Ni-doped TiO2 nanotubes photoanode for enhanced photoelectrochemical water splitting

Ni-doped TiO2 nanotubes photoanode for enhanced photoelectrochemical water splitting

Accepted Manuscript Full Length Article Ni-doped TiO2 nanotubes photoanode for enhanced photoelectrochemical water splitting Zhenbiao Dong, Dongyan Di...

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Accepted Manuscript Full Length Article Ni-doped TiO2 nanotubes photoanode for enhanced photoelectrochemical water splitting Zhenbiao Dong, Dongyan Ding, Ting Li, Congqin Ning PII: DOI: Reference:

S0169-4332(18)30691-3 https://doi.org/10.1016/j.apsusc.2018.03.031 APSUSC 38779

To appear in:

Applied Surface Science

Received Date: Revised Date: Accepted Date:

19 December 2017 28 February 2018 5 March 2018

Please cite this article as: Z. Dong, D. Ding, T. Li, C. Ning, Ni-doped TiO2 nanotubes photoanode for enhanced photoelectrochemical water splitting, Applied Surface Science (2018), doi: https://doi.org/10.1016/j.apsusc. 2018.03.031

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Ni-doped TiO2 nanotubes photoanode for enhanced photoelectrochemical water splitting Zhenbiao Donga, Dongyan Dinga,*, Ting Lia, Congqin Ningb a

Institute of Electronic Materials and Technology, School of Materials Science and Engineering,

Shanghai Jiao Tong University, Shanghai 200240, China. b

State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai

Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China.

ABSTRACT Photoelectrochemical (PEC) water splitting hydrogen production provides a promising way for sustainable development. In this work, we prepared Ni-doped TiO2 (Ti–Ni–O) nanotubes through anodizing different Ti-Ni alloys and further annealing them at elevated temperatures, and reported their PEC water splitting performance. It was found that Ni doping could improve light absorption and facilitate separation of photo-excited electron-hole pair. The nanotubes fabricated on Ti-1 wt.% Ni alloy and annealed at 550 ºC exhibited better PEC water splitting performance than those on Ti-10 wt.% Ni alloy. The photoconversion efficiency was 0.67%, which was 3.35 times the photoconversion efficiency of undoped TiO2. It demonstrated that it was feasible to fabricate high-performance Ti–Ni–O nanotubes on Ti-Ni alloys and used as photoanode for improving PEC water splitting.

Keywords: Ti-Ni alloys; Anodization; Ni-doped TiO2 nanotubes; Photoelectrochemical; Water splitting

*

Corresponding Author. Tel: +86 21 34202741.

E-mail address: [email protected] (D. Ding). 1

1. Introduction Photoelectrochemical (PEC) water splitting hydrogen evolution reaction by exploiting solar energy provides an efficient approach to solve the increasing energy crisis [1–3]. Since the pioneers Fujishima and Honda first reported the photo-oxidation of water on TiO 2 [4], TiO2 has become a well-known photocatalyst due to its merits of reliable chemical properties, low-cost and non-toxicity [5, 6]. However, it shows lower quantum efficiency due to wide band gap and easy recombination of photo-generated carriers, which significantly restricts its widespread applications [7, 8]. Therefore, various methods such as nonmetal and metal doping [9–12], semiconductor coupling [13–16] have been used in order to address these main obstacles. Due to large specific surface areas and good electron transport ability, TiO 2 nanotubes have been widely used in the fields of photocatalysis, biomedical devices and dye-sensitized solar cells [17–19]. Recently, Momeni et al. fabricated high visible light response Cu-doped, Mn-doped and Cr-sensitized TiO2 nanotube arrays [11, 12, 20]. Schmuki et al. prepared Nb-doped TiO2 nanotubes through anodizing different Ti-Nb alloys and reported the relevant PEC water splitting performance [21]. Grimes et al. synthesized Ti–Fe–O nanotubes and demonstrated that the special material architecture showed favorable hydrogen generation properties [22]. Roy et al. fabricated Ru-doped TiO2 nanotubes on Ti-Ru alloys and found a strong and stable enhancement of PEC water splitting activity [23]. In addition, Ni element was also proved to be an ideal dopant for improving photocatalytic activity and Lin et al. demonstrated that Ni element doping could promote light absorption by introducing Ni 3d states in the band gap [24–26]. To our knowledge, rare works reported the effects of Ni-doping and annealing temperature on PEC water splitting properties of Ni-doped TiO2 nanotubes prepared on different Ni-content Ti alloys. Herein, we fabricated Ni-doped TiO2 nanotubes photoanodes by anodizing Ti alloys with different Ni content and further annealing treatment. The influences of Ni-doping and annealing temperature on the PEC water splitting performances were investigated through microstructural characterization and PEC measurement. The doped nanotubes fabricated on Ti-1 wt.% Ni alloy and annealed at 550 °C were found to have a much higher PEC water splitting behavior compared with undoped TiO2.

