Synergistic Effect of the Electronic Structure and Defect Formation

Subscriber access provided by University of Winnipeg Library

C: Surfaces, Interfaces, Porous Materials, and Catalysis

Synergistic Effect of the Electronic Structure and Defect Formation Enhances Photocatalytic Efficiency of Gallium Tin Oxide Nanocrystals Vahid Ghodsi, Wenhuan Lu, and Pavle V. Radovanovic J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b08613 • Publication Date (Web): 06 Dec 2018 Downloaded from http://pubs.acs.org on December 7, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 54 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Synergistic Effect of the Electronic Structure and Defect Formation Enhances Photocatalytic Efficiency of Gallium Tin Oxide Nanocrystals Vahid Ghodsi, Wenhuan Lu, Pavle V. Radovanovic*

Department of Chemistry, University of Waterloo, 200 University Avenue West, Waterloo,

Ontario N2L 3G1, Canada

ACS Paragon Plus Environment

1

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 54

ABSTRACT

The design of photocatalysts with enhanced efficiency is pivotal to sustainable environmental

remediation

and

renewable

energy

technologies.

Simultaneous

optimization of different factors affecting the performance of a photocatalyst, including the density of active surface sites, charge carrier separation, and valence and conduction band redox potentials, remains challenging. Here we report the synthesis of ternary gallium tin oxide (GTO) nanocrystals with variable composition, and investigate the role of Ga3+ dopants in altering the electronic structure of rutile-type SnO2 nanocrystal

lattice

using

steady-state

and

time-resolved

photoluminescence

spectroscopies. Substitutional incorporation of Ga3+ increases the band gap of SnO2


2

Page 3 of 54 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


nanocrystals, imparting the reducing power to the conduction band electrons, and causes the formation of acceptor states, which, in conjunction with electron trapping by donors (oxygen vacancies), leads to stabilization of the photoexcited carriers. Combination of a decrease in the charge recombination rate and adjustment of the conduction band reduction potential to more negative values synergistically promote the photocatalytic efficiency of the GTO nanocrystals. The apparent rate constant for the photocatalytic degradation of rhodamine-590 dye by optimally prepared GTO NCs is 0.39 min-1, more than two times greater than that by benchmark Aeroxide TiO2 P25 photocatalyst. The results of this work highlight the concept of using rational aliovalent doping of judiciously chosen metal oxide nanocrystal lattices to simultaneously manipulate multiple photocatalytic parameters, enabling the design of versatile and highly efficient photocatalysts.


3


Page 4 of 54

INTRODUCTION

The discovery1 of the electrochemical photolysis of water at TiO2 electrode by Fujishima and Honda effectively incepted a field of semiconductor photocatalysis as a promising green technology for

solar energy conversion and environmental

remediation. Despite its photostability and abundance, TiO2 has some innate drawbacks that contribute to its low photocatalytic activity, with quantum yields often reported to be less than 1 %.2-4 The primary hindrance is that only about 10 % of photogenerated charge carriers have lifetimes longer than 10 ns,5-7 and only a small fraction of these carriers reaches the surface to react with adsorbates. Furthermore, the conduction band potential is marginally more negative than the water reduction potential, which generates a small driving force for the photoexcited electrons to reduce the surfacebound water molecules.6, 7 Finally, a wide optical band gap of TiO2 (3.2 and 3.0 eV for anatase and rutile phase, respectively)8 limits the portion of the solar spectrum that can be used for carrier generation.6 To address these deficiencies of TiO2 various


4

Page 5 of 54 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


semiconductors have been explored,9-14 including their modifications by size and shape,15-17 doping,18-20 structural transformation,8, 21 and heterojunction formation.22-24

Among different semiconductors showing photocatalytic activity, oxides of the maingroup metals (e.g., group 13 and 14) have attracted significant attention because of their chemical stability, high electron mobility, and the possibility to manipulate their electronic structure and carrier dynamics by controlling the crystal structure and native defects. Specifically, SnO2 is a wide band gap (3.6 eV) semiconductor with high n-type conductivity. Spontaneously formed oxygen vacancies are responsible for its high carrier density (up to 1020 cm-3), comparable to that of semimetals (1017 – 1020 cm-3).25 Moreover, free electrons experience very high mobility (>100 cm2 V−1 s−1)26 compared to TiO2 (0.1-4 cm2 V−1 s−1),27 making SnO2 particularly promising alternative to TiO2 in dye-sensitized solar cells and photocatalysis. However, in spite of these strengths, the conduction band edge potential of SnO2 is more positive than the H+/H2 reduction potential (0 V relative to the normal hydrogen electrode or NHE), which impedes its photocatalytic reduction power.28 Enhancement of the photocatalytic activity has been achieved by exploiting the conduction band offset with other metal oxides, such as TiO229 and ZnO,30


