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Cite This: Chem. Mater. XXXX, XXX, XXX−XXX
Controllable Anomalous n- and p‑Type Photothermoelectric Effects of Platinum Oxide and Tungsten Trioxide Layers with and without Chromic Reaction Kohei Shimoyama†,§ and Hiroshi Irie*,†,‡,§ †
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Special Doctoral Program for Green Energy Conversion Science and Technology, Integrated Graduate School of Medicine, Engineering and Agricultural Sciences and ‡Clean Energy Research Center, University of Yamanashi, 4-3-11 Takeda, Kofu, Yamanashi 400-8511, Japan S Supporting Information *
ABSTRACT: We attempted to develop both n-type and p-type anomalous photothermoelectric (photo-TE) effects, which are of interest for developing practical applications of TE devices for energy harvesting using sunlight, as well as for interpreting such phenomena from scientific viewpoints. We were able to demonstrate the n-type and p-type anomalous photo-TE effects using the same components, platinum-oxide species (PTO) and tungsten oxide (WO3), by controlling the thickness of the PTO, and with and without applying a chromic reaction to WO3, to form HyWO3. It was concluded that the directions of photoexcited electrons transfer from PTO to HyWO3 and from HyWO3 to PTO are responsible for these types of anomalous photo-TE effects.
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INTRODUCTION Thermoelectric (TE) materials have been attracting attention because of their potential for utilizing ambient thermal energy and recycling exhaust heat through a TE conversion effect.1−5 Therefore, TE materials are expected to generate environmentally clean energy. Solar power, including both photon and thermal energies, is also regarded as one of the important ambient energies and can be an environmentally clean energy source. For example, solar photon energy is converted to hydrogen energy through a photocatalytic effect.6−10 In addition, solar photon and solar thermal energies are converted to electricity through a photovoltaic effect11−13 and by the concentration of sunlight using mirrors or lenses,14,15 respectively. The combined use of thermal and solar energies has led to a new research topic based on the changes in the Seebeck coefficient (S) and electrical conductivity (σ) under light irradiation. Such changes involve two separate phenomena. One is induced by the temperature gradient caused by heating with light (thermal contribution of light),16−19 and the other is the contribution of the photon energy of light, which is termed as a photothermoelectric (photo-TE) effect.16,20−33 Most of the previous studies on the photo-TE effect have demonstrated a decrease in S and an increase in σ under light irradiation, which is called the normal photo-TE effect. The normal effect is explained as an effect of doping photoinduced charge carriers,16,20,24,26,28,33 and it is well-known that σ and S are inversely related as a function of carrier concentration, i.e., higher carrier concentrations are associated with higher σ and smaller S. Other interpretations have been suggested, such as considerable enhancement of the carrier mobility,16,25,27,29 although such explanations remain controversial. However, the general rule that S decreases and σ increases under light irradiation is still applicable. The TE conversion efficiency is © XXXX American Chemical Society
frequently represented by a power factor (PF), expressed as S2σ. From this equation, large S and σ values are necessary for high TE performance. Thus, the normal photo-TE effect does not fully satisfy the above conditions for high TE performance, although it has been reported that PF for a Cu2S film irradiated with light was markedly improved compared with that in the dark.33 In contrast, the anomalous photo-TE effect, in which both S and σ increase under light irradiation, fulfills the conditions for high TE performance. The anomalous photo-TE effect was reported for p-type silicon (Si) and lead chromium oxide (Pb2CrO5), which was explained by their different surface and volume electrical conductivities, that is, the two-carrier contribution to the transport properties.34,35 Recently, our group reported platinum-loaded tungsten oxide (Pt-loaded WO3) after a chromic reaction (Pt-loaded protonated WO3 (Pt-loaded HyWO3)), which