Non-Precious Molybdenum Nanospheres as A Novel Cocatalyst for Full-Spectrum-Driven Photocatalytic CO2 Reforming to CH4

Journal Pre-proof Non-Precious Molybdenum Nanospheres as A Novel Cocatalyst for Full-Spectrum-Driven Photocatalytic CO2 Reforming to CH4 Shaolong Huang (Conceptualization) (Methodology) (Software) (Validation) (Formal analysis) (Investigation) (Writing - original draft), Huan Yi (Conceptualization) (Methodology) (Software) (Investigation) (Writing - original draft), Luhong Zhang (Software) (Validation) (Formal analysis) (Investigation) (Resources) (Writing review and editing), Zhengyuan Jin (Software) (Validation) (Formal analysis) (Investigation) (Resources), Yaojia Long (Formal analysis) (Investigation) (Resources), Yiyue Zhang (Investigation) (Resources) (Data curation), Qiufan Liao (Data curation), Jongbeom Na (Writing - review and editing), Hongzhi Cui (Supervision), Shuangchen Ruan (Supervision), Yusuke Yamauchi (Writing review and editing) (Supervision), Toru Wakihara (Writing - review and editing), Yusuf Valentino Kaneti (Writing - review and editing), Yu-Jia Zeng (Writing - review and editing) (Supervision) (Project administration) (Funding acquisition)

PII:

S0304-3894(20)30312-5

DOI:

https://doi.org/10.1016/j.jhazmat.2020.122324

Reference:

HAZMAT 122324

To appear in:

Journal of Hazardous Materials

Received Date:

16 January 2020

Revised Date:

14 February 2020

Accepted Date:

14 February 2020

Please cite this article as: Huang S, Yi H, Zhang L, Jin Z, Long Y, Zhang Y, Liao Q, Na J, Cui H, Ruan S, Yamauchi Y, Wakihara T, Valentino Kaneti Y, Zeng Y-Jia, Non-Precious Molybdenum Nanospheres as A Novel Cocatalyst for Full-Spectrum-Driven Photocatalytic

CO2 Reforming to CH4 , Journal of Hazardous Materials (2020), doi: https://doi.org/10.1016/j.jhazmat.2020.122324

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier.

Non-Precious Molybdenum Nanospheres as A Novel Cocatalyst for Full-Spectrum-Driven Photocatalytic CO2 Reforming to CH4

Shaolong Huanga, Huan Yib, Luhong Zhanga, Zhengyuan Jina, Yaojia Longa, Yiyue Zhanga, Qiufan Liaoa, Jongbeom Nac, Hongzhi Cuid, Shuangchen Ruana, Yusuke Yamauchic, Toru

a

ro

of

Wakiharae, Yusuf Valentino Kanetic and Yu-Jia Zeng*a

Shenzhen Key Laboratory of Laser Engineering, College of Physics and Optoelectronic

b

-p

Engineering, Shenzhen University, Shenzhen, 518060, China

International Collaborative Laboratory of 2D Materials for Optoelectronics Science and

c

lP

University, Shenzhen, 518060, China

re

Technology of Ministry of Education, Institute of Microscale Optoelectronics, Shenzhen

International Research Center for Materials Nanoarchitechtonics (WPI-MANA), National

ur na

Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan d

College of Civil Engineering, Shenzhen University, Shenzhen, 518060, China

e

Graduate School of Engineering, The University of Tokyo, 7 Chome-3-1 Hongo, Bunkyo

Jo

City, Tokyo 113-8654, Japan

*

Corresponding author: [email protected]

Graphical abstract

1

of

The Mo nanospheres were loaded on the surface of g-C3N4 via a facile in-situ solvothermal

-p



ro

Highlights

method.

The Mo nanospheres induced the raised CO2 adsorption and photo-thermal-driven activation.



The decorated Mo nanospheres trapped the photo-induced electrons to retard the recombination of charge carriers.

The loading of Mo nanospheres resulted in the formation of strong band tails with extending

lP



re



Abstract

ur na

spectral response from the UV to the NIR region.

Photocatalytic CO2 reforming is considered to be an effective method for clean, low-cost, and environmentally friendly reduction and conversion of CO2 into hydrocarbon fuels by utilizing

Jo

solar energy. However, the low separation efficiency of charge carriers and deficient reactive sites have severely hampered the efficiency of photocatalytic CO2 reforming process. Therefore, cocatalysts are usually loaded onto the surface of semiconductor photocatalysts to reduce the recombination of charge carriers and accelerate the rates of surface reactions. Herein, molybdenum (Mo) nanospheres are proposed as a novel non-precious cocatalyst to significantly

2

enhance the photocatalytic CO2 reforming of g-C3N4. The Mo nanospheres boost the adsorption of CO2 and activate the surface CO2 via photothermal effect. The time-resolved fluorescence decay spectra reveals that the lifetime of photo-induced charge carriers is prolonged by the Mo nanospheres, which guarantees the migration of charge carriers from g-C3N4 to Mo nanospheres. Unexpectedly, Mo loaded g-C3N4 can effectively utilize spectral range from UV to near-infrared region (NIR, up to 800 nm). These findings highlight the potential of Mo nanospheres as a novel

ro

of

cocatalyst for photocatalytic CO2 reforming to CH4.

