# Photoreduction of CO2 to CH4 over Efficient Z-Scheme γ-Fe2O3/g-C3N4 Composites

Authors: Thanh-Binh Nguyen; Thuy Hang Dinh Thi; Doan Pham Minh; Hien Bui Minh; Ngoc Quynh Nguyen Thi; Bang Nguyen Dinh
Journal: Journal of Analytical Methods in Chemistry
 (2022)

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## Abstract

A series of compositeγ-Fe2O3/g-C3N4 (denoted as xFeCN with x equal 5, 10, 15, and 20 of γ-Fe2O3 percentage in weight) was prepared by calcination and precipitation-impregnation methods. X-ray diffraction (XRD), Fourier transform infrared (FTIR), and X-ray photoelectron spectrometry (XPS) characterizations indicated the successful synthesis of Z-scheme FeCN composites. A red shift of the light absorption region was revealed by UV-vis diffuse reflectance spectroscopy (UV-DRS). In addition, photoluminescence spectroscopy (PL) spectra showed an interface interaction of two phases Fe2O3 and g-C3N4 in the synthesized composites that improved the charge transfer capacity. The photocatalytic activity of these materials was studied in the photoreduction of CO2 with H2O as the reductant in the gaseous phase. The composites exhibited excellent photoactivity compared to undoped g-C3N4. The CH4 production rate over 10FeCN and 15FeCN composites (2.8 × 10−2 and 2.9 × 10−2 μmol h−1 g−1, respectively) was ca. 7 times higher than that over pristine g-C3N4 (0.4 × 10−2 μmol h−1 g−1). This outstanding photocatalytic property of these composites was explained by the light absorption expansion and the prevention of photogenerated electron-hole pairs recombination due to its Z-scheme structure.


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## Body

## 1. Introduction

Carbon dioxide from fossil energy consumption is the most important source of greenhouse gas emissions to the atmosphere, causing global warming [1, 2]. Carbon dioxide capture and storage (CCS) or utilization (CCU) has largely been studied during the last decades [3, 4]. Among them, photocatalytic CO2 conversion into valuable compounds, such as CH4 and CH3OH, is an attractive route [5–7]. Thus, many semiconductors have been investigated as photocatalysts with a particular focus on the development of photocatalytic heterojunction systems by combining various semiconductor materials to form different photocatalyst types such as type I, type II, or especially Z-scheme systems [8–12]. In the type I and type II photocatalysts, there occurs the photogenerated electrons transfer between conduction bands and holes, one between valance bands of each component in a composite. While in the Z-scheme type, this transfer takes place between the conduction and valance band of two adjacent components. By this characteristic, the Z-scheme photocatalyst has the advantage of mobilizing the potential position of the conduction band or valance band of each semiconductor when they are simultaneously combined for targeted reaction. Thereby, Z-scheme photocatalysts are expected to have stronger redox properties, better charge transfer, and higher improved light absorption yield than those of other simple photocatalyst types.Recently, graphitic carbon nitride, g-C3N4, a semiconductor, has attracted researchers as a potential photocatalyst thanks to its easy synthesis, low cost, and moderate bandgap energy of 2.7 eV, which allows the absorption of visible light [13, 14]. In particular, the conduction band (CB) position of g-C3N4 is quite negative (−1.14 eV), which is favorable for the photoreduction of CO2 into almost valuable hydrocarbons such as CH4, HCOOH, CH3OH, and C2H5OH [15]. However, one of the major disadvantages of g-C3N4 is the fast electron-hole pair recombination, the relative high bandgap energy to be able to absorb most of the visible light, occupying 44% solar spectra. To improve these drawbacks, one of the strategies is to combine g-C3N4 with other semiconductors [16–20].Among the semiconductors used as photocatalysts, iron oxide, Fe2O3, is an interesting candidate [21]. One of the outstanding features of this semiconductor is its low synthesis cost and relatively low band gap energy, Eg = 2.2 eV, which allows broad absorption in the visible region of sunlight. Therefore, Fe2O3 oxide has been extensively studied as a photocatalyst through the photodegradation of polluted organic compounds in water [22, 23]. However, for the photoreduction of CO2, the conduction band potential (CB) is positive (+1.58 eV). So, Fe2O3 oxide is not able to reduce this molecule. Combining Fe2O3 oxide with another semiconductor having enough negative conduction band potential is required to create an efficient photocatalyst for CO2photoreduction [24–29]. The improving photocatalytic activity by adding γ-Fe2O3 was also reported in some other works. In the research of Ding et al., for the photoreduction of CO2 in liquid phase to CH3OH [30], a-Fe2O3/g-C3N4 with the weight ratio a-Fe2O3: g-C3N4 of 60 : 40 had the best photocatalytic activity with 2.9-fold than pristine g-C3N4. Duan and Mei [31] also reported the highest amount of CH3OH obtained on 15%Fe2O3/g-C3N4(>3.5 fold than pristine g-C3N4). For CO2 photoreduction in the gas phase, Wong et al. [32] observed the CO formation on dendrite-structured a-Fe2O3/g-C3N4 (27.2 μmol h−1g−1), which was about 2.2 times higher than the one on pure g-C3N4 (10.3 μmol h−1 g−1). On the same type of catalyst with urchin-like a-Fe2O3 morphology, Yong Zhou et al. also recognized a CO production rate of 17.8 μmol g−1 h−1, 3 times higher than that of pristine g-C3N4 (6.1 μmol g−1 h−1) [33]. Besides, Fe2O3/g-C3N4 composites also showed higher photoactivity than pure a-Fe2O3 and g-C3N4 in the photodegradation of organic pollutants (Direct Red 81, Rhodamin B, and tetracycline hydrochloride) [34–36]. These studies showed that the morphology of Fe2O3 oxide seems to play an important role in the activity of catalyst. The different morphologies may change interface interaction between Fe2O3 and g-C3N4 phase, leading to the change in charge carrier, a key factor of photocatalysis.Based on the analysis above, in this study, we have synthesized Z-schemeγ-Fe2O3/g-C3N4 photocatalysts for CH4 production from CO2 photoreduction in the gas phase. This new Z-scheme materiel could take advantage of the low bandgap energy of γ-Fe2O3 oxide and the rather negative conduction band potential of g-C3N4 as well.


