NiO heterostructured photocatalyst with efficient reduction of CO2 into CH4

Separation and Purification Technology 142 (2015) 14–17

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Short Communication

A new Ni/Ni3(BO3)2/NiO heterostructured photocatalyst with efficient reduction of CO2 into CH4 Yanlong Yu a, Limei Guo a, Han Cao b, Yuekai Lv b, Enjun Wang c, Yaan Cao a,⇑ a

MOE Key Laboratory of Weak-Light Nonlinear Photonics, TEDA Applied Physics Institute and School of Physics, Nankai University, Tianjin 300457, China College of Physics and Material Science, Tianjin Normal University, Tianjin 300387, China c Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei 230031, China b

a r t i c l e

i n f o

a b s t r a c t

Article history: Received 3 August 2014 Received in revised form 2 December 2014 Accepted 4 December 2014 Available online 31 December 2014

A new Ni/Ni3(BO3)2/NiO heterostructured photocatalyst was synthesized. Owing to the formation of the heterostructure, the photo-excited charge carriers are utilized efficiently for the redox reaction, resulting in an enhanced photocatalytic activity on reduction of CO2 with H2O into CH4. Ó 2014 Elsevier B.V. All rights reserved.

Keywords: Ni3(BO3)2 Heterostructure Photo-reduction Carbon tetrahydroxide

The reduction of CO2 into CH4 by photocatalysis is one of the most valuable methods to solve the energy crisis and global warming problems [1]. Amounts of semiconductors, such as TiO2 [2–4], Zn2GeO4 [5,6], Bi2WO6 [7], InTaO4 [8] and CeO2 [9], have been reported as effective photocatalyst for the reduction of CO2. Besides, many strategies have been explored to improve the photo-reduction of CO2, e.g., the addition of noble metal onto the photocatalyst [3] and combination with other metal oxide to form heterostructure [10]. However, the photocatalytic activity is still too low to be applied in practice. Therefore, new photocatalysts with high photocatalytic performance is necessary to further improve the photo-reduction of CO2. The process of photocatalysis reduction of CO2 into CH4 with semiconductor as well as the redox potential (Eo) can be shown as follows [11]:

Photocatalyst þ hv ! e þ h þ

þ

ð1Þ

o

ð2Þ

CO2 þ 8e þ 8Hþ ! CH4 þ 2H2 O Eo ¼ 0:24 Vðvs NHEÞ

ð3Þ

þ

2H2 O þ 4h ! O2 þ 4H

E ¼ 0:82 Vðvs NHEÞ

Under irradiation, the photocatalyst creates photo-excited electrons and holes in the conduction band (CB) and valence band (VB), respectively. The holes on the VB would react with the adsorbed H2O to form H+ and oxygen. Meanwhile, the electrons on the CB

⇑ Corresponding author. http://dx.doi.org/10.1016/j.seppur.2014.12.014 1383-5866/Ó 2014 Elsevier B.V. All rights reserved.

are captured directly by the surface adsorbed CO2 molecules to form CO and oxygen. The resultant CO would further react with electrons and H+ to generate the final product, CH4. According to the aforementioned mechanism, the conduction band edge should be slightly above the reduction potential of Eq. (3) (Eo = 0.24 V, vs NHE), while the valence band edge should be slightly below the oxidation potential of Eq. (2) (Eo = 0.82 V, vs NHE), to provide energetic electrons and holes for the redox reaction. Owing to the appropriate energy level band, the photo-excited electrons and holes participate in the redox reaction on the surface more easily, leading to a high photocatalytic activity. Borates have been reported as low-cost and stable photocatalyst with excellent catalytic performance. Our previous work has reported InBO3 and its application in photodegradation of 4-chlorophenol [12]. Besides that, Pang et al. reported the synthesis of Ni3(BO3)2 nanoribbons with good antimicrobial activities [13]. However, as far as we know, there is no reports about Ni3(BO3)2 based photocatalyst with enhanced photocatalytic activity on the reduction of CO2 into CH4. In this work, we reported a new Ni/Ni3(BO3)2/NiO heterostructured photocatalyst with enhanced photocatalytic activity. Owing to the formation of heterostructure, the photogenerated charge carriers are separated effectively, especially after the addition of elemental Ni. The band structure of photocatalyst is tuned to match the redox potential, as a result, leading to a significant enhanced photocatalytic activity for the photo-reduction of CO2 into CH4.

