Cd2+-doped amorphous TiO2 hollow spheres for robust and

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Cd2+-doped amorphous TiO2 hollow spheres for robust and ultrasensitive photoelectrochemical sensing of hydrogen sulfide Hongbo Li, Jing Li, Yunyun Zhu, Wenyu Xie, Rong Shao, Xiaxi Yao, Aiqin Gao, and Yadong Yin Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b01178 • Publication Date (Web): 03 Apr 2018 Downloaded from http://pubs.acs.org on April 3, 2018

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Analytical Chemistry

Cd2+-doped amorphous TiO2 hollow spheres for robust and ultrasensitive photoelectrochemical sensing of hydrogen sulfide Hongbo Li,a,b Jing Li,a Yunyun Zhu,a Wenyu Xie,a Rong Shao,a* Xiaxi Yao,b Aiqin Gao,b Yadong Yinb* a

School of Chemistry and Chemical Engineering, Yancheng Institute of Technology, Yancheng 224051, PR China b

Department of Chemistry, University of California, Riverside, CA 92521, USA Tel: 951-827-4965; Fax: 951-827-4713 * E-mail: [email protected] (Y.Y.); [email protected] (R.S.).

Abstract：：Hydrogen sulfide is a highly toxic molecule to human health, but its high-performance detection remains a challenge. Herein, we report an ultrasensitive photoelectrochemical (PEC) sensor for H2S by modifying indium tin oxide (ITO) electrodes with Cd2+-doped amorphous TiO2 hollow spheres, which are prepared by templating against colloidal silica particles followed by a cadmium-sodium cation exchange reaction. The amorphous TiO2 hollow spheres act as both the probing cation carrier and the photoelectric beacon. Upon exposure to sulfide ions, the photocurrent of the functionalized photoanode proportionately decreases in response to the formation of CdS nanoparticles. The decreased photocurrent could be attributed to the mismatching bandgap between the amorphous TiO2 and CdS nanoparticles: the photoexcited electrons and holes from amorphous TiO2 are transferred to the conduction band and valence band of CdS, respectively, and then recombined. The 1

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decrease in photocurrent is linear with the concentration of sulfide ions in the range from 1 to 10000 pmol L-1 with a detection limit of 0.36 pmol L-1. Enabled by a unique sensitization mechanism, this PEC sensor features excellent performance in a wide linear range, high selectivity and sensitivity, high stability, and low fabrication cost. Keywords: Photoelectrochemistry, Sensor, TiO2 hollow sphere, Hydrogen sulfide, Ion exchange

Introduction As a molecule highly relevant to human health, hydrogen sulfide (H2S) has been widely researched in the fields of clinical diagnostics and environmental assay.1 It has been regarded as an endogenous signal molecule and plays a critical role in the central nervous system and other biological systems under the physiological and pathological conditions.2-4 On the other hand, it is one of the most common toxic gases frequently produced in various industrial activities such as petroleum extraction, fuel burning, and waste treatment.5 It falls under the category of chemical asphyxiant following carbon monoxide and cyanide gases. When the release concentration of H2S in the atmosphere is greater than the olfactory perception threshold of 300 ppb, it will harm human health and induce nausea, headache, and lung irritation.6 Even chronic, low-level exposures can also lead to irreversible health effects.7,8 From this point of view, there is an essential demand to develop a reliable and high-performance approach for H2S monitoring. Conventionally,

chromatography,9,10 spectrometry,11,12

and

electrochemical

protocols13-16 are used for the determination of H2S. Gas chromatography methods 2


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have proven to be precise, but their application is unpractical in environmental samples due to the requirement of a multi-stage testing protocol.10 Numerous optical approaches for sulfide analysis were thus developed based on sulfide-selective, organic- and organometallic-based dyes or nanoparticle-based sensors. 8, 17, 18 Albeit facile, these methods are of relatively low sensitivity and often need tedious procedures for preparation. The vast majority of H2S sensors are based on electrochemistry,14,