## 2. Experimental

### 2.1. Materials

Melamine (C3H6N6), iron (II) sulfate heptahydrate (FeSO4.7H2O), and sodium hydroxide (NaOH) with analytical purity were purchased from Sigma-Aldrich. Deionized water was used as solvent for all preparations.


### 2.2. Synthesis of g-C3N4 and γ-Fe2O3/g-C3N4 Composite

Carbon graphitic nitride, g-C3N4, was synthesized by calcination of melamine at 550oC for 3 hours under the nitrogen atmosphere.A series of x%γ-Fe2O3/g-C3N4 composites was prepared by the precipitation-impregnation method. First, 1 g of g-C3N4, which were synthesized from melamine calcination, was added in a 100 ml solution of 0.1 M NaOH. The mixture was covered by paraffin paper, stirred, and kept at 60oC for 2 hours. Then, a calculated amount of FeSO4.7H2O was slowly added in the solution above, and the pH was adjusted to 10 with 0.1 M NaOH solution, leading to the formation of a composite material of iron oxide precipitate and g-C3N4. The latter was filtered and washed by deionized water and dried at 70oC. Four Z-scheme γ-Fe2O3/g-C3N4 photocatalysts, denoted as xFeCN, with x equal to 5, 10, 15, or 20 wt.% of γ-Fe2O3 were obtained.


### 2.3. Characterization

All composites were characterized by X-ray diffraction (XRD, model Bruker D8), Fourier transform infrared (FTIR, model 8101M Shimazu), X-ray photoelectron spectroscopy (XPS, Thermo VG Scientific MultiLab 2000), UV-vis diffuse reflectance spectroscopy (UV-DRS, model Jaco V-530), photoluminescence spectroscopy (PL, model Horiba FluoroMax-4), transmission electron microscopy (TEM, JEM 1400, Plus Jeon), and scanning electron microscopy-energy dispersive X-ray spectrometry (SEM-EDS, Hitachi TM4000 Tabletop Microscope).


