Ti3+ Self-Doped Black TiO2 Nanotubes with Mesoporous Nanosheet

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Ti3+ Self-doped Black TiO2 Nanotubes with Mesoporous Nanosheet Architecture as Efficient Solar-Driven Hydrogen Evolution Photocatalysts Xiangcheng Zhang, Weiyao Hu, Kaifu Zhang, Jianan Wang, Bojing Sun, Haoze Li, Panzhe Qiao, Lei Wang, and Wei Zhou ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b01114 • Publication Date (Web): 30 Jun 2017 Downloaded from http://pubs.acs.org on July 1, 2017

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ACS Sustainable Chemistry & Engineering

Ti3+ Self-doped Black TiO2 Nanotubes with Mesoporous Nanosheet Architecture as Efficient Solar-Driven Hydrogen Evolution Photocatalysts Xiangcheng Zhang, Weiyao Hu, Kaifu Zhang, Jianan Wang, Bojing Sun, Haoze Li, Panzhe Qiao, Lei Wang, Wei Zhou* Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education of the People’s Republic of China, Heilongjiang University, Harbin 150080, P. R. China. E-mail: [email protected]

KEYWORDS: Mesoporous TiO2, Ti3+ self-doping, Nanotube, Photocatalysis, Solar-driven hydrogen evolution

ABSTRACT: Ti3+ self-doped black TiO2 nanotubes (TDBTNs) with mesoporous nanosheet architecture have been successfully synthesized by solvothermal method combined with ethylenediamine encircling strategy to protect mesoporous frameworks, then calcined at 600 °C under hydrogen atmosphere. In this case, ethylenediamine molecules play important roles on maintaining the mesoporous networks and inhibiting the phase transformation from anatasetorutile effectively. The as-prepared TDBTNs with mesoporous nanosheet architecture possess a relatively high specific surface area of ~ 95 m2 g-1 and an average pore size of ~ 15.6 nm. The reduced bandgap of ~ 2.87 eV extends the photoresponse from ultroviolet to visible light region due to the Ti3+ self-doping. The solar-driven photocatalytic hydrogen evolution rate for TDBTNs

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is approximately 3.95 mmol h-1 g-1, which is much better (about four times) than that of the pristine one (~ 0.94 mmol h-1 g-1). This improvement is attributed to the reduced bandgap increasing the utilization ratio of solar energy, the formed Ti3+ enhancing separation efficiency of photogenerated charge carriers, and the special one-dimensional mesoporous architecture offering more surface active sites.

■ INTRODUCTION Environmental pollution and energy crisis are the world's two major problems of plaguing humanity. As a result of the exhausted fossil fuel, series of environmental pollution problems caused by fossil fuel combustion concerns much attention and explores new alternative clean energy such as hydrogen energy is urgent. In recent years, photocatalytic hydrogen generation has become a promising prospect to realize energy conversion from solar energy to hydrogen energy.1-8 Compared with other semiconductor photocatalytic materials, TiO2 with advantages of biological and chemical inert, strong oxidizing power, non-toxic and less prone to light and chemical corrosion, has been the most widely studied photocatalyst.9-16 However, the photocatalytic efficiency of TiO2 is limited by the rapid recombination rate of photogenerated electrons and holes. Moreover, the anatase TiO2 with large bandgap (~ 3.2 eV) can only be excited by UV light, which limits the utilization ratio of solar energy obviously. Therefore, narrowing the bandgap of anatase TiO2 to extend the photoresponse to visible light region is vital for improving the solar-driven photocatalytic hydrogen evolution.17-19 As is reported by Mao and co-workers, surface hydrogenation has proved to be a new strategy to enhance light absorption by introducing disordered surface layers of the highly crystallized TiO2 nanoparticles to obtain black TiO2 materials.20 It opens up a new era for tuning the bandgap


