRS), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and photoelectric cell (PEC) measurements. At lower Cd(NO3)2 concentration, no obvious CdO crystalline particle formed on the TiO2 NTbs surface, while the EDS and XPS measurements shows the increasing doping amount of CdO as the Cd(NO3)2 concentration increasing. At a relatively high precursor concentration (800 mM), the formation of particle clusters and nanocrystals on the surface of the TiO2 nanotubes could be easily detected, and the sample presented XRD diffraction peaks indicative of CdTiO3. Meanwhile, the Ti 2p XPS spectra displayed an obvious shift (∼0.3 eV), which could be attributed to the change in the lattice structure. A negative shift in the flatband potential (Vfb) and a decrease in charge carrier density were observed after doping. The maximum incident photon to charge carrier efficiency (IPCE) value calculated for the CdO-TiO2 nanotubes was 10.16%, much higher than that of pure TiO2 nanotubes.
1. Introduction The utilization of solar energy, which is a clean resource, has been explored for several decades because the continued reliance on fossil fuels has led to pollution. TiO2 nanomaterials remain the most promising system used in solar cells, although a wide range of semiconductor materials have also been investigated for this purpose [1–4]. To achieve optimal performance of TiO2 in water splitting, a suitable architecture with the following characteristics is required: (1) a large surface area, (2) few defects that facilitate recombination, and (3) good electron transport. Recently, among TiO2 nanomaterials, TiO2 nanotubes (TiO2 NTbs) have received increasing attention due to their outstanding charge separation and transport capabilities. Among different approaches used to synthesize TiO2 NTbs, anodization of titanium foil is an easy method for fabricating vertically aligned tube-like arrays, which can reduce the number of trap states [5–7]. Our earlier work showed that fluoride ions and organic solvents in the electrolyte are
essential for the formation of vertically aligned TiO2 NTbs [8,9]. However, the electron kinetics of TiO2 have restricted its range of application [10,11]. In recent years, enormous efforts have been devoted to exploring TiO2 nanomaterials-based nanocomposites for extensive photocatalytic applications; these efforts have been mostly based on different ion doping techniques using different metals. Recently, many reports have been published on CuO/TiO2 [12], Cu2O/ TiO2 [13], CdS/TiO2 [14], and other nanocomposites. The coupled semiconductor materials have two different energy-level systems, which play an important role in retarding the recombination of photogenerated electron hole pairs (e−-h+) or enhancing the electron transfer rate to the electrolyte. CdO is an n-type degenerate semiconductor with a direct band-gap of 2.2 eV, making it a suitable candidate for water splitting [15,16]. Biplab Sarma et al. developed an electrodeposition method for CdO-TiO2 NTbs nanocomposites, and carried out deposition for various times of 200, 300, 500, and 1000 s. Interestingly, they found that the composite formed after 300 s of
⁎
Corresponding author at: College of Chemistry & Chemical Engineering, Northeast Petroleum University, Daqing 163318, China. E-mail address: [email protected] (H. Song). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.apsusc.2019.01.044 Received 20 August 2018; Received in revised form 6 December 2018; Accepted 5 January 2019 Available online 08 January 2019 0169-4332/ © 2019 Published by Elsevier B.V.
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obtained (TEM, JEM-2100). The UV–Vis diffuse reflection spectra of pure TiO2 NTbs and series CdO-TiO2 NTbs were obtained using a UV–Vis spectrophotometer (UV-2550, Shimadzu, Japan).
deposition (there was no evidence of particle agglomeration at the top of the nanotubes) showed the highest photocurrent density compared to the other samples [17]. This phenomenon may be attributed to agglomerate structures that hindered the absorption of photons by the TiO2 nanotube arrays. Besides, during the photocatalytic hydrogen generation reaction, the in situ bulk formation of CdO in CdO-TiO2 system plays an important role in the enhanced activity [18]. To the best of our knowledge, the direct impregnation-calcination method has not yet been introduced to prepare CdO-TiO2 NTbs. The simple preparation method could easily introduce CdO onto TiO2 NTbs surface to form an efficient CdO-TiO2 NTbs system for water splitting hydrogen evolution. Herein, we used a direct impregnation-calcination method to deposit CdO nanoparticles into TiO2 NTbs structures to improve the photocatalytic hydrogen evolution efficiency. The influence of the doping amount of CdO was also investigated. The physicochemical properties of the samples were characterized by scanning electron microscopy (SEM), transimission electron microscopy (TEM), UV–Vis diffuse reflection spectra (UV–Vis DRS), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD). In addition, the incident photon to current conversion efficiency of the CdO-TiO2 NTbs system was investigated by recording the photocurrent responses of all samples.
2.4. PEC performance characterizations All electrochemical measurements were carried out using a threeelectrode arrangement under deaerated conditions, which were achieved by continuous purging of a 1 M Na2SO4 electrolyte at room temperature with nitrogen (99.999%). The light source (365 nm, Optimax 365) has a peak emission at 365 nm with a full width at half maximum of 10 nm. The prepared CdO/TiO2 NTbs (10 × 10 mm) were used as the working electrode, a platinum net (20 × 20 mm) was used as the counter electrode, and a Ag/AgCl electrode (3 M KCl) was used as reference electrode. Mott-Schottky plots were used to characterize all samples by impedance-potential (AUTOLAB PGSTAT 30) at potentials ranging from −0.1 to +0.7 V vs. Ag/AgCl with an amplitude of 10 mV. The photoresponses of all samples were measured by chronoamperometry without additional voltage. 3. Results and discussion 3.1. Micromorphology of the as-prepared CdO-TiO2 NTbs
2. Experimental
The micromorphology of the CdO-TiO2 NTbs were characterized by SEM. Fig. 1 shows the top-view SEM images of pure TiO2 NTbs and the CdO-TiO2 NTbs prepared with different concentrations of Cd(NO3)2. We can found that, the top surface of pure TiO2 nanotubes was clean and tidy, without any cluster formation. The other SEM images for CdOTiO2 NTbs (Fig. 1b–h) also exhibited a one-dimensional (1-D) tubular structure, while some of the tube walls peeled off after impregnation and annealing at 450 °C. The average diameter and wall thickness of all samples were found to be 102–109 nm and 10–12 nm, respectively, similar with the pure TiO2 NTbs, which indicates that the surface morphological properties of the TiO2 NTbs remained unchanged after impregnation. At relatively low Cd(NO3)2 concentrations (10–200 mM, Fig. 1(b)–(f)), there was no evidence of particle cluster or nanocrystal formation on the surface of the TiO2 NTbs. At a relatively high Cd (NO3)2 concentration (800 mM, Fig. 1(g–h)), the obvious formation of particle clusters and nanocrystals on the surface of the TiO2 NTbs can be detected. Interestingly, the nanocrystals with a diameter of 21–66 nm have not blocked the nanotubes. However, partial nanocrystals were observed filling in the pores of the nanotubes. Fig. 2a–g shows cross-sectional SEM images of pure TiO2 NTbs and the as-prepared samples. The average tube length of all samples was found to be about 4 µm, which indicates that the vertically oriented, close-packed structure was not destroyed during the preparation process and the tubes are well separated into individuals. The above SEM images also reveal that the impregnation deposition process does not damage the ordered structure of the TiO2 NTbs and the tube diameter and length were not altered. Fig. 2h and i shows the TEM images of pure TiO2 NTbs and the nanocomposite prepared at Cd(NO3)2 concentration of 800 mM, respectively. The pure TiO2 NTbs was characterized by a hollow nanotube-structure, without any impurity inside the nantoubes. After direct impregnation at lower Cd(NO3)2 concentration (Fig. S1a and b), there was also no obvious particles formed inside the tubes. However, when the TiO2 NTbs was treated at higher Cd(NO3)2 concentration (Fig. 2i), the crystalline particles formed inside the nanotubes, indicating the generation of a new crystalline style.
