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Z-scheme NiTiO3/g-C3N4 heterojunctions with enhanced photoelectrochemical and photocatalytic performances under visible LED light irradiation Zhenyu Huang, Xiaoqiao Zeng, Kai Li, Shanmin Gao, Qingyao Wang, and Jun Lu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b12386 • Publication Date (Web): 07 Nov 2017 Downloaded from http://pubs.acs.org on November 8, 2017

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ACS Applied Materials & Interfaces

Z-scheme NiTiO3/g-C3N4 heterojunctions with enhanced photoelectrochemical and photocatalytic performances under visible LED light irradiation Zhenyu Huang a#, Xiaoqiao Zeng b#, Kai Li a, Shanmin Gao a,*, Qingyao Wang a, Jun Lu b,* a

b

School of Chemistry and Materials Science, Ludong University, Yantai, 264025, P R China

Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, IL 60439, USA

*corresponding authors E-mail: [email protected] (S. Gao); [email protected] (J. Lu) # These authors contribute equally.

Abstract: Direct Z-scheme NiTiO3/g-C3N4 heterojunctions were successfully assembled by using simple calcination method and the photoelectrochemical and photocatalytic performance were investigated by light emitting diode (LED). The photoanode composed by the heterojunction with about 50 wt% NiTiO3 content exhibits the best photoelectrochemical activity with photoconversion efficiency up to 0.066%, which is 4.4 and 3.13 times larger than NiTiO3 or g-C3N4. The remarkably enhanced photoelectrochemical and photocatalytic activity of the heterojunction can be due to the efficiently photogenerated electron-hole separation by a Z-scheme mechanism. Keywords: NiTiO3/g-C3N4, Z-scheme, photoelectrochemical, photocatalytic, visible LED light

The semiconductor photoelectrode and photocatalytic (PC) degradations of organic pollutants via photoelectrochemical (PEC) water-splitting are regarded as one of the most promising way for hydrogen energy source production and green environmental stretagy.1 Recently, the perovskite-like NiTiO3 has been attracting more attention because it shows high stability in oxidizing environments 1

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under visible light illumination.2-4 However, the narrow band gap energy leads to higher photo-generated hole-electron pair recombines rate and lower quantum efficiency, which seriously hinders the PC activity when using individual NiTiO3 as photocatalyst.4 Recently, the graphitic carbon nitride (g-C3N4) as another narrow band semiconductor has been widely studied.5,6 It is because the stacking layer in single g-C3N4 will decrease the active sites and separation efficiency of photogenerated electron-hole pairs, leading to inefficient carrier transfer and low PEC and PC activities.7,8 In order to overcome these shortcomings, one effective way is preparing exfoliated g-C3N4 nanosheets, which can provide abundant reactive sites.9 Other effective method is recombining with another semiconductor into the heterojunctions, to reduce the photogenerated charges recombination.7,8,10 The heterojunction structure is beneficial to improve PEC and PC performance, in efficient charge separation, light harvesting, and also the faster electron-hole transfer through heterojunction.11-14 Direct Z-scheme photocatalytic systems with two narrow band-gap semiconductors are suitable for harvesting solar energy.13-15 More importantly, the excellent redox ability can be preserved in the Z-scheme photocatalytic system.16 The band gap of NiTiO3 and g-C3N4 are about 2.2 and 2.7 eV, and the CB, VB positions are about 0.2, -1.2 and 2.4, 1.5 eV, respectively.3,5 Due to their suitable band edges, NiTiO3 and g-C3N4 can form a heterojunction. Recently, the reported coupling NiTiO3 with g-C3N4 was scarce.7,17 Narrow band gaps semiconductors are usually not able to provide redox potentials matching strong redox power.18 However, if they can form the Z-scheme photocatalytic system, the excellent redox ability can be preserved due to the extremely negative CB minimum of g-C3N4 and the extremely positive VB maximum of NiTiO3. Furthermore, in-depth investigation the 2