Y. Yu et al. / Separation and Purification Technology 142 (2015) 14–17

CH4/10-6mol/L

4

It is clearly seen that the XRD peaks could be indexed to elemental Ni, NiO and Ni3(BO3)2, whose patterns match their JCPDS files Nos. 04-0850, 44-1049 and 26-1284, respectively. All these XRD peaks are quite sharp, indicating their high crystallinity. Besides that, the relative amount of elemental Ni could be easily adjusted by controlling the pH value (Fig. S2). The average crystal size of Ni, Ni3 (BO3)2 and NiO calculated from the XRD patterns are about 62.3 nm, 44.8 nm and 54.7 nm, respectively. Morphology of the as-prepared samples was investigated by TEM, as shown in Fig. S3 and Fig. 2b. It is clearly seen from Fig. S3 that Ni /Ni3(BO3)2/ NiO samples consist of nanoparticles with average diameter of about 50 nm. Three different kinds of lattice with d spaces of 0.215 nm, 0.241 nm and 0.203 nm can be clearly found, corresponding to the (1 1 3) plane of Ni3(BO3)2, the (1 0 1) plane of NiO and (1 0 1) plane of elemental Ni. Therefore, it is concluded from the HR-TEM image that the prepared Ni/Ni3(BO3)2/NiO behaves well as a heterostructure on nanoscale. In order to reveal the band structure of the photocatalyst, the SPS of pure NiO and Ni3(BO3)2 were applied in Fig. S4. The band gap for NiO and Ni3(BO3)2 is evaluated to be 3.35 eV and 3.37 eV, respectively. The XPS valence band spectra are shown in Fig. 2c and the energy levels are aligned using the work function of the XPS instrument (4.28 eV). The valence band maximum (VB) is determined to be +0.80 eV and +1.97 eV for NiO [14,15] and Ni3 (BO3)2, respectively (+0.58 eV and +1.75 eV, vs NHE). The conduction band minimum for NiO and Ni3(BO3)2 is 2.55 eV and 1.4 eV (2.77 eV and 1.62 eV, vs NHE). The Fermi energy level of elemental Ni is 0.1 eV(0.32 eV, vs NHE) and accordingly the band structure of Ni/Ni3(BO3)2/NiO is drawn in Fig. 4. Therefore, the photogenerated electrons could transfer from the CB of NiO to Ni3(BO3)2 and enrich in the Fermi energy level of Ni; and the holes could move from the VB of Ni3(BO3)2 to that of NiO, leading to an efficient separation of charge carriers for Ni/Ni3(BO3)2/NiO. To testify the mechanism discussed above, PL spectra and timeresolved PL decay curve which are related to the behaviors of photogenerated electrons and holes are carried out, as shown in

blank TiO2(P25) Ni 3(BO3)2 NiO/Ni3(BO3)2 Ni/Ni3(BO3)2/NiO

3

2

1

0 0

2

4

6

10

8

Time/h Fig. 1. CH4 generation over TiO2, Ni3(BO3)2, NiO/Ni3(BO3)2 and Ni/Ni3(BO3)2/NiO.

Herein, we performed the photo-reduction of CO2 in water under UV light irradiation in the presence of several Ni3(BO3)2 based photocatalysts and TiO2 (Degussa P25) is also applied for comparison. CO is the intermediate product and CH4 is the final product [5]. The photocatalysis results are shown in Fig. 1, Fig. S1 and Table S1. About 0.18 lmol of CH4 is generated in the presence of TiO2(Degussa P25) after 10 h irradiation. The as-prepared Ni3 (BO3)2 represented limited photocatalytic performance, about 0.14 lmol of CH4 was provided. The photocatalytic activity for NiO/Ni3(BO3)2 is slightly enhanced and about 0.16 lmol of CH4 was produced compared with pure Ni3(BO3)2. After the addition with Ni, the photocatalytic activity of Ni/Ni3(BO3)2/NiO is enhanced significantly and almost 0.41 lmol of CH4 was generated, whose specific photocatalytic activity is about twice higher than that of P25. These photocatalytic results indicate that the addition of elemental Ni into NiO/Ni3(BO3)2 heterostructure plays an important role in the photocatalytic process for the Ni/Ni3 (BO3)2/NiO heterostructured photocatalyst. The synthesis procedure is described in the Supporting information. Fig. 2a shows the XRD pattern of the prepared heterostructure.

a

Ni

Ni/Ni3(BO3)2 /NiO

Intensity/a.u.

NiO Ni3 (BO3)2

20

30

40

50

60

70

80

2θ/degree

c Ni NiO Ni 3(BO3)2

Ef

-3

-2

-1

0

1

2

3

4

5

6

15

7

Binding Energy (eV) Fig. 2. XRD patterns (a) and HR-TEM image (b) of Ni/Ni3(BO3)2/NiO. (c) XPS valence band spectra of Ni, NiO and Ni3(BO3)2.

