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Sulfur Doped g-CN/BiVO Composite Photocatalyst for Water Oxidation under Visible Light Hyung Jun Kong, Da Hye Won, Jungmo Kim, and Seong Ihl Woo Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b04178 • Publication Date (Web): 15 Feb 2016 Downloaded from http://pubs.acs.org on February 15, 2016

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Chemistry of Materials

Sulfur Doped g-C3N4/BiVO4 Composite Photocatalyst for Water Oxidation under Visible Light Hyung Jun Kong,† Da Hye Won,‡ Jungmo Kim,∥ and Seong Ihl Woo*,†,‡ Graduate School of EEWS (BK21PLUS), ‡Department of Chemical and Biomolecular Engineering, ∥Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Daejeon, 34141, Republic of Korea. †

Summary: Composite photocatalyst with BiVO4 and sulfur doped g-C3N4 was developed through impregnated co-precipitation. During impregnation, the surface of sulfur doped g-C3N4 was modified resulting in advantageous composition with BiVO4. Photocatalytic water oxidation was enhanced by 129% and charge carrier lifetime was extended by 349 % than pristine BiVO4. ABSTRACT: To achieve sustainable utilization of solar energy, development of efficient photocatalyst for water oxidation, the driving force of reductive solar fuel formation, is highly required. Herein, composite photocatalysts with bismuth vanadate (BiVO4) and sulfur doped graphitic carbon nitride (SCN) are developed by using one-pot impregnated precipitation method. FT-IR and XPS analyses demonstrate that the surface of SCN is oxidized during impregnation and the oxidized surface becomes the synthetic site for BiVO4 composition. Among the composites with various ratios, B7S catalyst, which is our best achievement, shows 750 μmol h-1 g-1 of oxygen evolution rate that is over twofold higher than that of pristine BiVO4 (i.e. 328 μmol h-1 g-1) in identical reaction condition of 0.05 M AgNO3 aqueous solution under visible light irradiation (λ > 420 nm). The photonic efficiency of B7S is also measured as 19 %. The mechanism behind this is the enhanced charge carrier lifetime of B7S, which is lengthened up to 4 times compared to BiVO4 (3.14 ns and 0.70 ns, respectively) due to the facilitated charge separation through composite.

1. Introduction Solar energy is a promising alternative energy source which satisfies the estimated future energy demands.1 Hence, conversion of solar energy to viable form is recently receiving great attention. Solar fuels, such as methanol, formate, carbon monoxide and hydrogen, are formed through the photocatalytic reduction of CO2 or proton in water by consumption of electrons from photon-induced excitons.2 In order to sustain the reduction reaction, effective photocatalytic hole consuming process, such as water oxidation, should be included for prevention of exciton recombination. Therefore, enhancement of photocatalytic water oxidation efficiency is crucial to successful solar fuel production. Bismuth vanadate (BiVO4) is one of the most prominent photocatalysts for water oxidation which has suitable direct band gap (2.4 eV) to absorb visible light

with highly positive valence band edge of 2.86 eV vs NHE.3,4 However, poor migration rate of holes in BiVO4 is an obstacle to achieving efficient reaction because it leads to high exciton recombination rate. To reduce recombination rate of BiVO4, various strategies were developed; including doping heteroatom (e.g. phosphate or erbium) in BiVO4,5,6 modifying morphology of photocatalyst,7 and synthesizing composite photocatalyst.8,9 Among these, composite photocatalyst is renowned to be effective for reduction of recombination through Z-scheme procedure or Mott-Schottky heterojunction effect.10,11 Lately, several works reported that photocatalytic activity was enhanced by forming composites of BiVO4 and graphitic carbon nitride (g-C3N4).12-19 g-C3N4 is an excellent candidate material for BiVO4 composite photocatalyst because of its suitable band structure for formation of Z-scheme with BiVO4, and high electron mobility.20-23 Excited electrons from BiVO4

