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Influence of g-C3N4 Precursors in g-C3N4/NiTiO3 Composites on Photocatalytic Behavior and the Interconnection between g-C3N4 and NiTiO3 Thanh-Truc Pham, and Eun Woo Shin Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b02596 • Publication Date (Web): 18 Oct 2018 Downloaded from http://pubs.acs.org on October 19, 2018

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Influence of g-C3N4 Precursors in g-C3N4/NiTiO3 Composites on Photocatalytic Behavior and the Interconnection between g-C3N4 and NiTiO3 Thanh-Truc Pham, and Eun Woo Shin* School of Chemical Engineering, University of Ulsan, Daehakro 93, Nam-gu, Ulsan 44610, South Korea KEYWORDS: g-C3N4 precursor; Photocatalytic behavior; Composite; Dicyandiamide; Thiourea; Interconnection

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ABSTRACT

In this study, composite photocatalysts were produced from NiTiO3 and N2-rich precursors (dicyandiamide, melamine, urea and thiourea) under N2 flow conditions. The goal of the study was to investigate the interaction between NiTiO3 and the synthesized g-C3N4. The properties of the g-C3N4/NiTiO3 (CNT) composites were different depending on the starting materials. Dicyandiamide and thiourea created strong connections with NiTiO3 and resulted in the generation of Ti-N and Ti-O-S bonds. Urea and melamine, however, had difficulty forming gC3N4 structures or interconnections with NiTiO3. The Ti-N and Ti-O-S bridges in the composite photocatalysts led to increased photocatalytic activity as well as inhibition of the recombination rate. Additionally, the band diagrams of g-C3N4 prepared from dicyandiamide and thiourea exhibited positions suitable for the Z-scheme charge transfer model with NiTiO3, implying that the composite photocatalysts were applicable for photocatalytic degradation of organic contaminants under the visible-light irradiation. Higher reaction rate constants for the composites prepared with dicyandiamide and thiourea confirmed the significant role of the Ti-N/Ti-O-S bridge between g-C3N4 and NiTiO3.

TEXT Introduction Recent focus on photocatalysts has extensively grown due to their widespread applications, including photocatalytic environmental remediation and solar energy conversion

1-2.

Since

photocatalysts are a semiconductor that can be excited by light irradiation, numerous single 2


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metal oxides and sulfides such as titanium dioxide (TiO2), zinc oxide (ZnO), cadmium sulfide (CdS), molybdenum disulfide (MoS2), iron (III) oxide (Fe2O3), and zinc sulfide (ZnS) have been used commercially under UV or visible light irradiation

3-8.

Recently, many researchers have

paid attention to multi-metal oxide photocatalysts due to wider applications under better conditions, including less energy consumption and higher photocatalytic performance. The catalysis of multi-metal oxide materials is generated by intentionally introducing foreign elements into the single metal crystal structure, thus accidentally creating complicated transitions of charge carriers 9. Among the various multi-metal oxide photocatalysts, ilmenite nickel titanate (NiTiO3) materials have been introduced as novel visible-light-active photocatalysts with facile preparation based on the traditional single metal oxide TiO2. NiTiO3 is also a semiconductor that has a direct band gap with a wide range of 2.1-2.9 eV 10-12. Besides metal-containing materials, metal-free semiconductors have been noted as a new possibility for photocatalytic applications. At present, graphitic carbon nitride (g-C3N4) is described as one of the most well-known metal-free semiconductors for photocatalysis 13. g-C3N4 can be prepared by bottom-up methods, including microwave irradiation, ionic liquid strategy, and thermal polymerization from nitrogen-rich precursors such as cyanamide, dicyandiamide (DCDA), melamine, urea, and thiourea

13.

g-C3N4 has band gap of 2.7-2.9 eV, which

approximates that of NiTiO3. Although both NiTiO3 and g-C3N4 are visible light-active photocatalysts, both are limited by poor quantum efficiency and fast recombination rates when applied as individual photocatalysts. Composite photocatalysts of the individual materials can overcome the limit in the quantum efficiency since the difference in band positions of these two materials supports the type II heterojunction or direct Z-scheme charge transfer theory in the composites 14. 3


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g-C3N4/NiTiO3 composite have been fabricated by several research groups as a high activity photocatalyst 15-17. Zeng et al. applied g-C3N4/NiTiO3 composites for H2 production and showed that the production rate of the g-C3N4/NiTiO3 composite was higher than that of g-C3N4

15.

Mesoporous g-C3N4/NiTiO3 composites were prepared as a visible light-active photocatalyst for removing nitrobenzene and resulted in higher photocatalytic activity than that of NiTiO3 alone 17. Our previous study investigated how temperature affects the thermal formation of gC3N4/NiTiO3 composites and used these composites for the photocatalytic degradation of methylene blue

18.

The photocatalytic activities of g-C3N4/NiTiO3 composites depended on the

thermal treatment temperatures and the different interactions between g-C3N4 and NiTiO3 resulted in variations in the properties of the g-C3N4/NiTiO3 composites. Different types of g-C3N4 precursors can affect the thermal formation of g-C3N4 structures in the composites and consequently change the properties and photocatalytic behavior of the composites. The reaction pathways from different precursors to the g-C3N4 structure are described elsewhere 19-21. N2-rich chemicals such as DCDA, melamine, urea and thiourea, whose chemical structures are shown in Scheme 1, are used as precursors in the thermal formation of gC3N4. DCDA, urea and thiourea are small molecules which react at high temperature to release gases (NH3, CO2, H2S, etc.). They then convert into an intermediate compound (melamine), and finally, condense to polymeric g-C3N4

19.