Keywords: Photocatalytic CO2 reforming; CH4 evolution; Cocatalyst; Molybdenum

Jo

ur na

lP

re

-p

nanospheres; g-C3N4

3

1. Introduction The rising energy crisis and climate change problems contributed by excessive consumption of fossil fuels have boosted research interests on CO2 capture, storage, and utilization.[1-5] CO2 capture and reuse to fuels and fine chemicals are therefore highly demanding for energy and sustainable environmental research.[6] Artificial photosynthesis process that can effectively

of

utilize solar energy to transform CO2 to simpler and valuable hydrocarbon fuels is highly desired.[7-10] Such process can not only reduce the greenhouse effect but also produces valuable

ro

chemicals for alternative energy supplies.[11-13] However, various intrinsic issues, including low separation efficiency of charge carriers and deficiency of reactive sites are considered to be

-p

the bottlenecks of the photocatalytic conversion technology.[14-18] Even with the assistance of

re

various kinds of sacrificial agents, most photocatalysts exhibited relatively low conversion efficiency.[19, 20]

lP

To address the issues, cocatalysts are usually incorporated onto the surface of semiconducting photocatalysts to reduce the recombination rate of charge carriers and accelerate

ur na

surface reaction rates.[21-23] Among all cocatalysts, Pt is the most prominent candidate and often shows the highest performance.[24-26] However, the practical value of Pt-based cocatalysts is not high because of its scarcity and high cost.[27, 28] Therefore, the development of highly active and cost-efficient alternative cocatalysts is of utmost importance. The discovery of highly

Jo

efficient non-precious cocatalysts is expected to pave the avenue toward the practical application of novel and affordable photocatalysts. To date, a wide variety of transition metals, such as nickel (Ni), cobalt (Co), molybdenum

(Mo), iron (Fe), and tungsten (W) and their derivative compounds, has been investigated as cocatalysts.[28] Among these, the transition metal, Mo, is identified theoretically as a promising

4

candidate for CO2 reforming by reducing the reaction barriers and changing the reaction pathways.[29] The doping of Mo into g-C3N4 can significantly widen the spectral response range and increase the separation rate of photo-induced charge carriers.[30, 31] In addition, Mo-C and Mo-N bonds can be formed as a result of the strong interfacial interaction between Mo atoms and g-C3N4, which enables effective electron and hole transfer to enhance the separation efficiency of charge carriers.[32] However, Mo nanostructures have never been experimentally studied as a

of

cocatalyst for photocatalytic CO2 reforming to CH4. In addition, g-C3N4 is considered to be a

ro

rising non-metal photocatalyst which has gained tremendous attentions due to its visible-light response, easy preparation, nontoxicity, low cost, good stability.[7, 9, 33] Herein, for the first

-p

time, Mo nanospheres are demonstrated as a novel cocatalyst to g-C3N4. Following the loading of

re

Mo nanospheres, g-C3N4 exhibited enhanced adsorption and CO2 activation, while also prolonging the life of charge carriers. Furthermore, such loading also unexpectedly extends the

lP

spectral response from UV to NIR region, thereby providing a good platform for in-depth

ur na

understanding of the cocatalytic mechanism.

2. Experimental Details

Melamine powder (99%), hydrogen peroxide (H2O2, AR, 30 wt%), Mo powder (99.9%) and ethanol solution (AR) were obtained from Aladdin, China. All chemical reagents were used as

Jo

received without further purification. 2.1. Synthesis of g-C3N4

The g-C3N4 sample was prepared by a common method as follows: melamine powder (10 g)

was ground for at least 1 h in an agate mortar and subsequently placed into an alumina crucible with a cover. Next, the melamine powder was subjected to thermal treatment at 550 °C for 2 h

5

with a ramping rate of 20 °C min-1. The obtained g-C3N4 product was then ground to a fine powder for further use. 2.2. Synthesis of Mo/g-C3N4 hybrids A facile solvothermal method was employed to prepare the Mo/g-C3N4 hybrid. Briefly, H2O2 (150 μL) was first added to a 50 mL Teflon container containing 10 mg of Mo powder. Next, ethanol solution (29 mL) was added and magnetically stirred for at least 30 min to obtain

of

an ethanolic solution of (MoO2(OH)(OOH).[34] Following this, a certain amount of g-C3N4 was

ro

resuspended into the above solution by ultrasonic treatment. The Teflon vessels were transferred into stainless steel autoclaves and heated at 140 oC for 12 h. After naturally cooled to ambient

-p

temperature, the as-prepared product was collected by centrifugation at 10000 rpm for 30 min,

re

rinsed with ethanol three times, and finally dried under vacuum. The loading of the Mo nanospheres onto the surface of g-C3N4 was controlled by adjusting the amount of g-C3N4 used

lP

for the solvothermal process. The amount of g-C3N4 nanosheets used was 10, 50, 100, 200 and 300 mg during the solvothermal reaction. In our case, we selected the sample synthesized using

ur na

200 mg of g-C3N4 as the optimum sample for further investigations, which is referred to as Mo/g-C3N4 throughout this manuscript. 2.3. Characterization

The phase compositions of the samples were analyzed by X-ray diffraction (XRD) using a

Jo

Bruker D8 powder diffractometer with Cu-Kα radiation (λ=1.5418 Å). Functional group characterizations of the samples were conducted using a Nicolet 6700 Fourier transform infrared (FTIR) spectrophotometer. Morphological characterizations of the samples were performed using Carl Zeiss MERLIN scanning electron microscope (SEM) coupled with EDS mapping (Bruker QUANTAX 200 XFlash) at 10 kV. Transmission electron microscopy (TEM) observations were conducted with a JEM-2100 and Aztec Energy TEM SP X-MaxN 80T, respectively, at 200 kV. 6