15

and often constructed by metal oxide semiconductors or

conducting polymers, which are unfortunately not cost-effective and produce inconsistent results that change with humidity and temperature. Additionally, they suffer from the interference of many chemicals which compete with H2S due to their affinity to metal oxides. Therefore, it is of high urgency to develop a new approach to improve the performance in H2S monitoring significantly. Recently, photoelectrochemical (PEC) detection, as a new technique, has aroused much research interest.19 By integrating an electrochemical instrument and an irradiation source, such sensors have the advantage of complete separation of the light excitation source and photocurrent detection signal.20 The reduced background signals endow these PEC sensors a higher sensitivity than conventional electrochemical approaches. Moreover, the PEC sensor using electronic readout is easy to miniaturize compared to optical techniques. It must be noted that it is the photocurrent increment/decrement but not the photocurrent itself on which the sensitivity of the PEC sensors depends. Consequently, those PEC sensors that may bring about photosensitizers or electron donors/receptors typically have a better sensitivity.21-23 3



Based on this principle, several PEC sensors for hydrogen sulfide or sulfide ion have been reported. Zheng group proposed an indirect PEC sensor for probing the cellular generation of H2S by using Cu2+ to quench the photogenerated electrons on the N-C-dot/TiO2 nanowire photoanodes and then S2- ions to reset the charge transfer and the photocurrent therein.24 Other PEC sensors for H2S were developed based on CdS nanoparticles covalently grafted on TiO2 nanotubes.25, 26 Obviously, in these previous systems, the amount of sensitizer coated on TiO2 surface can have a significant influence on the charge transfer efficiency and then the PEC sensitivity: the more sensitizer, the higher sensitivity. Therefore, it is feasible to further improve the PEC sensitivity for H2S through regulating the amount of coating materials. Compared to crystalline anatase or rutile TiO2, sol-gel-derived amorphous TiO2 may be an alternative for constructing hybrid nanostructures for sensor applications because its porous structures can help support more probes.27 In addition, sol-gel-derived amorphous TiO2 nanostructures, especially those treated by base etching, contain amorphous metal titanates which allow the convenient incorporation of other metal cations through ion exchange processes. In this work, different from the previous strategies for PEC sensors of H2S,25, 26 amorphous TiO2 hollow spheres were successfully prepared controllably by templating against SiO2 spheres followed by doping with Cd2+ probing ions into the porous structures through ion exchange reactions (Scheme 1). The amorphous TiO2 shells act as both the carriers for probing ions and the photoelectric beacon substrate. Also, a different sensitization mechanism is proposed: the CdS nanoparticles formed in amorphous TiO2 shells proportionately 4


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decrease the photocurrent of amorphous TiO2 shells with the increased concentration of H2S. This new mechanism, mainly attributed to the mismatch of energy band levels between the two materials, has been verified through characterization using X-ray photoelectron spectroscopy (XPS) and cyclic voltammetry. Finally, we demonstrate the excellent performance of the as-designed PEC sensor for H2S monitoring and highlight its convenience of fabrication, low production cost, and high sensitivity, selectivity, and stability.

Experimental section The reagents and apparatus used in this work were described in the Supporting Information. Preparation of SiO2 microspheres SiO2 microspheres were prepared by the Stöber method with some minor modification. TEOS (99%, 4.2 mL), ethanol (115 mL), ultrapure water (9.5 mL), and ammonium hydroxide (26%, 3.68 mL) were mixed sequentially and stirred for 5 h at room temperature. The resulting colloidal silica particles were separated by centrifugation, washed three times with ethanol, and then dispersed in 25 mL of ethanol. Preparation of SiO2@TiO2 microspheres In the synthesis of SiO2@TiO2 microspheres, a solution containing 4 mL of the above colloid, 60 mg of HPC, 15 mL of ethanol, 10 mL of acetonitrile, and 0.2 mL of ammonium hydroxide was prepared and stirred for 10 min. Next, 4 mL of an ethanol solution containing 1 mL of TBOT was added to the above mixture, which was stirred 5