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favorably combine to holes in valence band of g-C3N4, which is placed between the conduction band and valence band of BiVO4. In this work, novel BiVO4-sulfur doped g-C3N4 (SCN) composite photocatalyst for effective water oxidation is introduced. To increase the efficiency, the band gap of gC3N4 can be narrowed by sulfur doping. The doped sulfur changes the band structure by stacking its 2p orbitals on the valence band of pristine g-C3N4.24 Composite photocatalyst with BiVO4 and SCN was simply synthesized via impregnated co-precipitation. Chemical transformation of SCN during impregnation is analyzed and the advantage of sulfur doping to synthesize composite is verified. As a result, the catalytic activity of BiVO4-SCN composite photocatalyst for water oxidation (750 μmol h-1 g-1) was enhanced by 129 % compared to pristine BiVO4 (328 μmol h-1 g-1) under the identical reaction condition. Moreover, reduced recombination rate of composite photocatalyst is confirmed by measuring lifetime of charge carriers. 2. Experimental Section 2.1. Preparation of composite photocatalyst g-C3N4 in this experiment was synthesized by the calcination of melamine at 823 K for 4 h in static air (temperature elevation rate: 5 K min-1).25 SCN was synthesized under same calcination condition with thiourea instead of melamine. Alumina crucible which was covered with lid was used for calcination. Pristine BiVO4 was synthesized by reported procedure.26 12.0 mmol of ammonium metavanadate (NH4VO3, Junsei Chemical, purity: 99.0 %) and 12.0 mmol of bismuth (III) nitrate

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pentahydrate (Bi(NO3)3∙5H2O, Sigma Aldrich, purity: 98 %) were dissolved in 50 ml of 2 M nitric acid solution (HNO3, Sigma Aldrich, assay: 60.0 %) separately with magnetic stirring. Two solutions were mixed slowly and yellow homogeneous solution was obtained. Next, 5 g of urea ((NH2)2CO, Sigma Aldrich) was dissolved in abovementioned mixed solution and stirred at 363 K for 24 h. Increasing pH of solution causes crystallization of BiVO4 in solution. The precipitate was filtered, washed, and dried, sequentially. Composite photocatalyst with BiVO4 and SCN was synthesized by the modified method from BiVO4 synthesis. Specific mass of SCN powder (i.e. 1 g for B1S sample) was impregnated in the 10 ml of homogeneous BiVO4 precursor solution and stirred for 1 h. After impregnation, 0.5 g of urea was added and synthesis was proceeded in 363 K for 24 h. Also, comparative acid treated samples (i.e. CNA and SCNA) were prepared with magnetic stirring in nitric acid for 24 h identical to the preparation condition of composite photocatalyst. All synthesized photocatalysts were well ground by agate mortar and sieved. 2.2. Characterization Weight percentage of sulfur in composite photocatalyst was measured by elemental analysis (EA, FLASH 2000 series, Thermo Scientific) (Table S1). The morphology of the samples were observed by field emission scanning electron microscope (FE-SEM, S-4800, Hitachi) at acceleration voltage of 15 kV and transmission electron microscopy (TEM, JEM-2100F, JEOL LTD.) at acceleration voltage of 200 kV. Optical property was detected by ultraviolet-visible spectrometer (UV-Vis, V570, Jasco). Crystal structure was measured by powder X-ray diffractometer using Cu Kα

Figure 1. Morphologies of prepared photocatalysts were detected by (a) TEM and (b) FE-SEM. (c) Digital image shows change in color due to the absorbance difference. (Scale bar : TEM = 0.2 μm, SEM = 1 μm)

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source at operating voltage of 40 kV and current of 200 mA (D/MAX-2500, Rigaku). The identification by thermal degradation was conducted with thermogravimetric analysis (TGA 92-18, Setaram, N2 atmosphere, 313 K ~ 1173 K). Chemical transformation of catalyst surface was detected by Fourier transform infrared spectroscopy (FT-IR, ALPHA, Bruker) and X-ray photoelectron spectroscopy (K-alpha, Thermo VG Scientific). The length of V-O bond was analyzed by Raman spectroscopy (Senterra, Bruker), and the lifetime of charge carriers were measured by time-correlated single photon counting (TCSPC, FL920, Edinburgh Instruments). The calculated lifetime (τ1) of charge carriers was estimated by fitting the plot of TCSPC data to exponential decay curve. 2.3. Photocatalytic activity Photocatalytic water oxidation was performed using customized Teflon reactor with quartz window. 20 mg of photocatalyst was dispersed in 12 ml of 0.05 M AgNO3 aqueous solution. The concentration of photocatalyst was optimized (Figure S1).27 For consistent activity test, AgNO3 was used as sacrificial agent to accept electrons from semiconductor photocatalyst. Before irradiation, the reactor was purged with Ar at the rate of 20 ml min-1 for 2 h to eliminate oxygen in the air. 300 W Hg (Xe) DC Arc lamp (66902, Newport) with UV cut-off filter (λ > 420 nm) was used as a light source. When irradiation began, 5 ml min-1 of Ar gas continuously carried product-gas to gas chromatography. Photocatalytic activity was measured for 6 h with on-line gas injection every 10 minutes.