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of TiO2.21-30 Numbers of approaches have been adopted for synthesizing black TiO2 materials, but not many have been used to prepare one-dimensional (1D) nanotube structures. Especially, the 1D nanotube with mesoporous architecture will possess large surface area and expose adequate surface active sites, which should be good candidate for further improving the photocatalytic performance. However, the mesoporous frameworks can be collapsed easily due to the poor thermal stability during the high-temperature hydrogen treatment process, so how to effectively stablize TiO2 nanotubes with mesoporous frameworks becomes a great challenge. Adopting ethylenediamine molecules to protect skeleton structure is confirmed to be an effective and feasible strategy to satisfy with the above requirements, just like our previous work.21 After surface hydrogenation, the absorption of black TiO2 materials can extend to visible light region obviously.31 Here, we report one-step hydrothermal method combined with ethylenediamine encircling strategy and surface hydrogenation to synthesize Ti3+ self-doped black TiO2 nanotubes (TDBTNs), which have mesoporous nanosheet architectures. Ethylenediamine molecules play important roles on maintaining the mesoporous networks and inhibiting the phase transformation from anatase-to-rutile. The TDBTNs are explicitly investigated by thoroughly structural and spectral characterization. The solar-driven photocatalytic hydrogen rate for TDBTNs is approximately 3.95 mmol h-1 g-1, which is about four times as high as the pristine one (~ 0.94 mmol h-1 g-1). ■ EXPERIMENTAL SECTION Synthesis. Titanium oxide sulphate (TiOSO4) was purchased from Sigma Aldrich. Ethanol (EtOH), ethylenediamine, diethyl ether and glycerol were purchased from TianJin Kermel


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limited company. All chemicals were of analytical regent and used directly without any further purification. In a typical experimental method, black TiO2 nanotubes were synthesized by two step approach described in detail as is shown in Scheme 1. Briefly, at first step, 1 g of TiOSO4 and 18 mL of EtOH were added to weighing bottle (40 × 70 mm) by fierce stirring for 30 min. Then, 9 mL of glycerol and 9 mL of diethyl ether were added to the mixed liquor, followed by fierce stirring overnight. Finally, the solution was transferred into a 50 mL Teflon lined stainless steel autoclave, which was heated to 170 °C for 10 h. The obtained products were filtered, washed with distilled water and ethanol 5 times, and dried at 60 °C for 3 h. Then, the products were refluxed with ethanediamine aqueous solution for 48 h. The pH value was kept at ~ 11. The obtained powders were washed by distilled water three times and dried at 100 °C overnight. Subsequently, the samples were calcined at 600 °C for 4 h in air to obtain the stable TiO2 nanotubes (denoted as TNs). The second step, the as prepared samples were calcined at 600 °C for 2 h under hydrogen atmosphere to obtain Ti3+ self-doped black TiO2 nanotubes (denoted as TDBTNs). Characterization. The prepared samples were analysed by X-ray diffrraction patterns (Bruker D8 Advance diffractometer) with monochromatized Cu Kα radiation (λ = 0.15418 nm). Raman spectra were presented by Jobin Yvon HR 800 micro-Raman spectrometer at 457.9 nm. Nitrogen adsorption-desorption isotherms at 77 K were collected on ASAP2420 (Micrometrics Instruments) nitrogen adsorption apparatus. The Brunauer–Emmett–Teller (BET) equation was used to calculate the specific surface area. Pore size distributions were obtained using the Barrett–Joyner–Halenda (BJH) method from the adsorption branch of the isotherm. Scanning electron microscopy (SEM) images were received with a Philips XL-30-ESEM-FEG instrument operating at 20 kV. Transmission electron microscopy (TEM) micrographs were obtained by a