for 2 h. The resulting core-shell microspheres were collected by centrifugation, washed three times with ethanol and ultrapure water, and finally dispersed in 20 mL of water. Preparation of TiO2 and Cd2+-doped TiO2 shells In the preparation of the TiO2 hollow spheres, a mixture of 20 mL of SiO2@TiO2 microspheres dispersion and 4 mL of 2.0 mol L-1 NaOH solution was prepared and then refluxed for 4 h. Etching of the SiO2 cores resulted in TiO2 shells, which were centrifuged, washed three times with ultrapure water, and then re-dispersed in 10 mL of 0.1 mmol L-1 cadmium nitrate. After stirring the solution for 6 h at room temperature, we collected the Cd2+-doped TiO2 shells by centrifugation. They were washed three times with ultrapure water and then dispersed in 5 mL of water. Preparation of Cd2+-doped TiO2 shells modified ITO electrodes Firstly, an ITO electrode (1 cm × 4 cm) was cleaned by a mixed solution containing hydrogen peroxide, ammonia, and water with a volume ratio of 1:1:50, and then rinsed with ultrapure water and dried in air. Meanwhile, the dispersion of Cd2+-doped TiO2 shells was diluted 30 times by water. A 20 µL solution of the diluent was dropped onto the surface of ITO electrode, dried at room temperature, rinsed with ultrapure water for several times to produce the Cd2+-doped hollow TiO2 photoanodes. TiO2 shell-based photoanodes were prepared for control experiment by the same method but without the incorporation of Cd2+. Energy levels calculation of nano CdS and TiO2 cyclic voltammetry Voltammetric experiments were performed with a CHI 660E electrochemical 6


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workstation to test the valence band of CdS according to the reference with minor modification.28 All experiments were carried out at room temperature using a conventional three-electrode system: glassy carbon or modified glassy carbon working electrode, a Pt wire as the counter electrode, and a Ag/AgCl/sat. KCl electrode as the reference electrode. Measurements were conducted in 0.1 mol L-1 PBS (phosphate buffered solution) as a supporting electrolyte in a nitrogen atmosphere. Oxidation potentials of CdS and TiO2 were determined in the windows of 0-2.0 V and 0-3.0 V, respectively.

Results and discussion Characterization of synthesized nanomaterials FESEM and HRTEM were used to investigate the morphology and microstructure of SiO2, SiO2@amorphous TiO2, amorphous TiO2 shells and Cd2+-doped amorphous TiO2 shells, as shown in Figure 1. The monodisperse SiO2 microspheres were ~ 260 nm in diameter (Figure 1A), and the average size increased to ~ 340 nm (Figure 1B) after the coating of amorphous TiO2 on the surface of SiO2 microspheres. Etching of the SiO2 cores by NaOH produced amorphous TiO2 hollow shells (Figure 1C and Figure S1), which were partially converted to sodium titanate phase as described previously.29 Afterwards, Cd2+ ions were doped to the amorphous TiO2 hollow shells by exchanging with Na+ ions (Figure 1D). The doped shells appeared to be darker in contrast to those without, which was consistent with the fact that Cd has a higher atomic number than Na. To further confirm the doping of Cd2+ in the amorphous TiO2 shells, we carried out elemental mapping and found that the Cd species were 7