The two materials can be distinguished with their different transparency as in individual images: BiVO4 is opaque while SCN is shown as semi-transparent. High resolution TEM image of interface between BiVO4 and SCN (Figure S2) also shows the connection between two materials. Morphological particularity is more pronounce in FE-SEM image (Figure 1b). In the composite, the BiVO4 part has smooth surface due to its high crystallinity, as observed in image of pristine BiVO4 micro-particles. On the contrary, SCN shows fractured layer structure. It is also observed that the junction between BiVO4 and SCN is limited to the surface and their inherent bulk morphologies are not changed. Moreover, the different optical property between composite photocatalyst and pristine materials can be detected with naked eyes. Digital image of photocatalysts (Figure 1c) suggests that color of the composite photocatalyst is different from those of pristine BiVO4, SCN, and mixture of BiVO4 and SCN. Different colors of photocatalysts indicate the differences in light absorption due to the alteration of optical band gap (Figure S3). The change in optical band gap for prepared photocatalysts was calculated by Kubelka-Munk equation and Tauc plot through diffuse reflectance spectroscopy analysis (Table

Photocatalytic methyl orange (Sigma Aldrich) degradation was conducted to investigate catalytic activity. 0.1 g of photocatalyst was dispersed in 100 ml of methyl orange aqueous solution (concentration: 10 mg L-1).28,29 2 h of irradiation was applied with 300 W Hg (Xe) DC Arc lamp (66902, Newport) with UV cut-off filter (λ > 420 nm). After photodegradation of methyl orange, the absorbance of solution was detected by UV-Vis spectrophotometer (Optizen, 3220 UV). Oxygen evolution rate was calculated by direct comparison to a detection of oxygen in the standard gas by on-line gas chromatography (Hewlett Packard 5890, thermal conductivity detector, Ar carrier, Molecular sieve 5A 60/80 column, 313 K for oven temperature) for minimizing error value. The area of peak for known concentration of oxygen in the standard gas was used to measure evolving oxygen concentration from photocatalytic reaction. Three-pointcalibration were conducted by 10 % O2, 0.51 % O2 standard gas and Ar gas which contains no oxygen. 3. Results and Discussion Successful synthesis of composite photocatalyst through one-pot impregnated co-precipitation was verified by observing its morphology with TEM and FE-SEM. The TEM image (Figure 1a) of composite photocatalyst shows that two distinctive materials are adhered to one another.

Figure 2. (a) XRD spectra and (b) TGA trace of prepared photocatalysts


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the weight ratio of BiVO4 in the composite photocatalysts by observing their residual mass after the temperature has risen to 1000 K. Following the methodology, change in the weight percentage of BiVO4 is observed; from 67.9 % in B1S to 20.4 % in B9S (Table S1). Interestingly, the composite photocatalysts show different TGA trace compared to SCN. Their drastic weight loss is completed around 800 K and gradual weight loss is detected below 800 K. To inspect the disparity, TGA was conducted for acid treated SCN (SCNA). While the drastic weight loss of SCNA is completed at 1000 K similar to SCN, the gradual weight change of SCNA under 800 K is similar to the composite photocatalysts. The impregnation procedure is predicted to affect the structure of SCN, resulting in the gradual change of weight in both SCNA and the composite photocatalysts. Besides, the drastic weight loss of SCNA was possibly caused by the strong oxidation effect of BiVO4, which weaken the structural integrity of SCN in the composite photocatalysts.30

Figure 3. Surface oxidation of SCN during impregnation was shown in (a) FT-IR spectrum, (b) XRD peak shift.