showed the n-type anomalous photo-TE effect, caused by the accumulation of electrons in Pt induced by the charge transfer from WO3 to Pt,36 which is completely different from the previous explanation of the twocarrier contribution to the transport properties. A material showing the p-type anomalous photo-TE effect is required for use with the n-type anomalous photo-TE effect material to construct TE power generation modules. In the present study, we observed controllable n-type and p-type anomalous photo-TE effects for a Pt oxide and WO3 layer (PtOx/WO3) by controlling the charge transfer direction from PtOx to HyWO3 (or WO3) and vice versa. Received: May 22, 2019 Revised: July 24, 2019
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DOI: 10.1021/acs.chemmater.9b02015 Chem. Mater. XXXX, XXX, XXX−XXX
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Chemistry of Materials
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after the PC reaction, that is, the photothermoelectricity (σphoto and Sphoto) and thermoelectricity (σ and S) were measured under a N2 flow containing HCOOH vapor and by alternating visible-light irradiation from the Xe lamp with the Y-52 optical filter (>500 nm) and dark storage. For PTO (50 nm)/WO3, the photo-TE effect was measured using the as-prepared sample. HCHO was utilized instead of HCOOH, to avoid the GC reaction. All of the other measuring conditions, except for the use of HCHO instead of HCOOH, were the same as in the cases of HyWO3 and PTO/HyWO3. To measure σ, S, σphoto, and Sphoto, four Pt wires were attached to the prepared thin films by Ag conductive paste. The σ, S, σphoto, and Sphoto values of each sample were measured at room temperature by the conventional two-probe steady-state method.37
EXPERIMENTAL SECTION
Sample Preparation. WO3 thin films were prepared on SiO2 substrate plates (50 × 50 mm2) by spin-coating using tungsten(VI) chloride (WCl6, purity 99.99%, Kanto Chemicals) as a tungsten source. WCl6 (3.00 g) (corresponding to 0.76 × 10−2 mol of WO3) was dissolved in 30 mL of ethanol, and the obtained WCl6 solution was thoroughly stirred under a dry N2 atmosphere to form the WO3 precursor solution. The WO3 precursor (200 μL) was spin-coated on a SiO2 substrate at 1500 rpm for 5 s, and then at 3000 rpm for 10 s, which was followed by drying on a hot plate at 150 °C for 15 m. These coating and drying procedures were repeated 5 times, and the spin-coated films were calcined in air at 500 °C for 10 min to form WO3 thin films. The thickness of the WO3 films was controlled to be 650 nm. Then, the obtained WO3 films were cut into a size of 5 × 20 mm2. Pt species was deposited on the WO3 thin films and directly on a SiO2 or Si substrate by sputtering a Pt metal target under an argon atmosphere at room temperature with a sputtering current of 5 mA using a DC sputtering apparatus (Sanyu Electron Co., Ltd., SC-701). In fact, as discussed later, the deposited Pt species was a mixture of Pt and PtOx, which is referred to as platinum-oxide species (PTO) hereinafter. Then, we obtained PTO/WO3 and PTO films. The thickness of the PTO films was controlled to be 50, 90, 200, and 250 nm. Characterization. The cross-sectional structure of the films was observed by scanning electron microscopy (JSM-6500F, JEOL). An X-ray diffractometer (XRD, PW-1700, PANalytical) was used to determine the structures of the samples. UV−visible absorption spectra (UV−vis) were obtained using a spectrometer (V-650, Jasco). The Pt 4f, W 4f, Si 2p, and O 1s core levels were measured by X-ray photoelectron spectroscopy (XPS; JEOL, JPS-9200) on the surface and the inner region from the surface subjected to in situ Ar+ etching of the PTO/WO3 and PTO films (on SiO2 and Si substrates) to examine the percentages of Pt (Pt0, Pt2+ + Pt4+), W, Si, and O. The etching rate was assumed to be 2.5 nm/min (Supporting Information 1 (SI 1)). The number of absorption photons of either PTO or HyWO3 was calculated from the light intensity measured using a spectroradiometer (USR-40, Ushio) and the UV−visible absorption spectra of PTO and PTO/HyWO3 (SI 2). Photo-TE Effect Measurement. A prepared WO3, PTO/WO3, or PTO thin film was set in a quartz vessel. After sealing the vessel with a lid, the atmosphere was replaced with nitrogen (N2). The photo-TE effect of WO3 (650 nm) that had undergone a