Electronic Supporting Information. Enhanced Photocatalytic Hydrogen Evolution from Water by Niobate Single Molecular Sheets and Ensembles

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1 Electronic Supplementary Material (ESI) for Chemical Communications. This journal is The Royal Society of Chemistry 2014 Electronic Supporting Information Enhanced Photocatalytic Hydrogen Evolution from Water by Niobate Single Molecular Sheets and Ensembles Keizo Nakagawa, Tiantian Jia, Weiran Zheng, Simon Michael Fairclough, Masahiro Katoh, Shigeru Sugiyama, Shik Chi Edman Tsang* 1. Material synthesis 1.1 Materials All the chemicals are in analytical grade of over 99% purity from Aldrich chemicals unless otherwise stated. 1.2 Preparations of single molecular layer niobate (hy-nb-teoa) and reduced layered niobate nanosheets by hydrothermal method The niobate molecular sheets were synthesized by similar hydrothermal method using amine surfactants in ammonia solution. [1] In a typical experiment, g of niobium(v) ethoxide (Nb(OEt) 5 ) ( mol) was mixed with g of triethanolamine (TEOA) (0.025 mol), which is stable against hydrolysis at room temperature. The mixed solution was added to 25 ml of NH 4 OH and the final ph of the mixture was kept at The mixture was then transferred to a 45mL of Teflon autoclave, which was aged at 160 C for 24 h. The resulting product was centrifuged at 6000 rpm for 10 min, a colloidal suspension with small amount of white precipitate was obtained. After the removal of the precipitate fraction, the yellowish supernatant colloid was collected, filtered and washed with distilled water. The final product was dried at 80 C overnight (~12 h) and was christened as hy-nb-teoa. A hydrothermal synthesis of Nb(OEt) 5 without TEOA in NH 4 OH solution were performed in a similar way to examine the role of TEOA on the formation of niobate sheets. After centrifugation at 6000 rpm 1

2 for 10 min, a white precipitate with a clear supernatant was collected. The precipitate was then collected, washed with distilled water and dried at 80 C overnight (~12 h). The obtained sample was christened as hy-nb. 1.3 Preparation of colloidal solutions by exfoliation method Colloidal solutions including niobate molecular sheets were prepared by exfoliation method according to literature. [2,3] Exfoliated niobate molecular sheets were prepared by first adding 15 wt% tetra(nbutylammonium) hydroxide (TBAOH) solution to 100 ml of distilled water containing 20 mg of preprepared niobate sample and bulkier HNb 3 O 8. The resulting suspension was set at the ph of After shaking for 3-5 days the final colloidal solution was collected. The obtained colloidal solutions were denoted TBA-Nb, TBA-Nb-TEOA and TBA-HNb 3 O 8, respectively. No separation and purification of the solids from these colloids prior to their testing since the colloids were highly unstable against precipitation and re-dispersion in the absence of excess TBAOH solution. 1.4 Synthesis of graphene oxide Graphene was synthesized by an improved Hummers method. [4,5] For this method, a 9:1 mixture of concentrated H 2 SO 4 /H 3 PO 4 (360:40 ml) was added to a mixture of graphite (3.0 g, Alfa Aesar) and KMnO 4 (18.0 g). The mixture was heated to 50 C and stirred for 12 h. Then the reaction was cooled to room temperature and poured onto ice (400 ml) with 30% H 2 O 2 (3 ml). After stirring another 30 min, the mixture was centrifuged at 1000 rpm for 5 min, and the precipitates were removed. This process was repeated 4-5 times until no obvious precipitate was left after centrifugation. After that, the suspension was centrifuged at 8000 rpm for 15 min and the supernatant was decanted away. The remaining solid material was then washed with 200 ml of water, 200 ml of 30% HCl, and 200 ml of ethanol successively. The material remaining after the wash process was vacuum-dried at 80 C overnight (~12 h). 2

