Background

In recent years, environmental pollution and energy shortage have created serious social and economic issues for human society. Semiconductor-based photocatalysis has been widely employed as a highly efficient technique to overcome these issues [1,2,3]. Among these semiconductor metal oxides, zinc oxide (ZnO) has been recognized as a promising photocatalyst owing to its outstanding electrical and optical properties, low cost, high biological safety, versatile shapes and structures, environment benign and strong photocatalytic degradation ability of organic pollutants under UV light. However, ZnO with a wide band gap (Eg = 3.3 eV) can only be activated by ultraviolet (UV) light, which restricts its practical applications for solar energy [4,5,6,7,8]. Another main drawback of ZnO is rapid recombination of photo-induced electron-hole pairs, which results in the low quantum yield for any photocatalytic reactions [9,10,11,12]. Therefore, how to extend absorption edge of ZnO to visible light region for the utilization of about 43% solar spectrum meanwhile suppress the photo-generated electron-hole pairs recombination is still a great challenge for scientists. Various modification strategies to activate ZnO photocatalysis under visible light have been employed in the past few years, including sensitization, semiconductor coupling and doping. An efficient strategy is coupling ZnO with another narrowband-gap semiconductor (e.g. CdS [13], CdSe [14], Cu2O [15], C3N4 [16], ZnFe2O4 [17], Ag3PO4 [18], CuInS2 [19], AgBr [20] and BiVO4 [21]) to form ZnO/narrow-band-conductor type п heterostructures. The formation of type II heterostructures has been recognized as an attractive route to overcome the limitations of ZnO because it promotes efficient charge separation, enlarges the effective contact interfaces and improves the optical absorption [22, 23].

In2O3 with a band gap of 2.56 eV has been proved as efficient sensitizer to extent the light absorption spectra by coupling other semiconductor. Also, its valence and conduction band alignments are staggered relative to those of ZnO [24, 25]. A lot of researches on In2O3-ZnO composite have been reported for degradation of organic compounds and hydrogen production by photocatalysis [26,27,28]. These results show that the incorporation of In2O3 in ZnO nanostructure can remarkably inhibit recombination of photo-generated electron-hole pairs and thus improve the photocatalytic activity. To the best of our knowledge, there has rarely been reported on the fabrication and improvement ZnO photocatalytic activities and stability by In2O3 nanoparticles hybrid.

In this paper, In2O3 nanoparticles hybrid THD ZnO with different ratios were fabricated by a hydrothermal method. The microstructure and optical properties of ZnO/In2O3 heterostructures were examined. The photocatalytic activity and photo-stability of ZnO/In2O3 composites were evaluated by MO and 4-NP under light irradiation. Finally, the charge transfer and probable photocatalytic mechanism have been discussed and proposed on the basis of optical characterization, band gap structure and reactive species reaction.

Experimental

Formation of ZnO/In2O3 heterostructure

First, 0.1 mol of ZnAc and a specific molar of In(NO 3 ) 2 with a designed atom percent of In to Zn (about 2.0, 5.0, 8.0, 12.0 and 15.0 atom%) were dissolved in 50 ml deionized water to form a clear solution. Then, 15 ml of triethanolamine (TEA) was dropwise into the above solution under magnetically stirring. After that, the mixed solution was heated at 90 °C for 4 h, the obtained precipitates were centrifuged and washed by deionized water and ethanol for several times and dried in an oven at 60 °C. The final ZnO/In2O3 composites were thus obtained by annealing at 200 °C for 1 h. According to the In/Zn molar ratios of 0, 2, 5, 8, 12 and 15%, the composites were marked as Zn-In-0, Zn-In-1, Zn-In-2, Zn-In-3, Zn-In-4 and Zn-In-5, respectively. For comparison, pure In2O3 were also fabricated under the same condition.

