Zscheme SnO2 xgC3N4 composite as an efficient photocatalyst for dye degradation and photocatalytic CO2 reduction

Solar Energy Materials & Solar Cells 137 (2015) 175–184 Contents lists available at ScienceDirect Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat Z-scheme SnO2 À x/g-C3N4 composite as an efficient photocatalyst for dye degradation and photocatalytic CO2 reduction Yiming He a,n, Lihong Zhang a, Maohong Fan c, Xiaoxing Wang a, Mikel L Walbridge c, Qingyan Nong a, Ying Wu b,n, Leihong Zhao b a Department of Materials Physics, Zhejiang Normal University, Jinhua 321004, China Institute of Physical Chemistry, Zhejiang Normal University, Jinhua 321004, China c School of Energy Resources, University of Wyoming, Laramie, WY 82071, United States b art ic l e i nf o a b s t r a c t Article history: Received 17 September 2014 Received in revised form 29 January 2015 Accepted 30 January 2015 Available online March 2015 Highly efficient SnO2 À x/g-C3N4 composite photocatalysts were synthesized using simple calcination of gC3N4 and Sn6O4(OH)4 The synthesized composite exhibited excellent photocatalytic performance for rhodamine B (RhB) degradation under visible light irradiation The optimal RhB degradation rate of the composite was 0.088 À 1, which was 8.8 times higher than that of g-C3N4 The SnO2 À x/g-C3N4 composite also showed high photocatalytic activity for CO2 reduction and photodegradation of other organic compounds Various techniques including Brunauer–Emmett–Teller method (BET), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), UV–vis diffuse reflectance spectroscopy (DRS), photoluminescence spectroscopy (PL) and an electrochemical method were applied to determine the origin of the enhanced photoactivity of SnO2 À x/g-C3N4 Results indicated that the introduction of SnO2 À x on g-C3N4 increased its surface area and enhanced light absorption performance More importantly, a hetero-junction structure was formed between SnO2 À x and g-C3N4, which efficiently promoted the separation of electron–hole pairs by a direct Z-scheme mechanism to enhance the photocatalytic activity This study might represent an important step for the conversion of solar energy using cost-efficient materials & 2015 Elsevier B.V All rights reserved Keywords: Photocatalysis SnO2 À x g-C3N4 Z-scheme Visible light Introduction Photocatalysis has attracted remarkable interest because it offers a sustainable pathway to drive chemical reactions such as degradation of organic pollutants, water splitting, and carbon fixation [1–3] A significantly efficient, stable, and inexpensive photocatalyst that can harvest visible light is considered the key factor for the economical application of photocatalysis Therefore, development of an efficient visible-light-driven photocatalyst has been extensively investigated Although various novel visible-light-responsive materials, such as CaBi2O4, BiVO4, Ag3PO4, etc [4–8], have been reported, only a few of these materials have attracted much interest Graphitic carbon nitride (g-C3N4) is an outstanding photocatalyst because of its high reducibility and visible-light adsorption [8] In addition, g-C3N4 is also inexpensive because it is a metal-free semiconductor and can be synthesized by simple heating of urea or melamine at 500–600 1C However, pure g-C3N4 exhibits non-satisfactory photocatalytic efficiency, which can be partly attributed to its low surface area Hence, n Corresponding authors Tel.: þ 86 579 83792294; fax: þ86 579 83714946 E-mail addresses: hym@zjnu.cn (Y He), ying-wu@zjnu.cn (Y Wu) http://dx.doi.org/10.1016/j.solmat.2015.01.037 0927-0248/& 2015 Elsevier B.V All rights reserved fabricating nanostructured g-C3N4 to increase the surface area has been suggested to enhance photocatalytic activity [9] However, the promotion effect of this approach is limited Increasing studies have shown that pure g-C3N4 photocatalyst is hardly competent for efficient organic pollutant degradation or solar fuel generation because of the disadvantageous rapid charge recombination More and more researchers pay attentions on multi-component photocatalysts that comprise of g-C3N4 and another semiconductor g-C3N4-based composite photocatalysts have become a hot topic in photocatalysis Up to date, numerous of g-C3N4-based composites, such as LnVO4 (Ln¼Sm, Dy, Bi, La)/g-C3N4 [10–13], TaON/g-C3N4 [14], Ag3VO4/gC3N4[15], CdS/g-C3N4[16], AgX (X¼ Cl, Br, I)/g-C3N4 [17,18], MoO3/gC3N4 [19], S-TiO2/g-C3N4 [20] and BiOCl/g-C3N4 [21], have been reported The composite photocatalysts present much higher activity than pure g-C3N4, which is mainly attributed to the coupling effect between g-C3N4 and the semiconductor Two mechanisms are usually applied to explain the synergetic effect The first mechanism is the double-charge transfer mechanism [10–18], in which the photogenerated electrons in the conduction band (CB) of g-C3N4 are