2.1. Preparation of TiO2 NTbs Titanium samples (0.1 × 30 × 40 mm, 99.6+%, Advent, UK) were degreased by sonicating in acetone, ethanol, and deionized (DI) water and dried in a nitrogen stream without any additional surface polishing. The electrochemical anodization process was performed in a twoelectrode system. Ti foil served as the anode and a carbon rod as the cathode. As an electrolyte for the electrochemical experiments, we used an ethylene glycol/DI water solution (10 vol% of H2O), citric acid (0.11 M) and ammonium fluoride (0.15 M). The first step of the anodization process lasted 30 min at 60 °C and was followed by rinsing with DI water. Afterward, the Ti foil was cleaned by sonicating in DI water to remove the oxide layer. Interestingly, the oxide layer was completely removed in less than 1 min as the electrochemical anodization process occurred at a relatively high temperature. The second step of the anodization process lasted 2 h at room temperature (∼25 °C) and was followed by rinsing with DI water and drying in a nitrogen stream. 2.2. Preparation of CdO-TiO2 NTbs The as-prepared TiO2 NTbs were irradiated with UV light (LED, Optimax 365, λ = 365 nm) for 10 min and then immediately immersed in a water bath containing cadmium nitrate tetrahydrate (99%, Macklin) at different concentrations (10, 30, 50, 70, 200, and 800 mM) under UV light irradiation for 20 min at room temperature (∼25 °C). Samples were then rinsed with DI water and dried at 60 °C. Finally, the as-prepared samples were annealed in air at 450 °C for 3 h with a heating rate of 5 °C/min. 2.3. Characterization of physicochemical properties For the structural and morphological characterization of the samples, top view and cross-sectional SEM observations were carried out using a ZEISS-IGMA HD/VP. X-ray diffraction (XRD, D/max-2200PC, Rigaku) was used for the crystalline phase characterization of the CdOTiO2 NTbs. X-ray photoelectron spectroscopy (XPS, AXIS Ultra, Kratos) was employed to investigate the chemical composition of the CdO-TiO2 NTbs. The binding energies of the obtained XPS spectra were calibrated based on the deviation of adventitious carbon (284.8 eV). The transimission electron microscopy (TEM) images of the samples were
3.2. Crystallite structure of as-prepared CdO-TiO2 NTbs XRD was employed to characterize the crystal phases of the CdOTiO2 NTbs, as shown in Fig. 3, which presents the XRD patterns of pure CdO and CdO-TiO2 NTbs. The CdO was prepared by annealing Cd (NO3)2 in air at 450 °C for 3 h with a heating rate of 5 °C/min and the 137
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Fig. 1. The top-view SEM images of the pure TiO2 NTbs (a) and CdO-TiO2 NTbs on Ti foil surface prepared with different Cd(NO3)2 concentrations: 10 mM (b), 30 mM (c), 50 mM (d), 70 mM (e), 200 mM (f) and 800 mM (g–h).
Fig. 2. The cross-section SEM images of pure TiO2 NTbs (a) and the CdO-TiO2 NTbs prepared with different Cd(NO3)2 concentrations: (a–g) 10 mM, 30 mM, 50 mM, 70 mM, 200 mM and 800 mM; TEM images of pure TiO2 NTbs (h) and CdO-TiO2 NTbs prepared at Cd(NO3)2 concentrations of 800 mM (i). 138
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Table 1 Chemical composition of pure TiO2 NTbs and CdO-TiO2 TNbs prepared at different Cd(NO3)2 concentration as analyzed by XPS. Sample
Ti/at%
Cd/at%
O/at%
F/at%
N/at%
C/at%
Pure TiO2 NTbs 10 mM 50 mM 200 mM 800 mM
22.61 24.4 18.65 12.87 14.43
/ 0.29 2.01 5.06 10.17
51.6 53.25 43.68 33.07 45.13
0.36 / 0.31 / 0.64
2.04 1.39 1.05 1.46 1.65
23.4 20.67 28.68 43.64 27.33
peaks at 2θ = 25.3°, 48.0° and 55.1°, which can be assigned to the (1 0 1), (2 0 0), and (2 1 1) planes of the anatase phase, respectively [20]. At a low Cd(NO3)2 concentrations (10 ∼ 200 mM), the (2 2 0) and (2 0 0) peaks for CdO are located at almost the same position as the (2 1 1) peak of anatase and titanium, making it difficult to identify the (2 2 0) and (2 0 0) peaks of CdO. Besides, no diffraction peaks could be assigned to the (1 1 1) plane of CdO, the strongest peak observed for pure CdO, which is probably due to the relatively low loading amount and high dispersion, which is in agreement with what can be observed in the SEM images (Fig. 1(a)–(e)) [21]. When the Cd(NO3)2 concentration reached 800 mM, the sample presented diffraction peaks at 2θ = 20.3°, 22.9°, 31.2°, 34.3°, 46.9°, 50.7°, 59.4° and 61.4° corresponding to the (1 0 1), (0 1 2), (1 0 4), (1 1 0), (0 2 4), (1 1 6), (2 1 4) and (3 0 0) planes of CdTiO3, respectively [JCPDS No. 29–2077]. Beyond that, a peak at 2θ = 33.10° corresponding to the (1 1 1) plane of CdO can be detected, which indicates the crystalline nature of CdO after the treatment. From further observation of Fig. 3, we find that when the
Fig. 3. The XRD patterns of CdO and CdO-TiO2 NTbs prepared at different cadmium nitrate tetrahydrate concentrations. All samples were calcinated at 450 °C in air for 3 h.
CdO-TiO2 NTbs were prepared using different Cd(NO3)2 concentrations of 10, 30, 50, 70, 200, and 800 mM. The XRD pattern confirms the polycrystalline nature of pure CdO prepared by annealing Cd(NO3)2. The principal peaks at 2θ = 33.10° and 38.52° correspond to the (1 1 1) and (2 0 0) planes, while the weak peaks at 2θ = 54.40°, 66.56° and 69.85° correspond to the (2 2 0), (3 1 1), and (2 2 2) planes, respectively [19]. All of the TiO2 NTbs and CdO-TiO2 NTbs presented diffraction
Fig. 4. (a) The XPS survey scan spectra of pure TiO2 NTbs and CdO-TiO2 NTbs prepared at different Cd(NO3)2 concentrations. (b-d) The high resolution XPS spectra of Cd 3d, O 1s and Ti 2p for pure TiO2 NTbs and CdO-TiO2 NTbs prepared at different Cd(NO3)2 concentrations, respectively. All samples were calcinated at 450 °C in air for 3 h. 139
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Fig. 5. The high resolution spectra of O 1s for pure TiO2 NTbs (a) and CdO-TiO2 NTbs prepared at different Cd(NO3)2 concentrations: (b) 10 mM and (c) 800 mM. All samples were calcinated at 450 °C in air for 3 h.