S1). The calculated optical band gaps of synthesized photocatalysts with different compositions are distributed between those of pristine BiVO4 (2.40 eV) and SCN (2.85 eV), and increased from B1S (2.42 eV) to B9S (2.68 eV) in correlation to increasing ratio of SCN. Crystallographic and quantitative analyses were conducted by XRD and TGA, respectively. Powder XRD spectra (Figure 2a) shows that every composite photocatalyst primarily has crystal structure of monoclinic scheelite BiVO4 (JCPDS No. 14-0688). Furthermore, the existence of SCN in the composite photocatalyst has been evidenced by evolution of additional peak at 27 ° of two theta value, which is a representative peak for graphitic layered structure, with respect to the increasing ratio of SCN. As the ratio of SCN is increased, the peak is shifted to the angle where the characteristic peak of pristine SCN is located, due to the increment of SCN crystal structure. The actual weight ratio of BiVO4 in composite photocatalyst was measured by TGA analysis (Figure 2b). Decomposition of SCN is completed at 1000 K, while BiVO4 remains stable in temperature over 1000 K. Thus, it is possible to determine

The effect of impregnation was verified through FT-IR analysis of pristine g-C3N4, SCN, acid treated g-C3N4 (CNA) and SCNA (Figure 3a). The commonly shown sharp peak at 800 cm-1 and multiple peaks between 1200 cm-1 and 1600 cm-1 indicate that C-N heterocycles of carbon nitride structure were not changed during acid treatment.31 However, only acid treated samples show broad strong peak at 3160 cm-1 which is originated by O-H bond from acid.32 The characteristic peak of O-H bond is shown in composite photocatalysts as well as SCNA and CNA, which means that same effect of acid treatment is applied to the composite photocatalysts (Figure S4). Moreover, the alteration of SCN crystal structure through acid treatment is detected by the additional XRD analysis (Figure 3b). Peak of pristine SCN at 27 °, which represents (002) graphitic planar space, is shifted to 27.4 ° due to the acid treatment. The peak shift is caused by the change in unit cell size of SCN via partial surface oxidation. On the other hand, the CNA, with no sulfur, shows negligible peak shift in XRD compare to gC3N4 (Figure 3b, inset). Also, the O-H peak of CNA in the FT-IR characterization is smaller than that of SCNA as shown in Figure 3a. Therefore, this indicates that the sulfur doping facilitates the surface oxidation during the impregnation procedure. In order to investigate the effect of acid treatment, XPS analysis was proceeded. The XPS survey spectrum (Figure 4) and atomic percentage (Table 1) clearly show that acid treated samples (CNA and SCNA) present higher level of oxidation compared to pristine samples while characteristic peak for sulfur is less prominent due to its low doping level as suggested by the EA result. In addition, peak deconvolution was performed to specify the bond formation involved in the surface oxidation (Figure S5). In deconvoluted spectra of C 1s, both g-C3N4 and SCN has two distinctive peaks which represent sp2-bounded carbon (C=N) and graphitic carbon (C-C) at 288.1 eV and 284.7 eV, respectively.31 In case of SCNA, the peak at 289.4 eV, assigned to carboxylate carbon (O-C=O), has significantly evolved because of the oxidation during acid treatment.33 The evidence of oxidation can also be found in N 1s XPS spectrum analysis. Main peak of N 1s is composed of three different peaks which are located on the binding energy of 398.6 eV,


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Chemistry of Materials Table 1. Atomic composition of prepared samples