uniformly dispersed in TiO2 shells with a distribution profile similar to that of Ti and O (Figure 2). Herein, the amorphous structure of the TiO2 hollow spheres was also confirmed by XRD spectrum in Figure S2. Figure 3A showed the UV-vis diffuse reflectance spectra of TiO2 shells, Cd2+-doped TiO2 shells, and CdS-doped amorphous TiO2 shells prepared by exposing Cd2+-doped TiO2 shells to sulfide. It could be clearly seen that the pure amorphous TiO2 shells were inactive in the visible range, and its energy band gap of 3.65 eV can be calculated from the absorption edge at 340 nm by following the relationship of Eg = 1240 / λ. The UV-vis diffuse reflectance spectrum for Cd2+-doped amorphous TiO2 shells showed almost no changes. While the sample still showed the absorption edge at 340 nm after being exposed to sulfide, additional absorption appeared in the range from 340 nm to 513 nm, which could be attributed to the absorption of the generated CdS nanoparticles. An energy band gap of 2.4 eV could be obtained based on the absorption edge of 513 nm, which is in accord with the reported value for CdS nanoparticles in literature.30 The energy band gap of 3.65 eV calculated from the amorphous TiO2 shells in this work is different from that of 3.2 eV for anatase and 3.0 eV for rutile TiO2.31-33 In order to obtain the valence band information, the amorphous TiO2 shells were characterized by XPS (Figure 3B), which indicated the valence band of ~ 2.6 eV vs. NHS as calculated based on the tangent line of the spectrum. Then the conduction band could be calculated at -1.05 eV vs. NHS. In order to confirm the obtained valence band of amorphous TiO2 by XPS, the electrochemical approach was also chosen to verify it.28 The oxidation potential of amorphous TiO2 had been 8


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determined to be 2.31 V against Ag/AgCl/sat. KCl electrode in Figure 4A. Therefore, the valence band and conduction band of amorphous TiO2 using electrochemical method can be tested and calculated by the following equations: (1) Eoxidation = 2.31 V vs. Ag/AgCl/sat. KCl = (2.31 + 0.20) V vs. NHE =2.51 V vs. NHE Therefore, EVBTiO2 = 2.51 eV. (2) EVBTiO2 = 2.51 eV and EgTiO2=3.65 eV Therefore, ECBTiO2 = -1.14 eV The result indicates that it is reasonably consistent with that by XPS. With the same electrochemical method, the valence band of 1.9 eV and conduction band of -0.5 eV for CdS nanoparticles can be obtained, respectively, as shown in Figure 4B. Thus, the energy band levels of amorphous TiO2 shells and the CdS nanoparticles were obtained. As illustrated in Figure 5B, the conduction band level of CdS nanoparticles was apparently lower than that of the amorphous TiO2 shells, suggesting that the photogenerated electrons from CdS nanoparticles could not be injected into the conduction band of the TiO2 shells. This band structure relationship is different from the conventional ones between anatase/rutile TiO2 and CdS nanoparticles which enables promoted carrier separation and enhanced photoelectric conversion efficiencies.34 The detailed sensitized and inactive mechanism comparison has been demonstrated as follows. The TiO2 particles used in photovoltaics and photocatalysis 9



applications are typically crystalline (often anatase), while our TiO2 is amorphous. The difference in crystallinity leads to different band structures, which further leads to different responses in photocurrent when combined with CdS. To make it more clear, we have illustrated the band structures of TiO2 and CdS in Figure 5 to clarify the differences between the case of photovoltaics (or photocatalysis) and our case of photoelectrochemical sensing. For crystalline TiO2, typically anatase, the bandgap is ~ 3.2 eV, with the conduction band at -0.29 eV and valence band at 2.91 eV. The bandgap of CdS nanoparticles is ~ 2.4 eV, with the conduction band at -0.52 eV and valence band at 1.88 eV. When these two materials are combined, the photoexcited electrons from the conduction band of CdS nanoparticles can be injected into the crystalline TiO2.35 In such cases, the CdS nanoparticles serve as sensitizers (Figure 5A). However, in our work, the TiO2 is amorphous, which has a bandgap of ~3.65 eV, with a conduction band at -1.05 eV and valence band at 2.6 eV. When combined with CdS nanoparticles, the band levels do not match so CdS nanoparticles do not have the sensitization effect. Instead, the photogenerated electrons and holes from amorphous TiO2 are transferred to the conduction and valence bands of CdS nanoparticles, respectively.28,36 As a result, the photoinduced electrons and holes are recombined in CdS nanoparticles, causing a decreased photocurrent (Figure 5B). It is obvious that the decrement of photocurrent is due to the mismatching energy band levels between the amorphous TiO2 shells and CdS nanoparticles. Therefore, an “on-off” PEC sensing mechanism for hydrogen sulfide detection can be constructed using Cd2+-doped amorphous TiO2 shells based on their proportionately decreased 10


