N-doped rutile TiO2 composite g-C3N4 with Z-Scheme boosts photocatalytic hydrogen production under visible light

doi:10.21203/rs.3.rs-2497545/v1

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N-doped rutile TiO2 composite g-C3N4 with Z-Scheme boosts photocatalytic hydrogen production under visible light

https://doi.org/10.21203/rs.3.rs-2497545/v1

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published 15 Apr, 2023

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The photocatalysis technology is one of the recent research hotspots to solve a series of social problems such as energy crisis and environmental pollution. It is an important way to improve the photocatalytic efficiency by constructing heterojunction and improving the utilization rate of sunlight. In this paper, N-doping rutile TiO₂ (N-rTiO₂) and graphite phase carbon nitride (g-C₃N₄), as cheap, stable and non-toxic catalysts, were successfully composited. Although N-rTiO₂ could not meet the hydrogen production condition, after formed a Z-scheme heterojunction with g-C₃N₄, which improved the photocatalytic hydrogen production performance by 2.57 times compared with pristine g-C₃N₄. Moreover, a DFT calculation has analyzed the band structure, density of state, work function, differential charge density and Bader charge distribution of samples to reveal the photocatalytic mechanism. It is further proved that the formation of Z-scheme effectively promotes the improvement of photocatalytic performance. This study provides a feasible idea for designing and expanding new and cheap photocatalysts in the renewable energy fields.

This study efficient hybrid catalysts for other applications in the renewable energy fields.

N-doping rutile TiO2

g-C3N4

Z-scheme

DFT calculation

photocatalysis

The massive and continuous demand for fossil fuels from human daily life and industrial production has accelerated circumstance pollution, globe warming and energy crisis[1–5]. Since Fujishima and Honda discovered the photocatalytic phenomenon, photocatalytic technology has become one of the hot spots to solve the social problems such as energy crisis and environmental pollution[6–11]. It is a promising and potential strategy to use solar energy to drive water to convert into high-value chemicals and fuels. So that, researchers have been committed to developing photocatalysts with excellent performance, low cost and stability[12–16].

Titanium dioxide (TiO₂) is the first semiconductor material discovered to be able to photocatalyze the decomposition of water. TiO₂ has been widely studied as a photocatalytic material due to its non-toxic, cheap and stable properties[17–22]. Generally, TiO₂ has three crystal structures, namely anatase structure, rutile structure and plate titanium structure. Rutile TiO₂ (rTiO₂) is the steady phase, and the others are the metastable phase. Because of the long carrier lifetime and good photocatalytic properties, anatase TiO₂ (aTiO₂) with metastable phase has been widely research and applied in the field of photocatalysis. However, aTiO₂ has wide band gap (~ 3.2 eV) and only absorbs the sunlight's ultraviolet part, which is only about 5% of the sunlight, its further development and utilization are limited in the photocatalytic field. Compared with aTiO₂, rTiO₂ has a narrower band gap (~ 3.0eV) and the utilization rate of sunlight reaches about 11%. However, compared with more than 1ms lifetime of considerable number of photogenerated electrons in aTiO2, the rTiO2 is deeply trapped within several picoseconds and the depth of the electron traps at powder defects were estimated to be more than 0.9 eV, but the deeply tapped electrons has low activity in catalysis procedure[23–25]. Although which can prolong the lifetime of photogenerated holes and apply in degradation and water oxidation, the deep electrons-trapping in rTiO2 makes it rarely used in the field of photocatalysis H₂ production.

Graphite-phase carbon nitride (g-C₃N₄) is another non-toxic, cheap and stable material. Which is a kind of two-dimensional layered semiconductor material composed of C and N heterocycles and discovered in recent years. Due to the characteristics of environmental friendliness, stable physical and chemical properties and low cost. Its band gap is about 2.7eV, which means it can absorb part of visible light. Its suitable band gap (E_g), conduction band potential (E_cb) and valence band potential (E_vb) have made it widely studied in photocatalytic hydrogen production, photocatalytic carbon dioxide reduction, photocatalytic degradation and photocatalytic sterilization[26–33], etc. However, its small specific surface area and easy recombination of photogenerated carriers restrict its further application in the field of photocatalysis.