Surface elemental states of the products were checked by X-ray photoelectron spectroscopy (XPS) using a Thermo Scientific ESCALAB 250Xi spectrometer employing an Al-Kα source. The obtained XPS spectra were calibrated in reference to the binding energy of C 1s peak at 284.6 eV. The UV-visible-NIR (UV-vis-NIR) spectra were collected using a Hitachi U-4100 spectrophotometer. CO2 adsorption isotherms were obtained using Micromeritics ASAP 2020. The photoluminescence (PL) spectra and time-resolved fluorescence decay spectra were collected

of

using a fluorescence spectrophotometer (Model FL920, Edinburgh Instruments, UK) which

ro

employed a xenon (Xe) lamp with an excitation wavelength of 325 nm. The time-resolved fluorescence decay was calculated by the following equation [35]:

-p

I ( t )  A1 e x p (  t /  1 )  A 2 e x p (  t /  2 )

(1)

re

where τ1 and τ2 represent the fluorescence lifetime, A1 and A2 represent the relative amplitudes. The fluorescence lifetime τ1 (short lifetime) was assigned to the non-radiative recombination of

lP

the photo-induced electrons with the surface defects; the fluorescence lifetime τ2 (long lifetime) was derived from the bandgap charge-carriers recombination. Furthermore, the average

ur na

fluorescence lifetimes were calculated according to the equation [36]: A1 1  A 2 2 2

A=

2

(2)

A1 1  A 2 2

2.4. Photocatalytic CO2 reforming to CH4

Jo

The typical photocatalytic CO2 reforming to CH4 experiment was carried out with a Xe lamp

(300 W) containing 100 mg of photocatalyst. Briefly, a square glass (4 cm × 4 cm) with the photocatalyst being spread equally, was placed horizontally in the center of a 492 mL cylindrical glass reactor. Next, some CO2 gas (99.999%) was aerated into the glass reactor with an airflow velocity of 0.3 L min-1 for 1 h to fill the reactor and exclude the other gas. Next, the photocatalyst was laid horizontally under the lamp at a distance of 10 cm to ensure direct irradiation on the 7

catalyst. After that, deionized water (~2 mL) was injected into the sealed reactor as holes sacrificial agent. The glass reactor temperature was maintained within 28 oC by the cooling system. Every 2 h, the gaseous mixture (400 μL) was extracted from the reactor and immediately injected into a gas chromatograph. This chromatograph consisted of a flame ionization detector (FID) and a packed column, in order to evaluate the generated amount of CH4. The Xe lamp coupled with a UV cut-off filter (>420 nm) was used to obtain the visible light. The NIR light

of

was implemented by using the Xe lamp coupled with a UV-vis cut-off filter (>800 nm).

ro

2.5. Photoelectrochemical measurements

The transient photocurrent of photocatalysts were measured on a CHI660E electrochemical

-p

workstation under a 300 W Xe lamp. The working electrode was prepared as follows: 10.0 mg of

re

the photocatalyst was dispersed in 450 μL of ethanol and ultrasonicated for 30 min to form a homogeneous solution. Next, 25 μL of the solution was coated onto a FTO glass electrode and

lP

dried in air as 200 oC. Pt wire and Ag/AgCl were employed as counter and reference electrodes,

ur na

respectively. The transient photocurrent was recorded in a 0.5 M Na2SO4 aqueous solution.

3. Results and Discussion

A two-step method was used to fabricate Mo/g-C3N4 hybrids. In the first step, a conventional polymerization process was employed to synthesize g-C3N4. In the second step, Mo

Jo

nanospheres were decorated on the g-C3N4 surface by a facile solvothermal approach. The morphologies and microstructures of the as-synthesized hybrids were investigated by SEM and TEM. The SEM image in Fig. 1a shows that the bulk g-C3N4 has a wrinkled surface.[37, 38] The TEM image (Fig. 1b) proves that the bulk g-C3N4 possesses layered structure, which can be stripped into the 2D morphology via ultrasonication.[39] After the solvothermal process, a large

8

amount of Mo nanospheres with diameters of 100-200 nm are embedded on the surface of g-C3N4, as shown in Fig. 1c. The intimate contact between the Mo nanospheres and g-C3N4 can be observed from the TEM image shown in Fig. S1. Further observations via EDS elemental mapping suggest that the Mo nanospheres are uniformly loaded on the g-C3N4 surface. XRD measurements were carried out to investigate the phase structure and composition of all samples. In Fig. 2a, two obvious diffraction peaks of g-C3N4 at 2θ = 13.2° (100) and 27.4°

of

(002) are observed, which correspond to the in-plane intervals of periodic tri-s-triazine and the

ro

interplanar stacking of aromatic systems, respectively.[40, 41] The new peaks observed on the XRD pattern of the Mo/g-C3N4 hybrid can be perfectly indexed as Mo (PDF No. 42-1120),[42]

-p

indicating that Mo nanospheres were successfully loaded onto the surface of g-C3N4 through a

re

simple solvothermal process. The coverage density of the Mo nanospheres on the g-C3N4 surface can be easily controlled by regulating the amount of g-C3N4 used during the solvothermal process

lP

(Fig. S2a). Furthermore, the (100) peak of g-C3N4 in the hybrid is not shifted compared to that of pure g-C3N4 (Fig. S2b), suggesting that Mo is not doped into the lattice of pristine g-C3N4.[30]

ur na

FTIR spectrum (Fig. 2b) of the pure g-C3N4 shows the typical stretching vibration bands belonging to C-N heterocycles (1200-1650 cm-1), N-H (3100-3300 cm-1), and the breathing mode of s-triazine ring units (811 cm-1). These IR bands indicate that the chemical structure of g-C3N4 has not been destroyed after exfoliation.[43] Compared with the IR spectrum of pure g-C3N4, the

