The Journal of Physical Chemistry C - ACS Publications - American

Article pubs.acs.org/JPCC

High H2 Evolution from Quantum Cu(II) Nanodot-Doped TwoDimensional Ultrathin TiO2 Nanosheets with Dominant Exposed {001} Facets for Reforming Glycerol with Multiple Electron Transport Pathways Meng Zhang, Runze Sun, Yajun Li, Qiaomeng Shi, Lihong Xie, Jinsheng Chen, Xinhua Xu, Huixiang Shi, and Weirong Zhao* Department of Environmental Engineering, Zhejiang University, Hangzhou 310058, China S Supporting Information *

ABSTRACT: Two-dimensional (2D) ultrathin TiO2 nanosheets doped with quantum Cu(II) nanodots (QCNs) were synthesized through an in situ photodeposition for reforming glycerol. The optimized QCNs dopant exhibited favorable H2 evolution rate of up to 25-fold compared with bare TiO2. The prepared QCNs permeated into the homogeneous {001} facets resulting in discontinuous quantum nanodot doping, which was confirmed by energy-dispersive X-ray (EDX) spectroscopy. Analysis suggests that the suppressed recombination rate of the photoinduced electron−hole pair and the prolonged lifetime are benefits for enhancing photocatalytic performance. A photoinduced interfacial charge-transfer (IFCT) effect introduced by doping QCN was certified by electron spin resonance (ESR) spectroscopy. This offered more photoinduced electrons and holes for redox reaction including H2 production and CO2 and CO yields. Electrochemical characterizations were carried out to reveal that QCN could improve the photocurrent density and shift the conduction band (CB) potentials to negative direction. With all the characterizations conducted in this paper, a probable mechanism was proposed to represent the multiple charge-transfer pathways. Here, we provide a new modification strategy for designing QCNs/TiO2 and explore the mechanism involved in 2D ultrathin nanostructures doped with variable-valence metal quantum nanodots. The multiple charge-transfer pathways system applied to photocatalytic H2 production and environmental waste disposition in a facile and green manner may be further employed to synthesize other variable-valence metal doping systems.

1. INTRODUCTION Photocatalysis is a mild and significant renewable route for converting solar energy to chemical energy like photocatalytic H2 production. TiO2 is one of the most promising photoactive nanomaterials in many application fields. However, the high recombination rate of photogenerated electrons (e−) and holes (h+) and the limited utilization of solar light hamper its further applications.1 Developing various methods for modifying TiO2 to improve its photocatalytic H2 production characteristics is still a central issue.2,3 The dimensionality and morphological control of the crystal facets of TiO2 have a great influence on its electronic structure properties thus altering photocatalysis.4 A two-dimensional (2D) structure with reactive {001} facets can facilitate the electron transport and enhance the catalytic activity. The first report on 2D micrometer-size TiO2 with reactive {001} facets was published by Yang et al. using HF as a capping agent,5 for fluorination can reduce the surface energy of {001} facets (0.90 J/m2, according to Gibbs−Wulff theorem6) to make them thermodynamically stable.7 Various studies showing improved © XXXX American Chemical Society

photocatalytic performance from crystal facet engineering have been reported.8−11 Modifying TiO2 with non-noble metals is a simple and effective way to improve the e−−h+ separation efficiency and mediate forbidden energy level for broadening the absorption spectra of visible light. Among these non-noble metals, Cu, as a less expensive earth-abundant element with variable valence (such as +1, +2), is a good selection for TiO2 modification.12,13 The visible light response for Cu(II) is referred to as the photoinduced interfacial charge transfer (IFCT) from TiO2 to the Cu species, which has been studied extensively by using electron spin resonance (ESR) on valence variation.14 The induced structural and morphological modifications on TiO2 with variable-valence Cu dopant could offer more effective pathways for charge carrier transfer. The synthesis methods have an influence on the surface construction thus affecting the Received: January 30, 2016 Revised: May 3, 2016

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DOI: 10.1021/acs.jpcc.6b01030 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. Scheme of formation of QCNs/TiO2 photocatalysts.

photocatalytic performance, such as impregnation,15 solvothermal,16 photodeposition,17 and equilibrium deposition filtration.18 Zhu et al.17 prepared Cu(I)-loaded TiO2 nanosheets with exposed {001} facets via a two-step method. The highly dispersed Cu2O clusters played a significant role in modulating the superficial properties of TiO2, which showed high e− mobility and a good CO2 adsorption capacity for photoreduction CO2. The relationships among the Cu species (shown in eq 1) are presented as follows:18 (1)

Moreover, electrochemical characterization results also confirmed a high separation rate and multiple photogenerated e−− h+ transfer pathways, which are crucial for good photocatalytic performance. This method could further the synthetic methods for a homogeneous phase of 2D ultrathin nanostructure doped with variable-valence metal quantum nanodots. The detailed characterizations deepened the understanding of the mechanisms of multiple charge carrier transport pathways that can be applied to the production of H2 and the disposal of environmental waste in a facile and green manner simultaneously.