photochromic (PC) reaction (HyWO3) was measured as follows. Formaldehyde (HCHO) was vaporized by a flow of N2 of 400 sccm, and a N2 flow containing the HCHO vapor, acting as a hole scavenger, was introduced into the vessel. Then WO3 was irradiated with light from a xenon (Xe) lamp (LA-251Xe, Hayashi Tokei Works) equipped with an optical filter (U360, Hoya, irradiated wavelength of light: 300−400 nm) for 1 h to form HyWO3. The photo-TE effect of the thus obtained HyWO3 was measured under a N2 flow containing formic acid (HCOOH) vapor (HCOOH was vaporized by a flow of N2 of 400 sccm, which was introduced into the vessel). At each point, the photothermoelectricity (σphoto and Sphoto) was measured for the sample in 3 min after starting visible-light irradiation from the Xe lamp equipped with an optical filter (Y-52, irradiation wavelength of light >500 nm). The same measurements were also performed in 3 min after stopping light irradiation. The measurements under visible-light and dark conditions were repeated several times to confirm the reproducibility of the σphoto and Sphoto data. The photo-TE effect of PTO (50 nm) on a SiO2 substrate was measured under exactly the same conditions as in the case of HyWO3 mentioned above, but the PTO had not undergone the PC reaction. For PTO/WO3 (PTO: 50, 90, 200, and 250 nm), the photo-TE effect was measured using samples that had undergone a gasochromic (GC) reaction for 1 h under a N2 flow containing HCOOH. Here, HCOOH acts as the source of protons, which contribute to the GC reaction. The photo-TE effect of PTO/HyWO3 after the GC reaction was measured under exactly the same conditions as those for HyWO3
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RESULTS AND DISCUSSION Characterization. The actual thickness of both PTO and WO 3 was confirmed from the cross-sectional images, demonstrating the formation of WO3 (thickness of 650 nm) on SiO2, PTO (50 nm)/WO3 (650 nm) on SiO2, PTO (90 nm)/WO3 (650 nm) on SiO2, PTO (200 nm)/WO3 (650 nm) on SiO2, PTO (250 nm)/WO3 (650 nm) on SiO2, PTO (50 nm) on SiO2, and PTO (40 nm) on Si (Figure S10 in SI 3). Figure 1a−c shows the XRD patterns of the prepared films.
Figure 1. XRD patterns of the prepared films. Bare WO3 and PTO (50, 90, 200, and 250 nm)/WO3 on SiO2 substrates (a), PTO (40 nm) on a Si substrate, PTO (50 nm) on a SiO2 substrate, and PTO (50 nm)/WO3 on a SiO2 substrate (b), and an enlargement of the part of (a) and (b) with the Pt peak (c). Precisely measured XRD patterns for bare WO3 and PTO (50 nm)/WO3 on SiO2 substrates and the data were collected with a step angle of 0.010° and a stepcounting time of 1.5 s. (d) The asterisks indicate the peak of Pt, and the plus marks indicate that of Si (substrate).
WO3 was confirmed to have a monoclinic crystal system (Figure 1a). The peak intensity of Pt increased with increasing thickness of PTO (Figure 1a−d). In contrast, no peaks from PtOx were observed, suggesting the existence of small PtOx particles or amorphous PtOx. In addition, Pt was not incorporated into the WO3 lattice because no peak shift was observed between the bare WO3 and PTO (50 nm)/WO3 (Figure 1d). The optical absorption spectra of the prepared films were also determined (Figure 2). The absorption of the films increased in the visible-light region as the thickness of PTO increased, providing clear evidence for the deposition of PTO. Photo-TE Properties of WO3 and WO3 after the PC Reaction (HyWO3). The values of σ and S could not be measured for the as-prepared WO3 owing to the insulating properties of WO3. In contrast, the detection and measurement of σ and S for WO3 after the PC reaction (HyWO3) could be B
DOI: 10.1021/acs.chemmater.9b02015 Chem. Mater. XXXX, XXX, XXX−XXX
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were confirmed by repeated measurements. A decrease in the absolute value of Sphoto relative to S followed by an increase in σphoto relative to σ is a common phenomenon, termed the “normal photo-TE effect”, explained by an increase in the number of photogenerated carriers, as we previously reported.30,36 Photo-TE properties of PTO (50 nm)/SiO2. σ, S, σphoto, and Sphoto for PTO on SiO2 are shown in Figure 4. Again, the Figure 2. UV−visible absorption spectra of the prepared films. Bare WO3, PTO (50, 90, 200, and 250 nm)/WO3, and PTO (50 and 90 nm) on SiO2 substrates.