injected into the CB of another semiconductor Meanwhile, the photogenerated holes from the semiconductor transport to the valence band (VB) of g-C3N4 As a result, the electrons and holes are separated, and the 176 Y He et al / Solar Energy Materials & Solar Cells 137 (2015) 175–184 photocatalytic efficiency is enhanced The other mechanism is the direct Z-scheme-type mechanism [19–21], in which the photogenerated electrons in the CB of the coupled semiconductor are injected into the VB and annihilate the holes of g-C3N4 This process also facilitates electron–hole separation, and suppresses charge recombination, thereby improving the photocatalytic activity Meanwhile, given the strong reducibility and oxidability of the electrons on g-C3N4 and holes on the coupled semiconductor, Z-scheme composites usually present high photocatalytic activity For example, Katsumata et al synthesized an Ag3PO4/g-C3N4 composite and applied it in the photocatalytic oxidation of methyl orange (MO) [22] Compared with pure Ag3PO4, only one-third of irradiation time was needed for the Z-scheme hybrid to completely degrade MO solution Moreover, we fabricated an efficient Z-scheme photocatalyst MoO3/g-C3N4 [19]; the prepared MoO3/g-C3N4 composite degraded MO 10.4 times faster than g-C3N4 under visible light The promotion effect of MoO3 is nearly the best of the reported dopants Hence, this Z-scheme-type composite shows great potential as an efficient photocatalyst for conversion of solar energy to chemical energy However, to the best of our knowledge, only a few studies have focused on this photocatalyst and the number of efficient Z-scheme photocatalysts is still limited Additional investigations are necessary to develop this type of photocatalysts further In this paper, an efficient Z-scheme type photocatalyst, SnO2 À x/ g-C3N4 composite is presented Sn2 þ -doped SnO2 (SnO2 À x), which was prepared by heating Sn6O4(OH)4 in N2, was chosen as the doping semiconductor because of its low CB position and capability of harvesting visible light [23] The decoration of SnO2 À x remarkably promoted the photocatalytic activity of g-C3N4 in rhodamine B (RhB) degradation and CO2 photoreduction Investigation of the structure, surface area, and optical property of the composite showed that the relatively high photoactivity of SnO2 À x/g-C3N4 composite could be ascribed to a direct Z-scheme mechanism The Z-scheme mechanism of the SnO2 À x/g-C3N4 photocatalyst was demonstrated and explained for the first time Experimental section 2.2 Photodegradation of RhB The photocatalytic degradation of RhB was carried out in an outer irradiation-type photoreactor Typically, 100 mL of RhB solution with an initial concentration of 10 mg/L and 0.1 g of photocatalyst were added to a 250 mL Pyrex glass cell The RhB solution containing the photocatalyst powder was magnetically stirred before and during photocatalytic reaction The visible light source for photocatalysis was a spherical Xe lamp (350 W) equipped with a UV cut and an IR cut filters (800 nm4 λ 4420 nm) Other filters (λ 4320 nm, λ 4360 nm, λ 4480 nm and λ 4580 nm) were also used to cut off the light with different wavelengths Prior to irradiation, the suspension was agitated for an hour to ensure adsorption/desorption equilibrium at room temperature At regular intervals, samples were withdrawn and centrifuged to remove photocatalyst for analysis The concentration of aqueous RhB was determined by measuring its absorbance of the solution at 554 nm using a UV–vis spectrophotometer The RhB degradation was calculated by Lambert–Beer equation In addition to RhB, MO, methyl blue (MB) and phenol were also used as the simulative pollutants to investigate the photoactivity of SnO2À x/gC3N4 composite The procedures of the scavenging experiments of reactive oxygen species were similar to that of the photodegradation experiment The detailed process was described elsewhere [24,25] 2.3 Photocatalytic reduction of CO2 The photocatalytic CO2 reduction was carried out in a stainlesssteel reactor with a quartz window on the top of the reactor (Fig S3) A 500 W Xe lamp was used as the light source In the photocatalytic CO2 reduction reaction system, 20 mg of solid catalyst was placed on a Teflon catalyst holder in the upper region of the reactor mL water was pre-injected into the bottom of the reactor Prior to the light irradiation, the above system was thoroughly purged by CO2 to remove air in the reactor During reaction, the pressure of CO2 was kept to be 0.3 MPa and the photoreaction temperature was kept at 80 1C After light irradiation for h, the gas product was analyzed by a gas chromatograph (GC-950) with a FID and a TCD detector Only the products of CO, CH4, and CH3OH were detected 2.1 Catalysts preparation 2.4 Characterizations Melamine (C3H6N6, 499%), tin dichloride dihydrate (SnCl2 Á 2H2O, 498%), potassium hydroxide (KOH, 