Colloids and Surfaces A: Physicochemical and Engineering Aspects
Photoelectrochemical properties of three inorganic/organic hybrid films formed from sandwich-type tetrazinc(II) tungstophosphorate and hemicyanines with varied alkyl chain lengths
Graphical abstract
Three electrostatically self-assembled films made of [P2W18Zn4(H2O)2O68]10− (P2W18Zn4) and a hemicyanine dimer linked by varied alkyl chain lengths, were uniformly prepared with large bathochromic shifts compared to the hemicyanine aqueous solutions. A 3-layer film gave stable, large cathodic photocurrent densities of 13.7 μA/cm2 at an applied potential of −0.3 V vs. saturated calomel electrode.
Highlights
► Electrostatically self-assembled films from tetrazinc tungstophosphorate and hemicyanines were prepared. ► Alkyl chain lengths in hemicyanines strongly affected cathodic photocurrents. ► A maximum photocurrent density of 1.33 μA/cm2 was achieved at zero bias voltage.
Introduction
Inorganic–organic hybrid film materials have attracted much attention as photoelectric conversion materials because they can exhibit cooperative properties typically associated with inorganic and organic components [1], [2]. Hemicyanine dyes, a class of important organic molecules, were extensively investigated as nonlinear optical [3], [4], [5], photoelectric conversion [6], [7], [8], [9], [10], [11], [12], [13], [14], and fluorescence materials [15], [16], [17],and so on. Huang’s group reported photoelectric conversion properties of Langmuir–Blodgett (LB) films based on amphiphilic hemicyanine dyes with donor-π-acceptor structure [6], [7], [8], [9], [10], [11], [12], [13], [14]. These researches showed that hemicyanines exhibited good photoelectric activities, but LB films required expensive apparatus to be made and had inherent instability; moreover, the hemicyanines in LB films easily form H-aggregates that are disadvantageous to photoelectric conversion.
Polyoxometalates (POMs) represent a large class of anionic metal–oxygen clusters with varied compositions and structures as well as diverse and adjustable properties for potential applications in various fields such as catalysis [18], [19], medical [20] and materials science [21], [22], [23], [24], [25]. Recently, POMs were reported to act as electron shuttle to enhance the photovoltaic response. Walsh et al. reported that thin films of Ru(II) metallopolymer/Dawson polyoxotungstate produced enhanced photocurrent [26]. Xu et al. reported that the incorporating of POMs into phthalocyanine, TiO2 and CdS film electrodes could effectively enhance their photovoltaic performance [27], [28], [29], [30]. Our group has long been interested in POM/hemicyanine electrostatically self-assembled films with electrochemical, electrocatalytic, second order nonlinear optical and photoelectric conversion properties [31], [32], [33], [34], [35], [36], [37], [38], [39], [40]. We recently reported the photoelectrochemical properties of a series of electrostatically self-assembled films made of POM anions of [P2W18M4(H2O)2O68]10− (abbreviated as P2W18M4, M = Ni, Cu, Zn, Cd) and a hemicyanine of (E)-1,1′-(hexane-1,6-diyl) bis (4-(4-(dimethylamino) styryl) pyridinium) (abbreviated as H6, see Fig. 1 for molecular structure) modified ITO electrodes, which exhibited enhanced photoelectric performance with photocurrent densities being 0.35–0.54 μA/cm2, as compared with analogous hemicyanine-based LB films [41]. Very recently, we have found that the alkyl chain lengths strongly affected the aggregate formation and photocurrent generation in the electrostatically self-assembled films of P2W18Zn4 and three hemicyanines of N3,6,10 (see Fig. 1 for their molecular structures); these hemicyanines have varied alky chain lengths and increasing π-delocalization conjugation of the naphthalene ring in place of phenylene ring in H6. Here we would like to present these interesting findings.
Section snippets
Materials
(3-Aminopropyl) trimethoxysilane (99%) and poly (4-styrenesulfonic acid-co-maleic acid) sodium salt (PSSMA, Mw = 20,000) were purchased from Acros and Aldrich, respectively, and used without further purification. (E)-1,1′-(Hexane-1,6-diyl) bis (4-((E)-2-(4-(dimethylamino) naphthalen-1-yl) vinyl)-pyridinium) bromide (N6) and K10[P2W18Zn4(H2O)2O68] were synthesized according to modified literature methods [34], [42]. Other chemicals were A.R. grade and commercially available, and were used as
Ultraviolet–visible absorption spectra
UV–vis absorption spectra for P2W18Zn4 and N3 aqueous solutions, and a 10-layer quartz/(P2W18Zn4/N3) film are compared in Fig. 4. Similar spectral comparisons for N6 and N10 solutions and their films are shown in Figs. S1 and S2 (Supporting information), respectively. It is clearly seen that the P2W18Zn4 aqueous solution exhibited an Ob,c ⟶ W charge transfer (CT) absorption band centered at 248 nm, which is characteristic absorption of Keggin heteropolytungstates [43]. The Na (a = 3, 6, 10) aqueous
Conclusions
Three electrostatically self-assembled multilayer films made of [P2W18Zn4(H2O)2O68]10- and hemicyanines of N3,6,10 with varied alkyl chain lengths, were successfully deposited up to at least 10 layer. The films exhibited large bathochromic shifts in visible ICT absorption bands, and large cathodic photocurrents that strongly depended on the alkyl chain lengths. The shortest alkyl chain-containing (P2W18Zn4/N3)n films were found to be of the lowest energy ICT band centered at 477 nm. The
Acknowledgements
Financial supports by the National Natural Science Foundation of China (20871011, 21171022, 20971016, and 90922004) and Analytical and Measurements Fund of Beijing Normal University.
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2016, Colloids and Surfaces A: Physicochemical and Engineering AspectsCitation Excerpt :In recent years, POMs have been introduced into the photoelectric or photoelecrochemical cells as electron mediators because POMs could reversibly accept and transport multi-electrons, exhibiting stable variable redox states [9–24]. We have observed that the photoelectric conversion properties of hemicyanine thin films could be enhanced by introducing different POMs [18–23]. However, these studies only concerned about Keggin [PM12O40]3− (M = Mo, W) and Dawson of [S2M18O62]4− (M = Mo, W) and [P2W18O62]4− (M = Mo, W) POMs, many other types of POMs have remained unexplored.
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2015, Electrochimica ActaCitation Excerpt :It is well documented that the hemicyanie dyes are prone to form H or J aggregations [40] with ICT absorption band being shifted to a short wavelength (hypsochromic shift) or a longer wavelength (bathochromic shift), respectively, with respect to their monomer ICT absorption. Comparing the absorption peak of H1Br2 aqueous solution with those of corresponding (POMs/H1)n (n ≥ 2) multilayer films (Table 1), the red-shifts in their visible absorption maximum, Δλvis, were found to be +30 nm for multilayer (BW12/H1)n (n ≥ 2) film, +48 nm for (PW12/H1)n (n ≥ 2) film, and +22 nm for (PMo12/H1)n (n ≥ 2) film, which are similar to a behavior (Δλvis = +36 nm) previously reported for (SiW12/H1)n (n ≥ 2) film and others [25–30], indicating the hemicyanine H1 molecules formed J-aggregations in these (XM12/H1)n films and the degree of aggregation can be effectively tuned by POMs. As we compare W atom-containing POMs, their Δλvis values are found to follow following order: (PW12/H1)n > (SiW12/H1)n > (BW12/H1)n.