accomplished owing to the “memory effect”, in which the mixed-valency states of W6+ and W5+ are maintained for a period of time, even after stopping UV-light irradiation. The PC reaction proceeded with the formation of lower-valency W5+ cations (W6+ + e− → W5+), which was followed by the injection of protons (H+) into the WO3 structure to form hydrogen-tungsten bronze, HyWO3 (WO3 + ye− + yH+ → HyWO3).30 This PC reaction is initiated by light-induced band-gap excitation, followed by the trapping of electrons at W6+ sites to form W5+ and the diffusion of holes to the film surface, where they react with the hole scavenger, HCHO, and water molecules. σ and S were measured during irradiation with visible light and are termed σphoto (photoconductivity) and Sphoto (photo-Seebeck coefficient), respectively. Figure 3
Figure 4. Detected photoconductivity (black) and photo-Seebeck effect (blue) under on-and-off alternation of visible-light irradiation (>500 nm) of the PTO thin film on the SiO2 substrate. The open plots show values for σphoto and Sphoto, respectively, obtained during visible-light irradiation (denoted on). The solid plots show values for σ and S, respectively, in the absence of visible-light irradiation (denoted off).
Figure 3. Detected photoconductivity (black) and photo-Seebeck effect (blue) under on-and-off alternation of visible-light irradiation (>500 nm) of the WO3 thin film on the SiO2 substrate, which underwent the PC reaction under UV-light irradiation (300−400 nm, N2 and HCHO atmosphere) for 1 h (HyWO3). The open plots show values for σphoto and Sphoto, respectively, obtained during visible-light irradiation (denoted on). The solid plots show values for σ and S, respectively, in the absence of visible-light irradiation (denoted off). The inset shows UV−visible absorption spectra of as-prepared WO3 (black) and HyWO3 (purple).
open plots are σphoto and Sphoto, which were measured during visible-light irradiation (>500 nm, denoted on) under a N2 and HCOOH atmosphere. The solid plots show the values for σ and S in the absence of visible light irradiation (denoted off) under the same atmosphere. Here, S and S photo are unexpectedly both positive, indicating p-type conduction. Under irradiation with visible light, an increase in σphoto and a decrease in Sphoto were observed. Values of σphoto and Sphoto in response to irradiation with visible light were confirmed by repeated measurements. Thus, we could also observe the normal photo-TE effect in the case of PTO on SiO2, similar to the case of HyWO3 on SiO2. However, PTO on SiO2 was demonstrated to exhibit p-type conduction, different from HyWO3 on SiO2. Note that PTO (40 nm) on Si was confirmed to exhibit n-type conduction (S = −0.3 μV/K). To explain the p-type conduction of PTO on SiO2, in contrast to the n-type conduction of PTO on Si, we performed XPS measurements on the surface and the inner region from the surfaces of PTO/SiO2 and PTO/Si, and show the results in Figures 5 and 6, respectively (Figures S1−S4, Tables S1−S4 in
shows the measurements of σ, S, σphoto, and Sphoto for HyWO3. The UV−visible absorption spectra of the as-prepared WO3 and HyWO3 are also shown (inset of Figure 3). The absorption of HyWO3, compared with that of WO3, increased in the visible-light region, providing evidence of the PC phenomenon. The open plots presented in Figure 3 are for σphoto and Sphoto and were measured during visible-light irradiation (>500 nm, denoted “on”) of the HyWO3 film under a N2 and HCOOH atmosphere. The solid plots show the values of σ and S, respectively, in the absence of visible-light irradiation (denoted “off”) under the same atmosphere. Here, S and Sphoto are both negative, indicating n-type conduction. Under irradiation with visible light, an increase in σphoto and a decrease in the absolute value of Sphoto were observed. Values of σphoto and Sphoto in response to irradiation with visible light
Figure 5. XPS depth profiles of PTO on the SiO2 substrate. Depth profiles showing the percentages of Pt0, (Pt2+ + Pt4+), Pt (=Pt0 + Pt2+ + Pt4+), Si, and O vs the sum of those of Pt, Si, and O (100%, Pt + Si + O) (a) and those showing the percentages of Pt0 and (Pt2+ + Pt4+) vs the sum of those of Pt (100%, Pt = Pt0 + Pt2+ + Pt4+) and that of O vs (Pt + Si + O) (b). C
DOI: 10.1021/acs.chemmater.9b02015 Chem. Mater. XXXX, XXX, XXX−XXX
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Figure 6. XPS depth profiles of PTO on the Si substrate. Depth profiles showing the percentages of Pt0, (Pt2+ + Pt4+), Pt (= Pt0 + Pt2+ + Pt4+), Si, and O vs (Pt + Si + O) (a) and those showing the percentages of Pt0 and (Pt2+ + Pt4+) vs Pt and that of O vs (Pt + Si + O) (b).
