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A new application of Z-scheme Ag3PO4/gC3N4 composite in converting CO2 to fuel Yiming He, Lihong Zhang, Bo-Tao Teng, and Maohong Fan Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 08 Dec 2014 Downloaded from http://pubs.acs.org on December 9, 2014
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Environmental Science & Technology
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A new application of Z-scheme Ag3PO4/g-C3N4 composite
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in converting CO2 to fuel 1,2
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Yiming He, 2Lihong Zhang, 1,3Botao Teng, 1,4Maohong Fan*
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Department of Chemical & Petroleum Engineering, University of Wyoming, Laramie, Wyoming, 82071,
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United States 2
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Department of Materials Physics, Zhejiang Normal University, Jinhua, 321004, China
College of Chemistry and Life Sciences, Zhejiang Normal University, Jinhua, 321004, China
Department of Chemical & Petroleum Engineering, University of Wyoming, Laramie, Wyoming, 82071, United States
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Corresponding author: Tel: +1-307-766-5633; Fax: +1-307-766-6667; E-mail:
[email protected] 14
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ABSTRACT. This research was designed for the first time to investigate the activities of photocatalytic
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composite, Ag3PO4/g-C3N4, in converting CO2 to fuels under simulated sunlight irradiation. The composite
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was synthesized using a simple in-situ deposition method and characterized by various techniques including
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Brunauer–Emmett–Teller method (BET), X-ray diffraction (XRD), Fourier transform infrared spectroscopy
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(FT-IR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray
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photoelectron spectroscopy (XPS), UV-vis diffuse reflectance spectroscopy (DRS), photoluminescence
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spectroscopy (PL), and an electrochemical method. Thorough investigation indicated that the composite
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consisted of Ag3PO4, Ag, and g-C3N4. The introduction of Ag3PO4 on g-C3N4 promoted its light absorption
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performance. However, more significant was the formation of hetero-junction structure between Ag3PO4
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and g-C3N4, which efficiently promoted the separation of electron-hole pairs by a Z-scheme mechanism, and
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ultimately enhanced the photocatalytic CO2 reduction performance of the Ag3PO4/g-C3N4. The optimal
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Ag3PO4/g-C3N4 photocatalyst showed a CO2 conversion rate of 57.5 µmol·h-1·gcat-1, which was 6.1 and 10.4
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times higher than those of g-C3N4 and P25, respectively, under simulated sunlight irradiation. The work
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found a new application of the photocatalyst, Ag3PO4/g-C3N4, in simultaneous environmental protection and
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energy production.
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Graphical abstract
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1. INTRODUCTION
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Carbondioxide (CO2) is one of the major contributors to the greenhouse effect on Earth’s atmosphere,
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with the resulting climate change responsible for environmental problems such as increased severe weather
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events and rising sea levels. Accordingly, extensive research has been directed toward CO2 conversion. The
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photocatalyic conversion of CO2 into useful solar fuels (such as CO, CH4, or CH3OH) is considered one of
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the more promising strategies to address this crisis1-7. Since Inoue et al.1 first reported photocatalytic CO2
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reduction in a semiconductor aqueous suspension to produce hydrocarbon fuels, many research groups have
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studied the photocatalytic mechanism and efficiency of CO2 reduction over various semiconductors
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including Zn2GeO4,8 ZnGaO4,9 NaNbO310 and CaFe2O4.11 However, these semiconductor photocatalysts
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exhibit unsatisfactory photoactivity, leading researchers to recognize that, due to the disadvantages of
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limited visible-light harvesting and rapid charge recombination, a single-component semiconductor
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photocatalyst is inadequate for efficient generating solar fuels. Increasingly, researchers have instead turned
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their attentions to composite photocatalysts consisting of different semiconductors, including TiO2/ZnO,12
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CdS/TiO2,13 and Pt-CuOx-TiO2.14 The resulting hetero-junctions in these composites can markedly diminish
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the recombination of electron-hole pairs, thereby improving their photocatalytic efficiency.