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charge separation and electron transport mechanism are desirable for practical application.10 Therefore, we report the studies of a direct Z-scheme heterojunction based on NiTiO3 and g-C3N4. The PEC and PC performances of this coupled system were evaluated by using a low-energy consumption light emitting diode (LED) as irradiation source. The mechanisms of the separated photogenerated electrons were proposed based on the results for the linear scan voltammetry, electrochemical impedance spectrum and photoluminescence analysis. The direct Z-scheme heterojunctions made significant contribution to PEC and PC performances. The NiTiO3/g-C3N4 heterojunction was formed by the following procedure: 2 g of obtained agglomerate g-C3N4 and NiTiO3 powder were mixed in 50 mL of methanol and well-dispersed in a sonicator for 20 min. In this process, the exfoliation will occur for agglomerate g-C3N4 and the agglomerated NiTiO3 nanorods will disperse into nanoparticles.19 The yellowish mixture was obtained by vacuum drying the slurry at 60 °C for 12 h. The powder was calcined at 400 °C for 2 h to form the NiTiO3/g-C3N4 heterojunction. To investigate the effect of NiTiO3 content on the PEC and PC performance of the heterojunctions, the products of NTCN-30, NTCN-50, and NTCN-70 were synthesized with various NiTiO3 mass ratios of 30, 50, and 70 wt%, respectively. The products were examined by TEM, XRD, HRHEM, UV-vis DRS, BET, XPS and PL. The PEC and PC performances were test under a 30 W LED irradiation light source. For the preparation experiments details, the characterization of photocatalysts, the PEC and PC experiments details, please see Supporting Information (SI). Figure 1a is the XRD spectra of pure g-C3N4, NTCN-30 and NTCN-50. The pure g-C3N4 gives two peaks found at 2θ = 13.2 and 27.4°, which were indexed as (110) and (200) plane of g-C3N4 3



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(JCPDS, 87-1526), respectively.5 The NTCN specimens are not showing apparent g-C3N4 (110) peaks profile, indicating the layered agglomerate g-C3N4 got exfoliated successfully in the ultrasonic treatment processing.9 In addition, the relative intensity of peaks at ~27.4° decreases due to the low g-C3N4 contents (SI, Figure S1). From TEM and HRTEM (Figure 1 and Figure S2), the pure NiTiO3 has nanorods structure aggregated by nanoparticles (Figure S2a, b). Serious lamellar structured aggregation is observed in large scale in the pure g-C3N4 (Figure 1b). For the NCTN samples, the NiTiO3 nanorods are well-disappeared into nanoparticles, but distributed on the exfoliated g-C3N4 nanosheets surface (Figure 1c,d). In addition, with the increase of the NiTO3 amount, leading to the aggregation of NiTiO3 nanoparticles on the g-C3N4 surface gradually (Figure 1c, d and Figure S2c). Such aggregation was verified to reduce the specific surface area from the BET tests (Figure S3a and inset table). The NiTiO3/g-C3N4 heterojunctions have larger surface area than that of NiTiO3 or g-C3N4. The increased surface area of heterojunctions may be caused by the highly dispersed NiTiO3 nanoparticles attached onto the exfoliated nanosheets (g-C3N4) surface. On the other hand, for the NTCN-X samples, there are more abundant pore structures (Figure S3b). These results indicated that the exfoliation was happened for g-C3N4 and the NiTiO3 nanoparticles have better dispersion in the heterojunctions.

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Figure 1.

(a) XRD profile of the pure g-C3N4, NTCN-50 and NTCN-30. TEM images of (b) pure g-C3N4, (c) NTCN-30, (d) NTCN-50.