### 2.4. Photocatalytic Procedure

In a typical test, 100 mg of catalyst was added in 15 mL deionized water in a beaker with a 5 cm diameter. The solvent in the mixture was completely evaporated in an oven at 70oC to well disperse the photocatalyst on the bottom of the beaker. After that, the catalyst-containing beaker was placed in a closed stainless reactor equipped with a quark window (Figure 1). The high purity (99,999%) CO2 flow of 500 ml/minute bubbled in a closed stainless contains deionized water kept at 25oC before entering the stainless reactor for 30 minutes. Then, a 150W Xenon lamp (Newport model 67005) was switched on for 18 h.Figure 1
Scheme of the photocatalytic reactor used in this work.


## 2.1. Materials

Melamine (C3H6N6), iron (II) sulfate heptahydrate (FeSO4.7H2O), and sodium hydroxide (NaOH) with analytical purity were purchased from Sigma-Aldrich. Deionized water was used as solvent for all preparations.


## 2.2. Synthesis of g-C3N4 and γ-Fe2O3/g-C3N4 Composite

Carbon graphitic nitride, g-C3N4, was synthesized by calcination of melamine at 550oC for 3 hours under the nitrogen atmosphere.A series of x%γ-Fe2O3/g-C3N4 composites was prepared by the precipitation-impregnation method. First, 1 g of g-C3N4, which were synthesized from melamine calcination, was added in a 100 ml solution of 0.1 M NaOH. The mixture was covered by paraffin paper, stirred, and kept at 60oC for 2 hours. Then, a calculated amount of FeSO4.7H2O was slowly added in the solution above, and the pH was adjusted to 10 with 0.1 M NaOH solution, leading to the formation of a composite material of iron oxide precipitate and g-C3N4. The latter was filtered and washed by deionized water and dried at 70oC. Four Z-scheme γ-Fe2O3/g-C3N4 photocatalysts, denoted as xFeCN, with x equal to 5, 10, 15, or 20 wt.% of γ-Fe2O3 were obtained.


## 2.3. Characterization

All composites were characterized by X-ray diffraction (XRD, model Bruker D8), Fourier transform infrared (FTIR, model 8101M Shimazu), X-ray photoelectron spectroscopy (XPS, Thermo VG Scientific MultiLab 2000), UV-vis diffuse reflectance spectroscopy (UV-DRS, model Jaco V-530), photoluminescence spectroscopy (PL, model Horiba FluoroMax-4), transmission electron microscopy (TEM, JEM 1400, Plus Jeon), and scanning electron microscopy-energy dispersive X-ray spectrometry (SEM-EDS, Hitachi TM4000 Tabletop Microscope).


## 2.4. Photocatalytic Procedure

In a typical test, 100 mg of catalyst was added in 15 mL deionized water in a beaker with a 5 cm diameter. The solvent in the mixture was completely evaporated in an oven at 70oC to well disperse the photocatalyst on the bottom of the beaker. After that, the catalyst-containing beaker was placed in a closed stainless reactor equipped with a quark window (Figure 1). The high purity (99,999%) CO2 flow of 500 ml/minute bubbled in a closed stainless contains deionized water kept at 25oC before entering the stainless reactor for 30 minutes. Then, a 150W Xenon lamp (Newport model 67005) was switched on for 18 h.Figure 1
Scheme of the photocatalytic reactor used in this work.


