Photocatalytic reduction of CO2 into hydrocarbon solar fuels over g

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Cite this: Phys. Chem. Chem. Phys., 2014, 16, 11492

Photocatalytic reduction of CO2 into hydrocarbon solar fuels over g-C3N4–Pt nanocomposite photocatalysts Jiaguo Yu,*ac Ke Wang,a Wei Xiao*b and Bei Chenga Photocatalytic reduction of CO2 into renewable hydrocarbon fuels is an alternative way to develop reproducible energy, which is also a promising way to solve the problem of the greenhouse effect. In this work, graphitic carbon nitride (g-C3N4) was synthesized by directly heating thiourea at 550 1C and then a certain amount of Pt was deposited on it to form g-C3N4–Pt nanocomposites used as catalysts for photocatalytic reduction of CO2 under simulated solar irradiation. The main products of photocatalysis were CH4, CH3OH and HCHO. The deposited Pt acted as an effective cocatalyst, which not only influenced the selectivity of the product generation, but also affected the activity of the reaction. The yield of CH4 first increased upon increasing the amount of Pt deposited on the g-C3N4 from 0 to 1 wt%, then decreased at 2 wt% Pt loading. The production rates of CH3OH and HCHO also increased with the content of Pt increasing from 0 to 0.75 wt% and the maximum yield was observed at 0.75 wt%. The Pt nanoparticles

Received 10th January 2014, Accepted 16th April 2014

(NPs) could facilitate the transfer and enrichment of photogenerated electrons from g-C3N4 to its surface

DOI: 10.1039/c4cp00133h

for photocatalytic reduction of CO2. At the same time, Pt was also used a catalyst to promote the oxidation of products. The transient photocurrent response further confirmed the proposed photocatalytic reduction

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photoactivity and selectivity of g-C3N4 for CO2 reduction.

mechanism of CO2. This work indicates that the deposition of Pt is a good strategy to improve the

1. Introduction The increasing concerns about inevitable depletion of fossil fuels and the environmental impact of their carbon-footprintintensive combustion have provoked more sustainable energy and environmental solutions.1 Among different alternatives, photocatalytic reduction of CO2 into renewable hydrocarbon solar fuels in the presence of H2O is a promising way to generate reproducible chemical energy.2 The first proofof-concept for CO2 photocatalytic reduction to hydrocarbon compounds was demonstrated with utilization of semiconductors such as TiO2, CdS and GaP by Inoue and co-workers in 1979.3 Afterwards, TiO2 nanocomposites, non-titanium metal oxide and metal-free catalysts were investigated as photocatalysts for CO2 reduction in the presence of water and under light irradiation.4–8 Generally, chemistry of photocatalytic reduction of CO2 is complex, with reported diverse intermediates/products a

State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, P. R. China. E-mail: [email protected]; Fax: +86-27-87879468; Tel: +86-27-87871029 b Department of Environmental Engineering, School of Resource & Environmental Sciences, Wuhan University, Wuhan 430072, P. R. China. E-mail: [email protected]; Fax: +86-27-68775799; Tel: +86-27-68775799 c Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia

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such as methane (CH4), methanol (CH3OH), formaldehyde (HCHO), ethanol (CH3CH2OH), formic acid (HCOOH), and even oxygen (O2).9,10 Thermodynamically, photocatalytic CO2 conversion to CH4, CH3OH, HCHO, CH3CH2OH and HCOOH needs 8, 6, 4, 12 and 2 e with equilibrium potentials appearing at 0.24, 0.38, 0.48, 0.33 and 0.61 V (vs. NHE at pH 7.00), respectively.11,12 Although many groups have studied the mechanism of photocatalytic reduction of CO2 using various catalysts, the mechanism of CO2 reduction still remains unclear. In addition, the vast majority of research focused on TiO2-based photocatalysts, which only showed appreciable photocatalytic activity under UV irradiation.13 In this context, development of novel photocatalysts with high-efficient utilization of the full solar spectrum and specification of the corresponding mechanism of photocatalytic CO2 reduction are of relevance. As an alternative to TiO2, graphitic carbon nitride (g-C3N4) is a stable, novel and non-toxic organic semiconductor material. The band gap of g-C3N4 is around 2.7 eV, corresponding to the strong absorbance in the visible-light region.14 Structurally, g-C3N4 is made up of layered structure which can be synthesized by facile thermolysis methods from cost-affordable precursors such as urea, thiourea, cyanamide, dicyandiamide, and melamine.15–17 And its applications in photodegradation of a variety of pollutions, hydrogen production by water splitting and photocatalytic

