Hierarchical Honeycomb Br, N-Codoped TiO2 with Enhanced Visible

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Hierarchical Honeycomb Br, N Codoped TiO2 with Enhanced Visible-Light Photocatalytic H2 Production Chao Zhang, Yuming Zhou, Jiehua Bao, Xiaoli Sheng, Jiasheng Fang, Shuo Zhao, Yiwei Zhang, and Wenxia Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b04947 • Publication Date (Web): 10 May 2018 Downloaded from http://pubs.acs.org on May 15, 2018

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ACS Applied Materials & Interfaces

Hierarchical Honeycomb Br, N Codoped TiO2 with Enhanced Visible-Light Photocatalytic H2 Production

Chao Zhang1, Yuming Zhou1* , Jiehua Bao1, Xiaoli Sheng1, Jiasheng Fang1, Shuo Zhao1, Yiwei Zhang1, Wenxia Chen1 1

School of Chemistry and Chemical Engineering, Southeast University, Jiangsu

Optoelectronic Functional Materials and Engineering Laboratory, Nanjing 211189, China

ABSTRACT The halogen elements modification strategy of TiO2 encounters a bottleneck in visible-light H2 production. Herein, we have for the first time reported a hierarchical honeycomb Br, N codoped anatase TiO2 catalyst (HM-Br,N/TiO2) with enhanced visible-light photocatalytic H2 production. During the synthesizing process, large amounts of meso–macroporous channels and TiO2 nanosheets were fabricated in massive TiO2 automatically, constructing the hierarchical honeycomb structure with large specific surface area (464 m2 g-1). CTAB and melamine played a key role in Corresponding author: Yuming Zhou E-mail: [email protected] Address: Southeast University Avenue Jiangning District, Nanjing, Jiangsu Province, China Tel: +86 25 52090617; Fax: +86 25 52090617. 1 ACS Paragon Plus Environment

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constructing the meso–macroporous channels. Additionally, HM-Br,N/TiO2 showed a high visible-light H2 production rate of 2247 µmol h−1 g−1, which is far more higher than single Br or N doped TiO2 (0 or 63 µmol h−1 g−1, respectively), thereby demonstrating the excellent synergistic effect of Br and N elements in H2 evolution. In HM-Br,N/TiO2 catalytic system, the codoped Br/N atoms could reduce the band gap of TiO2 to 2.88 eV and the holes on acceptor levels (N acceptor) can passivate the electrons on donor levels (Br donor), thereby preventing charge carriers recombination significantly. Furthermore, the proposed HM-Br,N/TiO2 fabrication strategy had a wide range of choices for N source (e.g. melamine, urea and dicyandiamide) and it can be applied to other TiO2 materials (e.g. P25) as well, thereby implying its great potential application in visible-light H2 production. Lastly, on the basis of experimental results, a possible photocatalytic H2 production mechanism for HM-Br,N/TiO2 was proposed.

KEYWORDS: Br, N, TiO2, H2, Photocatalyst

INTRODUCTION Since Fujishima and Honda reported photocatalytic splitting of water on TiO2 electrodes in 1972, visible-light photocatalytic hydrogen production has been considered as one of the most promising methods to produce hydrogen energy.1, 2 Among the oxide-based semiconductor photocatalysts, TiO2 is the most suitable candidate for water splitting due to its cost effectiveness, biological and chemical 2 ACS Paragon Plus Environment

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inertness, excellent photochemical stability and compatible band-edge positions.3-5 However, the wide band gap (3.1-3.2 eV), which leads to low visible light utilization, and high electron-hole pairs recombination seriously restrict its applications in photoinduced chemical reactions.6-8 Therefore, a number of methods have been proposed to fabricate efficient TiO2 photocatalysts for H2 production such as coupling with narrow band gap semiconductors and doping with noble metal nanoparticles (NPs) or various anionic elements.9-12 In the case of anionic elements doping, halogen elements (e.g. F, Cl, Br, I) modification is one of the most promising and widely investigated strategies to narrow the band gap of TiO2.12-14 This method has been applied in various TiO2 based catalytic systems including F/TiO2,13 graphene oxide-F/TiO2,14 F-N codoped TiO2,15, 16