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Due to its high stability and responsiveness to visible light, graphitic carbon nitride (g-C3N4) is a novel,
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non-metallic semiconductor that has attracted significant attention. It has been reported that g-C3N4
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demonstrates high photocatalytic performance for water splitting, dye degradation, and CO2 reduction.15-17
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However, the photoactivity of g-C3N4 is limited by its moderate band gap (Eg=2.7 eV) and fast
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recombination of electron-hole pairs. Therefore, scientists have made significant efforts to improve the
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photocatalytic activity of g-C3N4 by coupling it with other semiconductors, such as LnVO4 (Ln=Sm, Dy, Bi,
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Gd, La),18-22 CdS,23 AgX (X=Cl, Br, I),24-25 TaON 26 and S.27 Among these, Ag compounds have been shown
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to significantly promote the photoactivity of g-C3N4, due largely to the fact that Ag compounds are not
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stable and can generate Ag nanoparticles under light irradiation. The plasmonic effect induced by the
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formed Ag nanoparticles can drastically enhance the visible-light absorption of the photocatalyst, optimize
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the absorption of incident light in a thin layer (∼10 nm) under the surface, and promote the separation of 4 Environment ACS Paragon Plus
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electron-hole pairs, resulting in a high photoactive Ag doped g-C3N4 composite.24-25 An Ag3PO4
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semiconductor has been reported as an active visible-light-driven photocatalyst for dye degradation and
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oxygen evolution from water splitting,28-29 and has also been used as a semiconductor dopant to promote the
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photocatalytic activity of g-C3N4.30-31 For example, He et al. reported an Ag3PO4 photocatalyst hybridized
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with g-C3N4, along with its application for the photodegradation of RhB.30 The results showed that the
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photocatalytic activity of such a composite material was enhanced after the doping of g-C3N4. Kumar et al.
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reported that doping Ag3PO4 on g-C3N4 increased the photoactivity of g-C3N4 in methyl orange degradation
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by a factor of 5,31 with similar results also obtained by Jiang et al.32 It is interesting that both of these efforts
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investigated the catalyst’s photoactivity only in dye degradation and did not apply the composite in the
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reaction of H2 generation or CO2 photoreduction. One possible reason is that both of Kumar and Jiang might
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have thought the Ag3PO4/g-C3N4 composite followed a double charge transfer mechanism. The
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photogenerated electrons enrich on the Ag3PO4 semiconductor and holes on g-C3N4, and the low conduction
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band of Ag3PO4 (ECB=0.45 eV) indicates that the electrons cannot reduce H+ or CO2.31 Therefore, the
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Ag3PO4/g-C3N4 composite is not considered a suitable photocatalyst for CO2 reduction. However, many
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scientists have suggested that Ag compounds containing composite photocatalysts might follow a Z-scheme
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mechanism.33-35 For example, in a BiOBr/AgBr photocatalyst, the photogenerated electrons from the BiOBr
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semiconductor with a lower conduction band (CB) would recombine with photogenerated holes from the
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AgBr having a higher valance band (VB) with the help of Ag nanoparticles.33 By this means, electrons could
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remain on the AgBr in order to reduce O2 to O2- species (E=-0.046 eV),36 an effect which cannot be
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generated on BiOBr due to its positive conduction band potential (ECB=+0.22 eV). Hence, if the Ag3PO4/g-
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C3N4 photocatalyst follows the Z-scheme mechanism, the composite might also be an efficient photocatalyst
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for CO2 reduction. However, to the best of our knowledge, no corresponding research has been reported,
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which has led us to focus on the actual mechanism of the Ag3PO4/g-C3N4 photocatalyst.
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In order to resolve the question suggested above, an Ag3PO4/g-C3N4 composite photocatalyst was
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synthesized using an in-situ deposition method and investigated in photocatalytic CO2 reduction for the first
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time. The loading of Ag3PO4 remarkably promoted the photocatalytic activity of g-C3N4 on CO2
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photoreduction, proving that the system works in the Z-scheme way. After thorough investigation of 5 Environment ACS Paragon Plus
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structure, surface area, and optical properties, the origin of the higher photoactivity of the Ag3PO4/g-C3N4
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composite was discussed.