485%), ethanol (499.7%) were purchased from Sinopharm Chemical Reagent Corp., PR China P25 (TiO2, Degussa) was purchased from Beijing Entrepreneur Corp., China All these reagents were used without further purification Pure g-C3N4 powders were prepared by directly calcining melamine in a muffle furnace at 520 1C for h Pure SnO2À x was prepared by heating Sn6O4(OH)4 at 400 1C in N2 for h Sn6O4(OH)4 was prepared by a deposition method In a typical synthesis run, 6.768 g of SnCl2 Á 2H2O was dissolved in a mixture solvent of 50 mL H2O and 20 mL ethanol to obtain solution A 3.93 g of KOH was dissolved in 30 mL H2O to obtain solution B Then, solution B was added dropwise into solution A under stirring to generate white precipitate After stirring for two hours, the precipitate was filtered, and washed many times by water and ethanol to remove Cl À and K þ Yellow Sn6O4(OH)4 was obtained in a powder form after drying in oven at 60 1C for 12 h (Fig S1) The SnO2À x/g-C3N4 composites were prepared according to the following procedure A given amount of Sn6O4(OH)4 and g-C3N4 were mixed and ground in an agate mortar for 20 Then, the mixture was calcined at 400 1C in N2 for h to obtain the SnO2À x/g-C3N4 catalyst By this way, the SnO2À x/g-C3N4 (SC) composites with the SnO2À x concentration of 5.4 wt%, 17.5 wt%, 29.6 wt%, 42.2 wt%, 55.6 wt % were prepared and denoted as 5.4 wt%SC, 17.5 wt%SC, 29.6 wt%SC, 42.2 wt%SC, 55.6 wt%SC, respectively The concentration of SnO2À x was determined by thermogravimetry (TG) analysis (Fig S2) TG analysis (Netzsch STA449) was carried out in a flow of air (10 mL/min) at a heating rate of 10 1C/min The specific surface areas were measured on Autosorb-1 (Quantachrome Instruments) by the BET method The powder X-ray diffraction (XRD, Philips PW3040/60) was used to record the diffraction patterns of photocatalysts employing Cu Kα radiation (40 kV/40 mA) A field emission scanning electron microscope (LEO-1530) and a JEM-2010F transmission electron microscope were employed to observe the morphology of the catalysts The FT-IR spectra of the catalysts were recorded on Nicolet NEXUS670 with a resolution of cm À The XPS measurements were performed with a Quantum 2000 Scanning ESCA Microprobe instrument using AlKα The C 1s signal was set to a position of 284.6 eV The UV–vis diffuse reflectance spectra (DRS) of catalysts were recorded on a UV– vis spectrometer (PerkinElmer Lambda900) equipped with an integrating sphere The PL spectra were collected on FLS-920 spectrometer (Edinburgh Instrument), using a Xe lamp (excitation at 365 nm) as light source The electrochemical impedance spectroscopy (EIS) and photocurrent (PC) responses measurements were performed by using a CHI 660B electrochemical workstation with a standard threeelectrode cell at room temperature The prepared sample, Ag/AgCl (saturated KCl), and a Pt wire were used as the working electrode, the reference electrode, and the counter electrode, respectively The working electrodes were prepared as follows Indium tin oxide (ITO) glass pieces (1.5 Â cm2) were cleaned successively by Y He et al / Solar Energy Materials & Solar Cells 137 (2015) 175–184 177 acetone, boiling NaOH (0.1 mol/L), deionized water, and dried in an air stream Then, 0.018 g sample and 0.002 g polyvinylidene fluoride was mixed and ground for three minutes After adding of three drops of 1-Methyl-2-pyrrolidinone, the mixture was ultrasonicated for 20 to obtain a suspension, which was then coated onto the ITO glass substrate The coated area on the ITO glass was controlled to be 0.8 Â 0.8 cm2 Finally, the coated ITO glass was dried at 50 1C to obtain the working electrode The EIS experiment was performed in aqueous 0.1 M Na2SO4 solution in the dark The potential was varied between and À V (vs Ag/ AgCl) with an AC amplitude of 10 mV and frequencies in the 200– 4000 Hz range For PC measurement, a 350 W Xe arc lamp served as the light source and Na2SO4 (0.5 M) aqueous solution was used as the electrolyte Results and discussion 3.1 Characterizations of SnO2 À x/g-C3N4 composites The structure of the synthesized SnO2 À x/g-C3N4 composites was characterized by XRD and FT-IR Fig 1a shows the powder XRD patterns of g-C3N4, SnO2 À x, and SnO2 À x/g-C3N4 with different SnO2 À x concentrations Pure g-C3N4 has two distinct peaks at 27.41 and 13.11, which can be indexed to the (002) and (100) diffraction planes [26] Pure SnO2 À x exhibits several strong peaks at 26.61, 33.91, 38.01 and 51.81, which matches well with the standard diffraction data for the tetragonal phase of SnO2 (PDF 41-1445) This result indicates that the content of doped Sn2 þ may be very low and does not change the crystal structure of SnO2, which is consistent with Long's result [23] For the SnO2 À x/g-C3N4 hybrids, the XRD patterns display a combination of the two sets of diffraction data for both g-C3N4 and SnO2 À x With the increase in