XPS spectra confirmed the presence and chemical states of Ti, Ni, N, C and O in the samples (SI, Figure S4). The N 1s spectra of pure g-C3N4 and NTCN-50 sample were given in Figure 2a. For pure g-C3N4, the peaks at 398.4 eV, 400.7 eV, and 404.2 eV are deconvoluted N 1s peaks of the sp2-hybridized N (C=N-C), C-N-H and π excitation, respectively.20 For the NTCN-50 sample, there is no signal at 404.2 eV, indicating that the g-C3N4 structure changed during the formation of heterojunction by its interaction with NiTiO3.10,21 For the pure NiTiO3 (Figure 2b), two peaks at ~ 458.2 and 464.1 eV are corresponding to the Ti 2p3/2 and Ti 2p1/2 binding energies, respectively. For the NTCN-50 sample, the lower energy Ti 2p peak shifting indicates the interaction between NiTiO3 and g-C3N4 in heterojunctions.21 Above all, the NiTiO3/g-C3N4 heterojunction structures were successfully fabricated as designed.10 The UV-vis DRS results (Figure 2c) indicated that pure NiTiO3 and NiTiO3/g-C3N4 heterojunctions have intense absorption within the visible light range. The band gap values for pure NiTiO3 and g-C3N4 are converted as 2.22 and 2.7 eV, respectively (Figure 2d and SI). It 5



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can be expected that the light absorbing capacity in the visible-light range of the NiTiO3/g-C3N4 heterojunctions will provide a high visible-light PEC and PC activity to this heterojunctions.

Figure 2.

(a, b) High-resolution XPS profile of N 1s and Ti 2p on pure NiTiO3, g-C3N4 and sample

NTCN-50. (c) UV-Visible Diffuse Reflectance Spectroscopy of g-C3N4, NiTiO3 and NTCN heterojunctions. (d) calculated band gap of g-C3N4 and NiTiO3. From LSV, the PEC performances of the pure g-C3N4, NiTiO3 and NTCN-X photoelectrodes were studied under the 30 W visible light LED light illumination. The LSV results (Figure 3a) indicated that the pure NiTiO3 and g-C3N4 photoelectrodes exhibit lower photocurrent density. It is due to the high photogenerated electron-hole recombination. On the contrary, the increased photocurrent density shown in NiTiO3/g-C3N4 heterojunctions photoelectrodes are with higher the bias potential. In addition, the photocurrent of the NTCN-50 photoelectrode was significantly increased compared with others, indicating that the higher separation and transport efficiencies of photogenerated electron–hole pairs. This can be certified by the EIS and PL results (SI, Figure S5), confirming the significant contribution 6


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of the heterojunctions in inhibiting the recombination of electrons and holes.21 The photoconversion efficiency was plotted as a function of the applied bias (Figure 3b). The pure NiTiO3 and g-C3N4 show the maximum conversion efficiency of ~0.016% and ~0.021%. However, the NTCN-50 photoanode yielded a maximum efficiency of 0.066%, which is about 4.4 and 3.13 times higher than that of the pure NiTiO3 and g-C3N4, respectively. The PC activities were examined by degrading RhB under 30 W visible LED light irradiation. Before the degradation, the samples achieved adsorption/desorption equilibrium by kept stirring in the dark for 30 min (SI, Figure S6). Under 30 W visible-light LED light illumination, NTCN-X samples significantly enhance the visible PC performance on the decomposition of RhB in comparison to the pure g-C3N4 and NiTiO3(SI, Figure S7a). The RhB degradation rate constants are following: NTCN-50 > NTCN-30 > NTCN-70 > pure g-C3N4 > pure NiTiO3, where NTCN-50 shows a highest value of kapp = 0.0309 min-1, which is about 11.44 and 5.72 times greater than that of pure NiTiO3 and g-C3N4, respectively (Figure 3c). To better study the photodegradation of RhB, the total organic carbon (TOC) was measured in the photodegradation process and the results were shown in Figure 3d. Obviously, the removal efficiency reached 52% in the presence of NTCN-50 heterojunction after 100 min of visible-light LED irradiation, which confirmed that RhB was mineralized by the heterojunctions. Also, the TOC removal rate for both the pure g-C3N4 and NiTiO3 was lower than the heterojunctions. Obviously, more NiTiO3 content in the heterojunctions will lead to decreased degradation rate and mineralization efficiency, which may because of the agglomeration of NiTiO3 in the g-C3N4 sheet. Thus, the agglomerations can decrease the specific surface areas and PC activity. As a result, suitable amount and distribution of NiTiO3 are crucial for the improvement of PC activity. In addition, the 7