2. Experimental section 2

2.1. Fabrication of Ni-doped nanotubes photoanode Ti-Ni alloys with nominal Ni contents of 1 wt.% Ni and 10 wt.% were cast by vacuum arc melting process. After melting, the ingots were cut into plate samples with a desired dimension of 20 mm × 10 mm × 1 mm. According to our X-ray diffraction (XRD) analysis, Ti2Ni phase existed in the Ti-Ni alloys. Plate samples of different Ni-content Ti alloys as well as pure Ti (for reference) were mechanically polished by emery abrasive papers and ultrasonically cleaned with absolute alcohol. The alloy plate working electrode and graphite plate counter electrode constituted a two-electrode system for anodization. The electrolyte was based on ethylene glycol solution, which was composed of 0.5 wt.% NH4F and 3 vol.% H2O. All the samples were prepared at 40 V for 20 min at room temperature. Finally, the samples had a 2 h heat-treatment at elevated temperatures. For the sake of marking the different samples, Ti1NiO and Ti10NiO were used to label the nanotube samples for the Ti-1 wt.% Ni and Ti-10 wt.% Ni system, respectively. For reference, undoped TiO2 nanotubes prepared on Ti-0 wt.% Ni (pure Ti) was also fabricated and labeled as Ti0NiO. 2.2. Materials characterization Surface and cross-sectional morphology observation was characterized by field-emission scanning electron microscope (SEM, FEI Sirion 200). Element chemical composition was analyzed by energy dispersive spectrometer (EDS, OXFORD INCA). Typical single Ti1NiO nanotube and its energy dispersive X-ray (EDX) mapping images were also observed under transmission electron microscope (TEM, JEM-2001F, JEOL) to confirm the presence of Ni element in the Ni-doped system. X-ray diffractometer (XRD, Rigaku Ultima IV) with scan speed of 5°/min was used to identify the crystal phase structure. Raman microscope (Bruker Optics Senterra) analysis of the oxide film was also performed with a laser wavelength of 532 nm. The surface elements bonding states and valence band edge positions were investigated through X-ray photoelectron spectroscopy (XPS, Kratos Axis Ultra DLD). UV–visible (UV–vis, Perkin Elmer Inc., Lambda 750S) measurement was conducted to evaluate the optical absorption property. Photoluminescence (PL, Perkin Elmer Inc., LS 55) measurement was conducted with a 320 nm laser excitation wavelength to understand the behavior of trap and migration of photo-excited electron-hole pairs. 2.3. PEC measurement

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PEC water splitting performances of the different photoanodes were measured in 1.0 M KOH solution based on electrochemical station (CHI Instruments, model CHI660C). The three electrode system consisted of nanotubes photoanode, platinum foil and Ag/AgCl. A 150 W Xenon lamp provides simulate light, the light intensity was adjusted to 100 mW/cm2. Electrochemical impedance spectra were obtained at open circuit potential vs. Ag/AgCl under illumination. Mott–Schottky plots measurement was conducted in the dark at a fixed frequency of 1000 Hz.