SnO2 À x content, the peaks of g-C3N4 weaken No other phase is detected, which indicates that the SnO2 À x/g-C3N4 hybrids are composed of g-C3N4 and SnO2 À x; similar result is obtained by FT-IR Fig 1b shows that the SnO2 À x/g-C3N4 consists of two sets of characteristic vibration peaks The IR peak at 567 cm À can be ascribed to the characteristic peak of SnO2 À x, while the peaks in the range of 1245–1574 cm À can be assigned to the characteristic vibration peaks of C–N heterocyclics in g-C3N4 [27] This result is in excellent agreement with the XRD analysis The morphologies of g-C3N4, SnO2 À x and the SnO2 À x/g-C3N4 photocatalyst were investigated by SEM and TEM In Fig 2a and b, a stacked layer structure is clearly observed in the g-C3N4 sample, which is consistent with previous reports [15,19] The SnO2 À x sample displays a nanospherical shape with an average diameter of $50 nm (Fig 2c and d) In the SEM micrograph of the representative composite (42.2 wt% SC), g-C3N4 sheets are found to be covered by SnO2 À x nanoparticles (Fig 2e) The size of SnO2 À x in the composite is similar to that of the pristine SnO2 À x The TEM image provides a more evident observation about the two components (Fig 2f) The darker part with spherical shape should be SnO2 À x and the lighter part is g-C3N4, which further demonstrates the well dispersion of SnO2 À x on g-C3N4 An inserted highresolution TEM (HRTEM) image shows the microstructure of the SnO2 À x/g-C3N4 composite Two clear lattice fringes are observed in the HRTEM image of 42.2 wt% SC The interplanar spacings are approximately 0.3476 and 0.2740 nm, which are very close to the (110) and (101) planes of SnO2, respectively, in accordance with the XRD result in Fig 1a The lattice fringe is difficult to observe in g-C3N4 However, the SnO2 À x nanoparticles are evidently anchored on the g-C3N4 surface Some chemical bonds may be formed between SnO2 À x and g-C3N4, leading to a close interface between the two semiconductors in the as-prepared composite This tight coupling is favorable for the charge transfer between g-C3N4 and Fig XRD patterns (a) and FT-IR spectra (b) of SnO2 À x/g-C3N4 composites with different SnO2 À x concentrations SnO2 À x and promotes the separation of photogenerated electron– hole pairs Meanwhile, the HRTEM image also suggests that the SnO2 À x/g-C3N4 hybrids in structure are heterogeneous rather than a physical mixture of two separate phases of SnO2 À x and g-C3N4 The close interaction between SnO2 À x and g-C3N4 in the composite can also be observed via TG analysis Fig shows the TG profiles of g-C3N4, 42.2 wt% SC, and the physical mixture of SnO2 À x and g-C3N4 (42.2 wt% SC-PM) Compared with pure g-C3N4, sharp weight loss occurs at a lower temperature for SnO2 À x/g-C3N4, which can be attributed to the catalytic role of SnO2 À x [10,13] The amount of catalyst and the contact between SnO2 À x and g-C3N4 are two important factors that influence the catalytic oxidation of g-C3N4 Although 42.2 wt% SC and 42.2 wt% SC-PM have nearly the same SnO2 À x concentration, the difference in their sharp weight losses is still evident The catalytic oxidation of g-C3N4 in 42.2 wt% SC composite is faster than that in 42.2 wt% SC-PM, indicating the tight contact between SnO2 À x and g-C3N4 in SnO2 À x/g-C3N4 composite This result is consistent with the TEM analysis Fig shows the XPS spectra of the SnO2 À x/g-C3N4 composites The survey scan XPS spectra provide the C 1s and N 1s peaks for gC3N4 and 42.2 wt% SC, as well as the Sn 3p, 3d, and O 1s peaks for SnO2 À x and 42.2 wt% SC These results are consistent with the chemical composition of the photocatalyst, as proven by the XRD and FT-IR analyses The high-resolution X-ray photoelectron spectra of C 1s are shown in Fig 4b SnO2 À x shows one C 1s peak at 284.6 eV as a result of its external carbon contamination [28] In 178 Y He et al / Solar Energy Materials & Solar Cells 137 (2015) 175–184 Fig SEM and TEM images of g-C3N4 (a, b), SnO2 À x (c, d), and 42.2 wt% SC (e, f) photocatalysts Fig TG profiles of g-C3N4, 42.2 wt% SC and 42.2 wt% SC-PM the case of g-C3N4 and 42.2 wt% SC composite, another C 1s peak is found, which corresponds to the carbon atoms bonded with three N neighbors in its chemical bone structure, suggesting the existence of g-C3N4 [28] Notably, the C 1s binding energy of 42.2 wt% SC is slightly higher than that of pure g-C3N4, which is similar to the N 1s XPS peak of the SnO2 À x/g-C3N4 composite (Fig S4) Fig 4c displays the Sn 3d high-resolution XPS peak Two signals at binding energies of 486.7 eV (Sn 3d5/2) and 495.1 eV (Sn 3d3/2) are observed for the SnO2 À x sample The Sn 3d5/2 and Sn 3d3/2 peaks of Sn2 þ located at 486.3 and 494.7 eV, respectively, whereas those peaks centered at 486.9 and 495.3 eV could be assigned to Sn4 þ [29] The result in Fig 4c suggests that the existence of some Sn2 þ in the SnO2 À x