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reduction of CO2 were demonstrated.8,18–21 Despite the strong absorbance in the visible-light range, the photoactivity of intrinsic g-C3N4 remains unsatisfactory. Therefore, many attempts were made to modify g-C3N4 for enhanced performance. For example, g-C3N4 modified with a formate anion showed dramatically enhanced photoactivity and stability for Cr(VI) photoreduction under visible-light irradiation.22 It was also reported that g-C3N4–Pt-TiO2 nanocomposites exhibited remarkably enhanced photocatalytic activity for visible-lightdriven hydrogen evolution.23 In this work, we fabricated g-C3N4 and derived Pt-deposited g-C3N4 via facile thermolysis of thiourea and post-impregnation for Pt incorporation. The composites of g-C3N4–Pt were used for photocatalytic reduction of CO2 under simulated solar irradiation. The influence of introduced Pt on photocatalytic activity and reduction products was rationalized.

2. Experimental 2.1

Material syntheses

All chemicals are of analytical-grade and used without further purification. Graphitic carbon nitrides were synthesized by calcining thiourea in a muffle furnace. In a typical synthesis, 10 g of thiourea powder was put into an alumina crucible with a cover, and then the precursor was heated to 550 1C for 2 h at a heating rate of 15 1C min 1.24,25 When cooled to room temperature, the yellow solid was collected and ground to fine powders. The g-C3N4–Pt composites were prepared by using a combined NaOH-assisted impregnation of g-C3N4 with Pt precursors (H2PtCl6) and the NaBH4-reduction process.26 Briefly, 1.0 g of the g-C3N4 was put in a 100 mL beaker, and then 20 mL deionized water was added. After ultrasonication for about 1 h, a certain amount of H2PtCl6 aqueous solution was added into the mixture under magnetic stirring. After 1 h, 5 mL of the mixed solution of NaOH aqueous solution (0.5 M) and NaBH4 aqueous solution (0.1 M) was quickly added into the solution under continuous vigorous stirring for 30 min. Then the samples were collected by centrifugation and washed with deionized water for three times. Finally, the samples were dried at 80 1C overnight. The weight percentage ratios of Pt against g-C3N4 were 0, 0.25, 0.5, 0.75, 1.0 and 2.0 wt%, which were denoted as Pt0, Pt0.25, Pt0.5, Pt0.75, Pt1.0 and Pt2.0, respectively. The real content of Pt in the samples measured by inductively coupled plasma atomic emission spectrometry (ICP-AES) (an Optima 4300 DV spectrometer, Perkin Elmer) is listed in Table 1.

Table 1 Effect of Pt loading on the physical properties of g-C3N4–Pt composite samples

No.