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Y. Yu et al. / Separation and Purification Technology 142 (2015) 14–17

a Intensity/a.u.

Ni 3 (BO3)2 NiO/Ni3 (BO3)2 Ni/Ni 3 (BO3)2/NiO

400

500

600

Wavelength/nm

b

Ni/Ni3 (BO3)2 /NiO

PL intensity (a.u.)

NiO/Ni3(BO3)2 Ni3 (BO3)2

20

22

24

26 Time (ns)

28

30

Fig. 3. Photoluminescence (PL) emission spectra (a) and the time resolved PL decay curve (b) of Ni3(BO3)2, NiO/Ni3(BO3)2 and Ni/Ni3(BO3)2/NiO samples.

Fig. 3. As shown in Fig. 3a, the PL peaks of Ni3(BO3)2 are attributed to the electron transition from the oxygen vacancies to the valence band. In comparison with Ni3(BO3)2, the PL intensity is weakened for NiO/Ni3(BO3)3 and further quenched for Ni/Ni3(BO3)2/NiO. The decrease of PL intensity suggests the photogenerated charge carriers are separated efficiently, contributing from the formation of heterojunction between Ni, NiO and Ni3(BO3)2. Moreover, As shown in Fig. 3b, the s2 value (Table S3) arising from the radiative process of recombination for Ni/Ni3(BO3)2/NiO is much longer than that for NiO/Ni3(BO3)2 and Ni3(BO3)2, implying an effective separation of the photogenerated charge carriers. It is concluded from Fig. 3 that the photo-excited electrons and holes are separated effectively owing to heterostructure. As the photocatalytic activity is closely related to the band structure of heterostructured photocatalyst as well as the redox potential (Eqs. (2) and (3)), the enhanced photocatalytic mechanism for Ni/Ni3(BO3)2/NiO may be explained by the scheme shown

in Fig. 4. Ni3(BO3)2 shows a quite low photocatalytic activity, as its CB and VB (1.62 eV and +1.75 eV, vs NHE) is much more negative and positive than the redox potential (Eo = 0.24 V and 0.82 V, vs NHE)), respectively. Because of the high recombination rate for Ni3(BO3)2 (Fig. 3), seldom photogenerated electrons and holes can participate in the photocatalytic reaction. In comparison with Ni3(BO3)2, the photocatalytic activity of NiO/Ni3(BO3)2 is enhanced, due to the formation of heterostructure between NiO and Ni3(BO3)2. After the addition of elemental Ni, the photocatalytic activity of Ni/Ni3(BO3)2/NiO is improved significantly compared with NiO/Ni3(BO3)2 and Ni3(BO3)2. For Ni/Ni3(BO3)2/NiO, the photogenerated electrons could transfer from the CB of NiO to Ni3(BO3)2 and finally enrich in the Fermi energy level of Ni(0.32 eV vs NHE); and the holes could move from the VB of Ni3(BO3)2 to that of NiO, resulting in an efficient separation of charge carriers. Moreover, as the Fermi energy level of Ni and the VB of NiO (0.32 eV and +0.58 eV, vs NHE) is slightly above the redox potential (Eo = 0.24 V and 0.82 V, vs NHE), the electrons enriched in the Fermi energy level of Ni participate in the photocatalytic reaction to generate CH4 efficiently. Furthermore, the recombination life-time of electrons is effectively prolonged, which is also in favor of the photocatalytic activity. In addition, as the formation of CH4 would need six more electrons than CO, the enriched electron on Ni particles would benefit for the formation of CH4 rather than CO. This explains why Ni/Ni3(BO3)2/NiO heterostructure photocatalyzed more CH4 and less CO than TiO2(P25). Considering about the obtained samples with diameter as large as about 50 nm, the enhanced photocatalytic activity for Ni/Ni3(BO3)2/NiO is mainly attributed to the formation of heterostructure. In summary, Ni/Ni3(BO3)2/NiO heterostructured photocatalyst represents an enhanced photocatalytic activity on reduction of CO2 with H2O into CH4. The formation of heterostructure facilitated the separation of photogenerated charge carriers. The addition of elemental Ni could tune the band structure of photocatalyst to match the redox potential, improving the utilization of electrons and holes involved with the photocatalytic reduction of CO2 into CH4 and resulting in the best photocatalytic activity. Our results may offer a paradigm for designing and developing photocatalysts with high efficiency. Acknowledgement This work is supported by National Natural Science Foundation of China (51072082, 51372120, 51302269 and 21173121). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.seppur.2014. 12.014. References

Fig. 4. Scheme of photocatalytic mechanism for Ni/Ni3(BO3)2/NiO under UV light irradiation.

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