Fig. 6. (a) Amperometric I-t curves of the TiO2 NTbs and CdO-TiO2 NTbs without additional voltage with 30 s on/off light cycles, (b) Amperometric I-t curves of the TiO2 NTbs.
3.3. Characterization of the chemical composition of TiO2 NTbs and CdOTiO2 NTbs
Cd(NO3)2 concentration reached 30 mM, the relative intensity of anatase decreased with increasing Cd(NO3)2 concentration. Combined with the information provided by the SEM images, it can be concluded that the tube length of all samples did not change after the impregnation deposition process; therefore the lower diffraction peaks of anatase may be attributed to Cd2+ entering into the crystal lattice of TiO2 and thus destroying its crystalline structure [22].
The representative XPS survey scan and the high resolution XPS spectra for Cd 3d, O 1s and Ti 2p of pure TiO2 NTbs and CdO-TiO2 NTbs prepared at Cd(NO3)2 concentrations of 10 mM, 50 mM, 200 mM and 800 mM are shown in Fig. 4. Fig. 4a shows XPS survey scan spectra exhibiting Ti and O peaks for TiO2 NTbs and Ti, O, and Cd peaks for CdO-TiO2 NTbs. It reveals that the direct impregnation method could successfully deposit Cd onto TiO2 NTbs surface, which is consistent with the EDS analysis results (shown in Fig. S2). Besides these peaks, a trace 140
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Cd 3d spectrum (Fig. 4b), which can be attributed to the existence of Cd2+; this exactly matches the binding energy of Cd-O [23,24]. The high resolution spectra of O 1s of pure TiO2 NTbs and series CdO-TiO2 NTbs were compared in Fig. 4c. To further explore the existence formation of Cd2+, the O 1s spectra of representative samples were studied by means of XPS-peak- differentation analysis, as shown in Fig. 5a–c. The additional O 1s peaks observed at 529.5 eV after CdO doping (Fig. 5b and c) are identical to those already reported for CdO systems [24], which indicates a successful preparation of CdO-TiO2 NTbs nanocomposite. Fig. 4d shows the measured Ti 2p spectra. It should be noted that a slight shift was observed when the Cd(NO3)2 concentration was 10 mM (the doping amount was 0.29 at%), indicating the relatively stable chemical state of the TiO2 crystal. However, an obvious shift (∼0.3 eV) was observed when the Cd(NO3)2 concentration reached 800 mM (the doping amount was 10.17 at%). We concluded that the peak shifts should be attributed to the change in the lattice structure. The atomic radii of titanium and cadmium are 147 pm and 151 pm, respectively. In addition, titanium and cadmium have nearly the same electronegativity (1.54 and 1.69, respectively), which may cause a portion of the Cd2+ to enter the TiO2 crystal in the form of substitutional impurities or form a substitutional solid solution, causing lattice distortion. This can also explain the XRD diffraction peaks observed for CdTiO3 as the Cd(NO3)2 concentration reached 800 mM (Fig. 3).
Fig. 7. Instantaneous photocurrent and instantaneous photocurrent decay after 30 s for TiO2 and CdO-TiO2 NTbs.
3.4. Photocurrent response of TiO2 NTbs and CdO-TiO2 NTbs The photocurrent response of the TiO2 NTbs and CdO-TiO2 NTbs was observed using amperometric measurements under intermittent illumination. Fig. 8 shows the I-t curves obtained from the PEC cell using CdO-TiO2 NTbs (Fig. 6a) and pure TiO2 NTbs (Fig. 6b) (apparent surface area of 1 × 1 cm2) as the anode and a platinum mesh (apparent surface area of 2 × 2 cm2) as the cathode without additional potential. For pure TiO2 NTbs (Fig. 6b), when the light is switched on, a photocurrent spike is observed, indicating the sudden generation of electrons, and the photocurrent value decays to about 36.3% after 30 s. However, we can barely see any cathodic transient when the light is switched off, indicating that the photocurrent decay can be mostly attributed to the poor electron transport in TiO2 [25]. For CdO-TiO2 TNbs series samples, the data showed a very low dark current for all samples. As illumination started, the current increased instantaneously and reached a steady state for each sample. At very low Cd(NO3)2 concentrations (10 mM ∼ 30 mM), the photocurrent response of the CdO-TiO2 NTbs showed no significant improvement compared with the pure TiO2 NTbs. This phenomenon may be attributed to the very small doping amounts which arise from the loss of cadmium nitrate at high temperature. When the concentration of Cd(NO3)2·4H2O reached 50 mM, the photocurrent response of the CdO-TiO2 NTbs was obviously improved to
Fig. 8. IPCE values for the TiO2 NTbs and CdO-TiO2 NTbs prepared at different Cd(NO3)2 concentrations. All samples were calcinated at 450 °C in air for 3 h.