Pt loading/ wt% Composition Color

Pt0 Pt0.25 Pt0.5 Pt0.75 Pt1.0 Pt2.0

0 0.25 0.5 0.75 1.0 2.0

g-C3N4 g-C3N4–Pt g-C3N4–Pt g-C3N4–Pt g-C3N4–Pt g-C3N4–Pt

Yellow Light gray Light gray Gray Gray Gray


SBET/ m2 g 13.0 11.3 11.6 14.2 12.8 9.5

1

Vpore/ cm3 g 0.023 0.017 0.019 0.029 0.024 0.021

1

dpore/ nm 7.1 6.0 6.6 8.2 7.7 9.1

2.2

Characterization

X-ray diffraction (XRD) patterns were obtained on a D/Max-RB X-ray diffractometer (Rigaku, Japan), under Cu Ka radiation at a scan rate (2y) of 0.051 s 1. The accelerating voltage and applied current were 40 kV and 80 mA, respectively. Transmission electron microscopy (TEM) and high resolution TEM (HRTEM) analysis were conducted on a Tecnai G2 F20 S-TWIN microscope at an accelerating voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) measurements were done by using an ultra-high-vacuum VG ESCALAB 210 electron spectrometer equipped with a multichannel detector. The spectrum was excited by Mg Ka radiation of two anodes in the constant analyzer energy mode and operating at 200 W. All the binding energies were referenced to the C 1s peak at 285.0 eV of the surface adventitious carbon. UV-visible absorbance spectra were obtained by using the die-pressed disk sample and a UV-visible spectrophotometer (UV-2550, SHIMADZU, Japan), in which BaSO4 was used as the reflectance standard. Fourier transform infrared spectra (FTIR) of the samples were evaluated using an IR Affiinity-1 FTIR spectrometer which used conventional KBr pellets in the range of 4000–500 cm 1 at room temperature. The Brunauer–Emmett–Teller (BET) specific surface area (SBET) of powder was recorded by using a Micromeritics ASAP 2020 nitrogen adsorption apparatus (USA). All of the samples were degassed at 180 1C before measurement. The pore size distributions were determined using desorption data by the Barrett–Joyner– Halenda (BJH) method. The BET surface area was determined by a multipoint BET method using the adsorption data in the relative pressure (P/P0) range of 0.05–0.25. Photoluminescence (PL) emission spectra were used to investigate the fate of photogenerated electrons and holes in the samples, and were measured at room temperature on an F-7000 Fluorescence Spectrophotometer (Hitachi, Japan). The excitation wavelength is 380 nm, the scanning speed is 1200 nm min 1, and the photomultiplier tube (PMT) voltage is 700 V. The width of both the excitation slit and the emission slit was 1.0 nm. 2.3

Catalytic activity test

The photocatalytic reduction of CO2 was carried out in a 200 mL home-made Pyrex reactor at ambient temperature and atmospheric pressure. The reactor has two openings which were sealed using a silicone rubber septum. A 300 W simulated solar Xe arc lamp was used as the light source and positioned 10 cm above the photocatalytic reactor. In a typical photocatalytic experiment, 100 mg of the sample was put into the glass reactor and 10 mL of deionized water was added. The catalyst was dispersed by ultrasonication for about 30 min to form suspension. After evaporation of water at 80 1C for 2 h, the sample was deposited on the bottom of the reactor in the form of thin films. Before irradiation, the reactor was blown with nitrogen for 30 min to remove air and ensure that the reaction system was under anaerobic conditions. CO2 and H2O vapor were in situ generated by the reaction of NaHCO3 (0.12 g, introduced into the reactor before seal) and HCl aqueous solution (0.25 mL, 4 M) which was introduced into the reactor using a syringe. 1 mL of mixed gas was taken from the reactor at given

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intervals (1 h) during the irradiation and used for gas component analysis by a gas chromatograph (GC-2014C, Shimadzu, Japan) equipped with a flame ionized detector (FID) and a methanizer. Products were calibrated with a standard gas and determined by the retention time. The carrier gas used in the GC-2014C was high purity nitrogen. The product of the experiment was also analyzed by another gas chromatograph (GC-14C, Shimadzu, Japan, TCD, 5 Å molecular sieve column) which also used nitrogen as carrier gas. Blank experiments were carried out in the absence of CO2 or light irradiation to confirm that CO2 and light were two key influencing elements for photocatalytic CO2 reduction. Control experiments were also used to verify whether the carbon resource was derived from CO2 or the catalyst itself. 2.4

Photoelectrochemical measurements

Photocurrent measurements were conducted on a CHI 660D electrochemical workstation (Chenhua Instrument, Shanghai, China) in a conventional three-electrode configuration using a working electrode, a counter electrode and a reference electrode. The working electrodes have an active area of ca. 0.5 cm2, and a Pt wire and Ag/AgCl (saturated KCl) were used as the counter electrode and reference electrode, respectively. A low power LED (3 W, 420 nm) (Shenzhen LAMPLIC Science Co. Ltd. China) was used as the light source and the focused intensity was ca. 80.0 mW cm 2. The electrolyte utilized in the photocurrent measurement is 1 M Na2SO4 aqueous solution. The working electrodes were prepared as follows: 0.2 g of the as-prepared sample was mixed with 0.08 g of polyethylene glycol (PEG, molecular weight: 20 000) and 2 mL of water, then the mixture was ground to form a slurry. Next, the slurry was coated onto a pre-cleaned 2 cm 1.2 cm F-doped SnO2-coated glass (FTO glass) electrode by the doctor blade technique. The prepared electrodes were dried and then calcined at 450 1C for 30 min in an oven with a heating rate of 5 1C min 1.