Br-N codoped TiO2,17, 18 I-N codoped TiO219 and Cl-N codoped TiO2.20 The doped

halogen atoms can substitute Ti sites and induce donor level in TiO2, thereby enhancing photocatalytic performance.21, 22 But, most of the halogen modified TiO2 catalysts were used in the degradation of organics and so far no efficient halogen modified TiO2 catalysts for H2 production have been synthesized successfully. The halogen elements modification strategy of TiO2 encounters a bottleneck in visible-light H2 production. Generally, the doped halogen elements can form donor−acceptor pairs with other elements (e.g. N and C) in TiO2.21, 23 Efficient H2 production catalysts may be synthesized via special codoping strategies.

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Figure 1. Fabrication process of the hierarchical honeycomb Br, N codoped anatase TiO2 catalyst (HM-Br,N/TiO2) with enhanced visible-light photocatalytic H2 production.

Herein, we have for the first time successfully designed and fabricated a hierarchical honeycomb Br, N codoped anatase TiO2 catalyst (HM-Br,N/TiO2) with enhanced visible-light photocatalytic H2 production. The synthesizing process is very simple, which is mainly containing a one-step hydrothermal treatment of massive TiO2 with CTAB and melamine to serve as Br and N sources, respectively (Figure 1). During the hydrothermal process, massive TiO2 was converted into honeycomb structure automatically, meanwhile, numerous tiny TiO2 nanosheets (NSs) in situ grew on massive TiO2 surface due to the corrosive attack of OH- on TiO2, thereby constructing the hierarchical honeycomb structure facilely. The large amounts of meso–macroporous channels and TiO2 NSs could increase the specific surface area of massive TiO2 to 464 m2 g-1 dramatically. In H2 production, HM-Br,N/TiO2 showed a high visible-light H2 production rate of 2247 µmol h−1 g−1, which is far more higher 4 ACS Paragon Plus Environment

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than single Br or N doped TiO2 (0 or 63 µmol h−1 g−1, respectively), thereby demonstrating the excellent synergistic effect of Br and N elements in H2 evolution. The codoped Br/N atoms could substitute Ti/O sites in massive TiO2 and acted as donor-acceptor pairs, thereby reducing the band gap of TiO2 to 2.88 eV and increasing charge carriers separation. Besides, the proposed HM-Br,N/TiO2 fabrication strategy can also be used in modifying P25 for visible-light H2 production. Furthermore, except for melamine, both of urea and dicyandiamide could serve as N source in the strategy, thereby implying its great potential applications in H2 production.

EXPERIMENTAL SECTION Chemicals and materials Melamine,

Tetrabutyl

titanate

(TBOT),

dicyandiamide,

urea

and

cetyl

trimethylammonium bromide (CTAB) were purchased from Sigma-Aldrich. Ammonia solution (28%), ethanol and triethanolamine (TEOA) were purchased from Sinopharm Chemical Reagent Co. (Shanghai, China).

Synthesis of HM-Br,N/TiO2 Tetrabutyl titanate (TBOT, 2 mL) was mixed with de-ionized water (0.5 mL) and ethanol (40 mL) under vigorous stirring to form a homogeneous solution. After reaction for 6 h, the massive TiO2 was separated by centrifugation and then dried at 60 ℃. The obtained massive TiO2 (0.3 g) was dispersed in a solution of CTAB (3 mL, 5 ACS Paragon Plus Environment


0.2 g mL-1), melamine (3 mL, 0.1 g mL-1) and de-ionized water (50 mL). A certain amount of ammonia solution (10 mL) was added in the mixture. Then the solution was sealed into a 100 mL Teflon-lined autoclave, followed by hydrothermal treatment at 150 ℃ for 24 h in a static state. The resulting precipitates were isolated by centrifugation, dried under vacuum and calcined at 520 ℃ for 4 h in N2 to obtain the hierarchical honeycomb Br, N codoped anatase TiO2 catalyst (HM-Br,N/TiO2).

