Detailed Investigation on the Band Energetics of Nanostructured

Article pubs.acs.org/JPCC

Detailed Investigation on the Band Energetics of Nanostructured Zn2SnO4 Electrodes in Aqueous Electrolytes Huizhi Kou*,† and Shuming Yang‡ †

School of Chemical and Environmental Engineering, Wuhan Polytechnic University, Wuhan 430023, China College of Chemistry and Chemical Engineering, Institute of Applied Chemistry, Xinyang Normal University, Henan 464000, China

‡

ABSTRACT: Band energetics of nanostructured Zn2SnO4 electrodes were determined with spectroelectrochemical measurements, and its trap states were investigated with electrochemistry. The flat band edges (Efb) of nanostructured Zn2SnO4 electrodes were highly dependent on pH of electrolyte and measured to be −0.60, −0.73, −0.85, −0.94, and −1.08 V vs saturated Ag/AgCl in 0.5 mol L−1 LiClO4 of pH 3.0, 5.0, 6.9, 10.0, and 13.0, respectively, and followed the Nestain relationship of Efb = −0.49 − 0.046pH, V vs Ag/AgCl. The addition of acetate also changed the Efb. The Efb were measured to be −0.48, −0.54, −0.71, −0.86, and −1.03 V in 0.5 mol L−1 LiClO4 and 1.0 mol L−1 NaAc of pH 4.0, 5.0, 7.0, 10.0, and 13.2, respectively, and followed a Nestian relationship of Efb = −0.25 − 0.060pH, V vs Ag/AgCl, very close to that reported by Wu and Alpuche-Aviles [J. Am. Chem. Soc. 2009, 131, 3216]. The time-resolved currents at different applied potentials positive of flat band edge clearly indicate a trap-filling process. Trap state densities are also highly pH dependent. Total trap state densities of 2.79 × 1015, 6.75 × 1015, and 9.98 × 1015 cm−2 were determined in 0.5 mol L−1 LiClO4 solutions of pH 3.0, 6.9, and 13.0, respectively, with maximum located at −0.25 V, −0.30 V, and −0.60 V. The trap state densities of 4.17 × 1015, 6.88 × 1015, and 7.17 × 1015 cm−2 were calculated in 0.5 mol L−1 LiClO4 and 1.0 mol L−1 NaAc solutions of pH 4.0, 7.0, and 13.2, respectively. The results obtained from CVs are in good agreement with that obtained from the measurements of time-resolved currents, and the size of the peak potentials related to the trap states increases dramatically with an increase of pH, indicating that traps are mostly surface-related. electrodes.24 So far, there is rarely a report on the dependence of the trap states of nanostructured Zn2SnO4 electrodes on pH of an electrolyte. Electrochemical and spectroelectrochemical methods are valuable in the study of the band energetics of transparent nanostructured semiconductor electrodes and have been applied in the determination of the Efb and trap states of transparent nanostructured semiconductor electrodes.11,25−27 Fitzmaurice and co-workers determined Efb for nanostructured TiO2 electrodes under a wide range of conditions with the model.25−30 They found that in protic solvents, Efb of nanostructured TiO2 electrodes displayed a Nernstian dependence on pH and was independent of the nature and concentration of electrolyte cation.29 Our previous research showed that the Efb and trap states of the nanostructured SrTiO3 electrodes were heavily dependent on the pH of aqueous electrolyte solution.7 In this work, Zn2SnO4 nanoparticles were synthesized with the hydrothermal method and used to fabricate transparent nanostructured Zn2SnO4 electrodes. Its flat band potential and trap states were measured with spectroelectrochemical and electrochemical technique, respectively, in aqueous electrolytes

1. INTRODUCTION Nanostructured semiconductor electrodes possess high porosity and larger surface area, which gives nanostructured electrodes great advantages in many applications such as photoelectrochemical solar cells,1−3 electrochromic windows,4 etc. The nanostructured semiconductor electrodes frequently employed are typically wide band gap n-type semiconductors such as TiO2,1,5 SnO2,6 SrTiO3,7 etc. However, a striking characteristics of nanostructured electrodes is the relatively large density of surface states. Surface states are electronic energy levels located at the semiconductor surface and play a very important role in reactions at the liquid−solid surface. They are most important when these states lie in the band gap. Generally, their natures and properties could be measured with cyclic voltammetry,8 charge recombination kinetics,9,10 and spectroelectrochemistry.5,11 Ternary oxide zinc stannate, Zn2SnO4, is an important semiconducting material with a band gap energy of 3.6 eV.12 Zn2SnO4 has promising applications in photovoltaic devices,13,14 photocatalysts to decompose organic wastes,15 humidity detection,16−18 Li−ion batteries19 and transparent conducting substrate due to its high electron mobility, high electrical conductivity, and high transmittance.20−23 Wu and co-worker studied the dependence of flat band potential of Zn2SnO4 on pH of an aqueous electrolyte with photocurrent measurements and found that its Efb dependence on pH is similar to that of nanostructured TiO2 © 2012 American Chemical Society