3. Results and discussion 3.1

Phase and microstructure analyses

XRD was used to reveal phase structures of g-C3N4–Pt nanocomposite photocatalysts. Fig. 1 shows the XRD patterns of the Pt0, Pt0.25, Pt0.5, Pt0.75, Pt1.0 and Pt2.0 samples. Two distinct diffraction peaks can be found in the XRD patterns of all samples. The strong peak at around 27.41 is ascribed to interplanar stacking of aromatic systems, which is indexed to the (002) peak and corresponded to an interlayer distance of d = 0.326 nm.27 The smaller peak (100) at around 13.11 corresponding to a distance of d = 0.675 nm can be attributed to an in-planar structural packing motif.28 The two characteristic diffraction peaks show that the samples are graphitic carbon nitride, which is consistent with the previous reports.21,22,29 The XRD patterns of all samples show no characteristic diffraction peaks attributed to the phase structure of Pt because of its low loading content. Further observation of the XRD patterns shows that diffraction peak positions of all the samples remain


Fig. 1 XRD patterns of the Pt0, Pt0.25, Pt0.5, Pt0.75, Pt1.0 and Pt2.0 samples.

Fig. 2

TEM (a) and HRTEM images (b) of the Pt1.0 sample.

unchanged upon Pt incorporation, suggesting that the Pt is deposited on the surface of g-C3N4 instead of being incorporated into the lattice of g-C3N4. TEM was further used to investigate the microstructure and morphology of the as-prepared Pt1.0 composite. Fig. 2 depicts the TEM and HRTEM images of the Pt1.0 sample. As observed from Fig. 2a, the image shows the existing two-dimensional lamellar structure composed of numerous randomly organized nanosheets with surface deposition of irregular NPs with a diameter of about 3–7 nm.30 The TEM image also shows that nanoplates were curled to form a bubble-like structure and wrinkles (not shown here). Fig. 2b clearly shows that the lattice spacing of NPs is 0.221 nm, corresponding to the lattice spacing of (111) planes of the metallic Pt.26 3.2

XPS analysis

The XPS spectra obtained were used to analyze the chemical composition and the element chemical status of the pure g-C3N4 (Pt0) and g-C3N4–Pt (Pt1.0) composite samples. Fig. 3a shows the comparison of survey spectra of the Pt0 and Pt1.0 samples. The XPS survey spectra of Pt0 and Pt1.0 illustrate that C and N are the main elements and a small amount of O element is also


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Fig. 3 XPS survey spectra (a), high-resolution XPS spectra of the C 1s region (b) and N 1s region (c) for the Pt0 and Pt1.0 samples, and high-resolution XPS spectra of Pt 4f (d) for the Pt1.0 sample.

observed for two samples, and their respective photoelectron peaks emerge at binding energies of 23.5 eV (O 2s), 285 eV (C 1s), 399 eV (N 1s) and 532 eV (O 1s). In addition, the characteristic peak of Pt 4f appears at a binding energy of 71.5 eV, and the peaks at the binding energies of 979.1 and 999.1 eV are assigned to O KLL and the photoelectron peaks at 497.3 and 1072 eV correspond to Na KLL and Na 1s for the Pt1.0 sample.31 The sodium species results from the residual Na+, which is introduced by the process of Pt deposition. Fig. 3b–d display the high-resolution XPS spectra of the elements C 1s, N 1s and Pt 4f, respectively. As observed from Fig. 3b, the C 1s peak can be fitted into three peaks at binding energies of 284.8, 286.9 and 288.6 eV for both Pt0 and Pt1.0 samples, which can be ascribed to C–C, (C)3–N and C–NQC coordination, respectively.14,23 The C–C coordination was attributed to surface adventitious carbon, which comes from the sample and the XPS instrument itself. The high-resolution XPS spectrum of N 1s (Fig. 3c) can be deconvoluted into four peaks appearing at binding energies of 398.4, 399.4, 400.5, and 403.9 eV. The main peaks at around 398.4 and 399.4 eV are ascribed to sp2 hybridized aromatic N bonded to carbon atoms in the form of