Characterization

The crystal structures were studied by powder X-ray diffraction (XRD) with a 0.154178 nm Cu-Kα radiation. The morphologies and size of the ZnO/In2O3 composites were measured by field emission scanning electron microscopy (FESEM; JSM-6700F, Japan). Chemical compositions were analyzed by X-ray energy-dispersive spectroscopy (EDS) equipped to the SEM. The detailed microstructures of samples were characterized by high resolution transmission electron microscopy (FE-SEM SUPRA™ 40). Chemical states of the samples were analyzed using X-ray photoelectron spectroscopy (XPS; PHI-5300, ESCA, USA). The UV-vis diffused reflectance spectra (UV-vis DRS) of samples were measured on a UV-3600 spectrophotometer. Photoluminescence (PL; Renishaw1000, UK) spectra were measured at room temperature using a He-Cd laser as the excitation light source at 325 nm. The •OH-trapping PL spectra was collected in 5 * 10−3 M terephthalic acid solutions containing 0.01 M NaOH solution with different irradiation time; the excitation wavelength was 325 nm.

Photocatalytic test

The photocatalytic activities of the as-prepared samples were evaluated by the photocatalytic degradation of MO and 4-NP. The wavelength distribution of Xenon lamp was similar to that of solar light; thus, a 500 W Xenon lamp was employed as the light source. For each photocatalytic activity measurement, typically, 10 mg of the photocatalyst was dispersed in 50 ml of MO (5 mg/l) or 4-NP (1 mg/l) aqueous solution and then stirred in the dark for 30 min to achieve an adsorption-desorption equilibrium. The photocatalytic reaction was carried out by Xenon lamp as the solar light source with continuous stirring. At the given intervals, 3 mL of the aliquots was sampled and analyzed by recording variations in the absorption band (464 and 317 nm) in the UV-vis spectra of MO or 4-NP, respectively. To probe the photo-stability of the Zn-In-4 catalyst, cycle degradation was carried out. In this case, Zn-In-4 was repeatedly used, which was separated and collected by centrifugation. After being washed with water and ethanol for several times and dried at 60 °C overnight, the Zn-In-4 catalyst was reused with a fresh MO aqueous solution (5 mg/l) for subsequent reactions under the identical conditions.

Trapping experiments were performed to probe the main active species in the photocatalytic process. The experimental apparatus and procedures were identical to that of the photocatalytic activity tests except that different types of scavengers (1 mM) were added into the MO solution. Herein, a fluorescence technique was employed to detect the formation of free hydroxyl radicals (•OH) and terephthalic acid (TPA) was used as the probe molecule. In detail, the as-synthesized Zn-In-4 (0.025 g) was dispersed into 50 mL mixed solution of 0.25 mmol TPA and 1 mmol NaOH under magnetically stirring. After Xenon lamp (500 W) irradiation for 90 min, the supernatant of reaction solution was collected and examined by a FP-6500 fluorescence spectrophotometer with an excitation wavelength of 315 nm.

Results and discussion

Morphology and phase structure analysis

Figure 1 gives the SEM images of fabricated ZnO/In2O3 composites with different loading amounts of In2O3. It can be clearly seen from Fig. 1a that the pure ZnO present twins hexagonal disk shape. The twins hexagonal disk have an average side length value of about 700–1000 nm, and the height of every disks is about 300–400 nm. It is clearly indicated in Fig. 1 that all samples retain the THD morphology and the size of the samples does not change with In2O3 content increasing. The only difference is that the amount of In2O3 nanoparticles on the surface of ZnO/In2O3 composites increase with the increasing of In(NO3)3 content. It should be mentioned that the In2O3 nanoparticles are uniformly distributed on the surface of each THD ZnO, There is rare aggregations even for higher In2O3 contents sample. The EDS spectra of ZnO/In2O3 samples (inserted to corresponding SEM image) were detected by dispersed samples onto a conductive carbon tape. Elemental zinc, oxygen, and indium are detected, and the corresponding weight and atomic percentages for the all samples are listed in Table 1.