3 1.5 Synthesis of MoS 2 In a typical synthesis of MoS 2, [6] 1 mmol of Na 2 MoO 4 2H 2 O (BDH laboratory) and 5 mmol of CS(NH 2 ) 2 were dissolved in 60 ml of distilled water. The homogeneous solution was then transferred into a 100 ml Teflon-lined autoclave and aged at 210 C for 24 h. After that, the black precipitate was collected by centrifugation, washed three times with distilled water and ethanol, and then dried in an oven at 80 C for 12 h. 2. Photocatalytic Testing Photocatalytic reactions were performed by dispersing 20 mg of catalysts in a 100 ml of aqueous solution of methanol (10 ml) and distilled water (90 ml). The solution was bubbled with 5% CH 4 /Ar to remove the dissolved oxygen prior to irradiation under a 500 W mercury vapor lamp (UV 50F, helios italquartz). Throughout the irradiation process, agitation of the solution ensured uniform irradiation of the samples. A 30 ml sample of the generated gas was collected and analysed by gas chromatograph. The composition of gas produced from all samples was tested using an Agilent 7890A Gas Chromatograph (GC) fitted with both a Thermal Conductivity Detector (TCD) and a Flame Ionisation Detector (FID). This allowed for accurate detection of H 2, Ar, CH 4 and identification of other species that may have been present (N 2, CO 2 and O 2 ). (i) Blank experiments: There was no hydrogen evolution at all for the typical hy-nb-teoa catalyst in CH 3 OH/H 2 O when placed in the dark, which clearly indicates that the hydrogen production process is a photo-catalytic reaction but not a chemical reaction. On the other hand, the measured hydrogen evolution rate in the absence of catalyst under the UV irradiation was found to be 0.72 µmol/h. This small amount of background hydrogen produced under our 500 W UV lamp illumination was attributed to a small quantity of methanol molecules decomposed during the measurement. 3

4 (ii) Light-purge-light sequences: light on light off light on light off light on H 2 evolution / μmol Reaction time / h Figure S1. Cyclic hydrogen evolution testing of hy-nb-teoa+graphene+mos 2 Figure S1 shows three identical hydrogen evolution rates measured over three 2h cycles upon repeated presence and absence of UV irradiation (hy-nb-teoa-graphene-mos 2 = 99.0:0.5:0.5). Although more cycles are more desirable in our lab-scale testing (notice that the data were collected within the stable regime of our UV-lamp, the intensity and temperature of which can be fluctuated for longer test period), the working catalyst clearly indicates a good degree of stability and reproducibility for the catalytic hydrogen production. 3. Characterization Crystalline phase of the samples were identified by powder XRD (PANalytical X Pert Pro diffractormeter) operating in Bragg-Brentano focusing geometry and using Cu ka radiation (λ = Å) from a generator operating at 40 kv and 40 ma. TEM images of the samples were recorded with a JEOL 2010 equipped with a high-resolution pole piece. TG measurement over 5mg solid sample was conducted using Q50 TA thermogravimetric analysis with a heating rate of 5 C min -1 under a 30ml min -1 flowing stream of air. The UV/Vis adsorption spectra were recorded at room temperature with a UV/Vis 4

5 spectrometer (Lambda 750S, Perkin Elmer) with potassium bromide as the reference. Specific surface areas were derived from the corresponding adsorption isotherms with a conventional BET nitrogen adsorption apparatus (BELSORP18SP, Bell Japan Inc.). The pore size distribution was obtained from the desorption branches based on the Dollimore Heal method. Atomic force microscopy (AFM) images were recorded with by NanoWizard II (JPK Instruments) through the examination of dispersed molecular sheets on a flat Si single crystal surface. Raman spectra were measured with in Via Raman Microscpoe (Renishaw) with a laser excitation wavelength of 532 nm under ambient conditions. The Si Raman band at 520 cm -1 was used as an internal frequency reference. (i) (a) (c) (ii) (b) (d) Figure S2. (i) (a) TEM image of hy-nb; (b) TEM image of hy-nb-teoa; (c) HRTEM image of hy-nb of 4-20 stacked layers in the oxide bundles; (d) HRTEM image of discrete hy-nb-teoa single molecular sheets., (ii) A picture of colloidal suspension of hy-nb-teoa. 5