Dong et al. created a novel g-C3N4 (urea)/g-C3N4

(thiourea) heterojunction and demonstrated its enhanced photocatalytic activity due to the efficiency of the charge transfer in the heterojunction from the difference in the g-C3N4 (urea) and g-C3N4 (thiourea) band gaps

22.

Diverse g-C3N4 photocatalysts were synthesized using

different precursors in a burning explosion and then used in the photocatalytic degradation of tylosin 23. The type of g-C3N4 precursor had a crucial effect on the crystalline structure of the g4


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C3N4 and its photocatalytic performance. In addition, there are more studies related to g-C3N4 precursors

22-25,

g-C3N4/metal compound composites

26-37,

or g-C3N4/NiTiO3 composites

15-17.

However, to the best of our knowledge, there have been no reports describing the g-C3N4 precursor effect on the thermal formation of g-C3N4/NiTiO3 composites and their photocatalytic performance. The different g-C3N4 precursors can bring the different interaction between the gC3N4 precursors (organic) and metal oxide NiTiO3 (inorganic) during the thermal formation of gC3N4 structure, resulting in different composite structures and photocatalytic behavior. The aim of this study is to understand the influence of different types of g-C3N4 precursors (DCDA, melamine, urea and thiourea) on the thermal formation of g-C3N4/NiTiO3 composites and their properties. We also investigated their photocatalytic behavior using the photocatalytic degradation of methylene blue. Due to their different chemical structures, the photocatalytic performance and physicochemical properties of the g-C3N4/NiTiO3 composites are significantly different from one another. The presence of CN or sulfur in DCDA and thiourea plays an important role in the formation of pristine g-C3N4 and g-C3N4/NiTiO3 composites. Their photocatalytic performance was explained based on the Z-scheme charge transfer model and the resultant composite structures.

Materials and methods Preparation of g-C3N4 and their composites with NiTiO3 Pristine g-C3N4 compounds were synthesized with precursors DCDA, melamine, urea and thiourea. Original white powder precursors were purchased from Sigma-Aldrich Korea (Gyeonggi, South Korea) and ground for 10 min using a mortar. The powder was put on a 5


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crucible boat covered with aluminum foil containing holes and placed inside a tube furnace. N2 gas was purged through the tube furnace for 30 min to remove all of the air before starting thermal treatment. Thermal treatment was conducted at 550 C for 4 hours (ramping rate = 10 C/min). After the treatment, the solid sample was removed at room temperature, ground again, washed with a deionized water-ethanol mixture, and dried overnight in an 80 C oven. The gC3N4 samples were denoted as dC, mC, uC, and tC, corresponding to the starting g-C3N4 precursors DCDA, melamine, urea, and thiourea, respectively. NiTiO3 was synthesized by hydrothermal treatment method. The details of preparation and properties of NiTiO3 was reported in the previous study, and here it was denoted as NT 18. The composites were produced using the same procedure as the pristine g-C3N4 except for the step where NiTiO3 was ground into the mixture before being placed into the crucible boat. The mixture was heated at 500 C for 4 hours (ramping rate = 10 C/min). The mass ratio between the precursors and NiTiO3 was fixed at 1:1. The composites were designated as dCNT, mCNT, uCNT and tCNT corresponding to composites prepared from DCDA, melamine, urea, and thiourea, respectively. Characterization Field-emission scanning electron microscopy (FE-SEM; JSM-600F, JEOL, Tokyo, Japan) and high-resolution transmission electron microscopy (HR-TEM; JEM-2100F, JEOL, Tokyo, Japan) were employed to study the morphologies of the photocatalysts. Crystalline structures were determined via X-ray diffraction (XRD) with a Rigaku D/MAZX 2500V/PC high-power diffractometer (Tokyo, Japan) utilizing a Cu K X-ray source with a wavelength of 1.5415 Å and a scanning rate of 2° (2)/min. Functional groups were verified using a Nicolet 380 Fourier transform infrared (FTIR) spectrometer (Thermo Scientific Nicolet iS5 with an iD1 transmission 6


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accessory, Waltham, MA, USA). A Thermo Scientific K-Alpha X-ray photoelectron spectroscopy (XPS) system (Waltham, MA, USA) was used to measure the elemental composition empirical formula, chemical state, and electronic state of the elements in the materials. The optical properties of the materials were studied by measuring the absorbance of photons via UV-visible diffuse reflectance (UV-Vis-DRS; SPECORD 210 Plus spectroscope, Analytik Jena, Jena, Germany) and the recombination rate of charges using a Cary Eclipse fluorescence spectrophotometer (PL; Agilent Technologies, Santa Clara, CA, USA) at room temperature with a 473 nm diode laser. The ultraviolet photoelectron spectroscopy measurements (UPS; Thermo Fisher Scientific, model ESCALAB 250XI, Waltham, MA, USA) were obtained using a He(I) = 21.2eV light source at a resolution of 0.02eV. Photocatalytic degradation of methylene blue An aqueous solution containing 10 mg/L of methylene blue (MB; Sigma-Aldrich Korea, Gyeonggi, South Korea) was prepared for photocatalytic degradation. During the test, 10 mg of the photocatalyst was immersed in 50 ml of the aqueous MB solution, and continuously stirred in the dark for 30 min for adsorption. After reaching equilibrium adsorption, the solution was irradiated for 150 min using four visible light sources (model GB22100(B)EX-D Eltime, 100 W) located on four edges. The MB concentration was measured instantaneously by a UV-Vis absorbance microplate spectrophotometer (SpectraMax Plus 384, Molecular Devices, San Jose, CA, USA).