3. Results and discussion Figs. 1a-c presents the morphology of different doped nanotubes annealed at 550 ºC. Apparently, the doped nanotubes were prepared by anodizing the different Ti-Ni alloys. As shown in Fig. 1a, the average diameter was 59.5 nm and the average length was 2.1 μm for undoped TiO2. For the Ti1NiO nanotubes on Ti-1 wt.% Ni alloy(Fig. 1b ), the average diameter was 58.8 nm and the average nanotube length was 2.8 μm. When anodizing Ti-10 wt.% Ni alloy, the average diameter and nanotube length of the Ti10NiO decreased to 47.9 nm and 2.2 μm, respectively (Fig. 1c). This might be caused by the different roughness of Ti-Ni alloys and the different electronegativity between Ti and Ni during electrochemical anodization [27–30]. EDS analyze results were shown in Table. 1, the average Ni content was 0.3 wt.% for the Ti1NiO nanotubes and 2.1 wt.% for the Ti10NiO nanotubes. Typical single Ti1NiO nanotube (Fig. 1d) and its corresponding EDX mapping images (Figs. 1e-g) were also observed, which further revealed that the Ni element existed in Ti1NiO nanotubes. XRD measurement results of the doped nanotubes processed at 550 °C are shown in Fig. 2a. For the undoped TiO2 (Ti0NiO), the diffraction angle (2θ) centered at 25.48º, 37.02º and 48.18º corresponded to the anatase TiO2 diffraction peaks (JCPDS No. 21-1272) [31]. No Ni-related impurity oxides phase were found in the Ti–Ni–O nanotube samples. And the diffraction peaks of Ni-doped TiO2 samples shifted to lower diffraction angles slightly compared to undoped TiO 2, which might be resulted from Ni doping since the radius of Ni2+ was larger than that of Ti4+ [24, 25]. Compared to undoped TiO2 (Ti0NiO), the rutile peaks centered at 27.17º became stronger with increase of Ni doping content and the diffraction angles centered at 35.82º, 41.14º and 45.13º were indexed as rutile TiO2 (JCPDS No. 21-1276) [32]. This indicated, at the same annealing temperature of 550 °C, Ni-doping could facilitate the anatase to rutile phase transformation. Fig. 2b shows XRD patterns of the Ti1NiO nanotubes annealed at various annealing 4

temperatures. When the Ti1NiO was annealed at 500 °C, it mainly contained anatase phase. The rutile phase peaks appeared when it was annealed at 550 °C. With further increase of annealing temperature, the rutile peaks became more obvious, suggesting more rutile phase could form. Phase structures of the Ti1NiO annealed at various temperatures were also analyzed with Raman spectra. As shown in Fig. 3a, anatase phase Raman peaks at around 144 (E g), 197 (Eg), 393 (B1g), 514 (A1g) and 635 (Eg) cm-1 were found when the samples were annealed at 500 °C [33]. The rutile phase peaks at around 235 (E g), 443 (Eg), 610 (A1g) cm-1 begun to appear when the samples were annealed at 550 °C [34]. The rutile modes became stronger with increase the annealing temperature from 600 °C to 700 °C, indicating more rutile phase could form. The phase structures of the different doped nanotubes annealed at 550 °C were further analyzed by Raman spectra. As shown in Fig. 3b, the lowest mode at around 143 (E g) cm-1 of the Ti0NiO exhibited blue shift (inset of Fig. 3b) after Ni doping. And new rutile TiO 2 Raman peaks were detected at around 443 (Eg) cm-1 and 445 (Eg) cm-1 for Ti1NiO and Ti10NiO, respectively. Obviously, the microstructure gradually changed and Ni-doping was beneficial to the anatase to rutile phase transformation. These results were in agreement with our XRD analysis. XPS was employed to examine surface elements chemical bonding states of the doped nanotubes. Main elements of Ni, Ti, O and C existed in Ti–Ni–O nanotubes according to the full spectra (Fig. 4a). C 1s peak located at 284.5 eV was used to calibrate other elements. Ti 2p spectra were shown in Fig. 4b. Two obvious peaks around 458.7 eV and 464.4 eV were detected, which corresponded to typical peaks of Ti4+ [35, 36]. Fig. 4c shows XPS spectra of O 1s and the binding energies located at around 529.5 eV confirmed the Ti-O linkage [37]. The Ni 2p spectra are shown in Fig. 4d. Weaker Ni 2p spectrum was found for Ti1NiO, which mainly due to less Ni doping, as analyzed by EDS. For Ti10NiO, two most intense Ni 2p3/2 and Ni 2p1/2 peaks located at around 855.2 eV and 873.0 eV are characteristic peaks of Ni2+ ion in an oxygen environment. The peaks are accompanied by two broader shoulder peaks due to shake-up processes, and these peaks are the fingerprint of Ni 2+ species. Combined with comprehensive analyses of the element mappings, XRD patterns and Raman spectra, it revealed that no nickel oxide was formed and Ni was doped in the TiO2 nanotubes [38–40]. Fig. 4e shows valence band spectra of the different samples. The valence band edge was located at 2.73 eV for undoped TiO2. While for Ni-doped TiO2, the valence band edge position of the Ti1NiO and Ti10NiO gradually decreased to 2.56 eV and 1.75 eV, respectively. These results further suggested 5