## 3. Results and Discussion

Figure2(a) shows the XRD patterns of the fresh g-C3N4 and xFeCN photocatalysts. The formation of g-C3N4 by melamine calcination was confirmed by the presence of peaks at 2θ of 13.2 and 27.3° (JCPDS 87–1526). For the composite photocatalysts, the characteristic peaks of γ-Fe2O3 appeared in all patterns with 2θ at 30.2, 35.7, 43.2, 53.8, 57.2, and 62.9° (JCPDS 004–075). The peak intensity of the γ-Fe2O3 phase is proportional to its content in the composite materials.Figure 2
XRD patterns (a) and FTIR (b) spectra of g-C3N4, xFeCN composites.
(a)(b)Figure2(b) shows the FTIR spectra of all fresh photocatalysts. All these materials have similar characteristic signals of g-C3N4 structure. The peak at 804 and 887 cm−1 is attributed to the heptazine stretching mode [37], while those at 1203–1630 cm−1 correspond to stretching vibrations of C=N and C–N bonds of the heterocycle [38, 39]. For the broad band from 3000 to 3500 cm−1, the signals were ascribed to the vibration of NH2 and NH functional groups of g-C3N4 and the OH group of absorbed water [40]. In addition, the weak signals observed at586 cm−1 originated from the Fe–O bond vibrations [41]. These results confirmed the formation of the two expected crystalline phases of g-C3N4 and γ-Fe2O3 in the photocatalysts synthesized. In order to confirm the formation of composite materials between these two components, XPS analysis was performed for one photocatalyst containing 10 wt% of γ-Fe2O3 (Figure 3). The general XPS spectrum (Figure 3(a)) shows the presence of Fe, O, C, and N on the surface of this photocatalyst. On the high-resolution Fe elemental spectrum (Figure 3(b)), there are two major peaks at the binding energy 711.2 and 724.7 eV corresponding to Fe3+2p3/ 2 and Fe3+2p1/2, respectively. These two peaks are characteristics of the Fe3+ oxidation state as observed on the XPS spectrum of Fe2O3 oxide [42, 43]. Besides, two satellite peaks can be detected at positions 718.2 and 731.0 eV. With the C1s spectrum (Figure 3(e)), there are three peaks at 284.9, 286.5, and 288.5 eV. The first peak at 284.9 eV is assigned to C–C/C=C bonds, the second to the carbon of the C–NH2 group, and the third to bonded C in the heptazine structure (N–C=N) [44, 45]. In the N1s spectrum (Figure 3(d)), the peak deconvolution shows the presence of 3 peaks at 399.0, 400.3, and 401.2 eV, which correspond to Sp2 bonded N in triazine ring (C–N=C), the tertiary nitrogen N-(C)3 group in the heptazine structure, and N in the C–N–H group, respectively [39, 40]. For the oxygen element (Figure 3(c)), the peak deconvolution of O1s indicates the existence of three components: lattice oxygen of Fe–O bond (529.7 eV), oxygen of surface hydroxyl–OH (531.0 eV), and the one of adsorbed water H2O (533.3 eV) [43]. Hence, the results of XPS, XRD, and IR proved that γ-Fe2O3/g-C3N4 composites were successfully synthesized.Figure 3
XPS survey of 10FeCN composite (a) and its XPS spectra of Fe2p (b), O1s (c), N1s (d), and C1s (e).
(a)(b)(c)(d)(e)In order to predict the light absorption ability, all composites and g-C3N4 were measured by the UV-DRS method. Figures 4(a) and 4(b) show the obtained results. From the Kubelka–Munk function, the obtained bandgap energy was of 2.63, 2.54, 2.47, 2.25, 1.95, and 1.80 eV for g-C3N4, 5FeCN, 10FeCN, 15FeCN, 20FeCN, and γ-Fe2O3, respectively. So, the increase of γ-Fe2O3 content led to decrease of the bandgap energy. In other words, it means that the light absorption region of catalysts shifted more in the visible light wavelength by increasing the γ-Fe2O3 content. Hence, the photocatalytic activity of composites is expected to be improved.Figure 4
UV-DRS spectra (a) and Kubelka–Munk function (b) of xFeCN composites.
(a)(b)To estimate the recombination of electron-hole pair phenomena, the PL spectroscopy was carried out for g-C3N4, γ-Fe2O3, and 10FeCN composites (Figure 5(a)). As observed, the intensity of PL spectrum of g-C3N4 is nearly twice in comparison with the one of 10FeCN. No signal was observed on PL spectra of Fe2O3 at activated wavelength. This result reflected that the presence of γ-Fe2O3 and its interface interaction with g-C3N4 seem to inhibit the photogenerated electron-hole pairs recombination, which could improve photoactivity.Figure 5
PL spectra of g-C3N4, γ-Fe2O3, and 10FeCN composite. (a) SEM mapping images of different elements in 10FeCN: SEM image (b), Fe distribution (c), O distribution (d), C distribution (e), and N distribution (f).
(a)(b)(c)(d)(e)(f)Figures5(b), 5(c), and 5(d) show the SEM image and different element distribution of the 10FeCN sample. It is obvious that Fe and O elements or Fe2O3 phase were quite homogenously dispersed on the surface of g-C3N4. The elemental composition is given in Table 1. It is noted that the experimental composition is quite.Table 1
Weight percentages of different elements in the composition of 10FeCN.