In order to improve the photocatalytic performance of semiconductor materials, it is the important link to improve the utilization rate of sunlight and reduce photogenerated carrier’s recombination. In general, semiconductor doping can reduce band gap semiconductor and effectively improve the utilization rate of sunlight[34–37], while the construction of heterojunction between the interface to form internal electric field (IEF) is an effective way to promote the separation and reduce the recombination of the photogenerated carriers[38–40]. As shown in Fig. S1, the formation of type II, Z-scheme heterojunction structures between interfaces is a common method to improve photocatalytic performance[41–43]. It is noteworthy that the photogenerated electrons and holes in the type II structure at the semiconductor interface migrate towards the lower edge of conduction band (CB) and valence band (VB). However, although type II structure can effectively separate the photogenerated carriers, it also weakens the redox driving ability of the photogenerated carriers. And if the lower conduction potential is lower than the hydrogen production potential, the photogenerated electrons cannot be used for photocatalytic hydrogen production because they cannot meet the thermodynamic conditions of hydrogen production. Z-scheme can migrate electron-hole pairs and recombine at lower conduction band edge and valence band edge, which means that redox reactions mainly occur at higher conduction band edges and valence band edges, which can not only effectively separate photogenerated carriers, but also retain photogenerated electrons and holes with stronger redox ability. Therefore, it is an effective method to develop efficient composite photocatalysts by Z-scheme[44–46].

Herein, N-rTiO₂ composite with g-C₃N₄ was synthesized (schematic diagram is shown in Fig. 1a). Compared with aTiO₂, the N-rTiO₂ can respond to visible light, Although the deep electrons-trapping of the N-rTiO₂ cannot be used for photocatalytic hydrogen production. But when a Z-scheme heterojunction structure was constructed with g-C₃N₄. The recombination between N-rTiO₂’s conduction electrons and g-C₃N₄’s valence band holes improve the efficiency of photocatalytic hydrogen production. By constructing Z-scheme heterojunction structures, it provides an idea for the application of semiconductor materials which cannot meet the photocatalytic hydrogen production.

2.1 the synthesis of samples

TiO₂ nanoparticles can be synthesized by the sol-gel method. Typically, 10ml tetranobutyl titanate was added to 35ml absolute ethanol and stirred to form uniform yellow clear solution A; meanwhile, 4ml glacial acetic acid and 10ml deionized water were added to 35ml absolute ethanol and stirred to form uniform solution B. Then add 1–2 drops of dilute hydrochloric acid to B solution to adjust the PH ≤ 3. The light-yellow solution was obtained by slowly dropping solution A into solution B with stirring vigorously for 30min. Then, the yellow gel was obtained by water bath at 45℃. The yellow gel was dried at 105 ℃ and annealed at 500℃ for 2h to obtain TiO₂ powder.

The synthesized TiO₂ powders and urea mixed with the mass ratio of 1:4, and placed in tube furnace sealed on both ends with polymer ball, then kept 900 ℃ for 2h with 5 ℃ min^− 1 heating rate, and natural cooling to room temperature to get the powder, washing with deionized water and ethanol for 6 times, centrifuged and drying 6 h at 60 ℃, the N-doped rutile TiO₂ powder (N-rTiO₂) can be obtained.

The pre-synthesized 40mg N-rTiO₂ powder and a certain amount of urea were mixed evenly and put into a crucible with a lid. After calcining at 520℃ for 4h with a heating rate of 5℃ min^− 1 in the Muffle furnace. the composite of N-rTiO₂ and g-C₃N₄ were obtained. The materials were named NTU-x (x = 3,4,5,6), where x is the mass of urea is 3, 4, 5 and 6g, respectively. Purity g-C₃N₄ can be obtained by calcination of urea without adding N-rTiO₂ with same method.

2.2 Characterization and electrochemical tests

XRD patterns were gained on a PANalytical Empyrean X-ray diffractometer for 2θ = 5º~80º. FTIR spectra were carried out on a Shimadzu infrared spectrometer in the range of 4000 to 400 cm^− 1. SEM images were obtained on a ZEISS Gemini 300 field mission scanning electron microscope. TEM and HRTEM images were observed on FEI Telos electron microscopy. XPS survey and high-resolution spectra were recorded on an Escalab 250Xi X-ray photoelectron spectrometer. Diffuse reflection spectra (DRS) were performed on a Shimadzu UV-3600 plus UV-vis-NIR spectrometer equipped Integrating sphere. Photoluminescence (PL) and RAMAN spectra were acquired on a Horiba raman spectrometer with 325nm and 532nm laser, respectively. TRPL were measured on an Edinburgh FLS980 fluorescence lifetime spectrometer with 360nm laser.