Jo

IR spectrum of the Mo/g-C3N4 hybrid exhibits a weak shift to lower wavenumber in the vibration peak position of the s-triazine rings and the shift is increased with the decrease of g-C3N4 amount in the Mo/g-C3N4 hybrid (Fig. S3). This result shows that chemical interaction exists at the interface of Mo and g-C3N4,[32] giving rise to the structural distortion of g-C3N4.[44] To confirm how the Mo nanospheres are bonded on the g-C3N4 surface, XPS measurements were performed. For pure g-C3N4, the C 1s spectrum (Fig. 2c) shows three distinct peaks at 9

284.6, 286.1 and 288.2 eV, indexed to the sp2 C-C bonds of graphitic carbon, sp3-bonded carbon, and sp2-bonded carbon, respectively. Similarly, the N 1s spectrum exhibits three major peaks at 398.5, 399.0 and 400.5 eV, attributed to the sp2-bonded N (C-N=C), tertiary nitrogen groups, and amino groups (Fig. 2d), respectively, further confirming the formation of g-C3N4. After the loading of Mo nanospheres onto the surface of g-C3N4, both C 1s and N 1s peaks of g-C3N4 are shifted to higher binding energies. These results suggest that electron transfer occurs from g-C3N4

of

to Mo nanospheres. The electron transfer process is most likely to take place at the Mo/g-C3N4

ro

interface, leading to reduced and increased electron screening effects for g-C3N4 and Mo, respectively.[45] The peaks at 235.82 eV and 232.65 eV in the Mo 3d spectrum of Mo/g-C3N4

-p

(Fig. S4a) are attributed to Mo 3d3/2 and Mo 3d5/2, respectively, in MoOx. The O 1s spectrum of

re

Mo/g-C3N4 (Fig. S4b) shows the related binding energy for MoOx.[46] These results imply that the surface of Mo is inevitably oxidized. Note that no zero-valence-state Mo is detected probably

lP

due to the limited detection depth of XPS. The XPS results confirm the strong synergistic effect between Mo and g-C3N4, which leads to electron transfer process at the interface during the

ur na

photocatalytic reaction.

UV-vis-NIR spectroscopy was used to investigate the light absorption properties of the as-synthesized samples. As shown in Fig. 3a, pure g-C3N4 has an intrinsic absorption edge located at ~467 nm, suggesting a band gap of 2.65 eV. It is obvious that the loading Mo

Jo

nanospheres onto the g-C3N4 surface can realize a full-spectrum absorption. Therefore, the Mo/g-C3N4 hybrids were measured for photocatalytic CO2 reforming to CH4. The photocatalytic activity of the Mo/g-C3N4 hybrid (6.0 µmol g-1 h-1) for CH4 evolution is 3.2 times higher than that of pure g-C3N4 (1.9 µmol g-1 h-1) (Fig. 3b). Furthermore, the CH4 evolution amounts of the Mo/g-C3N4 hybrids are confirmedly relied on the amount of Mo nanospheres loaded on the

10

surface of g-C3N4 (Fig. S5). Upon visible-light (> 420 nm, Fig. 3c) irradiation, the enhancement factor defined as the ratio of CH4 evolution rate between the Mo/g-C3N4 hybrid (3.8 µmol g-1 h-1) and pristine g-C3N4 (1.5 µmol g-1 h-1), decreases to 2.5. It can be deduced that the broadened spectral region results in a weakened excitation of semiconductor, leading to reduced charge transfer at the interface of Mo and g-C3N4. Unexpectedly, when the spectral region is further extended to the NIR region (λ > 800 nm, Fig. 3d), the Mo/g-C3N4 hybrid also exhibits a passable

of

photocatalytic activity for CH4 evolution with a rate of 1.9 µmol g-1 h-1. This proves that the Mo

ro

nanospheres can work as a cocatalyst to collect the interfacial charge in the Mo/g-C3N4 hybrid for promoting the charge carrier separation of g-C3N4.[24, 47] As a result, the hybrid possesses a

-p

good full-spectrum-driven photocatalytic activity of CH4 evolution. In addition, the recycling test

re

(Fig. S6) indicates that the photocatalytic CH4 evolution activity of the Mo/g-C3N4 hybrid is nearly constant after three cycles for 24 h, suggesting the outstanding stability and activity of the

lP

hybrid photocatalyst.

It is widely accepted that introduction of transition metals into semiconductors contributes to

ur na

light absorption and CO2 adsorption with activation.[48] As shown in Fig. 4a, the introduced Mo nanospheres promote increased CO2 adsorption to g-C3N4, which is making for the electrons transfer and photocatalysis. The kinetics of charge transfer of g-C3N4 and Mo/g-C3N4 samples were studied by transient photocurrent responses. Fig. 4b reveals that the transient photocurrent

Jo

of g-C3N4 increases rapidly once exposed to light. Interestingly, after several cycles of intermittent on-off irradiation, the photocurrent remains steady and reproducible. The Mo/g-C3N4 hybrid exhibits a stronger photocurrent response than the pure g-C3N4, indicating the photo-induced electrons can rapidly transfer from g-C3N4 to Mo nanospheres, thereby restricting the direct recombination of electrons and holes.