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2. EXPERIMENTAL SECTION
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All chemicals were reagent grade (Alfa-Aesar, USA) and purchased commercially without further
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purification. Porous graphitic carbon nitride (g-C3N4) was synthesized by heating urea in air at 550 oC for 4
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h. For preparation of the Ag3PO4/g-C3N4 composite photocatalysts, 0.15 g of g-C3N4 powder and different
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amounts of AgNO3 were dissolved in 100 mL of deionized water and ultrasonicated for 30 min. A certain
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amount of Na2HPO4 solution was then dropped into the solution under vigorous stirring. After stirring for 4
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h, the product was irradiated by a 500 w Xe lamp for one hour. The resulting solid product was collected by
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centrifugation, washed with distilled water, and dried in an oven at 60 oC for 24 h. In this manner, different
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molar ratios of the Ag3PO4 to g-C3N4 samples (i.e. 10 %, 20%, 30%, 40%, and 50% Ag3PO4/g-C3N4), were
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obtained and denoted as 10AC, 20AC, 30AC, 40AC, and 50AC, respectively. Ag/g-C3N4 and Ag3PO4
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samples were prepared using the same procedures as 30AC except that no Na2HPO4 or g-C3N4 was added.
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The photocatalytic reduction of CO2 to fuels and the photocatalytic degradation of RhB were
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undertaken in order to investigate the photoactivity of the catalyst. The detailed information and
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characterizations are listed in the supporting information.
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3. RESULTS AND DISCUSSION
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3.1 Characterizations of the Ag3PO4/g-C3N4 composites
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The BET surface area of g-C3N4 is 50.3 m2/g, which is equal to that of P25 (50.0 m2/g) and much larger
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than that of Ag3PO4 (2.0 m2/g). The addition of Ag3PO4 decreases the surface area. The BET surface area of
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the 10 AC, 20 AC, 30 AC, 40 AC and 50AC samples are 35.4, 28.7, 20.4, 19.0 and 16.0 m2/g, respectively.
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The introduction of Ag nanoparticles shows little effect on the surface area. The specific surface area of
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Ag/g-C3N4 is equal to 47.8 m2/g.
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Figure 1. TEM images of g-C3N4, Ag3PO4 (a), and 30AC (b).
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The morphologies of pure g-C3N4, Ag3PO4, and the Ag3PO4/g-C3N4 composite were investigated by
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SEM and TEM. Pure Ag3PO4 exhibits as spherical particles with smooth surfaces (Figure S2a), while the g-
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C3N4 polymer is an aggregation of many wrinkled sheets with irregular shapes (Figure S2b). In the SEM
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image of the Ag3PO4/g-C3N4 composite, it can be observed that Ag3PO4 nanoparticles coat the surface of the
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g-C3N4 (Figure S2c). The hybrids structure can be further verified by the backscattered electron image
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(Figure S2d), since Ag3PO4 particles are brighter than g-C3N4 due to the latter’s heavy atomic weight. TEM
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images of the three samples show their microstructure. Small pores with an average size of 50 nm are
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observed in the g-C3N4 sheet (Figure 1a), indicating its mesoporous structure, which is consistent with
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previous results.37-38 The particle size of Ag3PO4 obtained from its TEM image (Figure 1a), is much smaller
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than that from the SEM image, suggesting the large particle size distribution of Ag3PO4. For the Ag3PO4/g-
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C3N4 composite (Figure 1b), the coated large Ag3PO4 particles are not observed, which might be attributed
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to these particles peeling off during the ultrasonic treatment (30 min) before TEM analysis. However, small
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Ag3PO4 nanoparticles definitely adhere closely to the g-C3N4 sheet. (At first glance, more black particles can
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be observed.) Under electron beam irradiation, these Ag3PO4 particles decompose and disappear rapidly
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(Figure S3).
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Figure 2a shows the XRD patterns of the Ag3PO4/g-C3N4 composite. Pure Ag3PO4 is in the body-
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centered cubic structure (JCPDS No. 06-0505), while g-C3N4 shows its characterization peaks at 27.4 o and
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13.0 o, which can be indexed to the (002) and (100) diffraction plane of the graphite-like carbon nitride.16 7 Environment ACS Paragon Plus
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The Ag3PO4/g-C3N4 composite samples exhibit diffraction peaks corresponding to both g-C3N4 and Ag3PO4.
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With the increase in Ag3PO4 concentration, the diffraction peaks of Ag3PO4 intensify gradually at the
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expense of g-C3N4 peaks, reflecting their contents in the Ag3PO4/g-C3N4 hybrids. It should be noted that,
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although all the samples were irradiated by Xe lamp for one hour, no diffraction peak corresponding to Ag
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nanoparticles can be observed, indicating that the formed Ag nanoparticles have a small particle size. The
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same phenomenon can also be observed in the Ag/g-C3N4 composite, which exhibits the same XRD patterns
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as pure g-C3N4.