sample Some Sn2 þ cations are not oxidized to Sn4 þ during the calcination process in nitrogen atmosphere When the SnO2 À x sample is calcined in air at 600 1C for h, the Sn 3d5/2 and Sn 3d3/2 peaks shift to 487.0 and 495.4 eV, respectively (Fig S5), which further proves the existence of Sn2 þ in the SnO2 À x sample For the 42.2 wt% SC sample, the Sn 3d XPS peak displays a negative shift compared with that of SnO2 À x; the binding energies of Sn 3d5/2 and 3d3/2 move to 486.5 and 494.9 eV, respectively Clearly, the coupled g-C3N4 shows its contribution in hindering the Sn2 þ oxidation Meanwhile, combined with the slight shift in the C 1s and N 1s spectra, the result in Fig 3c represents the interactions between SnO2 À x and g-C3N4 [30,31], which may be via the chemical bonds of Sn–O–N or Sn–O– C The XPS result demonstrates that the synthesized SnO2 À x/gC3N4 composite is not a physical mixture, which is consistent with the TEM analysis Fig 4d shows the VB X-ray photoelectron spectra of g-C3N4 and SnO2 À x The VB edge of g-C3N4 is 1.51 eV, which is close to the reported values [10] The value for the SnO2 À x sample Y He et al / Solar Energy Materials & Solar Cells 137 (2015) 175–184 179 Fig XPS spectra of SnO2 À x, g-C3N4 and 42.2 wt% SC composite, (a) survey spectra, (b) C 1s, (c) Sn 3d, (d) VB XPS of g-C3N4 and SnO2 À x is 2.70 eV, which is negative to that of SnO2 (EVB ¼3.48 eV) [32] This result indicates that the doped Sn2 þ generates an impurity energy level in the VB and elevates the VB edge [23] The interactions between the components are important for the formation of a hetero-junction structure in the composite photocatalysts, and this structure contributes to the separation of electron–hole pairs and subsequently results in their high photoactivity [33,34] In the case of SnO2 À x/g-C3N4, the enhanced separation efficiency of electron–hole pairs should be observed considering that the interactions between g-C3N4 and SnO2 À x has been proven Therefore, PL experiment was performed to verify the aforementioned hypothesis Fig shows the PL spectra of gC3N4, 42.2 wt% SC, and the physical mixture 42.2 wt% SC-PM Pure g-C3N4 has a strong emission band at 460 nm, which is attributed to the recombination process of self-trapped excitations [35] The PL spectrum of 42.2 wt% SC is similar to that of pure g-C3N4, which indicates that the emission band originates from the incorporate g-C3N4 Meanwhile, the emission peak is much lower than that of g-C3N4 In general, the decreased content of g-C3N4 and enhanced separation efficiency of charges would result in this change [35,36] Hence, a physical mixture of 42.2 wt% SC-PM was characterized as a reference sample The result suggests that the emission band of the physical mixture is weaker than that of gC3N4, but stronger than that of 42.2 wt% SC This condition confirms that the synthesized SnO2 À x/g-C3N4 has higher separation efficiency of electron–hole pairs than g-C3N4 Fig Photoluminescence spectra of pure g-C3N4, 42.2 wt% SC composite, and 42.2 wt% SC-PM The EIS and PC analyses were conducted to confirm the high efficiency of SnO2 À x/g-C3N4 hybrid in hindering the recombination of electron–hole pairs The EIS spectra of SnO2 À x, g-C3N4, and SnO2 À x/g-C3N4 composite are shown in Fig 6a The arc radius of the EIS Nyquist plot of the 42.2 wt% SC is smaller than that of g-C3N4 or SnO2 À x Given that the arc radius on the EIS spectra 180 Y He et al / Solar Energy Materials & Solar Cells 137 (2015) 175–184 Fig EIS (a) and transient photocurrent responses of pure g-C3N4, SnO2 À x and 42.2 wt% SC composite (b) reflects the reaction rate at the surface of an electrode [37,38], the data in Fig 6a suggest the more effective separation of photogenerated electron–hole pairs and a faster interfacial charge transfer on SnO2 À x/g-C3N4 hybrid under this condition Fig 6b displays the photocurrent transient responses for SnO2 À x, g-C3N4 and 42.2 wt% SC electrodes Fast and uniform photocurrent responses are evidently observed for each switch-on and switchoff event in both electrodes The photocurrent of the SnO2 À x/gC3N4 electrode is approximately and 20 times higher than those of the SnO2 À x and g-C3N4 electrodes, respectively This result is consistent with the EIS and PL analyses; and clearly indicates that the introduction of SnO2 À x into g-C3N4 can effectively enhance the separation efficiency of photogenerated electron–hole pairs [39,40] The optical properties of SnO2À x/g-C3N4 samples were probed by UV–vis diffuse reflectance spectroscopy (Fig 7) The doping of Sn2 þ on SnO2 generates an impurity energy level in the VB and narrows the band gap [23] Hence, SnO2À x can absorb visible light, and its band gap energy is determined to be 2.50 eV by the K–M equation, which is much smaller than that of SnO2 [41] For comparison, the SnO2À x sample calcined in air for h was also characterized by DRS