Mohamed and co-workers1,2 used a sol−gel associated hydrothermal method to prepare Cu/TiO2 for water splitting. They found that this method increased the amount of hydroxyl groups and controlled photocatalytic properties. Valero et al.19 revealed that different methods strongly affected the reactivity when comparing the chemical reduction and impregnation methods for copper-doped TiO2 preparation. Moreover, quantum dot-decorated TiO2 demonstrated enhanced photocatalysis, such as obvious quantum size effects and fast diffusion of the involved charge carriers across the interface for the reaction.20 The ultrasmall size of nanodots could make a significant contribution to decreasing the distance for photogenerated electrons transfer and facilitating the transfer rate.21 However, to the best of our knowledge, a detailed description of enhanced performance of QCN-doped 2D ultrathin TiO2 nanosheets has not yet been reported. Inspired by the previous achievements, we employed a modified in situ photodeposition method to design quantum Cu(II) nanodot (QCN)-doped 2D ultrathin TiO2 nanosheets with dominant exposed {001} facets. We applied the asprepared QCN/TiO2 for glycerol photoreformation, which is a low value-added product resulting from biodiesel production and weighing approximately 10 wt % (weight ratio).22 The synergistic effect of the crystal facet engineering and the QCNdoping modification was investigated by extensive structural and surface characterizations. The suppressed recombination rate of photoinduced e−−h+ was showed by photoluminescence (PL) due to the existence of QCNs. The time-resolved PL decay (TRPL) spectroscopy illustrated prolonged lifetimes of photocatalysts. Downconversion and upconversion PL suggested luminescence of QCNs/TiO2 was excited by one photon instead of two photons. ESR indicated IFCT offers more pathways for charge transfer by variable-valence Cu metal.

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. 2D ultrathin anatase TiO2 (ATiO2) nanosheets with dominant {001} facets were synthesized via a traditional hydrothermal method.17 In a typical synthesis process, 3 mL of hydrofluoric acid (HF) was added dropwise into a Teflon-lined autoclave containing 25 mL of tetrabutyl titanate (Ti(OBu)4) under magnetic stirring. The color turned white immediately when HF mixed with Ti(OBu)4. After 2 h, the mixture was submitted for thermal treatment at 180 °C for 24 h at a ramp rate of 5 °C/min. After cooling to room temperature, the obtained precipitate was centrifuged, washed with ultrapure water and ethanol several times, and dried at 80 °C overnight. The sample was designated S0 after calcination at 350 °C for 2 h. Figure S1 shows that the best H2 evolution was obtained at 350 °C compared among different calcination temperatures (CT) in the references. QCNs/TiO2 was prepared via an in situ photodeposition method.17,23 The as-prepared TiO2 was first mixed with the required amounts of a CuCl2 aqueous solution (0.01 mol/L) under ultrasonic dispersion. Then, a certain amount of glycerol was added to the system, which was the same as in the hydrogen production experiment (described in the next part). The above suspension was evacuated in air and irradiated with a 300 W xenon lamp for 2 h under magnetic stirring. The purple precipitate with brick-red dots (shown in Figure S2) was collected by washing, filtering, drying, and calcining. The QCN/TiO2 containing Cu/Ti molecule ratios of 0.1%, 0.5%, 1%, 1.5%, and 2% were designated S1, S2, S3, S4, and S5, respectively. The scheme of formation of QCNs/TiO2 is shown in Figure 1. 2.2. Catalyst Characterization. The crystal structures of the photocatalysts were determined by X-ray diffraction (XRD)

CuO ⇄ Cu 2O ⇄ Cu

B


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aqueous solution. Before illumination, high-purity Ar gas was purged for 1 h to completely remove oxygen. A 300 W xenon lamp (CEL-HXUV300, Aulight, Beijing, China) was used as the light source. The produced H2 was periodically determined using a gas chromatograph (Fuli 9790, Zhejiang, China) equipped with a thermal conductivity detector at intervals of 0.5 h, using Ar as the carrier gas. CO and CO2 were analyzed using a gas chromatograph/flame ionization detector (Fuli 9790, Zhejiang, China).