SI 1). The percentage of Pt (Pt0, Pt2+ + Pt4+) decreased and those of Si and O increased with the increasing depth of PTO on SiO2 (Figure 5a). In particular, O mainly existed near the interface of PTO and SiO2, at a distance of 40−50 nm from the surface, where the percentage of (Pt2+ + Pt4+) vs Pt (=Pt0 + Pt2+ + Pt4+) was higher and that of Pt0 vs Pt was lower than those of ∼30 nm from the surface (Figure 5b). This suggests the formation of PtOx in the region 40−50 nm from the surface. Only Pt was detected, and neither PtO2 nor PtO was detected by XRD, as shown in Figure 1, so we concluded the existence of amorphous PtOx. In contrast, the percentages of Pt (Pt0, Pt2+ + Pt4+) and O decreased and that of Si increased with increasing depth of PTO on Si (Figure 6a). The percentage of (Pt2+ + Pt4+) vs Pt (=Pt0 + Pt2+ + Pt4+) increased and that of Pt0 vs Pt decreased at the interface of PTO and Si, ∼40 nm from the surface (Figure 6b). As indicated in Figure 6a,b, there is only a small amount of O in the same region, so the generation of PtOx is less probable. In fact, the binding energies of the Pt 4f XPS peaks derived from Pt2+ and Pt4+ 38,39 are quite similar to those derived from the PtSi species, Pt2Si and PtSi, respectively.40,41 Although we assigned them to Pt2+ and Pt4+ and analyzed these peaks, it is more reasonable to conclude the generation of PtSi species than that of PtOx. Thus, the generation of PtOx is attributable to the p-type conduction behavior of PTO on SiO2 because it has already been reported that PtOx (PtO and PtO2) has the ptype properties.42,43 Although no PtOx was detected by XRD, the existence of PtOx was suggested by XPS, and the prepared PTO was a mixture of Pt and PtOx, in which PtOx dominated the p-type conducting behavior. In contrast, PTO on Si was ntype, which was caused by the presence of Pt and/or PtSi species and by the absence of PtOx. In fact, Pt and PtSi have been reported to be n-type.44,45 Photo-TE Properties of PTO (50, 90 nm)/WO3 after the GC Reaction (PTO (50, 90 nm)/HyWO3). Before discussing the photo-TE effect of PTO (50, 90 nm)/HyWO3, the XPS results for the surface and the inner region from the surface of PTO (50, 90 nm)/WO3 on SiO2 are shown in Figure 7a−d (Figures S5−S8, Tables S5−S8). The trends were similar for PTO (50 nm)/WO3 and PTO (90 nm)/WO3. That is, the percentage of Pt (Pt0, Pt2+ + Pt4+) decreased and those of W and O increased with increasing depth of PTO (50, 90 nm)/WO3 (Figure 7a,c). In the case of PTO (90 nm)/WO3, the percentages of W and O leveled out and remained constant from ca. 50 nm from the surface to the interface of PTO and WO3 (Figure 7c). Moreover, the percentages of Pt0 and (Pt2+ + Pt4+) vs Pt (=Pt0 + Pt2+ + Pt4+) tended to decrease and increase, respectively, with increasing depth of PTO (50 nm)/
Figure 7. XPS depth profiles of PTO (50, 90 nm)/WO3 on the Si substrates. Depth profiles of PTO (50 nm)/WO3 showing the percentages of Pt0, (Pt2+ + Pt4+), Pt (=Pt0 + Pt2+ + Pt4+), W, and O vs (Pt + Si + O) (a) and those showing the percentages of Pt0 and (Pt2+ + Pt4+) vs Pt and that of O vs (Pt + W + O) (b). Depth profiles of PTO (90 nm)/WO3 showing the percentages of Pt0, (Pt2+ + Pt4+), Pt (=Pt0 + Pt2+ + Pt4+), W, and O vs (Pt + Si + O) (c) and those showing the percentages of Pt0 and (Pt2+ + Pt4+) vs Pt and that of O vs (Pt + W + O) (d).