amount of N and F could be detected in all samples which originated from the electrolyte containg NH4F during anodization oxidation preparation of TiO2 NTbs. The chemical compositions analyzed by XPS were shown in Table 1. The results show that the loading amount of cadmium (0.29 at% for 10 mM and 10.17 at% for 800 mM, respectively) increased as the concentration of Cd(NO3)2 was increased. Two well-resolved peaks at 404.9 eV and 411.7 eV were observed from the
Fig. 9. Mott-Schottky plots for pure TiO2 NTbs and CdO-TiO2 NTbs prepared at a Cd(NO3)2·4H2O concentration of 50 mM. 141
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∼0.45 mA/cm2. Further increasing the Cd(NO3)2 concentration (70–200 mM) resulted in a lower photocurrent response. This phenomenon can be ascribed to the distortion of the TiO2 crystal lattice, which induced a decrease in the charge carrier density. As the concentration of Cd(NO3)2 reached 800 mM, the photocurrent response decreased nearly to 0.01 mA/cm2, which was mainly because of the formation of CdTiO3. The band gap energy of CdO-TiO2 NTbs was also considered to be an important factor which affected the photocatalytic performances. The UV–Vis diffuse reflection spectra of pure TiO2 NTbs and the CdO-TiO2 NTbs were shown in Fig. S3 in the supporting information. It can be found that, after a small amount of CdO doping, the band gap energy was narrowed from 3.24 eV (for pure TiO2 NTbs) to ∼3.0 eV for CdO doped TiO2 NTbs, which also indicating an improved light absorption ability for CdO-TiO2 NTbs. Fig. 7 shows Instantaneous photocurrent and instantaneous photocurrent decay after 30 s for TiO2 and CdO-TiO2 NTbs. The CdO-TiO2 NTbs prepared at Cd(NO3)2 concentrations of 10 mM and 30 mM showed higher photocurrent spikes, indicating more electron accumulation on the electrode surface. However, the photocurrent value decreased to about 66% after 30 s. In addition, cathodic transients can be observed in Fig. 6(a), which suggests hole accumulation on the electrode surface. Thus, the photocurrent decay is mostly attributed to charge carriers quickly recombining as well as poor electron transport in TiO2 [26]. The photocurrent spike observed for CdO-TiO2 NTbs prepared at a Cd(NO3)2 concentration of 50 mM increased sharply and decayed to about 53% after 30 s, indicating that accumulating more electrons on the electrode surface may reduce the attenuation rate of the photocurrent. Fig. 8 shows the incident photon to current conversion efficiency (IPCE) for the TiO2 NTbs and CdO-TiO2 NTbs prepared at different Cd (NO3)2 concentrations. The IPCE can be calculated using the following equation: [27]
IPCE =
where e is the electronic charge (1.6 × 10−19 ∁), ∊0 is the permittivity of free space (8.86 × 10−12 F/m ), ∊r is the relative permittivity of TiO2 (taken as 31 for anatase), CSC is the capacitance of the space charge layer, and E is the applied potential. Using the equation shown above and the physical constants for TiO2, the charge carrier densities of pure TiO2 NTbs and CdO-TiO2 NTbs were calculated to be 2.28 × 1017 cm−3 and 1.77 × 1017 cm−3 , respectively. This indicates that the charge carrier density decreased after doping, which is mainly attributed to Cd2+ ions occupying the TiO2 lattice acting as acceptors. Cadmium is provided with two valence electrons which can form covalent bonds with surrounding atoms. Thus, cadmium must capture two electrons from other atoms resulting in the appearance of holes in the lattice. Therefore, the Cd2+ ions will offer double-acceptor levels, resulting in a lower density of charge carriers. 4. Conclusions In this work, we have developed a promising and efficient strategy for preparing CdO-TiO2 NTbs with highly ordered micromorphology. It was found that the obvious formation of particle clusters and nanocrystals on the surface of the TiO2 NTbs can be detected only at a relatively high precursor concentration; meanwhile, a portion of TiO2 was converted into CdTiO3 after doping. The Mott-Schottky plots showed that the flatband potential (Vfb) of CdO-TiO2 NTbs has a negative shift compared with pure TiO2 NTbs, indicating higher energy level electrons. The measured Ti 2p XPS spectra and calculated charge carrier densities (Nd) showed that Cd2+ may enter into the crystal lattice. The amperometric I-t curves indicated that even a small amount of Cd2+ would also lead to more electron accumulation on the electrode surface, while on the other hand, the injection of Cd2+ would similarly cause hole accumulation on the electrode surface, leading to greater attenuation of the photocurrent. The maximum incident photon to charge carrier efficiency (IPCE) value found for CdO-TiO2 NTbs was 10.16%, much higher than that of pure TiO2 NTbs.
1240 × jp (mA/cm2) P (mW/cm2) × λ (nm)
Acknowledgement
where jP is the photocurrent density, P is the incident light power density (15 mW/cm2 , Optimax 365, 38 cm), and λ is the wavelength (365 nm ). The order of IPCE values of all the samples follows the same trend as the photocurrent values, with IPCE values of 5.36%, 5.15%, 5.24%, 10.16%, 7.75%, 5.72% and 0.29%, respectively. Indeed, both I-t curves and IPCE values indicate a significant promotion of the photoresponse of CdO-TiO2 NTbs prepared at a suitable concentration of Cd (NO3)2 compared to the pure TiO2 NTbs. To further explore the synergistic mechanism of CdO-TiO2 TNbs for photocatalytic water splitting, the Mott-Schottky plots of pure TiO2 NTbs and CdO doped TiO2 NTbs samples were conducted in 1 M Na2SO4 solution at a frequency of 1 kHz at room temperature, which were shown in Fig. 9. As can be seen, the Mott-Schottky plots possess a positive slope characteristic of n-type semiconductors. The flatband potential can be determined by extrapolating to C = 0. The flatband potentials (Vfb) of pure TiO2 NTbs and CdO-TiO2 NTbs prepared with 50 mM cadmium nitrate tetrahydrate can be deduced from Fig. 9b. The calculated Vfb for the pure TiO2 NTbs and CdO coated TiO2 NTbs were 0.110 V and 0.085 V vs. Ag/AgCl (3 M KCl), respectively, exhibiting a negative shift of Vfb inducing more electrons to migrate to the electrode surface. Correspondingly, the shifted Vfb leads to a higher quasi-Fermi energy level which indicates higher energy level electrons [28]. Furthermore, a linear relationship is predicted between the applied potential and the inverse square of the capacitance, arising from the space charge layer in the semiconductor. The Mott-Schottky equation, which allows for the determination of charge carrier density (Nd), can be expressed as follows: [29]
This work was supported by the Natural Science Foundation of Heilongjiang Province of China [grant number QC2017005], the Province Postdoctoral Fund [grant number LBH-Z15032], the Heilongjiang Youth Innovative Talents Training Project [grant number UNPYSCT-2018042], the Youth Fund, Cultivating Fund and Scientific Research Foundation of Northeast Petroleum University [grant number NEPUBS201508, 2017PYQZL-06]. We are also grateful for the measurement assistants from Analysis & Testing Center of Northeast Petroleum University. Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apsusc.2019.01.044. References [1] S.U. Khan, M. Alshahry, I.W. Jr, Efficient photochemical water splitting by a chemically modified n-TiO2, Science 34 (2003) 2243–2245. [2] G. Wang, H. Wang, Y. Ling, Y. Tang, X. Yang, R.C. Fitzmorris, C. Wang, J.Z. Zhang, Y. Li, Hydrogen-treated TiO2 nanowire arrays for photoelectrochemical water splitting, Nano Lett. 11 (2011) 3026–3033. [3] A. Wolcott, W.A. Smith, T.R. Kuykendall, Y. Zhao, J.Z. Zhang, Photoelectrochemical water splitting using dense and aligned TiO2 nanorod arrays, Small 5 (2009) 104–111. [4] P. Hartmann, D.K. Lee, B.M. Smarsly, J. Janek, Mesoporous TiO2: comparison of classical sol−gel and nanoparticle based photoelectrodes for the water splitting reaction, ACS Nano 4 (2010) 3147–3154. [5] J. Wang, Z. Lin, Anodic formation of ordered TiO2 nanotube arrays: effects of electrolyte temperature and anodization potential, J. Phys. Chem. C 113 (2012) 4026–4030. [6] Y. Liao, W. Que, P. Zhong, J. Zhang, Y. He, A facile method to crystallize amorphous
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Full Length Article
Enhanced photoelectrochemical performance of CdO-TiO2 nanotubes prepared by direct impregnation
T
Xueqin Wanga,1, Kai Chenga,1, Shuna Douc, Qihui Chena, Junlei Wanga, Zhenyuan Songa, ⁎ Jiaojing Zhanga, Hua Songa,b, a
College of Chemistry & Chemical Engineering, Northeast Petroleum University, Daqing 163318, China Provincial Key Laboratory of Oil & Gas Chemical Technology, Northeast Petroleum University, Daqing 163318, China c Strategic Planning and Development Division, Zhonghua Petrochemical Co. LTD, Qingzhou 262513, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: TiO2 nanotubes Cadmium oxide Impregnation-calcination Photoelectrochemical
A direct impregnation technique was adopted to prepare a series of CdO-TiO2 nanotubes. Self-organized TiO2 nanotubes were prepared using an optimized two-step anodization process. The morphology, crystallinity, elemental composition, and photoelectrochemical properties of the CdO-TiO2 nanotubes were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV–Vis diffuse reflection spectra (UV–Vis DRS), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and photoelectric cell (PEC) measurements. At lower Cd(NO3)2 concentration, no obvious CdO crystalline particle formed on the TiO2 NTbs surface, while the EDS and XPS measurements shows the increasing doping amount of CdO as the Cd(NO3)2 concentration increasing. At a relatively high precursor concentration (800 mM), the formation of particle clusters and nanocrystals on the surface of the TiO2 nanotubes could be easily detected, and the sample presented XRD diffraction peaks indicative of CdTiO3. Meanwhile, the Ti 2p XPS spectra displayed an obvious shift (∼0.3 eV), which could be attributed to the change in the lattice structure. A negative shift in the flatband potential (Vfb) and a decrease in charge carrier density were observed after doping. The maximum incident photon to charge carrier efficiency (IPCE) value calculated for the CdO-TiO2 nanotubes was 10.16%, much higher than that of pure TiO2 nanotubes.