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photocurrent. PEC sensing mechanism The ITO electrode modified by pure amorphous TiO2 shells showed a photocurrent of 38.2 nA at a bias of 0.2 V in 0.1 mol L-1 PBS (pH 7.0) with irradiation (Figure 6A, curve a), while the one modified by the Cd2+-doped amorphous TiO2 shells showed a photocurrent of 36.8 nA (Figure 6A, curve b). The small decrease in the photocurrent may be caused by the capture of photoelectrons by Cd2+. The exposure of the electrode to a 2 nmol L-1 Na2S solution (H2S in the aqueous medium) led to the production of CdS inside the amorphous TiO2 shells. As can be seen from the curve c of Figure 6A, the photocurrent was obviously decreased to 20.2 nA under the chopped light irradiation (150 W Xe lamp). This could be attributed to the mismatching energy band levels between the amorphous TiO2 shells and CdS nanoparticles, and the photoexcited electrons and holes from amorphous TiO2 are transferred to the conduction band and valence band of CdS, respectively.28,36 As a result, these carriers are recombined in CdS nanoparticles, causing the decreased photocurrent. This comparison in the photocurrent of these differently modified ITO electrodes demonstrates the feasibility of our design for the detection of H2S. Analytical performance The fabricated PEC sensor based on Cd2+-doped amorphous TiO2 shells was used to detect the H2S captured in an aqueous medium (For convenience, Na2S was used as the source here). As can be seen from Figure 7A, the photocurrent gradually decreased with the increased concentration of sulfide at a bias voltage of 0.2 V under 11



irradiation. Figure 7B clearly indicated that there was a linear relationship between log C and (I0 – I). Herein, log C is referred to the logarithm of the concentrations of sulfide, while I0 and I represented the photocurrent intensity of the photoanode in the absence and presence of sulfide, respectively. The linear range was from 1 pmol L-1 to 10000 pmol L-1 with a detection limit of 0.36 pmol L-1, which was lower than all of those reported previously (as shown in Table 1). The wide dynamic range up to four orders of magnitude benefits from the porous structures of the amorphous TiO2 shells that can support a relatively large amount of Cd2+ probe ions. The sensitivity at sub-picomole level is attributed to the recombination of the photoexcited electrons and holes that are transferred from the amorphous TiO2 to CdS. In addition to the excellent performance in sensing, the PEC sensor could be fabricated conveniently at a low cost. As an essential parameter for PEC sensing, the stability of the sensor was investigated. Figure 8 shows the result when the photoexcited process was repeated 3 times over 90 s in exposure to 2 nmol L-1 Na2S at a bias voltage of 0.2 V under the chopped irradiation. The photocurrent displayed almost no change during these photoexcitation cycles, indicating excellent stability of the system. Also, the reproducibility in the fabrication of the PEC sensor is also very good: we observed a relative standard deviation of 4.9% by estimation based on the slopes of the calibration plots of six freshly prepared PEC sensors. We also characterized the shelf life of the fabricated PEC sensor by storing it in a refrigerator at 4◦C and measuring its photocurrent response every week. There was no apparent change in the photocurrent 12