WO3 (Figure 7b). In the case of PTO (90 nm)/WO3, the percentages of Pt0 and (Pt2+ + Pt4+) vs Pt (=Pt0 + Pt2+ + Pt4+) remained nearly constant near the interface of PTO and WO3 (Figure 7d). By analogy to WO3 on SiO2, PtOx was suggested to form near the interface of PTO (50 nm)/WO3 and from ca. 50 nm from the surface to the interface of PTO (90 nm)/ WO3. Only Pt was detected, and neither PtO2 nor PtO was detected by XRD in Figure 1, so we concluded that amorphous PtOx was included in PTO and that PTO exhibited p-type conduction. Figure 8a shows σ, S, σphoto, and Sphoto for PTO (50 nm)/ HyWO3. Again, the open plots are for σphoto and Sphoto in the presence of visible light (>500 nm, denoted on) and the solid plots show the values for σ and S in the absence of visible light (denoted off) under N2 and HCOOH atmosphere. The UV− visible absorption spectra of the as-prepared PTO (50 nm)/ WO3 and PTO (50 nm)/HyWO3 are also shown (inset of Figure 8a). Compared with the absorption of PTO (50 nm)/ WO3, that of PTO (50 nm)/HyWO3 was greater in the visiblelight region, providing clear evidence for the GC phenomenon. In general, the GC reaction of Pt/WO3 in the presence of hydrogen (H2) is explained by the formation of HyWO3, similar to the PC reaction, by the following three steps:46 H2 → 2H, 2H → 2H+ + 2e−, and WO3 + xe− + yH+ → HyWO3. In the presence of HCOOH, the following reactions have been reported to produce e− and H+ at the Pt surface: HCOOH ⇌ H+ + HCOO−, Pt + HCOO− → CO2 + Hads + e−, and Hads ⇌ H+ + e− + Pt; the total reaction is HCOOH → CO2 + 2H+ + 2e−.47 Thus, it is reasonable to consider that PTO/WO3 in the present study undergoes the GC reaction, even in the presence of HCOOH instead of H2, owing to the catalytic generation of H+ and e− on Pt as a result of the fabrication of a mixture of Pt and PtOx. The PTO surface of PTO (50 nm)/WO3 was so porous that HCOOH presumably penetrated into the interface of PTO and WO3 (Figure S11a in SI 3). Then the generated H+ and e− on Pt was able to transfer to WO3, transforming into HyWO3. D
DOI: 10.1021/acs.chemmater.9b02015 Chem. Mater. XXXX, XXX, XXX−XXX
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S and Sphoto for PTO (90 nm)/HyWO3 are both positive in Figure 8b, indicating p-type conduction, in contrast with those for PTO (50 nm)/HyWO3 in Figure 8a. In the presence of visible light, increases in both σphoto and Sphoto were observed. Values of σphoto and Sphoto in response to irradiation with visible light were confirmed by repeated measurements. Thus, we demonstrated the p-type anomalous photo-TE effect in the case of PTO (90 nm)/HyWO3. Also, both n-type and p-type anomalous photo-TE effects were successfully demonstrated by controlling the thickness of PTO layers on HyWO3. The p-type anomalous phenomena were assumed to originate from the p-type nature of PTO and the photoinduced electron transfer from PTO to HyWO3, in the opposite direction in the case of PTO (50 nm)/HyWO3 (Figure S12 in SI 4). Upon irradiating light, the number of holes (photogenerated holes) in PTO increases, whereas the number of minor carriers in PTO, in this case electrons, decreases because the photoinduced electrons in PTO transfer to HyWO3. In addition, when the electrons are transferred to HyWO3, y increases, resulting in a decrease in the absolute value of S for HyWO3. Taken together, the p-type anomalous phenomena are presumed to have been observed. The numbers of absorbed photons for PTO (50 nm), PTO (90 nm), and HyWO3 in PTO (50 nm)/HyWO3 and PTO (90 nm)/HyWO3 were estimated (Figure S9 and Table S9 in SI 2). For PTO (50 nm)/HyWO3, the numbers of photons absorbed by PTO (50 nm) and HyWO3 are 1.5 × 1016 and 2.2 × 1015 quanta/s, respectively (absorbed photon ratio, PTO (50 nm)/HyWO3 = 87:13). For PTO (90 nm)/HyWO3, they are 2.1 × 1016 and 2.0 × 1015 quanta/s for PTO (90 nm) and HyWO3, respectively (PTO (90 nm)/HyWO3 = 92:8). No decisive difference was observed, although the actual number of photons absorbed by HyWO3 in PTO (50 nm)/HyWO3 was larger than that absorbed by HyWO3 in PTO (90 nm)/HyWO3. We next performed two additional types of photo-TE measurement: one was to increase the absorbed photon number by PTO through the increase in the PTO thickness further to guarantee the electron transfer from PTO to HyWO3, leading to the p-type anomalous photo-TE effect. The other