1. Introduction The utilization of solar energy, which is a clean resource, has been explored for several decades because the continued reliance on fossil fuels has led to pollution. TiO2 nanomaterials remain the most promising system used in solar cells, although a wide range of semiconductor materials have also been investigated for this purpose [1–4]. To achieve optimal performance of TiO2 in water splitting, a suitable architecture with the following characteristics is required: (1) a large surface area, (2) few defects that facilitate recombination, and (3) good electron transport. Recently, among TiO2 nanomaterials, TiO2 nanotubes (TiO2 NTbs) have received increasing attention due to their outstanding charge separation and transport capabilities. Among different approaches used to synthesize TiO2 NTbs, anodization of titanium foil is an easy method for fabricating vertically aligned tube-like arrays, which can reduce the number of trap states [5–7]. Our earlier work showed that fluoride ions and organic solvents in the electrolyte are
essential for the formation of vertically aligned TiO2 NTbs [8,9]. However, the electron kinetics of TiO2 have restricted its range of application [10,11]. In recent years, enormous efforts have been devoted to exploring TiO2 nanomaterials-based nanocomposites for extensive photocatalytic applications; these efforts have been mostly based on different ion doping techniques using different metals. Recently, many reports have been published on CuO/TiO2 [12], Cu2O/ TiO2 [13], CdS/TiO2 [14], and other nanocomposites. The coupled semiconductor materials have two different energy-level systems, which play an important role in retarding the recombination of photogenerated electron hole pairs (e−-h+) or enhancing the electron transfer rate to the electrolyte. CdO is an n-type degenerate semiconductor with a direct band-gap of 2.2 eV, making it a suitable candidate for water splitting [15,16]. Biplab Sarma et al. developed an electrodeposition method for CdO-TiO2 NTbs nanocomposites, and carried out deposition for various times of 200, 300, 500, and 1000 s. Interestingly, they found that the composite formed after 300 s of
⁎
Corresponding author at: College of Chemistry & Chemical Engineering, Northeast Petroleum University, Daqing 163318, China. E-mail address: [email protected] (H. Song). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.apsusc.2019.01.044 Received 20 August 2018; Received in revised form 6 December 2018; Accepted 5 January 2019 Available online 08 January 2019 0169-4332/ © 2019 Published by Elsevier B.V.
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obtained (TEM, JEM-2100). The UV–Vis diffuse reflection spectra of pure TiO2 NTbs and series CdO-TiO2 NTbs were obtained using a UV–Vis spectrophotometer (UV-2550, Shimadzu, Japan).
deposition (there was no evidence of particle agglomeration at the top of the nanotubes) showed the highest photocurrent density compared to the other samples [17]. This phenomenon may be attributed to agglomerate structures that hindered the absorption of photons by the TiO2 nanotube arrays. Besides, during the photocatalytic hydrogen generation reaction, the in situ bulk formation of CdO in CdO-TiO2 system plays an important role in the enhanced activity [18]. To the best of our knowledge, the direct impregnation-calcination method has not yet been introduced to prepare CdO-TiO2 NTbs. The simple preparation method could easily introduce CdO onto TiO2 NTbs surface to form an efficient CdO-TiO2 NTbs system for water splitting hydrogen evolution. Herein, we used a direct impregnation-calcination method to deposit CdO nanoparticles into TiO2 NTbs structures to improve the photocatalytic hydrogen evolution efficiency. The influence of the doping amount of CdO was also investigated. The physicochemical properties of the samples were characterized by scanning electron microscopy (SEM), transimission electron microscopy (TEM), UV–Vis diffuse reflection spectra (UV–Vis DRS), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD). In addition, the incident photon to current conversion efficiency of the CdO-TiO2 NTbs system was investigated by recording the photocurrent responses of all samples.
2.4. PEC performance characterizations All electrochemical measurements were carried out using a threeelectrode arrangement under deaerated conditions, which were achieved by continuous purging of a 1 M Na2SO4 electrolyte at room temperature with nitrogen (99.999%). The light source (365 nm, Optimax 365) has a peak emission at 365 nm with a full width at half maximum of 10 nm. The prepared CdO/TiO2 NTbs (10 × 10 mm) were used as the working electrode, a platinum net (20 × 20 mm) was used as the counter electrode, and a Ag/AgCl electrode (3 M KCl) was used as reference electrode. Mott-Schottky plots were used to characterize all samples by impedance-potential (AUTOLAB PGSTAT 30) at potentials ranging from −0.1 to +0.7 V vs. Ag/AgCl with an amplitude of 10 mV. The photoresponses of all samples were measured by chronoamperometry without additional voltage. 3. Results and discussion 3.1. Micromorphology of the as-prepared CdO-TiO2 NTbs
2. Experimental
The micromorphology of the CdO-TiO2 NTbs were characterized by SEM. Fig. 1 shows the top-view SEM images of pure TiO2 NTbs and the CdO-TiO2 NTbs prepared with different concentrations of Cd(NO3)2. We can found that, the top surface of pure TiO2 nanotubes was clean and tidy, without any cluster formation. The other SEM images for CdOTiO2 NTbs (Fig. 1b–h) also exhibited a one-dimensional (1-D) tubular structure, while some of the tube walls peeled off after impregnation and annealing at 450 °C. The average diameter and wall thickness of all samples were found to be 102–109 nm and 10–12 nm, respectively, similar with the pure TiO2 NTbs, which indicates that the surface morphological properties of the TiO2 NTbs remained unchanged after impregnation. At relatively low Cd(NO3)2 concentrations (10–200 mM, Fig. 1(b)–(f)), there was no evidence of particle cluster or nanocrystal formation on the surface of the TiO2 NTbs. At a relatively high Cd (NO3)2 concentration (800 mM, Fig. 1(g–h)), the obvious formation of particle clusters and nanocrystals on the surface of the TiO2 NTbs can be detected. Interestingly, the nanocrystals with a diameter of 21–66 nm have not blocked the nanotubes. However, partial nanocrystals were observed filling in the pores of the nanotubes. Fig. 2a–g shows cross-sectional SEM images of pure TiO2 NTbs and the as-prepared samples. The average tube length of all samples was found to be about 4 µm, which indicates that the vertically oriented, close-packed structure was not destroyed during the preparation process and the tubes are well separated into individuals. The above SEM images also reveal that the impregnation deposition process does not damage the ordered structure of the TiO2 NTbs and the tube diameter and length were not altered. Fig. 2h and i shows the TEM images of pure TiO2 NTbs and the nanocomposite prepared at Cd(NO3)2 concentration of 800 mM, respectively. The pure TiO2 NTbs was characterized by a hollow nanotube-structure, without any impurity inside the nantoubes. After direct impregnation at lower Cd(NO3)2 concentration (Fig. S1a and b), there was also no obvious particles formed inside the tubes. However, when the TiO2 NTbs was treated at higher Cd(NO3)2 concentration (Fig. 2i), the crystalline particles formed inside the nanotubes, indicating the generation of a new crystalline style.