The result shown in Fig S6 indicates that the band gap of the sample remarkably increases after calcination, which proves the contribution of Sn2 þ , as supported by the XPS results Pure g-C3N4 can absorb light with wavelength lower than 460 nm and has a band gap of 2.70 eV The SnO2À x/g-C3N4 samples display an absorption edge similar to that of g-C3N4, indicating their ability to respond to visible light Meanwhile, a noticeable correlation between the SnO2À x content and the UV–vis spectral change is observed The Fig UV–vis spectra of SnO2 À x/g-C3N4 (a) composites and estimated band gaps of g-C3N4 and SnO2 À x (b) absorption in the visible region increases with SnO2À x contents of the SnO2À x/g-C3N4 samples These results may have been caused by the interactions between SnO2À x and g-C3N4 (via the formed chemical bonds), which results in modifications of the fundamental process of formation of electron/hole pair during irradiation [37] The BET surface areas of SnO2 À x/g-C3N4 hybrids are listed in Table 1, as well as that of SnO2 À x and g-C3N4 for comparison The BET surface area of g-C3N4 is 13 m2/g, which is slightly higher than that of SnO2 À x (9 m2/g) Comparing with SnO2 À x or g-C3N4, the SnO2 À x/g-C3N4 composites exhibit much higher BET values Given that the BET value of 42.2 wt% SC-PM is only 11 m2/g, the high surface area of the composites indicates that some changes occur on the incorporate g-C3N4 or SnO2 À x In another word, some reactions might have occurred between g-C3N4 and the precursor of SnO2 À x during calcination, which is consistent with the aforementioned hypothesis on the interaction between g-C3N4 and SnO2 À x However, no regularity between the SnO2 À x contents and BET values is observed The 29.6 wt% SC sample shows the highest specific surface area of 43 m2/g 3.2 Photocatalytic activities of the SnO2 À x/g-C3N4 composites The photocatalytic activity of the as-prepared SnO2 À x/g-C3N4 hybrids was evaluated by RhB degradation under visible-light irradiation (Fig 8a) SnO2 À x and g-C3N4 samples are used for comparison Fig 8b shows the plots of ln(Ct/C0) vs irradiation time The reaction rate constants k are calculated by the kinetics Y He et al / Solar Energy Materials & Solar Cells 137 (2015) 175–184 equation: ln(Ct/C0)¼ À kt where k is the pseudo-first-order rate constant, C0 is the RhB concentration after adsorption, and Ct represents the concentration at reaction time t As shown in Fig 8a, the self-degradation of RhB can be negligible in the absence of a photocatalyst The pristine SnO2 À x shows weak ability in RhB degradation, while pure g-C3N4 exhibits certain photoactivity with a reaction rate constant of 0.01 À Compared with the g-C3N4 sample, the SnO2 À x/g-C3N4 hybrids display markedly higher photocatalytic activity because of the increased separation efficiency of electron–hole pairs The photocatalytic activity is enhanced gradually with increased SnO2 À x content from 5.4 wt% to 42.2 wt% The 42.2 wt% SC sample presents the highest efficiency for RhB degradation under visible light irradiation The k value is determined to be 0.088 À (Fig 8b), which is Table Specific surface area of g-C3N4, SnO2 À x, and SnO2 À x/g-C3N4 composites Catalysts S/m2 g À g-C3N4 SnO2 À x 5.4 wt% SC 15.5 wt% SC 29.6 wt%SC 42.2 wt% SC 55.6 wt%SC 42.2 wt% SC-PM 13 25 23 43 41 34 11 Fig Photocatalytic activities of SnO2 À x/g-C3N4 composites on photodegradation of RhB under visible-light irradiation (λ 4420 nm) (a) and the corresponding kinetic studies (b) 181 8.8 times higher than that of pure g-C3N4 However, further increase in the SnO2 À x content in the composites leads to the decrease in photocatalytic activity The stability of the optimized SnO2 À x/g-C3N4 composite (42.2 wt%SC) was investigated by a 10-run cycling test under the same condition For each run, the photocatalyst was recycled, cleaned, and dried The photodegradation efficiency of 42.2 wt%SC shows no apparent decrease after the 10 reuse cycles, indicating its stability (Fig 9a) The stability of SnO2 À x/g-C3N4 can also be proven by XRD analysis (Fig S7) The XRD pattern of the used SnO2 À x/g-C3N4 sample reveals that no change have occurred observed after the photocatalytic reaction The results in Figs 9a and S4 suggest that the SnO2 À x/g-C3N4 photocatalyst can be reused completely for wastewater treatment In addition to high stability, the SnO2 À x/g-C3N4 hybrid also exhibits the feasibility for the degradation of various organics Fig 9b shows the photocatalytic activity of the 42.2 wt% SC sample for photodegradation of RhB, MO, MB, and phenol under visible light irradiation The SnO2 À x/g-C3N4 hybrid exhibits high degradation efficiency for all three dyes For phenol, only 40% of the initial content is degraded under visible light irradiation for 90 However, considering the high concentration of phenol (50 mg/L), the SnO2 À x/g-C3N4 composite