Sara E. Skrabalak, PhD
, Indiana University (Bloomington)
Of primary importance is the development of efficient means to harness the huge energy potential of solar irradiation and use it to drive chemical reactions or convert it into stored chemical energy. To meet this goal, our support from ACS-PRF has been directed toward the validation of a potential z-scheme photocatalyst.
As outlined in the proposal, we sought to validate nitrogen-doped M1xOy/M2xOy composites prepared by an electrospray method as a new class of Z-scheme photocatalysts in which the nitrogen-doped unit would enhance visible light absorption while composite formation would enhance electron-hole separation. In particular, N-TiO2/SnO2 composites were synthetically targeted as studies of the photogenerated charge transfer processes in TiO2/SnO2 bilayers indicate the formation of a heterojunction upon composite synthesis. The creation of a heterojunction is the result of the favorable relative band positions of TiO2 and SnO2. The band gap of SnO2 is greater than that of anatase-phase TiO2 (Eg = 3.6 eV vs. 3.2 eV, respectively), but the conduction band of SnO2 is more positive than that of TiO2 (ECB = 0 V and -0.5 V vs. NHE at pH 7). As outlined in our previous progress report, NH4Cl was found to be a suitable nitrogen source; however, the high concentrations required for the syntheses inhibited the formation of high-quality samples by electrospray. Thus, a solution based route was developed instead.
In particular, N-TiO2/SnO2 composites were successfully synthesized by adding pre-formed SnO2 colloids to a sol-gel precursor solution for N-TiO2. The use of pre-formed SnO2 was critical to achieving the desired composites as i) sol-gel precursors to SnO2 (e.g., SnCl4 or Sn(OR)4) can yield solid solutions of TiO2 and SnO2 when mixed with TiO2 precursors and ii) the crystalline nature of the SnO2 particles limits diffusion and thus doping of the SnO2 with nitrogen. After gelation, the composites were annealed to crystallize the N-TiO2. The heat treatment conditions were controlled to achieve exclusively anatase-phase N-TiO2. The TiO2-to-SnO2 ratio was controlled by varying the amount of SnO2 added to the TiO2 precursor solution.
X-ray powder diffraction and energy-dispersive X-ray spectroscopy confirmed the formation of the composites and the controllable TiO2-to-SnO2 content. EPR spectroscopy and X-ray photoelectron spectroscopy confirmed dopant incorporation. The composites were also characterized by N2-adsorption, diffuse reflectance spectroscopy, and scanning electron microscopy. Finally, the photodegradation of rhodamine B (RhB) was selected as a model system to evaluate the photocatalytic potential of the composites. The degradation rates of RhB in the presence of composites with different TiO2-to-SnO2 ratio were measured under UV-visible and visible (> 400 nm) light irradiation. These results where compared to standards, including TiO2, N-TiO2, SnO2, and TiO2/SnO2 composites.
Interestingly, N-TiO2 provides the greatest rate of photodegradation under UV-visible irradiation, with the rate of degradation diminishing with increasing SnO2 content in the composites. This result is attributed to the overall decrease in light absorption as a function of SnO2 amount, as the surface areas of the N-TiO2/SnO2 composites are actually greater than N-TiO2. Such a dependence on SnO2 percentage has been reported in TiO2/SnO2 composites, but at sufficiently low SnO2 content enhanced performance has been observed. That is not the case with our system even in the absence of nitrogen doping. The origin of this difference is currently unclear but may be related to the selection of anatase-phase TiO2 exclusively in the composites. Mainly, powder XRD of TiO2/SnO2 composite photocatalysts from literature typically indicate rutile-phase TiO2 as a minority component in the predominately anatase-phase TiO2/SnO2 composites. Rutile TiO2 and SnO2 phases are isostructural, and the formation of a SnxTi1-xO2 mixed oxide interface may be integral to the formation of an appropriate heterojunction and enhanced photocatalytic performance; to the best of our knowledge this idea has yet to be tested. Regardless, it should be noted that the N-TiO2/SnO2 composites outperform the TiO2/SnO2 composites despite the TiO2/SnO2 composites having enhanced surface areas. This result can be attributed to the enhanced light absorption provided by the nitrogen-doped component.
Remarkably, when irradiated with visible light only (> 400 nm), the N-TiO2/SnO2 composites with low SnO2 content outperformed all the undoped samples as well as N-TiO2. We attribute this property to the greater absorption of visible light with the N-TiO2/SnO2 composites compared to the undoped composites. However, this explanation does not account for the enhanced performance of the N-TiO2/SnO2 composites under visible light irradiation when compared to N-TiO2 alone. When illuminated with visible light, only the N-TiO2 portion of the composite will be photoactive. However, in addition to N-TiO2 mediated decomposition, dye degradation can proceed via sensitization under visible light (injection of electrons from dye molecules into the conduction bands of the oxides) as RhB absorbs 554 nm light. The N-TiO2/SnO2 composites should still provide greater charge separation than N-TiO2. The enhanced performance of the N-TiO2/SnO2 composites with low SnO2 content suggests that the dye sensitization mechanism predominates under visible light illumination and a series of control experiments validated this hypothesis.
Broader Impact. The results outlined here should enable the design of future composite photocatalysts with enhanced performance and could inspire novel routes to more stable dye-sensitized solar cells. Also, the general synthetic technique demonstrated may facilitate a range of composites to be fabricated. This ACS-PRF grant has been effectively used to attract and support the work of two postdoctoral associates and two graduate students. The results from this project have been presented at the spring and fall 2009 MRS conferences as well as several regional conferences. One of the postdoctoral associates is now a faculty member at a PUI. One graduate student recently completed his master’s degree in chemistry and the other will be defending her PhD in summer 2012. The preliminary results presented in Progress Report #1 formed the basis for funded NSF CAREER Award proposal (DMR-0955028), which will financially support a portion of this work in the future.
Skrabalak, S. E.