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response to sulfide after two weeks, and 95.6% of the initial photocurrent response was maintained after storage for five weeks, suggesting the robustness of the as-designed PEC sensor. The selectivity of the fabricated PEC sensor for 1 nmol L-1 S2- in the aqueous medium was also investigated against other competitive anions such as SO42−, HSO3−, NO3−, NO2−, and HCO3− at different concentrations. As shown in Figure 9A, the interference ratios were no more than 6.8% in the presence of 10-fold SO42−, 30-fold HSO3−, 50-fold NO3−, 50-fold NO2−, 5-fold HCO3− or the same concentration of each above, indicating a high selectivity which can be attributed to the high affinity between sulfide ion and Cd2+. In order to further confirm that the above anions did not interfere with the detection of S2- or H2S (Figure 9B), 1.0 mmol L-1 of SO42−, HSO3−, NO3−, NO2−, HCO3− was used to react with Cd2+-doped amorphous TiO2 shells, and then the corresponding products were characterized by UV-vis spectrum. Compared to the spectrum of the Cd2+-doped amorphous TiO2 shells, the above anions did not cause apparent changes, indicating that the anions did not bind strongly to the surface of the TiO2 shells, which is consistent with the much smaller Ksp of CdS than the compounds based on other anions. In order to confirm that the proposed PEC sensor can be used to detect H2S in a practical setting, we collected 1.0 L of H2S-containing industry waste gas and reacted it with 100 mL of 1.0 mmol L-1 NaOH solution, and then injected a 30 µL of the resulting solution to an electrochemical cell containing 30 mL pH 7.0 PBS for PEC sensing. A concentration of 8.3 nmol L-1 (218.7 ppb) was detected, and recovery 13



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between 94.8% and 104.9% was obtained, indicating that the fabricated PEC sensor is efficient for the detection of H2S in industry waste gas under practical settings.

Conclusions In this work, we report a facile and robust "on-off" PEC sensor for selective detection of H2S by modifying ITO electrodes with Cd2+-doped amorphous TiO2 shells. The amorphous TiO2 shells act as both the carriers for probing ions and the photoelectric beacon substrates. Different from the reported ones, this PEC sensor is established based on the mismatch of the energy band levels between the amorphous TiO2 shells and the generated CdS nanoparticles. This new PEC sensor exhibits excellent performance in sensing S2- and H2S, and displays advantages such as simplicity, wide linear range, and high selectivity, sensitivity, stability, and reproducibility.

Acknowledgments We gratefully acknowledge the financial support from the Natural Science Foundation of China (21305123, 21505117, 21775135), the Natural Science Foundation of Jiangsu Province (BK20161309), the research fund of Jiangsu Collaborative Innovation Center for Ecological Building Materials and Environmental Protection

Equipment

(GX2015103,

CP201502),

and

this

work

was also

sponsored by QingLanProject of Jiangsu Province. Acknowledgment is also made to the Donors of the American Chemical Society Petroleum Research Fund for partial support of this research (55904-ND10).

Supporting Information 14


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The Supporting Information is available free of charge on the ACS Publications website at DOI: XXXX.

Experimental procedures and additional characterizations by HRTEM and XRD (PDF)