Received: October 4, 2011 Revised: February 7, 2012 Published: February 9, 2012 6376

dx.doi.org/10.1021/jp209560k | J. Phys. Chem. C 2012, 116, 6376−6382

The Journal of Physical Chemistry C

Article

of different pHs with or without acetate. Correlations of its band energetics with pH and acetate are to be established.

2. EXPERIMENTAL SECTION 2.1. Materials and Solutions. Optically transparent electrodes (OTE) were fabricated on an F-doped SnO2-coated glass substrate. Water (R = 18.3 MΩ) from an Easy Pure RF water purification system from Thermo Scientific was used in the preparation of all solutions. ZnCl2, SnCl4·5H2O, ethylene glycol, HClO4, acetic acid, and sodium acetate were purchased from Tianjin Chemical Company. LiClO4 and tert-butylamine were purchased from Shanghai Nuotai Chemical Company. All the chemicals used were reagent grade. 2.2. Preparation of Nanostructured Zn2SnO4 Films. Nanoparticles of Zn2SnO4 were prepared by the hydrothermal method following the reported procedures.13 The Zn2SnO4 precipitate from autoclaving was washed with water and ethanol successively. A certain amount of ethylene glycol was added to the Zn2SnO4 paste. The mixture was then dispersed under sonication, and ethanol was removed by a rotary evaporator. The final paste was grounded in an agate mortar for 15 min. The Zn2SnO4 paste was spread on conductive substrates by a glass rod with adhesive tapes as spacers. The films were dried at 125 °C and sintered at 500 °C for 1 h in air and finally cooled to room temperature. The film thickness was about 3 μm 2.3. Instrumentation. X-ray diffraction (XRD) measurements were performed on a D8 diffractometer (Bruker Co.) with Cu Kα (λ = 1.5405 Å) to identify the phase structure of samples. Film was characterized by scanning electron microscopy (SEM, S-4800, Hitachi, Japan). Film thickness was determined by an XP stylus profiler (Ambios Technology, Inc.). The absorption spectra were recorded on a UV-1240 spectrophotometer (Shimadzu, Japan). Electrochemical experiments were performed on a CH800 electrochemical analyzer (CH Instrument). All electrochemical and spectroelectrochemical experiments were carried out in a three-electrode system in which a nanostructured Zn2SnO4 electrode, a platinum wire, and a saturated Ag/AgCl electrode acted as working, counter, and reference electrodes, respectively. Spectroelectrochemistry measurements were undertaken according to published literature.7 A quartz cell with three electrodes and electrolyte was incorporated into the sample compartment of a Shimadzu UV−vis spectrophotometer and connected to a CHI 800 potentiostat. All aqueous electrolyte solutions were prepared in ion-free water with LiClO4 and/or NaAc as supporting electrolyte, and the pH of the electrolyte solutions was adjusted by HClO4, acetic acid, or NaOH. The electrolyte solutions were thoroughly degassed by bubbling with N2 prior to experiments. All potentials are hereafter given with reference to the saturated Ag/AgCl electrode. The working area of nanostructured Zn2SnO4 electrodes was 3 cm2.

Figure 1. XRD pattern of Zn2SnO4 nanoparticles sintered at 500 °C for 1 h.