2.1. Preparation of TiO2 NTbs Titanium samples (0.1 × 30 × 40 mm, 99.6+%, Advent, UK) were degreased by sonicating in acetone, ethanol, and deionized (DI) water and dried in a nitrogen stream without any additional surface polishing. The electrochemical anodization process was performed in a twoelectrode system. Ti foil served as the anode and a carbon rod as the cathode. As an electrolyte for the electrochemical experiments, we used an ethylene glycol/DI water solution (10 vol% of H2O), citric acid (0.11 M) and ammonium fluoride (0.15 M). The first step of the anodization process lasted 30 min at 60 °C and was followed by rinsing with DI water. Afterward, the Ti foil was cleaned by sonicating in DI water to remove the oxide layer. Interestingly, the oxide layer was completely removed in less than 1 min as the electrochemical anodization process occurred at a relatively high temperature. The second step of the anodization process lasted 2 h at room temperature (∼25 °C) and was followed by rinsing with DI water and drying in a nitrogen stream. 2.2. Preparation of CdO-TiO2 NTbs The as-prepared TiO2 NTbs were irradiated with UV light (LED, Optimax 365, λ = 365 nm) for 10 min and then immediately immersed in a water bath containing cadmium nitrate tetrahydrate (99%, Macklin) at different concentrations (10, 30, 50, 70, 200, and 800 mM) under UV light irradiation for 20 min at room temperature (∼25 °C). Samples were then rinsed with DI water and dried at 60 °C. Finally, the as-prepared samples were annealed in air at 450 °C for 3 h with a heating rate of 5 °C/min. 2.3. Characterization of physicochemical properties For the structural and morphological characterization of the samples, top view and cross-sectional SEM observations were carried out using a ZEISS-IGMA HD/VP. X-ray diffraction (XRD, D/max-2200PC, Rigaku) was used for the crystalline phase characterization of the CdOTiO2 NTbs. X-ray photoelectron spectroscopy (XPS, AXIS Ultra, Kratos) was employed to investigate the chemical composition of the CdO-TiO2 NTbs. The binding energies of the obtained XPS spectra were calibrated based on the deviation of adventitious carbon (284.8 eV). The transimission electron microscopy (TEM) images of the samples were
3.2. Crystallite structure of as-prepared CdO-TiO2 NTbs XRD was employed to characterize the crystal phases of the CdOTiO2 NTbs, as shown in Fig. 3, which presents the XRD patterns of pure CdO and CdO-TiO2 NTbs. The CdO was prepared by annealing Cd (NO3)2 in air at 450 °C for 3 h with a heating rate of 5 °C/min and the 137
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Fig. 1. The top-view SEM images of the pure TiO2 NTbs (a) and CdO-TiO2 NTbs on Ti foil surface prepared with different Cd(NO3)2 concentrations: 10 mM (b), 30 mM (c), 50 mM (d), 70 mM (e), 200 mM (f) and 800 mM (g–h).
Fig. 2. The cross-section SEM images of pure TiO2 NTbs (a) and the CdO-TiO2 NTbs prepared with different Cd(NO3)2 concentrations: (a–g) 10 mM, 30 mM, 50 mM, 70 mM, 200 mM and 800 mM; TEM images of pure TiO2 NTbs (h) and CdO-TiO2 NTbs prepared at Cd(NO3)2 concentrations of 800 mM (i). 138
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Table 1 Chemical composition of pure TiO2 NTbs and CdO-TiO2 TNbs prepared at different Cd(NO3)2 concentration as analyzed by XPS. Sample
Ti/at%
Cd/at%
O/at%
F/at%
N/at%
C/at%
Pure TiO2 NTbs 10 mM 50 mM 200 mM 800 mM
22.61 24.4 18.65 12.87 14.43
/ 0.29 2.01 5.06 10.17
51.6 53.25 43.68 33.07 45.13
0.36 / 0.31 / 0.64
2.04 1.39 1.05 1.46 1.65
23.4 20.67 28.68 43.64 27.33
peaks at 2θ = 25.3°, 48.0° and 55.1°, which can be assigned to the (1 0 1), (2 0 0), and (2 1 1) planes of the anatase phase, respectively [20]. At a low Cd(NO3)2 concentrations (10 ∼ 200 mM), the (2 2 0) and (2 0 0) peaks for CdO are located at almost the same position as the (2 1 1) peak of anatase and titanium, making it difficult to identify the (2 2 0) and (2 0 0) peaks of CdO. Besides, no diffraction peaks could be assigned to the (1 1 1) plane of CdO, the strongest peak observed for pure CdO, which is probably due to the relatively low loading amount and high dispersion, which is in agreement with what can be observed in the SEM images (Fig. 1(a)–(e)) [21]. When the Cd(NO3)2 concentration reached 800 mM, the sample presented diffraction peaks at 2θ = 20.3°, 22.9°, 31.2°, 34.3°, 46.9°, 50.7°, 59.4° and 61.4° corresponding to the (1 0 1), (0 1 2), (1 0 4), (1 1 0), (0 2 4), (1 1 6), (2 1 4) and (3 0 0) planes of CdTiO3, respectively [JCPDS No. 29–2077]. Beyond that, a peak at 2θ = 33.10° corresponding to the (1 1 1) plane of CdO can be detected, which indicates the crystalline nature of CdO after the treatment. From further observation of Fig. 3, we find that when the
Fig. 3. The XRD patterns of CdO and CdO-TiO2 NTbs prepared at different cadmium nitrate tetrahydrate concentrations. All samples were calcinated at 450 °C in air for 3 h.