Fig. 1
figure 1

SEM images of ZnO/In2O3 composites with different In2O3 amount (a–f). The insets are the EDS spectra corresponding samples

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Table 1 Weights and atomic percentages of elements in ZnO/In2O3 composites
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Figure 2 presents the XRD patterns of the ZnO/In2O3 composites. For Zn-In-0 sample, all diffraction peaks match well with wurtzite ZnO structure (JCPDS 36–1451). For the ZnO/In2O3 heterostructure composites, three new characteristic peaks appear at 2θ values of 30.6, 51.1, and 60.7 can be indexed to (222), (440), and (622) crystal planes of body-centered cubic structure of In2O3 (JCPDS, No. 71–2194), respectively. However, with the molar ratio increasing of In:Zn in ZnO/In2O3 composites, the intensities of typical characteristic peaks indexed to In2O3 increase. No characteristic peaks for other impurities are observed, confirming that the successfully fabricated ZnO/In2O3 composites have high purity.

Fig. 2
figure 2

XRD patterns of ZnO/In2O3 composites with different In2O3 amount

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To further obtain the morphology and structure information, Fig. 3 presents HR-TEM images of Zn-In-4 sample. It can be found that hexagonal disk structures have a diameter of about 800 nm and the surfaces are covered with In2O3 nanoparticles. It is obvious that ZnO/In2O3 heterostructure is composed of twinned hexagonal disks ZnO and In2O3 nanoparticles. Fig. 3(b) indicates the edges of twinned hexagonal disks covered by nanoparticles with diameters of 20–50 nm. The HR-TEM image of the white-square area of Fig. 3b is shown in Fig. 3c, a clearly distinguished interface can be observed from Fig. 3c. The spacing with 0.248 nm is consistent with the inter-planar spacing of the (002) planes of hexagonal ZnO phase [12]. The left part clearly exhibits the In2O3 (222) facets with a spacing value of 0.285 nm, which is consistent with the reported value [24]. The good crystalline quality and the sharp interface between ZnO and In2O3 would be advantageous for the separation of the photo-generated charge carriers. Figure 3d is the selected area electron diffraction (SAED) pattern of the interface, which consists of two sets of zone diffraction spots. These mixed diffraction patterns further indicate the presence of the In2O3 crystalline nucleus on the interface of ZnO hexagonal disk.

Fig. 3
figure 3

TEM image (a) and HR-TEM images (b, c) of Zn-In-4 sample

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XPS analysis

XPS measurement was carried out to further identify the surface elemental and chemical states of ZnO/In2O3 composites. The survey spectra (Fig. 4a) reveal the presence of the Zn2p, In3d, O1s and C1s energy regions. The high-resolution Zn2p spectrum in Fig. 4b showed two major fitting peaks centered at 1044.21 and 1021.36 eV, which are assigned to Zn2p1/2 and Zn2p3/2, respectively, indicating the Zn (II) oxidation state in ZnO [20]. In terms of the In 3d spectrum (Fig. 4c), there are two characteristic peaks centered at 444.16 and 451.73 eV that can be attributed to In 3d5/2 and In 3d3/2, which indicate the presence of In3+ in the ZnO/In2O3 composites [27, 29]. In the O 1 s XPS spectrum (Fig. 4d, the asymmetric profile can be divided to two symmetrical peaks centered at 530.06 and 531.74 eV, respectively. The peak located at 530.06 eV is assigned to lattice oxygen binding with In and Zn (denoted as In-O and Zn-O). In addition, the peak centered at 531.74 eV is associated with the surface-absorbed oxygen species [26, 30]. Many documents have recorded that the surface oxygen species can produce primary active superoxide radicals and hydroxyl radicals, which are capable to trap photo induced electrons and holes to enhanced photocatalytic activities [8, 31].