efficient Ni element doping, as the hybridization of metal (Ti and Ni) 3d states with O 2p should be responsible for the shift of valence band [26]. Fig. 5a shows UV–vis absorption spectra of the different doped nanotubes. The optical absorption intensity of undoped TiO 2 nanotubes was improved after Ni-doping. The undoped TiO2 (Ti0NiO) was used as a reference and the absorption edge showed a red-shift with increase of Ni-doping. Thus, corresponding band gaps of Ti1NiO and Ti10NiO would be narrowed based on the Kubelka–Munk function [41]. According to Lin's work, new impurity states below the conduction band formed due to hybridization of metal (Ti and Ni) 3d and O 2p orbits. The electrons in the valence band could be first excited to the impurity levels by absorbing lower energy photons, and subsequently transitioned to the conduction band. This decreased the effective photo-excited energy and made a red-shift of absorption edge [26]. PL spectra measurement was an important method to investigate the migration and trapping behavior of photo-excited electrons and holes [42]. As shown in Fig. 5b, the samples including undoped TiO2 showed an obvious emission peak at about 420 nm. According to previous reports [43, 44],this signal might be ascribed to free excitons luminescence and caused by surface defects of the semiconductor nanotubes. Apparently, the overall PL emission intensity of Ni-doped TiO2 samples decreased in comparison with that of undoped TiO 2 sample (Fig. 5b), which could attributed to the longer electron lifetime induced by Ni-doping. It is worthy to note that the PL intensity of Ti1NiO was substantially lower than that of Ti10NiO, indicating that lower Ni-doping facilitated the separation of photogenerated electrons and holes while higher Ni-doping may form more recombination centers of the charge carriers [45]. Linear sweep voltammetry (LSV) and amperometric photocurrent density versus time (I-t) curves were employed to study the PEC water splitting behaviors of the different doped nanotubes photoanodes. It scanned from -1 to 0.6 V for the LSV curves and the I-t response were measured at 0 V versus the reference electrode (Ag/AgCl). Both of them were recorded under dark or light illumination conditions. All of the nanotubes were controlled at ca. 2.2 um by adjusting the anodizing time. Figs. 6a and b show the transient LSV curves and I-t responses of different Ti–Ni–O nanotubes photoanodes fabricated at 550 °C, respectively. Apparently, at the same condition of 0 V vs. Ag/AgCl, the Ti–Ni–O nanotubes photoanodes exhibited favorable photocurrent response and showed higher photocurrent density than that of undoped TiO2 (0.40 mA/cm2). It was 0.85 mA/cm2 for the Ti1NiO 6

