Adjustment and Matching of Energy Band of TiO2-Based

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Adjustment and Matching of Energy Band of TiO-Based Photocatalysts by Metal Ions (Pd, Cu, Mn) for Photoreduction of CO into CH 2

4

Yabin Yan, Yanlong Yu, Shaolong Huang, Yajun Yang, Xiaodan Yang, Shougen Yin, and Yaan Cao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b07180 • Publication Date (Web): 03 Jan 2017 Downloaded from http://pubs.acs.org on January 3, 2017

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The Journal of Physical Chemistry

Adjustment and Matching of Energy band of TiO2-based Photocatalysts by Metal Ions (Pd, Cu, Mn) for Photoreduction of CO2 into CH4 Yabin Yana, Yanlong Yuc, Shaolong Huangd, Yajun Yanga, Xiaodan Yanga, Shougen Yinb* and Yaan Caoa* a

Key Laboratory of Weak-Light Nonlinear Photonics, Ministry of Education, TEDA Applied Physics Institute and School of Physics, Nankai University, Tianjin 300457, China.

b

Key Laboratory of Display Materials and Photoelectric Devices (Ministry of Education), Institute of Material Physics, and Tianjin Key Laboratory for Photoelectric Materials and Devices, Tianjin University of Technology, Tianjin 300384, China. c

Department of Materials Chemistry, College of Chemistry, Nankai University, Tianjin 300457, China

d

Shenzhen Key Laboratory of Laser Engineering, College of Optoelectronic Engineering, Shenzhen University, Shenzhen, 518060, China

ABSTRACT: A series of the metal ions (Pd, Cu and Mn) modified TiO2 photocatalysts are synthesized via simple sol-gel method. Characterized by XRD, Raman, UV-vis absorption spectra, XPS, EDAX, time-resolved PL decay curves and PL, it was revealed these introduced metal ions existed as O-Me-O species (Me: Pd, Cu and Mn ) on the surface of TiO2. The corresponding theory calculation is used to investigate the electronic density of states and band structure of the metal ions (Pd, Cu and Mn) modified TiO2. The modified TiO2 photocatalysts exhibit an imrpvoed photocatalytic performance on reduction of CO2 and H2O into methane (CH4), attributed to the contribution of surface species by enhancing the visible absorption efficiently, separating charge carriers and matching of the redox potential on the photoreduction of CO2 into CH4. This manuscript could provide a wider understanding about the adjustment and matching of the energy level for the synthesis and design of functional materials with excellent photocatalytic performance

Introduction The greenhouse effect and energy shortage have gained increasing concern over the recent years. A possible avenue for solving these problems is to use the photocatalysts for photoreduction of CO2 into hydrocarbons fuels1. The photocatalytic reduction mechanism of CO2 into CH4 can be summarized in equation (1)-(3)2-4. The photocatalyst generates photo-excited electrons and holes under irradiation (hν) (1); Then the photogenerated holes (h+) react with the adsorbed water molecules to produce O2 and H+ (2) (0.82 V, vs NHE; pH=7), and the electrons and H+ would react to generate the final product CH4 (3) ( -0.24V ,vs NHE; pH=7). Catalyst + hν → e- + h+ (1) 2H2O + 4h+ → O2+ 4H+ Eoredox = 0.82 V vs NHE (2) CO2 + 8e- + 8H+ → CH4 + 2H2O Eoredox = -0.24 V vs NHE (3) TiO2 is always considered as the most promising candidate owing to the good stability, charge transport properties as well as high photocatalytic performance2, 5-7.

Up to now, the practical application for titania is impeded by the large band gap8, the rapid recombination efficiency of charge carriers and the band potential does not match with the redox potential of photoreduction of CO2 into CH4. Hence, it is important to develop the TiO2-based photocatalyst with excellent photocatalytic performance for photo-reduction of CO2 into methane. Doping TiO2 with metal elements has been regarded as one of the most promising ways. It is reported that the Pd/TiO2 shows an improved catalytic activity for photoreduction of CO29-10. It is reported that Cu particles dispersed on the surface of TiO2 presents improved performance of CO2 photoreduction11. In et al. prepared CuO-TiO2 nanocubes with improved photocatalytic activity12. Benjwal et al. synthesized Mn doped TiO2 with enhanced photocatalytic activity on removal of methylene blue13. Li and his co-workers synthesized Pd modified TiO2 with significant photocatalytic performance14-15. However, seldom reports focus on the structure and energy levels of metal ions surface species (O-Me-O), the relationship with redox potential on photoreduction of CO2 into CH4, the effect on the behaviors of photogener-