Figure 2 is the absorption spectrum of a nanostructured Zn2SnO4 film deposited on quartz substrate. An absorption tail

Figure 2. Absorption spectrum of a nanostructured Zn2SnO4 film.

at longer wavelength is due to the diffusion and reflection of the nanostructured Zn2SnO4 film. The onset is at 342 nm, corresponding to a band gap of 3.63 eV.13 3.2. Dependence of Flat Band Potentials on pH of Electrolytes. Spectroelectrochemical measurement was normally applied for the monitoring of the electron filling in the conduction band.25−30 Because the excitation of electrons in the conduction band needs small energy, the absorbance changes at long wavelength can be used to monitor the filling of electrons in the conduction band, and as a result, the flat band potential of a semiconductor can be calculated. Figure 3 shows the potential-dependent absorption spectra of nanostructured Zn2SnO4 electrodes measured in 0.5 mol L−1 LiClO4 aqueous solutions of different pHs. Application of a sufficiently negative potential results in an absorbance increase at longer wavelengths. The spectra measured at different pHs are similar, but the absorption onset shifted to more negative potential as pH increased. The absorbance values at 500 nm were plotted against applied potentials and shown in insets of Figure 3. It can be seen that the flat band edges are strongly dependent on pH of

3. RESULTS AND DISCUSSIONS 3.1. Characterization and Band Gap of Nanostructured Zn2SnO4 Electrodes. The X-ray diffraction pattern for the synthesized Zn2SnO4 nanoparticles sintered at 500 °C for 1 h is presented in Figure 1. This pattern matches the Powder Diffraction File (PDF #24-1470) of the International Center for Diffraction Data. The Zn2SnO4 sample has a cubic inverse spinel structure. Because of the same procedure adopted in the synthesis of Zn2SnO4 nanoparticles, our sample is almost the same as that obtained in literature.13 6377



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− 0.06pH (V vs SCE).29 The linear correlation was also found for the nanostructured Zn2SnO4 electrodes. Figure 4 shows the

Figure 4. Flat band potential of nanostructured Zn2SnO4 films as a function of pH.

dependence of the flat band edge of the nanostructured Zn2SnO4 electrode on the pH of the electrolytes. A linear relationship was obtained from fitting data linearly and is expressed as Efb = −0.49 − 0.046pH (V vs Ag/AgCl). Wu and AlpucheAviles reported a linear relationship of Efb = −0.28 − 0.059pH (V vs Ag/AgCl) for nanostructured Zn2SnO4 electrodes.24 There is some discrepancy between these two results. In their studies, they added acetate salt in electrolyte. On the basis of our earlier report, solvent chelation could influence the flat band potential of nanostructured electrodes.31 So, it is necessary for us to measure the flat band potential of nanostructured Zn2SnO4 electrodes in electrolyte solutions containing acetate salt. In order to investigate the effect of acetate on Efb, the potential-dependent absorption spectra of nanostructured Zn2SnO4 electrodes were measured in the electrolyte solution of 0.5 mol L−1 LiClO4 and 1.0 mol L−1 NaAc, and the results are shown in Figure 5. The absorbance at 500 nm were plotted against applied potentials and shown in the insets of Figure 5 On the basis of the potential-dependent absorption spectra, the Efb could be determined to be −0.48, −0.71, and −1.03 V at pH 4.0, 7.0, and 13.2, respectively. A linear relationship was obtained from fitting data linearly and is expressed as Efb = −0.25 − 0.060pH (V vs Ag/AgCl), in good agreement with the Wu’ s results.24 This behavior was attributable to the chelation of the carboxy group by the acetate ion, which can coordinate to the surface of Zn2SnO4 electrodes, which would change the charge distribution in Helmholtz layer and, as a result, the Efb. 3.3. Dependence of Trap State Distribution on pH of Electrolytes. 3.3.1. Time-Resolved Current at pH 13.0. The current−time curves of a nanostructured Zn2SnO4 electrode were measured in 0.5 mol L−1 LiClO4 solution of pH 13.0 under different potentials, and the results are shown in Figure 6a. It is shown that the current is significantly influenced by the applied potential. At potential from 0 to −0.5 V, the currents decrease to almost zero very quickly. At −0.6 V, the current decrease slows down, and this behavior is found at all potentials more negative than −0.6 V. The results can be understood in terms of trap filling in the band gap region. A nanostructured Zn2SnO4 electrode has a flat band edge of −1.08 V in 0.5 mol L−1

Figure 3. Differential spectra of nanostructured Zn2SnO4 electrodes in 0.5 mol L−1 LiClO4 at (a) pH 3.0; (b) pH 6.9; and (c) pH 13.0. The insets show absorbance changes at 500 nm. Spectra are recorded after being polarized for 2 min at indicated potentials. The spectrum measured after stabilization for 5 min at +0.5 V has been subtracted.