11

In order to further verify the effect of the Mo nanospheres, PL measurements were carried out on both g-C3N4 and Mo/g-C3N4 samples. It is well-known that PL can study the efficiency of charge carrier separation and transfer in semiconductors.[49] The PL spectra in Fig. 4c shows the obvious decrease in the characteristic peak of g-C3N4 (466 nm) after the loading of Mo nanospheres. Furthermore, the time-resolved fluorescence decay curves of the Mo/g-C3N4 hybrid and pure g-C3N4 were also measured. As shown in Fig. 4d, the τ1 (2.9 ns) and τ2 (23.8 ns) values

of

for the Mo/g-C3N4 hybrid are longer than those of pure g-C3N4 (τ1 = 2.6 ns; τ2 = 18.4 ns).

ro

Obviously, the average fluorescence lifetime of g-C3N4 (14.1 ns) is prolonged to 18.5 ns after the loading of Mo nanospheres. The decreased PL intensity and increased fluorescence lifetime

-p

provide strong evidence for the accelerated electron transfer and suppressed recombination of

re

charge carriers by decorating Mo nanospheres on the surface of g-C3N4.[44] Based on the above results, the possible mechanism for the photocatalytic CO2 reforming to

lP

CH4 over the Mo/g-C3N4 hybrid is proposed in Fig. 5. The photocatalytic CO2 reforming process generally involves three main processes (I) CO2 adsorption and activation; (II) carriers excitation

ur na

and transfer to the surface; (III) photocatalytic reaction.[32] For pure g-C3N4, the low light absorption and CO2 adsorption, the low separation efficiency of charge carriers, and the high reaction barrier, remarkably limit its photocatalytic activity. The introduction of Mo nanospheres to the g-C3N4 surface causes increased CO2 adsorption for enhancing the photocatalytic activity.

Jo

Upon illumination, the surface temperature of Mo/g-C3N4 is higher than that of pure g-C3N4 (Fig. S7), indicating that the loading of Mo nanospheres activates the adsorbed CO2 and decreases the CO2 reduction barrier via photothermal effect.[50] Furthermore, the decorated Mo nanospheres can trap the photo-induced charge carriers to restrict recombination of charge carriers, thereby leading to the overall improvement in photocatalytic activity. Moreover, the loading of Mo

12

nanospheres remarkably extends the absorbance edge to 800 nm, which contributes to the NIR photocatalytic CO2 reforming to CH4.[51]

4. Conclusions In summary, Mo nanospheres have been investigated as a novel cocatalyst for g-C3N4 to

of

enhance the photocatalytic CO2 reforming to CH4. SEM, FTIR and XPS characterizations prove that the Mo nanospheres are well-bonded to the g-C3N4 surface. The loaded Mo nanospheres can

ro

increase the adsorption and photo-activation of surface CO2. The studies of the transient photocurrent, PL and the time-resolved fluorescence decay indicate that Mo nanospheres with

-p

finely designed configuration can trap photo-induced electrons and prevent the recombination of

re

charge carriers. At the same time, Mo nanospheres can broaden the absorption spectra of g-C3N4 beyond 800 nm. Such unique properties of Mo nanospheres lead to superior photocatalytic CH4

lP

evolution activity. These findings may encourage further researches on the design and preparation of efficient cocatalytic materials for photocatalytic CO2 reforming and many other

NOTES

ur na

energy conversion systems.

Jo

The authors declare no competing financial interest.

Credit Author Statement Shaolong Huang: Conceptualization, Methodology, Software, Validation, Formal analysis, Investigation, Writing-Original Draft. Huan Yi: Conceptualization, Methodology, Software, Investigation, Writing-Original Draft.

13

Luhong Zhang: Software, Validation, Formal analysis, Investigation, Resources, Writing-Review & Editing. Zhengyuan Jin: Software, Validation, Formal analysis, Investigation, Resources. Yaojia Long: Formal analysis, Investigation, Resources. Yiyue Zhang: Investigation, Resources, Data Curation. Qiufan Liao: Data Curation.

of

Jongbeom Na: Writing-Review & Editing.

ro

Hongzhi Cui: Supervision. Shuangchen Ruan: Supervision.

-p

Yusuke Yamauchi: Writing-Review & Editing, Supervision.

re

Toru Wakihara: Writing-Review & Editing. Yusuf Valentino Kaneti: Writing-Review & Editing.

lP

Yu-Jia Zeng: Writing-Review & Editing, Supervision, Project administration, Funding

ur na

acquisition.

Jo

The authors declare no competing financial interest.

Acknowledgements S. Huang and H. Yi contributed equally to this work. This work was supported by the

Shenzhen

Peacock

Technological

Innovation

Project

under

Grant

No.

KQJSCX20170727101208249, and the Shenzhen Science and Technology Project under Grant

14

Nos. JCYJ20170412105400428 and JCYJ20180507182246321. The authors thank the technical support from The Photonics Centre of Shenzhen University

Appendix A. Supplementary data

of

Supplementary data associated with this article can be found in the online version.

ro

References

[1] L. Yuan, Y.-J. Xu, Photocatalytic conversion of CO2 into value-added and renewable fuels,

-p

Appl. Surf. Sci. 342 (2015) 154-167.

[2] S. Holloway, Storage of fossil fuel-derived carbon dioxide beneath the surface of the earth,

re

Annu. Rev. Environ. Resour. 26 (2001) 145-166.

lP

[3] M. Sevilla, P. Valle-Vigón, A. B. Fuertes, N-Doped Polypyrrole-Based Porous Carbons for CO2 Capture, Adv. Funct. Mater. 21 (2011) 2781-2787.

ur na

[4] J. Wang, L. Huang, R. Yang, Z. Zhang, J. Wu, Y. Gao, Q. Wang, D. O'Hare, Z. Zhong, Recent advances in solid sorbents for CO2 capture and new development trends, Energy Environ. Sci. 7 (2014) 3478-3518.