Fig. 4
figure 4

XPS survey spectrum of Zn-In-4 and corresponding high-resolution XPS spectra: (b) Zn2p, (c) In3d and (d) O1s. The units of Fig. 4 (a),(b),(d) should be "Binding Energy" rather than "Banding Energy". The replaceable Fig. 4 (a), (b), (d) shown in attachment

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Optical characteristics

Figure 5a shows the UV-vis diffuse reflectance spectra (UV-vis DRS) of the obtained ZnO/In2O3 composites. The bare ZnO show a broad absorbance with an absorption edge at 385 nm owing to the intrinsic wide band gap, whereas the cut off wavelength of pure In2O3 nanoparticles located at 450 nm. With the increase of In2O3 content from 2 to 15 at%, the absorption band edges of samples shifts from 380 to 420 nm and the color of the prepared samples also from whitish-yellow to brilliant yellow. This result implies that In2O3 nanoparticles successfully incorporate into ZnO. The inset is the magnified view of UV-vis DRS with the wavelength from 350 to 420 nm. According to Kubelka-Munk method [32], the band gap energy values for ZnO and In2O3 is estimated 3.18 and 2.75 eV, respectively [25, 33]. The plots of (F(R)hν)1/2 vs. hν of the photocatalysts are presented in Fig. 5b.

Fig. 5
figure 5

a UV-vis diffuse reflectance spectra of the prepared composites and b band gap energy of pure ZnO and In2O3 samples

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Photoluminescence (PL) technique is widely used to investigate the migration, transfer and separation efficiency of the photo-induced electrons-holes pairs in a photocatalyst. The higher PL intensity indicates the faster recombination rate of the photo-generated charge carriers; the fewer photogenerated electrons and holes participated in the photocatalytic redox reactions results in lower photocatalytic activity [15, 34, 35]. Consequently, in order to investigate the effect of In2O3 nanoparticles to ZnO, the PL emission spectra of ZnO/In2O3 composites with different contents of In2O3 were measured at room temperature under the excitation wavelength of 325 nm, as shown in Fig. 6. In this investigation, Zn-In-0 exhibits a strong UV luminescence emission peak centered at about 380.0 nm. The UV emission is attributed to the near band edge emission of ZnO. After the modification of In2O3, the emission intensity of ZnO/In2O3 samples dropped significantly. This result indicates that the recombination efficiency of photo-induced electron-hole pairs can be effectively inhibited through the formation of heterojunction structure. However, the Zn-In-4 samples showed the lowest intensity of PL emission peak, which means the Zn-In-4 has highest photocatalytic activity for all ZnO/In2O3 samples.

Fig. 6
figure 6

PL spectra of ZnO/In2O3 composites with different In2O3 amount

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Photocatalytic activity

The photocatalytic activities of ZnO/In2O3 samples were evaluated by degradation of MO and 4-NP under simulated solar irradiation. As illustrated in Fig. 7, self-degradation of MO and 4-NP is negligible without addition of photocatalysts, indicating these two types of organic dyes are photochemical stable. Figure 7a shows the degradation curves of MO on the ZnO/In2O3 samples. Under solar irradiation, only 35% of MO is decomposed after 90 min for Zn-In-0 photocatalysts, which attributed to its higher band gap energy. In comparison, the degradation rate of MO is about 64 and 82% for the Zn-In-1 and Zn-In-2 after 90 min of treatment, respectively. MO can be almost degraded thoroughly after 90 min treatment by Zn-In-3, 70 min by Zn-In-5, and only 60 min by Zn-In-4 composite photocatalyst. The temporal evolution of spectral changes accompanying the photodecomposition of MO over as-prepared Zn-In-4 is shown in Additional file 1: Figure S1. The characteristic absorption peak intensity of MO at 664 nm gradually decreased with the increase of irradiation time and the color of MO-containing solution was also changed from initial lemon yellow to almost transparent color after 60 min reaction, indicating that the MO have been completely decomposed during the photocatalytic process. According to the apparent pseudo-first-order kinetics equation, relative rate constants kapp for different catalysts are calculated and summarized in Fig. 7b. Their corresponding rate constants (k) are determined as 0.0058, 0.010, 0.0193, 0.0450, 0.0687 and 0.0584 min−1 for Zn-In-0, Zn-In-1, Zn-In-2, Zn-In-3, Zn-In-4, Zn-In-5, respectively. It can be found that rate constants k is first increase and then decrease with the increasing of In2O3 content in of ZnO/In2O3 composites. Zn-In-4 shows the highest photocatalytic activity. 4-NP was selected as another typical target compound for evaluating the photocatalytic activity of ZnO/In2O3 composites, and the photocatalytic degradation curves of 4-NP by ZnO/In2O3 composites are shown in Fig. 7c, d. With the In2O3 content increasing, the 4-NP degradation rate firstly increases and then decreases. Furthermore, the highest degradation rate is obtained from Zn-In-4 sample with almost 100% of 4-NP removal after 80 min solar light irradiation. The rate constants k of Zn-In-4 is about 12 times higher than Zn-In-0. Based on the above analysis, we can conclude that the photocatalytic activity of ZnO is enhanced significantly by In2O3 nanoparticles hybrid. With molar ratio increasing of In2O3 to ZnO, the degradation efficiency is firstly increased and then decrease, implying that the optimal loading amount of In2O3 is important for enhancing photocatalytic activity of ZnO/In2O3 composites.