electrolytes and calculated to be −0.60, −0.85, and −1.08 V at pH 3.0, 6.9, and 13.0, respectively. It was reported that the flat band edge of TiO2 is correlated linearly with pH and has the relationship of Efb = −0.40 6378



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Figure 6. (a) Current−time curves of a nanostructured Zn2SnO4 electrode in 0.5 mol L−1 LiClO4 of pH 13.0. The electrode was initially polarized at 0.5 V for 2 min and then measured at different applied potential. (b) Cathodic charges at different potentials derived by integrating the current−time curves in panel a. The inset shows the energy level of trap distribution against dQ/dU.

was at −0.7 V, and a further negative shift of the potential significantly shortens the trap-filling time. This should be related to the kinetics of trap filling. A faster trap filling is expected at a more negative potential since the driving force for the trap filling is larger. The accumulated charge Q under the current−time curves in Figure 6a is calculated, and some interesting features appear. Figure 6b shows the accumulated charge Q against potentials. At potentials more positive than −0.5 V, the accumulated charge is pretty small. It is striking that there is a sharp increase in accumulated charge up to −0.6 V. At potential more negative than −0.6 V, the accumulated charges increase slowly. If the accumulated charge Q from trap-filling reflects the density of states, eq 1 can be obtained11

Figure 5. Differential spectra of nanostructured Zn2SnO4 electrodes in 0.5 mol L−1 LiClO4 and 1.0 mol L−1 NaAc at (a) pH 4.0; (b) pH 7.0; and (c) pH 13.2. The insets show absorbance changes at 500 nm. Spectra are recorded after being polarized at indicated potentials. The spectrum measured after stabilization at +0.5 V has been subtracted.

Ntrap(U ) =

LiClO4 solution of pH 13.0. At potentials more positive than −0.5 V, trap density is low, and thus trap-filling time is short, resulting in fast current decay. However, at potentials more negative than −0.5 V, the trap density increases, so a longer time is needed to fill these traps. The longest trap-filling time

1 dQ q dU

(1)

where Q is accumulated charge, Ntrap(U) density of trap states at potential U, and q electron charge. Equation 1 clearly indicates that trap density is directly proportional to dQ/dU, which provides a direct measurement of trap distribution. 6379



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(not shown). The cathodic charges were calculated by integrating current−time curves measured in electrolyte solutions at pH 4.0, 7.0, and 13.2, respectively, and the results are shown in Figure 8a−c.

By differentiating the accumulated charge to the applied potential, a plot of dQ/dU against the energy level is obtained and shown in the inset of Figure 6b. This plot reflects the distribution of traps below the flat band edge. It is seen that most traps are located at potentials around −0.6 V. Total trap densities were calculated to be 9.98 × 1015 cm−2. 3.3.2. Time-Resolved Currents at pH 3.0 and 6.9. The current−time curves in the pH 3.0 and 6.9 solutions (not shown) are similar to those in the pH 13.0 solution; the cathodic charges at different potentials by integrating current−time curves measured in both solutions are shown in Figure 7a,b, respectively.

Figure 7. Cathodic charges accumulated at different potentials as derived by integrating the current−time curves in 0.5 mol L−1 LiClO4 at (a) pH 3.0 and (b) pH 6.9. The inset shows the energy level of trap distribution against dQ/dU.

Figure 7a,b shows the charge required to fill the traps. By differentiating the accumulated charge to the applied potential at pH 3.0 and 6.9, plots of dQ/dU against the energy level are obtained and shown in the insets of Figure 7a,b, respectively. Therefore the total trap densities can be calculated to be 2.79 × 1015 and 6.75 × 1015 cm−2 at pH 3.0 and 6.9, respectively. Again, in order to further investigate the effects of the chelation of acetate on trap states, the current−time curves of nanostructured Zn2SnO4 electrodes were measured in electrolytes of 0.5 mol L−1 LiClO4 and 1.0 mol L−1 NaAc

Figure 8. Cathodic charges accumulated at different potentials as derived by integrating the current−time curves in 0.5 mol L−1 LiClO4 and 1.0 mol L−1 NaAc at (a) pH 4.0; (b) pH 7.0; and (c) pH 13.2. The inset shows the energy level of trap distribution against dQ/dU. 6380



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The total trap states were calculated to be 4.17 × 1015, 6.88 × 1015, and 7.17 × 1015 cm−2 at pH 4.0, 7.0, and 13.2, respectively. Therefore, acetate also has influence on the trap states of nanostructured Zn2SnO4 electrodes. The above estimation of trapping density leads us to consider the origin of traps, which has been related to the nanostructured semiconductor/electrolyte interface.32 By comparing trap densities at different pHs, it is noticed that much lower values are obtained in neutral and acid electrolytes. To get detailed information on this point, the cyclic voltammograms of nanostructured Zn2SnO4 electrodes were recorded in 0.5 mol L−1 LiClO4 solution at different pHs. The results are shown in Figure 9.

dramatically with increasing pH, so this strongly indicates that the traps are mostly surface-related.