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JEOL JEM-2100F microscope (Japan) operated at 200 kV. The X-ray photoelectron spectroscopy (XPS) analysis was measured on a ULTRAAXIS DLD with an Al Ka (1253.6 eV) achromatic X-ray source by referencing to the C 1s peak at 284.6 eV. The diffuse reflectance spectroscopy (DRS) was obtained on a UV-vis spectrophotometer (Shimadzu UV-2550) in the range of 200-800 nm. The surface photovoltage spectroscopy (SPS) measurements were taken using a home-built apparatus described elsewhere.16 The photoluminescence (PL) spectra were measured by a PE LS 55 spectrofluoro-photometer with excitation wavelength of 332 nm. Scanning Kelvin Probe (SKP) measurements (SKP5050 system, Scotland) were tested under normal laboratory conditions (in ambient atmosphere). The electrochemical impedance spectroscopy (EIS) and the Mott–Schottky plots were analysed by an electrochemical workstation (Princeton VersaSTAT). Using high-temperature pyrolysis (350 °C for 2 h under N2 atmosphere) to make films on FTO (Fluorine doped SnO2 layer, 20 Ω per square, Nippon sheet glass, Japan) as a working electrode, Pt foil as the counter electrode, Ag/AgCl as a reference electrode, and 1 M KOH was employed as electrolyte. In photoelectrochemical tests, the coating area and illumination area were both ~ 1.5 cm2. Photocatalytic hydrogen evolution. The photocatalytic hydrogen evolution experiments were conducted in an online photocatalytic hydrogen generation system (AuLight, Beijing, CELSPH2N) at room temperature (20 °C). The photocatalysts (50 mg) were scattered uniformly to ultrapure water (80 mL) and methanol (20 mL), then loaded with H2PtCl6·6H2O (0.5 wt%) as cocatalyst. Before the reaction, the reactor and the entire gas circulating system were de-aerated using a vacuum pump for 30 min. The reaction was happened by irradiating the mixture with AM 1.5 light from a 300 W Xe lamp with a full spectrum reflection filter. The UV-vis spectrum of solar simulator equipped with an AM 1.5G filter was shown in Figure S1. H2 evolution was


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observed only under photoirradiation with a power density of 100 mW cm-2. The photocatalytic hydrogen generation was analyzed using a gas chromatograph (SP7800, TCD, molecular sieves 5 Å, N2 carrier, Beijing Keruida Limited). ■ RESULTS AND DISCUSSION As show in Figure 1A, after surface hydrogenation, the XRD patterns of both TDBTNs and TNs show five obvious crystal peaks at 2θ = 25.2–55.2°, which could be indexed as the (101), (004), (200), (105), and (211) for the anatase phase (JCPDS no. 21-1272), but after hydrogenation the intensity for the former decreases and broadens obviously, indicating that the crystalline structure of TDBTNs has some variation.32-33 Raman spectra is a powerful technique, which is also used to investigate the structural changes. As shown in Figure 1B, five peaks at 141, 193, 387, 505 and 626 cm-1 could be belonged to typical anatase Raman bands of Eg, Eg, B1g, A1g(B1g), and Eg modes, respectively.34-35 We can clearly see that, after hydrogen gas annealing, the peaks of TDBTNs have a slight red-shift, which is due to the original symmetry of the TiO2 lattice distorted and should be one reason for the narrowed bandgap.36 UV-vis reflectance spectra are used to study the absorbance and bandgap. As shown in Figure 2A, the TNs can only absorb UV light, but after surface hydrogenation, a strong absorption between 400 ~ 800 nm has been observed as shown in Figure 2A. This strong absorption may be ascribed to the surface disorders and the existence of Ti3+ defects, which induce a continuous vacancy band of electronic states just below the conduction band edge of TDBTNs.37 From the photo in the inset of Figure 2 we can clearly see that the color of Ti3+ self-doped TiO2 is black, but the unhydrogenation one is white, which confirms the efficient surface hydrogenation for TDBTNs. The optical gaps demonstrate that the bandgaps of TDBTNs (~ 2.87 eV) and TNs (~