Photocatalytic H2 Production Test Generally, the photocatalyst (10 mg) and TEOA (10 mL) were dispersed in 90 mL de-ionized water. Then, the obtained mixture was degassed to remove air fully. A 300 W Xenon lamp was used as light source and the UV light was removed by UV-light cutoff filter (λ>420 nm). The amount of H2 was quantified by an online gas chromatograph (GC9890A/T, TCD detector, Ar carrier gas, 5A molecular sieve column).

RESULTS AND DISCUSSION


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Figure 2. (a-c) SEM images, (d) TEM image and (e) elemental mapping analysis of HM-Br,N/TiO2. The inset in (d) is high resolution TEM image of HM-Br,N/TiO2.

The morphology and structure of HM-Br,N/TiO2 were investigated by SEM and TEM. As shown in Figure 2a and b, after hydrothermal treatment of massive TiO2 (Figure S1), many macropores (ca. 1 µm in diameter) appeared on TiO2 surface, showing honeycomb morphology. Meanwhile, due to the corrosive effect of OH-, numerous tiny TiO2 NSs in situ overgrew on honeycomb TiO2 (Figure 2c). From the large-scale SEM image of HM-Br,N/TiO2 in Figure S2, it can be found that most massive TiO2 had been converted into hierarchical honeycomb structure via the hydrothermal 7 ACS Paragon Plus Environment


treatment. Besides, from the TEM image of Figure 2d, HM-Br,N/TiO2 presented many transparent areas, which is corresponding to the macropore structure. TiO2 NSs could be observed in the margin area, thereby further demonstrating the hierarchical honeycomb structure of HM-Br,N/TiO2. The high resolution TEM image in inset showed the lattice fringes of (101) atomic plane of anatase, thereby confirming the crystalline nature of HM-Br,N/TiO2. Furthermore, the elemental mapping analysis of HM-Br,N/TiO2 in Figure 2e proved the uniform distribution of Ti, O, N and Br elements in HM-Br,N/TiO2. All the elements exhibited similar outline with the TEM image of Figure 2d. During the hydrothermal treatment, melamine not only served as N source, but also as macropore-forming agent. Just as shown in Figure S3a, in the absence of melamine, no macropore structure was constructed via hydrothermal treatment. Only numerous tiny TiO2 NSs grew on massive TiO2 surface (Figure S3b). When adding 1 mL melamine solution (0.1 g mL-1) in the hydrothermal system, macropore structure started to appear (Figure S3c) and some macropore in initial formative stage could be observed clearly (Figure S3d).


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Figure 3. (a) Nitrogen adsorption-desorption isotherm and (b) corresponding pore size distribution curve of massive TiO2, H-Br/TiO2 (synthesized in melamine dosage of 0 mL and CTAB dosage of 3 mL), HM-Br,N/TiO2-1 (synthesized in melamine dosage of 1 mL and CTAB dosage of 3 mL) and HM-Br,N/TiO2. The pore size distribution curve was calculated using the data of adsorption branch.

Nitrogen adsorption-desorption isotherm was employed to further investigate the pore-forming factors. As shown in Figure 3, pristine massive TiO2 showed low specific surface area (ca. 35 m2 g-1) and porosity. After hydrothermal treatment in the present of CTAB only, the specific surface area of H-Br/TiO2 increased to 241 m2 g-1 significantly because of the growth of TiO2 NSs and some mesoporous (ca. 2-4 nm in diameter) appeared. Thus, CTAB served as pore-forming agent during the hydrothermal treatment. Besides, when adding 1 mL melamine solution (0.1 g mL-1) in the hydrothermal system, more mesoporous structures of ca. 2-3 nm in diameter were constructed in HM-Br,N/TiO2-1 and, accordingly, the specific surface area increased to 363 m2 g-1, thereby indicating that melamine could contribute to the 9 ACS Paragon Plus Environment


formation of mesoporous in massive TiO2. Furthermore, as melamine dosage increased to 3 mL, the specific surface area of HM-Br,N/TiO2 further improved to 464 m2 g-1 and, from the pore size distribution curve, the mesoporous structures (ca. 2-3 nm in diameter) became more clearly. The specially fabricated mesoporous and macropore in HM-Br,N/TiO2 served as channels for the access of reactants in H2 production. From all the above analysis, it can be concluded that both of CTAB and melamine played a key role in forming pore structures in HM-Br,N/TiO2.