4. CONCLUSIONS The band energetics of nanostructured Zn2SnO4 electrodes have been investigated with electrochemical and spectroelectrochemical methods. The flat band edges (Efb) of nanostructured Zn2SnO4 electrodes have been determined by the spectroelectrochemical technique. The Efb strongly depends on pH of electrolytes and shifts toward more negative potentials with the increase of pH value. A Nestian relationship of Efb = −0.49 − 0.046pH, V vs Ag/AgCl, was obtained in 0.5 mol L−1 LiClO4 solution. It was found that acetate also influence the Efb of the nanostructured Zn2SnO4 electrodes and a Nestian relationship of Efb = −0.25 − 0.060pH, V vs Ag/AgCl, was measured in 0.5 mol L−1 LiClO4 and 1.0 mol L−1 NaAc. The trap state distribution was investigated by the measurements of time-resolved current and electrochemistry. The results showed that trap state densities are also highly pH dependent. Total trap state densities of 2.79 × 1015, 6.75 × 1015, and 9.98 × 1015 cm−2 were determined in 0.5 mol L−1 LiClO4 solutions of pH 3.0, 6.9, and 13.0, respectively, with a maximum located at −0.25, −0.30, and −0.60 V. The trap state densities in 0.5 mol L−1 LiClO4 and 1.0 mol L−1 NaAc solutions of pH 4.0, 7.0, and 13.2 were 4.17 × 1015, 6.88 × 1015, and 7.17 × 1015 cm−2, respectively. The results obtained from CVs are in good agreement with that obtained from the measurements of time-resolved currents. The size of the peak at the potentials related to the trap states shown at −0.26, −0.33, and −0.65 V for 0.5 mol L−1 LiClO4 solutions at pH 3.0, 6.9, and 13.0, respectively, in the cyclic voltammograms, increases dramatically with an increase in pH, indicating that traps are mostly surface-related.

Figure 9. Cyclic voltammograms of nanostructured Zn2 SnO 4 electrodes measured in 0.5 mol L−1 LiClO4 at different pH values. The inset shows the trap state distribution peaks at each pH. The electrodes were initially polarized at 0.5 V before scanning; the scan rate was 5 mV/s.

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Corresponding Author

*E-mail: [email protected].

Cyclic voltammetry is a choice for detecting and characterizing surface traps in nanocrystalline electrodes. A shoulder in the current−potential curves that appears at potentials more positive than the conduction band edge has been generally assigned to the presence of electron traps.8,11 The flat band edge of the nanostructured Zn2SnO4 electrode at pH 3.0 is approximately at −0.60 V (seen Figure 3), and a feature at −0.26 V in the cyclic voltammogram corresponds to trap state filling below the flat band edge, confirming the presence of surface traps.5,33,34 To explain the peaks in Figure 9, eq 1 can be rewritten as11 Ntrap(U ) =

1 dQ /dt 1 i = q dU /dt q dU /dt

AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported financially by the National Natural Science Foundation of China (Grant No.20773103), the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry (2008101), the selected programs for scholars back from overseas, Ministry of Personnel (2006164), and Science & Technology Program of Education Department of Henan (2008A150022).

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(2)

where i is the current density, t is time, and dU/dt is the scanning rate in the cyclic voltammograms. Equation 2 illustrates that the trap distribution at constant scanning rates is directly proportional to current density. Alternatively, current density distribution is a direct measure of trap density. Since the scanning rate was the same for all the experiments (5 mV/s), the curves in Figure 9 make it possible to compare the trap distribution at different pHs. The trap state distribution peaks (shown in the inset) at each pH are located at −0.26, −0.33, and −0.65 V at pH 3.0, 6.9, and 13.0, respectively, in good agreement with the results obtained from Figure7a,b and Figure 6b. It should be noticed that the size of the peak increases

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Detailed Investigation on the Band Energetics of Nanostructured

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