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3.2 eV) are different obviously, from Figure 2B. After surface hydrogenation, the narrowed bandgap for TDBTNs is due to the raised vacancy band of electronic states. This surface modification strategy for anatase TiO2 materials could indeed extend the photoresponse to visible light region and increase the utilization ratio of solar energy obviously.38 Figure 3 shows the N2 adsorption-desorption isotherms (A) and the corresponding pore size distribution plots (B) of TDBTNs (a) and TNs (b). The curves both show typical type IV hysteresis loop, which indicate that they belong to mesoporous material.39-40 The specific surface areas are calculated by Brunauer−Emmett−Teller method, which are ~ 95 m2 g-1 for TDBTNs and ~ 71 m2 g-1 for TNs, respectively. The main pore size for TDBTNs and TNs are ~ 15.6 and 14.5 nm respectively. After surface hydrogenation, the increased surface area for TDBTNs is due to the formed surface disorders and the pore size has little changes, which may be ascribed to the efficient ethylenediamine encircling strategy. Ethylenediamine molecules are strong alkali with two primary amine groups (two positive charges), which could interact with TiO2 nanotubes strongly and bind on the surface of TiO2 nanotubes, which could maintain TiO2 nanotubes framework against collapsing, inhibit undesirable grain growth, prolong the improvement of crystallization for anatase and retard the phase transformation of anatase-to-rutile.21 The mesoporous frameworks are not destroyed and collapsed during high temperature surface hydrogenation process due to the effective protection, which is in good agreement with our previous results.21 Obviously, without introducing ethylenediamine, the nanotube structures are collapsed and rutile phase is present (Figure S2). The results further confirming the efficient protection effect of ethylenediamine on maintaining the mesoporous networks and inhibiting the phase transformation from anatase-to-rutile. Large surface area and the integrated mesoporous


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networks can supply more surface activity sites, which are significant for improving the photocatalytic performance.41 The X-ray photoelectron spectroscopy is a useful tool to investigate the surface change of Ti 2p and O 1s regions. Figure 4A shows survey spectrum of TDBTNs, we can clearly see that there exists Ti 3p, Ti 3s, and the O 1s peaks, confirming the pure TiO2 material. The Ti 2p spectra for TDBTNs is also found Ti3+ peaks located at 457.9 eV and 463.5 eV that consistent to the characteristic of Ti 2p3/2 and Ti 2p1/2 peaks, as shown in Figure 4B, which is different from the pristine one (Figure S3). These peaks transfer to lower binding energy, indicating achieving a different bonding environment and the presence of Ti3+ in TDBTNs.35 The TDBTNs also exhibit a broader O 1s peak at 529.8 eV with an extra shoulder at 531.8 eV compared to the pristine TiO2 nanotubes (Figure S3). This broad peak is divided into two peaks (Figure 4C), the peak at 531.8 eV which can be attributed to Ti-OH, is higher than that of the pristine TiO2.42 It confirms the fact that the existence of more oxygen vacancies on TiO2 surface after surface hydrogenation. According to literatures,43-44 the oxygen vacancies for TDBTNs and TNs are ~ 9.09 and ~ 2.2%, respectively (Table S1). Moreover, the N species are negligible from Figure S4, confirming not resulting in N-dopant into the nanostructure. The valence band spectra of the pristine TiO2 nanotubes and TDBTNs are tested to analyse the effect of the electronic band structure after surface hydrogenation. From Figure 4D of the valence band (VB) XPS, we can find that the valence band maximum (VBM) for TDBTNs is located at ~ 1.94 eV, which is lower than that of TNs (~ 3.13 eV). The results suggest that hydrogenation increases the VBM significantly, which narrows the bandgap obviously. The morphology and structure of the obtained samples are further confirmed by SEM and TEM observations. Figure 5 (a, b) shows the SEM images of TDBTNs, we can no hard to find