Results and discussion

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Figure 1 shows XRD patterns of the pristine g-C3N4 and the corresponding composites. As seen, dC has a clear graphitic structure with characteristic peaks at 13.10 and 27.07 corresponding to the (100) and (002) diffraction planes of g-C3N4 (JCPDS 87-1526), respectively 38.

In contrast, the g-C3N4 in uC contains an amorphous g-C3N4 structure since its characteristic

XRD peak is very broad. Based on the intensity of the peak, the crystallinity of the g-C3N4 samples decrease in the order dC > mC > tC>uC. According to previous studies, the thermal polymerization process from urea/thiourea to g-C3N4 requires NH3 for self-supporting reactions 19, 21, 39-40.

In a continuous N2 flowing environment, NH3 released from organic compounds

readily flows outside. The insufficient amount of NH3 gas leads to a lack of reagents, thus causing the reduction in uC and tC crystallinity

19, 22.

Bragg’s law was applied to calculate the

interlayer d-spacing and distance between layers based on (100) and (002) peaks listed in Table 1. The d-spacing of g-C3N4 sheets in the dC and mC samples can be calculated whereas it is hard to find the diffraction peak on the (100) plane in the uC and tC samples due to their low crystallinity. The peak at the (002) plane was used to calculate the distance between layers. The calculated values are in the order tC > dC > mC (Table 1). In the peak positions of the (002) plane, the mC sample shifts towards a slightly higher diffraction angle compared to the dC sample, while the peak for the tC sample shifts towards a lower angle. These observations are consistent with previously reported results, implying that the mC sample has denser interlayer packing than the dC or tC samples. However, in the case of the CNT composites, all of the samples clearly show characteristic peaks for the ilmenite phase (JCPDS 33-0960) at 23.96, 32.95, 35.54, 40.70, 49.31, 53.85, 57.33, 62.31, 63.96, and 71.66, corresponding to the (0 1 2), (1 0 4), (1 1 0), (1 1 3), (0 2 4), (1 1 6), (0 1 8) (1 2 4), (3 0 0), and (1 0 10) planes, respectively. There are no shifts in peak positions, indicating that the thermal formation process 8


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does not affect the NiTiO3 crystal structure. Based on the peak for the (104) diffraction plane at 33.11, average NiTiO3 crystallite sizes can be calculated using the Scherrer equation. The values are similar despite different g-C3N4 precursors, confirming the g-C3N4 precursors have no influence on the NiTiO3 phase in the CNT composites. Peaks at approximately 13.10 and 27.07 disappeared in all composites, implying a low crystallinity or amorphous g-C3N4 sample 29-30, 33, 36

.

FTIR spectroscopy was used to investigate the functional groups in g-C3N4, NiTiO3, and the gC3N4/NiTiO3 composites. As seen in Figure 2, the unique functional groups in g-C3N4 are maintained in the dC, mC, uC, and tC samples. A sharp peak at 807 cm-1 corresponds to the breathing mode of s-triazine, which is the most important characteristic peak of g-C3N4

41.

Several peaks in the wavenumber range of 1200-1700 cm-1 represent the stretching vibration of aromatic rings 42. The peaks centered at 1241, 1323 and 1410 cm-1 are assigned to vibrations of aromatic C-N. The C=N stretching in the tri-s-triazine ring produces signals at 1567 and 1642 cm-1 13. These characteristic peaks can also be detected with a small absorption intensity in the dCNT, mCNT and tCNT composites. For the uCNT composite, the broad band of 1200-1700 cm-1 implies that there are some C-N bonds, but it is not a clear if this is the case for g-C3N4. On the other hand, all of the composites show signals of Ni-O, Ti-O, and Ni-O-Ti vibrations in the NiTiO3 phase. These appear at 453, 561, and 695 cm-1, respectively 43-44. The composites’ thermal stability and organic component contents were studied via TGA-DTG analysis (Figure S5 in Supporting Information). The samples’ weight loss below 200 C can be attributed to the evaporation of adsorbed water

45.

The TGA curves of the dC, mC, uC, and tC

samples show the same shape in the decomposition stage, but different initial decomposition temperatures (Tonset). The Tonset of the dC and mC samples are 598 C and 569 C, respectively, 9


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whereas the Tonset of the uC sample is 467 C and the tC sample is 555 C. The Tonset values show that the dC and mC samples have a higher thermal stability than the uC and tC samples. The DTG peaks for the uC and tC samples are broader and occur sooner, at 549.3C and 630.2C, respectively. The lower heat resistance of the uC and tC samples is due to the presence of impurities such as oxygen and sulfur, which weaken the structure and cause poor thermal stability at less than 600 C. The order of the decomposition temperatures in the DTG profiles is the same as that of the g-C3N4 crystallinity. Moreover, the higher weight loss of the uC and tC samples at 200 C < T < Tonset reveals that these samples contain a large amount of low molecular weight components (LMWC). After Tonset points, weight loss drops to zero at T = 600650 C for pristine g-C3N4, which is in agreement with other reports

25, 39, 46.

However, the

thermal stability in the composites tends to differ from pristine g-C3N4. The Tonset values for the composites are tCNT (434 C) > dCNT (411 C) > mCNT (396 C) > uCNT (264 C). The change of the composites’ thermal stability is due to the strength of the connection between gC3N4 and NiTiO3, which is explained more clearly using the XPS spectra

47.