CdO-TiO2 NTbs were prepared using different Cd(NO3)2 concentrations of 10, 30, 50, 70, 200, and 800 mM. The XRD pattern confirms the polycrystalline nature of pure CdO prepared by annealing Cd(NO3)2. The principal peaks at 2θ = 33.10° and 38.52° correspond to the (1 1 1) and (2 0 0) planes, while the weak peaks at 2θ = 54.40°, 66.56° and 69.85° correspond to the (2 2 0), (3 1 1), and (2 2 2) planes, respectively [19]. All of the TiO2 NTbs and CdO-TiO2 NTbs presented diffraction
Fig. 4. (a) The XPS survey scan spectra of pure TiO2 NTbs and CdO-TiO2 NTbs prepared at different Cd(NO3)2 concentrations. (b-d) The high resolution XPS spectra of Cd 3d, O 1s and Ti 2p for pure TiO2 NTbs and CdO-TiO2 NTbs prepared at different Cd(NO3)2 concentrations, respectively. All samples were calcinated at 450 °C in air for 3 h. 139
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Fig. 5. The high resolution spectra of O 1s for pure TiO2 NTbs (a) and CdO-TiO2 NTbs prepared at different Cd(NO3)2 concentrations: (b) 10 mM and (c) 800 mM. All samples were calcinated at 450 °C in air for 3 h.
Fig. 6. (a) Amperometric I-t curves of the TiO2 NTbs and CdO-TiO2 NTbs without additional voltage with 30 s on/off light cycles, (b) Amperometric I-t curves of the TiO2 NTbs.
3.3. Characterization of the chemical composition of TiO2 NTbs and CdOTiO2 NTbs
Cd(NO3)2 concentration reached 30 mM, the relative intensity of anatase decreased with increasing Cd(NO3)2 concentration. Combined with the information provided by the SEM images, it can be concluded that the tube length of all samples did not change after the impregnation deposition process; therefore the lower diffraction peaks of anatase may be attributed to Cd2+ entering into the crystal lattice of TiO2 and thus destroying its crystalline structure [22].
The representative XPS survey scan and the high resolution XPS spectra for Cd 3d, O 1s and Ti 2p of pure TiO2 NTbs and CdO-TiO2 NTbs prepared at Cd(NO3)2 concentrations of 10 mM, 50 mM, 200 mM and 800 mM are shown in Fig. 4. Fig. 4a shows XPS survey scan spectra exhibiting Ti and O peaks for TiO2 NTbs and Ti, O, and Cd peaks for CdO-TiO2 NTbs. It reveals that the direct impregnation method could successfully deposit Cd onto TiO2 NTbs surface, which is consistent with the EDS analysis results (shown in Fig. S2). Besides these peaks, a trace 140
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Cd 3d spectrum (Fig. 4b), which can be attributed to the existence of Cd2+; this exactly matches the binding energy of Cd-O [23,24]. The high resolution spectra of O 1s of pure TiO2 NTbs and series CdO-TiO2 NTbs were compared in Fig. 4c. To further explore the existence formation of Cd2+, the O 1s spectra of representative samples were studied by means of XPS-peak- differentation analysis, as shown in Fig. 5a–c. The additional O 1s peaks observed at 529.5 eV after CdO doping (Fig. 5b and c) are identical to those already reported for CdO systems [24], which indicates a successful preparation of CdO-TiO2 NTbs nanocomposite. Fig. 4d shows the measured Ti 2p spectra. It should be noted that a slight shift was observed when the Cd(NO3)2 concentration was 10 mM (the doping amount was 0.29 at%), indicating the relatively stable chemical state of the TiO2 crystal. However, an obvious shift (∼0.3 eV) was observed when the Cd(NO3)2 concentration reached 800 mM (the doping amount was 10.17 at%). We concluded that the peak shifts should be attributed to the change in the lattice structure. The atomic radii of titanium and cadmium are 147 pm and 151 pm, respectively. In addition, titanium and cadmium have nearly the same electronegativity (1.54 and 1.69, respectively), which may cause a portion of the Cd2+ to enter the TiO2 crystal in the form of substitutional impurities or form a substitutional solid solution, causing lattice distortion. This can also explain the XRD diffraction peaks observed for CdTiO3 as the Cd(NO3)2 concentration reached 800 mM (Fig. 3).
Fig. 7. Instantaneous photocurrent and instantaneous photocurrent decay after 30 s for TiO2 and CdO-TiO2 NTbs.
3.4. Photocurrent response of TiO2 NTbs and CdO-TiO2 NTbs The photocurrent response of the TiO2 NTbs and CdO-TiO2 NTbs was observed using amperometric measurements under intermittent illumination. Fig. 8 shows the I-t curves obtained from the PEC cell using CdO-TiO2 NTbs (Fig. 6a) and pure TiO2 NTbs (Fig. 6b) (apparent surface area of 1 × 1 cm2) as the anode and a platinum mesh (apparent surface area of 2 × 2 cm2) as the cathode without additional potential. For pure TiO2 NTbs (Fig. 6b), when the light is switched on, a photocurrent spike is observed, indicating the sudden generation of electrons, and the photocurrent value decays to about 36.3% after 30 s. However, we can barely see any cathodic transient when the light is switched off, indicating that the photocurrent decay can be mostly attributed to the poor electron transport in TiO2 [25]. For CdO-TiO2 TNbs series samples, the data showed a very low dark current for all samples. As illumination started, the current increased instantaneously and reached a steady state for each sample. At very low Cd(NO3)2 concentrations (10 mM ∼ 30 mM), the photocurrent response of the CdO-TiO2 NTbs showed no significant improvement compared with the pure TiO2 NTbs. This phenomenon may be attributed to the very small doping amounts which arise from the loss of cadmium nitrate at high temperature. When the concentration of Cd(NO3)2·4H2O reached 50 mM, the photocurrent response of the CdO-TiO2 NTbs was obviously improved to
Fig. 8. IPCE values for the TiO2 NTbs and CdO-TiO2 NTbs prepared at different Cd(NO3)2 concentrations. All samples were calcinated at 450 °C in air for 3 h.