The photoelectrochemical properties were conducted in 0.5 M Na₂SO₄ solution using a Shanghai Chenhua electrochemical station with a standard three-electrode system, in which a Pt sheet used as a counter electrode and a saturated calomel electrode (SCE) as a reference electrode (converted to NHE using E_NHE=E_SCE +0.24 at PH = 7). The working electrode was fabricated via drop-coated method on ITO glass. Typically, 5 mg of samples were firstly dispersed into ethanol aqueous solution containing 10 vol% of Nafion under ultrasonic treatment for 30 min, and then drop-coated 20 µl on 1 cm² of ITO glass. After it was dried at ambient temperature, the sample was then heated at 150°C for 2h. The transient photocurrent responses (I-t curve) were recorded at bias voltage of -0.2 V vs. SCE, the electrochemical impedance spectroscopy (EIS) was performed in the frequency range between 1 Hz to 100 kHz at an open-circuit voltage, and the Mott-Schottky tests were conducted on the potential range from − 1.0 to -0.4 V vs. SCE for g-C₃N₄ and 0 to 0.5V vs. SCE for N-rTiO₂ at 1.0 kHz.

2.3 Photocatalytic performance evaluation

The photocatalytic activity of the samples was evaluated by H₂ production under visible light irradiation, which obtained on a 300W xenon lamp equipped with 400nm cut-filter. Typically, 25 mg samples were dispersed into 45ml DI water and added 5ml triethanolamine (TEOA) as sacrificial agent, Then, 3wt% Pt was in situ deposited on the surface of catalyst as co-catalyst by adding H₂PtCl₆ and then irradiating for 30 minutes with 300W Xe lamp. The photocatalytic evaluation process was run at 6℃ and have been removed the air of the reaction system by vacuum pump. The H₂ production was tested on GC7920 gas chromatograph (equipped with TDX-01 column and TCD detector, N₂ as carrier gas).

2.4 DFT calculation

The electronic structure and total energy calculations were performed using density functional theory (DFT) as implemented in the Vienna ab initio simulation package (VASP) [47, 48], version 5.4.4. The ion-electron interactions were treated with the frozen-core projector-augment-wave (PAW) approach[49]. The exchange correlation effects were adopted by using generalized gradient approximation (GGA) under Perdew-Burke-Ernzerhof (PBE) scheme[50]. The cut-off energy for plane-wave basis sets was 400 eV and the Gaussian smearing [51] σ = 0.1 eV was employed to help the convergence. Brillouin zone was sampled by the Γ-centered Monkhorst-Pack [52] k-points grids with resolution of 0.2 Å⁻¹. A vacuum slab of 15 Å was used to minimize the interactions between periodic cells. Dipole corrections (IDIPOL = 3) were also included to minimize the possible inaccuracies. The Grimme DFT-D3 dispersion correction [53] with the Becke Jonson (BJ) damping was added to consider the effect of van der Waals interactions. The band structures and density of states (DOS) were calculated by the DFT + U [54], where the value of U was 4 eV for Ti d states. The energy convergence is set to 1.0 ×10^− 6 eV, and the force convergence is set to 0.005 eV/Å.

3.1 Structure and morphology

Shown in Fig. 1b, the structures of samples were described by XRD patterns. g-C₃N₄ has two broad peaks located at 13.2° and 27.4°, which corresponds to the (100) and (002) crystal plane of g-C₃N₄, respectively. (110) crystal plane is ascribed to the in-plane repetitive unit of tri-s-triazine and the (002) crystal plane is ascribing to the interplanar stacking peak of conjugated aromatic rings (JCPDS CARD 87-1526) [37]. The diffraction peaks of anatase TiO₂ are exhibited at 25.4°, 37.1°, 37.9°, 38.7°, 48.2°, 54.1°, 55.2°, 62.9°, 69.0°, 70.5° and 75.3°, assigning to the crystal indices of (101), (103), (004), (112), (200), (105), (211), (204), (116), (220) and (215), respectively, which indicated that aTiO₂ was the tetragonal system structure (JCPDS CARD 73-1746) [55]. The diffraction peaks of N-rTiO₂ are observed at 27.4°, 36.1°, 39.2°, 41.1°, 44.0°, 54.4°, 56.6°, 62.7°, 64.1°, 69.0° and 69.7°, assigning to the crystal indices of (110), (101), (200), (111), (210), (211), (220), (002), (310), (301) and (112), respectively, which is a typical rutile TiO₂ structure, indicated that N-doped does not change the tetragonal crystal structure of rTiO₂ (JCPDS CARD 21-1276)[56]. the diffraction peaks of NTU-4 are similar with N-rTiO₂, the (100) peak of g-C₃N₄ cannot be observed because of too weak, and the (002) broad peak of g-C₃N₄ and the (110) narrow peak of N-rTiO₂ are duplicated, it is obvious that the bottom part of the peak at 27.4° is widen. Seen from Fig. S2a, NTU-x has similar crystal structure patterns, with the increase of g-C₃N₄ content, the bottom part of the peak at 27.4° gradually widens, indicating that NTU-x is composed of N-rTiO₂ and g-C₃N₄.