and 0.60 mA/cm2 for the Ti10NiO. This suggested that low-Ni doping could act as electron trap and inhibit recombination of electron-hole during irradiation, thereby increased the lifetime of charge carriers. Nevertheless, high-Ni doping may induce defects that acted as recombination centers, which caused the PEC activity decreased [46]. Figs. 6c and d shows transient LSV curves and I-t response of the Ti1NiO nanotubes annealed at various temperatures. The photocurrent density could reach 0.93 mA/cm2 when the Ti1NiO annealed at 550 °C. With decrease or increase of the annealing temperature of Ti1NiO, it did not show any more beneficial effect. According to the XRD and Raman analyses, the Ti1NiO annealed at 500 °C was mainly composed of anatase TiO2. Anatase and rutile mixed phases coexisted when the Ti1NiO nanotubes were annealed at 550 °C. The photo-excited carriers could separate and transport effectively to some extent in such a mixed crystalline phase [8]. More rutile phase could form with increase of the annealing temperatures above 550 °C, which was not favorable for enhancing PEC water splitting performance [28]. Photoeletrode stability as a function of irradiation time is also important for durable PEC water splitting [47]. Fig. 7a shows the I-t response of Ti1NiO recorded for about 3 h continuous irradiations. It can be seen that there was no significant decay of the photocurrent density during long-term illumination, suggesting that the Ti1NiO nanotubes photoanode exhibited good PEC stability for water splitting. The solar-to-hydrogen efficiency (ƞ) can be obtained according to Eq. (1) and (2) [48]: ƞ = I (1.23–VRHE) /Jlight VRHE=VAg/AgCl+0.059PH+0.199

(1) (2)

Where Jlight and I is the irradiation intentsity (100 mW/cm2) and corresponding photocurrent density. VAg/AgCl and VRHE is the applied bias and relative hydrogen potential, respectively. PH value is 13.6 for the KOH electrolyte. Compared to the calculated maximum photoconversion efficiency of undoped TiO2 (0.20%) (Fig. 7b), photoconversion efficiency of the Ti1NiO reached 0.67%. This was almost 3.35 times that of the undoped TiO 2. Electrochemical impedance spectra were employed to further understand the modification of TiO2 electronic properties through Ni-doping. Nyquist plots of different TiO2 nanotubes photoanodes fabricated at 550 C are shown in Fig. 8a. According to previous reports, smaller arc radius in the Nyquist plots corresponded to a lower charge transfer resistance (R ct) [43, 49, 50]. The Rct is related to 7

the water oxidation reaction occurring at the photoanode surface, i.e., the lower R ct means more holes participated in water oxidation reaction. Thus, the better separation efficiency was acquired due to lower density of holes available for electrons recombination. We can clearly see that Ti1NiO exhibited smaller arc radius in comparison with undoped TiO2 and Ti10NiO. Therefore, we concluded that the Ti1NiO nanotubes annealed at 550 C presented better electron transfer and separation efficiency across the interface of Ti1NiO nanotubes photoanode and electrolyte solution [34]. Fig. 8b shows Mott–Schottky (MS) plots of the doped nanotubes samples. As shown in Fig. 8b, the slopes of the linear part of MS plots were positive, which indicated that n-type semiconducting nature of TiO2 was not changed after Ni-doping [51]. The flat band potential (VFB) was –0.50 V for undoped TiO2, while the VFB for Ti1NiO and Ti10NiO were –0.76 V and –0.67 V, respectively. More negative flat band potential of Ti1NiO nanotubes photoanodes suggested more efficient charge separation and transport compared with undoped TiO 2 and Ti10NiO [52, 53]. Therefore, the Ti1NiO nanotubes photoanode could demonstrate higher PEC water splitting performance. Furthermore, the Ti1NiO had a smaller slope of linear part in the MS plots than that of undoped TiO2 and Ti10NiO, indicating much higher donor density in the Ti1NiO [54]. Higher donor density could raise the Fermi level near to the conductive band, which facilitated electron transfer to the current collector [37]. The result of lower donor density of Ti10NiO compared with the Ti1NiO photoanode also support that doping more Ni element may induce recombination centers of photo-excited carriers and thus caused the photocurrent density decreased. As a result, the Ti1NiO nanotubes annealed at 550 C could be used as a desirable photoanode for enhancing PEC water splitting. According to above experimental results and analyses, possible mechanism diagram of Ti-Ni-O nanotubes photoanode for PEC water splitting was proposed and illustrated in Fig. 9. Ni element doping improved the optical absorption property of undoped TiO2, as UV-vis analyzed. Meanwhile, impurity Ni 3d states were introduced into the forbidden band and located near the CB [26]. Therefore, the photo-induced electrons in VB can be first excited to the impurity states and subsequently to CB. Ni 3d states could act as trapping sites for charge carries and decrease the recombination rate. Once illuminated, the photo-excited electrons would be quickly migrated to the surface of 1D Ni-doped TiO2 nanotubes and participated in oxygen evolution reaction ( H 2O  2H+ + 1/ 2O2 ). Meanwhile,

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the photo-excited electrons were efficiently transferred to cathode and participated in hydrogen evolution reaction ( 2 H   2e  H 2 ). As a result, the Ni-doped TiO2 nanotubes resulted in enhanced PEC water splitting properties than that of undoped TiO2.