5


Page 6 of 54

in heterojunctions to separate photogenerated carriers and suppress charge recombination. Coupling with graphene31-33 or small band gap semiconductors,34 and iodination35 have also been used to increase the absorption of the visible portion of the solar spectrum to achieve better photocatalytic activity of SnO2. In contrast to SnO2, the potential of Ga2O3 conduction band minimum is more negative than the H+/H2 reduction potential, and the band gap of ca. 4.9 eV fully encompasses the water redox potentials.36 Ga2O3 crystalizes in five different polymorphs (α, β, γ, δ, and ε phases), and the photocatalytic activities of α, β, and γ phases have been investigated.21,

22, 37-39

We have recently

demonstrated that a control of the crystal structure of nanocrystalline Ga2O3 can result in the enhancement of the photocatalytic performance.21 Specifically, we have shown that nanocrystalline γ-Ga2O3 has a superior activity over β-Ga2O3, which originates from high concentration of native defects (i.e., oxygen vacancies) and their proximity to the semiconductor surface. Trapping of the photogenerated charge carriers in the localized native defect states prolongs their lifetime,40-43 leading to a higher rate of interfacial charge transfer and the corresponding redox reactions.21, 44


6

Page 7 of 54 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Preparation of a semiconductor photocatalyst in the nanocrystalline form has also proven to be beneficial for the photocatalytic efficiency in multiple different ways. Nanostructured materials generally possess high surface-to-volume ratios, providing a higher density of catalytically active sites relative to their bulk counterparts. Quantumconfinement-induced widening of the band gap of semiconductors having conduction band minima more positive than the NHE reduction potential can enhance their activity by making the conduction band potential more negative in reduced dimensions. Nanostructuring of the semiconductor photocatalysts also has important consequences on the charge carrier dynamics. The competition between charge recombination and separation determines the probability of converting photon energy into redox reactions.45,46 Reduced path length of the photogenerated carriers to the reactive sites at the surfaces of nanostructures relative to bulk leads to an increase in the quantum efficiency of a photocatalyst. Finally, reducing the semiconductor size has important consequences on the formation and density of surface defects, which tend to form new electronic states within the band gap. The role of these new electronic states in controlling the dynamics of charge separation has been a subject of vigorous debate.


7


Page 8 of 54

While defect states have traditionally been associated with charge recombination which is detrimental to photocatalysis, recent studies have revealed that bulk rather than surface defect states primarily determine charge carrier lifetime and photocatalytic activity.47-49 The defect sites in the proximity of semiconductor surfaces could, therefore, potentially be utilized to affect the rate of photocatalytic reactions and enhance photocatalytic performance. However, the nature, properties, and control of these sites remain controversial.

Here, we prepared ternary gallium tin oxide (Sn1-xGaxO2-0.5x) NCs, further in the text referred to as GTO NCs, to specifically explore the effect of aliovalent doping/alloying on the electronic structure, defect formation, and carrier dynamics in complex metal oxides, and their role in the photocatalytic activity. Doping SnO2 NCs with Ga3+ widens their bandgap and extends the lifetime of charge carriers trapped in defects states, leading to a significant increase in the photocatalytic activity. However, at sufficiently high Ga3+ content, closer proximity of donor and acceptor defect sites increases the probability of charge recombination, which is detrimental to the interfacial charge


8

Page 9 of 54 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


transfer and the overall catalytic activity. The rate of photodegradation of organic dye molecules using GTO NCs with optimal composition surpasses that using benchmark photocatalyst, Aeroxide TiO2 P25. The results of this work demonstrate an effective control of the electronic structure and charge carrier dynamics in high surface area metal oxide semiconductors by aliovalent doping, and allow for new opportunities to develop highly efficient photocatalysts.

EXPERIMENTAL SECTION Materials. All reagents and solvents are commercially available, and were used without further purification. Gallium(III) nitrate hydrate (Ga(NO3)3·xH2O, 99.99 %), and tin(IV) chloride pentahydrate (SnCl4·5H2O, 98 %) were purchased from STREM Chemicals. Dodecylamine (DDA, 98 %), trioctylphosphine oxide (TOPO, 90 %), ammonium hydroxide (NH4OH, 28.0−30.0 %), 1,4-dioxane (> 99.0 %), ethanol (HPLC grade), rhodamine-590 (Rh-590), and hexane (HPLC grade) were purchased from Sigma-Aldrich.