Fig. 7
figure 7

Photocatalytic degradation curves and Kinetic linear simulation rate constants of MO (a, b) and 4-NP (c, d) by Zn-In-0, Zn-In-1, Zn-In-2, Zn-In-3, Zn-In-4, Zn-In-5

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As we all known, the mass ratio of components has a great effect on the photocatalytic performance in heterostructural photocatalyst system [27, 36]. With mass ratio increasing of In2O3 in ZnO/In2O3 composites, there was no significant difference in the degradation tendency of MO and 4-NP, and appears the maximum degradation efficiency for the sample Zn-In-4. However, PL intensities of the samples show an opposite variation tendency. This result indicates that an appropriate amount of In2O3 in the composites was beneficial to the fast separation of photo-generated charge carriers and thus enhanced the photocatalytic activity [5, 37].

It is well known that ZnO has poor stability in degradation of organic pollutants. So, to investigate the photo-stability and repeatability of ZnO-base photocatalyst is very important for photocatalytic performance. The recycling experiments were carried out by the degradation of MO (Fig. 8a) and 4-NP (Fig. 8b) solutions over Zn-In-4 under solar light irradiation. The degradation efficiency of MO drops from 99.7 to 82.6% and 4-NP from 99.5 to 85.4% after five cycles. There has a small variation in the degeneration efficiently of Zn-In-4 for different dyes after five recycling tests under solar light. In addition, the crystal structure as well as morphology of Zn-In-4 has no discernible change before and after five recycling tests under solar light irradiation. So, Zn-In-4 c is stable in photodegradation of organic pollutants.

Fig. 8
figure 8

Recycling experiments of solar light photocatalytic degradation of MO (a) and 4-NP (b) over the Zn-In-4

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Proposed photocatalytic mechanism

As been well known, it is important to investigate the active species in the photocatalytic process in order to better understand the mechanism of photocatalysis. The photocatalytic degradation of dyes mainly involved several active radical species, such as hole (h+), superoxide anion radical (•O2 −) and hydroxyl radicals (•OH) [19, 38]. To evaluate the roles of these active species, a series of quenchers were employed during photodegradation processes. Benzoquinone (BQ), disodium salt ethylenediaminetetraacetic acid (EDTA-2Na), isopropanol (IPA) were used as scavengers for •O2 −, photogenerated holes, and •OH in degradation of MO, respectively. As shown in Fig. 9, under solar light irradiation, the photocatalytic activity of Zn-In-4 composite is greatly suppressed by the addition of BQ or EDTA-2Na, suggesting that both photo-generated •O2 − and holes are the main oxidative species and played a crucial role in the degradation process of MO. However, there are little changes of the photodegradation performance when IPA added into photocatalytic system, suggesting that the •OH has a very small effect on the photocatalytic reaction system. In order to probe the photoactive hydroxyl radicals (•OH), the OH-trapping photoluminescence (PL) spectra (as shown in Fig. 9b) over Zn-In-4 suspension were collected in which terephthalic acid was used as trapping reagents for •OH. It was clear that the emission peak at 426 nm appeared under illumination and the intensity of the emission peak present a little change with the illumination time [39, 40]. Consequently, it can be furtherly confirmed that the photocatalytic degradation of MO over the as-prepared ZnO/In2O3 composite was mainly governed by •O2 − and h+ rather than •OH under solar light irradiation.