was to drive electron transfer from PTO to HyWO3, even in PTO (50 nm)/HyWO3, to confirm the switching from the n-type anomalous photo-TE effect to the p-type anomalous effect. Photo-TE Properties of PTO (200, 250 nm)/WO3 after the GC Reaction (PTO (200, 250 nm)/HyWO3). In Figure 9a,b, σ, S, σphoto, and Sphoto for PTO (200 nm)/HyWO3 and PTO (250 nm)/HyWO3 are shown, respectively. Here, the UV−visible absorption spectra of PTO (200 nm)/HyWO3 and PTO (250 nm)/HyWO3 are not shown because we were unable to observe the increase in their absorption spectra compared with those of PTO (200 nm)/WO3 and PTO (250 nm)/WO3 owing to their absorption of nearly 100% (Figure 2). S and Sphoto for both PTO (200 nm)/HyWO3 and PTO (250 nm)/HyWO3 are positive, indicating p-type conduction. As expected, the increase in both σphoto and Sphoto could be repeatedly observed, demonstrating the p-type anomalous photo-TE effect. Figure 10 shows the PTO thickness dependence of σ and S and that of Δσ (=σphoto − σ) and ΔS (= Sphoto − S). In the p-type region, σ, S, Δσ, and ΔS increased with increasing PTO thickness. It is reasonable to consider that this is due to the increase in the contribution of PTO.
Figure 8. Detected photoconductivity (black) and photo-Seebeck effect (blue) under on-and-off alternation of visible-light irradiation (>500 nm) of PTO (50 nm)/WO3 (a) and PTO (90 nm)/WO3 (b) on the SiO2 substrate, which underwent the GC reaction (N2 and HCOOH) for 1 h to form PTO (50 nm)/HyWO3 and PTO (90 nm)/HyWO3, respectively. The inset of (a) shows UV−visible absorption spectra of the as-prepared PTO (50 nm)/WO3 (black) and PTO (50 nm)/HyWO3 (purple). The inset of (b) shows those of the as-prepared PTO (90 nm)/WO3 (black) and PTO (90 nm)/ HyWO3 (purple). The symbols for σ, S, σphoto, and Sphoto are the same as those in Figures 3 and 4.
In Figure 8a, S and Sphoto are both negative, indicating n-type conduction. In the presence of visible light, increases in σphoto and the absolute value of Sphoto were observed. Values of σphoto and Sphoto in response to irradiation with visible light were confirmed by repeated measurements. Thus, we demonstrated the n-type anomalous photo-TE effect in the case of PTO (50 nm)/HyWO3. This phenomenon coincided with a previous report of one of the authors (H.I.) on a Pt (PtOx)-dispersed HyWO3 film.36 In the report, it was concluded that the n-type anomalous photo-TE effect was caused by the n-type conduction of HyWO3, the photoinduced electrons, and the accumulation of electrons in Pt (PtOx) transferred from HyWO3. That is, the photoinduced electrons and the accumulated electrons in Pt (PtOx) contribute to the increase in σphoto. In addition, electrons are extracted from the W5+ state, decreasing the number of W5+ in HyWO3 (increasing the number of W6+ in the direction of generating WO3) and thus contributing to the increase in Sphoto (Figure S12 in SI 4). After light irradiation, the accumulated electrons in Pt (PtOx) return to the energetically favorable W5+ state (increasing the number of W5+ in the direction of generating HyWO3), and Pt (PtOx)/ HyWO3 returns to the initial state. Then both σ and S decrease. In Figure 8b, σ, S, σphoto, and Sphoto for PTO (90 nm)/ HyWO3 are shown. The UV−visible absorption spectra of the as-prepared PTO (90 nm)/WO3 and PTO (90 nm)/HyWO3 are also shown (inset of Figure 8b). The absorption of PTO (90 nm)/HyWO3 was greater than that of PTO (90 nm)/WO3 in the visible-light region. Thus, the GC phenomenon proceeded. The PTO surface of PTO (90 nm)/WO3 was denser than that of PTO (50 nm)/WO3; however, it included numerous voids and pores (Figure S11b in SI 3). Therefore, HCOOH was probably accessible to the interface of PTO and WO3. Then, the generated H+ and e− on Pt transferred to WO3, transforming into HyWO3. E
DOI: 10.1021/acs.chemmater.9b02015 Chem. Mater. XXXX, XXX, XXX−XXX
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Figure 11. Detected photoconductivity (black) and photo-Seebeck effect (blue) under on-and-off alternation of visible-light irradiation (>500 nm) of the as-prepared PTO (50 nm)/WO3 thin film on the SiO2 substrate. The symbols for σ, S, σphoto, and Sphoto are the same as those in Figures 3, 4, 8, and 9.
transfer direction of the photoinduced electrons plays an important role in the generation of the n-type or p-type anomalous photo-TE effect.