6 In Figure S2b and S2d, TEM image shows typical single molecular sheets from hy-nb-teoa which was obtained from colloidal suspension after centrifugation at 6000 rpm (the colloidal is highly stable over a few weeks, indicating the formation of dispersive homogeneous 2D molecular sheets). (a) (b) (c) Figure S3. AFM image of hy-nb-teoa in the viewing area of 1.0 µm 1.0 µm: (a) a topography image, (b) lock-in phase image, and (c) a corresponding logarithmic histogram calculated from image shown in (a). 6

7 Weight (%) 80 hy-nb hy-nb-teoa Temperature ( C) Universal V4.5A TA Inst Figure S4. TG curves of hy-nb and hy-nb-teoa. The composition of sample was studied by thermogravimetric (TG) analysis. Apart from the solvent evaporation from the samples the TG curve shows a further gradual weight loss occurred from ca C in the hy-nb-teoa sample. The total weight loss was ca. 5 wt.-% for hy-nb-teoa as compared with hy-nb. These results imply that TEOA molecules were present as strong structural stabilizers (stabilizing molecular sheets from self-stacking) even through the sample was washed extensively with distilled water and ethanol (a) (b) F(R) 1.5 F(R) hv (ev) hv (ev) Figure S5. Plot of transformed Kubelka Munk function versus the energy of light, (a) hy-nb and (b) hy-nb-teoa. The band-gap energies of the materials were estimated using the following equation αhν = (hν - E g ) n, where α, E g, and n are the absorption coefficient (Kubelka Munk function), band gap, and an exponent 7

8 that is 2 for indirect-gap semiconductors and ½ for direct-gap semiconductors. From the plot of (αhν)1/n vs. hν, the band gap (Eg) can be obtained by extrapolating the linear portion to the hν axis intercept. Assuming that the materials are direct band-gap semiconductors (n = ½), one can estimate the band-gap energies of hy-nb and hy-nb-teoa to be 3.68 and 3.76 ev, respectively. Table S1 Band-gap energies and conduction band potentials. Eg (ev) ECB (V vs NHE) HNb3O hy-nb hy-nb-teoa It is reported that an empirical correlation between the conduction band potential (ECB) of metal oxide semiconductors containing d0 and d10 metal ions and the band gaps (Eg) as follows: ECB Eg/2.[7] Thus, we have estimated the conduction band potentials of hy-nb, hy-nb-teoa and bulk HNb3O8 from their band-gap energies accordingly. Figure S6a. A TEM image of a typical single molecular sheet from hy-nb-teoa. 8

9 Figure S6b. TEM images of single molecular sheets from TBA-Nb (left) and TBA-Nb-TEOA (right) (a) (b) (c) Figure S7. (a) AFM image of hy-nb-teoa, (b) a thickness evaluation of hy-nb-teoa sheets by atomic force microscopy, (c) Height profile of a hy-nb-teoa sheet and corresponding single molecular layer dimension of HNb3O8 (atomic model). Atoms are color labeled as follows: Nb (green), H (white) and O (red): this image of which was generated by Crystal Maker based on the H substitution of KNb3O8. Arrow shows the thickness of the HNb3O8 monolayer of 0.9nm. The AFM image of hy-nb-teoa and the corresponding height profile display a smooth and sheetlike morphology with a thickness of 0.8± 0.1 nm, which agrees well with the 0.9 nm of a single molecular sheet thickness along the [020] direction in our material model. 9

10 (a) (b) Figure S8. AFM images of (a) hy-nb and (b) HNb 3 O 8. Table S2 showing the size estimations of niobate nanosheets by TEM, XRD and AFM Sample TEM (nm) Av. XRD Av. Av. Layer (020) Layer AFM Layer (nm) (nm) (TEM) (XRD) (AFM) HNb 3 O 8 45 ± ± 3 50 hy-nb 9 ± ± 2 15 hy-nb-teoa <4 0.8± TBA-Nb-TEOA The average numbers of niobate layers were estimated from the bright field TEM images, the 020 peak broadening using Scherrer equation from the XRD and AFM images, respectively. There were discrepancies in the average size estimations due to the errors of each techniques, preferential dispersion of the materials in solvent prepared by the TEM and background interference to the signals, etc. 10