9


Page 10 of 54

Synthesis of GTO Nanocrystals. The GTO NCs were synthesized by a sol-gel method.50,

51

In a typical synthesis 1.4 g of SnCl4·5H2O and varying amounts of

Ga(NO3)3·xH2O (from 0 to 100 mol%) were added to 25 mL of deionized water. All precursors were stirred and dissolved, and the reaction mixture was cooled down in an ice bath for 15 minutes. The nucleation of NCs was initiated by a dropwise addition of NH4OH until pH=6. The reaction beaker was left for three hours to allow for settling of a white precipitate. The nucleated NCs were subsequently washed with deionized water three times. Next, NH4OH was added gradually over ca. 15 minutes, until the suspension became completely transparent. After refluxing this suspension for 15 hours at 90 °C and allowing it to cool to room temperature, the NCs were reprecipitated by the addition of 1,4-dioxane, collected by centrifuging, and washed with ethanol three times. The main portion of the product was dried on a watch glass at room temperature for photocatalytic measurements. A small fraction of precipitated NCs was capped with a coordinating ligand to form colloidal suspension for optical measurements.50 These NCs were resuspended in an excess amount of melted DDA and heated at 120 °C for 30 minutes, resulting in a clear suspension. The suspensions were then precipitated and


10

Page 11 of 54 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


washed with ethanol. Finally, DDA-capped NCs were treated with TOPO at 140 °C for 1 hour and washed with ethanol three times. The resulting NCs were suspended in a small amount of hexane for spectroscopic measurements. Sample Characterization and Measurements. XRD patterns were obtained using an INEL

powder

X-ray

diffractometer

having

a

position-sensitive

detector

and

monochromatic Cu Kα radiation (λ = 1.5418 Å). Transmission electron microscopy (TEM) imaging and energy dispersive X-ray spectroscopy (EDX) elemental analysis and mapping were performed with JOEL-2010F microscope. TEM samples were prepared by dropping a hexane suspension of NCs on a copper grid containing lacey Formvar/carbon support film (Ted Pella, Inc.). Brunauer−Emmet−Teller (BET) surface area measurements were conducted with a Quantachrome Autosorb system. X-ray photoelectron spectra (XPS) were collected with VG Scientific ESCALAB 250 spectrometer, using Al Kα radiation (1486.6 eV photon energy) as the excitation source. Optical absorption spectra were recorded with a Varian Cary 5000 UV-vis-NIR spectrophotometer.

Photoluminescence

(PL)

spectra

were

collected

at

room

temperature with a Varian Cary Eclipse fluorescence spectrometer by exciting the


11


Page 12 of 54

samples at 240 nm. Photoluminescence quantum yield (PLQY) was calculated as an average value of two relative quantum yields (see Supporting Information for more detail). Time-resolved PL measurements were recorded with a Horiba Jobin Yvon IBH Ltd. spectrometer using the time-correlated single photon counting (TCSPC) method. To collect the instrument response function (IRF), ludox colloidal silica suspension in water (acquired from Sigma Aldrich) was used, and the PL time decay data were fit with a multiexponential function. We used 250 nm nanoLED (IBH Ltd.) as the excitation source, and the emission signal was recorded at the emission band maximum for each sample. Photocatalytic Evaluations. In a 100 mL beaker, 12 mg of air-dried GTO NCs were suspended in 50 mL of 5 mg/L Rh-590 solution. The reaction mixture having pH ≈ 6.5 was sonicated for 15 minutes to establish adsorption-desorption equilibrium, and the beaker was placed on a stir plate 18 cm below a dual 40 W 254 nm fluorescent tube lamp. The mixture was irradiated with the UV light and continually stirred to ensure uniform exposure of the suspension to UV radiation throughout the process. At periodic time intervals, 3 mL aliquots of the suspension were withdrawn to determine the


12

Page 13 of 54 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


concentration of unreacted Rh-590 by monitoring the absorbance at the maximum of the Rh-590 (S0→ S1) band at 520 nm. Identical procedure was followed for all GTO NCs. To verify the recyclability, photodegradation measurements of Rh-590 were repeated for five consecutive cycles under the same conditions. At the end of each cycle, the catalyst was collected and washed with deionized water, followed by drying at 80 °C for 90 minutes. To identify the species involved in the photocatalytic process, different scavengers were used. Before irradiation, a specific scavenger (NaHCO3, methanol, or FeCl3) was added to the beaker to achieve the concentration of 10 mM. Nitrogen gas was utilized as a superoxide radical scavenger; it was blown into the solution for 15 minutes prior to turning on the UV lamp. The measurements were then carried out as described. RESULTS AND DISCUSSION XRD patterns of GTO NCs having different composition are shown in Figure 1a. The patterns of undoped SnO2 and Ga2O3 NCs are indexed to bulk rutile SnO2 (JCPDS 0880287, red sticks), and bulk γ-Ga2O3 (JCPDS 020-0426, blue sticks), respectively. Significant broadening of the peaks of all diffraction patterns is indicative of small