amount of N and F could be detected in all samples which originated from the electrolyte containg NH4F during anodization oxidation preparation of TiO2 NTbs. The chemical compositions analyzed by XPS were shown in Table 1. The results show that the loading amount of cadmium (0.29 at% for 10 mM and 10.17 at% for 800 mM, respectively) increased as the concentration of Cd(NO3)2 was increased. Two well-resolved peaks at 404.9 eV and 411.7 eV were observed from the
Fig. 9. Mott-Schottky plots for pure TiO2 NTbs and CdO-TiO2 NTbs prepared at a Cd(NO3)2·4H2O concentration of 50 mM. 141
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∼0.45 mA/cm2. Further increasing the Cd(NO3)2 concentration (70–200 mM) resulted in a lower photocurrent response. This phenomenon can be ascribed to the distortion of the TiO2 crystal lattice, which induced a decrease in the charge carrier density. As the concentration of Cd(NO3)2 reached 800 mM, the photocurrent response decreased nearly to 0.01 mA/cm2, which was mainly because of the formation of CdTiO3. The band gap energy of CdO-TiO2 NTbs was also considered to be an important factor which affected the photocatalytic performances. The UV–Vis diffuse reflection spectra of pure TiO2 NTbs and the CdO-TiO2 NTbs were shown in Fig. S3 in the supporting information. It can be found that, after a small amount of CdO doping, the band gap energy was narrowed from 3.24 eV (for pure TiO2 NTbs) to ∼3.0 eV for CdO doped TiO2 NTbs, which also indicating an improved light absorption ability for CdO-TiO2 NTbs. Fig. 7 shows Instantaneous photocurrent and instantaneous photocurrent decay after 30 s for TiO2 and CdO-TiO2 NTbs. The CdO-TiO2 NTbs prepared at Cd(NO3)2 concentrations of 10 mM and 30 mM showed higher photocurrent spikes, indicating more electron accumulation on the electrode surface. However, the photocurrent value decreased to about 66% after 30 s. In addition, cathodic transients can be observed in Fig. 6(a), which suggests hole accumulation on the electrode surface. Thus, the photocurrent decay is mostly attributed to charge carriers quickly recombining as well as poor electron transport in TiO2 [26]. The photocurrent spike observed for CdO-TiO2 NTbs prepared at a Cd(NO3)2 concentration of 50 mM increased sharply and decayed to about 53% after 30 s, indicating that accumulating more electrons on the electrode surface may reduce the attenuation rate of the photocurrent. Fig. 8 shows the incident photon to current conversion efficiency (IPCE) for the TiO2 NTbs and CdO-TiO2 NTbs prepared at different Cd (NO3)2 concentrations. The IPCE can be calculated using the following equation: [27]
IPCE =
where e is the electronic charge (1.6 × 10−19 ∁), ∊0 is the permittivity of free space (8.86 × 10−12 F/m ), ∊r is the relative permittivity of TiO2 (taken as 31 for anatase), CSC is the capacitance of the space charge layer, and E is the applied potential. Using the equation shown above and the physical constants for TiO2, the charge carrier densities of pure TiO2 NTbs and CdO-TiO2 NTbs were calculated to be 2.28 × 1017 cm−3 and 1.77 × 1017 cm−3 , respectively. This indicates that the charge carrier density decreased after doping, which is mainly attributed to Cd2+ ions occupying the TiO2 lattice acting as acceptors. Cadmium is provided with two valence electrons which can form covalent bonds with surrounding atoms. Thus, cadmium must capture two electrons from other atoms resulting in the appearance of holes in the lattice. Therefore, the Cd2+ ions will offer double-acceptor levels, resulting in a lower density of charge carriers. 4. Conclusions In this work, we have developed a promising and efficient strategy for preparing CdO-TiO2 NTbs with highly ordered micromorphology. It was found that the obvious formation of particle clusters and nanocrystals on the surface of the TiO2 NTbs can be detected only at a relatively high precursor concentration; meanwhile, a portion of TiO2 was converted into CdTiO3 after doping. The Mott-Schottky plots showed that the flatband potential (Vfb) of CdO-TiO2 NTbs has a negative shift compared with pure TiO2 NTbs, indicating higher energy level electrons. The measured Ti 2p XPS spectra and calculated charge carrier densities (Nd) showed that Cd2+ may enter into the crystal lattice. The amperometric I-t curves indicated that even a small amount of Cd2+ would also lead to more electron accumulation on the electrode surface, while on the other hand, the injection of Cd2+ would similarly cause hole accumulation on the electrode surface, leading to greater attenuation of the photocurrent. The maximum incident photon to charge carrier efficiency (IPCE) value found for CdO-TiO2 NTbs was 10.16%, much higher than that of pure TiO2 NTbs.
1240 × jp (mA/cm2) P (mW/cm2) × λ (nm)
Acknowledgement
where jP is the photocurrent density, P is the incident light power density (15 mW/cm2 , Optimax 365, 38 cm), and λ is the wavelength (365 nm ). The order of IPCE values of all the samples follows the same trend as the photocurrent values, with IPCE values of 5.36%, 5.15%, 5.24%, 10.16%, 7.75%, 5.72% and 0.29%, respectively. Indeed, both I-t curves and IPCE values indicate a significant promotion of the photoresponse of CdO-TiO2 NTbs prepared at a suitable concentration of Cd (NO3)2 compared to the pure TiO2 NTbs. To further explore the synergistic mechanism of CdO-TiO2 TNbs for photocatalytic water splitting, the Mott-Schottky plots of pure TiO2 NTbs and CdO doped TiO2 NTbs samples were conducted in 1 M Na2SO4 solution at a frequency of 1 kHz at room temperature, which were shown in Fig. 9. As can be seen, the Mott-Schottky plots possess a positive slope characteristic of n-type semiconductors. The flatband potential can be determined by extrapolating to C = 0. The flatband potentials (Vfb) of pure TiO2 NTbs and CdO-TiO2 NTbs prepared with 50 mM cadmium nitrate tetrahydrate can be deduced from Fig. 9b. The calculated Vfb for the pure TiO2 NTbs and CdO coated TiO2 NTbs were 0.110 V and 0.085 V vs. Ag/AgCl (3 M KCl), respectively, exhibiting a negative shift of Vfb inducing more electrons to migrate to the electrode surface. Correspondingly, the shifted Vfb leads to a higher quasi-Fermi energy level which indicates higher energy level electrons [28]. Furthermore, a linear relationship is predicted between the applied potential and the inverse square of the capacitance, arising from the space charge layer in the semiconductor. The Mott-Schottky equation, which allows for the determination of charge carrier density (Nd), can be expressed as follows: [29]
This work was supported by the Natural Science Foundation of Heilongjiang Province of China [grant number QC2017005], the Province Postdoctoral Fund [grant number LBH-Z15032], the Heilongjiang Youth Innovative Talents Training Project [grant number UNPYSCT-2018042], the Youth Fund, Cultivating Fund and Scientific Research Foundation of Northeast Petroleum University [grant number NEPUBS201508, 2017PYQZL-06]. We are also grateful for the measurement assistants from Analysis & Testing Center of Northeast Petroleum University. Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apsusc.2019.01.044. References [1] S.U. Khan, M. Alshahry, I.W. Jr, Efficient photochemical water splitting by a chemically modified n-TiO2, Science 34 (2003) 2243–2245. [2] G. Wang, H. Wang, Y. Ling, Y. Tang, X. Yang, R.C. Fitzmorris, C. Wang, J.Z. Zhang, Y. Li, Hydrogen-treated TiO2 nanowire arrays for photoelectrochemical water splitting, Nano Lett. 11 (2011) 3026–3033. [3] A. Wolcott, W.A. Smith, T.R. Kuykendall, Y. Zhao, J.Z. Zhang, Photoelectrochemical water splitting using dense and aligned TiO2 nanorod arrays, Small 5 (2009) 104–111. [4] P. Hartmann, D.K. Lee, B.M. Smarsly, J. Janek, Mesoporous TiO2: comparison of classical sol−gel and nanoparticle based photoelectrodes for the water splitting reaction, ACS Nano 4 (2010) 3147–3154. [5] J. Wang, Z. Lin, Anodic formation of ordered TiO2 nanotube arrays: effects of electrolyte temperature and anodization potential, J. Phys. Chem. C 113 (2012) 4026–4030. [6] Y. Liao, W. Que, P. Zhong, J. Zhang, Y. He, A facile method to crystallize amorphous
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