11 (a) 200 (b) 0.04 hy-nb-teoa hy-nb HNb3O8 150 Amount adsorbed / cm 3 (STP) g Relative Pressure / dv p /dr p Pore radius / nm 20 Figure S9. (a) N 2 adsorption desorption isotherms of HNb 3 O 8, hy-nb and hy-nb-teoa. Open and closed circles express adsorption and desorption branches, respectively. (b) Pore size distribution of hy-nb-teoa. Figure S9(a) shows N 2 adsorption desorption isotherms. In the case of hy-nb and hy-nb-teoa, the results exhibit a typical type IV curve with an H3-type hysteresis loop at a relative pressure (p/p 0 ) of indicating the existence of silt-shaped mesopores, [8] whilst the hysteresis is small for hy-nb- TEOA. This agrees well with the textural feature of the sheets morphology as shown in Figure 1. The Brunauer Emmett Teller (BET) specific surface areas and the total pore volumes of these niobate sheets are shown in Table S3. Table S3 Specific surface area and pore volume estimated by N 2 adsorption. Sample S BET / m 2 g -1 V / cm 3 g -1 hy-nb-teoa hy-nb HNb 3 O The given surface areas of commercial TiO 2 (54 m 2 g -1 ) and Nb 2 O 5 (3.3m 2 g -1 ) 11

12 Absorbance / a.u. hy-nb hy-nb-teoa HNb 3 O Wavenumber / cm Figure S10. Raman spectra of HNb 3 O 8, hy-nb and hy-nb-teoa. Various niobates with highly distorted NbO 6 units with characteristic short Nb-O bonds can give Raman stretching bands at around cm -1. [9] Kudo et al have investigated the Raman spectra of H + -exchanged KNb 3 O 8. They assigned that the band at 955 cm -1 to the stretching mode of short Nb-O bonds of K + (~1.73 Å) and corresponding terminal Nb-O-H bond at 900 cm -1 on surface. [10] In addition, a weak band at higher wavenumbers was observed at higher degree of H + -exchange in the case of [10,11] KNb 3 O 8 and K 4 Nb 6 O 17. From the Raman spectra of hy-nb and hy-nb-teoa, the dominant feature of terminal oxygen of NbO 6 is clearly associated with the Nb-O-H on the single molecular sheet. (a) (b) hy-nb-teoa+gr+mos 2 hy-nb-teoa+gr+mos 2 Absorbance / a.u. Absorbance / a.u. Graphene oxide MoS Wavenumber / cm Wavenumber / cm Figure S11. Raman spectra of Graphene oxide, MoS 2 and hy-nb-teoa+gr+mos 2, (97.0:0.5:2.5). 12

13 The Raman spectroscopy shown above give characteristic peaks of E 1 2g (380 cm -1 ) and A 1 g (407 cm -1 ) for MoS [12] 2 and D band (1365 cm -1 ) and G band (1600cm -1 ) for graphitized structure [13] in the hy- Nb-TEOA+Gr+MoS 2. Although the peaks of MoS 2 and graphene oxide are very small due to the low loading, they confirm the presence of MoS 2 and graphene oxide in the composite prepared by a simple physical mixing. (001) Intensity / a.u. (002) (10) (100) (103) Graphen oxide (105) (110) MoS degree Figure S12. XRD patterns of graphene oxide and MoS 2 those were used for the preparation of the niobate molecular sheets ensembles. The XRD patterns of graphene oxide reveal a sharp peak at 2θ = 10.3 due to the (001) reflection, indicative of a good layer regularity with a repeating interlayer distance of 0.86 nm, and the detectable (10) peak at around 2θ= 42.3 which is characteristic of an in-plane repeating structure. [14] All the diffraction peaks of MoS 2 can be well indexed according to MoS 2 phase with hexagonal structure (PDF no ). 13