13


Page 14 of 54

average NC sizes and/or impurity-induced lattice disorder. The NCs appear to retain SnO2 rutile structure, without the evidence of other phases, until the starting Ga3+ concentration reaches ca. 50 %.52 Substitutional doping of SnO2 NCs with Ga3+, which has a smaller crystal radius than Sn4+ by ca. 0.07 Å (or 10 %), should cause shrinking of the NC lattice, as evidenced by a shift of the XRD peaks to larger angles (Figure S1 in Supporting Information).52 Above 50 % Ga3+ precursor concentration, γ-Ga2O3 likely begins to coexist with SnO2, becoming the dominant phase for the starting Ga3+ content of about 90 %. In this work we predominantly focus on GTO NCs having rutile SnO2 crystal structure and manipulation of their carrier dynamics and photocatalytic activity via doping/alloying with Ga3+. The results of the elemental analysis (Table S1 in Supporting Information) together with the SnO2 (101) XRD peak shift and the absence of observable reflections characteristic for Ga2O3 or other Ga3+ related secondary phases (Figure S1 in Supporting Information) generally suggest significantly higher solubility of Ga3+ in SnO2 NCs than in bulk.53, 54 This observation is consistent with a number of previous reports on achievable Ga doping levels in SnO2 nanostructures synthesized under nonequilibrium conditions.52, 55-57 This increased solubility of Ga3+ in nanocrystalline relative


14

Page 15 of 54 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


to bulk SnO2 can be explained by lower synthesis temperature which favors kinetic doping and prevents expulsion of the dopant impurities.58 While we do not completely exclude possible formation of small gallium oxide clusters at high Ga3+ starting concentrations, they cannot account for the carrier dynamics and photocatalytic results reported here (vide infra). An overview and high resolution TEM images of typical GTO NCs are displayed in Figure 1b and c, respectively. TEM images of other GTO NC samples are shown in Figure S2 (Supporting Information) for comparison. The NCs have quasi-spherical morphology with an average diameter below ca. 5 nm, and show some degree of aggregation. The average lattice spacing for {110} plane (Figure 1c) is slightly smaller than lattice spacing for the same plane for undoped SnO2 due to the smaller ionic radius of Ga3+ compared to Sn4+. To address the distribution of Sn and Ga in the NC samples, we performed EDX elemental mapping for typical GTO NCs (Figure 1d-f). The comparison of the Sn and Ga elemental maps confirms the coexistence of Ga and Sn in the NC samples and is consistent with their uniform distribution within the spatial limitations of the measurement.


15


Page 16 of 54

Figure 1. (a) XRD patterns of GTO NCs prepared with different starting fractions of gallium (0-100 %) as indicated in the graph. Red and blue sticks represent the patterns for bulk rutile SnO2 and γ-Ga2O3, respectively. (b, c) Overview (b) and high resolution (c) TEM images of GTO NCs synthesized with starting Ga fraction of 25 %. The average lattice spacing in (c) corresponds to rutile SnO2, as assigned in the image. (d-f) STEM image (d) and the corresponding EDX elemental maps (Ga (e) and Sn (f)) for typical GTO NCs.


16

Page 17 of 54 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


While the wide band gap of γ-Ga2O3 supplies photogenerated carriers with both oxidative and reductive power, low-lying conduction band minimum of SnO2 fails to produce catalytically active electrons. Hence, we hypothesized that doping SnO2 NCs with Ga3+ could lead to widening of the band gap and allow for generation of photoactive electrons and holes, while simultaneously controlling the native defect formation and retaining the advantages of SnO2 lattice as a photocatalyst. The absorption spectra of GTO NCs having different composition is shown in Figure 2a. The blue shift of the band edge absorption with increasing Ga content attests to the composition-dependent electronic structure of alloyed NCs, and suggests an increase in the energy of the conduction band minimum. Formation of the structural defects in SnO2 NCs is an inevitable consequence of aliovalent doping.59, 60 Considering that Ga3+ has lower oxidation state compared to Sn4+, Ga3+ dopants act as acceptors in SnO2 NCs, and form energy states above the valence band maximum. These acceptors tend to form complexes with native donor defects (i.e., oxygen vacancies). Such donoracceptor pair (DAP) complexes generally act as radiative recombination centers for