can still be seen as an efficient photocatalyst To demonstrate the high photocatalytic activity of SnO2 À x/gC3N4, the prepared composite samples were evaluated using the reaction of photocatalytic CO2 reduction into fuels that is known to be a challenging but promising application for sustainable energy resources [42–44] The test results are shown in Fig 10 The blank Fig Cycling runs of 42.2 wt% SC composite (a) and its photocatalytic activity for different organics (b) under visible light irradiation (λ 4420 nm) 182 Y He et al / Solar Energy Materials & Solar Cells 137 (2015) 175–184 Fig 10 Photocatalytic activities of SnO2 À x/g-C3N4 composites on photocatalytic CO2 reduction under simulated sunlight irradiation Fig 11 Possible schemes for electron–hole separation and transport at the visiblelight-driven SnO2 À x/g-C3N4 composite interface test indicates that the reduced products could be ignored in the absence of either a photocatalyst or simulated sunlight irradiation À1 À1 Pure g-C3N4 shows a CO2 reduction rate of 5:32 μmol h gcat , which is slightly higher than that of P25 The detected products are CO, CH3OH, and CH4 For P25, only CO and CH4 are observed which is due to the low conduction band position of P25 and the easy formation of CO and CH4 products [45] No reduced carbon product is observed in the presence of SnO2 À x as a result of its low CB band potential However, the decoration of SnO2 À x on g-C3N4 can effectively promote the catalytic performance for CO2 photoreduction With increased the SnO2 À x concentration, the photocatalytic activities of SnO2 À x/g-C3N4 composites increase gradually and then decrease The highest photocatalytic performance is obtained with the use of 42.2 wt% SC sample The CO2 reduction À1 À1 rate reaches 22:7 μmol h gcat , which is 4.3 and times higher than those of g-C3N4 and P25, respectively The decoration of SnO2 À x on g-C3N4 generates an efficient photocatalyst for both CO2 photoreduction and dye photodegradation hetero-junction structure to suppress the recombination of electron–hole pairs, as proved by the PL, EIS and photocurrent analyses However, the route of charge transfer remains controversial because both the double-charge-transfer and Z-scheme mechanism can promote the separation of electron and holes For the first mechanism, the photogenerated electrons on the g-C3N4 surfaces would transfer to SnO2 À x because of the difference in CB edge potentials, whereas the holes in SnO2 À x would move to the VB of g-C3N4 Thus, the electrons and holes are separated and accumulated on the surface of SnO2 À x and g-C3N4, respectively However, the enriched electrons on the SnO2 À x cannot reduce CO2 to fuel because of the low CB potential If SnO2 À x/g-C3N4 follows a double-charge-transfer mechanism, the decoration of SnO2 À x would not promote the photocatalytic CO2 reduction of g-C3N4 This result is inconsistent with the photocatalytic experiment Therefore, a Z-scheme mechanism is more suitable for the SnO2 À x/ g-C3N4 hybrids In Fig 11, the photogenerated electrons from the SnO2 À x semiconductor recombine with photogenerated holes from the g-C3N4 This process can also markedly improve the photogenerated electron–hole pair separation and retain the electrons on the CB of g-C3N4, which results in the high photoactivity of SnO2 À x/g-C3N4 composites in the photocatalytic CO2 reduction to fuels under simulated sunlight irradiation A series of radicals trapping experiments were performed using benzoquinone (BQ), KI, and isopropanol (IPA) scavengers to further prove the direct Z-scheme mechanism of SnO2 À x/g-C3N4 Fig 12 shows the photocatalytic activity of 42.2 wt% SC in the presence of these quenchers The inset is the corresponding kinetic constants of 42.2 wt% SC and g-C3N4 The addition of BQ (quencher of O2À ) [24,25] and KI (quencher of H þ and dOH) [24,25] results in a significant suppression of the degradation rate, whereas IPA (quencher of dOH) [24,25] has nearly no effect on the RhB degradation in the presence of 42.2 wt% SC catalyst This result indicates that the O2À and H þ are the main reactive species during the photocatalytic oxidation of RhB A similar result is also obtained on g-C3N4 Considering that the CB edge potential of SnO2 À x is more positive than EO2 =O À ( À0.046 V) and the electrons on SnO2 À x cannot reduce O2 to O2À species[46], the active trapping experiments indicate that the photoexcited electrons in SnO2 À x/g-C3N4 hybrids accumulate on the CB of g-C3N4 This result demonstrates that the direct Z-scheme mechanism works in the composite In addition to the scavenging experiments of the reactive species, the photocatalytic activity of composite photocatalyst under different light sources can also provide useful information 3.3 Possible photocatalytic mechanism in the SnO2 À x/g-C3N4 system The surface area, light absorption ability, and separation