Figure 4. XPS spectra of (a) Ti 2p, (b) N 1s, (c) Br 3d and (d) O 1s in HM-Br,N/TiO2.

XPS analysis was carried out to analyze the surface electronic status of Ti, N, Br and O elements in HM-Br,N/TiO2. As shown in Figure 4a, Ti 2p spectrum presented two peaks at 458.4 (Ti 2p3/2) and 464.1 eV (Ti 2p1/2), which indicates the normal state of Ti4+ in TiO2.24 N 1s could be fitted by Gaussian curves with two dominant 10 ACS Paragon Plus Environment

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components at 399.3 and 401.2 eV, related to N−Ti−O and Ti−N−O, respectively.17, 18, 25

The doped N atoms could fill in the oxygen deficiency of TiO2, thereby forming the

N−Ti−O and Ti−N−O bonds. Besides, N exhibits higher atomic p-orbital energy than O and it could induce an acceptor level above the valence band maximum (VBM) of TiO2.26, 27 Br element shows no obvious XPS peak at 67.8 eV (Br 3d5/2) and 69.4 eV (Br 3d3/2) due to low Br content (0.3 wt%, Figure S4).28, 29 Besides, similarly with I or F doping, Br atoms can substitute Ti sites in TiO2, constructing the lattice oxygen of Ti-O-Br.28-31 Just as confirmed in Figure 4d, the O 1s spectrum is deconvoluted into three peaks and the peak at 529.5 eV is belonged to Ti−O−Br.28, 29 The doped Br could adjust the conduction band maximum (CBM) position uphill and broaden the photoresponse region.12, 32 In HM-Br,N/TiO2, the codoped Br and N elements can form an intermediate band within the band gap of TiO2 and serve as charge-compensated donor-acceptor pairs.

Figure 5. (a) XRD analysis and (b) FT-IR spectra of the prepared samples. HM-XBr,N/TiO2 was synthesized by adding X mL CTAB (0.2 g mL-1) in the hydrothermal system.



The

crystalline

structures

of

TiO2,

HM-Br/TiO2,

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HM-N/TiO2

and

HM-XBr,N/TiO2 were characterized by XRD (Figure 5a). It is apparent that all the samples showed classical crystal structure of anatase TiO2 (JCPDS no. 21-1272), indicating that doped Br and N elements could not alter the crystalline structure of TiO2. FT-IR spectra of the samples are shown in Figure 5b. In the spectrum of pristine TiO2, the absorption band at 446 cm−1 corresponds to the Ti-O-Ti stretching vibration.33 Generally, Br could substitute the Ti site of TiO2, forming Ti-O-Br bond.28, 29, 32 Thus, after doping Br elements in TiO2, the obvious absorption band at 510 cm−1 of HM-Br/TiO2 can be assigned to O-Br. Besides, when increasing CTAB dosage from 1 to 3 mL, the characteristic peaks of O-Br in HM-XBr,N/TiO2 (X=1, 2, 3) enhanced gradually, demonstrating the successful doping of Br. For HM-N/TiO2, the absorption band at 1381 cm−1 is related to hyponitrite groups, thereby indicating the decomposition of melamine during hydrothermal process.34 Furthermore, with CTAB dosage increasing, the characteristic peaks of hyponitrite groups in HM-XBr,N/TiO2 (X=1, 2, 3) became weak obviously, suggesting that CTAB could promote the decomposition of N source. The peak at 1050 cm−1 is ascribed to the C-O bond.27, 35 Additionally, compared with HM-3Br,N/TiO2, HM-5Br,N/TiO2 exhibited no characteristic peak of O-Br at 510 cm−1 because of the changed structure. Just as confirmed in Figure S5, HM-5Br,N/TiO2 showed an irregular structure instead of hierarchical honeycomb structure due to the excess amount of CTAB dosage.


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Figure 6. (a) UV−visible absorbance spectra and (b) corresponding Kubelka−Munk transformed

reflectance

spectra

of

TiO2,

HM-Br/TiO2,

HM-N/TiO2

and

HM-Br,N/TiO2.