CQN–C and the tertiary N bonded to carbon atoms (N–(C)3). The peak at 400.5 eV can be assigned to N–H, and the weakest peak at 403.9 eV is attributed to p-excitations.25 In Fig. 3d, four characteristic peaks are presented at binding energies of 70.9, 72.7, 74.4 and 75.8 eV for Pt1.0. The two peaks emerging at binding energies of 70.9 and 74.4 eV correspond to Pt 4f7/2 and Pt 4f5/2, respectively, which indicate that the partial Pt deposited on the g-C3N4 is metallic.32,33 The peaks appearing at 72.7 and 75.8 eV correspond to Pt 4f7/2 and Pt 4f5/2 of Pt2+, respectively, implying that partial Pt was not completely reduced into metallic Pt in the reduction reaction.34 3.3

BET surface area and pore size distributions

The specific surface areas and porous structures of Pt0 and Pt1.0 were investigated by nitrogen adsorption–desorption isotherms. Fig. 4 presents the nitrogen adsorption–desorption isotherms and the corresponding pore-size distribution curves (inset) of the Pt0 and Pt1.0 samples. According to Brunauer–Deming–Deming–Teller (BDDT) classification,35,36 the isotherms of all the samples are of type IV, indicating the presence of mesopores (2–50 nm).37,38 Further observation indicates that the isotherms of Pt0 shift up


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Fig. 5 UV-vis diffuse reflectance spectra of the Pt0, Pt0.25, Pt0.5, Pt0.75, Pt1.0 and Pt2.0 samples. The inset shows the photos of the Pt0, Pt1.0 and Pt2.0 samples. Fig. 4 Nitrogen adsorption–desorption isotherms and the corresponding pore-size distribution curves (inset) of the Pt0 and Pt1.0 samples.

in contrast to Pt1.0 in all relative pressure ranges, suggesting Pt0 with higher surface areas and larger pore volume. The hysteresis loops of the Pt0 and Pt1.0 samples are of type H3, reflecting the existence of slit-like mesopores due to the aggregation of plate-like particles. This also agrees well with the observed morphology in the TEM images (Fig. 2).39 The pore size distribution curves (inset) calculated from nitrogen desorption isotherms by the BJH method show that the range of pore diameter is 2–120 nm. Fig. 4 (inset) shows that the Pt1.0 sample has less mesopores and smaller pore volume, because some mesopores were sealed with Pt or other chemical reactants in the process of Pt deposition and NaBH4-reduction.40 Table 1 lists the BET surface areas, pore volumes and the average pore diameter of all samples, indicating that the BET surface areas, pore volumes and the average pore diameter of the Pt0, Pt0.25, Pt0.5, Pt0.75, Pt1.0 and Pt2.0 samples almost remain unchanged due to the low Pt loading amount. 3.4

The color change of the samples from Pt0 to Pt1.0 and Pt2.0 also further confirms the presence of metallic Pt NPs in the composite sample. 3.5

IR spectra

Fig. 6 reveals the comparison of FT-IR spectra of pure g-C3N4 (Pt0) and the g-C3N4–Pt composite (Pt1.0). No great difference can be found in the spectra of two samples. All absorption bands are similar to the IR features of the pure g-C3N4 found in the previous reports.43,44 The strong absorption peaks at 1635, 1570, 1404, 1325 and 1243 cm 1 are attributed to the characteristic stretching modes of C–N heterocycles. The typical breathing mode of the tri-s-triazine units at 806 cm 1 can also be found in the spectra.45 The absorption band at 883 cm 1 can be assigned to the deformation mode of N–H.46 The broad bands at around the 3200 cm 1 region can be attributed to the adsorbed H2O molecules and N–H vibration of the uncondensed amine groups.47 The peak at 2380 cm 1 observed in two samples is due to the adsorbed CO2 from the atmosphere.41 No other peaks can be

UV-vis analysis

The electronic structure of the as-prepared g-C3N4–Pt nanocomposites (Pt0, Pt0.25, Pt0.5, Pt0.75, Pt1.0 and Pt2.0) was measured by UV-vis diffuse reflectance spectroscopy. For the pure g-C3N4 (Fig. 5), the absorbance abruptly increases at a wavelength shorter than ca. 455 nm, which can be ascribed to the intrinsic bandgap of g-C3N4. The corresponding bandgap is about 2.7 eV, which is in line with the reported value of g-C3N4 in the literature.41,42 The absorption edge of the composites does not change with the Pt introduction. This also indicates that the Pt is only supported on the surface of g-C3N4 and not incorporated into the lattice of g-C3N4, which is consistent with the above XRD results. Further observation indicates that the background absorption in the visible-light region increases with increasing Pt content, which is in agreement with the color of the sample changing from yellow to gray (see insets in Fig. 5). This is due to the absorbance of black Pt NPs.33


Fig. 6

IR spectra of the Pt0 and Pt1.0 samples.