ElementTheoretical percentExperimental percentFe6.997.26O3.013.24C35.2236.57N54.7852.93The morphology of composite 10FeCN was characterized by TEM (Figure6). It is obvious that the particle Fe2O3 is in cubic form with average size of 10 nm. Apart from γ-Fe2O3 particles dispersed on the g-C3N4 surface, it seems that some these particles were covered by g-C3N4 layer to form core-shell structure Fe2O3@g-C3N4.Figure 6
TEM images of 10FeCN.
(a)(b)The photocatalytic activity was evaluated through photoreduction of CO2. Figure 7(a) shows the obtained results. Methane was the main production of the reaction, while CO was not noticeable. No product was detected on γ-Fe2O3. This is explained by the more positive value of its conduction band potential in comparison of that reduction potential of CO2/CH4 (−0.24 V) [2]. It was noted that the presence of γ-Fe2O3 remarkably impacted the CH4 formation. A volcano-like evolution of methane formation as a function of γ-Fe2O3 is observed. Concretely, under the same operational conditions, the amount of CH4 formed is 0.4 × 10−2, 2.1 × 10−2, 2.8 × 10−2, 2.9 × 10−2, and 1.0×10−2μmol g−1 h−1 for g-C3N4, 5FeCN, 10FeCN, 15FeCN, and 20FeCN, respectively. Hence, the maximum CH4 formation quickly increased when rising the γ-Fe2O3 content and reached the maximum with 10FeCN and 15FeCN composites. Thus, compared to pristine g-C3N4, the produced amount of CH4 over 10FeCN and 15FeCN composites was 7 fold. However, at higher γ-Fe2O3 content, e.g., 20wt.% Fe2O3, CH4 production dropped rapidly to 1.0×10−2μmol g−1 h−1. In comparison with the research on urchin-like Fe2O3/g-C3N4 and dendrite-structured Fe2O3/g-C3N4 catalysts for CO2 photoreduction in the gas phase, CH4 gas was the preferred product on this catalyst, instead of CO [32, 33]. Hence, the obtained results show interesting photocatalytic activity of the synthesized composite materials and also prove the good combination of two phases, γ-Fe2O3 and g-C3N4, to make new active photocatalysts. The small formed CH4 amount was probably due to the low power of xenon lamp (only 150W). To justify these results, a blank test (without catalyst) and a test on g-C3N4 under N2 gas were carried out. No CH4 amount was detected for these ones. In addition, the test was performed also on the mixture of 10% (wt) of γ-Fe2O3 with g-C3N4 (Figure 7(a)), and the CH4 product is under detectable limit that could be evidence for the absence of phase interaction between γFe2O3 and g-C3N4 as well as the presence of γ-Fe2O3 hindering the illumination on g-C3N4.Figure 7
CH4 production on g-C3N4 and different xFeCN composites (a); proposed mechanism of CO2 photoreduction on Z-scheme structure of γ-Fe2O3/g-C3N4 (b).
(a)(b)Generally, the outstanding photoactivity ofγ-Fe2O3/g-C3N4 composite materials compared to g-C3N4 is assigned to better light-harvesting ability and charge transfer than those of single phase of γ-Fe2O3 or g-C3N4. It should be emphasized that the charge transfer process in the Z-scheme structure not only reduces photoelectron-hole pair recombination but also makes photocatalyst, the redox potential, stronger: CB potential more negative and VB potential more positive. In addition, it seems that the quite small size particles of γ-Fe2O3 as observed in TEM images, about 10 nm of diameter, have improved the interface interaction of composite γ-Fe2O3/g-C3N4. The increase of γ-Fe2O3 quantity superior of 15% (wt) has led to the decrease of photoactivity. This is probably due to the strong sintering of γ-Fe2O3 nanoparticles, causing less interface interaction. Besides, the accumulation of γ-Fe2O3 nanoparticles could cover g-C3N4 surface and inhibits the irradiation on this phase. Hence, the γ-Fe2O3 quantity of 10% (wt) was the most suitable to obtain the highest photoactivity.From UV-DRS spectrum and Mulliken electronegativity theory, the valance band (VB) and conduction band (CB) ofγ-Fe2O3 and g-C3N4 were calculated [30]. According to this theory, ECB = X−Ec−0.5Eg, where ECB is the conduction band edge energy, X is the electronegativity of the semiconductor (equal 5.825 eV for Fe2O3 and 4.72 eV for g-C3N4), Ec, equal 4.5 eV, is the energy of free electrons with hydrogen scale, and Eg is the bandgap energy of semiconductor. Basing on this equation and concrete value of each parameter, it is found that VB and CB of γ-Fe2O3 and g-C3N4 are +2.23, +0.43 eV and +1.53, −1.10 eV, respectively. Figure 7(b) shows a possible photoreduction mechanism of CO2 photoreduction into CH4. According to this Z-scheme composite, all the two phases, γ-Fe2O3 and g-C3N4, harvested the irradiated light and generated electron-hole pairs. The photogenerated electron located on CB of γ-Fe2O3 migrated and recombined with photogenerated holes located on the valance band (VB) of g-C3N4. This process allows broadening the light absorption region and also prevents recombination of photogenerated electron-hole pairs that both improved the photocatalytic activity. The details of this process are presented through the following equations:(1)γ−Fe2O3/g−C3N4+hν⟶γ−Fe2O3hVB++g−C3N4eCB−,(2)CO2+8H++8eCB−⋅g−C3N4⟶CH4+2H2O,(3)8H2O+8HVB+γ−Fe2O3⟶8H+⋅+8⋅OH,(4)8⋅OH⟶4H2O+2O2.