Fig. 9
figure 9

Trapping experiment of active species during the photocatalytic degradation of MO over Zn-In-4 under solar light irradiation (a) and the OH-trapping PL spectra over Zn-In-4 sample (b)

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Therefore, to fully understand the photocatalytic reaction mechanism occurring during the photodegradation of as-prepared ZnO/In2O3 composites, the band edge positions of the valence band (VB) and conduction band (CB) of both In2O3 and ZnO are necessary to be determined. For a semiconductor, the VB and CB can be calculated according to the empirical equation [41]:

$$ {E}_{CB}=X-{E}_C-\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.{E}_{\mathrm{g}} $$
(1)
$$ {E}_{VB}={E}_{CB}+{E}_g $$
(2)

Where EVB is the valence band, E CB is conduction band, E C is the energy of free electrons with respect to normal hydrogen electrode (about 4.5 eV vs. NHE) and Eg is the band gap of the semiconductor. X is the absolute electronegativity of the semiconductor, according to previous literatures, the values of X for In2O3 and ZnO were 5.24 and 5.94 eV [27, 42], respectively. Based on the result in Fig. 5(b), the band gap energies of In2O3 and ZnO are estimated as 2.75 and 3.18 eV, respectively. Given the equations above, the E CB of In2O3 and ZnO are estimated to be −0.635 and −0.15 eV, respectively; while the E VB of In2O3 and ZnO are estimated to be 2.12 and 3.03 eV, respectively. Figure 10a shows the energy band structure of ZnO/In2O3 heterostructure. The Femi energy level (Ef) of In2O3 is more negative than that of ZnO [33, 43]. So, in order to achieve Fermi energy level equilibration in In2O3/ZnO heterojunction-type photocatalyst, the Fermi level of ZnO is maintained its position due to the pinning effect of wide-band semiconductor while the Fermi level of In2O3 could shift up until reaching equilibrium. Under solar irradiation, both In2O3 and ZnO absorb light, the electrons will be excited and migrates to the CBs and holes remain on the VB of both In2O3 and ZnO. The electrons on the CB of In2O3 could easily transfer to the CB of ZnO. Simultaneously, the holes in the VB of ZnO migrate into the VB of In2O3. The electrons left at the CB of ZnO reduce O2 to yield •O2 −, which is a powerful oxidant for organic dyes degradation [15, 44]. Holes stored in the VB of In2O3 could directly oxidize the pollutants to harmless products, as shown in Fig. 10b. Based on the above analysis, we can conclude that photo-generated h+ and •O2 − are the primary active species to determine photocatalytic performance, while the enhanced photocatalytic activity is attributed to the efficient separation and transfer of the photo-generated carriers at the heterojuction interfaces driven by the well-matched band-structures of ZnO and In2O3.

Fig. 10
figure 10

Schematic diagram of the energy alignment (a) and charge transfer in the ZnO/In2O3 composites under solar light irradiation (b)

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Conclusions

In summary, In2O3 nanoparticles hybrid THD ZnO with different ratios were fabricated via the hydrothermal process. Significantly, compared with pure ZnO, the fabricated ZnO/In2O3 exhibits much better photocatalytic activities for the degradation of MO and 4-NP under simulated solar light irradiation, which can be ascribed to the synergetic effect between ZnO and In2O3, including the maximum heterostructure interface with intimate contact and excellent solar light response in the composite, which both can enhanced photogenerated charge separation efficiency. This work could give insights into the importance of rational design of heterostructure systems and provide a potential method for the construction of efficient heterostructure photocatalysts with controllable sizes and space distributions.