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found in the spectrum of Pt1.0 assigned to the Pt bonded to other elements. This appears to coincide with the previous XRD results. 3.6

Photocatalytic activity

Control experiments indicated that no hydrocarbon compounds including CH4, CH3OH and HCHO were detected in the absence of either a photocatalyst or irradiation, suggesting that catalysts and light irradiation were two indispensable factors for photocatalytic reduction of CO2, and production of hydrocarbon compounds. To further understand the photocatalytic reduction mechanism of CO2 to hydrocarbon compounds and to exclude possibilities of contamination from pure g-C3N4 and the g-C3N4–Pt composite, further control experiment was performed. The Pt1.0 composite sample or Pt0 itself was illuminated without introducing CO2 as the carbon source, where N2 was used as the carrier gas. If there was any reactive residual carbon in the prepared catalysts, it would react with protons (H+) or H2 generated from competitive water splitting to form hydrocarbon fuels. However, no hydrocarbon compounds were found, indicating that hydrocarbon compounds were not obtained from component carbon or other carbon contamination in our samples. This is also in good agreement with our recently reported results.2b In this study, CO2 was in situ produced by the reaction of NaHCO3 and HCl as the carbon source, and the hydrogen source was provided by H2O in the HCl aqueous solution. The photocatalytic reduction of CO2 was performed at atmospheric pressure and under simulated solar irradiation for 4 h. It is shown in Fig. 7 that the main products of the photocatalytic reaction of CO2 are CH4, CH3OH and HCHO. And other products such as oxygen and carbon monoxide can also be detected using the gas chromatograph equipped with a TCD. No hydrogen is detected. Since the main products (CH4, CH3OH and HCHO) are important industrial resources and fuels, this work mainly focuses on the yield and the generated sequence of the above main products and the effect of Pt content on photocatalytic activity and selectivity of g-C3N4. For the pure g-C3N4 (Fig. 7a), CH3OH is the main product under the UV-visible light irradiation, CH4 and HCHO can only be detected by the GC-2014 after 3 h. It is then concluded that CH3OH is firstly produced in the photocatalytic CO2 reduction. Fig. 7b (Pt0.25) shows that the CH3OH is firstly generated, with the appearance of CH4 and HCHO after 2 h and 3 h. The yield of CH3OH decreased after 2 h when CH4 and HCHO can be detected. It is shown in Fig. 7c (Pt0.5) that CH4 and HCHO can be found after 2 h and the yield of CH3OH increases continuously in 4 h. Upon increasing the Pt amount in the composite to 0.75 wt% (Fig. 7d), CH4 and CH3OH can be detected after 1 h irradiation and HCHO appears after 2 h. And the yield of CH4 becomes comparable with that of CH3OH after 4 h, indicating that the generation rate of CH4 increases faster in comparison with CH3OH. For the Pt1.0 sample (Fig. 7e), CH4, CH3OH and HCHO were detected after 1 h irradiation, suggesting that the three products were generated at the same time. Surprisingly, the yield of CH4 is larger than that of CH3OH after 3 h. However, upon further increasing the Pt amount in the composite to 2.0 wt% (Pt2.0), HCHO can only be detected after 3 h of irradiation and


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the yields of CH4 and CH3OH are lower than Pt1.0 (Fig. 7f), which illustrates that the activity of Pt2.0 is inferior to Pt1.0. It is clearly seen from Fig. 8 that the yield of CH4 increases along with the content of Pt deposited on the g-C3N4 increasing from 0 to 1.0 wt%. And the yields of CH3OH and HCHO increase upon increasing Pt content from 0 to 0.75 wt% on the g-C3N4. Therefore, the content of Pt is a key influencing factor for the photocatalytic CO2 reduction on the g-C3N4–Pt catalyst. Fig. 7 also shows that the generated rates of all products become decreased or leveled off after 4 h of continuous irradiation under UV-visible light. 3.7