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heterojunction exhibits a good recyclability with a slight decline after 10 cycling experiment tests (SI, Figure S7b, Figure S8).

Figure 3.

(a) LSV and (b) calculated photoconversion efficiencies results for the pure g-C3N4,

NiTiO3 and NTCN-X heterojunctions. (c) Photodegradation rate and (d) mineralization efficiency of RhB by using different samples as photocatalyst under cold visible LED light irradiation. The band-edge potential level is another important factor for the determination of photogenerated electron-holes flowchart in a heterojunction. Based on the DRS results, the pure NiTiO3 and g-C3N4 band gap value were around 2.22 eV and 2.7 eV, respectively. To approach the mechanism of the enhanced PEC and PC activities of the heterojunctions, the relative NiTiO3 bands positions were investigated (SI, Figure S9). The CB and VB edges were calculated as 0.21, 2.43 eV, respectively. Based on the published reports, the of g-C3N4 CB and VB potentials were located at −1.2 and 1.5 eV, respectively.5 As a result, NiTiO3 and g-C3N4 have suitable CB and VB levels, and consequently can form the heterojunctions to reduce the recombination of photoinduced carriers, consisting with the PL, LSV, and EIS analyses. 8


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When the NiTiO3/g-C3N4 follows a double-charge transfer mechanism, the photogenerated electrons in the g-C3N4 CB well would transfer to the NiTiO3 CB. Meanwhile, the holes in the NiTiO3 VB well would inject into the g-C3N4VB. Since the standard redox potential of O2/•O2- and H2O/•OH are -0.33 and 2.27 V,

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the NiTiO3 CB potential and the g-C3N4 HOMO potential are obviously

lower than O2/•O2- and H2O/•OH redox potential, respectively. Therefore, there is no •O2- or •OH radicals formed if the NiTiO3/g-C3N4 follows the double-charge transfer rule. From the g-C3N4 and O2/•O2- potential values, only the electrons in the g-C3N4 CB can capture O2 to generate •O2- specie, implying the electron transfer direction is from the NiTiO3 CB to the g-C3N4 VB. Therefore, the NiTiO3/g-C3N4 heterojunctions should follow the Z-scheme-type mechanism. According to these results and analysis, we illustrated the mechanism diagram as shown in Figure 4. According to Z-scheme-type mechanism, the narrow band gaps of NiTiO3 and g-C3N4 allow them easily absorb the visible light. The visible light wavelength from LED light source completely coincides with our samples. It allows electrons jump from the VB to the CB of NisTiO3 and g-C3N4, respectively [Figure 4(1)]. The excited electrons on CB of NiTiO3 will inject into the VB of g-C3N4 to recombine with photogenerated holes, thereby restraining the electron–hole pairs recombination [Figure 4(2)]. It was confirmed by EIS and PL spectra (SI, Figure S5). Obviously, the electrons on the CB of g-C3N4 behave as good reductants. It can capture the dissolved oxygen to generate •O2− since the CB level are lower than the O2/•O2− potential [Figure 4(3)]. Therefore, the RhB molecules were degraded through a series of reaction [Figure 4(4)]. In addition, the NiTiO3 VB holes can oxidize the surface OH- and H2O to generate •OH [Figure 4(5)]. As a result, these highly active species mainly contributed to the promoted RhB degradation.

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Figure 4.

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Schematic diagram of mechanism of the NiTiO3/g-C3N4 heterojunction under visible-light LED irradiation.