Figure 4. XPS Survey spectrum of prepared samples

400.1 eV, and 401.1 eV. g-C3N4, CNA, and SCN show similar peak shape which is composed of a main peak for C=N-C group (398.6 eV), and minor peaks for N-(C)3 group (400.1 eV) and –NH2 group (401.1 eV).31 On the other hand, the main peak of SCNA is very broad because of decreased C=N-C peak. Moreover, only SCNA shows significant increase in peak at 406.3 eV which stands for nitrate (NO3-).34 This suggests that acid treatment causes partial disintegration of the pyridinic bond in SCN and oxidize both carbon and nitrogen atoms in the surface. Deconvoluted peaks of O 1s are also in good agreement with the bond formation described above by showing exceptional increase of NO3- peak in SCNA. Based on the results, we suggest a mechanism that the junction between BiVO4 and SCN is resulted from the surface oxidation of SCN (Scheme 1). The formation mechanism of monoclinic BiVO4 is the reaction between negatively charged vanadates and Bi(NO3)3 or BiO+.3 In our experiment, BiVO4 structure is formed by the coordination of Bi3+ ion on the VO43- tetrahedron structure. One oxygen atom is required to form stable VO43- tetrahedron structure with vanadate ion (VO3-) which originates from precursor NH4VO3. As the result, VO43- tetrahedron is formed onto oxidized surface of SCN where an extra oxygen atom preexists. Hence, the sulfur doping has a critical role in the formation of composite photocatalyst, which is advantageous in charge separation through the chemical bond between SCN and BiVO4.

C at.%

N at.%

O at.%

g-C3N4

41.6

46.6

3.5

CNA

40.1

44.3

7.2

SCN

42.3

44.6

5.1

SCNA

36.5

39.8

15.2

Raman analysis for prepared photocatalysts was conducted to evidence the suggested composite formation mechanism (Figure 5). Peaks near 828 cm-1, which represent the stretching mode of V-O bond can be correlated to V-O bond length through an empirical formula (Table S1).35-37 The peaks are red shifted with regard to the increasing ratio of SCN in composite, indicating the elongation of V-O bond length. Change of bond length represents that the chemical bond between vanadium atom and oxygen atom on the oxidized surface of SCN is formed. Since SCN is composed of elements which have high electronegativity, the composition between SCN and BiVO4 causes lengthened V-O bond and partial polarization of VO43- tetrahedron which merits for photocatalytic activity.26 The photocatalytic water oxidation performance of prepared catalysts was measured by gas chromatography for 6 h under visible light (λ > 420 nm) irradiation (Figure 6). Both pristine BiVO4 and SCN showed similar oxygen evolution rate of 328 μmol h-1 g-1 and 315 μmol h-1 g-1, respectively. BiVO4 has 2.4 eV band gap which can absorb visible light, but its poor hole mobility inhibits photocatalytic water oxidation reaction. Hence, despite its wider band gap of 2.7 eV, the oxygen evolution rate of SCN is comparable to BiVO4 because of its better charge carrier mobility. Moreover, it should be noted that acid treated SCN shows similar water oxidation performance than SCN, suggesting that acid treatment itself does not primarily affect the photocatalytic activity but enhance the viability for composite formation with BiVO4. The composite photocatalyst has improved charge separation and the reduced recombination rate results in the enhancement of photocatalytic activity. The oxygen evolution rate of B1S is 428 μmol h-1 g-1 which is higher than that of pristine BiVO4, and the composite photocatalyst shows higher oxygen evolution rate with increasing SCN content. Through optimization, B7S shows the highest activity of 750 μmol h-1 g-1 which is more

Scheme 1. The effect of SCN acid treatment leads to the formation of composite between SCN and BiVO4 (Gray: SCN, Red: Oxygen, Blue: Vanadium atom)


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Figure 5. Tendency of Raman shift for prepared photocatalysts

than twofold higher than pristine BiVO4. However, B9S which has more SCN composition than B7S, shows slightly decreased catalytic activity (i.e. 738 μmol h-1 g-1). B9S would have residual impregnated SCN which can inhibit the photocatalytic water oxidation by blocking illumination. Therefore, B7S can be regarded as the saturated composite photocatalyst between BiVO4 and SCN. Also, the photonic efficiency of photocatalyst was calculated by using oxygen evolution rate and measured power of irradiation (Table S1). Since the actual number of absorbed photon is in correlation to that of incident photon, photonic efficiency can provide the relative evaluation for catalytic activities under the identical reactor and identical reaction condition.27,3840 The calculated photonic efficiency of pristine BiVO4 is 8.32 % which is in close resemblance to the reported quantum yield (9 %).3 The best photonic efficiency we achieved is 18.90 % of B7S. To certify the improvement of synthesized composite photocatalysts, two samples were additionally tested: i) physically mixed BiVO4 and SCN; ii) composite photocatalyst with BiVO4 and g-C3N4. Physically mixed BiVO4 and SCN was examined under same condition and only 394 μmol h-1 g-1 of oxygen evolution rate was detected. Hence, the chemical adsorption between BiVO4 and SCN clearly improves the photocatalytic activity. The composite photocatalyst with BiVO4 and g-C3N4 which contains no sulfur was also tested for verifying the effect of sulfur doping. Composite photocatalyst synthesized with 7 g of g-C3N4 impregnation (B7C) instead of SCN, showed 316 μmol h-1 g-1 of oxygen evolution rate, which is similar to that of pristine BiVO4 in same condition. This result verifies that the sulfur doping on g-C3N4 catalyzes the formation of composite with BiVO4 by promoting the oxidation of surface during impregnation in nitric acid precursor solution. In addition, photocatalytic methyl orange degradation was conducted to measure the catalytic activity (Figure S6). Methyl orange has a specific absorbance peak at 460 nm. The decrement of absorbance indicates the photocatalytic