with Cu Kα radiation (λ = 0.15406 nm, PANalytical, Netherlands) over the 2θ range from 10° to 80°. The accelerating voltage and the applied current were set to 40 kV and 40 mA, respectively. The actual amount of total Cu was determined by inductively coupled plasma atomic emission spectrometer (ICP-AES, Prodigy, America) at the measurement wavelength of 259 nm. The morphological and lattice structures of the photocatalysts were observed on a scanning electron microscope (SEM, Supra55, Zeiss, Germany), transmission electron microscope (TEM), and high-resolution TEM (HRTEM, JEM-2010, JEOL, Japan) at an accelerating voltage of 200 kV, equipped with an energy-dispersive X-ray spectroscopy (EDX) detector. The elemental composition of photocatalyst surfaces and the chemical statuses of Ti, Cu, and O were detected by X-ray photoelectron spectroscopy (XPS) using an Al X-ray source (Al Kα 150 W, hv = 1486.6 eV, Escalab 250Xi, Thermo, England). All binding energies were referenced to carbon (C 1s = 284.8 eV). The Brunauer−Emmett−Teller (BET) surface areas of the photocatalysts were measured with automatic analyzer (3H2000PS2, Beishide, Beijing, China) using nitrogen as the adsorbate, and the pore-size distributions were determined by using absorption curves and the Barrett−Joyner−Halenda (BJH) method. UV−vis diffuse reflectance spectra (DRS) were measured using a UV−vis spectrophotometer (TU-1901, Pgeneral, Beijing, China) equipped with an integrating sphere at room temperature. Photoluminescence emission spectra were measured at room temperature with a fluorescence spectrophotometer (FLS920, Edinburgh, England) using a 325 nm wavelength laser as the excitation source. Time-resolved PL decay spectroscopy was performed with an excitation wavelength of 405 nm and a measurement wavelength of 500 nm. Downconversion PL (DCPL) spectra were conducted using different excitation wavelengths (290, 300, 310, 320, 330, and 340 nm). The electron spin resonance spin-trap technique was conducted using a spectrometer (JES FA200, JEOL, Japan) operated at the X-band frequency and equipped with an Oxford cryostat. Samples were irradiated with or without visible light (wavelength longer than 420 nm) at 77 K under vacuum with a 5 vol % glycerol solution. 5,5-Dimethyl-1-pyrroline N-oxide (DMPO) was used to capture the superoxide (·O2−) and (· OH). All photoelectrochemical measurements (photocurrent densities vs time curves i−t, Nyquist plots, and Mott−Schottky MS) were carried out with an electrochemical workstation (CH Instruments 660D, Shanghai, China) in a standard threeelectrode quartz cell, as illustrated in our previous work.24 Intermediate products, after reacting for 3 h with solutions containing glycerol, were determined with a nano liquid chromatography electrospray ionization quadrupole time-offlight mass spectrometer (Q-TOF-MS, Triple TOF 5600+, AB Sciex, America). All intermediates were tested under the negative ion mode. 2.3. Photocatalytic Activity Measurement. In this study, the photocatalytic hydrogen evolution reaction was carried out in a top-irradiation jacketed quartz photoreactor, in which water flowed through to maintain a constant temperature for the reactor. In a typical reaction, 50 mg of photocatalyst was dispersed by magnetic stirring in 100 mL of 5% glycerol

3. RESULTS AND DISCUSSION 3.1. Morphologies and Crystal Surface Structures. Figure 2 shows the XRD patterns of S0−S5. For all the as-

Figure 2. XRD patterns of QCNs/TiO2 photocatalysts.