[5] S. Huang, Y. Long, S. Ruan, Y.-J. Zeng, Enhanced Photocatalytic CO2 Reduction in ACS Omega 4 (2019)

Jo

Defect-Engineered Z-Scheme WO3-x/g-C3N4 Heterostructures, 15593-15599.

[6] P. Bhanja, A. Modak, A. Bhaumik, Supported Porous Nanomaterials as Efficient Heterogeneous Catalysts for CO2 Fixation Reactions, Chem. Eur. J. 24 (2018) 7278-7297.

15

[7] P. Xia, M. Antonietti, B. Zhu, T. Heil, J. Yu, S. Cao, Designing Defective Crystalline Carbon Nitride to Enable Selective CO2 Photoreduction in the Gas Phase, Adv. Funct. Mater. 29 (2019) 1900093. [8] X. Chang, T. Wang, J. Gong, CO2 photo-reduction: insights into CO2 activation and reaction on surfaces of photocatalysts, Energy Environ. Sci. 9 (2016) 2177-2196. [9] W.-J. Ong, L.-L. Tan, Y.H. Ng, S.-T. Yong, S.-P. Chai, Graphitic Carbon Nitride

of

(g-C3N4)-Based Photocatalysts for Artificial Photosynthesis and Environmental Remediation:

ro

Are We a Step Closer To Achieving Sustainability?, Chem. Rev. 116 (2016) 7159-7329.

[10] C. Dong, C. Lian, S. Hu, Z. Deng, J. Gong, M. Li, H. Liu, M. Xing, J. Zhang,

-p

Size-dependent activity and selectivity of carbon dioxide photocatalytic reduction over platinum

re

nanoparticles, Nat. Commun. 9 (2018) 1252.

[11] T. Inoue, A. Fujishima, S. Konishi, K. Honda, Photoelectrocatalytic reduction of carbon

lP

dioxide in aqueous suspensions of semiconductor powders, Nature 277 (1979) 637-638. [12] W. Tu, Y. Zhou, Z. Zou, Photocatalytic Conversion of CO2 into Renewable Hydrocarbon

ur na

Fuels: State-of-the-Art Accomplishment, Challenges, and Prospects, Adv. Mater. 26 (2014) 4607-4626.

[13] S. Rani, N. Bao, S.C. Roy, Solar Spectrum Photocatalytic Conversion of CO 2 and Water Vapor Into Hydrocarbons Using TiO2 Nanoparticle Membranes, Appl. Surf. Sci. 289 (2014)

Jo

203-208.

[14] M. Ou, W. Tu, S. Yin, W. Xing, S. Wu, H. Wang, S. Wan, Q. Zhong, R. Xu, Amino-Assisted Anchoring of CsPbBr3 Perovskite Quantum Dots on Porous g-C3N4 for Enhanced Photocatalytic CO2 Reduction, Angew. Chem. 130 (2018) 13758-13762.

16

[15] Z. Sun, J.M.T.A. Fischer, Q. Li, J. Hu, Q. Tang, H. Wang, Z. Wu, M. Hankel, D.J. Searles, L. Wang, Enhanced CO2 photocatalytic reduction on alkali-decorated graphitic carbon nitride, Appl. Catal. B: Environ. 216 (2017) 146-155. [16] C. Xue, F. Zhang, Q. Chang, Y. Dong, Y. Wang, S. Hu, J. Yang, MIL-125 and NH2-MIL-125 Modified TiO2 Nanotube Array as Efficient Photocatalysts for Pollute Degradation, Chem. Lett. 47 (2018) 711-714.

of

[17] Y.-T. Liao, Y.-Y. Huang, H.M. Chen, K. Komaguchi, C.-H. Hou, J. Henzie, Y. Yamauchi,

ro

Y. Ide, K.C.W. Wu, Mesoporous TiO2 Embedded with a Uniform Distribution of CuO Exhibit Enhanced Charge Separation and Photocatalytic Efficiency, ACS Appl. Mater. Interfaces 9 (2017)

-p

42425-42429.

re

[18] Y.V. Kaneti, S. Dutta, M.S.A. Hossain, M.J.A. Shiddiky, K.-L. Tung, F.-K. Shieh, C.-K. Tsung, K.C.W. Wu, Y. Yamauchi, Strategies for Improving the Functionality of Zeolitic

29 (2017) 1700213.

lP

Imidazolate Frameworks: Tailoring Nanoarchitectures for Functional Applications, Adv. Mater.

ur na

[19] Y. Fu, D. Sun, Y. Chen, R. Huang, Z. Ding, X. Fu, Z. Li, An Amine-Functionalized Titanium Metal-Organic Framework Photocatalyst with Visible-Light-Induced Activity for CO2 Reduction, Angew. Chem. Int. Ed. 51 (2012) 3364-3367. [20] A. Ali, W.-C. Oh, Preparation of Nanowire like WSe2-Graphene Nanocomposite for

Jo

Photocatalytic Reduction of CO2 into CH3OH with the Presence of Sacrificial Agents, Sci. Rep. 7 (2017) 1867.