The multi stage

decomposition found in the dCNT and tCNT composites denotes the existence of intermediate nitrogen-rich oligomers 39. This means the carbon nitride precursors have not completely reacted during the thermal treatment process in the dCNT and tCNT composites, thus generating various intermediate structures that can decay at different temperatures. For the dCNT composite, decomposition proceeds slowly from 350 C to 600 C, indicating the firmness of the g-C3N4 skeleton structure. Conversely, three separate decomposition steps for the tCNT composite means that the g-C3N4 in the tCNT composite is not fully developed. The quantity of g-C3N4 in each composite was roughly calculated using the Tonset point to the stable weight loss point. The 10


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results are reported in Table 1. The quantity of the organic component, C3N4, is the summation of the quantities of LMWC and g-C3N4. The dCNT component shows a high concentration of gC3N4 and has a great potential for creating g-C3N4 under the current conditions. The concentration of g-C3N4 in the mCNT composite is also high but decomposes at a lower temperature, suggesting a weak g-C3N4 skeleton structure. The significantly lower quantity of gC3N4 in the uCNT composite can be explained by the low heat resistant nature of g-C3N4, which polymerized from urea as mentioned previously. The amount of g-C3N4 in the dCNT composite is considered an approximate value due to intermediate compounds. Figure 3, Figure S1 and Figure S2 are FE-SEM and HR-TEM images displaying the morphology of each material. The mC and tC samples are thick, smooth layers on a large scale and look very similar to the compact structure of the dC sample in a previous study 18. The uC sample exhibits wrinkled and fragile sheets with curved edges and large pores (Figure S1). Even though melamine is an intermediary in the formation of g-C3N4, the morphology of the mC sample is far different from the uC sample. The unique structure of the uC sample is due to gas release during thermal polymerization, generating ultrathin 2D sheets

23.

In the heating stage,

urea condenses and releases NH3, H2O, and CO2 at the same time. As a result, urea forms thin, porous layered g-C3N4 with low crystallinity at 500 C. Although urea and thiourea differ by one atom, O and S, respectively, gas emission in the tC sample is less than in the uC sample. Hence, the tC sample composes dense and thick layers

22.

In the composites, the morphology strongly

depends on the precursors. For the mCNT composite, g-C3N4 and NiTiO3 are located separately. There are some segregated fragments with layer-stacked structures. These are randomly separated from NiTiO3 nanoparticles, and it is hard to determine any interconnections between the two components (Figure 3 and Figure S2). The morphology of the uCNT composite is 11


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similar to that of NiTiO3 due to the very low content of g-C3N4 (1.26 wt%). The tCNT composite shows a layered g-C3N4 structure that encloses the metal oxide nanoparticles. The morphological structure is coincidentally similar to the dCNT composite; NiTiO3 nanoparticles are covered with the thick layers of the g-C3N4 structure (Figure 3 and Figure S2). The details of elemental mapping for dCNT and tCNT are displayed in Figure S3 and Figure S4, respectively (Supporting Information). All of elements are evenly distributed over the composite materials. NiTiO3 accounts for a majority part in the composites with high weight percent of Ni, Ti and O. The sum of C and N contents representing organic part in the composites are 37.43 wt% and 9.72 wt% for dCNT and tCNT, respectively, which are nearly close to the values (LMWC + g-C3N4) in Table 1. In summary, the morphological structures of the dCNT and tCNT composites indicate strong interconnections between g-C3N4 and NiTiO3 nanoparticles during the thermal polymerization process. Figure 4 displays the XPS data for C 1s and N 1s. These represent the surface composition and chemical states of g-C3N4 in the prepared materials. No significant change was seen in the pristine g-C3N4 C 1s XPS data. The C 1s spectra can be deconstructed into four peaks at 284.1, 285.5, 287.4 and 288.7 eV. Peaks at 284.1 eV are assigned to C-C for adventitious carbon contaminants and defects containing sp2 hybridized carbon atoms in graphitic domains. The major peak at 287.4 eV is related to tertiary C-(N)3 coordination. The peaks at 285.5 and 288.7 eV are ascribed to C-NH2/C-S and sp2 bonded C in N-C=N/O-C=O, respectively

17, 48.

As

compared to the pristine g-C3N4, the XPS data of C 1s for the CNT composites reveal a significant change in relative peak area ratio reflecting the relative surface compositions. Based on the deconvolution of the C 1s spectra, a g-C3N4 structure in the CNT composite has less graphitic domains (C-(N)3 and N-C=N) than that of the pristine g-C3N4. Meanwhile, in the case 12


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of N 1s spectra, the peaks appearing at 397.6, 398.4 and 399.8 eV are characteristic of C-N-C, N(C)3 and C-N=C bonds, respectively 40, 49. An additional peak of Ti-N is found only in the dCNT and tCNT composites at 396.6 eV. This suggests that N atoms in the g-C3N4 structure play an important role in creating chemical bonds between the organic and inorganic components 50. The XPS data of Ti 2p, Ni 2p and O 1s were obtained from the NiTiO3 phase. Characteristic peaks at 457.4 and 463.1 eV in Ti 2p spectra correspond to Ti3+ state and peaks at 458.6 and 464.2 eV are assigned to the Ti4+ state 28. For the dCNT and tCNT composites, additional peaks appearing at 456.1 and 460.9 eV indicate the interconnection between Ti in the NiTiO3 lattice and N in the graphitic skeleton of the g-C3N4 structure

51-53.