prepared QCN/TiO2, the diffraction peaks at 2θ = 25.281°, 37.800°, 38.575°, 48.050°, 53.890°, 55.060°, 62.121°, 70.311°, and 75.032° correspond to (101), (004), (112), (200), (105), (211), (213), (220), and (215) planes of the tetragonal anatase phase of TiO2 (JCPDS card 21-1272), respectively. When compared with TiO2, there were no significant changes in the intensities and peak positions for the QCNs/TiO2, which indicated that doping had no remarkable influence on the crystal structure of the photocatalyst during the synthetic process.16,25,26 In addition, no Cu species, including metallic copper, cuprous oxide, or copper oxide, were detected, even with high Cu content (2 mol %). These results could infer the thorough dispersion of QCNs on the TiO2 surfaces. This is in accordance with the results reported by Sun et al.26 and Lalitha et al.27 Figure 3 shows the SEM, TEM, and HRTEM images of S3. The images show the clear lattice fringes of S3, indicating good crystallinity. The ultrathin nanosheets of S3 with ca. 3−5 nm thickness and ca. 50 nm lateral size are shown in Figure 3a−d. The insets of Figure 3, parts c and d, show HRTEM images of the detailed structure of S3. The interplanar distances parallel to the top and bottom facets are 0.238 nm, corresponding to the A-TiO2 {001} planes, and the 0.352 nm spacing is in accordance with the {101} facet of A-TiO2. As seen from Figure 3e, the black dots ranging from 1 to 3 nm can be ascribed to the doped QCNs. When the black dots are magnified , it can be seen clearly that the lattice fringes of the dot structure corresponds to the (1̅11)/(002) plane of monoclinic CuO (tenorite phase, JCPDS card 45-937) in the inset of Figure 3e. The diffraction rings, from inner to outer in C


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Figure 3. SEM image (a), TEM image (b), HRTEM images (c−e), and electron diffraction pattern (f) of S3 nanosheets. The insets are the corresponding lattice fringes.

Figure 4. XPS fully scanned spectra (a) and high-resolution XPS spectra of Ti 2p (b), Cu 2p (c), and O 1s (d) for S0 and S3.

the electron diffraction pattern, have d-spacings of 0.352, 0.238, 0.189, 0.167 nm corresponding to the (101), (004), (200), (211) lattice planes (shown in Figure 3f) indexed to A-TiO2, respectively. This result is in accordance with the XRD result. The QCNs were uniformly doped as indicated by the X-ray elemental mapping analysis and EDX spectroscopy results (Figure S3).

XPS analysis was conducted to provide the surface chemistry information and structural environment of elements for the QCN/TiO2 photocatalysts.28 Full-scan spectra of S0 and S3 are shown in Figure 4a. The C 1s is detected due to adventitious carbon-based contaminants. In the full-scan spectra, S0 shows Ti and O peaks, while the Cu, Ti, and O elements are detected in S3, which is consistent with the EDX result. Figure 4b shows D


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Figure 5. Nitrogen adsorption (solid icon)−desorption (hollow icon) isotherms of S0 (a), S1 (b), S2 (c), S3 (d), S4 (e), and S5 (f). The insets are the corresponding pore size distribution.

Table 1. Microstructure and MS Parameters of S0−S5 for Comparison sample

Cu/Ti (mol %)a

Cu/Ti (mol %)b

specific surface area (m2/g)

pore volumes (mL/g)c

Eb (eV)

S0 S1 S2 S3 S4 S5

0 0.100 0.500 1.000 1.500 2.000

0 0.105 0.576 0.924 1.574 1.913

37.4 38.3 41.3 54.4 48.2 36.2

0.278 0.397 0.426 0.448 0.403 0.348

−0.12 −0.14 −0.18 −0.20 −0.24 −0.27

4

−2

K (107 cm ·F 3.84 3.29 1.03 0.50 0.80 1.90

·V−1 d

)

Nd (1020 cm−3) 4.59 5.36 17.1 35.3 22.1 9.29

a

Theoretical values. bActual values determined by inductively coupled plasma atomic emission spectrum. cDetermined by BJH method using the nitrogen absorption branch of the isotherm. dThe slope of MS plot determined by experimental data.

energy located at 529.68 eV is characterized as OCu−O.31 Moreover, the peaks centered at ca. 532.28 eV are attributed to the absorbed −OH on the surface of photocatalysts.1,34 The microstructural characteristics of the photocatalysts were conducted via N2 adsorption−desorption analyses, as shown in Figure 5. All adsorption isotherms exhibit similar type-IV patterns with an evident type-H3 hysteresis in the relative pressure range of 0.4−1.0. This phenomenon suggests a slit aperture formed by nanosheets, which is typical for mesoporous materials.35 The aggregation of TiO2 nanosheets may result in the formation of a mesoporous structure.36 The insets of Figure 5 show the pore size distributions. All the curves show two components of the peaks values: 2−4 and 50 nm. The addition of Cu increases the size and the amounts of the mesoporous pores compared with S0. This result infers that the QCNs augment the surface attractive forces to forming larger mesoporous pores without changing its first-type pores. The structural parameters, including specific surface area (SSA) and pore volumes (PV), are summarized in Table 1. Larger SSAs