Figure 9. Detected photoconductivity (black) and photo-Seebeck effect (blue) under on-and-off alternation of visible-light irradiation (>500 nm) of the PTO (200, 250 nm)/WO3 thin film on the SiO2 substrate, which underwent the GC reaction (N2 and HCOOH) for 1 h to form PTO (200 nm)/HyWO3 (a) and PTO (250 nm)/HyWO3 (b). The symbols for σ, S, σphoto, and Sphoto are the same as those in Figures 3, 4, and 8.
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CONCLUSIONS We demonstrated four types of photo-TE effects: the n-type normal effect of HyWO3, the p-type normal effect of PTO on SiO2, the n-type anomalous effect of PTO (50 nm)/HyWO3, and the p-type anomalous effect of PTO (90, 200, 250 nm)/ HyWO3 and PTO (50 nm)/WO3. The anomalous photo-TE effect, i.e., the increase in both S and σ under light irradiation, is important because it fulfills the conditions for high TE performance. In particular, we achieved both n-type and p-type anomalous photo-TE effects using the same components, PTO and WO3 (HyWO3), by controlling the thickness of PTO, with and without applying the chromic reaction to WO3. We need to enhance the photo-TE properties as well as the transparency before the PC reaction and the spectral change before and after the reaction. Such studies are now underway in our laboratory. However, on the basis of the fact that the TE devices are composed of n- and p-type materials, the present findings would be expected to lead to a functional material for use as a component of photochromic smart windows, which are anticipated to provide shade from sunlight while simultaneously generating electric power.
Figure 10. Dependences of the average values of S (a) and σ (b) (denoted by solid circles) in the absence of visible-light irradiation, and the difference between the average values of S from Sphoto (a) and those of σ from σphoto (b) (denoted by open squares) on the thickness of PTO in PTO/WO3.
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Photo-TE Properties of As-prepared PTO (50 nm)/ WO3. σphoto, Sphoto, σ, and S for PTO (50 nm)/WO3 were measured under a N2 flow containing HCHO vapor, instead of HCOOH vapor, to avoid the GC reaction and under the alternation of visible-light irradiation with a wavelength of >500 nm and dark storage. Under the light conditions, WO3 is unable to absorb the incident light, and only PTO absorbs the light. Thus, only the transfer of photoexcited electrons from PTO to WO3 occurs. In contrast to PTO (50 nm)/HyWO3 in Figure 8a, PTO (50 nm)/WO3 showed p-type conduction because of the positive S and Sphoto values in Figure 11, as we expected. Importantly, we could repeatedly observe the increase in both σphoto and Sphoto under the alternation of visible-light irradiation, demonstrating the p-type anomalous photo-TE effect, which is different from the case of PTO (50 nm)/HyWO3 in Figure 8a. Thus, we also successfully demonstrated both n-type and p-type anomalous photo-TE effects regardless of whether or not the chromic reaction was applied to WO3. The difference between PTO (50 nm)/ HyWO3 and PTO (50 nm)/WO3 strongly indicates that the
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.9b02015. Procedure for XPS analyses; explanation of the process for obtaining numbers of absorbed photons; crosssectional and surface images of the prepared samples; schematic band diagrams of WO3 and PTO (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Hiroshi Irie: 0000-0002-1286-3271 Author Contributions §
H.I. and K.S. contributed equally to this work.
Notes
The authors declare no competing financial interest. F
DOI: 10.1021/acs.chemmater.9b02015 Chem. Mater. XXXX, XXX, XXX−XXX
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Chemistry of Materials
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ACKNOWLEDGMENTS This work was supported by JKA and its promotion funds from KEIRIN RACE (2017M-142, 2019M-178).
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ABBREVIATIONS
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REFERENCES
TE, thermoelectric; S, Seebeck coefficient; σ, electrical conductivity; PC, photochromic; GC, gasochromic; PTO, platinum-oxide species
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