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ated charge carriers as well as the influence of surface species on the photocatalytic perfoemance. Herein, the metal ions surface species (O-Me-O, Me: Pd, Cu and Mn) modified TiO2 photocatalysts were synthesized by sol-gel method. The structure of metal ions surface species (O-Me-O), energy level of the surface species, the match relationship with the redox potential on photoreduction of CO2 into CH4, the behavior of charge carrier and the mechanism of photocatalytic reaction is also investigated and discussed. Experimental Catalyst Preparation At room temperature, 40 mL of anhydrous ethanol was mixed with 1 mL of deionized water. Then 1 ml of concentrated HCl (12 mol/L) was added to adjust the pH value. After mixing, 3 ml of PdCl2 solution was added into the mixture. Under vigorous stirring, 12 ml of tetrabutyl titanate was added dropwise into the solution. The mixture was stirred continuously until the formation of TiO2 gel. The resultant precipitate was dried at 100 °C for 12 h after aging at room temperature for 24 h and calcined at 450 °C for 2.5 h. A series of Pd modified TiO2 catalysts with different concentrations of palladium was prepared by changing the concentration of PdCl2 solution added in the mixture. The catalysts were designated as TiO2-Pd x%, where “x%” represented the nominal molar percentage content of Pd2+ ions in all metal ions (Pd and Ti) in TiO2. Pure TiO2, TiO2-Cu x% and TiO2-Mn x% samples were prepared with the same procedure, but by replacing the PdCl2 solution with deionized water, CuCl2 and MnCl2 solution, respectively. Characterization X-ray diffraction (XRD) patterns were collected on a Rigaku D/max 2500 X-ray diffraction spectrometer(Cu Kα, λ=1.54056 Å) in the range of 20°~80°. The average crystal size was calculated based on Scherrer equation (D = kλ/Bcosθ, parameter B is the peak half width). The BET surface areas of samples were determined by nitrogen absorption-desorption isotherm measurement at 77K (Micromeritics Automatic Surface Area Analyzer Gemini 2360, Shimadzu). Raman spectra were taken on a Renishaw in Via Raman microscope by using the 785 nm line of Renishaw HPNIR 785 semiconductor laser. X-ray photoelectron spectroscopy (XPS) measurements were carried out with a VG-Scientific ESCA Lab 220i-XL spectrometer by using an unmonochromated Al Kα(1486.6eV) X-ray source. All of the XPS spectra were calibrated with respect to the binding energy of the adventitious C1s peak at 284.8 eV. Energy-dispersive X-ray spectroscopy (EDAX) was taken on a scanning electron microscope (FEI Helios Nanolab 600i, USA). The high-resolution transmission electron microscopy (HRTEM) analysis were performed using a Philips Tecnai G2F20 instrument at an accelerating voltage of 200 kV, for which the samples were prepared by applying a drop of ethanol suspension onto an amorphous carbon-coated copper grid and dried natural-

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ly. Diffuse reflectance UV-visible (UV-vis) absorption spectra were recorded on a UV-vis spectrometer (U-4100, Hitachi). The photoluminescence (PL) spectra were measured by fluorescence spectrophotometer (Edinburgh Instruments, FLS920) using the 340 nm line of a Xe light as the excitation source. The time-resolved fluorescence decay spectra were measured on FL900 (Edinburgh, England) by using the light source of nf900. The experimental setup consists of a gas chromatograph (GC7890F, Shanghai Techcomp Instrument Co., Ltd.), a hydrogen generator (SPH-300A, Beijing BCHP Analytical Technology Institute), an automatic air source (SPB-3, Beijing BCHP Analytical Technology Institute) and a computer for data processing. All the measurements were carried out at room temperature (25±2) °C unless stated otherwise. Evaluation of Photocatalytic Activity The photoreduction of CO2 and H2O into CH4 was carried out in a 225 mL cylindrical glass reactor with built-in light source of a 32 W Hg lamp. A stirred annular reactor (length 20 cm, width 4 cm) with a suspended catalyst was illuminated by a peak light wavelength at 253.7 nm situated in the center of the quartz tube. The whole reactor contains solution layer (125 ml) and gas layer (100 ml). First, 125 mg of catalysts was suspended in 125 mL of NaOH solutions (0.2 mol/L) in the reactor and the whole reactor was continuously inflated by CO2 gas (99.999%) at a flux of 0.3 L min-1 for at least 45 min, which was used to exclude the O2 and other gas in the whole reactor. The pH value of the solution is evaluated to be at 6.5. Except for the NaOH solutions in the reactor, the 100 mL of gas layer filled with CO2 is applied for quantitative test of the generated product CH4 and CO. The cooling water in the outer layer of reactor was used to maintain the temperature balance of the whole reactor ((25±2) °C). Every 2 h, 0.4 mL gas of gas layer was extracted to measure the produced amount of CH4 and CO by a gas chromatograph (GC7890F, Shanghai Techcomp Instrument Co., Ltd.). The chemical reagents were all of analytical grade in the experiments and water was deionized water (>18.2 MΩ cm). Calculation The calculation was carried out by a first-principle calculation software package CASTEP. Generalized gradient approximation (GGA)16-17 based density-functional theory (DFT) was used to calculate the electronic band structure and density of states (DOS) for TiO2, TiO2-Pd, TiO2-Cu and TiO2-Mn, respectively. An anatase TiO2 model of 76 atoms with exposed (101) facet is created. The vacuum region has the same thickness as the TiO2 slab. For TiO2-Pd, TiO2-Cu and TiO2-Mn, one metal ion is linked with two surface bridge O ions on the (101) facet. The pseudopotential used is Vanderbilt-type ultrasoft pseudopotential with 2s22p4, 3s23p63d24s2, 4d10, 3d104s1 and 3d54s2 as the valence-electron configurations for the oxygen, titanium, palladium, copper and manganese atoms, respectively. The plane wave energy cutoffs were taken to be 420 eV. In all the cases, geometry optimizations were