The mechanism of photocatalytic reduction of CO2

Photocatalytic reduction of CO2 is a multiple-electron step, which involves photoexcited electron and hole generation as well as transfer, C–O bond breaking, C–H and O–H bond formation. Thermodynamically, one CO2 molecule needs 8, 6 and 4 e to be converted into CH4, CH3OH and HCHO, respectively (as shown in eqn (1–4)).11,13,48 While the photoexcited holes react with adsorbed water to produce oxygen (eqn (5)).49,50 The generated oxygen tends to react with the products to generate CO2 and H2O, resulting in the observed decrease in the generation rate of products. At the same time, CH3OH might be oxidized to HCHO by generating oxygen in the closed system, contributing to a decreased generation rate of CH3OH. g-C3N4 + UV-visible light - h+ + e

(1)

CO2 + 8H+ + 8e - CH4 + 2H2O

(2)

CO2 + 6H+ + 6e - CH3OH + H2O

(3)

CO2 + 4H+ + 4e - HCHO + H2O

(4)

2H2O + 4h+ - O2 + 4H+

(5)

The deposition of Pt on the g-C3N4 is a key factor for the photocatalytic reduction of CO2, which influences the yield and the generation rate of products. The metal Pt is a good catalyst for many oxidation and reduction reactions.51 In this work, the Pt is a cocatalyst which facilitates the interfacial electron transfer from g-C3N4 to Pt NPs and prevents the recombination of photoexcited electron–hole pairs. So upon increasing the content of Pt from 0 to 1 wt%, the contact area of g-C3N4 and Pt NPs increased. Thus, more electrons transferred and gathered on the surface of the Pt NPs to react with CO2 and H2O to convert them into hydrocarbon fuels.52 As can be seen from Fig. 8, the evolution of CH3OH and HCHO increases as the Pt deposited on the g-C3N4 increases from 0 to 0.75 wt%, but the yield decreases upon introduction of more Pt (above 1.0 wt%). Such a tendency is ascribed to the simultaneous evolution and oxidation decomposition of HCHO under light irradiation.26 The Pt NPs deposited on the surface of g-C3N4 can also lower the overpotential, as well as promote the electron transfer for the CO2 reduction.53,54 To further confirm the above suggested photocatalytic mechanism, the transient photocurrent responses of the Pt0, Pt1.0 and Pt2.0 electrodes were monitored by using typical switch on–off cycles of irradiation. Fig. 9 shows a comparison of photocurrent–time (I–t)


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Fig. 7 Time courses of photocatalytic CH4, CH3OH, and HCHO production over the Pt0 (a), Pt0.25 (b), Pt0.5 (c), Pt0.75 (d), Pt1.0 (e) and Pt2.0 (f) samples under simulated solar irradiation.

curves of the Pt0, Pt1.0 and Pt2.0 samples with typical switch on–off cycles of intermittent visible light irradiation at a bias potential of 0.6 V vs. Ag/AgCl. The photocurrent responded quickly in three electrodes when the samples were exposed to the visible light irradiation, and the photocurrent value gradually decreased to zero as soon as the irradiation of light was switched off, which displayed


good reproducibility of the samples. The completely reversible phenomenon of photoresponsivity indicated that most of the photogenerated electrons were transported to the back contact across the sample to produce photocurrent under visible light irradiation.23,55 For the Pt0 sample, the photocurrent response rapidly boosts and slowly decreases to reach a constant value as the


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Fig. 8 Comparison of the photocatalytic CH4, CH3OH and HCHOproduction rate of the Pt0, Pt0.25, Pt0.5, Pt0.75, Pt1.0 and Pt2.0 samples under simulated solar irradiation for 4 h.