## 4. Conclusions

In this work, different Z-scheme photocatalysts,γ-Fe2O3/g-C3N4, were synthesized by simple methods of calcination and impregnation-precipitation. XRD, IR, and XPS characterizations of these materials confirmed the formation of composite structure of these photocatalysts, wherein γ-Fe2O3 grew on g-C3N4 surface. SEM analysis highlighted a good distribution of Fe on the surface of g-C3N4 support. The UV-DRS and PL spectra of the photocatalyst containing 10 wt.% γ-Fe2O3 evidenced interface interaction of the two phases of γ-Fe2O3 and g-C3N4.The catalytic performance of these materials was evaluated through the CO2 photoreduction in the gaseous phase. Methane was found as the main product, while no trace of CO was observed. According to CH4 production, the photocatalytic activity followed the following order: 15FCN (2.9×10−2 μmol g−1 h−1)   10FeCN (2.8×10−2 μmol g−1 h−1) > 5FeCN (2.1×10−2 μmol g−1 h−1) > 20FeCN (1.0 × 10−2 μmol g−1 h−1) > g-C3N4 (0.4×10−2 μmol g−1 h−1). Hence, 10FeCN and 15FeCN composites had the highest photoactivity, which is approximately 7 times higher than that on bulk g-C3N4. This could be explained by a synergy combination and interaction of the two phases γ-Fe2O3 and g-C3N4, leading to the Z-scheme structure composites, which improved light absorption with red-shift light and also diminishes photogenerated electron-hole pairs recombination. These outstanding results are promising for the design of a low-cost and highly efficient photocatalyst for CO2 reduction on the basis of γ-Fe2O3 and g-C3N4.



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