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carried out first, and convergence was assumed when the forces on atoms were less than 50 meV/Å. Compared with experimental results, theoretical calculation usually results in a underestimated band gap, caused by the shortcoming of the exchange-correction functional in describing the excited states18-19. Results and discussion

anatase rutile

Intensity(a.u.)

♦

●

♦

Intensity(a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


♦

TiO2-Mn 1% ●

●

♦

●

24

♦♦

25

●

26

♦2θ(degree) ♦♦

27

♦

TiO2-Cu 1% TiO2-Pd 1% TiO2 10

20

30

40

50

60

70

80

2θ(degree)

Figure 1. XRD patterns of TiO2, TiO2-Pd 1%, TiO2-Cu 1% and TiO2-Mn 1% samples. Inset is the magnified crystal plane (101) of anatase. Table 1. Lattice Parameter, Crystal Size and Specific Surface Area for TiO2, TiO2-Pd 1%, TiO2-Cu 1% and TiO2-Mn 1% samples. Lattice parameter (Å) a=b c

cell ume (Å3)

TiO2

3.783

9.507

TiO2-Pd 1%

3.784

TiO2-Cu 1% TiO2-Mn 1%

Samples

vol-

crystal size (nm)

SBET (m2g1 )

136.15

18.1

54.4

9.509

136.19

10.9

64.4

3.786

9.484

136.18

14.8

46.6

3.788

9.491

136.16

9.0

81.7

the major phase20. Except for anatase peaks, three weak peaks located at 27.5°, 36.1° and 41.3° are found21, respectively. This result indicates that there are tiny amount of rutile existed in four samples. No other XRD peaks related to Pd, Cu and Mn (such as PdO, CuO and MnO) are detected in TiO2-Pd 1%, TiO2-Cu 1% and TiO2-Mn 1% samples, respectively. The lattice parameters, cell volumes and crystal size for all four samples are evaluated and summarized in Table 1. It is easily observed that after the introduction of metal ions (Pd, Cu and Mn) into TiO2 system, the speciﬁc surface area (BET) increases and the crystallite sizes of the catalysts decrease. These results suggest the addition of Pd, Cu and Mn ions could inhibit titanium dioxide’s grain growth. Compared with TiO2, the peak position remains almost unchanged (Inset of Figure 1). The lattice parameters and cell volume remain unchanged for TiO2-Pd 1%, TiO2-Cu 1% and TiO2-Mn 1% samples (Table 1), implying that no metal ions (Pd, Cu and Mn) are doped into the lattice of TiO2 for TiO2-Pd 1%, TiO2-Cu 1% and TiO2-Mn 1% samples. According to the doping mechanism, it is hardly for the Pd, Cu or Mn ion to weave into the crystal cell of TiO2 via interstitial mode, as the ionic radius of Pd2+ ion (86 pm)22, Cu2+ ion (72 pm)23 and Mn2+(80 pm)24 are larger than Ti4+ ion (68 pm)25. Furthermore, if the introduced Pd, Cu or Mn ions replace lattice Ti4+ through substitutional mode, the lattice parameters and cell volume increase. As a result, the XRD peaks shift to lower diffraction angles for TiO2-Pd 1%, TiO2-Cu 1% and TiO2-Mn 1% samples. However, the shift of the peaks position can't be observed for TiO2-Pd 1%, TiO2-Cu 1% and TiO2-Mn 1% in Figure 1. Thus, the Pd, Cu and Mn ions in substitutional mode can be excluded. Therefore, the conjecture that the metal ions (Pd, Cu and Mn) might exist on the surface of TiO2 as some kinds of species for TiO2-Pd 1%, TiO2-Cu 1% and TiO2-Mn 1% is very reasonable.