the high-resolution XPS spectra of Ti 2p in S0 and S3. The core level peaks at 458.9 and 464.6 eV with a split of 5.7 eV are attributed to Ti 2p3/2 and 2p1/2, respectively, indicating the normal state of Ti4+.29,30 Compared with S0, the Ti 2p3/2 peak of S3 shifted to lower binding energies. The existence of an intermediate oxidation state of Ti3+ is a reasonable explanation of the phenomenon, for which Ti4+ trapped electrons, thereby resulting in a denser electron cloud lowering the binding energy.28 No obvious Cu 2p peak is observed for S0 in Figure 4c. The curve of S3 shows that the binding energies of Cu 2p2/3 and Cu 2p1/2 spin−orbital splitting are 933.68 and 935.68 eV, respectively.31 The characteristic shakeup satellite lines of Cu2+ are attributed to the shakeup transitions from the ligand−metal 3d charge transfer.32 O 1s spectra for S0 and S3 are illustrated in Figure 4d. The main contribution of O 1s for S0 is 530.38 eV, which corresponds to the lattice oxygen (OTi−O),33,34 while the O 1s peak of S3 centered at 530.18 eV is consistent with the literature.1,29 It may be that the doped QCNs interacted with Ti−O resulting in the lower binding energy. The binding E


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difference in the absorption edge among the S1−S5 samples, and it can be that the amount of QCNs doped is too small to be detected. The PL emission spectra were employed to disclose the surface states of the charge carrier transfer and the recombination and to understand the fate of photogenerated electrons and holes in the semiconductors.37,38 Figure 6b shows the typical PL spectral changes in the wavelength ranges of 250−800 nm. The peak located at 410 nm, existing in all curves, is attributed to the band−band PL phenomenon.39 The other PL peak at 470 nm arises from band-edge free excitons.40 The quenching of fluorescence indicates a suppressed charge recombination of photogenerated e−−h+ pairs because the PL emission is the result of the recombination of free carriers.41 When the amount of doped QCNs is greater than 1 mol %, the intensity does not decrease as much as for others. The small decrease may result from the excessive dopant decreasing the fluorescence rather than from the suppressed charge recombination, which is confirmed by the reduced H2 evolution rates of S4 and S5 in the inset. According to Cao et al.’s42 and He et al.’s43 researches, DCPL was the response of a one-photon excitation process and upconversion PL (UCPL) was the response of a two-photon excitation process for quantum dots. It can be seen that DCPL spectra of QCNs/TiO2 (shown in Figure S4) were independent of excitation wavelengths (from 290 to 340 nm), which is in accordance with the result of Zhuo et al.’s experiment,44 whereas no obvious luminescence was shown up in the UCPL experiment. Therefore, it is rational to infer that luminescence of QCNs/TiO2 was excited by one photon instead of two photons. To further confirm the fate of the photogenerated electrons, the TRPL curves are shown in Figure 4c. Each curve is fitted into eq S1 using doubleexponential decay response functions. The faster decay time component (τ1) is related to the radiative recombination, and the slower one (τ2) correlated with the trap emission.37 Average lifetimes (τ) values of the photocatalysts are calculated through the following eq S2.45 The fitted parameters of the PL decay are listed in Table S1. The values of the goodness of fit parameter (χ2) show a good fit to the experimental data. It can be observed that doped QCN significantly increased the decay time from 1.31 to 4.43 ns. 3.3. ESR. The ESR spectra were conducted on S0 and S3 under different atmospheric conditions (vacuum and air) with or without visible light irradiation (shown in Figure 7a−c). For S0, a significant increase of the higher field signal (g = 1.989) occurs, ascribed to the electrons trapped at the Ti3+ centers.3 For S3, a decrease in the large signal attributed to Cu2+ is evidenced by a negative signal at ca. 320 mT compared with the dark one.46 This observation of the signal decrease may illustrate that electrons transferred from the valence band (VB) to the Cu2+ leading to the formation of diamagnetic Cu+ ions by IFCT under visible light as reported by other researchers.46−49 The weak intensity of the Ti3+ signal in S3 may be explained by the reduction of Cu2+ to Cu+ through the following process: Cu2+ + Ti3+ → Cu+ + Ti4+.17 When the light was turned off, the signal of the curve was in keeping with its initiated state, which is a good confirmation of the reversibility of the Cu2+/Cu+ couple. Meanwhile, there is no decrease in the Cu2+ signal when exposed to air, indicating that the formed Cu+ was oxidized to Cu2+. The curves of DMPO−·O2− for S0 (Figure 7d) and S3 (Figure 7e) are shown. There is no signal detected for S0 regardless of the light off or on conditions, since TiO2 does not have sufficient potential to reduce O2 to ·O2−