The optical characteristics were investigated via UV−visible diffuse reflectance spectroscopy (Figure 6a). The basal absorption edge of pristine TiO2 occured at ca. 400 nm, in contrast, the absorption intensity of HM-Br/TiO2 raised slightly to ca 420 nm due to the doping of Br.12, 28 For HM-N/TiO2, an enhanced absorption in the visible range between 400 and 460 nm can be observed clearly. When doping Br and N elements in TiO2 simultaneously, except for the extended absorption edge between 400 and 460 nm, the visible light absorption at the range of 460-720 nm enhanced weakly, this might lead to the further improved visible-light absorption capacity of HM-Br,N/TiO2. Additionally, the band gap energies of all the samples were determined by Kubelka−Munk transformation, αhv = A(hv − Eg)2, where Eg is band gap energy, α is absorption coefficient, A represents a constant, v is

light frequency

and h is Planck's constant.36 From the plot of (αhv)½ vs photon energy in Figure 6b, the band gap energy values of TiO2, HM-Br/TiO2, HM-N/TiO2 and HM-Br,N/TiO2 13 ACS Paragon Plus Environment


were estimated to be 3.12, 3.1, 2.98 and 2.88 eV, respectively. The band gap energy of HM-Br,N/TiO2 was reduced by 0.24 eV obviously via codoping N and Br elements in TiO2. Additionally, the valence band (EVB) and conduction band (ECB) edge position of the samples can be calculated according to the two formulas: ECB= Xanatse-Ee-1/2Eg EVB=ECB+Eg where Xanatse is the electronegativity of anatse (5.8 eV), Ee is the energy of free electrons on the hydrogen scale (4.5 eV).37, 38 After calculation, the EVB/ECB values of TiO2, HM-Br/TiO2, HM-N/TiO2 and HM-Br,N/TiO2 are 2.86/-0.26, 2.85/-0.25, 2.79/-0.19 and 2.74/-0.14 respectively and close to other reports.36,

39

These results confirmed that Br and N elements can

modulate the band gap of TiO2 and enhance the visible-light absorption capacity effectively.


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Figure 7. (a) Visible-light H2 production rates for TiO2, HM-Br/TiO2, HM-N/TiO2 and HM-Br,N/TiO2. (b) PL emission spectra, (c) EPR spectra, (d) photocurrent response under visible-light irradiation and (e) electrochemical impedance spectroscopy plots of TiO2, HM-Br/TiO2, HM-N/TiO2 and HM-Br,N/TiO2. (f) Correlation between visible-light H2 production and CTAB dosage.

Visible-light H2 production was employed to study the photocatalytic performance of all the samples. As shown in Figure 7a, both of pristine TiO2 and HM-Br/TiO2 produced no detectable H2 because of the wide band gap (3.12 and 3.1 15 ACS Paragon Plus Environment


eV, respectively) and low visible light utilization. HM-N/TiO2 was able to generate a minimal amount of H2 (63µmol h−1 g−1). In sharp contrast, HM-Br,N/TiO2 exhibited a significantly enhanced H2 production of 2247 µmol h−1 g−1, approximately thirty-five fold higher than HM-N/TiO2. From Table S1, it could be found that HM-Br,N/TiO2 possessed higher visible-light H2 production than most TiO2 based catalysts (even higher than noble metal loaded TiO2 catalysts). The high H2 production of HM-Br,N/TiO2 could be attributed to the improved visible-light absorption capacity (Figure 6a) and narrow band gap of 2.88 eV (Figure 6b). Besides, codoped Br and N elements could serve as charge-compensated donor-acceptor pairs in anatase TiO2, thereby enhancing charge carriers separation efficiently.12, 21, 26, 40 Just as shown in Figure 7b, compared with TiO2, HM-Br/TiO2 and HM-N/TiO2, HM-Br,N/TiO2 showed a very lower PL emission peak at about 370 nm and the PL emission spectra shifted to longer wavelength obviously, indicating the high charge carriers separation and enhanced visible-light absorption capacity.36 Additionally, the PL peak at 470 is related to band edge free excitons.41 The oxygen vacancies induced by Br/N donor-acceptor pairs were investigated by electron spin resonance (EPR) analysis to further demonstrate the excellent charge carriers separation of HM-Br,N/TiO2. From Figure 7c, HM-Br,N/TiO2 presented an enhanced paramagnetic signals at g=2.003 due to surface oxygen vacancy, directly proving the efficient electrons generation and separation.42, 43 The doped Br and N atoms in HM-Br,N/TiO2 can act as donor-acceptor pairs and thus passivate the photogenerated electrons. The high charge carriers separation of HM-Br,N/TiO2 was 16 ACS Paragon Plus Environment