Pure TiO2, TiO2-Pd 1%, TiO2-Cu 1% and TiO2-Mn 1% samples are studied by the EDAX spectra, shown is Figure S1. For pure TiO2 (Figure S1a), Ti, O and C are major elements. Compared with pure TiO2, the Pd, Cu and Mn elements are found for TiO2-Pd 1% (Figure S1b), TiO2-Cu 1% (Figure S1c) and TiO2-Mn 1% (Figure S1d), respectively. This result indicates that the introduced metal ions (Pd, Cu and Mn) really exist in TiO2 photocatalyst. The N2 adsorption isotherms of four samples are shown in Figure S16. To investigate the crystal structure of the metal ions (Pd, Cu and Mn) modified TiO2, Figure 1 shows the XRD patterns of TiO2, TiO2-Pd 1%, TiO2-Cu 1% and TiO2-Mn 1% samples. TiO2 modified with different amount of the metal ions (Pd, Cu and Mn) are also tested by XRD, shown in Figure S2-S4, respectively. It can be found that anatase is

Figure 2. HR-TEM images of TiO2, TiO2-Pd, TiO2-Mn and TiO2-Cu. It is observed from Figure 2 and S15 that TiO2 consists of nanoparticles with an average diameter of 18 nm. After modification with Pd, Cu and Mn ions, the particles size



TiO2-Pd 0.5%

(a)

TiO2-Pd 1% TiO2-Pd 2%

336.7 eV

342.1 eV

Intensity(a.u.)

of the nanoparticles decreased effectively, compared with pure TiO2. Moreover, the fringe spacing (d) corresponding to the (101) plane of anatase TiO2 is 3.52 Å for all samples, suggesting the Pd, Cu and Mn ions didn’t enter into TiO2 lattice in interstitial or substitutional mode. There is no other phase, such as PdO, CuO and MnOx observed in the TEM images. These TEM and HR-TEM images further demonstrate the introduced Pd, Cu and Mn ions exist on the surface of TiO2 as some kind of surface species.

335

340

345

Binding Energy(eV)

TiO2 TiO2-Pd 1% TiO2-Cu 1% TiO2-Mn 1%

Intensity(a.u.)

TiO2-Cu 0.5% TiO2-Cu 1%

(b)

TiO2-Cu 2% 952.8eV

Intensity(a.u.)

933.0eV

200

400

600

800

1000

925

930

-1 Wavelength(cm )

935

940

945

950

955

960

Binding Energy(eV)

Figure 3. Raman spectra of pure TiO2, TiO2-Pd 1%, TiO2Cu 1% and TiO2-Mn 1% samples Raman spectra of pure TiO2, TiO2-Pd 1%, TiO2-Cu 1% and TiO2-Mn 1% samples are plotted in Figure 3, to further investigate the structure of the obtained samples. Raman spectra of TiO2 modified with different amount of the metal ions (Pd, Cu and Mn) are shown in Figure S5S7, respectively. The peaks at about 144 cm−1 (Eg), 194 cm−1 (Eg), 396 cm−1 (B1g), 516 cm−1 (A1g and B1g) and 638 cm−1 (Eg) are found for TiO2, TiO2-Pd 1%, TiO2-Cu 1% and TiO2-Mn 1% samples, ascribed to typical anatase structure26. Compared with TiO2, no shift of Raman peaks is detected for TiO2-Pd 1%, TiO2-Cu 1% and TiO2-Mn 1%, implying that the metal ions are not weaved into TiO2 lattice and might exist on the surface as some kinds of species. In addition, the new Raman peaks at about 302 cm-1 in TiO2-Pd 1% and 284 cm-1 in TiO2-Cu 1% are observed, ascribed to stretching modes of O-Pd and O-Cu bonds on surface, respectively27-28. Another new Raman peak at about 364 cm-1 is observed for TiO2-Mn 1%, ascribed to stretching modes of O-Mn bonds on surface30. The existing states of the metal ions (Me: Pd, Cu and Mn) would be further studied by XPS.

TiO2-Mn 0.1% TiO2-Mn 0.5%

(c)

TiO2-Mn 1%

640.8 eV

652.5 eV

Intensity(a.u.)

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635

640

645

650

655

660

Binding Energy(eV)

Figure 4. (a) Pd 3d XPS spectra of TiO2-Pd x%; (b) Cu 2p XPS spectra of TiO2-Cu x%;(c) Mn 2p XPS spectra of TiO2-Mn x%. The XPS spectra for TiO2-Pd x%, TiO2-Cu x% and TiO2-Mn x% samples are plotted in Figure 4a, b and c, respectively. Figure 4a shows the Pd 3d XPS spectra of TiO2-Pd x% sample. The doublet peaks at 336.7 eV and 342.1 eV are assigned to the Pd 3d5/2 and Pd 3d3/2 of O-PdO species on surface of TiO2 (one Pd2+ ions linked with two unsaturated O2- ions)22. In the Cu 2p XPS spectra of TiO2-Cu x% (Figure 3b), there are double peaks observed at 952.8 eV and 933.0 eV, ascribed to the Cu 2p1/2 and Cu 2p3/2of O-Cu-O species on surface of TiO2-Cu x%, which was reported by our previous work23, since the peak position of Cu 2p3/2 (933.0 eV) is between those of CuO (933.9 eV)29 and Cu2O (931.7 eV)30. As shown in Figure 3c, Mn 2p (652.5 and 640.8eV) is ascribed to the Mn 2p1/2 and Mn 2p3/2 of O-Mn-O species on surface of TiO2-Mn x%, respectively, because the valence state of Mn in this work is +2 and the binding energy of Mn 2p3/2 is between that of Mn (638.9 eV)31 and MnO (641.4 eV)32 , implying the Mn ion for TiO2-Mn x% linked with less O atoms than that for MnO. This results indicate that introduced metal ions are connected with two unsaturated oxygen ions on sur-