In addition, the relative atomic

ratios of Ti3+ to Ti4+ for the dCNT and tCNT composites are lower than those for NiTiO3, uCNT and mCNT. This implies that the difference is caused by the interconnection between g-C3N4 and NiTiO3 in the dCNT and tCNT composites. To make the Ti-N linkage, some of Ti atoms in the NiTiO3 lattice structure should interconnect with the precursors. In the interconnection, the precursors prefer Ti3+ sites to Ti4+ in the lattice structure, resulting in a decrease of Ti3+/Ti4+ in the XPS data. Therefore, the lower ratios of Ti3+/Ti4+ in the XPS data indicates the interconnection between the NiTiO3 lattice and the precursors. In reverse, the mCNT and uCNT composites contain a larger amount of Ti3+ than Ti4+, which is the same trend as in NiTiO3. There are no strong interconnections between NiTiO3 and g-C3N4 in the mCNT and uCNT composites. There are no remarkable differences in the Ni 2p core level spectra of all the CNT composites, implying that Ni atoms have a durable oxidation state and barely make any bonds with the g-C3N4 structure. 13


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The XPS data demonstrate that the dCNT and tCNT composites create interconnections between NiTiO3 and the g-C3N4 structure whereas the mCNT and uCNT composites do not. Since the precursors are involved at the beginning of the thermal polymerization process in the presence of NiTiO3, they strongly influence the thermal polymeric condensation. In DCDA, the C≡N triple bond (two  bonds and one  bond) in the chemical structure shows strong activity and makes it susceptible to attack. The  bonds in the precursor compounds break down to either make a single  bond with another DCDA molecule for the final formation of g-C3N4 structure or to attract Ti atoms in the NiTiO3 lattice for a Ti-N bonding formation. In thiourea, sulfur plays an important role. Sulfur is in group VIA of the periodic table with a higher electron affinity than nitrogen and a higher reactivity in polymeric reactions. Therefore, the active thiourea S atoms can link with O atoms in the NiTiO3 lattice. Evidenced for this is seen by the existence of Ti-O-S bonding in the S 2p XPS data (Figure 4) 54. Although urea has oxygen, which is also reactive, the oxygen atoms accompany other carbon atoms and tend to escape as gases under the N2 flow. This results in the thermal formation of a negligible amount of g-C3N4 structure in the uCNT composite. Melamine has the most stable aromatic ring structure among the precursors and prefers the segregated formation of g-C3N4 structure to the interconnection with the NiTiO3 lattice under the thermal formation conditions. The schematic diagrams in Scheme 2 illustrate each thermal polymerization process forming g-C3N4/NiTiO3 composites. As discussed earlier, the dCNT and tCNT composites have strong interconnections between g-C3N4 and NiTiO3 due to their highly reactive precursors. The reactivity of DCDA and thiourea molecules play a decisive role in the formation of Ti-N and TiO-S within the composites, respectively. Finally, two composites contain the interconnected 14


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structures displayed in Scheme 2. Melamine generates a segregated g-C3N4 structure with no linkage to the NiTiO3 phase. Urea releases a large amount of oxygen-containing gas during the thermal formation process and leads to low condensation in the g-C3N4 composites. Figure 5 shows the photocatalytic degradation of methylene blue (initial MB concentration = 10 ppm) over pristine g-C3N4 and the composites. The adsorption stage obeys the pseudosecond-order kinetic model and the adsorption capacities of MB. qe was obtained from the slope and intercept of plot t/qt vs t as follows: 𝑡 1 1 = 𝑡+ 𝑞𝑡 𝑞𝑒 𝑘𝑎𝑑𝑠𝑞2𝑒 where qt (mg/g) is the quantity of MB adsorbed on the photocatalyst at a specified time, t, qe (mg/g) is adsorption capacity, the quantity of MB adsorbed on the photocatalyst at equilibrium, and kads (g/mg.min) is the pseudo-second-order adsorption rate constant. The adsorption process achieves steady state after 30 min. There is no significant difference between the adsorption capacity of pristine g-C3N4 and the composites. The MB degradation experiments were conducted in aqueous solution under simulated visible light irradiation to assess the photocatalytic activity of the photocatalysts. The MB concentrations of the aqueous solution were calculated as a function of reaction time and plotted for the fit to the kinetic model. The reaction rate constants were determined from the apparentfirst-order kinetic model described as the following equation 55-56: ln

𝐶𝑡 𝐶0

= ― 𝑘𝑎𝑝𝑝𝑡

Through the plots of ln (Ct/C0) versus irradiation time, t, kapp values were observed from the slopes (Figure 5(b)). Among the g-C3N4 catalysts, the high kapp value of the uC sample is due to 15


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its porous structure; dye molecules migrate easily through the thin layers and degrade. The dCNT and tCNT composites displayed the most prominent photocatalytic activity, increasing kapp values compared with those of the dC and tC samples. This implies that the photocatalytic activity of the composite photocatalysts was strongly dependent on the intrinsic interconnection of the composites, i.e., the interconnected structure through the Ti-N and Ti-O-S bridges. Since the mCNT and uCNT composites consist of segregated structures with no bonding between the g-C3N4 and NiTiO3, the photocatalytic reaction activity is not influenced by the combination of g-C3N4 and NiTiO3. In reverse, their photocatalytic activities are inhibited by their mutual blocking. Thus, the kapp values of the mCNT and uCNT composites are lower than those of pristine g-C3N4 (Figure S6) and NiTiO3 (kapp = 3.41 × 10-3 min-1) 18. UV-Vis spectroscopy was utilized to study the optical properties of these materials, including the absorption region and band gap. The absorption peak of all samples located in the UV range ( < 400 nm), and their shoulder widened into the visible range (Figure 6(a,b)). Since dC, mC, uC and tC are the same material, the shape of their UV-Vis curves is relatively analogous. Due to a direct transition of g-C3N4, their band gaps can be estimated by the Tauc plot (inset of Figure 6(a)) and the estimated values are presented in Table 1. As a result, the bandgaps of g-C3N4 are calculated to be approximately 2.4 – 2.7 eV that are applicable to the photodegradation under the visible light irradiation. Meanwhile, in the case of the composite materials, since the heterogeneous transition cannot identify whether the band is direct or indirect, their bandgaps can be estimated from the equation: Eg = 1240/h where h is a value where the extrapolation of the straight line in certain region intercepts 16