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consistent with its excellent visible-light H2 production (2247 µmol h−1 g−1, Figure 7a). In strong contrast, HM-Br/TiO2 and HM-N/TiO2 exhibited no obvious oxygen vacancy signal at g=2.003 because of the limited inducing effect of single Br or N atoms in TiO2. Photoelectrochemical measurements were carried out in three-electrode cell to investigate charges generation properties. As shown in Figure 7d, compared with TiO2, HM-Br/TiO2 and HM-N/TiO2 presented slightly higher photocurrent response because of the doping of Br or N elements, respectively. Besides, the photocurrent response of HM-Br,N/TiO2 was enhanced dramatically because of the excellent charges generation ability. The sharp increased photocurrent response at 30 s is peak current, which is related with the properties of catalysts and test device. Figure 7e showed the electrochemical impedance spectra (EIS) in dark. Among all the samples, TiO2 showed the largest EIS semicircular, suggesting high electron migration resistance.9 Compared with HM-Br/TiO2 and HM-N/TiO2, HM-Br,N/TiO2 exhibited an obviously smaller EIS semicircular due to the co-doped Br/N donor-acceptor pairs. This result indicated that Br/N donor-acceptor pairs could provide more freedom for the migration of charge carriers and hinder the recombination of photogenerated electron-hole pairs efficiently. The effect of CTAB dosage on visible-light H2 production had been investigated as well through changing CTAB dosage (Figure 7f). With CTAB dosage increasing from 0 to 3 mL, the H2 production of HM-Br,N/TiO2 enhanced from 63 to 2247 µmol h−1 g−1 significantly because of the excellent synergistic effect of Br and N elements. 17 ACS Paragon Plus Environment


Then, when further increasing CTAB dosage to 5 mL, the catalytic performance of HM-Br,N/TiO2 reduced seriously because of the collapsed hierarchical honeycomb structure. The synthesized HM-Br,N/TiO2 had an excellent stability and it showed no obvious decrease in H2 production after four consecutive photocatalytic experiments (Figure S6). Moreover, compared with unused HM-Br,N/TiO2, the recycled HM-Br,N/TiO2 showed no obviously changes in photochemical properties and morphology (Figure S7). Furthermore, except for melamine, both of dicyandiamide and urea could also serve as N source to fabricate the Br, N codoped anatase TiO2 catalyst (denoted as H-BrN/TiO2-D and HM-Br,N/TiO2-U, respectively). As shown in Figure S8, all the obtained Br, N codoped anatase TiO2 catalysts presented hierarchical structures as well and the H2 production of H-BrN/TiO2-D and HM-Br,N/TiO2-U could reached up to 2127 or 1930 µmol h−1 g−1, respectively. Additionally, the proposed Br/N codoping strategy is also applied to other TiO2 materials (such as, P25, Figure S9), thereby implying its great potential application in H2 production.


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Figure 8. (a) Energy diagram showing the positions of valence and conduction bands of TiO2, HM-Br/TiO2, HM-N/TiO2 and HM-Br,N/TiO2, (b) Schematic for the passivation effect of Br/N donor-acceptor pairs in HM-Br,N/TiO2. On the basis of above results, a possible photocatalytic H2 production mechanism for HM-Br,N/TiO2 was proposed. In HM-Br,N/TiO2, the codoped Br/N atoms can not only narrow the band gap of TiO2 but also hinder the charge carriers recombination. Just as shown in Figure 8a, after codoping Br and N elements in TiO2, the band gap reduced from 3.12 to 2.88 eV significantly, thereby improving the visible light utilization. Furthermore, the doped N element in TiO2 showed higher atomic p-orbital energy than O element and it tended to induce acceptor level above the VBM of TiO2.21, 26, 27 Besides, the doped Br atom could occupy Ti site of TiO2 lattice and acted as donor, leading to a uphill shift of CBM position.12, 22, 32 In these cases, the holes on the acceptor levels (N acceptor) could passivate the same amount of electrons on the donor levels (Br donor), thereby prolonging the lifetime of charge carriers and enhancing H2 production (Figure 8b).21, 44, 45 Just as shown in Figure 7a, HM-Br/TiO2 and HM-N/TiO2 exhibited a much lower H2 production of 0 and 63 µmol h−1 g−1 than 19 ACS Paragon Plus Environment