4. Conclusions In summary, we successfully prepared Ti–Ni–O nanotubes through anodizing different Ti-Ni alloys and investigated the influences of Ni-doping and annealing temperature on PEC water splitting properties. It was found that moderate Ni-doping resulted in enhanced PEC water splitting properties of undoped TiO2, since it could improve light absorption and facilitate separation of photo-generated electron-hole pair. The nanotubes fabricated on Ti-1 wt.% Ni alloy and annealed at 550 °C presented higher PEC water splitting performance. A higher photocurrent density of 0.93 mA/cm2 at 0 V vs. Ag/AgCl was achieved for the Ti1NiO nanotubes photoanode. And the maximum photoconversion efficiency was 0.67%, which was 3.35 times the photoconversion efficiency of undoped TiO 2. We expected that these findings could contribute to develop high-performance TiO2-based nano-photoanodes for enhancing PEC water splitting.

Acknowledgements This work was supported by National Natural Science Foundation of China (No. 51572170). We thank the help from SEM, Raman and XPS labs at Instrumental Analysis Center of SJTU.

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Figure captions Fig. 1. Surface and cross-section view (insets) morphology of (a) Ti0NiO. (b) Ti1NiO. (c) Ti10NiO. (d) Typical TEM image of single Ti1NiO nanotube and corresponding EDX mapping image of (e) Ti, (f) Ni and (g) O.

Fig. 2. XRD patterns of (a) the different Ti–Ni–O nanotubes processed at 550 °C and (b) the Ti1NiO samples fabricated at various annealing temperatures.

Fig. 3. Raman spectra of (a) the Ti1NiO nanotubes annealed at various temperatures and (b) different Ti–Ni–O nanotubes processed at 550 °C.

Fig. 4. XPS results of the different doped nanotubes. (a) Full spectra, (b) Ti 2p, (c) O 1s, (d) Ni 3d and (e) valence band.

Fig. 5. (a) UV–vis absorption spectra and (b) PL spectra of the different Ti–Ni–O nanotubes.

Fig. 6. (a) Transient LSV curves and (b) I-t responses of different nanotubes photoanodes processed at 550 °C. (c) Transient LSV curves and (d) I-t responses of Ti1NiO nanotubes photoanodes fabricated at different temperatures.

Fig. 7. (a) PEC stability of the Ti1NiO nanotubes photoanode. (b) Photoconversion efficiency of the different TiO2 nanotubes phohtoanodes.

Fig. 8. (a) Nyquist plots and (b) MS plots of the different doped nanotubes phohtoanodes.

Fig. 9. Mechanism diagram of the Ti-Ni-O nanotubes photoanode for PEC water splitting.

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Table Table 1. Elemental composition of the different doped nanotubes annealed at 550 ºC. Sample

Ti(wt.% / at.%)

Ni(wt.% / at.%)

O(wt.% / at.%)

Ti0NiO

47.0 / 22.9

0/0

53.0 / 77.1

Ti1NiO

50.2 / 25.2

0.3 / 0.1

49.5 / 74.7

Ti10NiO

37.1 / 16.8

2.1 / 0.8

60.8 / 82.4

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Highlights in this paper [1] Ni-doped TiO2 nanotubes were fabricated by anodizing different Ti-Ni alloys. [2] Ni doping improved the light absorption property of undoped TiO2. [3] Moderate Ni-doping facilitated the separation of photogenerated electron-hole pairs. [4] Moderate Ni-doping was found to be an important design for enhancing PEC property. [5] Ti1NiO nanotubes annealed at 550 ºC showed higher PEC water splitting performance.

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