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Figure 6. Photocatalytic activities of prepared photocatalysts

Figure 7. Charge carriers lifetime of prepared photocatalysts

degradation of methyl orange. SCN shows negligible degradation effect after 2 h of photoreaction while BiVO4 partially degraded methyl orange and the absorbance decreases by 27 % compared to blank sample. Moreover, B1S showed increased degradation of 52 % and degradation of over 80 % was measured for other composite photocatalysts; B3S, B5S, B7S and B9S. This photocatalytic degradation of methyl orange claims that the composite photocatalyst has significant improvement on the photocatalytic activity compared to both pristine BiVO4 and SCN. Photocatalytic activity of composite photocatalyst was enhanced due to reduced recombination rate of excited charge carriers by Z-scheme in the composite photocatalyst. In order to confirm the recombination inhibition, analysis of charge carrier lifetime was proceeded. The duration of charge carrier from photocatalyst can be measured by TCSPC (Figure 7). BiVO4 has no remaining charge carriers after 120 ns from photonic excitation, while SCN has detectable charge carriers until 300 ns after excitation. The calculated lifetime (τ1) of SCN is 2.29 ns which is much


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longer than that of BiVO4 (i.e. 0.70 ns). As the ratio of SCN increases, the lifetime is extended. Moreover, B7S and B9S have longer lifetime of charge carriers than SCN does (i.e. 3.14 ns and 3.04 ns, respectively). Thus, the extension of lifetime originates from the Z-scheme formation in composite as well as the intrinsic characteristic of SCN. 4. Conclusion In summary, we developed heterojunction photocatalysts with various compositions of BiVO4 and SCN by the impregnated co-precipitation for enhancing water oxidation reaction. The synthetic method via impregnated coprecipitation is not only simple but also effective to formulate junction between BiVO4 and SCN, and the formation mechanism of which was revealed. The sulfur doping facilitates the surface oxidation of SCN during impregnation procedure, and consequently VO43- tetrahedron is formed onto oxidized site of SCN. In result, the lifetime of B7S charge carriers is extended by 449 % compared to pristine BiVO4 according to TCSPC analysis. Through optimization, B7S exhibits the excellent oxygen evolution rate of 750 μmol h-1 g-1 under visible light irradiation which is 129 % increased to pristine BiVO4 and 19 % of photonic efficiency due to the diminished recombination of charge carriers. Introducing the advantageous effect of sulfur doping is a significant improvement from researches on composite photocatalysts between BiVO4 and pristine g-C3N4. Since SCN has photocatalytic activity to produce hydrogen from water, the composite photocatalyst between BiVO4 and SCN is one of the strong possibilities to achieve total water splitting without any sacrificial agent.

ASSOCIATED CONTENT Supporting Information Available: Tables of supplementary data: weight percent of sulfur, optical bandgap, weight percent of BiVO4, Raman shift frequency, V-O bond length, and photonic efficiency, and graph of optimal ratio of photocatalytic reaction, high resolution TEM image, UV-Vis absorbance, XPS peak deconvolution spectra, FT-IR spectrum for prepared photocatalysts and methyl orange degradation. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]

ACKNOWLEDGMENT This research is supported by BK21PLUS program through the National Research Foundation of Korea.


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Photoelectrochemical Water Splitting. J. Phys. Chem. Lett. 2010, 1,

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