www.sciencemag.org/cgi/content/full/science.1200448/dc1 Supporting Online Material for Increasing Solar Absorption for Photocatalysis with Black Hydrogenated Titanium Dioxide Nanocrystals This PDF file includes: Methods SOM Text Figs. S1 to S6 References Xiaobo Chen, Lei Liu, Peter Y. Yu, Samuel S. Mao* *To whom correspondence should be addressed. E-mail: ssmao@lbl.gov Published 20 January 2011 on Science Express DOI: 10.1126/science.1200448
Supporting Online Materials Increasing Solar Absorption for Photocatalysis with Black, Hydrogenated Titanium Dioxide Nanocrystals Xiaobo Chen, Lei Liu, Peter Y. Yu, and Samuel S. Mao Methods X-ray analysis: The crystal size from the XRD pattern was calculated using the Scherrer formula, D=0.9λ/βcosθ, where D is the crystal size, λ is the wavelength of the X-ray radiation (0.15418 nm for Cu K α radiation), β is the full width at half maximum, and θ is the diffraction angle. Hydrogen production measurements: Photocatalytic hydrogen generation was measured using a Varian gas chromatograph; data were taken approximately every hour during solar irradiation. XPS measurements: XPS data were collected using an X-ray photoelectron spectrometer PHI 5400 (Physical Electronics) with a nominal energy resolution of 0.7 ev. Measurements were performed using an ultrahigh vacuum chamber with a base pressure below 5 10-9 Torr. Spectra were acquired using a photon beam of 1486.6 ev, selected from an Al/Mg dual-anode X-ray source. Data were collected at room temperature
SOM Text and Figures S.1 Photocatalytic decomposition of methylene blue and phenol Solar-driven photocatalysis measurements were conducted by irradiating the sample solution with a Newport Oriel full spectrum solar simulator, installed with an AM 1.5 filter that generates about 1 sun power. Figure S1 shows the absorption spectra of methylene blue solution over time after irradiating with simulated solar light and incorporating with black TiO 2 nanocrystals as the photocatalyst. O. D. / a.u. 1.0 0.8 0.6 0.4 0 min 1 min 2 min 3 min 4 min 5 min 6 min 7 min 8 min A 0.2 0.0 500 550 600 650 700 750 Wavelength / nm Figure S1. Absorption spectra of methylene blue solution at different time after irradiating with simulated solar light and using black TiO 2 nanocrystals as the photocatalyst. In addition to methylene blue, we examined photocatalytic decomposition of phenol using black TiO 2 as the photocatalysts. Experiments were conducted with 3 ml of 10 mg/l phenol aqueous solution and 1 mg catalysts, under irradiation from the same solar simulator used in methylene blue decomposition experiments. Concentration change of phenol was monitored by the change of the 270 nm absorption peak. As shown in Figure S2, which compares solar-driven photocatalytic activity of the disorder-engineered black TiO 2 nanocrystals against that of the unmodified white TiO 2 nanocrystals under the same testing conditions, photo-degradation of phenol is complete approximately 40 minutes after the start of solar irradiation when the photocatalysts were black TiO 2 nanocrystals, which represents a significant improvement over the cases where white TiO 2 nanocrystals were used as the photocatalysts. 1.0 0.8 black TiO2 white TiO2 C/C0 0.6 0.4 0.2 0.0 0 20 40 60 80 Time / min Figure S2. Comparison of solar-driven photocatalytic activity of the black TiO 2 nanocrystals against that of the white TiO 2 nanocrystals for phenol decomposition. 2
S.2 Examination of hydrogen concentration in photocatalysts We performed experiments to quantify how much hydrogen could be absorbed by the TiO 2 nanocrystals through the hydrogenation process that produces disorder-engineered black TiO 2. Measurements were conducted using a designated hydrogen storage capacity testing system (Intelligent Gravimetric Analyzer) manufactured by Hiden Isochema. Figure S3 plots hydrogen sorption-desorption data measured at 200 o C, which yield the amount of hydrogen (in weight percentage) absorbed in TiO 2 nanocrystals under different hydrogenation pressure. The data indicate that approximately 0.3 wt% of hydrogen can be absorbed in disorder-engineered TiO 2 nanocrystals after hydrogenation (sorption process). After desorption cycle, about 0.25 wt% hydrogen would remain in black TiO 2. 0.3 Wt % 0.2 0.1 Desorption Sorption 0.0 0 5000 10000 15000 20000 Pressure / mbar Figure S3. Gravimetric hydrogen sorption-desorption measurements at 200 C. For photocatalytic hydrogen production experiments, we use 20.0 mg of black TiO 2 photocatalysts, hence, approximately 0.05 mg (20 mg x 0.25%) of hydrogen by weight are contained in the photocatalysts. The measured hydrogen production rate is about 0.2 mmol (0.4 mg) hydrogen per hour, i.e., 20 mmol (40 mg) hydrogen for a 100-hour experiment, as reported in the manuscript. Thus, the amount of hydrogen generated in 100 hours (40 mg) using 20.0 mg photocatalysts is significantly greater than the amount of hydrogen stored (0.05 mg) in black TiO 2. This result eliminates the possibility that the photocatalysts would act as the hydrogen reservoir for hydrogen production and is also consistent with the observation that black TiO 2 photocatalysts remain black after cycling experiments. S.3 X-ray photoelectron spectroscopy Using X-ray photoelectron spectroscopy (XPS), we examined the change of surface chemical bonding of TiO 2 nanocrystals due to hydrogenation to form disorder-engineered black TiO 2. As shown in Figure S4, the Ti 2p XPS spectra were almost identical. In the Ti 2p spectral region, there are four peaks at 458.9, 464.4, 472.3 ev, and 477.9 ev, respectively. These peaks can be attributed to Ti 2p 3/2, Ti 2p 1/2 and their satellite peaks with typical characteristic Ti 4+ binding energies. All Ti 2p signals are symmetric with no shoulders at the lower energy sides, which are significantly different from the spectra of TiO 2 doped with carbon or other impurities. 3