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the X axis in the UV-Vis spectrum. The band gaps of the composites are in a range of 2.25-2.64 eV (Table 1), indicating that the composites can be used as a visible-light active photocatalyst. The PL emission spectra were employed to investigate the recombination of free charge carriers. Figure 6(b) shows the broad luminescence peak of g-C3N4 centered at around 450 nm with a shift in peak position. The intensity of these peak reflects the radiative recombination of charge carriers. Hence, the mC sample has a very strong PL emission peak revealing fast electrons-holes recombination. In contrast, the uC and tC samples show charge separation because of relatively low PL emission intensities. Imperfections in the g-C3N4 structure, such as the uncondensed -NH-, -NH2 groups, can capture the charge carriers, resulting in a low-intensity PL emission signal 22. The shift in peak position is in agreement with the variation in the band gaps estimated form UV-Vis absorption (uC < dC < tC < mC). Meanwhile, the combination of two components in the composites reduces the PL emission intensities, reflecting the lower recombination rate of the composites. Although the combination of the mC and uC samples with NiTiO3 in the mCNT and uCNT composites result in much low PL emission intensities, the shapes of the PL emission spectra are almost similar to those of the mC and uC samples with a higher PL intensity than that of NiTiO3 (inset of Figure 6(b)). This can be explained by the segregated structure of the mCNT and uCNT composites; the g-C3N4 structure in the mCNT and uCNT composites is segregated from NiTiO3 nanoparticles, maintaining the PL emission nature of g-C3N4. The combination of two components in the dCNT composite produces a new interconnected structure between g-C3N4 and NiTiO3, showing lower PL emission intensity than that of pristine NiTiO3. For the tCNT composite, a peak around 475-490 nm centered at 485 nm causes a fast recombination at this excitation wavelength (inset of Figure 6(b)). Consequently,

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the high photocatalytic activity of the dCNT composite is attributed to the low recombination rate, as evidenced by the PL emission spectra. One important piece of information about the CNT composite is that it satisfies the criterion of the conduction band (CB), which decides whether charge separation is possible or not. UPS data can be used to determine the position of the valence band (VB) in the samples. The work function values () determining the distance between the vacuum level and Fermi energy (EF) were obtained from the cut-off region in Figure S7(a) (Supporting Information). The positions of VB energy (EVB) located below EF were acquired from Figure S7(b) (Supporting Information). The CB is calculated by the following equation: ECB = EVB – Eg, where Eg is the band gap estimated from UV-Vis spectroscopy. After the axial displacement calculation from E vs. vacuum to E vs. NHE (Evac = 0 is equal to ENHE = -4.5eV), the final band edges for dC, mC, uC, tC and NiTiO3 with both CB and VB are shown in Figure 7. The charge transfer in the composite materials can be explained by one of two models: type II heterojunction transfer or Z-scheme transfer. In this case, the type II heterojunction model is inadequate to explain the photocatalytic behavior. When holes in VB of NiTiO3 transfer to VB of g-C3N4, they become inactive and cannot form OH since VB potentials of g-C3N4 are more negative than E (OH-/OH) 57. Similarly, electrons excited from VB of both phases are rapidly transferred to CB of NiTiO3. Since the CB of NiTiO3 (ECB = -0.05 eV) is close to E (O2/O2-) = -0.046 eV

57-58,

the probability of generating O2- is very small. If the g-

C3N4/NiTiO3 composite materials follow the type II heterojunction model, the photocatalytic 18


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activity must be lower than NT or g-C3N4 themselves

58.

Accordingly, dCNT and tCNT are

considered to follow the Z-scheme charge transfer model where the reduction occurs at the CB of g-C3N4 and oxidation takes place at VB of NiTiO3 by the reaction of O2- and OH, respectively. The interconnected composite structure in dCNT and tCNT facilitates the charge transfer in the composites while the segregate composite structure in mCNT and uCNT doesn’t (Scheme 2).

Conclusions In this study, various pristine g-C3N4 samples and corresponding NiTiO3 composites were synthesized under N2 flow condition using g-C3N4 precursors DCDA, melamine, urea and thiourea. Characterization shows that the dCNT and tCNT composites produce strong interconnections between the inorganic-organic components through Ti-N and Ti-O-S bonds, respectively. Due to the strong interconnections, the dCNT and tCNT components have an interconnected g-C3N4 and NiTiO3 structure, resulting in better optical properties and lower recombination rates compared to the mCNT and uCNT composites. These composites have segregated structures because there is no interconnection between the two components. The band diagrams of the dC and tC samples demonstrate that their band position is suitable to create a Zscheme charge transfer model with NiTiO3, improving the photocatalytic activities of the dCNT and tCNT composites in methylene blue photocatalytic degradation. The enhanced photocatalytic activity in the dCNT and tCNT composites confirms the significant role the TiN/Ti-O-S bridge plays between g-C3N4 and NiTiO3. 19


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FIGURES

Figure 1. XRD patterns of g-C3N4 and their composites as a function of different precursors (middle). (a) an enlarged area of (0 0 2) diffraction peak from g-C3N4 and (b) an enlarged area in on (1 0 4) and (1 1 0) diffraction peaks of NiTiO3 in the composites.