and suitable PV values could accommodate better adsorption of reactant molecules and a larger number of active sites for H2 production. But it made little difference among these photocatalysts in the experiment. 3.2. Optical Properties. Figure 6a shows the UV−vis DRS of the photocatalysts. The red shift of absorption edge for QCN/TiO2 can be caused by the IFCT. The absorption in the visible region is enhanced accordingly from 400 to 800 nm with increases in the Cu content. However, there is no conspicuous

Figure 6. UV−vis DRS of QCNs/TiO2 photocatalysts (a). The PL emission spectra (b) and the time-resolved PL decay curves (c) of photocatalysts. The inset in panel b shows the relative lines between the PL intensity and H2 evolution rate of photocatalysts. F


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the theory of the Cu2+/Cu+ level reached the O2/·O2− level at pH 7.46 The introduction of IFCT allowed more pathways for photogenerated carriers transfer resulting in suppressed e−−h+ recombination. 3.4. Photoelectrochemical Experiment. The transient photocurrent responses were tested to study the interfacial transfer charge dynamics of photoelectrons at wavelengths longer than 350 nm with a light on/off cycle of 50 s. As shown in Figure 8a, a fast photocurrent response with good reproducibility is observed for each light on/off cycle for all photocatalysts; it shows no significant photocurrent density in the dark, but a rapid increase appears for the light on. Then, the photocurrent remains steady following prolonged light irradiation. When the light is off, the photocurrent dramatically decreases, followed by a mild decay. It may be presumed that more mobile electrons prolonged the decay time of the response photocurrent of QCNs/TiO2.53 The photocurrent responses of photocatalysts follow the order S3 > S4 > S2 > S5 > S1 > S0. The small decrease in the photocurrent of the second and third cycles may be attributed to the generated Cu(I) reducing the density of electrons transfer by IFCT. The highest photocurrent density of S3 is nearly 10 times that of S0. MS was conducted to obtain the electronic properties, as shown in Figure 8b.54 All plots show positive slopes indicating n-type semiconductors and confirming that QCN doping into the TiO2 lattice did not change the properties of the TiO2.55 The Nd was calculated from the slope values shown in Table 1 using eq 254

Figure 7. ESR spectra of S0 and S3 measured at 77 K under vacuum (a and b) and air (c) in darkness and under the wavelength longer than 420 nm irradiation. The DMPO−·O2− of S0 (d) and S3 (e) with irradiation for 5 min.

under visible light irradiation, whereas S3 with the QCN doping has four characteristic peaks with similar intensity attributed to DMPO−·O2−.50,51 This observation is attributed to IFCT excitation, which was proposed based on Li et al.’s work,52 on

Figure 8. Photocurrent action spectra of all photocatalysts in the photoelectric process irradiated with wavelengths longer than 350 nm for 50 s (a). Mott−Schottky plots of all photocatalysts measured at 1 kHz in 0.01 M Na2SO4 solution (b). Nyquist plots were obtained in 0.01 M Na2SO4 solution for all photocatalysts (c), and the suggested equivalent circuit model (d). G


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The Journal of Physical Chemistry C Table 2. Model Parameters of the Photocatalysts Based on EIS Results parameters Rs (Ω/cm2) Rct (Ω/cm2) Cdl × 108 (F/cm2) Zw × 104 (S0.5/Ω cm2)