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54, 12758-12761. (38) Tian, X.; Li, Z.; Lau, C.; Lu, J. Anal. Chem. 2015, 87, 11325-11331. (39) Wang, Y.; Ge, S.; Zhang, L.; Yu, J.; Yan, Mei.; Huang, J. Biosens. Bioelectron. 2017, 89, 859-865. Figure and table captions Scheme 1. Schematic illustration of the synthesis of Cd2+-doped amorphous TiO2 shells and their use for PEC sensing of hydrogen sulfide. Figure 1. FESEM images of SiO2 (A) and SiO2@amorphous TiO2 (B); HRTEM images of amorphous TiO2 shells (C) and Cd2+-doped amorphous TiO2 shells (D). Figure 2. HRTEM image of Cd2+-doped amorphous TiO2 shells (A) and elemental mappings of Ti (B), O (C) and Cd (D) corresponding to the image of (A). Figure 3. (A) UV–vis diffuse reflectance spectra of amorphous TiO2 shells (a), Cd2+-doped amorphous TiO2 shells (b), CdS-doped amorphous TiO2 shells (c). (B) XPS valence band of amorphous TiO2 shells measured relative to the Fermi level. Figure 4. (A) Cyclic voltammetry of glassy carbon electrode (curve a) and TiO2 shell modified glassy carbon electrode (curve b); (B) Cyclic voltammetry of glassy carbon electrode (curve c) and TiO2/CdS modified glassy carbon electrode (curve d) in 0.1 mol L-1PBS under a nitrogen flow. Figure 5. The conventional sensitized mechanism by anatase TiO2 combined with CdS nanoparticles (A), and schematic illustration of the proposed “on-off” PEC sensing mechanism for hydrogen sulfide using Cd2+-doped amorphous TiO2 shells (B). 18


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Figure 6. Photocurrent responses of amorphous TiO2 shells/ITO (a), Cd2+-doped amorphous TiO2 shells/ITO (b), CdS-doped amorphous TiO2 shells/ITO (c), measured in 0.1 mol L-1 PBS (pH 7.0) at a bias voltage of 0.2 V with chopped light irradiation (150 W Xe lamp). Figure 7. (A) Photocurrent responses of the prepared Cd2+-doped amorphous TiO2 shells/ITO in 0.1 mol L-1 PBS (pH 7.0) containing 0, 1, 2, 4, 6, 8, 10, 20, 40, 60, 80, 100, 200, 400, 600, 800, 1000, 2000, 4000, 6000, 8000 and 10000 pmol L-1 Na2S at a bias potential of 0.2 V with chopped light irradiation (150 W Xe lamp); (B) The linear calibration curve. Figure 8. Time-based typical photocurrent of the CdS-doped amorphous TiO2 shells/ITO in the presence of 2 nmol L-1 Na2S in 0.1 mol L-1 PBS (pH 7.0) at a bias voltage of 0.2 V in response to repeated chopped light irradiation every 10 s (150 W Xe lamp). Figure 9. (A)The investigation of interference by common species such as SO42−, HSO3−, NO3−, NO2−, HCO3−or the mixture of each above. The measurements were carried out by adding the above species in 0.1 mol L-1 PBS (pH 7.0) containing 1.0 nmol L-1 Na2S at a bias potential of 0.2 V with chopped light irradiation (150 W Xe lamp); (B) UV–vis diffuse reflectance spectra of Cd2+-doped amorphous TiO2 shells before and after reacting with 1.0 mmol L-1 of SO42−, HSO3−, NO3−, NO2−, HCO3−. Table 1 The different detection methods for H2S.

19



Scheme 1

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Figure 1

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Figure 2

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Figure 3

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Figure 4

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Figure 5

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Figure 6

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Figure 7

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Figure 8

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Figure 9

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Table 1 Detection methods

Linear range

Limit of detection

Reference

Electrochemical

0.08-0.38 µmol L-1

0.038 µmol L-1

16

Optical

0.65-2.54 nmol L-1

0.11 nmol L-1

6

Colorimetry

3-10 µmol L-1

0.2 µmol L-1

18

SERS

0.12-84 µmol L-1

0.1 µmol L-1

37

Fluorescence

10-1000 µmol L-1

0.1 µmol L-1

38

PEC

10-100 nmol L-1

10 nmol L-1

24

PEC

10 nmol-1 mmol L-1

0.31 nmol L-1

26

PEC

10 nmol-1 mmol L-1

0.7 nmol L-1

25

PEC

1 nmol-5 mmol L-1

29 ng mL-1

39

PEC

1-10000 pmol L-1

0.36 pmol L-1

This work

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TOC

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Cd2+-doped amorphous TiO2 hollow spheres for robust and

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