Aiming to verify the active species, we detected the main activity species by adding the trapping agent of radicals and holes in the PC process (SI, Figure S10). The RhB degradation efficiency slightly decreased by adding TBA, excluding the •OH radical from the reactive species. The PC activity is obviously inhibited when AO is added. It reveals that the holes (h+) have a great impact on the PC process. The rate of degradation of RhB decreased heavily after the adding of BQ, exhibiting that the •O2- specie makes a crucial contribution in the PC process. Moreover, a control experiment in a N2 atmosphere showed the decreased PC efficiency, proving that O2 primarily acts as efficient electron traps generateing •O2-. Furthermore, the ESR results also indicated that the •O2- radicals playing a crucial role during the photodegradation (SI, Figure S11). Therefore, the h+ and •O2- are concluded as the pivotal reactive species in the RhB degradation process over the NTCN-x heterojunctions. The aforementioned results demonstrate that the enhanced PEC and PC activities of the NiTiO3/g-C3N4 heterogeneous junctions are due to: (i) The suitable NiTiO3 and g-C3N4 band gaps can utilize more visible light and the formation of heterojunction can promote the electron–hole pairs separation at the heterojunction interface; (ii) the exfoliated g-C3N4 nanosheets increase the consequent 10


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contact surface with NiTiO3 nanoparticles, facilitating the transportation of reactants and products on photocatalyst surfaces and resulting in facile chemical reactions. In summary, we reported the fabrication and characterization of direct Z-scheme NiTiO3/g-C3N4 heterojunctions. In as-prepared heterojunction, the NiTiO3 nanoparticles dispersing on the g-C3N4 surface which its aggregation can be restrained. Benefiting from a unique nanostructure and direct Z-scheme heterojunction, such NiTiO3/g-C3N4 composites exhibit an enhanced PEC and PC performances comparing with the pure NiTiO3 and g-C3N4. The irradiation was carried out by using 30 W visible light LED light with low-energy consumption. The efficient separation of electrons and holes originated from the formation of NiTiO3/g-C3N4 heterojunctions was beneficial to the PEC and PC activity. The results of the promoted PEC and PC activities provided meaningful guidance for the research of Ti-based PEC and PC materials, which are promising for degrading organic pollutants and water splitting under solar irradiation and even for related applications. Supporting information Additional text and figures for the following information are listed in the supporting information: XRD patterns of the pure NiTiO3 and NTCN-70 sample; TEM images of pure NiTiO3 and NTCN-70 sample; HRTEM image of NCTN-30, NTCN-50 and NTCN-70 samples; The sample specific surface area and corresponding Barret–Joyner–Halenda (BJH) pore-size distribution profile; survey XPS spectrum and high-resolution XPS spectra of C 1s, Ni 2p, and O 1s for pure NiTiO3, g-C3N4 and sample NTCN-50; EIS spectra of Nyquist plots collected at open circuit potential and PL spectra of the obtained samples; adsorption/desorption results for photodegradation of RhB and recycling test result by using the NTCN-50 sample; XRD patterns of NTCN-50 sample before and after irradiation; trapping experiment

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of the active species for the photodegradation of RhB on sample NTCN-50; DMPO spin-trapping ESR spectra of hydroxyl radicals and superoxide radicals from NTCN-50 sample under LED light irradiation. Acknowledgments This work was supported by the Science and Technology Development Plan Project of Shandong Province (2014GSF117015), the National Basic Research Program of China (Grant No. 2013CB632401) and the National Nature Science Foundation of China (51402145). J. Lu gratefully acknowledges support from the U. S. Department of Energy (DOE), Office of Energy Efficiency and Renewable Energy, Vehicle Technologies Office.

Argonne National Laboratory is operated for DOE

Office of Science by UChicago Argonne, LLC, under contract number DE-AC02-06CH11357. Notes: The authors declare no competing finanical interest. References (1)

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