S0 22.1 99.3 0.492 5.36

± ± ± ±

S1 1.48 1.55 0.018 0.250

⎛ 2 ⎞⎛ kT ⎞ Csc−2 = ⎜ ⎟⎜ E − E b − ⎟ q ⎠ ⎝ εε0qNd ⎠⎝

21.4 85.0 0.565 7.63

± ± ± ±

S2 0.760 1.18 0.025 0.500

19.0 79.3 0.841 7.00

± ± ± ±

S3 1.33 1.73 0.039 0.540

18.9 68.5 1.10 7.64

± ± ± ±

1.11 1.34 0.0370 0.520

S4 20.5 73.5 0.980 7.40

± ± ± ±

S5 0.850 1.13 0.044 0.430

22.3 79.4 0.622 6.28

± ± ± ±

1.44 1.50 0.028 0.330

(2)

in which ε is the dielectric constant of the semiconductor, ε0 is the permittivity of a vacuum (8.854 × 10−12 F/m), q is the electric charge (1.6 × 10−19 C), Nd is the current carrier density of the semiconductor, E is the applied bias voltage relative to the reference electrode, Eb is the flat-band potential, k is the Boltzmann constant (1.38 × 10−23 J/K), and T is Kelvin temperature (298 K). In general, kT/q can be omitted due to its small value (0.0257). S3 has the smallest slope suggesting that the greatest current carrier density may lead to the most charge carriers involved into the reaction. In addition, another important parameter derived from the MS plots is the flat-band potential (Eb). Conduction band (CB) values can be estimated from the Eb values because Eb is approximately 0.1−0.3 V more positive than CB for the n-type semiconductors.56,57 The more positive the Eb value is, the greater the band bending is and the greater the barrier is for the corresponding semiconductor electrode in the reaction process. By increasing the amount of QCNs, the values of Eb becomes more negative, which means a more negative potential to reduce water to hydrogen.2 From the results above, we can presume that a more negative Eb and a larger Nd result in the promotion of the photocatalytic efficiency. To gain a better understanding of the charge migration process of the photocatalysts on the electrochemical performance, electrochemical impedance spectroscopy (EIS) was performed. The Nyquist curves of the EIS results, shown in Figure 8c, were measured from 100 to 1000 kHz. All plots illustrate a high-frequency semicircle and a low-frequency line. The suggested equivalent circuit model is shown in Figure 8d, which includes the solution resistance Rs, charge-transfer resistance Rct, double-layer capacitance Cdl, and Warburg impedance Zw. The semicircles resulted from the Rct and Cdl at the electrolyte/counter electrode interface corresponding to a kinetically controlled process.11,58 The values of the corresponding parameters were obtained by fitting the data with the ZSimpWin 3.10d program and are summarized in Table 2. The Rs values vary from 18.89 to 22.29 Ω/cm2. This suggested that a small difference occurs with the different Cu ratios of these photocatalysts. Generally, smaller arc radii describe higher charge mobility. As shown, S3, with the smallest semicircle, has the smallest Rct and the largest Cdl values, indicating the highest carrier transfer efficiency and the superior catalytic activity, respectively.58,59 The line segment in the lowfrequency range is associated with the Nernst diffusion process.11 No apparent difference among the Zw values of the photocatalysts is noted. 3.5. Photocatalytic Activity. Figure 9 displays the photocatalytic hydrogen production activities of all the photocatalysts under UV−vis light irradiation (300 W xenon lamp without AM1.5 filter; the light spectrum is provided in Figure S5) for reforming glycerol. A control experiment was

Figure 9. Photocatalytic activity of H2 evolution for all photocatalysts (a), cycle experiment of S3 (b), and the turnover efficiencies of H2, CO2, and CO for all photocatalysts (c). The inset in panel c is the magnified vision of the turnover efficiencies of CO2 and CO.

H


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Figure 10. Schematic illustration of the photoreforming glycerol by QCNs-doped 2D ultrathin TiO2 nanosheet.

depth, the concentration of Cu partially decreased. In pathway I, TiO2 generated e− and h+, and Cu(II) trapped e−61 (namely, IFCT). Pathway II represents the transportation of photogenerated e− and h+ after pathway I. The photocatalytic redox reactions are illustrated in pathway III, including the photocatalytic reduction reaction that yields H2 and the photocatalytic oxidation reaction for the selective reformation of glycerol. H2 is more probably produced from water, and maybe there is a minority H2 production from the exchange between water and glycerol.62 The valence variation process of Cu+ turning to Cu2+ offered two routes for charge transfer: one is for H2 production (route a), the other is for h+ consumption (route b). As a result, the number of e− increased and the charge-transfer rate was enhanced by of QCN-doped 2D ultrathin TiO2 nanosheets. For glycerol, the molecules adsorbed onto the TiO2 hydroxyl sites (Ti−OH) first. Then, glycerol was oxidized to CO2, CO, and other intermediates, such as glycolaldehyde, glyoxylic acid, glycolic acid, glyceraldehyde, and glyceric acid, etc., by the photogenerated holes or hydroxyl radicals (·OH, detected in Figure S9), which are products of the trapping holes by the surface Ti−OH groups or adsorbed water molecules.