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face of TiO2-Me x% sample (Me = Pd, Cu and Mn), existing as O-Me-O species.

2

Eg=2.729eV

Energy(eV)

(b) Sum Ti 3d O 2p Pd 4d

DOS(electrons/eV)

(a) 4

0

-2

-4

G

Q

F

Z

G

-5

-4

-3

-2

-1

0

1

2

3

4

Energy(eV)

(b) 4

(c)

3

Energy(eV)

2

Sum O 2p Ti 3d Cu 3d

1

-2 -3 -4

G

F

Q

Z

G

DOS(electrons/eV)

0 -1

(c) 4

-6

Energy(eV)

2

-4

-2

0

4

(d)

-2

-4

G

F

Z

Q

Z

G

Sum O 2p Ti 3d Mn 3d

DOS(electrons/eV)

G

(d) 4

2

Energy(eV)

2

Energy(eV) 0

0

-2

-6

-4

Q

F

G

Figure 5. Theoretically calculated band structure for TiO2 (a), TiO2-Pd (b), TiO2-Cu (c) and TiO2-Mn (d).

(a) Sum O 2p Ti 3d

DOS(electrons/eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-6

-4

-2

0

Energy(eV)

2

4

-4

-2

0

2

4

Energy(eV)

Figure 6. Projected density of states (PDOS) for TiO2 (a), TiO2-Pd (b), TiO2-Cu (c) and TiO2-Mn (d) To obtain a deep understanding of the qualitative tendency on electronic band structure and density of states of the metal ions (Pd, Cu and Mn) modified TiO2 photocatalysts, the models of TiO2, TiO2-Pd, TiO2-Cu and TiO2-Mn are established and DFT calculation was carried out. Band structure for TiO2, TiO2-Pd, TiO2-Cu and TiO2Mn are shown in Figure 5. In Figure 5a, the zero energy level locates at the top of the valence band (VB), corresponding to highest state level that electrons occupy. Since the bottom of conduction band (CB) and top of valence band (VB) are at the different K point, pure TiO2 exhibits an indirect band gap of 2.729 eV. For TiO2-Pd (Figure 4b) and TiO2-Cu (Figure 4c), a new energy level forms above the VB of TiO2, contributing from the O-PdO and O-Cu-O- surface species, respectively. Moreover, a new energy level is observed below the CB of TiO2 for TiO2-Mn (Figure 4d), due to the introduced O-Mn-O species. The density of states for TiO2, TiO2-Pd, TiO2-Cu and TiO2-Mn are shown in Figure 5. The VB is made up of O 2p orbitals, hybridized with a small fraction of Ti 3d, and


The Journal of Physical Chemistry the CB is made up of Ti 3d orbitals and a small amount of O 2p. After the introduction of the metal ions into TiO2 system, the VB top and CB bottom for TiO2-Pd, TiO2-Cu and TiO2-Mn almost remain unchanged compared with pure TiO2. However, for TiO2-Pd (Figure 5b) and TiO2-Cu (Figure 5c), the small hump occurs near to VB, due to the the mixed O 2p state with the Pd 4d or Cu 3d state, respectively. For TiO2-Mn (Figure 5d), a small peak forms close to the conduction of TiO2 owing to the mixed O 2p state with the Mn 3d state. According to the discussion above, the introduction of metal ions (Pd, Cu and Mn) into TiO2 system might enhance the visible response and inhibit the recombination of charge carriers, which would be in favor to improve the photocatalytic activity for photoreduction CO2 into CH4. To prove the theoretical prediction, the corresponding characterizations are performed, as shown in the following sections.

(a)

Intensity(a.u.)

TiO2-Mn 1% TiO2-Cu 1%

TiO2-Pd 1% TiO2 0

2

4

6

8

Bingding Energy(eV)

TiO2 TiO2-Cu1%

(b)

TiO2-Pd 1% TiO2-Mn 1%

Intensity(a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

300

400

500

600

700

Wavelength(nm)

Figure 7. (a) XPS valence band spectra of pure TiO2, TiO2-Pd 1%, TiO2-Cu 1% and TiO2-Mn 1%; (b) Diffuse reflectance UVVis spectra of pure TiO2, TiO2-Pd 1%, TiO2-Cu 1% and TiO2Mn 1%.