In the FTIR spectra of samples (Fig. 1c and Fig. S2b), the structure properties and chemical groups was exhibited. The typical fingerprint peak of g-C₃N₄ is at 808 cm^− 1, deriving from the bending vibration of the heterocyclic structure. The peaks at ~ 1240, 1323, 1407, 1463, 1565 and 1638 cm^− 1 are ascribed to the stretching vibration of C = N or C-N in heterocycle of g-C₃N₄. And the broad peak between 3000–3500 cm^− 1 attributes to the stressing vibration of the terminal groups. The board bands at 400–700 cm^− 1 are due to Ti–O stretching vibrations and Ti–O–Ti bridging stretching vibrations of N-rTiO₂. It is worth noting that the peaks of g-C₃N₄ and N-rTiO₂ can be observed in the FTIR spectrum of NTU-4 and the NTU-x has similar FTIR spectra, suggesting that NTU-x is formed by the physical combination of g-C₃N₄ and N-rTiO₂.

RAMAN spectrum is an important tool used to study material structure and groups. Although aTiO₂ and rTiO₂ are tetragonal phase structures, they have different space groups and their RAMAN spectral characteristics are obviously different. As shown in Fig. 1d, anatase TiO₂ consists of four obvious peaks located at 143, 397, 518 and 639cm^-1, respectively. The peak located at 143cm^-1 is stronger, which is attributed to the symmetric O-Ti-O vibration peak and corresponds to Eg vibration mode, while the peak located at 397 cm^-1 corresponds to B1g vibration mode, the peak located at 518cm^-1 corresponds to A1g + B1g vibration mode, and the peaks located at 639cm^-1 corresponds to Eg vibration mode. RAMAN peaks of N-rTiO₂ are obviously different from aTiO₂ peaks. Peaks located at 137, 230, 445 and 610cm^-1 belong to B1g, the multi-photon process, Eg and A1g vibration mode, respectively, which is consistent with the literature[57], indicating that TiO₂ with rutile structure is synthesized.

SEM exhibit the morphological characteristics of the samples. Shown in Fig. 1e and Fig. S3, g-C₃N₄ has a two-dimensional crimped nanosheet structure with large gaps between the nanosheet structures. aTiO₂ is an irregular block formed by the aggregation of small nano-spherical particles. Different from aTiO₂, the surface of N-rTiO₂ is relatively smooth, which is distributed some obvious pits on the surface, which should be the traces left on the surface when urea is decomposed and N-doped. NTU-4 is a composite material composed of g-C₃N₄ and N-rTiO₂, but N-rTiO₂ can be seen hardly, which may be caused by the high content of g-C₃N₄ and N-rTiO₂ coated by g-C₃N₄ in the synthesis process. The EDS plots of NTU-4 showed that g-C₃N₄ and N-rTiO₂ are composited successfully (Fig. 1f), which was consistent with the XRD and FTIR results.

X-ray photoelectron spectroscopy (XPS) is an important surface analysis technique. It can not only provide information about molecular structure and valence state, but also provide information about elemental composition and chemical state of various compounds. Fig. S4a shows the Survey spectra of the samples. It is obvious that N-rTiO₂ has strong Ti2p, O1s peaks and a weak N1s peak, the weak N1s peak indicates that a small amount of N is doped into the rutile TiO₂. While g-C₃N₄ is mainly composed of C and N, and adsorbed oxygen (O_a) exists on the surface. The peaks of N1s, C1s, O1s and Ti2p can be observed in the survey spectrum of XPS of NTU-4, indicating that g-C₃N₄ and N-rTiO₂ are successfully composited.