[21] J. Ran, M. Jaroniec, S.-Z. Qiao, Cocatalysts in Semiconductor-based Photocatalytic CO2 Reduction: Achievements, Challenges, and Opportunities, Adv. Mater. 30 (2018) 1704649. [22] X. Li, J. Yu, M. Jaroniec, X. Chen, Cocatalysts for Selective Photoreduction of CO 2 into Solar Fuels, Chem. Rev. 119 (2019) 3962-4179. 17

[23] J. Yang, D. Wang, H. Han, C. Li, Roles of Cocatalysts in Photocatalysis and Photoelectrocatalysis, Acc. Chem. Res. 46 (2013) 1900-1909. [24] W.-N. Wang, W.-J. An, B. Ramalingam, S. Mukherjee, D.M. Niedzwiedzki, S. Gangopadhyay, P. Biswas, Size and Structure Matter: Enhanced CO2 Photoreduction Efficiency by Size-Resolved Ultrafine Pt Nanoparticles on TiO2 Single Crystals, J. Am. Chem. Soc. 134 (2012) 11276-11281.

of

[25] C. Wang, R.L. Thompson, J. Baltrus, C. Matranga, Visible Light Photoreduction of CO2

ro

Using CdSe/Pt/TiO2 Heterostructured Catalysts, J. Phys. Chem. Lett. 1 (2010) 48-53.

[26] Y.-P. Yuan, S.-W. Cao, Y.-S. Liao, L.-S. Yin, C. Xue, Red phosphor/g-C3N4 heterojunction

-p

with enhanced photocatalytic activities for solar fuels production, Appl. Catal. B: Environ.

re

140-141 (2013) 164-168.

[27] G. Dukovic, M.G. Merkle, J.H. Nelson, S.M. Hughes, A.P. Alivisatos, Photodeposition of Pt

4306-4311.

lP

on Colloidal CdS and CdSe/CdS Semiconductor Nanostructures, Adv. Mater. 20 (2008)

ur na

[28] H. Zhang, P. Zhang, M. Qiu, J. Dong, Y. Zhang, X.W. Lou, Ultrasmall MoOx Clusters as a Novel Cocatalyst for Photocatalytic Hydrogen Evolution, Adv. Mater. 31 (2019) 1804883. [29] P. Li, F. Wang, S. Wei, X. Li, Y. Zhou, Mechanistic insights into CO 2 reduction on Cu/Mo-loaded two-dimensional g-C3N4 (001), Phys. Chem. Chem. Phys. 19 (2017) 4405-4410.

Jo

[30] Y. Wang, Y. Zhang, S. Zhao, Z. Huang, W. Chen, Y. Zhou, X. Lv, S. Yuan, Bio-template synthesis of Mo-doped polymer carbon nitride for photocatalytic hydrogen evolution, Appl. Catal. B: Environ. 248 (2019) 44-53. [31] Y. Wang, Y. Xu, Y. Wang, H. Qin, X. Li, Y. Zuo, S. Kang, L. Cui, Synthesis of Mo-doped graphitic carbon nitride catalysts and their photocatalytic activity in the reduction of CO 2 with H2O, Catal. Commun. 74 (2016) 75-79. 18

[32] R. Zhang, P. Li, F. Wang, L. Ye, A. Gaur, Z. Huang, Z. Zhao, Y. Bai, Y. Zhou, Atomically Dispersed Mo Atoms on Amorphous g-C3N4 Promotes Visible-Light Absorption and Charge Carriers Transfer, Appl. Catal. B: Environ. 250 (2019) 273-279. [33] Z. Jin, J. Chen, S. Huang, J. Wu, Q. Zhang, W. Zhang, Y.-J. Zeng, S. Ruan, T. Ohno, A facile approach to fabricating carbonaceous material/g-C3N4 composites with superior photocatalytic activity, Catal. Today 315 (2018) 149-154.

of

[34] H. Cheng, T. Kamegawa, K. Mori, H. Yamashita, Surfactant-Free Nonaqueous Synthesis of

ro

Plasmonic Molybdenum Oxide Nanosheets with Enhanced Catalytic Activity for Hydrogen Generation from Ammonia Borane under Visible Light, Angew. Chem. Int. Ed. 53 (2014)

-p

2910-2914.

re

[35] X. Liu, Q. Zhang, G. Xing, Q. Xiong, T.C. Sum, Size-Dependent Exciton Recombination Dynamics in Single CdS Nanowires beyond the Quantum Confinement Regime, J. Phys. Chem.

lP

C 117 (2013) 10716-10722.

[36] Y. Zhang, Y. Tang, X. Liu, Z. Dong, H.H. Hng, Z. Chen, T.C. Sum, X. Chen,

ur na

Three-Dimensional CdS-Titanate Composite Nanomaterials for Enhanced Visible-Light-Driven Hydrogen Evolution, Small 9 (2013) 996-1002. [37] L. Zhang, Z. Jin, S. Huang, X. Huang, B. Xu, L. Hu, H. Cui, S. Ruan, Y.-J. Zeng, Bio-inspired carbon doped graphitic carbon nitride with booming photocatalytic hydrogen

Jo

evolution, Appl. Catal. B: Environ. 246 (2019) 61-71. [38] Z. Jin, Q. Zhang, J. Chen, S. Huang, L. Hu, Y.-J. Zeng, H. Zhang, S. Ruan, T. Ohno, Hydrogen bonds in heterojunction photocatalysts for efficient charge transfer, Appl. Catal. B: Environ. 234 (2018) 198-205.