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el36. In Figure 6a, the onset edge of the VB for pure TiO2 is 2.75 eV (2.35 eV, vs NHE). Meanwhile, it can be observed from Figure 6a that the onset edge of the valence band maximum for TiO2-Pd 1%, TiO2-Cu 1% and TiO2-Mn 1% is almost the same as that of pure TiO2, indicating the valence band position of TiO2 is not changed by modified metal ions (Pd, Cu and Mn). Moreover, the hump about 2.25 eV (1.85 eV, vs NHE) for TiO2-Pd 1% and 1.75 eV (1.35 eV, vs NHE) for TiO2-Cu 1% are found, caused by the contribution of the Pd 3d states and Cu 3d states for O-Me-O species on surface of TiO2, respectively. So the energy levels of O-Pd-O and O-Cu-O species are at about 0.5 eV and 1 eV above the VB top of TiO2. In addition, the energy level of O-Mn-O for TiO2-Mn 1% is not observed, implies the energy levels related to O-Mn-O species are close to the CB of TiO2. In Figure 7b, the strong peak in UV light region is observed for all samples, because of the band-band transition. The band gap is estimated to be about 3.0 eV for pure TiO2, as the absorption onset edge is about 412.9 nm21. According to the onset edge of the valence band maximum (In Figure 7a, 2.75 eV (2.35 eV, vs NHE)), the CB bottom is -0.65 V (vs NHE) for pure TiO2. Moreover, the stronger peaks at around 496 nm for TiO2-Pd 1%, at about 450 and 750 nm for TiO2-Cu 1% and at about 435 nm for TiO2-Mn 1% are observed, respectively. Moreover, with an increase of the modified metal ions (Pd, Cu and Mn), the absorption in visible region is further enhanced for TiO2-Pd x%, TiO2-Cu x% and TiO2-Mn x% ( Figure S8), indicating that the visible-light absorption is really caused by the contribution of O-Me-O species (Me: Pd, Cu and Mn). According to the results of XPS valence band spectra and DFT calculation, a peak at around 496 nm for TiO2Pd 1% is attributed to the electron transition from the surface energy level of O-Pd-O to the CB of TiO2. Two peaks about 450 and above 750 nm for TiO2-Cu 1% are attributed to the electron transition from the surface energy level of O-Cu-O to the CB of TiO2 and from VB of TiO2 to the surface energy level of O-Cu-O, respectively. So, energy levels of O-Pd-O and O-Cu-O species can be determined at 0.5 eV and 1.0 eV above VB of TiO2. For TiO2-Mn 1%, the strong absorption around 435 nm can be ascribed to electron transition from VB to the surface energy level of O-Mn-O, which is determined at around 0.15 eV below the CB. Therefore, the schematic band structure of TiO2 modified metal ions (Pd, Cu and Mn) photocatalysts are drawn and shown in Figure 10.

To explore the band structure of pure TiO2, TiO2-Pd 1%, TiO2-Cu 1% and TiO2-Mn 1% samples, XPS valence band (VB) spectra and diffuse reflectance UV−vis absorption spectra are shown in Figure 7. The XPS valence band spectra of TiO2, TiO2-Pd 1%, TiO2-Cu 1% and TiO2-Mn 1% are plotted in Figure 7a. The energy levels are in alignment with the work function of the XPS instrument (4.10 eV, Fermi level). The binding energies of the onset edge reveal the energy gap between the VB top and Fermi lev-


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in Table 2. The PL lifetime of TiO2-Mn 1%, TiO2-Cu 1% and TiO2-Pd 1% is effectively prolonged, compared with pure TiO2and the corresponding τ2 values are 0.936, 0.993 and 1.146 ns, respectively, indicating that the photogenerated charge carriers is separated. Among these samples, the τ2 value of TiO2-Pd 1% is longest, which is consistent with the result of PL spectra. These results further suggest that the modification with metal ions (Pd, Cu and Mn) can inhibit effectively the recombination of photogenerated charge carriers.

TiO2 TiO2-Cu 1% TiO2-Mn 1% TiO2-Pd 1%

Intensity(a.u.)

450

500

550

600

Wavelength(nm)

5

Amount of CH4(10-6mol/L)

Figure 8. Photoluminescence spectra (PL) of pure TiO2, TiO2Pd 1%, TiO2-Cu 1% and TiO2-Mn 1% samples. The PL spectra is employed to investigate charge carriers’ behavior of TiO2, TiO2-Pd 1%, TiO2-Cu 1% and TiO2-Mn 1% photocatalysts, shown in Figure 8. The peaks centered at about 467 and 528 nm are attributed to the transition from the oxygen vacancies with two trapped electrons and one 33-36 trapped electron to the VB of TiO2, respectively . The energy level of the oxygen vacancies is calculated to be about 0.34 and 0.65 eV below the CB of TiO2, respectively. The generated electrons in CB would migrate to the energy level of oxygen vacancies via a nonirradiative process, and further recombine with the holes in the VB, along with the ﬂuores37 cence emission . So, the decreased PL intensity suggests the efficient separation of photogenerated carriers. Shown in Figure 7, the photoluminescence intensity of TiO2-Pd 1%, TiO2-Cu 1% and TiO2-Mn 1% are weakened significantly compared with pure TiO2, indicating that the introduction of metal ions (Pd, Cu and Mn) could effectively separate of photogenerated charge carriers. Moreover, TiO2-Pd 1% sample shows the lowest fluorescence intensity in the four samples.