Fig. 9 Transient photocurrent responses of the Pt0, Pt1.0 and Pt2.0 samples in 1 M Na2SO4 aqueous solution under visible-light irradiation at 0.6 V vs. Ag/AgCl.

irradiation of light is switched on. While the photocurrent curve of Pt1.0 has an obvious photocurrent spike at the initial time of irradiation. After the spike current is obtained, the photocurrent value continuously decreases to a constant current with time. The photocurrent value decay indicates that the recombination of photogenerated electron–hole pairs occurs in the process of irradiation. The holes accumulating on the surface of the samples (Pt0, Pt1.0 and Pt2.0) competitively recombine with electrons from the conduction band rather than being trapped or captured by reduced species in the electrolyte, which leads to the photocurrent decay.56 When the irradiation is switched off, the photocurrent values of the Pt0 and Pt1.0 samples decrease gradually to zero due to the charge carriers released from the traps. This indicates that many traps existing in the g-C3N4 and the Pt NPs could separate and store the photogenerated electrons, reducing the recombination rate of the photogenerated electrons and holes and enhancing the value of photocurrent.56


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Fig. 10 Comparison of photoluminescence (PL) spectra of the Pt0, Pt1.0 and Pt2.0 samples.

Therefore, the photocurrent value of Pt1.0 is higher than that of Pt0 because the recombination rate of electron–hole pairs of the former is lower than that of the latter, thus causing the enhancement of the photocatalytic reduction rate. In contrast, for the Pt2.0 sample, its photocurrent value is smaller than that of the Pt1.0 sample. This is because Pt overload possibly shields the light absorption of the Pt2.0 sample and becomes the recombination center of photogenerated electrons and holes. Further observation indicates that when light is switched off, the photocurrent of the Pt2.0 sample decreases more rapidly than other two samples, implying that overloaded Pt NPs possibly become the conductor of photogenerated electrons. To verify the above explanation, the PL analysis of the Pt0, Pt1.0 and Pt2.0 samples was performed. Generally, PL emission is from the recombination of photogenerated electrons and holes, so PL intensity is used to compare the fate of photogenerated charge carriers. The stronger the PL peak intensity, the higher the recombination efficiency is.57 Fig. 10 presents the PL spectra of the Pt0, Pt1.0 and Pt2.0 samples. As can be seen, the shape of PL spectra of Pt1.0 and Pt2.0 is similar to that of Pt0. The PL emission peak at around 460 nm, which is very close to the absorption of g-C3N4, is ascribed to the band–band PL phenomenon.14 However, the peak intensity of Pt1.0 and Pt2.0 is obviously lower than that of Pt0, indicating that the loading of Pt greatly reduces the recombination of photogenerated charge carriers. This is because the photogenerated electrons can transfer to Pt NPs, thus reducing the recombination of photogenerated charge carriers. Particularly, the PL peak intensity of Pt1.0 is the lowest among three samples, also implying Pt1.0 with the highest photocatalytic activity.58

4. Conclusions Graphitic carbon nitride g-C3N4 was synthesized by directly heating thiourea at 550 1C and a certain amount of Pt was deposited on g-C3N4 to form g-C3N4–Pt nanocomposites, which


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were used as catalysts for photocatalytic reduction of CO2 under simulated solar irradiation. The content of Pt displayed a significant influence on the activity and selectivity of g-C3N4 for photocatalytic reduction of CO2 into CH4, CH3OH and HCHO. The Pt NPs were shown to be an effective cocatalyst, which not only influenced the selectivity of the product generation, but also affected the photoactivity of the g-C3N4. The yields of CH4 and CH3OH increased upon increasing the content of Pt deposited on the g-C3N4 from 0 to 1 wt%, and the yield of HCHO increased as the Pt content increased from 0 to 0.75 wt% but decreased when more Pt is deposited. What’s more, CH4 and HCHO can be generated more quickly upon increasing the Pt loading from 0 to 1 wt%. The Pt acted as a cocatalyst which could facilitate electron transfer and enrich more photogenerated electrons on its surface for photocatalytic reduction of CO2. At the same time, Pt also acts as a catalyst to promote the oxidation of the HCHO product. The transient photocurrent responses further confirm the proposed mechanism of photocatalytic CO2 reduction. This work not only indicates that the photoactivity and selectivity of g-C3N4 for CO2 reduction can be mediated by a small amount of deposited Pt NPs, but also shows that CO2 can be easily converted into valuable hydrocarbon solar fuels at ambient temperature and pressure.

Acknowledgements This work was partially supported by the 973 program (2013CB632402), NSFC (51320105001, 51372190 21177100 and 51272199), Fundamental Research Funds for the Central Universities (WUT: 2013-VII-030) and Self-determined and Innovative Research Funds of SKLWUT (2013-ZD-1).

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