To obtain more insight into the molecular structure and chemical bonding between the atoms in the composite, the high-resolution spectra and corresponding fits of N1s, O1s, Ti2p and C1s of samples were shown in Fig. 2a-c and Fig. S4b. From N1s spectrum of NTU-4 (Fig. 2a), Five fitting peaks are observed at 398.60, 399.23, 400.77, 401.45 and 404.53eV, corresponding to the C-N = C structure of the heterocyclic ring in g-C₃N₄, the N-Ti-O structure caused by the N-doping in N-rTiO₂, the N-C₃ structure of the heterocyclic ring in g-C₃N₄, the -NH/NH₂ group adsorbed on the surface of g- C₃N₄, and the π-π* structure of g-C₃N₄ heterocyclic structure, respectively. Interestingly, compared with g-C₃N₄, the C-N = C, N-C₃, -NH/NH₂ and π-π* fitting peaks of NTU-4 is positively shifted from 398.57 to 398.60eV, from 400.10 to 400.77eV, from 401.25 to 401.45eV and from 404.53 to 404.53eV, respectively. On the contrary, compared with N-rTiO₂, the N-Ti-O fitting peak of NTU-4 is negatively shifted from 399.66 to 398.60eV. The O1s XPS spectrum of NTU-4 has three fitting peaks at 529.68, 531.59 and 532.97eV. which correspond to the Ti-O structure of N-rTiO₂, oxygen vacancy (O_v) of N-rTiO₂ and the surface adsorbed oxygen (O_a) of g-C₃N₄. Similarly, in NTU-4, the binding energy of adsorbed oxygen from g-C₃N₄ shifts positively from 532.30 to 532.97eV, while the lattice oxygen Ti-O and oxygen vacancy from N-rTiO₂ shift negatively from 529.83 and 531.65eV to 529.68 and 531.59eV, respectively. The Ti2p_3/2 and Ti2p_1/2 bipeaks of NTU-4 located at 458.40 and 464.06eV are shown in the Ti2p fitting peaks (Fig. 2c). The bipeaks have an energy interval of 5.66eV, which belongs to a typical Ti⁴⁺ species with Ti-O structure. And compared with N-rTiO2, the Ti2p_3/2 and Ti2p_1/2 binding energies shift negatively from 458.52, 464.21eV to 458.40 and 464.06eV, respectively. C1s XPS spectrum of NTU-4 is composed of three fitting peaks at 284.80, 286.43 and 288.26eV, respectively (Fig. S4b). The fitting peak located at 284.80eV corresponds to the C-C/C = C structure of contaminated carbon, the fitting peak located at 286.43eV corresponds to the C-O-C structure of adsorbed oxygen on the g-C₃N₄ surface, and the fitting peak located at 288.26eV corresponds to the N-C = N structure of the heterocyclic ring in g-C₃N₄.

It is noted that the XPS spectra shifts of binding energy indicate the strong interaction and the heterojunctions formed between N-rTiO₂ and g-C₃N₄ in NTU-4. Furthermore, with the contact of N-rTiO₂ and g-C₃N₄, the positive shifts of binding energy from g-C₃N₄ imply the g-C₃N₄ is the donor of surface N-rTiO₂ and g-C₃N₄ in NTU-4, and the negative shifts of binding energy from N-rTiO₂ suggest the N-rTiO₂ is the acceptor of surface between N-rTiO₂ and g-C₃N₄ in NTU-4. The strong donor–acceptor coupling at their interface must build an internal electric field (IEF) on the interface between N-rTiO₂ and g-C₃N₄ in NTU-4. When light is irradiated into the composite material, IEF will effectively separate the photogenerated carriers to promote the improvement of photocatalytic efficiency.

In order to further confirm the composition of NTU-4, TEM and HRTEM were measured through a micro-grid copper network. Figure 2d-g shows TEM, HRTEM, FFT pattern and EDS of N-rTiO₂, respectively. It can be seen from the figure that N-rTiO₂ is a three-dimensional block structure. High resolution crystal lattice images were captured and the N-rTiO₂ structure was determined by calibration of its fast Fourier transform (FFT) pattern. Crystal images belonged to the [1–11] crystal band axis, crystal planes were (110) and (10 − 1) crystal planes, and the lattice spacing of 0.32nm corresponded to the direction of (110) crystal plane of N-rTiO₂. Obviously, N-doping did not change the rutile lattice structure, which was consistent with XRD results. As can be seen from EDS, N was successfully doped into rutile TiO₂. Fig. S5 shows the TEM and EDS images of g-C₃N₄, from which it can be seen that g-C₃N₄ has a typical amorphous two-dimensional layered structure composed of C and N with evenly distributed voids. TEM, HRTEM, and EDS of NTU-4 are shown in Fig. 2h-j, indicating that N-rTiO₂ and g-C₃N₄ is well composited.

3.2 photo and photoelectrochemical performance

The band gap E_g of the samples can be characterized by UV-vis DRS. From Fig. 3a, in the visible region, the absorption of N-rTiO₂ is significantly higher than that of g-C₃N₄ and aTiO₂. N-doping also makes the light absorption obviously redshift to the visible region. In order to further study the change of band gap, the band gaps of g-C₃N₄, aTiO₂ and N-rTiO₂ are obtained by using the Kubelka-Munk formula, The band gap E_g can deduce by the extending line of line part of [F(R)hν]ⁿ to the photon energy (hν) curve. Where n = 1/2 for indirect semiconductors aTiO₂ and g-C₃N₄ and n = 2 for direct semiconductor N-rTiO₂. Figure 3b shows that the E_g of g-C₃N₄, aTiO₂ and N-rTiO₂ are 2.92, 3.06 and 3.17 eV, respectively. The results show that both N-rTiO₂ and g-C₃N₄ can respond to part of visible light, which is beneficial to make full use of sunlight. Further separation of photogenerated carriers through constructing heterojunction will further promote the improvement of photocatalytic performance.