4

TiO2 TiO2-Cu 1%

3

TiO2-Mn 1% TiO2-Pd 1%

2 1

0

2

20

TiO2 TiO2-Cu 1%

15

TiO2-Mn 1% TiO2-Pd 1%

TiO2-Cu 1%

TiO2-Pd 1%

τ1（ns）

0.196

0.272

0.268

0.208

τ2（ns）

0.842

0.936

0.993

1.146

8

10

12

(b)

5

0 2

4

6

8

10

12

Time(h)

Figure 9. Photocatalytic activity on reduction of CO2 into (a) CH4 and (b) CO of TiO2, TiO2-Pd 1%, TiO2-Cu 1% and TiO2Mn 1% samples. Table 3. Photocatalytic activity of pure TiO2, TiO2-Pd 1%, TiO2-Cu 1% and TiO2-Mn 1% under UV irradiation for 12 hours. sample

The time-resolved PL decay curves of TiO2, TiO2-Mn 1%, TiO2-Cu 1% and TiO2-Pd 1% are shown in Figure S9 and Table 2. The τ1 and τ2 value for TiO2, TiO2-Mn 1%, TiO2-Cu 1% and TiO2-Pd 1% samples was counted through the double exponential decay fitting37. The PL decay curve derives from a nonradiative (τ1) process and a radiative (τ2) process38-40. The fast decay (τ1) comes from the nonradiative relaxation process in connection with oxygen vacancies, and the slow decay process (τ2) is usually ascribed to the radiative process about the recombination of carriers37. It is shown that the τ2 value for pure TiO2 is 0.842 ns

6

10

0

TiO2-Mn 1%

4

Time(h)

Table 2. Fluorescence Lifetimes of pure TiO2, TiO2-Mn 1%, TiO2-Cu 1% and TiO2-Pd 1%. TiO2

(a)

0

Amount of CO(10-6mol/L)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TiO2 TiO2Pd 1% TiO2Cu 1% TiO2Mn 1%

CH4 generation amount (10-6mol) 1.52±0.06

Specific photocatalytic activity a (106 mol·g-1·h-1) 1.01±0.04

CO generation amount(106 mol) 10.43±0.46

specific photocatalytic activity b (106 mol·g-1·h-1) 6.95±0.31

4.43±0.17

2.96±0.11

14.73±1.36

9.82±0.95

2.92±0.14

1.94±0.09

15.07±0.69

10.05±0.46

2.72±0.11

1.81±0.08

20.02±1.11

13.35±0.72

a

specific photocatalytic activity of CH4, CH4 generation amount per unit

mass catalyst per hour; b

specific photocatalytic activity of CO, CO generation amount per unit

mass catalyst per hour.



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To evaluate the photocatalytic performance, the photoreduction of CO2 and H2O into CH4 under UV light irradiation is employed for all samples. CO is the intermediate product and CH4 is the final product49. The photocatalytic results of pure TiO2, TiO2-Pd 1%, TiO2-Cu 1% and TiO2-Mn 1% are shown in Figure 9 and Table 3. Pure TiO2 represents limited photocatalytic performance and about 1.52 μmol of CH4 is produced after irradiation for 12 hours. Compared with pure TiO2, the TiO2-Pd 1%, TiO2-Cu 1% and TiO2-Mn 1% samples show an enhanced photocatalytic activity and 4.433, 2.916, 2.720 μmol of CH4 is produced, respectively. The specific photocatalytic activity of TiO2Cu 1% and TiO2-Mn 1% is 1.92 and 1.79 times higher than pure TiO2, respectively. For all samples, the TiO2-Pd 1% photocatalyst shows the better photocatalytic performance than any other samples, whose specific photocatalytic activity is 2.92 times higher than pure TiO2. In addition, the photocatalytic experiment of TiO2-Pd x%, TiO2Cu x% and TiO2-Mn x% is carried out, as shown in Figure S10-S12. These photocatalytic results indicate that the introduction of metal ions (Pd, Cu and Mn) is the core role for an improved photocatalytic activity on the reduction of CO2 into CH4 for TiO2-based photocatalysts.

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Figure 10. Schematic diagram of photocatalytic mechanism for TiO2-Pd, TiO2-Cu and TiO2-Mn photocatalysts.