17


Page 18 of 54

photoexcited charge carriers.61 The energy of DAP emission (E) can be described by the following equation:

E  Eg  ( ED  EA ) 

e2 4 r

(1)

where Eg is the band gap energy, ED and EA are donor and acceptor binding energies, respectively, and r is the average donor-acceptor separation. Other parameters have their usual meaning. With increasing Ga content DAP recombination emission shifts to higher energies, following the blue shift in the band edge absorption (Figure 2b). The NC PL quantum yield depends on the competition between the radiative and nonradiative recombination of the photoexcited charge carriers: PLQY 

kr (kr  knr )

(2) where kr and knr are radiative and nonradiative recombination rates, respectively. These recombination rates also determine the average PL lifetime (τ):


18

Page 19 of 54 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60




1 (kr  knr )

(3) The lifetime of the DAP PL of GTO NCs is on the order of a microsecond, and increases with increasing concentration of Ga up to ca. 20 % Ga (Figure 2c). Upon excitation of GTO NCs, the coupled donor-acceptor pairs undergo a slow PL decay owing to the tunnel transfer form donor to acceptor sites as the rate-determining step.40,

42, 43, 62

Consequently, at low to moderate Ga content, the DAP emission lifetime and intensity increase with increasing Ga3+ doping concentration. However, when the concentration of Ga3+ exceeds ca. 20 %, further increase in the density of donors and acceptors causes a reduction in their average separation, resulting in a higher probability of recombination and a shorter PL lifetime (Figure 2c and Table S2 in Supporting Information). Interestingly, Ga2O3 NCs synthesized by the sol-gel method are only weekly emissive, in contrast to the NCs synthesized by a typical colloidal method,43 due to concentration-induced quenching processes (see Supporting Information for additional discussion).


19


Page 20 of 54

Short-lived excited-state carriers are generally detrimental to the photocatalytic activity. By measuring PL time-decay curves and PLQYs of the same samples, we can determine the radiative and nonradiative recombination rates for GTO NCs synthesized by the sol-gel method. The two recombination rates and PLQY are plotted together as a function of Ga content in Figure 2d. The radiative and nonradiative recombination rates follow the opposite trend with respect to the NC composition, as expected. The composition dependences of the radiative recombination rate and PLQY are also in excellent agreement. These results suggest that electrons and holes are effectively trapped in the corresponding long-lived defect states formed upon substitutional incorporation of Ga3+ into SnO2 NCs. The photocatalytic efficiency of semiconductors is largely determined by the competition between the charge carrier recombination and separation. Upon absorption of light, the first competition takes place between exciton recombination and carrier trapping in defect states.45,63 The trapped carriers can subsequently undergo charge recombination or diffuse to the surface, where they can perform interfacial redox reactions.45,

63

It is believed that the recombination rate is

suppressed by extending the lifetime of the carriers in the trap states, leading to a


20

Page 21 of 54 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


higher photocatalytic efficiency.45, 49, 64 We presumed that generation of long-lived trap states in nanostructured SnO2 by Ga3+ doping could lead to an effective charge separation and ensuing charge transfer to the active surface sites.

Figure 2. (a) Absorption spectra of GTO NCs with different Ga content as indicated in the graph. (b) PL spectra of GTO NCs with different Ga content from panel (a). (c) Time-resolved PL decays of GTO NCs having different Ga content, as indicated in the graph, in the microsecond time range. The data for remaining samples were left out for clarity. (d) Comparison between radiative (kr) and nonradiative (knr) rate constants (right ordinate) and PLQY (left ordinate) with respect to the NC composition for GTO NCs.


21


Page 22 of 54

To evaluate the effect of Ga content on the photocatalytic activity of GTO NCs, we measured the rate of degradation of Rh-590 in the presence of GTO catalysts having different composition under 254 nm light irradiation. Rhodamine-590 is chosen as a model system, because it has a strong absorption band in the yellow-orange part of the spectrum arising from S0S1 transition, which does not overlap with the band edge absorption of GTO NCs, and can serve to accurately monitor the photocatalytic degradation. The kinetic data were analyzed using Langmuir–Hinshelwood model, expressed as:65 

kK adsC dC  dt 1  K adsC

(4)

where C is the concentration of the dye, t is the reaction time, k is the reaction rate constant, and Kads is the adsorption coefficient of the dye. When KadsC

Synergistic Effect of the Electronic Structure and Defect Formation

Recommend Documents