From Fig. 3c and 3d, the PL peak of g-C₃N₄ and N-rTiO₂ decreases significantly after composite, indicating that photogenerated carriers of NTU-x are further separated near the interface than g-C₃N₄, which reduces the probability of recombination and promotes the improvement of photocatalytic performance. Among the PL peaks of NTU-x, NTU-4 has the weakest peak, implying NTU-4 inhibits the photoluminescence better, indicating that the probability of photogenerated carriers migrating to the surface is the greatest. This will be more conducive to the improvement of photocatalytic performance.

Table 1

Short lifetime, long lifetime and average lifetime of samples.
Sample	A₁	τ₁ /ns	A₂	τ₂ /ns	τ_avg /ns
g-C₃N₄	0.75	2.47	0.25	11.65	8.08
NTU-3	0.89	1.97	0.11	9.87	5.07
NTU-4	0.86	1.74	0.14	8.36	4.59
NTU-5	0.78	2.00	0.22	9.45	6.25

To obtain an insight on more information photogenerated carriers, the TRPL fluorescence attenuation curve of the sample was fitted by the following equation (Eq. 1).

$${y={{y}_{0}+A}_{1}e}^{{-t}_{1}/{\tau }_{1}}+{{A}_{2}e}^{{-t}_{2}/{\tau }_{2}}$$

Where τ₁ and τ ₂ corresponding to the short lifetime and long lifetime, respectively. A₁ and A₂ represent the proportions of τ₁ and τ₂, respectively.

While photogenerated charges return to equilibrium from the unstable excited state, energy will release by radiative and non-radioactive decay process, corresponding to short lifetime τ₁ and long lifetime τ₂. By the following equation (Eq. 2) can deduce carrier's average lifetime τ_avg of fluorescent light.

$${\tau }_{avg}=\frac{{A}_{1}{\tau }_{1}^{2}{+A}_{2}{\tau }_{2}^{2}}{{A}_{1}{\tau }_{1}{+A}_{2}{\tau }_{2}}$$

As shown in the Fig. 3e, the τ₁, τ₂ and τ_avg of NTU-4 are 1.74, 8.38 and 4.59ns respectively, which are 30%, 28% and 43% less than the 2.47, 11.65 and 8.08ns of g-C₃N₄, respectively. The reduced carrier lifetime of NTU-4 is attributed to the fact that g-C₃N₄ and N-rTiO₂ recombination provides non-radiative decay channels for rapid carrier migration, which causes fluorescence quenching. It can be seen from Table 1 and Fig. 3f that the short life, long life and average life of NTU-x are all smaller than g-C₃N₄, indicating that the composite of g-C₃N₄ and N-rTiO₂ promotes carrier migration and separation. It is note that, among NTU-x, NTU-4 has the smallest lifetime, indicating that NTU-4 is the easiest to separate carriers than other composites. This is consistent with the representation of PL.

The same results were also reflected in the transient photocurrent (I-t) and electrochemical impedance spectroscopy (EIS) tests. In the three-electrode electrochemical system, when light was irradiated to the working electrode, the photocurrent of NTU-4 was significantly stronger than that of g-C₃N₄ and N-rTiO₂ (Fig. 4a), which indicated that the heterojunction composed of g-C₃N₄ and N-rTiO₂ effectively promoted the carrier separation. From Fig. 4b, the EIS also shows that the Nyquist curve radius of NTU-4 is the smallest, which means the photogenerated carriers’ migration resistance is smallest, that makes it easier for carriers to migrate to the surface of the material and thus improve the photocatalytic performance more effectively. The Mott-Schottky plot is shown in Fig. 4c and 4d, the positive slope indicates that both g-C₃N₄ and N-rTiO₂ are n-type semiconductors. The flat-band potential of g-C₃N₄ and N-rTiO₂ are deduced by the intercept of the extrapolated straight-line part of Mott-Schottky plots. The flat-band potential of g-C₃N₄ and N-rTiO₂ are − 1.29 and − 0.24V vs SCE at PH = 7, respectively. It is note that although the conduction band potential (E_cb) of n-type semiconductor is more negative about − 0.1 to -0.3 V than E_fb, the E_cb of N-rTiO₂ is not more negative than − 0.3V vs NHE at PH = 7, which means that N-rTiO₂ cannot meet the thermodynamic conditions for photocatalytic hydrogen production (more negative than − 0.41V at PH = 7). While the conduction potential of g-C₃N₄ reaches − 1.05V vs NHE at PH = 7 without considering the difference with E_fb, which is sufficient to satisfy the thermodynamic conditions of photocatalytic hydrogen production. If the difference between E_cb and E_fb don't be take into account, based on the formula E_vb = E_cb + E_g, the valence band potential of g-C₃N₄ and N-rTiO₂ are be deduced 1.63 and 2.82V vs SCE at PH = 7 (1.87 and 3.06V vs NHE), respectively.