performed without any photocatalysts showing negligible H2 production under the same condition. H2 evolution is clearly observed for samples in the following order: S3 > S4 > S2 > S5 > S1 > S0. The cycle experiment of the photocatalytic hydrogen generation of S3 is presented in Figure 9b. A slight increase is observed in the second-run experiment compared with the first one due to the reduction of Cu(II) restricting the H2 evolution rate in the first stage (as confirmed in Figure S6).22 Meanwhile, the CO2 evolution rate (Figure S6) accelerates because Cu(II) can trap photoinduced electrons through IFCT leaving more separated holes for the oxidation of glycerol. Moreover, the rate of CO2 evolution then starts to decrease (shown in Figure S6) due to the oxidation process of Cu(I) to Cu(II), which is in competition with the glycerol oxidation, and this accelerates the H2 evolution rate (eq 3) temporally (in Figure S7). 2H+ + 2eCB− → H 2

(3)

For the next two cycles, the photocatalytic performance is continuous and the activity is stable. The fifth run shows a slight decrease. This decrease can be attributed to the shortage of glycerol. The turnover efficiencies (TOF) of the main gas products are depicted in Figure 9c, showing that the optimal dopant amount is 1 mol %. It can be concluded that the high H2 evolution photocatalyst demonstrated high CO2 and CO production (in Figure S7), attributed to the efficient e− and h+ separation rate. CO, CO2, and other intermediates (like glycolaldehyde, glyoxylic acid, glycolic acid, glyceraldehyde, and glyceric acid, etc.) can be detected (eq 4, as confirmed by Q-TOF in Figure S8), which is in agreement with Montini et al.’s experimental results.60

4. CONCLUSION QCN-doped 2D ultrathin TiO2 nanosheets with different ratios were synthesized via a modified in situ photodeposition method. QCNs with clear lattice fringes ranging from 1 to 3 nm have an influence on hydrogen production and glycerol photoreformation. The optimized sample S3 with 1 mol % had its advantages over others upon almost all the characterizations, especially on the most important factor of the separation rate of the photoinduced e−−h+ pairs. The induced IFCT effect of QCNs/TiO2 was indicated by ESR providing multiple routes for charge transfer. Moreover, the increasing density of charge carriers with a rapid transfer rate across the interface, the suppressed recombination rate of e−−h+, and the prolonged lifetime of electrons with QCN doping are beneficial for boosting photocatalytic performance. The luminescence of QCNs/TiO2 was excited by one photon instead of the two photons through DCPL and UCPL. This material modification design provides another avenue for the explorations of the

C3H8O3 + 3H 2O + 14hVB+ → intermediates (C2H4O2 , C2H 2O3 , C2H4O3 , C3H6O3 , C3H6O4 , etc.) → 3CO2 + 14H+

(4)

3.6. Mechanism of Photocatalytic Reforming of Glycerol. Figure 10 demonstrates the photoreformation of glycerol by QCN-doped 2D ultrathin TiO2 nanosheets with exposed {001} facets via the spatial separation of e− and h+. During the preparation, QCNs adsorbed on the {001} facets with a sequential Ti−O arrangement, and then gradually permeated into the surface. In addition, with the increase in I


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mechanism for homogeneous phase 2D ultrathin structures doped with variable-valence metal quantum nanodots Cu. It is hoped that it could be applied to other variable-valence metaldoped systems such as Fe, V, Sn, Mn, and Ce, etc. This work exhibits the multiple charge-transfer pathways that efficiently enhance H2 evolution and the selective photoreformation of glycerol for dealing with environmental waste simultaneously and in an ecologically friendly manner.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b01030. Table of the fitted parameters of the PL decay, PLrelated equations, and additional figures showing the effect of calcination temperature, preparation of photoreduction of QCNs/TiO2, EDX and DCPL spectra, xenon lamp light spectrum, turnover rates, Q-TOF MS spectra, and generation of hydroxl radical by irradiation (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-571-8898-2032. Fax: +86-571-8898-2032. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Support from the National Natural Science Foundation of China (Grant Nos. 51278456 and 5117841) and the State Science and Technology Support Program (Grant No. 2013BAC16B01) is gratefully acknowledged.

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REFERENCES

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