19

[39] L. Jing, R. Zhu, D.L. Phillips, J.C. Yu, Effective Prevention of Charge Trapping in Graphitic Carbon

Nitride

with

Nanosized

Red

Phosphorus

Modification

for

Superior

Photo(electro)catalysis, Adv. Funct. Mater. 27 (2017) 1703484. [40] X. Lu, Y. Wang, X. Zhang, G. Xu, D. Wang, J. Lv, Z. Zheng, Y. Wu, NiS and MoS2 nanosheet co-modified graphitic C3N4 ternary heterostructure for high efficient visible light photodegradation of antibiotic, J. Hazard. Mater. 341 (2018) 10-19.

of

novel

Z-scheme

Bi2WO6

quantum

dots/g-C3N4

ultrathin

nanosheets

ro

fabrication

of

[41] W. Chen, T.-Y. Liu, T. Huang, X.-H. Liu, J.-W. Zhu, G.-R. Duan, X.-J. Yang, In situ

heterostructures with improved photocatalytic activity, Appl. Surf. Sci. 355 (2015) 379-387.

-p

[42] N.R. Denny, F. Li, D.J. Norris, A. Stein, In situ high temperature TEM analysis of sintering

re

in nanostructured tungsten and tungsten-molybdenum alloy photonic crystals, J. Mater. Chem. 20 (2010) 1538-1545.

lP

[43] J. Li, Y. Yin, E. Liu, Y. Ma, J. Wan, J. Fan, X. Hu, In situ growing Bi2MoO6 on g-C3N4 nanosheets with enhanced photocatalytic hydrogen evolution and disinfection of bacteria under

ur na

visible light irradiation, J. Hazard. Mater. 321 (2017) 183-192. [44] Z. Zhang, J. Huang, Y. Fang, M. Zhang, K. Liu, B. Dong, A Nonmetal Plasmonic Z-Scheme Photocatalyst with UV- to NIR-Driven Photocatalytic Protons Reduction, Adv. Mater. 29 (2017) 1606688.

Jo

[45] S. Huang, Z. Jin, H. Yi, Z. Yang, Y. Long, Q. Liao, J. Chen, Y. Cao, S. Ruan, Y.-J. Zeng, Stannous oxide promoted charge separation in rationally designed heterojunction photocatalysts with a controllable mechanism, Dalton Trans. 47 (2018) 12734-12741. [46] M. Anwar, C.A. Hogarth, R. Bulpett, Effect of substrate temperature and film thickness on the surface structure of some thin amorphous films of MoO3 studied by X-ray photoelectron spectroscopy (ESCA), J. Mater. Sci. 24 (1989) 3087-3090. 20

[47] O.K. Varghese, M. Paulose, T.J. LaTempa, C.A. Grimes, High-Rate Solar Photocatalytic Conversion of CO2 and Water Vapor to Hydrocarbon Fuels, Nano Lett. 9 (2009) 731-737. [48] L. Wang, M. Ghoussoub, H. Wang, Y. Shao, W. Sun, A.A. Tountas, T.E. Wood, H. Li, J.Y.Y. Loh, Y. Dong, M. Xia, Y. Li, S. Wang, J. Jia, C. Qiu, C. Qian, N.P. Kherani, L. He, X. Zhang, G.A. Ozin, Photocatalytic Hydrogenation of Carbon Dioxide with High Selectivity to Methanol at Atmospheric Pressure, Joule 2 (2018) 1369-1381.

of

[49] X. She, J. Wu, H. Xu, J. Zhong, Y. Wang, Y. Song, K. Nie, Y. Liu, Y. Yang, M.-T.F.

ro

Rodrigues, R. Vajtai, J. Lou, D. Du, H. Li, P.M. Ajayan, High Efficiency Photocatalytic Water Splitting Using 2D α-Fe2O3/g-C3N4 Z-Scheme Catalysts, Adv. Energy Mater. 7 (2017) 1700025.

-p

[50] J. Li, Y. Ye, L. Ye, F.-Y. Su, Z. Ma, J. Huang, H. Xie, D.E. Doronkin, A. Zimina, J.-D.

re

Grunwaldt, Sunlight Induced Photo-thermal Synergistic Catalytic CO2 Conversion via Localized Surface Plasmon Resonance of MoO3-x, J. Mater. Chem. A 7 (2019) 2821-2830.

lP

[51] M.-Q. Yang, M. Gao, M. Hong, G.W. Ho, Visible-to-NIR Photon Harvesting: Progressive Engineering of Catalysts for Solar-Powered Environmental Purification and Fuel Production,

Jo

ur na

Adv. Mater. 30 (2018) 1802894.

21

of ro

Jo

ur na

lP

re

-p

Fig. 1. a) SEM and b) TEM images of g-C3N4; c) SEM and corresponding EDS elemental mapping images of the Mo/g-C3N4 hybrid. The Mo nanospheres are decorated on the surface of g-C3N4.

22

of ro -p re lP

Jo

ur na

Fig. 2. The structure, surface chemistry state, and elemental composition of g-C3N4 and Mo/g-C3N4 hybrid: a) XRD patterns; b) FTIR spectra; XPS spectra of c) C 1s and d) N 1s.

23

of ro -p re lP

Jo

ur na

Fig. 3. a) UV-vis-NIR absorption spectra of g-C3N4 and Mo/g-C3N4 hybrid; Time-dependent photocatalytic CH4 evolution curves of the g-C3N4 and Mo/g-C3N4 hybrid under b) Xe lamp irradiation, c) visible-light irradiation, and d) NIR-light irradiation.

24

of ro -p re lP

Jo

ur na

Fig. 4. a) CO2 adsorption isotherms; b) Transient photocurrent responses; c) PL spectra and d) time-resolved fluorescence decay spectra of g-C3N4 and Mo/g-C3N4 hybrid.

25

of ro -p

re

Fig. 5. Schematic diagram of the photocatalytic mechanism: photo-induced generation of charge

Jo

ur na

lP

carriers/transfer and photocatalytic reforming of CO2 to CH4.

26