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lots of nanotubes and a large amount of nanosheets grown on the surface of nanotubes. The nanotubes with several micrometers long and width of ~ 500 nm can be observed clearly. The sizes of the nanotubes are not symmetry but possess sensational dispersibility, this particular structure not only possesses a large specific surface area but also provides adequate surface active sites. The TEM images (Figure 5c, d) further demonstrate that the as-prepared samples are hollow section and on the surface are overgrown with mesoporous nanosheets. The mesopores are formed by nanoparticles aggregation due to the Ostwald ripening.24 It also offers many surface active sites, which could further improve the photocatalytic hydrogen performance and should be good candidate in fields of photocatalysis. In addition, the morphology and structure are nearly constant before and after surface hydrogenation (Figure S5), indicating the high thermal stability of the obtained TNs. HRTEM (Figure 5e) shows that the TDBTNs still remain anatase phase. The lattice fringes correspond to (101) (d101= 0.35 nm) of TiO2, which indicate that the pore walls are highly crystalline. After surface hydrogenation, we can clearly see a thin disordered layer covering on surface of TiO2 nanoparticles. The high crystallinity core can also be observed, which is demonstrated by the selected-area electron diffraction pattern (Figure 5f). The photocatalytic activity is evaluated by photocatalytic hydrogen generation. Figure 6A shows a much higher photocatalytic hydrogen evolution rate of 3.95 mmol h-1 g-1 than that of the pristine TiO2 nanotubes (0.94 mmol h-1 g-1) under AM 1.5 irradiation. From Figure 6A, we can clearly see that TDBTNs are very stable because after four cycles it has little changes in the test process. In order to investigate the effect of surface area and oxygen vacancies, the corresponding photocatalytic H2 generation rate is also calculated. The photocatalytic H2 generation rates (AM 1.5) for TDBTNs and TNs are 41.6 and 13.2 µmol h-1 g-1 per unit surface area (m2 g-1), respectively. In addition, the photocatalytic H2 generation rates (AM 1.5) for


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TDBTNs and TNs are ~ 43.9 and 42.7 µmol h-1 g-1 per oxygen vacancy, respectively. These results all illustrate the better photocatalytic performance for TDBTNs than that of TNs. The photocatalytic activity of TDBTNs under visible light irradiation (λ > 420 nm) is shown in Figure S6. It indicates that UV light is the main contributor to the high photocatalytic activity of the TDBTNs materials under AM 1.5, which is consisted with literatures.21 In order to confirm the excellent photocatalytic performance, a standard commercial anatase TiO2 photocatalyst is chosen as comparison. The photocatalytic performance is shown in Figure S7. Obviously, the photocatalytic performance of black TiO2 nanotubes is much better than that of the standard commercial anatase TiO2 photocatalyst. Scanning Kelvin probe maps (Figure 6B) are also performed to further demonstrate the electrons are easier excited from the valence band and then transferred to the cocatalysts surface for photocatalytic hydrogen evolution. The working function of TDBTNs and TNs are ~ 5.54 eV and ~ 5.71 eV, respectively. According to the result, the Fermi level of TDBTNs is higher than that of TNs, which changes the built-in electric field and surface band bending. It could accelerate the photogenerated electron transferring to the surface and then to cocatalysts, thus greatly reduce the electron-hole recombination and improve the photocatalytic performance significantly. The surface photovoltage spectroscopy (SPS) is used to testify the separation efficiency of electron and hole (e--h+). As shown in Figure 6C, a strong SPS peak at around 350 nm is attributed to the electron transitions from the valence to the conduction band (band-to-band transitions, O2p-Ti3d).16 The stronger SPS peak intensity for TDBTNs than that of TNs illustrates the high-efficiency of separation of photogenerated electron-hole pairs and long excitation lifetimes for the former. Furthermore, the onset of TDBTNs is red-shifted obviously, indicating the visible light photoactivity. The decreased fluorescence intensity for TDBTNs further demonstrates the high separation efficiency of