efficiency of electron–hole pairs are closely correlated with the catalytic performance of a photocatalyst In the case of SnO2 À x/gC3N4 hybrids, the introduction of SnO2 À x promotes the surface area of g-C3N4, which is beneficial for dye adsorption and the subsequent photocatalytic reaction However, no regularity between the BET surface areas of SnO2 À x/g-C3N4 hybrids and photoactivities can be observed The SnO2 À x/g-C3N4 sample with the highest surface area does not exhibit the highest photocatalytic activity The adsorption experiment in the dark also verifies that the RhB adsorption ability of the SnO2 À x/g-C3N4 photocatalyst shows certain consistency with the BET surface area (Fig S8), but not in agreement with the photocatalytic activity This result indicates that the specific surface area and the light absorption capability (as shown by DRS analysis), are not the dominant factors affecting the photocatalytic activity of SnO2 À x/g-C3N4 Therefore, the high activity of SnO2 À x/g-C3N4 may have been caused by the excellent separation efficiency of electron–hole pairs The VB edges of SnO2 À x and g-C3N4 are determined to be 2.70 and 1.51 eV, respectively via the VB XPS experiment Using the equation of ECB ¼EVB À Eg, the CB edge potentials of the two semiconductors can be obtained From Fig 11, the CB potentials of g-C3N4 and SnO2 À x are À 1.19 and 0.20 eV, respectively The two semiconductors have suitable band potentials and can form the Y He et al / Solar Energy Materials & Solar Cells 137 (2015) 175–184 183 because excessive coupling of SnO2 À x leads to the shielding of the active site on g-C3N4 surfaces, similar to the results obtained by Wang et al [48–50]; they found that co-exposure of both semiconductors on the surface is necessary to enhance photocatalytic activity in the hetero-junction system Conclusion Fig 12 Photodegradation of RhB over 42.2 wt%SC photocatalyst with different quenchers (λ 4420 nm) Sn2 þ -doped SnO2 was hybridized with g-C3N4 to generate an efficient photocatalyst for dye photodegradation and photocatalytic CO2 reduction The experimental data indicate that SnO2 À x introduction leads to the formation of SnO2 À x–g-C3N4 heterojunction, which hinders the recombination of electron–hole pairs and results in enhanced photoactivity Meanwhile, the reactive species trapping experiment verifies that the SnO2 À x/g-C3N4 composite follows a direct Z-scheme mechanism This study might provide a promising approach to address the low photoactivity of pristine gC3N4 for water purification and CO2 reduction Acknowledgments This work was financially supported by Natural Science Foundation of Zhejiang Province in China (LY14B030002) Appendix A Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.solmat.2015.01.037 References Fig 13 Photocatalytic activities of g-C3N4 and 42.2 wt% SC in RhB degradation under light with different wavelength on the Z-scheme mechanism Sasaki et al found that the photocatalytic activity of a Z-scheme composite was dominated by the absorption of the semiconductor with a wider band gap [47] This rule was also applied by Kondo et al to verify the Z-scheme mechanism of S-TiO2/g-C3N4 [20] In the current study, the photoactivity of SnO2 À x/g-C3N4 and g-C3N4 in RhB degradation was tested by irradiation at different wavelengths The result indicates that the photocatalytic activity of SnO2 À x/g-C3N4 is much higher than that of g-C3N4 when the wavelengths of the cut-off filter are 320, 360, and 420 nm (Fig 13) However, when the wavelength is 480 nm, both the photoactivities of g-C3N4 and SnO2 À x/g-C3N4 significantly decrease Since all incoming photons with wavelengths lower than 480 nm are stopped during the experiment, resulting the excitation of SnO2 À x but not g-C3N4, the decreased activity for pure g-C3N4 is reasonable However, for the SnO2 À x/gC3N4 sample, the result in Fig 13 indicates that the present photocatalysis system (SnO2 À x/g-C3N4) works through a direct Zscheme mechanism The photoexcitation of both semiconductors is required to highlight the promotion effect of SnO2 À x Otherwise, the invalidation of the Z-scheme mechanism would lead to a significant decrease in photocatalytic activity of the composite Meanwhile, although the coupling of SnO2 À x can greatly enhance the photocatalytic efficiency, the concentration of SnO2 À x plays a critical role The increase in the 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1s peaks for SnO2 À x and 42.2 wt%... stirring for two hours, the precipitate was filtered, and washed many times by water and ethanol to remove Cl À and K þ Yellow Sn6O4(OH)4 was obtained in a powder form after drying in oven at 60 1C for. .. and times higher than those of g-C3N4 and P25, respectively The decoration of SnO2 À x on g-C3N4 generates an efficient photocatalyst for both CO2 photoreduction and dye photodegradation hetero-junction