Intensity / a.u. A Ti2p XPS 477.9 ev 472.3 ev (b) black TiO2 464.4 ev 458.9 ev (a) white TiO2 480 470 460 450 Binding Energy / ev Figure S4. Ti 2p XPS spectra of the white and black TiO 2 nanocrystals. The red and black curves are XPS data, and the blue curves are individual fittings of peaks identified in the figure. The green curves are fittings including all identified peaks. We evaluated carbon impurity in both un-modified and disorder-engineered TiO 2 nanocrystals. Figure S5 shows C 1s XPS measurements of the white and black TiO 2 nanocrystals in the energy range aimed to detect possible doped carbon in TiO 2 lattice. Doped carbon would exhibit carbon ion characteristics peaked at approximately 281.8 ev (S1), which is different from the background peak at 284.6 ev (carbon tape with C 0 ). As shown in Figure S5, there is no carbon ion peak (281.8 ev) for both the white and black TiO 2 nanocrystals. This result is consistent with the synthesis procedure that effectively removes carbon impurity. White TiO2 Black TiO2 Intensity / a.u. 288 286 284 282 280 Binding Energy / ev Figure S5. C 1s X-ray photoelectron spectra of white and black TiO 2. In addition to C 1s XPS, Ti-C bonding in TiO 2 would also be reflected in Ti 2p XPS spectra, with non-vanishing features such as a peak or shoulder in the vicinity of 455 ev (S2). Nevertheless, the Ti 2p XPS spectrum for black TiO 2 is almost identical to that for un-modified white TiO 2 nanocrystals, as shown in Figure S4, and there is no peak or shoulder around 455 ev. S.4 Calculations of electronic structures of TiO 2 nanocrystals without disorders We started theoretical modeling by constructing a network of TiO 2 nanocrystals without disorders, but having surface dangling bonds that are allowed to relax fully. Our DFT calculations 4
were performed using Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation (GGA). The Kohn-Sham density-functional equations were solved with the projected augmented wave (PAW) method as implemented by the VASP software. A kinetic energy cutoff of 400 ev and k- point sampling with 0.05 Å -1 separation in the Brillouin zone were applied. Figure S6(A) shows a supercell for modeling TiO 2 nanocrystals (with surface dangling bonds but no disorders), as compared to the supercell for a bulk anatase TiO 2. The calculated total and partial density of states (DOS) of TiO 2 nanocrystals are plotted in Figure S6(B), which indicate that the primary effect of surface reconstruction in TiO 2 nanocrystals is producing strong band tailing near the edge of the valence band. A DOS (states/ev/unit cell) 125 100 75 50 25 Total O_s O_p O_d Ti_s Ti_p Ti_d B 0-5 -4-3 -2-1 0 1 2 3 4 Energy (ev) 5 6 7 8 DOS I Ti OH Surface I H2 I H IO V O V Ti Nano Bulk C -5-4 -3-2 -1 0 1 2 3 4 5 6 7 8 Energy (ev) Figure S6. (A) A supercell for modeling TiO 2 nanocrystals (with surface dangling bonds but no disorders), as compared to the supercell of a bulk anatase TiO 2. (B) Calculated DOS of TiO 2 nanocrystals. The energy of the valence band maximum of the bulk phase is taken to be zero. As an example, label Ti_s represents the partial DOS for the s orbital of Ti atoms. (C) Calculated DOS of TiO 2 nanocrystals with different types of defects and hydrogen impurities. The DOS of defect-free TiO 2 nanocrystals and bulk anatase TiO 2 are also shown. It is well-known that DFT tends to underestimate the absolute values of semiconductor band gaps, and several approaches have been proposed to correct the calculated values. Some authors consider phenomenological parameters; for example, Morgan and Watson (S3) introduced a Coulomb repulsion term (labeled as U) for both the d-electrons of Ti and the p-electrons of O. Since we are more interested in relative changes in energies induced by defects, impurities, and disorders, rather than the absolute energy values, we applied a scissor operator approach with which the conduction band energy was shifted upwards by 1.21 ev to agree with the experimental value of 3.30 ev for un-modified TiO 2 nanocrystals. Similarly, the calculated energies of mid-gap states have been linearly enlarged by a scaling parameter of 1.58 accordingly. 5
We also calculated the band structures of TiO 2 nanocrystals without a disorder layer but containing four types of intrinsic defects with low formation energies: Ti vacancy (V Ti ), O vacancy (V O ), interstitial titanium (I Ti ), and interstitial Oxygen (I O ), and three types of defects involving hydrogen impurities: interstitial H atoms (I H ), interstitial H 2 molecule (I H2 ), and H atoms forming surface OH bonds with oxygen (OH surface ). The calculated DOS for these defects in TiO 2 nanocrystals are summarized in Figure S6(C). Neither the three native defects: V Ti, V O and I O, nor the three hydrogen impurities introduce mid-gap states. The only defect that produces a gap state is I Ti, about 0.5 ev below the conduction band minimum and indicated by an arrow in Figure S6(C). References S1. X. Chen, et al., J. Am. Chem. Soc. 130, 5018 (2008). S2. Y. Zhang, P. Xiao, X. Zhou, D. Liu, B. B. Garcia, and G. Cao, J. Mater. Chem. 19, 948 (2009). S3. B. J. Morgan and G. W. Watson, Phys. Rev. B 80, 233102 (2009). 6