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Figure 2. FTIR spectra of photocatalysts as a function of different precursors and the 2D structures of NiTiO3 (peaks in the range of 400-760 nm) and g-C3N4 (peaks in the range of 7601900 nm).

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Figure 3. HR-TEM images of (a) dCNT, and (c) uCNT and FE-SEM images of (b) mCNT, and (d) tCNT.

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18

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Figure 4. The XPS data of C 1s and N 1s in g-C3N4, Ti 2p, Ni 2p, and O 1s in NiTiO3 and S 2p in tCNT.

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Figure 5. Photocatalytic degradation of MB with dC, tC and corresponding composites (dCNT, tCNT): (a) adsorption in the dark and (b) photocatalytic degradation under visible light irradiation. The inset tables of (a) and (b) represent the corresponding adsorption capacity qe and apparent photocatalytic reaction rate constant kapp, respectively.

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Figure 6. UV-Vis absorption spectra of (a) pristine g-C3N4, and (b) corresponding composites (inset of (a) is the Tauc plot of E (eV) vs (Ahν)2) and (c) photoluminescence emission spectra of NiTiO3, pristine g-C3N4 and corresponding composites.

Figure 7. Band diagrams of NiTiO3 (NT) and different types of g-C3N4.

2 3

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4

SCHEMES

5 6

Scheme 1. Chemical structures of the different g-C3N4 precursors.

7

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8 9

Scheme 2. Schematic illustrations of the thermal polymerization process over each g-C3N4 /NiTiO3 composite: (a) interconnected

10

composte structure via Ti-N and Ti-O-S bridges between g-C3N4 and NiTiO3 in dCNT and tCNT and (b) segregate composite

11

structure in mCNT and uCNT (early decomposition of g-C3N4 in uCNT). The blue octahedral structure represents for trigonal NiTiO3

12

ilmenite phase.

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TABLES Table 1. Structural parameters and band gaps of the prepared photocatalysts.

Sample

d-spacing (nm)a d(100)

d(002)

dC

0.676

0.329

mC

0.680

uC

Grain size

(nm)b

Component by wt%c

Eg (eV)d

LMWC

g-C3N4

NiTiO3

-

1.06

95.07

-

2.62

0.326

-

0.46

97.85

-

2.71

-

-

-

3.86

91.49

-

2.38

tC

-

0.333

-

1.40

93.04

-

2.64

dCNT

-

-

0.270

1.08

23.23

72.63

2.31

mCNT

-

-

0.270

0.60

15.93

81.92

2.48

uCNT

-

-

0.271

0.15

1.52

97.66

2.25

tCNT

-

-

0.271

1.12

11.28

85.86

2.64

a Lattice

spacing calculated from (100) and (002) planes of XRD diffraction spectra by the Bragg’s law n = 2d(hkl)sin. b Crystallite

kλ

size of NiTiO3 phase from (104) diffraction plane observed from the Scherrer equation d(104)

= βcosθ . The content of components in the materials after polymerization calculate by the weight loss from TGA data. Note: LMWC – low molecular weight components. c

d

Estimated band gap from UV-Vis spectra.

ASSOCIATED CONTENT The following files are available free of charge. The Supporting Information contains: HR-TEM and FE-SEM images, EDS and elemental mapping, TGA-DTG curves, photocatalytic degradation process and UPS spectra of pristine gC3N4 and composites (file type: PDF). ACS Paragon Plus Environment

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AUTHOR INFORMATION Corresponding Author * Corresponding author Tel.: +82 52 259 2253 Fax.: +82 52 259 1689 Email: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT This research was supported by the Basic Science Research Program administered through the National Research Foundation of Korea (NRF) and funded by the Ministry of Science, and ICT (no. 2018R1A2B6004219). REFERENCES This article references 58 other publications. 1. Paleocrassas, S. N., Photocatalytic hydrogen production: A solar energy conversion alternative? Sol. Energy 1974, 16 (1), 45-51. 2. Zamaraev, K. I.; Parmon, V. N., Potential Methods and Perspectives of Solar Energy Conversion via Photocatalytic Processes. Catal. Rev. 1980, 22 (2), 261-324. 3. Macwan, D.; Dave, P. N.; Chaturvedi, S., A review on nano-TiO2 sol–gel type syntheses and its applications. Journal of materials science 2011, 46 (11), 3669-3686. 4. Ong, C. B.; Ng, L. Y.; Mohammad, A. W., A review of ZnO nanoparticles as solar photocatalysts: Synthesis, mechanisms and applications. Renewable Sustainable Energy Rev. 2018, 81, 536-551.