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photogenerated charge carriers (Figure 6D). The SPS and fluorescence both illustrate that after surface hydrogenation the samples possess superior separation efficiency of photogenerated charge carriers and visible light photoactivity, which results in the high solar-driven photocatalytic hydrogen performance. The photoelectrochemical properties of the as-prepared samples are analyzed by Princeton VersaSTAT. The Linear sweep voltammograms (Figure 7A) of TDBTNs and TNs show a high current density of 92.4 and 51.7 µA cm-2 under AM 1.5 irradiation, respectively. The higher current density for TDBTNs demonstrates the more efficient separation of photogenerated charge carriers than that of TNs. However, when the two samples treated under dark condition, they are both present low current density, illustrating the more efficient separation of photogenerated electron and hole pairs under irradiation conditions. The chronoamperometry results are tested at a bias voltage (0.4 V), as show in Figure 7B, the photocurrent for TDBTNs and TNs are both stable, the current density of the TDBTNs is probably two times as high as TNs, indicating that more efficient charge separation and transport for TDBTNs, which is corresponding to the result of linear sweep voltammograms. The electrochemical impedance (EIS) tests show the smaller interfacial resistance for TDBTNs (Figure 7C) under irradiation, which indicates that TDBTNs favor the separation of electrons and holes. That is to say, it has higher carrier mobility than that of TNs. When treated in the dark, both TDBTNs and TNs exhibit large interfacial resistance, suggesting that the charge separation can be promoted through the AM 1.5 irradiation. The type of semiconductor for TDBTNs and TNs is confirmed by Mott-Schottky (M-S) plots (Figure. 7D). Both TDBTNs and TNs show the positive plots, which are the direct proofs of n-type semiconductor character.45 According to the Mott-Schottky plots, the flat band positions of the TDBTNS and TNs are -0.82 and -0.80 V, respectively, which


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indicate the slightly up-shift of Fermi level for the former caused by self-doping. The results suggest that hydrogenation significantly increases the valence band maximum but slightly affects the conductance band minima, which favors the visible light absorption and photocatalytic hydrogen evolution. Moreover, comparing to TNs, the smaller slope for TDBTNs could be observed, indicating that the donor density has an obvious increase after surface hydrogenation. The carrier density can be roughly estimated through the following equation:

Nd =

2 / e0 εε 0 d 1 / C 2 / dV

(

)

We take ε = 55 for anatase TiO2.46 The electron densities of TDBTNs and TNs are roughly ~ 3.8 × 1018 and ~ 1.9 × 1018 cm-3, respectively. The higher electron density for TDBTNs is due to the presence of self-doped Ti3+ in frameworks, which can promote the separation of electron-hole pairs. That would be beneficial to the high photocatalytic hydrogen evolution. ■ CONCLUSIONS In summary, we have demonstrated a facile solvothermal method combined with ethylenediamine encircling and surface hydrogenation strategy to synthesize Ti3+ self-doped black TiO2 nanotubes with mesoporous nanosheet architecture. The ethylenediamine molecules protected the mesoporous frameworks against collapsing efficiently during high-temperature surface hydrogenation process, which could offer efficient surface active sites, and thus favored the photocatalytic performance. The efficient Ti3+ self-doping and disordered layers narrowed the bandgap and enhanced the separation efficiency of photogenerated charge carriers. The solardriven photocatalytic hydrogen production performance for TDBTNs (~ 3.95 mmol h-1 g-1) was


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approximately four times as high as TNs (~ 0.94 mmol h-1 g-1). The novel strategy affords an effective path to improve the solar-driven photocatalytic performance, and maybe also a promising method to fabricate other semiconductor oxide photocatalysts.