HM-Br,N/TiO2 (2247 µmol h−1 g−1), although HM-Br/TiO2 and HM-N/TiO2 owned narrower band gap than TiO2, thereby indicating the passivation effect of Br-N donor-acceptor pairs. Finally, the separated electrions and holes migrated in HM-Br,N/TiO2 catalytic system and reacted with surrounding sacrificial reagent (TEOA) and H+, respectively, generating large volumes of H2.42 CONCLUSIONS In summary, we have for the first time successfully prepared a hierarchical honeycomb Br, N codoped anatase TiO2 catalyst (HM-Br,N/TiO2) with enhanced visible-light photocatalytic H2 production. The synthesizing process is very simple, mainly containing a one-step hydrothermal treatment of massive TiO2 in the present of CTAB and melamine. CTAB and Melamine not only served as Br and N source, respectively, but also as pore-forming agent. Besides, numerous tiny TiO2 nanosheets (NSs) in situ grew on massive TiO2 surface due to the corrosive attack of OH-, thereby fabricating the hierarchical honeycomb structure with large specific surface area of 464 m2 g-1. In this work, HM-Br,N/TiO2 showed a high visible-light H2 production rate of 2247 µmol h−1 g−1, which is far more higher than single Br or N doped TiO2 (0 or 63 µmol h−1 g−1, respectively), proving the excellent H2 production enhancement of Br/N elements. The charge-compensated Br/N donor-acceptor pairs can narrow the band gap of TiO2 to 2.88 eV and enhance the charge carriers separation significantly due to the passivation effect of Br-N donor-acceptor pairs. Furthermore, except for melamine, both of urea and dicyandiamide can be used as N source to fabricate HM-Br,N/TiO2 and the proposed HM-Br,N/TiO2 fabrication 20 ACS Paragon Plus Environment

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strategy could be applied to other TiO2 materials (such as, P25) as well, implying its great potential application in H2 production.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on ACS Publications website.

Characterization. SEM images of massive

TiO2, HM-Br,N/TiO2, recycled

HM-Br,N/TiO2, H-BrN/TiO2-D and HM-Br,N/TiO2-U. EDX data. Stability test results. TEM images. Table of comparing visible-light H2 production.

AUTHOR INFORMATION Corresponding Author *Yuming Zhou. E-mail: [email protected]. Tel: +86 25 52090617. Fax: +86 25 52090617. Notes The authors declare no competing financial interest. ACKNOWLEDGEMENT The authors are grateful to the financial supports of the National Natural Science Foundation of China (Grant No. 21676056, 21376051 and 51673040), ‘‘Six Talents Pinnacle Program’’ of Jiangsu Province of China (JNHB-006), Qing Lan Project of



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Jiangsu Province (1107040167), Graduate student scientific research innovation program of Jiangsu Province (KYCX17_0134 and KYCX17_0136), Scientific Research Foundation of Graduate School of Southeast University (YBJJ1732, YBJJ1733, YBJJ1731), Postgraduate Research ＆ Practice Innovation Program of Jiangsu Province (KYCX17_0134, KYCX17_0136), Fund Project for Transformation of Scientific and Technological Achievements of Jiangsu Province of China (Grant No. BA2014100),

Fundamental

Research

Funds

for

the

Central

Universities

(2242018k30008, 3207047402, 3207046409) and A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) (1107047002).

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Performance of Rutile TiO2 by Sb-N Donor-Acceptor Coincorporation from First Principles Calculations. Appl. Phys. Lett. 2011, DOI: 10.1063/1.3662968.



Graphic abstract

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Hierarchical Honeycomb Br, N-Codoped TiO2 with Enhanced Visible

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