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38. Yan, S. C.; Li, Z. S.; Zou, Z. G., Photodegradation Performance of g-C3N4 Fabricated by Directly Heating Melamine. Langmuir 2009, 25 (17), 10397-10401. 39. Su, Q.; Sun, J.; Wang, J.; Yang, Z.; Cheng, W.; Zhang, S., Urea-derived graphitic carbon nitride as an efficient heterogeneous catalyst for CO2 conversion into cyclic carbonates. Catal. Sci. Technol. 2014, 4 (6), 1556-1562. 40. Lazauskas, A.; Baltrusaitis, J.; Puodžiukynas, L.; Andrulevičius, M.; Bagdžiūnas, G.; Volyniuk, D.; Meškinis, Š.; Niaura, G.; Tamulevičius, T.; Jankauskaitė, V., Characterization of urea derived polymeric carbon nitride and resultant thermally vacuum deposited amorphous thin films: Structural, chemical and photophysical properties. Carbon 2016, 107, 415-425. 41. Ong, W.-J.; Tan, L.-L.; Ng, Y. H.; Yong, S.-T.; Chai, S.-P., Graphitic Carbon Nitride (gC3N4)-Based Photocatalysts for Artificial Photosynthesis and Environmental Remediation: Are We a Step Closer To Achieving Sustainability? Chem. Rev. 2016, 116 (12), 7159-7329. 42. Groenewolt, M.; Antonietti, M., Synthesis of g-C3N4 Nanoparticles in Mesoporous Silica Host Matrices. Adv. Mater. 2005, 17 (14), 1789-1792. 43. Yuvaraj, S.; Nithya, V. D.; Fathima, K. S.; Sanjeeviraja, C.; Selvan, G. K.; Arumugam, S.; Selvan, R. K., Investigations on the temperature dependent electrical and magnetic properties of NiTiO3 by molten salt synthesis. Materials Research Bulletin 2013, 48 (3), 1110-1116. 44. Sobhani-Nasab, A.; Hosseinpour-Mashkani, S. M.; Salavati-Niasari, M.; Taqriri, H.; Bagheri, S.; Saberyan, K., Synthesis, characterization, and photovoltaic application of NiTiO3 nanostructures via two-step sol–gel method. Journal of Materials Science: Materials in Electronics 2015, 26 (8), 5735-5742. 45. Shi, Y.; Jiang, S.; Zhou, K.; Bao, C.; Yu, B.; Qian, X.; Wang, B.; Hong, N.; Wen, P.; Gui, Z.; Hu, Y.; Yuen, R. K. K., Influence of g-C3N4 Nanosheets on Thermal Stability and Mechanical Properties of Biopolymer Electrolyte Nanocomposite Films: A Novel Investigation. ACS Appl. Mat. Interf. 2014, 6 (1), 429-437. 46. Jiang, J., Improving the surface-enhanced Raman scattering activity of carbon nitride by two-step calcining. RSC Adv. 2016, 6 (53), 47368-47372. 47. Vellaichamy, B.; Periakaruppan, P., Catalytic hydrogenation performance of an in situ assembled Au@g-C3N4-PANI nanoblend: synergistic inter-constituent interactions boost the catalysis. New J. Chem. 2017, 41 (15), 7123-7132. 48. Fettkenhauer, C.; Weber, J.; Antonietti, M.; Dontsova, D., Novel carbon nitride composites with improved visible light absorption synthesized in ZnCl2-based salt melts. RSC Advances 2014, 4 (77), 40803-40811. 49. Zhang, G.; Savateev, A.; Zhao, Y.; Li, L.; Antonietti, M., Advancing the n → π* electron transition of carbon nitride nanotubes for H2 photosynthesis. J. Mater. Chem. A 2017, 5 (25), 12723-12728. 50. Wang, H.; Yuan, X.; Wu, Y.; Zeng, G.; Chen, X.; Leng, L.; Li, H., Synthesis and applications of novel graphitic carbon nitride/metal-organic frameworks mesoporous photocatalyst for dyes removal. Applied Catalysis B: Environmental 2015, 174, 445-454. 51. Bellam, J. B.; Ruiz-Preciado, M. A.; Edely, M.; Szade, J.; Jouanneaux, A.; Kassiba, A. H., Visible-light photocatalytic activity of nitrogen-doped NiTiO3 thin films prepared by a cosputtering process. RSC Advances 2015, 5 (14), 10551-10559. 52. Lin, T.; Yang, C.; Wang, Z.; Yin, H.; Lü, X.; Huang, F.; Lin, J.; Xie, X.; Jiang, M., Effective nonmetal incorporation in black titania with enhanced solar energy utilization. Energ. Environ. Sci. 2014, 7 (3), 967-972.


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53. Oktay, S.; Kahraman, Z.; Urgen, M.; Kazmanli, K., XPS investigations of tribolayers formed on TiN and (Ti,Re)N coatings. Appl. Surf. Sci. 2015, 328, 255-261. 54. Lei, T.; Xie, Y.; Wang, X.; Miao, S.; Xiong, J.; Yan, C., TiO2 Feather Duster as Effective Polysulfides Restrictor for Enhanced Electrochemical Kinetics in Lithium–Sulfur Batteries. Small 2017, 13 (37), 1701013. 55. Nguyen-Phan, T.-D.; Kim, E. J.; Hahn, S. H.; Kim, W.-J.; Shin, E. W., Synthesis of hierarchical rose bridal bouquet- and humming-top-like TiO2 nanostructures and their shapedependent degradation efficiency of dye. Journal of Colloid and Interface Science 2011, 356 (1), 138-144. 56. Pham, T.-T.; Nguyen-Huy, C.; Lee, H.-J.; Nguyen-Phan, T.-D.; Son, T. H.; Kim, C.-K.; Shin, E. W., Cu-doped TiO2/reduced graphene oxide thin-film photocatalysts: Effect of Cu content upon methylene blue removal in water. Ceramics International 2015, 41 (9, Part A), 11184-11193. 57. Yu, W.; Xu, D.; Peng, T., Enhanced photocatalytic activity of g-C3N4 for selective CO2 reduction to CH3OH via facile coupling of ZnO: a direct Z-scheme mechanism. J. Mater. Chem. A 2015, 3 (39), 19936-19947. 58. Yu, J.; Wang, S.; Low, J.; Xiao, W., Enhanced photocatalytic performance of direct Zscheme g-C3N4-TiO2 photocatalysts for the decomposition of formaldehyde in air. Phys. Chem. Chem. Phys. 2013, 15 (39), 16883-16890.


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