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Figure captions Figure 1. Typical XRD patterns (A) and Raman spectra (B) of TDBTNs (a) and TNs (b), respectively. Figure 2. The UV-vis reflectance spectra (A) and the corresponding optical bandgaps (B) of TDBTNs (a) and TNs (b), respectively. The inset is the photo of TDBTNs (a) and TNs (b). Figure 3. N2 adsorption-desorption isotherms (A) and the corresponding pore size distribution plots (B) of TDBTNs (a) and TNs (b), respectively. Figure 4. XPS spectra for survey spectrum (A), Ti 2p (B), O 1s (C) of TDBTNs and valence band spectra (D) of TDBTNs (a) and TNs (b), respectively. Figure 5. SEM (a, b), TEM (c, d), HRTEM images (e) and SAED pattern (f) of TDBTNs. Figure 6. Cycling tests of photocatalytic hydrogen generation under AM 1.5 irradiation (A), scanning Kelvin probe maps (B), surface photovoltage spectroscopy (C), and fluorescence spectra (D) of TDBTNs (a) and TNs (b), respectively. Figure 7. Photoelectrochemical properties of TDBTNs (a) and TNs (b). (A) Linear sweep voltammograms in the dark and under AM 1.5, (B) chronoamperometry results under AM 1.5, (C) Nyquist plots of electrochemical impedance in the dark and under AM 1.5, and (D) Mott– Schottky plots. Scheme 1. Schematic illustration of synthesis of the stable black TiO2 nanotubes with mesoporous nanosheet architecture.


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Figure 1. Typical XRD patterns (A) and Raman spectra (B) of TDBTNs (a) and TNs (b), respectively.

Figure 2. The UV-vis reflectance spectra (A) and the corresponding optical bandgaps (B) of TDBTNs (a) and TNs (b), respectively. The inset is the photo of TDBTNs (a) and TNs (b).


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Figure 3. N2 adsorption-desorption isotherms (A) and the corresponding pore size distribution plots (B) of TDBTNs (a) and TNs (b), respectively.


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Figure 4. XPS spectra for survey spectrum (A), Ti 2p (B), O 1s (C) of TDBTNs and valence band spectra (D) of TDBTNs (a) and TNs (b), respectively.


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Figure 5. SEM (a, b), TEM (c, d), HRTEM images (e) and SAED pattern (f) of TDBTNs.


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Figure 6. Cycling tests of photocatalytic hydrogen generation under AM 1.5 irradiation (A), scanning Kelvin probe maps (B), surface photovoltage spectroscopy (C), and fluorescence spectra (D) of TDBTNs (a) and TNs (b), respectively.


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Figure 7. Photoelectrochemical properties of TDBTNs (a) and TNs (b). (A) Linear sweep voltammograms in the dark and under AM 1.5, (B) chronoamperometry results under AM 1.5, (C) Nyquist plots of electrochemical impedance in the dark and under AM 1.5, and (D) Mott– Schottky plots.


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Scheme 1. Schematic illustration of synthesis of the stable black TiO2 nanotubes with mesoporous nanosheet architecture.

Displayed equations

Nd =

2 / e0 εε 0 d 1 / C 2 / dV

(

)

■ASSOCIATED CONTENT Supporting Information


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The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. The UV-vis spectrum of solar simulator equipped with an AM 1.5G filter, structures of TiO2 nanotubes without the treatment of ethylenediamine, XPS of Ti 2p and N 1s for TDBTNs and TNs, quantitative analyses of oxygen vacancies, SEM and TEM images for TNs, photocatalytic hydrogen generation for TDBTNs under visible light irradiation and under AM 1.5 irradiation for black TiO2 nanotubes and commercial anatase TiO2 sample. ■ AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. X. Zhang and W. Hu contributed equally. ■ ACKNOWLEDGMENTS We gratefully acknowledge the support of this research by the National Natural Science Foundation of China (21376065, 51672073), the University Nursing Program for Young Scholars with Creative Talents in Heilongjiang Province (UNPYSCT-2015014), and the Key Project of Undergraduate Creative Entrepreneurship Training Plan in Heilongjiang Province (2016102127244). ■ REFERENCES


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■ SYNOPSIS

Ti3+ self-doped black TiO2 nanotubes with mesoporous nanosheet architecture are synthesized via solvothermal combined with ethylenediamine encircling strategy and surface hydrogenation, which narrow the bandgap and exhibit excellent solar-driven photocatalytic hydrogen evolution due to self-doped Ti3+ enhancing separation of photogenerated charge carriers and mesoporous architecture offering more surface active sites.


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Ti3+ Self-Doped Black TiO2 Nanotubes with Mesoporous Nanosheet

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