3.3 photocatalytic performance evaluation

The photocatalytic performance of the sample is evaluated by photocatalytic hydrogen production. Figure 4e shows the hydrogen production rate of the sample under the irradiation of Xenon lamp at λ > 400nm. It is note that the hydrogen production of N-rTiO₂ is zero, on the one hand, for N-doping rutile TiO₂, the photogenerated electrons quickly are captured by the deep level in several picoseconds and lost the reaction activity, on the other hand, the E_cb level of N-rTiO₂ is more positive than that potential of H⁺/H₂ and cannot meet the thermodynamic conditions of photocatalytic hydrogen production, which is elucidated on Mott-Schottky potential. Therefore, even if the bandwidth can suit for absorbing more sunlight and generating more photogenerated carriers, it still cannot produce hydrogen because it cannot meet the thermodynamic conditions. When composited with g-C₃N₄, not only can hydrogen be produced, but also the hydrogen production rate is higher than that of g-C₃N₄ alone. When g-C₃N₄ was used alone, the hydrogen production rate was 825.5mmol g^-1 h^-1, while the hydrogen production rate of NTU-3, NTU-4 and NTU-5 was 1510.9, 2121.7 and 1570.1mmol g^-1 h^-1, respectively. They are 1.9, 2.57 and 2.1 times higher than pure g-C₃N₄, respectively. Among them, NTU-4 has the best performance, indicating that the composite of N-rTiO₂ and g-C₃N₄ promotes the separation of photogenerated carriers, Repeated testing of NTU-4 (Fig. 4f) showed that NTU-4 had good stability.

3.4 DFT calculation and possible mechanism analysis

To further study the photocatalytic mechanism of g-C₃N₄, N-rTiO₂ and NTU-x, it was carried out by the first-principles calculations of band structure, total density of state (TDOS) and projected density of state (PDOS), work function (W_F), differential charge density and Bader charge of g-C₃N₄, N-rTiO₂ and their composite. Fig. S6a is the band structure plots of g-C₃N₄ and N-rTiO₂ nanostructures. It can be seen that the nanostructures are an indirect band-gap semiconductor structure, the valence band top is located at point Γ, while the conduction band bottom is located at point B. In order to quantitatively study the electronic structure of the interface and identify the distribution of electron orbitals, the TDOS and PDOS of g-C₃N₄ (001)/N-rTiO₂ (110) nanostructures were calculated. According to the TDOS and PDOS plots (Fig. S6b), it can be seen that the valence band top of g-C₃N₄/N-rTiO₂ nanostructure is mainly occupied by O1s, N2p and Ti3d orbitals, and the conduction band bottom is mainly occupied by C2p and N2p orbitals. It can be seen that the conduction band base and valence band top of g-C₃N₄/N-rTiO₂ nanostructure belong to different components respectively, that is, they are occupied by the electron orbital of g-C₃N₄ and N-rTiO₂ respectively. According to the above analysis, it can be seen that g-C₃N₄/N-rTiO₂ nanostructure is a band staggered energy level.

The g-C₃N₄/N-rTiO₂ nano-heterostructure can also be described by work function. The W_F of g-C₃N₄ (001) surface, N-rTiO₂ (110) surface and g-C₃N₄/N-rTiO₂ heterostructure surface can be calculated according to the following formula:

W_F = Evac − EF (3)