14 Figures S13 (upper) and S14 (lower). TEM and HRTEM images of hy-nb-teoa+gr+mos2 with corresponding EDX spectra; the figures clearly show that the stacking of graphene oxide and MoS2 (fringe distances identified) on niobate-rich sheets (EDX rich) as the physical mixture. (b) (a) (c) Graphene oxide D(001)=0.87 nm (d) Graphene oxide D(001)=0.86 nm D(002)=0.63 nm Graphene oxide MoS2 (e) (f) hy-nb-teoa:gr =99.0:1.0 hy-nb-teoa:gr =99.0:1.0 hy-nb-teoa:mos 2 =99.0:1.0 (g) Graphene oxide (h) MoS2 D(002)=0.64 nm D(001)= 0.86 nm MoS2 D(002)=0.63 nm MoS2 D(002)=0.64 nm hy-nb-teoa:mos2 =99.0:1.0 D(001)= 0.85 nm hy-nb-teoa:mos2:gr =99.0:0.5:0.5 hy-nb-teoa:mos2:gr =99.0:0.5:0.5 Fig.S14 TEM and HRTEM images of (a) graphene oxide, (b) MoS2, (c,d) hy-nb-teoa:gr=99.0:1.0, (e,f) hy-nb-teoa:mos2 and (g,h) hy-nb-teoa:mos2:gr=99.0:0.5:

15 e - e - e - e - e - Gr H 2 e - H 2 O H 2 O H 2 e MoS h + h + h + Niobate single molecular sheets h + MoS 2 Methanol h + Methanol h + e - e - e - Niobate single molecular sheets Graphene Figure S15. Schematic diagram of the proposed mechanism for photocatalytic activity of hydrogen production over single molecular layer niobate sheets+graphene+mos 2 ensemble system under UV irradiation 4. Final remarks During the process of submitting this manuscript a new paper by Liang et al is just appeared. [15] By employing a conventional exfoliation preparative method a high photoactivity of the monolayer of HNb 3 O 8 for oxidation of benzylic alcohols is claimed. No study on the activity of different layered thickness or assembly with other layered compounds is reported but the paper can clearly give another good indication of the superior photoactivity of the molecular niobate in solution. 5. Acknowledgements The authors acknowledge the support from the EPSRC, UK. KN is grateful to the JSPS (KAKENHI Grant Number ) for a fellowship to work at Oxford University. References [1] K. Nakagawa, K. Yamaguchi, K. Yamada, K.-I. Sotowa, S. Sugiyama, M. Adachi, Eur. J. Inorg. Chem. 2012, 2012,

16 [2] A. Takagaki, M. Sugisawa, D. Lu, J. N. Kondo, M. Hara, K. Domen, S. Hayashi, J. Am. Chem. Soc. 2003, 125, [3] O. Compton, E. Carroll, J. Y. Kim, D. S. Larsen, F. E. Osterloh, J. Phys. Chem. C 2007, 111, [4] D. C. Marcano, D. V. Kosynkin, J. M. Berlin, A. Sinitskii, Z. Sun, A. Slesarev, L. B. Alemany, W. Lu, J. M. Tour, ACS Nano 2010, 4, [5] W. H. Jr, R. Offeman, J. Am. Chem. Soc. 1958, 80, [6] Q. Xiang, J. Yu, M. Jaroniec, J. Am. Chem. Soc. 2012, 134, 6575 [7] Y. Matsumoto, J. Solid State Chem. 1996, 126, 227. [8] K. S. W. Sing, D. H. Everett, R. A. W. Haul, L. Moscou, R. A. Pierotti, J. Rouquerol, T. Siemieniewska, Pure Appl. Chem. 1985, 57, [9] J.-M. Jehng, I. E. Wachs, Chem. Mater. 1991, 3, 100. [10] A. Kudo, T. Sakata, J. Phys. Chem. 1996, 100, [11] L. Li, J. Deng, R, Yu, J, Chen, X. Wang, X. Xing, Inorg. Chem. 2010, 49, [12] Q. Xiang, J. Yu, M. Jaroniec, J. Am. Chem. Soc. 2012, 134, [13] Y. Li, H. Wang, L. Xie, Y. Liang, G. Hong and H. Dai, J. Am. Chem. Soc., 2011, 133, 7296 [14] W. Peng, Z. Wang, N. Yoshizawa, H. Hatori, T. Hirotsu, K. Miyazawa, J. Mater. Chem. 2010, 20, [15] S. Liang, L. Wen, S. Lin, J. Bi, P. Feng, X. Fu, L. Wu. Angew Chem., 2014, 53, 1. 16