Because the photocatalytic performance is mainly connected with band structure of the metal ions modified TiO2 photocatalysts as well as the redox potential for reduction of CO2 into CH4, the enhancement of photocatalytic mechanism can be explained by the schematic diagram of TiO2-Pd 1%, TiO2-Cu 1% and TiO2-Mn 1% (Figure 10). Pure TiO2 shows limited photocatalytic activity, due to its high recombination rate of photogenerated charge carriers and large band gap. Since the CB and the VB (0.65 V and2.35 V, vs NHE) for pure TiO2 are much more negative and positive than the redox potential of CO2/CH4 (Eoredox, -0.210 V, vs NHE) and H2O/H+(Eoredox, 0.845 V, vs NHE), respectively, only seldom photogenerated carriers could participate in the photoreduction of CO2 into CH4. Compared with pure TiO2, the photocatalytic activity for TiO2-Pd 1%, TiO2-Cu 1% and TiO2-Mn 1% are enhanced significantly due to introduction of surface O-MeO (Me: metal ions, Cu, Pd and Mn) species. Since the existence of O-Me-O species, the visible response (400800nm) is improved and more photogenerated charge carriers can be excited. Moreover, the photogenerated charge carriers are separated and the photogenerated electrons’ lifetime is prolonged effectively. For TiO2-Mn 1%, the surface energy level of O-Mn-O located at -0.5 eV (vs NHE), which is about 0.15 eV below CB of TiO2 (-0.65 eV, vs NHE). Excited electrons would transfer from CB to the surface energy level of O-Mn-O, which further participate in the photoreduction of CO2 into CH4 (eqn 2). Moreover, the energy level of O-Pd-O and O-Cu-O surface species (1.85 eV and 1.35 eV, vs NHE, respectively) is 0.5 eV and 1.0 eV above VB of TiO2 (2.35 eV, vs NHE). Excited holes would migrate from VB to the surface levels of O-Pd-O and O-Cu-O, respectively, which further react with H2O and CO2 to generate CH4 (eqn 3). Compared with pure TiO2, the energy levels of O-Pd-O and O-Cu-O surface species(1.85 eV and 1.35 eV, vs NHE, respectively) are more matching with the redox potential (Eo= -0.845 eV, vs NHE). At the same time, the potential of O-Mn-O (-0.5 eV, vs NHE) is also better matching with the redox potential (Eo= -0.210 V, vs NHE), benefiting more photogenerated charge carriers to take part in the pahotoreduc-


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tion of CO2 into CH4, resulting in enhanced photocatalytic activity, compared with pure TiO2. Compared with TiO2-Cu 1% and TiO2-Mn 1% samples, the PL intensity for TiO2-Pd 1% is further quenched and the lifetime of the photogenerated electrons is prolonged, indicating the photogenerated charge carriers are separated more efficiently. More excited carriers participate in photoreduction reaction for TiO2-Pd 1% sample, leading to a much better photocatalytic activity than TiO2-Cu 1% and TiO2-Mn 1% samples. The large BET surface area for TiO2-Pd 1% also benefits for the photocatalytic reaction. Moreover, though the lifetime of photogenerated electrons for TiO2-Mn 1% is almost the same as that for TiO2-Cu 1%, the PL intensity is further quenched and BET surface area is larger. That is reason why TiO2Mn 1% exhibit better photocatalytic activity than TiO2-Cu 1%. Conclusions Three metal ions (Pd, Cu and Mn) modified TiO2 photocatalysts are prepared via sol-gel method, exhibiting improved photocatalytic activity for photoreduction of CO2 into CH4, in comparison with TiO2. It is revaled the introduction of surface O-Pd-O, O-Mn-O and O-Cu-O species can enhance the visible response and separate of charge carriers efficiently, and the energy level of O-PdO, O-Mn-Oand O-Cu-O surface species is better matching with the redox potential of H2O/H+(Eoredox, 0.845 eV, vs NHE) and CO2/CH4 (Eoredox, -0.210 eV, vs NHE). As a result, more photogenerated carriers participate in reduction process, enhancing photocatalytic activity efficiently.

Acknowledgements This work was supported by the National Natural Science Foundation of China (Nos, 51372120). We thank Prof. Ying Ma from Institute of Chemistry, Chinese Academy of Sciences, and Prof. Yihong Ding from State Key Lab of Theoretical and Computational Chemistry, Jilin University for helping us to carry out the theoretical calculation.

ASSOCIATED CONTENT Supporting Information. EDAX, XRD, Raman, Absorption spectra, Time-resolved PL decay curves, Photocatalytic Activity. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author. *Tel: 86 22 66229431. Email: [email protected] (Y.C.). Email: [email protected] (S.Y.).

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Insert Table of Contents artwork here

Schematic diagram of photocatalytic mechanism for TiO2-Pd, TiO2-Cu and TiO2-Mn photocatalysts


11

Adjustment and Matching of Energy Band of TiO2-Based

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