To further explore the charge transfer and separation between the interface of g-C₃N₄/N-rTiO₂, differential charge density and were calculated (Fig. 6a-f). The electron accumulation zone is shown in yellow, while the electron loss zone is shown in cyan. The redistribution of charge density mainly appears near the g-C₃N₄/TiO2 interface, while the charge density deep inside N-rTiO₂ hardly changes. which is mainly attributed to the weak vdW forces between g-C₃N₄ and N-rTiO₂ interfaces. Figure 6c and 6f show the calculated surface average differential charge density along the z-axis, electrons at the interface are mainly transferred from g-C₃N₄ surface to N-rTiO₂ surface, while holes remain on g-C₃N₄ surface. Furthermore, Bader charge analysis was calculated the charge transfer and separation between the interfaces of g-C₃N₄(001)/N-rTiO₂(110). The results show that approximately 0.2925e electrons are transferred from g-C₃N₄ (001) surface to N-rTiO₂ (110) surface. When the electron transfer reaches an equilibrium state, the accumulation of net charge on the surface results in the formation of an IEF at the g-C₃N₄ (001) /N-rTiO₂ (110) interface, and the IEF direction is from the g-C₃N₄ (001) surface to the N-rTiO₂ (110) surface. The presence of IEF is conducive to the effective separation of photogenerated h⁺-e⁻ pairs, which further promotes the improvement of photocatalytic activity of g-C₃N₄/N-rTiO₂.

The mechanism of photocatalysis is analyzed in Fig. 6. Although type II and Z-scheme are both staggered level structures, For Type-II heterojunctions (Fig. 6g), the photogenerated electrons migrate from the conduction band of g-C₃N₄ to the conduction band of N-rTiO₂, if the traditional type II heterojunction is formed, the conduction potential is too low to meet the thermodynamic conditions of photocatalytic hydrogen production. However, experiments have proved that the composite material can improve the effect of photocatalytic hydrogen production. Therefore, it is impossible for g-C₃N₄/N-rTiO₂ to form type II heterojunction. Similarly, XPS, work function, IEF, differential charge density and Bader analysis show that the g-C₃N₄/N-rTiO₂ interface forms not traditional type II heterojunction, but Z-scheme. For the Z-scheme (Fig. 6h), the electrons in the N-rTiO₂ conduction band will be recombined through the holes in the valence band of g-C₃N₄, the photogenerated electrons accumulate at the Ecb of g-C₃N₄ and the photogenerated holes accumulate at the E_vb of N-rTiO₂, the direct Z-scheme photocatalyzed heterostructures exhibit stronger redox capabilities due to the more negative E_cb and the more positive E_vb. Specifically (Fig. 6i), the edge potential of the conduction band of g-C₃N₄ is − 1.05V vs NHE, which is more negative than the potential of H ⁺ /H₂ (-0.41V) to product H₂. Therefore, the direct Z-scheme of g-C₃N₄/N-rTiO₂ exhibit higher photocatalytic hydrogen production performance than pristine N-rTiO₂ and g-C₃N₄.

In conclusion, for N-rTiO₂ material whose conduction band energy level cannot meet the thermodynamic conditions of hydrogen production, a Z-scheme was constructed by simple co-burning method, which not only promoted the separation and migration of photogenerated carriers, but also met the thermodynamic conditions of hydrogen production, making the photocatalytic efficiency increased to 2.57 times than that of g-C₃N₄. The design of the Z-scheme heterojunction provides a feasible idea for the photocatalytic hydrogen production of materials that cannot meet the conditions of hydrogen production. It is an effective way to expand the selection range of photocatalytic materials.

Funding

This work was supported by the project from the National Natural Science Foundation of China (Grant No. 11804005, 11375136, 12204014), and the corresponding project’s PO were Dr. Yuhua Wang, Dr. Qianqian Gao and Dr. Huibin Zheng, respectively. We also appreciate the support from the Anyang Institute of Technology Research Cultivation Fund (Grant No. YPY2019002), and the corresponding project’s PO was Jianxin Li.

Declaration of interest

The authors declare no competing financial interest.

Contributions

Jianxin Li wrote the main manuscript text, Yuhua Wang and Haixiang Song reviewed the manuscript and provided some advice and suggestions, Yao Guo did the VASP calculation and simulation, Shuaishuai Hu, Huibin Zheng, Shiding Zhang, Xianchang Li, Qianqian Gao, Chengbo Li, Zunwei Zhu and Yitong Wang prepared figures 1-6. All authors reviewed the manuscript.

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N-doped rutile TiO2 composite g-C3N4 with Z-Scheme boosts photocatalytic hydrogen production under visible light

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Abstract

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1 Introduction

2 Experiments

2.1 the synthesis of samples

2.2 Characterization and electrochemical tests

2.3 Photocatalytic performance evaluation

2.4 DFT calculation

3 Results And Discussion

3.1 Structure and morphology

3.2 photo and photoelectrochemical performance

3.3 photocatalytic performance evaluation

3.4 DFT calculation and possible mechanism analysis

4 Conclusion

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References

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