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Figure 2. XRD patterns (a), FT-IR (b) and XPS (c,d) spectra of g-C3N4, Ag3PO4, Ag/g-C3N4 and Ag3PO4/g-
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C3N4 composites.
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The FT-IR spectra of Ag3PO4, g-C3N4 and a series of Ag3PO4/g-C3N4 samples are shown in Figure 2b.
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Pure g-C3N4 exhibits several strong characteristic peaks in the range of 1200-1700 cm-1, which may be
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ascribed to the typical stretching vibration of CN heterocycles.16 In addition, the peak at 808 cm-1 may
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correspond to the breathing mode of triazine units.16 For Ag3PO4, the observed strong peaks at 541 cm-1 and
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928 cm-1 may be attributed to the characteristic peaks of PO43-.30,32 The FT-IR spectra of Ag3PO4/g-C3N4
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composites represent the overlap of the spectra of both g-C3N4 and Ag3PO4. The intensity of the peak at 928
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and 541 cm−1 increases with an increase of Ag3PO4 content. Meanwhile, a blue shift of the two IR peaks in
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the Ag3PO4/g-C3N4 hybrids can be observed. This increase in frequency (blue shift) is a general
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phenomenon observed in nanostructured materials due to their small size effects.39 However, the SEM and
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TEM experiments cannot prove that the Ag3PO4 particles in Ag3PO4/g-C3N4 have smaller particle size than
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pure Ag3PO4. It is possible that the interactions between Ag3PO4 and g-C3N4 phases might contribute to the
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blue shift of the PO43- IR peak, indicating that the synthesized Ag3PO4/g-C3N4 composite is not a simple
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physical mixture of the two semiconductors.
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The interaction between g-C3N4 and Ag3PO4 in the Ag3PO4/g-C3N4 composite was also verified by
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XPS. All signals of C, N, Ag, P and O are detected in the survey XPS spectrum of the 30AC composite
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(Figure 2c), indicating its hybrids structure, which is consistent with the XRD and FT-IR experiments.
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Actually, a similar result is also shown in the high-resolution XPS spectra of C1s (Figure S3). Two peaks at
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284.6 eV and 287.7 eV, which can be attributed separately to the contaminated carbon and the N-C-N
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coordination in graphitic carbon nitride,40 are observed in the C1s spectrum of 30AC, indicating the
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existence of a g-C3N4 phase in the composite. The binding energy (BE) of C1s is not changed, nor is that of
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P2p. The BE of P 2p is determined to be 132.4 eV (Figure S4), corresponding to P5+ in Ag3PO4.41 However,
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due to the doping of Ag3PO4, the BE of N1s (Figure 2d) shifts from 397.7 eV to 398.0 eV which verifies the
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interaction of Ag3PO4 and g-C3N4 as previously reported in the literature.42-43 This type of interaction is
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beneficial for the formation of hetero-junction between the two semiconductors and subsequently promotes
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the separation of electron-hole pairs in the composites.42-43 The Ag 3d5/2 and 3d3/2 BE of Ag3PO4 are located
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at 367.8 eV and 373.8 eV, respectively (Figure S4), which are very close to the BE values of Ag+ in Ag2O.44
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Fore Ag/g-C3N4, the two values are 368.5 eV and 374.5 eV, corresponding to metallic Ag (Figure 2c and 9 Environment ACS Paragon Plus
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Figure S4)45. The Ag 3d peaks of 30AC sample exhibit a slight positive shift as compared to the pure
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Ag3PO4, which originates in the formed Ag nanoparticles during the simulated sunlight irradiation process.
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Figure 3. EIS changes (a), transient photocurrent responses (b), and PL spectra (c) of g-C3N4, Ag3PO4, and
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30AC samples; (d) UV-vis spectra of Ag3PO4/g-C3N4 composites.
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The effect of the formed Ag3PO4/g-C3N4 hetero-junction on the separation efficiency of electron-hole
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pairs was investigated by electrochemical impedance spectroscopy (EIS) and photoelectric current (PC)
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experiments. Figure 3a shows the EIS changes of g-C3N4, Ag3PO4, and 30AC electrodes. In general, the
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smaller arc in an EIS Nyquist plot indicates a smaller charge–transfer resistance on the electrode surface.46-47
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The data in Figure 3a show that relative arc sizes for the three electrodes is 30AC
